Control of Actuator in Automation Mechanism

control of actuator in automation mechanism

1. Chapter 4 Control Of Actuators In Automation Mechanism

2. At the end of this topic, student should be able to:- Identify Stepper Motors 4.1.1 Describe principles of stepper motor operation 4.1.2 Explain Half Step Mode Operation 4.1.3 Explain Micro-step Mode 4.1.4 Describe additional Methods of Damping Rotor Oscillations 4.1.5 Describe permanent Magnet Stepper Motors 4.1.6 Describe stepper motor drives 4.1.7 Describe linear stepper motors Apply control method of actuators 4.2.1 Apply control method for Brushless DC Motors 4.2.2 Apply control method for Direct Drives Actuator 4.2.3 Apply control method for Hydraulic Actuators 4.2.4 Apply control method for Pneumatic Actuators

3. Stepper Motors A stepper motor is a “pulse-driven” motor that changes the angular position of the rotor in “steps” Define β = the step angle (per input pulse) Resolution = the number of steps/revolution θ = total angle traveled by the rotor = β X # of steps n = the shaft speed = ( β X f p ) / 360° f p = # of pulses/second

4. Stators Rotor Cross Section of a Stepper Motor

5. Rotor Stator Coils Outside Casing Stator Rotor Internal components of a Stepper Motor

6. Variable-Reluctance Stepper Motor Toothed Rotor and Toothed Stator Principle of Operation: Reluctance of the magnetic circuit formed by the rotor and stator teeth varies with the angular position of the rotor Here, energize coils A and A’ (Phase A) Rotor “steps” to align rotor teeth 1 and 4 with stator teeth 1 and 5

7. Variable-Reluctance Stepper Motor Energize coils B and B’ (Phase B) Rotor steps “forward” Rotor teeth 3 and 6 align with Stator teeth 1 and 5 Let N s = # of teeth on the stator N r = # of teeth on the rotor β = Step Angle in space degrees

8. Variable-Reluctance Stepper Motor Energize Phase C Rotor steps forward another 15 °

9. Variable-Reluctance Stepper Motor Energize Phase D Rotor steps forward another 15 °

10. Variable-Reluctance Stepper Motor Repeat the sequence Energize Phase A Rotor steps forward again

11. Stepper motors and drives, what is full step, half step and microstepping ? Stepper drives control how a stepper motor operates, there are three commonly used excitation modes for stepper motors, full step, half step and microstepping . These excitation modes have an effect on both the running properties and torque the motor delivers . A stepper motor converts electronic signals into mechanical movement each time an incoming pulse is applied to the motor. Each pulse moves the shaft in fixed increments. If the stepper motor has a 1.8° step resolution, then in order for shaft to rotate one complete revolution, in full step operation, the stepper motor would need to receive 200 pulses, 360° ÷ 1.8 = 200.

12. Four Steps per revolution i.e. 90 deg. steps. Full Step Operation

13. There are two types of full step excitation modes . In one-phase on - full step, Fig1, the motor is operated with only one phase energized at a time . This mode requires the least amount of power from the driver of any of the excitation modes . In two-phase on - full step, Fig2, the motor is operated with both phases energized at the same time. This mode provides improved torque and speed performance. Two-phase on provides about 30% to 40% more torque than one phase on, however it requires twice as much power from the driver.

14. Eight steps per. revolution i.e. 45 deg. steps. Half Step Operation

15. Half step excitation mode is a combination of one phase on and two phase on full step modes. This results in half the basic step angle . This smaller step angle provides smoother operation due the increased resolution of the angle . Half step produces about 15% less torque than two phase on - full step, however modified half stepping eliminates the torque decrease by increasing the current applied to the motor when a single phase is energized. See Fig3

16. Microstepping for greater control and smoother operation Microstepping can divide a motor’s basic step by up to 256 times, making small steps smaller. A Micro drive uses two current sinewaves 90° apart, this is perfect for enabling smooth running of the motor. You will notice that the motor runs is quietly and with no real detectable stepping action . By controlling direction and amplitude of the current flow in each winding, the resolution increases and the characteristics of the motor improve, giving less vibration and smoother operation. Because the sinewaves work together there is a smooth transition from one winding to the other. When current increases in one it decreases in the other resulting in a smooth step progression and maintained torque output. See Fig4

17. M ethod use in Damping Rotor Oscillations It is well known that rotor oscillation is one of the principal problems in the switched drive of a stepping motor , and nowadays several methods for damping this oscillation have been suggested in which the switching sequence is changed in some manner. In such methods, the excitation time of the stator windings must be tuned appropriately, or the effect of damping is insufficient and oscillation may be even amplified in some circumstances. To resolve this problem, adaptive methods for tuning of the excitation time have been developed. However, they suffer the disadvantage that they require a tuning period for the excitation time to attain the optimal value at the beginning of control or when the machine parameters are varied by changing the driving condition.

18. At very low stepping rates the motor comes to rest at the appropriate equilibrium position after each excitation change. The response of the system to each excitation change – known as the single-step response. For example, if a stepping motor is used to drive a printer carriage then the system must come to rest for the printing of each letter. The operating speed of the printer is limited by the time taken for the system to settle to within the required accuracy at each letter position. The frequency of oscillation can be predicted for any motor/load combination from the static torque/rotor position characteristic, provided the system is lightly damped.

19. Stepping Motor to move read-write head Stepper motor applications

20. Paper feeder on printers CNC lathes Stepper motors Stepper motor applications

21. Rotor Stator coils CNC Stepping Motor

22. Advantages:- Low cost for control achieved Ruggedness Simplicity of construction Can operate in an open loop control system Low maintenance Less likely to stall or slip Will work in any environment Disadvantages:- Require a dedicated control circuit Use more current than D.C. motors High torque output achieved at low speeds Advantages / Disadvantages

23. Permanent-magnet stepper motors have smooth armatures and include a permanent magnet core that is magnetized widthwise or perpendicular to its rotation axis . These motors usually have two independent windings, with or without center taps. The most common step angles for PM motors are 45 ° and 90 ° , but motors with step angles as fine as 1.8 ° per step as well as 7.5, 15, and 30 ° per step are generally available. Armature rotation occurs when the stator poles are alternately energized and de-energized to create torque . A 90° stepper has four poles and a 45° stepper has eight poles, and these poles must be energized in sequence. Permanent-magnet steppers step at relatively low rates, but they can produce high torques and they offer very good damping characteristics.

24. Linear actuators are available with axial integral threaded shafts and bolt nuts that convert rotary motion to linear motion. Powered by fractional horsepower permanent-magnet stepper motors, these linear actuators are capable of positioning light loads. Digital pulses fed to the actuator cause the threaded shaft to rotate, advancing or retracting it so that a load coupled to the shaft can be moved backward or forward. The bidirectional digital linear actuator shown in Figure can provide linear res- solution as fine as 0.001 in. per pulse . Travel per step is determined by the pitch of the lead screw and step angle of the motor. The maximum linear force for the model shown is 75 oz.

25. Motors Electromagnetic direct current (DC) motors Usually runs high speed and low torque (Gear down) Electromagnetic alternating current (AC) motors Seldom used in Robots because power supply is battery

26. DC Motors The most common actuator in mobile robotics simple, cheap, and easy to use. come in a great variety of sizes, to accommodate different robots and tasks.

27. Principles of Operation DC motors convert electrical into mechanical energy. They consist of permanent magnets and loops of wire inside. When current is applied, the wire loops generate a magnetic field, which reacts against the outside field of the static magnets. The interaction of the fields produces the movement of the shaft/armature. Thus, electromagnetic energy becomes motion.

28. The Basic Idea A motor uses magnets to create motion. The fundamental law of all magnets: Opposites attract and likes repel. Inside an electric motor, these attracting and repelling forces create rotational motion .

29. Advantage Brush Motor Advantages - Brush DC Motor The DC Brush Motor is one of the earliest of all electrical motor designs. It is usually the motor of choice for the majority of torque control and variable speed applications. The following discusses the advantages and disadvantages of using a Brush DC Motor in machinery and automated processes. • The Brush DC Motor has a simple construction , therefore may not require a controller. When a controller is chosen, it is typically a simple and inexpensive drive design. • The design of the Brush DC Motor is quite simple , in that a permanent magnetic field is created in the by either of two means; permanent magnets or electro-magnetic windings. Controlling the speed of a Brush DC Motor is simple . The higher the armature voltage, the faster the rotation. This relationship is linear to the Brush DC Motor's maximum speed . Simple and inexpensive control design Simple linear relationship between voltage and speed. Easy to control . Low cost approach to variable speed drives and positioning . Well down the manufacturing cost curve.

30. Brush DC PM Disadvantages

31. Negative Impact on Environment Brush-to-commutator (metal-on-metal) sparking presents potential hazard. Brush-to-commutator (metal-on-metal) also creates excessive EMI/RFI electrical noise/interference. Brush wear creates particle contamination. Limited speed range due to mechanical brush-commutator resistance as brushes ‘ride’ on the metal commutator. Brush DC PM Disadvantages

32. Electrical Wear of Brushes and Commutator Ring Is Not Predictable Wear characteristics non-linear and depend on current, motor inductance, temperature, and other environmental conditions. Wear varies from 2 nd power to 4 th power. Brush DC PM Disadvantages

33. Mechanical Wear Is Not Easily Predicted Brush DC motors have been used for over 120 years, and we still do not know how to predict brush wear with any mathematical certainty. It is operating life uncertainty that has plagued the brush DC motor. Stiction creates initial resistance to motion. Brush DC PM Disadvantages

34. Enhancements, Improvements, etc., to Brush DC PM Technology

35. Brush DC PM Better Brushes Better Materials Motor Consistent Manufacturing Processes

36. B rush DC PM Material improvements in magnets (stronger, more rugged), iron (better steels), and insulation (magnet wire, potting, slot liners, separators) will continue to evolve, raising motor performance characteristics. Application of new, higher performing plastics and other materials to replace aluminum and metal parts will help lower manufacturing costs.

37. Brush DC PM Suppliers of Note

38. Applications for Brush DC PM Motors

39. Application Characteristics Favorable to Brush DC Motors Speed control with low cost. Speeds under 2000 rpm for larger motors but capable of 20,000 rpm in micro motor sizes. Lower duty cycle applications. Fractional horsepower needs. Power Source: Battery powered.

40. The Universe of Application Segments Brush DC PM technology is broadly used in many major application segments: Transportation Personal Vehicles Commercial Vehicles Off-Road Vehicles

41. About 80% of electric motors in a car are Brush DC PM

42. Sample of Automotive Applications Door Locks Window Lifts Seat Adjust Engine Fan Climate Control Fuel and Exhaust

43. The Universe of Application Segments Brush DC PM technology is broadly used in many major application segments: Factory Automation Conveyor Systems General Industrial Equipment Material Handling Packaging Semiconductor Processing Equipment Special Industrial Equipment

44. Brush DM PM technology…more application segments with broad penetration: Medical Equipment Air moving (Sleep Apnea, Ventilators, Respirators, etc.) Diagnostic Equipment (Centrifuges, Scanning Machines, etc.) Tools, Hand Helds , Surgery and Dental Mobility Equipment The Universe of Application Segments Medical: Pump and Ventilator Motors Sponsored by:

45. T he U niverse of Application Segments Brush DC PM technology application segments with moderate penetration: Aerospace Commercial Aviation Military Aviation Missiles/Drones/UAVs Military/Defense Vehicles UAVs and UGVs Autonomous Vehicles in Commercial, Security, Farming Applications

46. T he U niverse of Application Segments Brush DC PM technology application segments in home and commercial uses: Home Appliances Hand Tools Commercial and Industrial HVAC Sponsored by:

47. Precision Applications Using Small Motors

48. Remote Presence R obot Challenge? Need expertise located remotely with a patient in immediate need? Need remote control to move about, see, hear, and be seen and be heard? Need to be able to operate on batteries; quiet; reliable? Motor Need? Highly reliable, efficient, lighter weight, small, higher power per volume. Motion Solution? Coreless brush motor, geared (planetary, low backlash, precise) to translate higher speed to higher torque. Electronics for precision camera positioning; mitigation of EMI/RFI emissions. Innovative feedback combining potentiometers and magnetic encoders to achieve absolute and relative positioning inexpensively.

49. Autonomous All Terrain Vehicle Challenge? How do you go where it may be unsafe or difficult for humans to assess a problem; rescue miners or earthquake victims; inspect or analyze farm land; inspect nuclear plants; inspect construction in progress or hazardous industrial areas? All in extremely varied and at times difficult terrain? Motor Need? Operate at very low voltages (battery, solar powered); high torque per weight; easy to control. Motion Solution? Coreless brush motor, geared (planetary, low backlash, precise) with precious metal commutator for minimum size and ability to operate at lower voltages. Small and light weight are also important in mobile applications.

50. Rosetta Comet Exploring Space Probe Challenge? Ever land on a comet orbiting the Sun for exploration? No one has! So…you need to be ready for almost any situation with motorized sub-systems for controlled landing, anchoring, orienting the landing probe. Motor Need? Reliability of the highest order; simple controls; space ready. Small, lightweight. High torque per volume. Motion Solution? 14 brush DC motors and controls, geared (planetary, low backlash, precise) to translate higher speed to higher torque. Motor functions include: precision camera positioning to search for good landing site; act as motors and generators to capture precious power from kinetic motion; draw anchor lines taught; actuating analytical instruments. Landing Probe Anchoring Actuator

51. P iezo A ctuators Overview Piezo technology, while not electromagnetic in nature, is an interesting and alternative approach to actuation where physical contact (like brushes in a DC PM motor) help convey the energy needed for motion. Like a DC motor, piezo technology is energized by applying a voltage to the mechanism and that electrical energy is converted to mechanical energy in the form of vibrations. By controlling the application of the energy and the structure of the stationary and moving elements, linear or rotary motion is initiated. Many camera auto-focuses use piezo rotating motors. Metrology applications (small movements) are also an important area of use.

52. Piezo Advantages/Disadvantages Advantages: Capable of very small moves ( nano motion). Very high holding torque with almost no power consumed. Fast response (micro-seconds) compared to electromagnetic devices (high milli -seconds). Operate in harsh environments such as vacuums, extreme cold, high magnetic fields, high energy radiation where typical electro-magnetic and PM actuators require special designs or materials. Disadvantages (compared to electromagnetic devices) Operate primarily at very slow speeds. Travel is limited. Limited at high temperature levels (200 deg C). Made of brittle material. High hysteresis (does not return to original position) and creep, but both controllable.

53. Surgical Robot Inside An MRI Challenge? If you can position a surgical probe remotely, while you view an active MRI image you can increase the accuracy of the surgical procedure and reduce surgery time. But the extremely strong magnetic field of the MRI creates significant challenges for standard electric motor-driven robots. Screws, gears, motors become hazards within the MRI unit. Motor Need? Unaffected by the strong magnetic field of the MRI machine. Accurate, micro positioning. Audibly and electrically quiet. Motion Solution? Piezo -electric ceramic actuator. Creates motion when a voltage is applied. No magnetic/metal parts. Special design to increase speed. Inherently safe as loss of power brings the motion to a stop.

54. Brushless DC (BLDC) Motors Brushless DC Motors are a type of synchronous motor magnetic fields generated by the stator and rotor rotate at the same frequency no slip Available in single-phase, 2-phase, and 3-phase configurations

55. BLDC Motor Stator

56. BLDC Motor Rotors

57. Hall-Effect If a current-carrying conductor is kept in a magnetic field, the magnetic field exerts a force on the moving charge carriers, tending to push them to one side of the conductor, producing a measurable voltage difference between the two sides of the conductor.

58. Hall-Effect Sensors Need 3 sensors to determine the position of the rotor When a rotor pole passes a Hall-Effect sensor, get a high or low signal, indicating that a North or South pole

59. Transverse Sectional View of Rotor

60. Commutation Sequence Each sequence has one winding energized positive (current into the winding) one winding energized negative (current out of the winding) one winding non-energized

61. Torque-Speed Characteristic

62. Six-Step Commutation (4-pole) Hall-Effect Sensors spaced 60 electrical degrees apart 6 steps to complete one electrical cycle Phase current switching updated every 60 electrical degrees

63. Essential Elements of a Typical BLDC Motor

64. BLDC Control

65. CW

66. CCW

67. A dvantages of DC motors : Speed control over a wide range both above and below the rated speed:  The attractive feature of the dc motor is that it offers the wide range of speed control both above and below the rated speeds. This can be achieved in dc shunt motors by methods such as armature control method and field control method. This is one of the main applications in which dc motors are widely used in fine speed applications such as in rolling mills and in paper mills.  High starting torque:  dc series motors are termed as best suited drives for  electrical traction  applications used for driving heavy loads in starting conditions. DC series motors will have a staring torque as high as 500% compared to normal operating torque. Therefore  dc series motor  are used in the applications such as in electric trains and cranes. Accurate steep less speed with constant torque:  Constant torque drives is one such the drives will have motor shaft torque constant over a given speed range. In such drives shaft power varies with speed. Quick starting, stopping, reversing and acceleration Free from   harmonics, reactive power consumption and many factors which makes dc motors more advantageous compared to ac induction motors.

68. Disadvantages of DC motors : High initial cost. Increased operation and maintenance cost due to presence of commutator and brush gear . High maintennce Cannot operate in explosive and hazard conditions due to sparking occur at brush ( risk in commutation failure ) Require tuning

69. Top Servo Motor advantages are : High output power relative to motor size and weight. Encoder determines accuracy and resolution. High efficiency. It can approach 90% at light loads. High torque to inertia ratio. Servo Motors can rapidly accelerate loads. Has 2-3 times more continuous power for short periods. Has 5-10 times more rated torque for short periods. Servo motors achieve high speed at high torque values. Quiet at high speeds. Encoder utilization provides higher accuracy and resolution with closed-loop control.

70. The top Servo Motor disadvantages are : Servos Motors requires tuning to stabilize the feedback loop. Servo Motor will become unpredictable when something breaks. So, safety circuits are required. Complex controller requires encoder and electronic support. Peak torque is limited to a 1% duty cycle. Servo Motors can be damaged by sustained overload. Gear boxes are often required to deliver power at higher speeds. Higher overall system cost and the installation cost of a Servo Motor system may be higher than that of a stepper motor due to the requirement for feedback components.

71. Apply control method for Direct Drives Actuator Solenoid Type Devices Solenoids, is the simplest electromagnetic actuators that are used in linear as well as rotary actuations for valves, switches, and relays. As the name indicates, a solenoid consists of a stationary iron frame (stator), a coil (solenoid), and a ferromagnetic plunger (armature) in the center of the coil,

72. As the coil is energized, a magnetic field is induced inside the coil. The movable plunger moves to increase the flux linkage by closing the air gap between the plunger and the stationary frame. The magnetic force generated is approximately proportional to the square of the applied current I and is inverse proportional to the square of the air gap , which is the stroke of the solenoid,

73. Apply control method for Hydraulic Actuators

74. Hydraulic Actuation Systems The components of a hydraulic actuation system are: the pump , that is, the hydraulic power generation system; the actuator , that is, the element which converts hydraulic power into mechanical power; the valve , that is, the hydraulic power regulator; the pipes for connecting the various components of the actuation system; the filters , accumulators, and reservoirs; the fluid , which transfers the power between the various circuit elements; the sensors and transducers ; the system display, measurement, and control devices.

75. Apply control method for Pneumatic Actuators the components of a pneumatic actuation system are: • the compressed air generation system, consisting of the compressor, the cooler, possibly a dryer, • the storage tank, and the intake and output filters; • the compressed air treatment unit, usually consisting of the FRL assembly (filter, pressure regulator, • and possibly a lubrifier ), which permits filtration and local regulation of the supply pressure to • the actuator valve; • the valve, that is, the regulator of the pneumatic power; • the actuator, which converts the pneumatic power into mechanical power; • the piping; • the sensors and transducers; • the system display, physical magnitude measurement, and control devices.