How does a servo motor work?
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Feb 15, 2017
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About This Presentation
EMP
Slide Content
How does a servo motor work?
The simplicity of a servo is among the features that make them so reliable. The heart of a servo is a small
direct current (DC) motor, similar to what you might find in an inexpensive toy. These motors run on
electricity from a battery and spin at high RPM (rotations per minute) but put out very low torque (a
twisting force used to do work— you apply torque when you open a jar). An arrangement of gears takes
the high speed of the motor and slows it down while at the same time increasing the torque. (Basic law of
physics: work = force x distance.) A tiny electric motor does not have much torque, but it can spin really
fast (small force, big distance). The gear design inside the servo case converts the output to a much
slower rotation speed but with more torque (big force, little distance). The amount of actual work is the
same, just more useful. Gears in an inexpensive servo motor are generally made of plastic to keep it
lighter and less costly (see Figure 3 below). On a servo designed to provide more torque for heavier work,
the gears are made of metal (see Figure 4 below) and are harder to damage.
Figure 3. The gears in a typical standard-size servo are made of plastic and convert the fast, low-power motion of the motor (on the right) to the
output shaft (on the left).
The simplicity of a servo is among the features that make them so reliable. The heart of a servo is a small
direct current (DC) motor, similar to what you might find in an inexpensive toy. These motors run on
electricity from a battery and spin at high RPM (rotations per minute) but put out very low torque (a
twisting force used to do work— you apply torque when you open a jar). An arrangement of gears takes
the high speed of the motor and slows it down while at the same time increasing the torque. (Basic law of
physics: work = force x distance.) A tiny electric motor does not have much torque, but it can spin really
fast (small force, big distance). The gear design inside the servo case converts the output to a much
slower rotation speed but with more torque (big force, little distance). The amount of actual work is the
same, just more useful. Gears in an inexpensive servo motor are generally made of plastic to keep it
lighter and less costly (see Figure 3 below). On a servo designed to provide more torque for heavier work,
the gears are made of metal (see Figure 4 below) and are harder to damage.
Figure 3. The gears in a typical standard-size servo are made of plastic and convert the fast, low-power motion of the motor (on the right) to the
output shaft (on the left).
Figure 4. In a high-power servo, the plastic gears are replaced by metal ones for strength. The motor is usually more powerful than in a low-cost
servo and the overall output torque can be as much as 20 times higher than a cheaper plastic one. Better quality is more expensive, and high-
output servos can cost two or three times as much as standard ones.
With a small DC motor, you apply power from a battery, and the motor spins. Unlike a simple DC motor,
however, a servo's spinning motor shaft is slowed way down with gears. A positional sensor on the final
gear is connected to a small circuit board (see Figure 5 below). The sensor tells this circuit board how far
the servo output shaft has rotated. The electronic input signal from the computer or the radio in a remote-
controlled vehicle also feeds into that circuit board. The electronics on the circuit board decode the
signals to determine how far the user wants the servo to rotate. It then compares the desired position to
the actual position and decides which direction to rotate the shaft so it gets to the desired position.
servo and the overall output torque can be as much as 20 times higher than a cheaper plastic one. Better quality is more expensive, and high-
output servos can cost two or three times as much as standard ones.
With a small DC motor, you apply power from a battery, and the motor spins. Unlike a simple DC motor,
however, a servo's spinning motor shaft is slowed way down with gears. A positional sensor on the final
gear is connected to a small circuit board (see Figure 5 below). The sensor tells this circuit board how far
the servo output shaft has rotated. The electronic input signal from the computer or the radio in a remote-
controlled vehicle also feeds into that circuit board. The electronics on the circuit board decode the
signals to determine how far the user wants the servo to rotate. It then compares the desired position to
the actual position and decides which direction to rotate the shaft so it gets to the desired position.
Figure 5. The circuit board and DC motor in a high-power servo. Did you notice how few parts are on the circuit board? Servos have evolved to
a very efficient design over many years.
Imagine you are playing catch with a friend on a sports field. You stand at one end and want your friend
to go out for a long throw. You could keep calling out "farther, farther, farther" until she got as far away as
you wanted. But if she went out farther than you can throw, you would have to call out "closer" until she
got back to the right spot. If she were a simple motor in a robot arm and you were the microprocessor,
you would have to spend some of your time watching what she did and giving her commands to move her
back to the right spot (this is called a feedback loop). If she were a servo motor, you could just say "go
out exactly 4.5 meters" and know that she would find the right spot. That is what makes servo motors so
useful: once you tell them what you want done, they do the job without your help. This automatic seeking
behavior of servo motors makes them perfect for many robotic applications.
Types of servo motors
Servos come in many sizes and in three basic types: positional rotation, continuous rotation, and linear.
Positional rotation servo: This is the most common type of servo motor. The output shaft
rotates in about half of a circle, or 180 degrees. It has physical stops placed in the gear
mechanism to prevent turning beyond these limits to protect the rotational sensor. These
common servos are found in radio-controlled cars and water- and aircraft, toys, robots, and many
other applications.
Continuous rotation servo: This is quite similar to the common positional rotation servo motor,
except it can turn in either direction indefinitely. The control signal, rather than setting the static
position of the servo, is interpreted as the direction and speed of rotation. The range of possible
commands causes the servo to rotate clockwise or counterclockwise as desired, at varying
speed, depending on the command signal. You might use a servo of this type on a radar dish if
you mounted one on a robot. Or you could use one as a drive motor on a mobile robot.
a very efficient design over many years.
Imagine you are playing catch with a friend on a sports field. You stand at one end and want your friend
to go out for a long throw. You could keep calling out "farther, farther, farther" until she got as far away as
you wanted. But if she went out farther than you can throw, you would have to call out "closer" until she
got back to the right spot. If she were a simple motor in a robot arm and you were the microprocessor,
you would have to spend some of your time watching what she did and giving her commands to move her
back to the right spot (this is called a feedback loop). If she were a servo motor, you could just say "go
out exactly 4.5 meters" and know that she would find the right spot. That is what makes servo motors so
useful: once you tell them what you want done, they do the job without your help. This automatic seeking
behavior of servo motors makes them perfect for many robotic applications.
Types of servo motors
Servos come in many sizes and in three basic types: positional rotation, continuous rotation, and linear.
Positional rotation servo: This is the most common type of servo motor. The output shaft
rotates in about half of a circle, or 180 degrees. It has physical stops placed in the gear
mechanism to prevent turning beyond these limits to protect the rotational sensor. These
common servos are found in radio-controlled cars and water- and aircraft, toys, robots, and many
other applications.
Continuous rotation servo: This is quite similar to the common positional rotation servo motor,
except it can turn in either direction indefinitely. The control signal, rather than setting the static
position of the servo, is interpreted as the direction and speed of rotation. The range of possible
commands causes the servo to rotate clockwise or counterclockwise as desired, at varying
speed, depending on the command signal. You might use a servo of this type on a radar dish if
you mounted one on a robot. Or you could use one as a drive motor on a mobile robot.
Linear servo: This is also like the positional rotation servo motor described above, but with
additional gears (usually a rack and pinion mechanism) to change the output from circular to
back-and-forth. These servos are not easy to find, but you can sometimes find them at hobby
stores where they are used as actuators in larger model airplanes.
Selecting a servo motor
When starting a project that uses servos, look at your application requirements. How fast must the servo
rotate from one position to another? How hard will it have to push or pull? Do I need a positional rotation,
continuous rotation, or linear servo? How much overshoot is allowable? The less you pay for the servo,
the less mechanical power it will have to muster and the less precision it will have in its movements. You
can pay a bit more and get one that moves quickly, but it may not have a lot of power. You can also buy
one that will pull or push large loads, but it may not move quickly or precisely. Manufacturers' websites
and online hobby guides will have a lot of this information you can use to compare models. You will also
find that hobby stores have a selection of servos and can usually help you decide which one is right for
your project and budget.
Controlling a servo motor
Servos take commands from a series of pulses sent from the computer or radio. A pulse is a transition
from low voltage to high voltage which stays high for a short time, and then returns to low. In battery
devices such as servos, "low" is considered to be ground or 0 volts and "high" is the battery voltage.
Servos tend to work in a range of 4.5 to 6 volts, so they are extremely hobbyist computer-friendly.
Have you ever picked up one end of a rope that was tied to a tree or held one end of a jump rope while a
friend held the other? Imagine that, while holding your end of the rope, you moved your arm up and down.
The rope would make a big hump that would travel from your end to the other. What you have done is
applied a pulse, and it traveled down the rope as a wave. As you raise your hand up and down, if you
keep your hand in the air longer, someone watching this experiment from the side would see that the
pulse in the rope would be longer or wider. If you bring your hand down sooner, the pulse is shorter or
more narrow. This is the pulse width. If you keep your end going up and down, making a whole bunch of
these pulses one after another, you have created a pulse train (see Figure 6 below). How often did you
raise and lower your end? This is the frequency of your pulse train and is measured in pulses per
second, or Hz (abbreviation of "hertz").
Note: The microprocessor in your computer uses pulses from special clock circuitry to get the job done.
Have you heard of your computer speed referred to as something like 1.7 gigahertz (GHz)? This is a way
of saying that the pulses are coming at 1.7 billion pulses per second, or 1,700,000,000 Hz. Imagine trying
to move your rope that fast!
additional gears (usually a rack and pinion mechanism) to change the output from circular to
back-and-forth. These servos are not easy to find, but you can sometimes find them at hobby
stores where they are used as actuators in larger model airplanes.
Selecting a servo motor
When starting a project that uses servos, look at your application requirements. How fast must the servo
rotate from one position to another? How hard will it have to push or pull? Do I need a positional rotation,
continuous rotation, or linear servo? How much overshoot is allowable? The less you pay for the servo,
the less mechanical power it will have to muster and the less precision it will have in its movements. You
can pay a bit more and get one that moves quickly, but it may not have a lot of power. You can also buy
one that will pull or push large loads, but it may not move quickly or precisely. Manufacturers' websites
and online hobby guides will have a lot of this information you can use to compare models. You will also
find that hobby stores have a selection of servos and can usually help you decide which one is right for
your project and budget.
Controlling a servo motor
Servos take commands from a series of pulses sent from the computer or radio. A pulse is a transition
from low voltage to high voltage which stays high for a short time, and then returns to low. In battery
devices such as servos, "low" is considered to be ground or 0 volts and "high" is the battery voltage.
Servos tend to work in a range of 4.5 to 6 volts, so they are extremely hobbyist computer-friendly.
Have you ever picked up one end of a rope that was tied to a tree or held one end of a jump rope while a
friend held the other? Imagine that, while holding your end of the rope, you moved your arm up and down.
The rope would make a big hump that would travel from your end to the other. What you have done is
applied a pulse, and it traveled down the rope as a wave. As you raise your hand up and down, if you
keep your hand in the air longer, someone watching this experiment from the side would see that the
pulse in the rope would be longer or wider. If you bring your hand down sooner, the pulse is shorter or
more narrow. This is the pulse width. If you keep your end going up and down, making a whole bunch of
these pulses one after another, you have created a pulse train (see Figure 6 below). How often did you
raise and lower your end? This is the frequency of your pulse train and is measured in pulses per
second, or Hz (abbreviation of "hertz").
Note: The microprocessor in your computer uses pulses from special clock circuitry to get the job done.
Have you heard of your computer speed referred to as something like 1.7 gigahertz (GHz)? This is a way
of saying that the pulses are coming at 1.7 billion pulses per second, or 1,700,000,000 Hz. Imagine trying
to move your rope that fast!
Figure 6. An example of a pulse train you might generate to control a servo, as shown in a screen capture from an inexpensive
digital oscilloscope, an instrument for observing voltages). Here, a pulse is generated once every 20 milliseconds, or at about 50 Hz. In this
example, the pulse width is about 2 milliseconds, which would have a servo rotate almost all the way to one end of its rotation. An oscilloscope
is incredibly useful for testing and debugging systems that use servos.
Your servo must be connected to a source of power (4.5 to 6 volts) and the control signal must come from
a computer or other circuitry. Each servo's requirements vary slightly, but a pulse train (as in Figure 6
above) of about 50 to 60 Hz works well for most models. The pulse width will vary from approximately 1
millisecond to 2 or 3 milliseconds (one millisecond is 1/1000 of a second). Popular hobbyist computers
such as the Arduino
TM
have software commands in the language for generating these pulse trains. But
any microcontroller can be programmed to generate these waveforms. A system that passes information
based on the width of pulses uses pulse width modulation (or PWM) and is a very common way of
controlling motor speeds and LED brightness as well as servo motor position.
Resources
The following selection guide can help you determine which Futaba® servo fits your needs:
Hobbico, Inc. (2012). Futaba® servo selection. Retrieved September 13, 2012, from www.futaba-
rc.com/servos/servo-select.php
================================================================================
What are Servo Motors?
A servo motor is a linear or rotary actuator that provides fast precision position control
for closed-loop position control applications. Unlike large industrial motors, a servo
motor is not used for continuous energy conversion.
Servo motors have a high speed response due to low inertia and are designed with
small diameter and long rotor length. Then how do servo motors work?
digital oscilloscope, an instrument for observing voltages). Here, a pulse is generated once every 20 milliseconds, or at about 50 Hz. In this
example, the pulse width is about 2 milliseconds, which would have a servo rotate almost all the way to one end of its rotation. An oscilloscope
is incredibly useful for testing and debugging systems that use servos.
Your servo must be connected to a source of power (4.5 to 6 volts) and the control signal must come from
a computer or other circuitry. Each servo's requirements vary slightly, but a pulse train (as in Figure 6
above) of about 50 to 60 Hz works well for most models. The pulse width will vary from approximately 1
millisecond to 2 or 3 milliseconds (one millisecond is 1/1000 of a second). Popular hobbyist computers
such as the Arduino
TM
have software commands in the language for generating these pulse trains. But
any microcontroller can be programmed to generate these waveforms. A system that passes information
based on the width of pulses uses pulse width modulation (or PWM) and is a very common way of
controlling motor speeds and LED brightness as well as servo motor position.
Resources
The following selection guide can help you determine which Futaba® servo fits your needs:
Hobbico, Inc. (2012). Futaba® servo selection. Retrieved September 13, 2012, from www.futaba-
rc.com/servos/servo-select.php
================================================================================
What are Servo Motors?
A servo motor is a linear or rotary actuator that provides fast precision position control
for closed-loop position control applications. Unlike large industrial motors, a servo
motor is not used for continuous energy conversion.
Servo motors have a high speed response due to low inertia and are designed with
small diameter and long rotor length. Then how do servo motors work?
Servo motors work on servo mechanism that uses position feedback to control the
speed and final position of the motor. Internally, a servo motor combines a motor,
feedback circuit, controller and other electronic circuit.
Servo motors
It uses encoder or speed sensor to provide speed feedback and position. This feedback
signal is compared with input command position (desired position of the motor
corresponding to a load), and produces the error signal (if there exist a difference
between them).
The error signal available at the output of error detector is not enough to drive the
motor. So the error detector followed by a servo amplifier raises the voltage and power
level of the error signal and then turns the shaft of the motor to desired position.
Types of Servo Motors
Basically, servo motors are classified into AC and DC servo motors depending upon the
nature of supply used for its operation. Brushed permanent magnet DC servo motors
are used for simple applications owing to their cost, efficiency and simplicity.
These are best suited for smaller applications. With the advancement of microprocessor
and power transistor, AC servo motors are used more often due to their high accuracy
control.
speed and final position of the motor. Internally, a servo motor combines a motor,
feedback circuit, controller and other electronic circuit.
Servo motors
It uses encoder or speed sensor to provide speed feedback and position. This feedback
signal is compared with input command position (desired position of the motor
corresponding to a load), and produces the error signal (if there exist a difference
between them).
The error signal available at the output of error detector is not enough to drive the
motor. So the error detector followed by a servo amplifier raises the voltage and power
level of the error signal and then turns the shaft of the motor to desired position.
Types of Servo Motors
Basically, servo motors are classified into AC and DC servo motors depending upon the
nature of supply used for its operation. Brushed permanent magnet DC servo motors
are used for simple applications owing to their cost, efficiency and simplicity.
These are best suited for smaller applications. With the advancement of microprocessor
and power transistor, AC servo motors are used more often due to their high accuracy
control.
DC Servo Motors
A DC servo motor consists of a small DC motor, feedback potentiometer, gearbox,
motor drive electronic circuit and electronic feedback control loop. It is more or less
similar to the normal DC motor.
The stator of the motor consists of a cylindrical frame and the magnet is attached to the
inside of the frame.
DC Servo Motor
The rotor consists of brush and shaft. A commutator and a rotor metal supporting frame
are attached to the outside of the shaft and the armature winding is coiled in the rotor
metal supporting frame.
A brush is built with an armature coil that supplies the current to the commutator. At the
back of the shaft, a detector is built into the rotor in order to detect the rotation speed.
With this construction, it is simple to design a controller using simple circuitry because
the torque is proportional to the amount of current flow through the armature.
And also the instantaneous polarity of the control voltage decides the direction of torque
developed by the motor. Types of DC servo motors include series motors, shunt control
motor, split series motor, and permanent magnet shunt motor.
A DC servo motor consists of a small DC motor, feedback potentiometer, gearbox,
motor drive electronic circuit and electronic feedback control loop. It is more or less
similar to the normal DC motor.
The stator of the motor consists of a cylindrical frame and the magnet is attached to the
inside of the frame.
DC Servo Motor
The rotor consists of brush and shaft. A commutator and a rotor metal supporting frame
are attached to the outside of the shaft and the armature winding is coiled in the rotor
metal supporting frame.
A brush is built with an armature coil that supplies the current to the commutator. At the
back of the shaft, a detector is built into the rotor in order to detect the rotation speed.
With this construction, it is simple to design a controller using simple circuitry because
the torque is proportional to the amount of current flow through the armature.
And also the instantaneous polarity of the control voltage decides the direction of torque
developed by the motor. Types of DC servo motors include series motors, shunt control
motor, split series motor, and permanent magnet shunt motor.
Working Principle of DC Servo Motor
A DC servo motor is an assembly of four major components, namely a DC motor, a
position sensing device, a gear assembly, and a control circuit. The below figure shows
the parts that consisting in RC servo motors in which small DC motor is employed for
driving the loads at precise speed and position.
Internal diagram
A DC reference voltage is set to the value corresponding to the desired output. This
voltage can be applied by using another potentiometer, control pulse width to voltage
converter, or through timers depending on the control circuitry.
The dial on the potentiometer produces a corresponding voltage which is then applied
as one of the inputs to error amplifier.
In some circuits, a control pulse is used to produce DC reference voltage corresponding
to desired position or speed of the motor and it is applied to a pulse width to voltage
converter.
In this converter, the capacitor starts charging at a constant rate when the pulse high.
Then the charge on the capacitor is fed to the buffer amplifier when the pulse is low and
this charge is further applied to the error amplifier.
A DC servo motor is an assembly of four major components, namely a DC motor, a
position sensing device, a gear assembly, and a control circuit. The below figure shows
the parts that consisting in RC servo motors in which small DC motor is employed for
driving the loads at precise speed and position.
Internal diagram
A DC reference voltage is set to the value corresponding to the desired output. This
voltage can be applied by using another potentiometer, control pulse width to voltage
converter, or through timers depending on the control circuitry.
The dial on the potentiometer produces a corresponding voltage which is then applied
as one of the inputs to error amplifier.
In some circuits, a control pulse is used to produce DC reference voltage corresponding
to desired position or speed of the motor and it is applied to a pulse width to voltage
converter.
In this converter, the capacitor starts charging at a constant rate when the pulse high.
Then the charge on the capacitor is fed to the buffer amplifier when the pulse is low and
this charge is further applied to the error amplifier.
So the length of the pulse decides the voltage applied at the error amplifier as a desired
voltage to produce the desired speed or position.
In digital control, microprocessor or microcontroller are used for generating the PWM
pluses in terms of duty cycles to produce more accurate control signals.
The feedback signal corresponding to the present position of the load is obtained by
using a position sensor. This sensor is normally a potentiometer that produces the
voltage corresponding to the absolute angle of the motor shaft through gear
mechanism. Then the feedback voltage value is applied at the input of error amplifier
(comparator).
The error amplifier is a negative feedback amplifier and it reduces the difference
between its inputs. It compares the voltage related to current position of the motor
(obtained by potentiometer) with desired voltage related to desired position of the motor
(obtained by pulse width to voltage converter), and produces the error either a positive
or negative voltage.
This error voltage is applied to the armature of the motor. If the error is more, the more
output is applied to the motor armature.
As long as error exists, the amplifier amplifies the error voltage and correspondingly
powers the armature. The motor rotates till the error becomes zero. If the error is
negative, the armature voltage reverses and hence the armature rotates in the opposite
direction.
Difference between the DC and AC Servo Motors
voltage to produce the desired speed or position.
In digital control, microprocessor or microcontroller are used for generating the PWM
pluses in terms of duty cycles to produce more accurate control signals.
The feedback signal corresponding to the present position of the load is obtained by
using a position sensor. This sensor is normally a potentiometer that produces the
voltage corresponding to the absolute angle of the motor shaft through gear
mechanism. Then the feedback voltage value is applied at the input of error amplifier
(comparator).
The error amplifier is a negative feedback amplifier and it reduces the difference
between its inputs. It compares the voltage related to current position of the motor
(obtained by potentiometer) with desired voltage related to desired position of the motor
(obtained by pulse width to voltage converter), and produces the error either a positive
or negative voltage.
This error voltage is applied to the armature of the motor. If the error is more, the more
output is applied to the motor armature.
As long as error exists, the amplifier amplifies the error voltage and correspondingly
powers the armature. The motor rotates till the error becomes zero. If the error is
negative, the armature voltage reverses and hence the armature rotates in the opposite
direction.
Difference between the DC and AC Servo Motors
DC SERVO MOTOR AC SERVO MOTOR
It delivers high power output Delivers low output of about 0.5 W to 100 W
It has more stability problems It has less stable problems
It requires frequent maintenance due to
the presence of commutator
It requires less maintenance due to the absence of
commutator
It provides high efficiency The efficiency of AC servo motor is less and is about 5 to
20%
The life of DC servo motor depends on
the life on brush life
The life of AC servo motor depends on bearing life
It includes permanent magnet in its
construction
The synchronous type AC servo motor uses permanent
magnet while induction type doesn’t require it.
These motors are used for high power
applications
These motors are used for low power applications
==============================================================================
It delivers high power output Delivers low output of about 0.5 W to 100 W
It has more stability problems It has less stable problems
It requires frequent maintenance due to
the presence of commutator
It requires less maintenance due to the absence of
commutator
It provides high efficiency The efficiency of AC servo motor is less and is about 5 to
20%
The life of DC servo motor depends on
the life on brush life
The life of AC servo motor depends on bearing life
It includes permanent magnet in its
construction
The synchronous type AC servo motor uses permanent
magnet while induction type doesn’t require it.
These motors are used for high power
applications
These motors are used for low power applications
==============================================================================
How servo motors work?
What is a servo?
A servo is a small motor that you can position at any angle very accurately. It contains
internal circuits that will automatically maintain that particular angle. However, you cannot
do full revolutions with a servo. You are restricted to a certain range, usually from 180-270
degrees. Servos are very powerful for their sizes. There exist servos that provide a torque
of 4kg-cm from a 50 gram servo!
Servos are often used in small sized humanoid robots (not Asimo). Space is a constraint,
but you need a lot of power to move without increasing the weight.
Positioning a servo
A servo has three wires. Two are for power (usually coloured black or brown for ground
and red for the positive terminal). The third wire is for signals to position the servo.
The signal wire expects input from a pulse width modulator. The period should be 20
milliseconds long and the duty cycle "encodes" the position of the motor.
If the duty cycle is 1 millisecond, the servo is positioned at 0 degrees. If the duty cycle is 2
milliseconds, the servo is positioned at the maximum possible angle (180 degrees, 270
degrees, or whatever is the maximum limit).
Positioning the servo
What is a servo?
A servo is a small motor that you can position at any angle very accurately. It contains
internal circuits that will automatically maintain that particular angle. However, you cannot
do full revolutions with a servo. You are restricted to a certain range, usually from 180-270
degrees. Servos are very powerful for their sizes. There exist servos that provide a torque
of 4kg-cm from a 50 gram servo!
Servos are often used in small sized humanoid robots (not Asimo). Space is a constraint,
but you need a lot of power to move without increasing the weight.
Positioning a servo
A servo has three wires. Two are for power (usually coloured black or brown for ground
and red for the positive terminal). The third wire is for signals to position the servo.
The signal wire expects input from a pulse width modulator. The period should be 20
milliseconds long and the duty cycle "encodes" the position of the motor.
If the duty cycle is 1 millisecond, the servo is positioned at 0 degrees. If the duty cycle is 2
milliseconds, the servo is positioned at the maximum possible angle (180 degrees, 270
degrees, or whatever is the maximum limit).
Positioning the servo
Internals of a servo
A servo contains a normal DC motor. This motor is connected to a potentiometer (or a
variable resistance) through gears. As the motor rotates, the potentiometer's resistance
changes. So the circuit can measure exactly what direction the motor's shaft is pointing.
When the shaft of the motor is at the desired position, power supply to the motor is
stopped. If not, the motor is turned in the appropriate direction.
The desired position is sent in through the signal wire. As long as the signal wire has a
position, the servo will ensure that the motor's shaft remains at the correct position.
Also, the speed with which the motor turns is proportional to the difference between its
actual position and desired position. So if the motor is near the desired position, it will turn
slow. Otherwise it will turn fast. This is called proportional control.
The internal components of a servo
Now for the electronics part. The circuit contains a chip, M51660L (or another proprietary
chip of the manufacturer). This chip compares the error in positioning the motor.
The chip contains a timer that produces pulse signals from the potentiometer. These signals
are similar to the ones you supply. These two pulse signals (the ones you are sending and
the ones generated by the potentiometer) are fed into a pulse width comparator. This
comparator produces the signals indicating which direction the motor should turn in. These
are fed into an H-bridge (a big H Bridge - L293D) to drive the motor.
All of this is contained within the chip. Only a few extra components like resistors and
capacitors are required.
A servo contains a normal DC motor. This motor is connected to a potentiometer (or a
variable resistance) through gears. As the motor rotates, the potentiometer's resistance
changes. So the circuit can measure exactly what direction the motor's shaft is pointing.
When the shaft of the motor is at the desired position, power supply to the motor is
stopped. If not, the motor is turned in the appropriate direction.
The desired position is sent in through the signal wire. As long as the signal wire has a
position, the servo will ensure that the motor's shaft remains at the correct position.
Also, the speed with which the motor turns is proportional to the difference between its
actual position and desired position. So if the motor is near the desired position, it will turn
slow. Otherwise it will turn fast. This is called proportional control.
The internal components of a servo
Now for the electronics part. The circuit contains a chip, M51660L (or another proprietary
chip of the manufacturer). This chip compares the error in positioning the motor.
The chip contains a timer that produces pulse signals from the potentiometer. These signals
are similar to the ones you supply. These two pulse signals (the ones you are sending and
the ones generated by the potentiometer) are fed into a pulse width comparator. This
comparator produces the signals indicating which direction the motor should turn in. These
are fed into an H-bridge (a big H Bridge - L293D) to drive the motor.
All of this is contained within the chip. Only a few extra components like resistors and
capacitors are required.
The servo control circuit
Summary
You learned about how to control a servo motor and how its internal circuit works. You
even got to know how to start building your own controller if you ever wanted to.
- See more at: http://aishack.in/tutorials/servo-motors/#sthash.LUwYw4pr.dpuf
=================================================
TYPES OF MOTORS
SERVO CONTROL FACTS A HANDBOOK EXPLAINING THE BASICS OF MOTION BALDOR ELECTRIC
COMPANY MN1205 TABLE OF CONTENTS TYPES OF MOTORS . . . . . . . . . . . . . . 3 OPEN LOOP/CLOSED
LOOP . . . . . 9 WHAT IS A SERVO . . . . . . . . . . . . . . 11 COMPENSATION . . . . . . . . . . . . . . . 13 TYPES OF
CONTROLS . . . . . . . . . . . 15 TYPES OF FEEDBACK DEVICES . 17 TYPES OF ACTUATORS . . . . . . . . . . 22 Page
2 Page 3 Servo Control Facts TYPES OF MOTORS The direct current (DC) motor is one of the first
machines devised to convert electrical energy to mechanical power. Its origin can be traced to machines
conceived and tested by Michael Faraday, the experimenter who formulated the fundamental concepts
Summary
You learned about how to control a servo motor and how its internal circuit works. You
even got to know how to start building your own controller if you ever wanted to.
- See more at: http://aishack.in/tutorials/servo-motors/#sthash.LUwYw4pr.dpuf
=================================================
TYPES OF MOTORS
SERVO CONTROL FACTS A HANDBOOK EXPLAINING THE BASICS OF MOTION BALDOR ELECTRIC
COMPANY MN1205 TABLE OF CONTENTS TYPES OF MOTORS . . . . . . . . . . . . . . 3 OPEN LOOP/CLOSED
LOOP . . . . . 9 WHAT IS A SERVO . . . . . . . . . . . . . . 11 COMPENSATION . . . . . . . . . . . . . . . 13 TYPES OF
CONTROLS . . . . . . . . . . . 15 TYPES OF FEEDBACK DEVICES . 17 TYPES OF ACTUATORS . . . . . . . . . . 22 Page
2 Page 3 Servo Control Facts TYPES OF MOTORS The direct current (DC) motor is one of the first
machines devised to convert electrical energy to mechanical power. Its origin can be traced to machines
conceived and tested by Michael Faraday, the experimenter who formulated the fundamental concepts
of electromagnetism. These concepts basically state that if a conductor, or wire, carrying current is
placed in a magnetic field, a force will act upon it. The magnitude of this force is a function of strength of
the magnetic field, the amount of current passing through the conductor and the orientation of the
magnet and conductor. The direction in which this force will act is dependent on the direction of current
and direction of the magnetic field. Electric motor design is based on the placement of conductors
(wires) in a magnetic field. A winding has many conductors, or turns of wire, and the contribution of
each individual turn adds to the intensity of the interaction. The force developed from a winding is
dependent on the current passing through the winding and the magnetic field strength. If more current
is passed through the winding, then more force (torque) is obtained. In effect, two magnetic fields
interacting cause movement: the magnetic field from the rotor and the magnetic field from the stators
attract each other. This becomes the basis of both AC and DC motor design. AC MOTORS Most of the
world's motor business is addressed by AC motors. AC motors are relatively constant speed devices. The
speed of an AC motor is determined by the frequency of the voltage applied (and the number of
magnetic poles). There are basically two types of AC motors: induction and synchronous. INDUCTION
MOTOR. If the induction motor is viewed as a type of transformer, it becomes MAGNETIC FIELD
CURRENT FORCE Fig. 1 - CONCEPT OF ELECTROMAGNETISM ROTOR FIELD STATOR FIELD INDUCED
VOLTAGE AND CURRENT Fig. 2 - INDUCTION MOTOR INDUCED V I Page 4 Servo Control Facts easy to
understand. By applying a voltage onto the primary of the transformer winding, a current flow results
and induces current in the secondary winding. The primary is the stator assembly and the secondary is
the rotor assembly. One magnetic field is set up in the stator and a second magnetic field is induced in
the rotor. The interaction of these two magnetic fields results in motion. The speed of the magnetic field
going around the stator will determine the speed of the rotor. The rotor will try to follow the stator's
magnetic field, but will "slip" when a load is attached. Therefore induction motors always rotate slower
than the stator's rotating field. Typical construction of an induction motor consists of 1) a stator with
laminations and turns of copper wire and 2) a rotor, constructed of steel laminations with large slots on
the periphery, stacked together to form a "squirrel cage" rotor. Rotor slots are filled with conductive
material (copper or aluminum) and are short-circuited upon themselves by the conductive end pieces.
This "one" piece casting usually includes integral fan blades to circulate air for cooling purposes. The
standard induction motor is operated at a "constant" speed from standard line frequencies. Recently,
with the increasing demand for adjustable speed products, controls have been developed which adjust
operating speed of induction motors. Microprocessor drive technology using methods such as vector or
phase angle control (i.e. variable voltage, variable frequency) manipulates the magnitude of the
magnetic flux of the fields and thus controls motor speed. By the addition of an appropriate feedback
sensor, this becomes a viable consideration for some positioning applications. Controlling the induction
motor's speed/torque becomes complex since motor torque is no longer a simple function of motor
current. Motor torque affects the slip frequency, and speed is a function of both stator field frequency
and slip frequency. Induction motor advantages include: Low initial cost due to simplicity in motor
design and construction; availability of many standard sizes; reliability; and quiet, vibration-free
operation. For very rapid start-stop positioning applications, a larger motor would be used to keep
temperatures Fig. 3 - CUTAWAY OF INDUCTION MOTOR STATOR LAMINATIONS STATOR WINDINGS
SQUIRREL CAGE ROTOR FAN BLADES SHAFT HOUSING Page 5 Servo Control Facts within design limits. A
low torque to inertia ratio limits this motor type to less demanding incrementing (start-stop)
applications. SYNCHRONOUS MOTOR. The synchronous motor is basically the same as the induction
motor but with slightly different rotor construction. The rotor construction enables this type of motor to
placed in a magnetic field, a force will act upon it. The magnitude of this force is a function of strength of
the magnetic field, the amount of current passing through the conductor and the orientation of the
magnet and conductor. The direction in which this force will act is dependent on the direction of current
and direction of the magnetic field. Electric motor design is based on the placement of conductors
(wires) in a magnetic field. A winding has many conductors, or turns of wire, and the contribution of
each individual turn adds to the intensity of the interaction. The force developed from a winding is
dependent on the current passing through the winding and the magnetic field strength. If more current
is passed through the winding, then more force (torque) is obtained. In effect, two magnetic fields
interacting cause movement: the magnetic field from the rotor and the magnetic field from the stators
attract each other. This becomes the basis of both AC and DC motor design. AC MOTORS Most of the
world's motor business is addressed by AC motors. AC motors are relatively constant speed devices. The
speed of an AC motor is determined by the frequency of the voltage applied (and the number of
magnetic poles). There are basically two types of AC motors: induction and synchronous. INDUCTION
MOTOR. If the induction motor is viewed as a type of transformer, it becomes MAGNETIC FIELD
CURRENT FORCE Fig. 1 - CONCEPT OF ELECTROMAGNETISM ROTOR FIELD STATOR FIELD INDUCED
VOLTAGE AND CURRENT Fig. 2 - INDUCTION MOTOR INDUCED V I Page 4 Servo Control Facts easy to
understand. By applying a voltage onto the primary of the transformer winding, a current flow results
and induces current in the secondary winding. The primary is the stator assembly and the secondary is
the rotor assembly. One magnetic field is set up in the stator and a second magnetic field is induced in
the rotor. The interaction of these two magnetic fields results in motion. The speed of the magnetic field
going around the stator will determine the speed of the rotor. The rotor will try to follow the stator's
magnetic field, but will "slip" when a load is attached. Therefore induction motors always rotate slower
than the stator's rotating field. Typical construction of an induction motor consists of 1) a stator with
laminations and turns of copper wire and 2) a rotor, constructed of steel laminations with large slots on
the periphery, stacked together to form a "squirrel cage" rotor. Rotor slots are filled with conductive
material (copper or aluminum) and are short-circuited upon themselves by the conductive end pieces.
This "one" piece casting usually includes integral fan blades to circulate air for cooling purposes. The
standard induction motor is operated at a "constant" speed from standard line frequencies. Recently,
with the increasing demand for adjustable speed products, controls have been developed which adjust
operating speed of induction motors. Microprocessor drive technology using methods such as vector or
phase angle control (i.e. variable voltage, variable frequency) manipulates the magnitude of the
magnetic flux of the fields and thus controls motor speed. By the addition of an appropriate feedback
sensor, this becomes a viable consideration for some positioning applications. Controlling the induction
motor's speed/torque becomes complex since motor torque is no longer a simple function of motor
current. Motor torque affects the slip frequency, and speed is a function of both stator field frequency
and slip frequency. Induction motor advantages include: Low initial cost due to simplicity in motor
design and construction; availability of many standard sizes; reliability; and quiet, vibration-free
operation. For very rapid start-stop positioning applications, a larger motor would be used to keep
temperatures Fig. 3 - CUTAWAY OF INDUCTION MOTOR STATOR LAMINATIONS STATOR WINDINGS
SQUIRREL CAGE ROTOR FAN BLADES SHAFT HOUSING Page 5 Servo Control Facts within design limits. A
low torque to inertia ratio limits this motor type to less demanding incrementing (start-stop)
applications. SYNCHRONOUS MOTOR. The synchronous motor is basically the same as the induction
motor but with slightly different rotor construction. The rotor construction enables this type of motor to
rotate at the same speed (in synchronization) as the stator field. There are basically two types of
synchronous motors: self excited ( as the induction motor) and directly excited (as with permanent
magnets). The self excited motor (may be called reluctance synchronous) includes a rotor with notches,
or teeth, on the periphery. The number of notches corresponds to the number of poles in the stator.
Oftentimes the notches or teeth are termed salient poles. These salient poles create an easy path for
the magnetic flux field, thus allowing the rotor to "lock in" and run at the same speed as the rotating
field. A directly excited motor (may be called hysteresis synchronous, or AC permanent magnet
synchronous) includes a rotor with a cylinder of a permanent magnet alloy. The permanent magnet
north and south poles, in effect, are the salient teeth of this design, and therefore prevent slip. In both
the self excited and directly excited types there is a "coupling" angle, i.e. the rotor lags a small distance
behind the stator field. This angle will increase with load, and if the load is increased beyond the motor's
capability, the rotor will pull out of synchronism. The synchronous motor is generally operated in an
"open loop" configuration and within the limiFig. 4 - CUTAWAY OF AC SYNCHRONOUS MOTOR STATOR
SHAFT ROTOR STATOR LAMINATIONS STATOR WINDINGS ROTOR WITH TEETH OR NOTCHES HOUSING
SHAFT Page 6 Servo Control Facts SHUNT WOUND MOTORS. With the shunt wound, the rotor and stator
(or field windings) are connected in parallel. The field windings can be connected to the same power
supply as the rotor, or excited separately. Separate excitation is used to change motor speed (i.e. rotor
voltage is varied while stator or field winding is held constant). The parallel connection provides a
relative flat speed-torque curve and good speed regulation over wide load ranges. However, because of
demagnetization effects, these motors provide starting torques comparatively lower than other DC
winding types. SERIES WOUND MOTORS. In the series wound motor, the two motor fields are connected
in series. The result is two strong fields which will produce very high starting torque. The field winding
carries the full rotor current. These motors are usually employed where large starting torques are
required such as cranes and hoists. Series motors should be avoided in applications tations of the
coupling angle (or "pull-out" torque) it will provide absolute constant speed for a given load. Also, note
that this category of motor is not self starting and employs start windings (split-phase, capacitor start),
or controls which slowly ramp up frequency/voltage in order to start rotation. A synchronous motor can
be used in a speed control system even though a feedback device must be added. Vector control
approaches will work quite adequately with this motor design. However, in general, the rotor is larger
than that of an equivalent servomotor and, therefore, may not provide adequate response for
incrementing applications. Other disadvantages are: While the synchronous motor may start a high
inertial load, it may not be able to accelerate the load enough to pull it into synchronism. If this occurs,
the synchronous motor operates at low frequency and at very irregular speeds, resulting in audible
noise. Also for a given horsepower, synchronous motors are larger and more expensive than non-
synchronous motors. DC MOTORS Most of the world's adjustable speed business is addressed by DC
motors. DC motor speeds can easily be varied, therefore they are utilized in applications where speed
control, servo control, and/or positioning needs exist. The stator field is produced by either a field
winding, or by permanent magnets. This is a stationary field (as opposed to the AC stator field which is
rotating). The second field, the rotor field, is set up by passing current through a commutator and into
the rotor assembly. The rotor field rotates in an effort to align itself with the stator field, but at the
appropriate time (due to the commutator) the rotor field is switched. In this method then, the rotor
field never catches up to the stator field. Rotational speed (i.e. how fast the rotor turns) is dependent on
the strength of the rotor field. In other words, the more voltage on the motor, the faster the rotor will
turn. The following will briefly explore the various wound field motors and the permanent magnet
synchronous motors: self excited ( as the induction motor) and directly excited (as with permanent
magnets). The self excited motor (may be called reluctance synchronous) includes a rotor with notches,
or teeth, on the periphery. The number of notches corresponds to the number of poles in the stator.
Oftentimes the notches or teeth are termed salient poles. These salient poles create an easy path for
the magnetic flux field, thus allowing the rotor to "lock in" and run at the same speed as the rotating
field. A directly excited motor (may be called hysteresis synchronous, or AC permanent magnet
synchronous) includes a rotor with a cylinder of a permanent magnet alloy. The permanent magnet
north and south poles, in effect, are the salient teeth of this design, and therefore prevent slip. In both
the self excited and directly excited types there is a "coupling" angle, i.e. the rotor lags a small distance
behind the stator field. This angle will increase with load, and if the load is increased beyond the motor's
capability, the rotor will pull out of synchronism. The synchronous motor is generally operated in an
"open loop" configuration and within the limiFig. 4 - CUTAWAY OF AC SYNCHRONOUS MOTOR STATOR
SHAFT ROTOR STATOR LAMINATIONS STATOR WINDINGS ROTOR WITH TEETH OR NOTCHES HOUSING
SHAFT Page 6 Servo Control Facts SHUNT WOUND MOTORS. With the shunt wound, the rotor and stator
(or field windings) are connected in parallel. The field windings can be connected to the same power
supply as the rotor, or excited separately. Separate excitation is used to change motor speed (i.e. rotor
voltage is varied while stator or field winding is held constant). The parallel connection provides a
relative flat speed-torque curve and good speed regulation over wide load ranges. However, because of
demagnetization effects, these motors provide starting torques comparatively lower than other DC
winding types. SERIES WOUND MOTORS. In the series wound motor, the two motor fields are connected
in series. The result is two strong fields which will produce very high starting torque. The field winding
carries the full rotor current. These motors are usually employed where large starting torques are
required such as cranes and hoists. Series motors should be avoided in applications tations of the
coupling angle (or "pull-out" torque) it will provide absolute constant speed for a given load. Also, note
that this category of motor is not self starting and employs start windings (split-phase, capacitor start),
or controls which slowly ramp up frequency/voltage in order to start rotation. A synchronous motor can
be used in a speed control system even though a feedback device must be added. Vector control
approaches will work quite adequately with this motor design. However, in general, the rotor is larger
than that of an equivalent servomotor and, therefore, may not provide adequate response for
incrementing applications. Other disadvantages are: While the synchronous motor may start a high
inertial load, it may not be able to accelerate the load enough to pull it into synchronism. If this occurs,
the synchronous motor operates at low frequency and at very irregular speeds, resulting in audible
noise. Also for a given horsepower, synchronous motors are larger and more expensive than non-
synchronous motors. DC MOTORS Most of the world's adjustable speed business is addressed by DC
motors. DC motor speeds can easily be varied, therefore they are utilized in applications where speed
control, servo control, and/or positioning needs exist. The stator field is produced by either a field
winding, or by permanent magnets. This is a stationary field (as opposed to the AC stator field which is
rotating). The second field, the rotor field, is set up by passing current through a commutator and into
the rotor assembly. The rotor field rotates in an effort to align itself with the stator field, but at the
appropriate time (due to the commutator) the rotor field is switched. In this method then, the rotor
field never catches up to the stator field. Rotational speed (i.e. how fast the rotor turns) is dependent on
the strength of the rotor field. In other words, the more voltage on the motor, the faster the rotor will
turn. The following will briefly explore the various wound field motors and the permanent magnet
(PMDC) motors. % RATED SPEED % RATED TORQUE 100 100 EMF SHUNT FIELD Fig. 5 - TYPICAL SPEED-
TORQUE CURVE FOR SHUNT WOUND MOTORS Page 7 Servo Control Facts COMPOUND WOUND
MOTOR. Compound motors use both a series and a shunt stator field. Many speed torque curves can be
created by varying the ratio of series and shunt fields. In general, small compound motors have a strong
shunt field and a weak series field to help start the motor. High starting torques are exhibited along with
relatively flat speed torque characteristics. In reversing applications, the polarity of both windings must
be switched, thusrequiring large, complex circuits. where they are likely to lose load because of the
tendency to "run away" under no-load conditions. SERIES EMF Fig. 6 TYPICAL SPEED-TORQUE CURVE
FOR SERIES WOUND MOTORS MOTORS % RATED SPEED % RATED TORQUE 100 200 STEPPER MOTOR.
Step motors are electromechanical actuators which convert digital inputs to analog motion. This is
possible through the motor's controller electronics. There are various types of step motors such as
solenoid activated, variable reluctance, permanent magnet and synchronous inductor. Independent of
stepper type, all are devices which index in fixed angular increments when energized in a programmed
manner. Step motors' normal operation consists of discrete angular motions of uniform magnitude
rather than continuous motion. A step motor is particularly well suited to applications where the
controller signals appear as pulse trains. One pulse causes the motor to increment one angle of motion.
This is repeated for one pulse. Most step motors are used in an open loop system configuration, which
can result in oscillations. To overcome this, either complex circuits or feedback is employed – thus
resulting in a closed loop system. Stepper motors are, however, limited to about one horsepower and
2000 rpm, therefore limiting them in many applications. DIGITAL TRAIN OF PULSES ROTATION Fig. 8 -
STEPPER MOTOR % RATED SPEED % RATED TORQUE 100 100 SERIES EMF SHUNT FIELD Fig. 7 TYPICAL
SPEED-TORQUE CURVE FOR COMPOUND WOUND MOTORS Page 8 Servo Control Facts PMDC MOTOR.
The predominant motor configuration utilized in demanding incrementing (start-stop) applications is the
permanent magnet DC (PMDC) motor. This type with appropriate feedback is quite an effective device in
closed loop servo system applications. Since the stator field is generated by permanent magnets, no
power is used for field generation. The magnets provide constant field flux at all speeds. Therefore,
linear speed torque curves result. This motor type provides relatively high starting, or acceleration
torque, is linear and predictable, and has a smaller frame and lighter weight compared to other motor
types and provides rapid positioning. HOUSING BRUSH COVERS PERMANENT MAGNETS ROTOR
COMMUTATOR MOUNTING BRUSHES Fig. 9 - TYPICAL DC MOTOR CONSTRUCTION Page 9 Servo Control
Facts OPEN LOOP/CLOSED LOOP In a system. the controller is the device which activates motion by
providing a command to do something, i.e. start or change speed/position. This command is amplified
and applied onto the motor. Thus motion commences . . . but how is this known? There are several
assumptions which have been made. The first assumption is that power is applied onto the motor and
the second is that the motor shaft is free to rotate. If there is nothing wrong with the system, the
assumptions are fine – and indeed motion commences and the motor rotates. If for some reason, either
the signal or power does not get to the motor, or the motor is somehow prevented from rotating, the
assumptions are poor and there would be no motion. Systems that assume motion has taken place (or is
in the process of taking place) are termed "open loop". An open loop drive is one in which the signal
goes "in one direction only". . . from the control to the motor. There is no signal returning from the
motor/load to inform the control that action/motion has occurred. A stepper drive is a perfect example
of an open loop system. One pulse from the control to the motor will move the motor one increment. If
for some reason the stepper does not move, for example due to jamming, the control is unaware of the
problem and cannot make any corrections. As an example, suppose an application calls for automatically
TORQUE CURVE FOR SHUNT WOUND MOTORS Page 7 Servo Control Facts COMPOUND WOUND
MOTOR. Compound motors use both a series and a shunt stator field. Many speed torque curves can be
created by varying the ratio of series and shunt fields. In general, small compound motors have a strong
shunt field and a weak series field to help start the motor. High starting torques are exhibited along with
relatively flat speed torque characteristics. In reversing applications, the polarity of both windings must
be switched, thusrequiring large, complex circuits. where they are likely to lose load because of the
tendency to "run away" under no-load conditions. SERIES EMF Fig. 6 TYPICAL SPEED-TORQUE CURVE
FOR SERIES WOUND MOTORS MOTORS % RATED SPEED % RATED TORQUE 100 200 STEPPER MOTOR.
Step motors are electromechanical actuators which convert digital inputs to analog motion. This is
possible through the motor's controller electronics. There are various types of step motors such as
solenoid activated, variable reluctance, permanent magnet and synchronous inductor. Independent of
stepper type, all are devices which index in fixed angular increments when energized in a programmed
manner. Step motors' normal operation consists of discrete angular motions of uniform magnitude
rather than continuous motion. A step motor is particularly well suited to applications where the
controller signals appear as pulse trains. One pulse causes the motor to increment one angle of motion.
This is repeated for one pulse. Most step motors are used in an open loop system configuration, which
can result in oscillations. To overcome this, either complex circuits or feedback is employed – thus
resulting in a closed loop system. Stepper motors are, however, limited to about one horsepower and
2000 rpm, therefore limiting them in many applications. DIGITAL TRAIN OF PULSES ROTATION Fig. 8 -
STEPPER MOTOR % RATED SPEED % RATED TORQUE 100 100 SERIES EMF SHUNT FIELD Fig. 7 TYPICAL
SPEED-TORQUE CURVE FOR COMPOUND WOUND MOTORS Page 8 Servo Control Facts PMDC MOTOR.
The predominant motor configuration utilized in demanding incrementing (start-stop) applications is the
permanent magnet DC (PMDC) motor. This type with appropriate feedback is quite an effective device in
closed loop servo system applications. Since the stator field is generated by permanent magnets, no
power is used for field generation. The magnets provide constant field flux at all speeds. Therefore,
linear speed torque curves result. This motor type provides relatively high starting, or acceleration
torque, is linear and predictable, and has a smaller frame and lighter weight compared to other motor
types and provides rapid positioning. HOUSING BRUSH COVERS PERMANENT MAGNETS ROTOR
COMMUTATOR MOUNTING BRUSHES Fig. 9 - TYPICAL DC MOTOR CONSTRUCTION Page 9 Servo Control
Facts OPEN LOOP/CLOSED LOOP In a system. the controller is the device which activates motion by
providing a command to do something, i.e. start or change speed/position. This command is amplified
and applied onto the motor. Thus motion commences . . . but how is this known? There are several
assumptions which have been made. The first assumption is that power is applied onto the motor and
the second is that the motor shaft is free to rotate. If there is nothing wrong with the system, the
assumptions are fine – and indeed motion commences and the motor rotates. If for some reason, either
the signal or power does not get to the motor, or the motor is somehow prevented from rotating, the
assumptions are poor and there would be no motion. Systems that assume motion has taken place (or is
in the process of taking place) are termed "open loop". An open loop drive is one in which the signal
goes "in one direction only". . . from the control to the motor. There is no signal returning from the
motor/load to inform the control that action/motion has occurred. A stepper drive is a perfect example
of an open loop system. One pulse from the control to the motor will move the motor one increment. If
for some reason the stepper does not move, for example due to jamming, the control is unaware of the
problem and cannot make any corrections. As an example, suppose an application calls for automatically
placing parts into bins A, B and C. The control can trigger one pulse, resulting in shaft rotation and
placement of a part in bin A. Two pulses cause shaft rotation and part placement in bin B and three
pulses for part placement in bin C. If for some reason the shaft cannot rotate to bins B and C, the control
is unaware of the problem and all parts are placed in bin A – a big problem if not discovered
immediately by an operator. If a signal is returned to provide information that motion has occurred,
then the system is described as having a signal which goes in "two directions": The command signal goes
out (to move the motor), and a signal is returned (the feedback) to the control to inform the control of
what has occurred. The information flows back, or returns. This is an example of a "closed loop" drive.
SIGNAL GOES IN ONE DIRECTION MOTOR CONTROL Fig. 10 - OPEN LOOP DRIVE CONTROL BIN A BIN B
BIN C Fig. 11 EXAMPLE OF AN APPLICATION USING OPEN LOOP DRIVE MOTOR A SIGNAL GOES OUT...
CONTROL MOTOR FEEDBACK DEVICE ...AND A SIGNAL RETURNS Fig. 12 - CLOSED LOOP DRIVE Page 10
Servo Control Facts The return signal (feedback signal) provides the means to monitor the process for
correctness. From the automatic pick and place application example previously cited, if the shaft cannot
rotate to bins B and C, the feedback will inform the control of an error and the control can activate a
light or a horn to alert the operator of the problem. When would an application use an open loop
approach? First of all, just think of how simple it would be to hook up – a few wires and no adjustments.
Stepper motors are traditionally employed in open loop systems . . . they are easy to wire, they interface
easily with the user's digital computer and they provide good position repeatability. Stepper motors,
however, are limited to approximately one horsepower. Their upper speed limit is about 2000 rpm. The
weaknesses of the open loop approach include: It is not good for applications with varying loads, it is
possible for a stepper motor to lose steps, its energy efficiency level is low and it has resonance areas
which must be avoided. What applications use the closed loop technique? Those that require control
over a variety of complex motion profiles. These may involve the following: control of either velocity
and/or position; high resolution and accuracy; velocity may be either very slow, or very high; and the
application may demand high torques in a small package size. Because of additional components such as
the feedback device, complexity is considered by some to be a weakness of the closed loop approach.
These additional components do add to initial cost (an increase in productivity is typically not
considered when investigating cost). Lack of understanding does give the impression to the user of
difficulty. In many applications, whether the open loop or closed loop techniques employed often comes
down to the basic decision of the user . . . and the approach with which he/she is most
knowledgeable/comfortable with. Page 11 Servo Control Facts WHAT IS A SERVO? What is a servo? This
is not easily defined nor self-explanatory since a servomechanism, or servo drive, does not apply to any
particular device. It is a term which applies to a function or a task. The function, or task, of a servo can
be described as follows. A command signal which is issued from the user's interface panel comes into
the servo's "positioning controller". The positioning controller is the device which stores information
about various jobs or tasks. It has been programmed to activate the motor/load, i.e. change
speed/position. The signal then passes into the servo control or "amplifier" section. The servo control
takes this low power level signal and increases, or amplifies, the power up to appropriate levels to
actually result in movement of the servo motor/load. These low power level signals must be amplified:
Higher voltage levels are needed to rotate the servo motor at appropriate higher speeds and higher
current levels are required to provide torque to move heavier loads. This power is supplied to the servo
control (amplifier) from the "power supply" which simply converts AC power into the required DC level.
It also supplies any low level voltage required for operation of integrated circuits. As power is applied
onto the servo motor, the load begins to move . . . speed and position changes. As the load moves, so
placement of a part in bin A. Two pulses cause shaft rotation and part placement in bin B and three
pulses for part placement in bin C. If for some reason the shaft cannot rotate to bins B and C, the control
is unaware of the problem and all parts are placed in bin A – a big problem if not discovered
immediately by an operator. If a signal is returned to provide information that motion has occurred,
then the system is described as having a signal which goes in "two directions": The command signal goes
out (to move the motor), and a signal is returned (the feedback) to the control to inform the control of
what has occurred. The information flows back, or returns. This is an example of a "closed loop" drive.
SIGNAL GOES IN ONE DIRECTION MOTOR CONTROL Fig. 10 - OPEN LOOP DRIVE CONTROL BIN A BIN B
BIN C Fig. 11 EXAMPLE OF AN APPLICATION USING OPEN LOOP DRIVE MOTOR A SIGNAL GOES OUT...
CONTROL MOTOR FEEDBACK DEVICE ...AND A SIGNAL RETURNS Fig. 12 - CLOSED LOOP DRIVE Page 10
Servo Control Facts The return signal (feedback signal) provides the means to monitor the process for
correctness. From the automatic pick and place application example previously cited, if the shaft cannot
rotate to bins B and C, the feedback will inform the control of an error and the control can activate a
light or a horn to alert the operator of the problem. When would an application use an open loop
approach? First of all, just think of how simple it would be to hook up – a few wires and no adjustments.
Stepper motors are traditionally employed in open loop systems . . . they are easy to wire, they interface
easily with the user's digital computer and they provide good position repeatability. Stepper motors,
however, are limited to approximately one horsepower. Their upper speed limit is about 2000 rpm. The
weaknesses of the open loop approach include: It is not good for applications with varying loads, it is
possible for a stepper motor to lose steps, its energy efficiency level is low and it has resonance areas
which must be avoided. What applications use the closed loop technique? Those that require control
over a variety of complex motion profiles. These may involve the following: control of either velocity
and/or position; high resolution and accuracy; velocity may be either very slow, or very high; and the
application may demand high torques in a small package size. Because of additional components such as
the feedback device, complexity is considered by some to be a weakness of the closed loop approach.
These additional components do add to initial cost (an increase in productivity is typically not
considered when investigating cost). Lack of understanding does give the impression to the user of
difficulty. In many applications, whether the open loop or closed loop techniques employed often comes
down to the basic decision of the user . . . and the approach with which he/she is most
knowledgeable/comfortable with. Page 11 Servo Control Facts WHAT IS A SERVO? What is a servo? This
is not easily defined nor self-explanatory since a servomechanism, or servo drive, does not apply to any
particular device. It is a term which applies to a function or a task. The function, or task, of a servo can
be described as follows. A command signal which is issued from the user's interface panel comes into
the servo's "positioning controller". The positioning controller is the device which stores information
about various jobs or tasks. It has been programmed to activate the motor/load, i.e. change
speed/position. The signal then passes into the servo control or "amplifier" section. The servo control
takes this low power level signal and increases, or amplifies, the power up to appropriate levels to
actually result in movement of the servo motor/load. These low power level signals must be amplified:
Higher voltage levels are needed to rotate the servo motor at appropriate higher speeds and higher
current levels are required to provide torque to move heavier loads. This power is supplied to the servo
control (amplifier) from the "power supply" which simply converts AC power into the required DC level.
It also supplies any low level voltage required for operation of integrated circuits. As power is applied
onto the servo motor, the load begins to move . . . speed and position changes. As the load moves, so
does some other "device" move. This other "device" is either a tachometer, resolver or encoder
(providing a signal which is "sent back" to the controller). This "feedback" sigCOMMAND SIGNAL "AC"
POWER LOW LEVEL POWER HIGH LEVEL POWER SERVO MOTOR FEEDBACK LOAD SERVO CONTROL
(AMPLIFIER) PROGRAMMABLE POSITIONING CONTROLLER INTERFACE PANEL POWER SUPPLY "DC"
POWER Fig. 13 - THE CONCEPT OF A SERVO SYSTEM Page 12 Servo Control Facts nal is informing the
positioning controller whether the motor is doing the proper job. The positioning controller looks at this
feedback signal and determines if the load is being moved properly by the servo motor; and, if not, then
the controller makes appropriate corrections. For example, assume the command signal was to drive the
load at 1000 rpm. For some reason it is actually rotating at 900 rpm. The feedback signal will inform the
controller that the speed is 900 rpm. The controller then compares the command signal (desired speed)
of 1000 rpm and the feedback signal (actual speed) of 900 rpm and notes an error. The controller then
outputs a signal to apply more voltage onto the servo motor to increase speed until the feedback signal
equals the command signal, i.e. there is no error. Therefore, a servo involves several devices. It is a
system of devices for controlling some item (load). The item (load) which is controlled (regulated) can be
controlled in any manner, i.e. position, direction, speed. The speed or position is controlled in relation to
a reference (command signal), as long as the proper feedback device (error detection device) is used.
The feedback and command signals are compared, and the corrections made. Thus, the definition of a
servo system is, that it consists of several devices which control or regulate speed/position of a load.
Page 13 Servo Control Facts COMPENSATION Why must servos be compensated? Simply stated, it is
required so that the controller and motor/load i.e. machine will operate properly. The machine must
produce accurate parts and have high productivity. In order for the machine to produce good, accurate
parts, it must operate in two distinct modes: transient and steady state. The first mode of operation, the
transient state (may also be termed dynamic response state), occurs when the input command changes.
This causes the motor/load to accelerate/decelerate i.e. change speed. During this time period, there is
an associated 1) time required for the motor/load to reach a final speed/position (rise time) , 2) time
required for the motor/load to settle and 3) a certain amount of overshoot which is acceptable. The
second mode of operation, steady state, occurs when the motor/load has reached final speed, i.e.
continuous operation. During this time, there is an associated following accuracy (how accurate the
machine is performing). This is typically called steady state error. The machine must be capable of
operating in these two distinct modes in order to handle the variety of operations required for machine
performance. And in order that the machine will perform without excessive overshoot, settle within
adequate time periods, and have minimum steady state error, the servo must be adjusted – or
compensated. Compensation involves adjustment or tuning the servo's gain and bandwidth. First of all,
a look at the definition of these terms is in order and then how they affect performance. Gain is a ratio
of output versus input. As an example, examine a home stereo system. The ratio of the input signal (as
received from the radio station) versus the output signal (what your ear hears) is gain. If the volume
knob is low, the sound is soft – low gain; if the volume is turned up high, the sound is loud – high gain.
Gain, therefore is a measure of the amplification of the input signal. In a servo controller, gain effects
the accuracy (i.e. how close to the desired speed, or position is the motor's actual speed or position).
High gain will allow small accurate movement and the machine will be capable of producing precise
parts. Bandwidth is expressed or measured in frequency. The home stereo system will again provide an
example for the definition. If the frequency of the sound heard is low (base drum), there is no difficulty
in hearing the sound. As the frequency is increased, the listener has more difficulty hearing the sound.
At some point, the human ear cannot detect the sound. This is attributed to the range of frequencies
(providing a signal which is "sent back" to the controller). This "feedback" sigCOMMAND SIGNAL "AC"
POWER LOW LEVEL POWER HIGH LEVEL POWER SERVO MOTOR FEEDBACK LOAD SERVO CONTROL
(AMPLIFIER) PROGRAMMABLE POSITIONING CONTROLLER INTERFACE PANEL POWER SUPPLY "DC"
POWER Fig. 13 - THE CONCEPT OF A SERVO SYSTEM Page 12 Servo Control Facts nal is informing the
positioning controller whether the motor is doing the proper job. The positioning controller looks at this
feedback signal and determines if the load is being moved properly by the servo motor; and, if not, then
the controller makes appropriate corrections. For example, assume the command signal was to drive the
load at 1000 rpm. For some reason it is actually rotating at 900 rpm. The feedback signal will inform the
controller that the speed is 900 rpm. The controller then compares the command signal (desired speed)
of 1000 rpm and the feedback signal (actual speed) of 900 rpm and notes an error. The controller then
outputs a signal to apply more voltage onto the servo motor to increase speed until the feedback signal
equals the command signal, i.e. there is no error. Therefore, a servo involves several devices. It is a
system of devices for controlling some item (load). The item (load) which is controlled (regulated) can be
controlled in any manner, i.e. position, direction, speed. The speed or position is controlled in relation to
a reference (command signal), as long as the proper feedback device (error detection device) is used.
The feedback and command signals are compared, and the corrections made. Thus, the definition of a
servo system is, that it consists of several devices which control or regulate speed/position of a load.
Page 13 Servo Control Facts COMPENSATION Why must servos be compensated? Simply stated, it is
required so that the controller and motor/load i.e. machine will operate properly. The machine must
produce accurate parts and have high productivity. In order for the machine to produce good, accurate
parts, it must operate in two distinct modes: transient and steady state. The first mode of operation, the
transient state (may also be termed dynamic response state), occurs when the input command changes.
This causes the motor/load to accelerate/decelerate i.e. change speed. During this time period, there is
an associated 1) time required for the motor/load to reach a final speed/position (rise time) , 2) time
required for the motor/load to settle and 3) a certain amount of overshoot which is acceptable. The
second mode of operation, steady state, occurs when the motor/load has reached final speed, i.e.
continuous operation. During this time, there is an associated following accuracy (how accurate the
machine is performing). This is typically called steady state error. The machine must be capable of
operating in these two distinct modes in order to handle the variety of operations required for machine
performance. And in order that the machine will perform without excessive overshoot, settle within
adequate time periods, and have minimum steady state error, the servo must be adjusted – or
compensated. Compensation involves adjustment or tuning the servo's gain and bandwidth. First of all,
a look at the definition of these terms is in order and then how they affect performance. Gain is a ratio
of output versus input. As an example, examine a home stereo system. The ratio of the input signal (as
received from the radio station) versus the output signal (what your ear hears) is gain. If the volume
knob is low, the sound is soft – low gain; if the volume is turned up high, the sound is loud – high gain.
Gain, therefore is a measure of the amplification of the input signal. In a servo controller, gain effects
the accuracy (i.e. how close to the desired speed, or position is the motor's actual speed or position).
High gain will allow small accurate movement and the machine will be capable of producing precise
parts. Bandwidth is expressed or measured in frequency. The home stereo system will again provide an
example for the definition. If the frequency of the sound heard is low (base drum), there is no difficulty
in hearing the sound. As the frequency is increased, the listener has more difficulty hearing the sound.
At some point, the human ear cannot detect the sound. This is attributed to the range of frequencies
which the human ear can detect, i.e. the bandwidth to which the human ear can hear or respond to. In a
servo, bandwidth is a measure of how fast the controller/motor/machine can respond. The wider the
bandwidth, the faster the machine can respond. Fast response will enable the machine to react rapidly,
producing many parts. FOLLOWING ACCURACY OR STEADY STATE ERROR RISE TIME SETTLE TIME
TRANSIENT STATE STEADY STATE Fig. 14 - SERVO RESPONSE Page 14 Servo Control Facts Why then, are
not all servos designed with high gain (high accuracy) and wide bandwidth (fast response)? This is
attributed to 1) limitations of the components and 2) resonant conditions. Limits of the components –
they can handle only so much power. In addition, increasing gain adds components, cost, complexity.
Resonant conditions – To explain this, imagine a yard stick held in your hand. Slowly move it up and
down. . . note that the far end of the rod will follow your hand movement. As movement is increased
(increasing frequency of motion) the far end of the yard stick will bend in its attempt to keep up with
your hand movements. At some frequency it is possible to break the stick . . . this is the resonant point.
Just as with this example, all systems have a resonant point, whether that system is a bridge, a tank or a
servo. Machines must not be operated at the resonant point otherwise instability and severe damage
will occur. In conclusion, servos are compensated or "tuned" via adjustments of gain and response so
that the machine will produce accurate parts at a high productivity rate. Page 15 Servo Control Facts
TYPES OF CONTROLS The control of a motor will employ some type of power semiconductor. These
devices regulate the amount of power being applied onto the motor, and moving the load. One type of
semiconductor is the SCR (silicon controller rectifier) which will be connected to the AC line voltage. This
type of device is usually employed where large amounts of power must be regulated, motor inductance
is relatively high and accuracy in speed is not critical (such as constant speed devices for fans, blowers,
conveyor belts). Power out of the SCR, which is available to run the motor, comes in discrete pulses. At
low speeds a continuous stream of narrow pulses is required to maintain speed. If an increase in speed
is desired, the SCR must be turned on to apply large pulses of instant power, and when lower speeds are
desired, power is removed and a gradual coasting down in speed occurs. A good example would be
when one car is towing a second car. The driver in the first car is the SCR device and the second car,
which is being towed is the motor/load. As long as the chain is taut, the driver in the first car is in control
of the second car. But suppose the first car slows down. There would be slack in the chain and, at that
point, the first car is no longer in control (and won't be until he gets into a position where the chain is
taut again). So, for the periods of time when the first car must slow down, the driver is not in control.
This sequence occurs repeatedly, resulting in a jerky, cogging operation. This type of speed control is
adequate for many applications If smoother speed is desired, an electronic network may be introduced.
By inserting a "lag" network, the response of the control is slowed so that a large instant power pulse
will not suddenly be applied. Filtering action of the lag network gives the motor a sluggish response to a
sudden change in load or speed command changes. This sluggish response is not important in
applications with steady loads or extremely large inertia. But for wide range, high performance systems,
in which rapid response is important, it becomes extremely desirable to minimize sluggish reaction since
a rapid changes to speed commands are desirable. Transistors may also be employed to regulate the
amount of power applied onto a motor. With this device, there are several "techniques", or design
methodology, used to turn transistors "on" and "off". The "technique" or mode of operation may be
"linear", "pulse width modulated" (PWM) or "pulse frequency modulated" (PFM). The "linear" mode
uses transistors which are activated, or turned on, all the time supplying the appropriate amount of
power required. Transistors act like a water faucet, regulating the appropriate amount of power to drive
the motor. If the transistor is turned on half way, then half of the power goes to the motor. If the
servo, bandwidth is a measure of how fast the controller/motor/machine can respond. The wider the
bandwidth, the faster the machine can respond. Fast response will enable the machine to react rapidly,
producing many parts. FOLLOWING ACCURACY OR STEADY STATE ERROR RISE TIME SETTLE TIME
TRANSIENT STATE STEADY STATE Fig. 14 - SERVO RESPONSE Page 14 Servo Control Facts Why then, are
not all servos designed with high gain (high accuracy) and wide bandwidth (fast response)? This is
attributed to 1) limitations of the components and 2) resonant conditions. Limits of the components –
they can handle only so much power. In addition, increasing gain adds components, cost, complexity.
Resonant conditions – To explain this, imagine a yard stick held in your hand. Slowly move it up and
down. . . note that the far end of the rod will follow your hand movement. As movement is increased
(increasing frequency of motion) the far end of the yard stick will bend in its attempt to keep up with
your hand movements. At some frequency it is possible to break the stick . . . this is the resonant point.
Just as with this example, all systems have a resonant point, whether that system is a bridge, a tank or a
servo. Machines must not be operated at the resonant point otherwise instability and severe damage
will occur. In conclusion, servos are compensated or "tuned" via adjustments of gain and response so
that the machine will produce accurate parts at a high productivity rate. Page 15 Servo Control Facts
TYPES OF CONTROLS The control of a motor will employ some type of power semiconductor. These
devices regulate the amount of power being applied onto the motor, and moving the load. One type of
semiconductor is the SCR (silicon controller rectifier) which will be connected to the AC line voltage. This
type of device is usually employed where large amounts of power must be regulated, motor inductance
is relatively high and accuracy in speed is not critical (such as constant speed devices for fans, blowers,
conveyor belts). Power out of the SCR, which is available to run the motor, comes in discrete pulses. At
low speeds a continuous stream of narrow pulses is required to maintain speed. If an increase in speed
is desired, the SCR must be turned on to apply large pulses of instant power, and when lower speeds are
desired, power is removed and a gradual coasting down in speed occurs. A good example would be
when one car is towing a second car. The driver in the first car is the SCR device and the second car,
which is being towed is the motor/load. As long as the chain is taut, the driver in the first car is in control
of the second car. But suppose the first car slows down. There would be slack in the chain and, at that
point, the first car is no longer in control (and won't be until he gets into a position where the chain is
taut again). So, for the periods of time when the first car must slow down, the driver is not in control.
This sequence occurs repeatedly, resulting in a jerky, cogging operation. This type of speed control is
adequate for many applications If smoother speed is desired, an electronic network may be introduced.
By inserting a "lag" network, the response of the control is slowed so that a large instant power pulse
will not suddenly be applied. Filtering action of the lag network gives the motor a sluggish response to a
sudden change in load or speed command changes. This sluggish response is not important in
applications with steady loads or extremely large inertia. But for wide range, high performance systems,
in which rapid response is important, it becomes extremely desirable to minimize sluggish reaction since
a rapid changes to speed commands are desirable. Transistors may also be employed to regulate the
amount of power applied onto a motor. With this device, there are several "techniques", or design
methodology, used to turn transistors "on" and "off". The "technique" or mode of operation may be
"linear", "pulse width modulated" (PWM) or "pulse frequency modulated" (PFM). The "linear" mode
uses transistors which are activated, or turned on, all the time supplying the appropriate amount of
power required. Transistors act like a water faucet, regulating the appropriate amount of power to drive
the motor. If the transistor is turned on half way, then half of the power goes to the motor. If the
transistor is turned fully on, then all of the power goes to the motor and it operates harder/faster. Thus
for the linear type of control, power is delivered constantly, not in discrete pulses (like the SCR control).
Thus better speed stability and control is obtained. Another technique is termed pulse width modulation
(PWM). With PWM techniques, power is regulated by applying pulses of variable width, i.e. by changing
or modulating the pulse widths of the power. In comparison with the SCR control (which applies large
pulses of power), the PWM AVAILABLE VOLTAGE PULSES OF POWER TO MOTOR MAINTAIN SPEED
INCREASE SPEED SLOW DOWN Fig. 15 - AN SCR CONTROL Page 16 Servo Control Facts technique applies
narrow, discrete (when necessary) power pulses. Operation is as follows: With the pulse width small, the
average voltage applied onto the motor is low, and the motor's speed is slow. If the width is wide, the
average voltage is higher, and therefore motor speed is higher. This technique has the advantage in that
the power loss in the transistor is small, i.e. the transistor is either fully "on" or fully "off" and, therefore,
the transistor has reduced power dissipation.This approach allows for smaller package sizes. The final
technique used to turn transistors "on" and "off" is termed pulse frequency modulation (PFM). With
PFM, the power is regulated by applying pulses of variable frequency, i.e. by changing or modulating the
timing of the pulses. The system operates as follows: With very few pulses, the average voltage applied
onto the motor is low, and motor speed is slow. With many pulses, the average voltage is increased, and
motor speed is higher. DRIVE TYPES OPEN LOOP •SIGNAL STARTS MOTION •NO FEEDBACK SIGNAL
EXAMPLE: STEPPER CLOSED LOOP • SIGNAL COMMANDS MOTION •FEEDBACK SIGNAL RETURNS
EXAMPLE: SERVOMOTOR + FEEDBACK DEVICE TYPES OF CONTROLS AC DC •CONVERTS AC TO DC TO AC
EXAMPLE: VECTOR •CONVERTS AC TO DC EXAMPLE: DC SERVO OUTPUT POWER DEVICES SCR •LARGE
PULSES OF POWER EXAMPLE: SCR SPEED CONTROL TRANSISTOR •SMOOTH OPERATION EXAMPLE:
SERVO CONTROL TECHNIQUES TO TURN TRANSISTORS OFF AND ON PULSE FREQUENCY MODULATION
(PFM) •TRANSISTOR EITHER OFF OR ON •AMPLITUDE OF VOLTS CONSTANT •TURN ON TIME VARIED
•LOW POWER DISSIPATION PULSE WIDTH MODULATION (PWM) •TRANSISTOR EITHER ON OR OFF
•AMPLITUDE OF VOLTS CONSTANT •WIDTH OF PULSE VARIED •LOW POWER DISSIPATION LINEAR
•TRANSISTOR ALWAYS ON •AMPLITUDE OF VOLTS VARIED •HIGH INTERNAL POWER DISSIPATED Fig. 18
- SUMMARY OF DRIVE TYPES NARROW PULSE WIDE PULSE t1 t2 t1 t = 2 Fig. 16 PULSE WIDTH
DETERMINES AVERAGE VOLTAGE AVG. VOLTS AVG. VOLTS AVG. VOLTS AVG. VOLTS t1 t = 2 = VARIABLE
FREQUENCY t1 t2 Fig. 17 PULSE FREQUENCY MODULATION TO DETERMINE AVERAGE VOLTAGE Page 17
Servo Control Facts Servos use feedback signals for stabilization, speed and position information. This
information may come from a variety of devices such as the analog tachometer, the digital tachometer
(optical encoder) or from a resolver. In the following, each of these devices will be defined and the
basics explored. TYPES OF FEEDBACK DEVICES ANALOG TACHOMETERS Tachometers resemble miniature
motors. However, the similarity ceases there. In a tachometer, the gauge of wire is quite fine, thus the
current handling capability is small. But the tachometer is not used for a power delivering device.
Instead, the shaft is turned by some mechanical means and a voltage is developed at the terminals (a
motor in reverse!). The faster the shaft is turned, the larger the magnitude of voltage developed (i.e. the
amplitude of the tach signal is directly proportional to speed). The output voltage shows a polarity (+ or
-) which is dependent on direction of rotation. Analog, or DC tachometers, as they are often termed,
play an important role in drives, because of their ability to provide directional and rotational
information. They can be used to provide speed information to a meter (for visual speed readings) or
provide velocity feedback (for stabilization purposes). The DC tach provides the simplest, most direct
method of accomplishing this feat. As an example of a drive utilizing an analog tach for velocity
information, consider a lead screw assembly which must move a load at a constant speed. The motor is
for the linear type of control, power is delivered constantly, not in discrete pulses (like the SCR control).
Thus better speed stability and control is obtained. Another technique is termed pulse width modulation
(PWM). With PWM techniques, power is regulated by applying pulses of variable width, i.e. by changing
or modulating the pulse widths of the power. In comparison with the SCR control (which applies large
pulses of power), the PWM AVAILABLE VOLTAGE PULSES OF POWER TO MOTOR MAINTAIN SPEED
INCREASE SPEED SLOW DOWN Fig. 15 - AN SCR CONTROL Page 16 Servo Control Facts technique applies
narrow, discrete (when necessary) power pulses. Operation is as follows: With the pulse width small, the
average voltage applied onto the motor is low, and the motor's speed is slow. If the width is wide, the
average voltage is higher, and therefore motor speed is higher. This technique has the advantage in that
the power loss in the transistor is small, i.e. the transistor is either fully "on" or fully "off" and, therefore,
the transistor has reduced power dissipation.This approach allows for smaller package sizes. The final
technique used to turn transistors "on" and "off" is termed pulse frequency modulation (PFM). With
PFM, the power is regulated by applying pulses of variable frequency, i.e. by changing or modulating the
timing of the pulses. The system operates as follows: With very few pulses, the average voltage applied
onto the motor is low, and motor speed is slow. With many pulses, the average voltage is increased, and
motor speed is higher. DRIVE TYPES OPEN LOOP •SIGNAL STARTS MOTION •NO FEEDBACK SIGNAL
EXAMPLE: STEPPER CLOSED LOOP • SIGNAL COMMANDS MOTION •FEEDBACK SIGNAL RETURNS
EXAMPLE: SERVOMOTOR + FEEDBACK DEVICE TYPES OF CONTROLS AC DC •CONVERTS AC TO DC TO AC
EXAMPLE: VECTOR •CONVERTS AC TO DC EXAMPLE: DC SERVO OUTPUT POWER DEVICES SCR •LARGE
PULSES OF POWER EXAMPLE: SCR SPEED CONTROL TRANSISTOR •SMOOTH OPERATION EXAMPLE:
SERVO CONTROL TECHNIQUES TO TURN TRANSISTORS OFF AND ON PULSE FREQUENCY MODULATION
(PFM) •TRANSISTOR EITHER OFF OR ON •AMPLITUDE OF VOLTS CONSTANT •TURN ON TIME VARIED
•LOW POWER DISSIPATION PULSE WIDTH MODULATION (PWM) •TRANSISTOR EITHER ON OR OFF
•AMPLITUDE OF VOLTS CONSTANT •WIDTH OF PULSE VARIED •LOW POWER DISSIPATION LINEAR
•TRANSISTOR ALWAYS ON •AMPLITUDE OF VOLTS VARIED •HIGH INTERNAL POWER DISSIPATED Fig. 18
- SUMMARY OF DRIVE TYPES NARROW PULSE WIDE PULSE t1 t2 t1 t = 2 Fig. 16 PULSE WIDTH
DETERMINES AVERAGE VOLTAGE AVG. VOLTS AVG. VOLTS AVG. VOLTS AVG. VOLTS t1 t = 2 = VARIABLE
FREQUENCY t1 t2 Fig. 17 PULSE FREQUENCY MODULATION TO DETERMINE AVERAGE VOLTAGE Page 17
Servo Control Facts Servos use feedback signals for stabilization, speed and position information. This
information may come from a variety of devices such as the analog tachometer, the digital tachometer
(optical encoder) or from a resolver. In the following, each of these devices will be defined and the
basics explored. TYPES OF FEEDBACK DEVICES ANALOG TACHOMETERS Tachometers resemble miniature
motors. However, the similarity ceases there. In a tachometer, the gauge of wire is quite fine, thus the
current handling capability is small. But the tachometer is not used for a power delivering device.
Instead, the shaft is turned by some mechanical means and a voltage is developed at the terminals (a
motor in reverse!). The faster the shaft is turned, the larger the magnitude of voltage developed (i.e. the
amplitude of the tach signal is directly proportional to speed). The output voltage shows a polarity (+ or
-) which is dependent on direction of rotation. Analog, or DC tachometers, as they are often termed,
play an important role in drives, because of their ability to provide directional and rotational
information. They can be used to provide speed information to a meter (for visual speed readings) or
provide velocity feedback (for stabilization purposes). The DC tach provides the simplest, most direct
method of accomplishing this feat. As an example of a drive utilizing an analog tach for velocity
information, consider a lead screw assembly which must move a load at a constant speed. The motor is
required to rotate the lead screw at 3600 rpm. If the tachometer's output voltage gradient is 2.5
volts/Krpm, the voltage read on the tachometer terminals should be: 3.600 Krpm x 2.5 volts/Krpm = 9
volts If the voltage read is indeed 9 volts, then the tachometer (and motor/load) is rotating at 3600 rpm.
The servo drive will try to maintain this voltage to assure the desired speed. Although this example has
been simplified, the basic concept of speed regulation via the tachometer is illustrated. Some of the
terminology associated with tachometers which explains the basic characteristics of this device are:
voltage constant, ripple and linearity. The following will define each. A tachometer's voltage constant
may also be referred to as voltage gradient, or sensitivity. This represents the output voltage generated
from a tachometer when operated at 1000 rpm, i.e. V/Krpm. Sometimes converted and expressed in
volts per radian per second, i.e. V/rad/sec. Ripple may be termed voltage ripple or tachometer ripple.
Since tachs are not ideal devices, and design and manufacturing tolerances enter into the product, there
are deviations from the norm. When the shaft is rotated, a DC signal is produced as well as a small
amount of an AC signal + MECHANICALLY ROTATE OUTPUT VOLTS SPEED OUTPUT VOLTAGE TACH
OUTPUT PROPORTIONAL TO SPEED Fig. 19 - TACHOMETER Page 18 Servo Control Facts which is
superimposed upon the DC level. In reviewing literature, care must be exercised to determine the
definition of ripple since there are three methods of presenting the data: 1) Peak-to-peak – the ratio of
peak-to-peak ripple expressed as a percent of the average DC level; 2) RMS – the ratio of the RMS of the
AC component expressed as a percent of the average DC level and 3) Peak to Average – the ratio of
maximum deviation from the average DC value expressed as a percent of the average DC level. Linearity
– The ideal tachometer would have a perfect straight line for voltage vs. speed. Again, design and
manufacturing tolerances enter the picture and alter this straight line. Thus, linearity is a measure of
how far away from perfect this product or design is. The maximum difference of the actual versus
theoretical curves is linearity (expressed in percentage). RIPPLE DC VOLTS 0 Fig. 20 - TACH RIPPLE SCOPE
VOLTS VS. TIME TIME VOLTS ACTUAL IDEAL SPEED VOLTS Fig. 21 - TACH LINEARITY DIGITAL
TACHOMETERS A digital tachometer, often termed an optical encoder or simply encoder, is a
mechanical-to-electrical conversion device. The encoder's shaft is rotated and an output signal results
which is proportional to distance (i.e. angle) the shaft is rotated through. The output signal may be
square waves, or sinusoidal waves, or provide an absolute position. Thus encoders are classified into
two basic types: absolute and incremental. ABSOLUTE ENCODER. The absolute encoder provides a
specific address for each shaft position throughout 360 degrees. This type of encoder employs either
contact (brush) or non-contact schemes of sensing position. The contact scheme incorporates a brush
assembly to make direct electrical contact with the electrically conductive paths of the coded disk to
read address information. The non-contact scheme utilizes photoelectric detection to sense position of
the coded disk. The number of tracks on the coded disk may be increased until the desired resolution or
accuracy is achieved. And since position information is directly on the coded disk assembly, the disk has
a Page 19 Servo Control Facts built-in "memory system" and a power failure will not cause this
information to be lost. Therefore, it will not be required to return to a "home" or "start" position upon
reenergizing power. EXAMPLE BRUSH DISK Fig. 22 - ABSOLUTE ENCODER INCREMENTAL ENCODER. The
incremental encoder provides either pulses or a sinusoidal output signal as it is rotated throughout 360
degrees. Thus distance data is obtained by counting this information. The disk is manufactured with
opaque lines. A light source passes a beam through the transparent segments onto a photosensor which
outputs a sinusoidal waveform. Electronic processing can be used to transform this signal into a square
pulse train. In utilizing this device, the following parameters are important: 1) Line count: This is the
number of pulses per revolution. The number of lines is determined by the positional accuracy required
volts/Krpm, the voltage read on the tachometer terminals should be: 3.600 Krpm x 2.5 volts/Krpm = 9
volts If the voltage read is indeed 9 volts, then the tachometer (and motor/load) is rotating at 3600 rpm.
The servo drive will try to maintain this voltage to assure the desired speed. Although this example has
been simplified, the basic concept of speed regulation via the tachometer is illustrated. Some of the
terminology associated with tachometers which explains the basic characteristics of this device are:
voltage constant, ripple and linearity. The following will define each. A tachometer's voltage constant
may also be referred to as voltage gradient, or sensitivity. This represents the output voltage generated
from a tachometer when operated at 1000 rpm, i.e. V/Krpm. Sometimes converted and expressed in
volts per radian per second, i.e. V/rad/sec. Ripple may be termed voltage ripple or tachometer ripple.
Since tachs are not ideal devices, and design and manufacturing tolerances enter into the product, there
are deviations from the norm. When the shaft is rotated, a DC signal is produced as well as a small
amount of an AC signal + MECHANICALLY ROTATE OUTPUT VOLTS SPEED OUTPUT VOLTAGE TACH
OUTPUT PROPORTIONAL TO SPEED Fig. 19 - TACHOMETER Page 18 Servo Control Facts which is
superimposed upon the DC level. In reviewing literature, care must be exercised to determine the
definition of ripple since there are three methods of presenting the data: 1) Peak-to-peak – the ratio of
peak-to-peak ripple expressed as a percent of the average DC level; 2) RMS – the ratio of the RMS of the
AC component expressed as a percent of the average DC level and 3) Peak to Average – the ratio of
maximum deviation from the average DC value expressed as a percent of the average DC level. Linearity
– The ideal tachometer would have a perfect straight line for voltage vs. speed. Again, design and
manufacturing tolerances enter the picture and alter this straight line. Thus, linearity is a measure of
how far away from perfect this product or design is. The maximum difference of the actual versus
theoretical curves is linearity (expressed in percentage). RIPPLE DC VOLTS 0 Fig. 20 - TACH RIPPLE SCOPE
VOLTS VS. TIME TIME VOLTS ACTUAL IDEAL SPEED VOLTS Fig. 21 - TACH LINEARITY DIGITAL
TACHOMETERS A digital tachometer, often termed an optical encoder or simply encoder, is a
mechanical-to-electrical conversion device. The encoder's shaft is rotated and an output signal results
which is proportional to distance (i.e. angle) the shaft is rotated through. The output signal may be
square waves, or sinusoidal waves, or provide an absolute position. Thus encoders are classified into
two basic types: absolute and incremental. ABSOLUTE ENCODER. The absolute encoder provides a
specific address for each shaft position throughout 360 degrees. This type of encoder employs either
contact (brush) or non-contact schemes of sensing position. The contact scheme incorporates a brush
assembly to make direct electrical contact with the electrically conductive paths of the coded disk to
read address information. The non-contact scheme utilizes photoelectric detection to sense position of
the coded disk. The number of tracks on the coded disk may be increased until the desired resolution or
accuracy is achieved. And since position information is directly on the coded disk assembly, the disk has
a Page 19 Servo Control Facts built-in "memory system" and a power failure will not cause this
information to be lost. Therefore, it will not be required to return to a "home" or "start" position upon
reenergizing power. EXAMPLE BRUSH DISK Fig. 22 - ABSOLUTE ENCODER INCREMENTAL ENCODER. The
incremental encoder provides either pulses or a sinusoidal output signal as it is rotated throughout 360
degrees. Thus distance data is obtained by counting this information. The disk is manufactured with
opaque lines. A light source passes a beam through the transparent segments onto a photosensor which
outputs a sinusoidal waveform. Electronic processing can be used to transform this signal into a square
pulse train. In utilizing this device, the following parameters are important: 1) Line count: This is the
number of pulses per revolution. The number of lines is determined by the positional accuracy required
in the application. 2) Output signal: The output from the photosensor can be either a sine or square
wave signal. 3) Number of channels: Either one or two channel outputs can be provided. The two
channel version provides LIGHT SOURCE DISK GRID ASSEMBLY PHOTO SENSOR PICKUP SQUARING
CIRCUITRY Fig. 23 - INCREMENTAL ENCODER Page 20 Servo Control Facts a signal relationship to obtain
motion direction (i.e. clockwise or counterclockwise rotation). In addition, a zero index pulse can be
provided to assist in determining the "home" position. A typical application using an incremental
encoder is as follows: An input signal loads a counter with positioning information. This represents the
position the load must be moved to. As the motor accelerates, the pulses emitted from the incremental
(digital) encoder come at an increasing rate until a constant run speed is attained. During the run period,
the pulses come at a constant rate which can be directly related to motor speed. The counter, in the
meanwhile, is counting the encoder pulses and, at a predetermined location, the motor is commanded
to slow down. This is to prevent overshooting the desired position. When the counter is within 1 or 2
pulses of the desired position, the motor is commanded to stop. The load is now in position. RESOLVERS.
Resolvers look similar to small motors – that is, one end has terminal wires, and the other end has a
mounting flange and a shaft extension. Internally, a "signal" winding rotor revolves inside a fixed stator.
This represents a type of transformer: When one winding is excited with a signal, through transformer
action the second winding is excited. As the first winding is moved (the rotor), the output of the second
winding changes (the stator). This change is directly proportional to the angle which the rotor has been
moved through. As a starting point, the simplest resolver unit contains a single winding on the rotor and
two windings on the stator (located 90 degrees apart). A reference signal is applied onto the primary
(the rotor), then via transformer action this is coupled to the secondary. The secondary's output signal
would be a sine wave proportional to angle VAC VOUT Fig. 25 - RESOLVER: A ROTATING TRANSFORMER
SPEED INPUT SIGNAL UP/DOWN COUNTER SERVO CONTROL SERVO ENCODER ENCODER PULSES Fig. 24 -
EXAMPLE USING ENCODER PULSES MECHANICAL REVOLUTION 360° MECHANICAL REVOLUTION ROTOR
STATOR V1 OUT SINE Fig. 26 - TYPICAL RESOLVER OUTPUT V2 OUT COSINE 360° Page 21 Servo Control
Facts (the other winding would be a cosine wave), with one electrical cycle of output voltage produced
for each 360 degrees of mechanical rotation. These are fed into the controller. Inside the controller, a
resolver to digital (R to D) converter analyzes the signal, producing an output representing the angle
which the rotor has moved through, and an output proportional to speed (how fast the rotor is moving).
There are various types of resolvers. The type described above would be termed a single speed resolver;
that is, the output signal goes through only one sine wave as the rotor goes through 360 mechanical
degrees. If the output signal went through four sine waves as the rotor goes through 360 mechanical
degrees, it would be called a 4 -speed resolver. Another version utilizes three windings on the stator –
and would be called a synchro. The three windings are located 120 degrees apart. The basic type of
resolver discussed thus far may also be termed a "resolver transmitter" – one phase input and two
phase outputs (i.e. a single winding of the rotor is excited and the stator's two windings provide position
information). Resolver manufacturers may term this a "CX" unit, or "RCS" unit. Another type of resolver
is termed "resolver control transformer" – two phase inputs and one phase output (i.e. the two stator
windings are excited and the rotor single winding provides position information). Resolver
manufacturers term this type "CT" or "RCT" or "RT". The third type of resolver is termed a "resolver
transmitter" – two phase inputs and two phase outputs (i.e. two rotor windings are excited, and position
information is derived from the two stator windings). This may be referred to as "differential" resolver,
or "RD", or "RC" depending on the manufacturer. Page 22 Servo Control Facts The basic actuators for
controlling motion (which involve control of either speed, torque or positional accuracy) would include:
wave signal. 3) Number of channels: Either one or two channel outputs can be provided. The two
channel version provides LIGHT SOURCE DISK GRID ASSEMBLY PHOTO SENSOR PICKUP SQUARING
CIRCUITRY Fig. 23 - INCREMENTAL ENCODER Page 20 Servo Control Facts a signal relationship to obtain
motion direction (i.e. clockwise or counterclockwise rotation). In addition, a zero index pulse can be
provided to assist in determining the "home" position. A typical application using an incremental
encoder is as follows: An input signal loads a counter with positioning information. This represents the
position the load must be moved to. As the motor accelerates, the pulses emitted from the incremental
(digital) encoder come at an increasing rate until a constant run speed is attained. During the run period,
the pulses come at a constant rate which can be directly related to motor speed. The counter, in the
meanwhile, is counting the encoder pulses and, at a predetermined location, the motor is commanded
to slow down. This is to prevent overshooting the desired position. When the counter is within 1 or 2
pulses of the desired position, the motor is commanded to stop. The load is now in position. RESOLVERS.
Resolvers look similar to small motors – that is, one end has terminal wires, and the other end has a
mounting flange and a shaft extension. Internally, a "signal" winding rotor revolves inside a fixed stator.
This represents a type of transformer: When one winding is excited with a signal, through transformer
action the second winding is excited. As the first winding is moved (the rotor), the output of the second
winding changes (the stator). This change is directly proportional to the angle which the rotor has been
moved through. As a starting point, the simplest resolver unit contains a single winding on the rotor and
two windings on the stator (located 90 degrees apart). A reference signal is applied onto the primary
(the rotor), then via transformer action this is coupled to the secondary. The secondary's output signal
would be a sine wave proportional to angle VAC VOUT Fig. 25 - RESOLVER: A ROTATING TRANSFORMER
SPEED INPUT SIGNAL UP/DOWN COUNTER SERVO CONTROL SERVO ENCODER ENCODER PULSES Fig. 24 -
EXAMPLE USING ENCODER PULSES MECHANICAL REVOLUTION 360° MECHANICAL REVOLUTION ROTOR
STATOR V1 OUT SINE Fig. 26 - TYPICAL RESOLVER OUTPUT V2 OUT COSINE 360° Page 21 Servo Control
Facts (the other winding would be a cosine wave), with one electrical cycle of output voltage produced
for each 360 degrees of mechanical rotation. These are fed into the controller. Inside the controller, a
resolver to digital (R to D) converter analyzes the signal, producing an output representing the angle
which the rotor has moved through, and an output proportional to speed (how fast the rotor is moving).
There are various types of resolvers. The type described above would be termed a single speed resolver;
that is, the output signal goes through only one sine wave as the rotor goes through 360 mechanical
degrees. If the output signal went through four sine waves as the rotor goes through 360 mechanical
degrees, it would be called a 4 -speed resolver. Another version utilizes three windings on the stator –
and would be called a synchro. The three windings are located 120 degrees apart. The basic type of
resolver discussed thus far may also be termed a "resolver transmitter" – one phase input and two
phase outputs (i.e. a single winding of the rotor is excited and the stator's two windings provide position
information). Resolver manufacturers may term this a "CX" unit, or "RCS" unit. Another type of resolver
is termed "resolver control transformer" – two phase inputs and one phase output (i.e. the two stator
windings are excited and the rotor single winding provides position information). Resolver
manufacturers term this type "CT" or "RCT" or "RT". The third type of resolver is termed a "resolver
transmitter" – two phase inputs and two phase outputs (i.e. two rotor windings are excited, and position
information is derived from the two stator windings). This may be referred to as "differential" resolver,
or "RD", or "RC" depending on the manufacturer. Page 22 Servo Control Facts The basic actuators for
controlling motion (which involve control of either speed, torque or positional accuracy) would include:
• Air Motors • Hydraulic Motors • Clutch/Brake • Stepper Motors • AC Induction Motors • Servomotors
The following presents a synopsis, of the strengths and weaknesses of each basic motion control
technique. Air Motors – use compressed air to create motion. Pressure and flow determine speed and
torque positional accuracy is usually not a requirement. Principle strengths: 1. Low cost 2. Available
components 3. Easy to apply 4. Easy to maintain 5 .Easy to understand 6 .Centralized power source
Hydraulic motors – use pressurized oil to move a piston. Higher pressure results in higher torque (i.e.
brute force). Principle strengths: 1 .Easy to apply 2. High torques available 3. Centralized power source
4. Easy to understand Clutch/Brake – a device coupling a continuously rotating shaft and a load.
Uncoupling the load results in stopping. Varying on/off time results in varying distances. Principle
strengths: 1. Easy to apply 2. Low comparative cost 3. Good for start/stop with light loads 4. Easy to
provide speed matching Principle weaknesses: 1. Audible compressor noise 2. Difficult to regulate speed
3 Prone to contamination 4. Energy inefficient TYPES OF ACTUATORS Principle weaknesses: 1. Audible
noise 2. Difficult to control speed 3. Slow positioning 4. Prone to leaks 5. Energy inefficient 6. Fire hazard
7. High maintenance required Principle weaknesses: 1.Uncontrolled acceleration 2. Inaccurate 3. Prone
to wear 4. Non-repeatable performance Stepping Motors – electromechanical device which converts
one digital pulse into a specific rotational movement or displacement. A "train of pulses" results in
rotational speed. Principle strengths: 1. Simple control 2. Moderate cost 3. Good for constant loads 4.
Good positional accuracy AC Induction Motors – widely used for constant speed requirements. Electric
"starters" provide connections/start-up/overload protection. Newer technology provides variable speed
capability. Principle strengths: 1. Simple motor 2. Low cost 3. Mature technology 4. Straightforward
on/off control 5. Affordable coarse speed control 6 .Simple wiring 7. Wide product variety 8. Many
vendors available Servomotors – A motor with a "feedback" device. Electronic packages control speed
and position accuracy. Principle strengths: 1. High performance 2. Small size 3. Wide variety of
components 4. High speeds available with specialized controls Page 23 Servo Control Facts Principle
weaknesses: 1. Prone to losing steps 2. Not good for varying loads 3. Energy inefficient 4. Large motor
size 5. Resonance problems Principle weaknesses: 1. Limited position control 2. Relatively larger size
Principle weaknesses: 1. Slightly higher cost 2. High performance limited by controls 3. High speed
torque limited by commutator or electronics TYPES OF ACTUATORS (cont.) BALDOR ELECTRIC COMPANY
5711 South 7th Street Fort Smith, Arkansas 72901 (501) 646-4711 Fax (501) 648-5792 `3/94 5M CMc
The following presents a synopsis, of the strengths and weaknesses of each basic motion control
technique. Air Motors – use compressed air to create motion. Pressure and flow determine speed and
torque positional accuracy is usually not a requirement. Principle strengths: 1. Low cost 2. Available
components 3. Easy to apply 4. Easy to maintain 5 .Easy to understand 6 .Centralized power source
Hydraulic motors – use pressurized oil to move a piston. Higher pressure results in higher torque (i.e.
brute force). Principle strengths: 1 .Easy to apply 2. High torques available 3. Centralized power source
4. Easy to understand Clutch/Brake – a device coupling a continuously rotating shaft and a load.
Uncoupling the load results in stopping. Varying on/off time results in varying distances. Principle
strengths: 1. Easy to apply 2. Low comparative cost 3. Good for start/stop with light loads 4. Easy to
provide speed matching Principle weaknesses: 1. Audible compressor noise 2. Difficult to regulate speed
3 Prone to contamination 4. Energy inefficient TYPES OF ACTUATORS Principle weaknesses: 1. Audible
noise 2. Difficult to control speed 3. Slow positioning 4. Prone to leaks 5. Energy inefficient 6. Fire hazard
7. High maintenance required Principle weaknesses: 1.Uncontrolled acceleration 2. Inaccurate 3. Prone
to wear 4. Non-repeatable performance Stepping Motors – electromechanical device which converts
one digital pulse into a specific rotational movement or displacement. A "train of pulses" results in
rotational speed. Principle strengths: 1. Simple control 2. Moderate cost 3. Good for constant loads 4.
Good positional accuracy AC Induction Motors – widely used for constant speed requirements. Electric
"starters" provide connections/start-up/overload protection. Newer technology provides variable speed
capability. Principle strengths: 1. Simple motor 2. Low cost 3. Mature technology 4. Straightforward
on/off control 5. Affordable coarse speed control 6 .Simple wiring 7. Wide product variety 8. Many
vendors available Servomotors – A motor with a "feedback" device. Electronic packages control speed
and position accuracy. Principle strengths: 1. High performance 2. Small size 3. Wide variety of
components 4. High speeds available with specialized controls Page 23 Servo Control Facts Principle
weaknesses: 1. Prone to losing steps 2. Not good for varying loads 3. Energy inefficient 4. Large motor
size 5. Resonance problems Principle weaknesses: 1. Limited position control 2. Relatively larger size
Principle weaknesses: 1. Slightly higher cost 2. High performance limited by controls 3. High speed
torque limited by commutator or electronics TYPES OF ACTUATORS (cont.) BALDOR ELECTRIC COMPANY
5711 South 7th Street Fort Smith, Arkansas 72901 (501) 646-4711 Fax (501) 648-5792 `3/94 5M CMc
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