Motor function operation UNIT 5 FINAL.pptx

Embedded systems motor

1. UNIT 5 By Durga Prasad Tumula

2. Syllabus Industrial Control: Introduction, Open and Closed loop control, Sensors, Output devices, Thyristors, Programmable Logic controllers.

3. INTRODUCTION Industrial control is one of the four major applications of electronics. In industrial control, electronic components, circuits, and equipment are used to operate various types of machines in manufacturing plants. Most industrial machines such as robots are mechanical in nature but electrically operated. Most are powered by electric motors or other actuators. To operate the machine, electrical power must be properly controlled. Industrial control electronics is used to turn machines off and on at the appropriate time, control their speed of operation, and otherwise produce the desired manipulations. Electronic control is not confined to industry. There are lots going on in the home and car as well. The principles given here apply to those applications as well.

4. OPEN- AND CLOSED-LOOP CONTROL The basic industrial control process is illustrated in Fig. 12.1. Electrical inputs, such as switches, sensors, or other devices, are used to initiate an electronic control process. The signal-processing circuits generate control output signals that are used to operate the industrial machines. This basic process is known as open-loop control. The typical device being controlled by the output signals is an electric motor that operates a machine. Other outputs could operate lights, relays, solenoids, or a variety of other devices.

5. Open-Loop Control A simple example of an open-loop control system is shown in Fig. 12.2. The input is a switch that applies electrical power to a motor. The motor, in turn, operates a pump that causes liquid to be put into a vat or tank. As soon as the vat is full, the operator turns the switch off. The basic control process in this system is simply turning power off and on to control the pump. The operator visually monitors the level in the vat until the correct level is reached.

6. Closed-Loop Control An improved form of industrial control system is illustrated in Fig. 12.3. It is known as a closed-loop control system. The system is given an initial input reference called the set point. The set point is usually a voltage that represents some physical value that is to be achieved. It is applied to an error detector that compares this input to a signal from the controlled devices. If the two signals are different, an error output is produced. This error signal is processed (usually amplified), and an output control signal is derived from it. It is this output signal that operates the controlled device. A transducer monitors the controlled device to see that the desired outcome is obtained. The signal from this transducer is called feedback. The feedback signal is compared to the input to produce the error signal. The control signal operates the controlled device until the error is reduced to zero. At that time, the desired output condition, where the feedback equals the set point, is obtained.

7. Cont.. The key to closed-loop control is the feedback. Feedback signals tell the system whether the machine is performing correctly. The feedback signal allows the system to adjust itself in such a way that the desired output is continuously accomplished. Control is automatic. A closed-loop version of our system to fill a vat with liquid is shown in Fig. 12.4. An initial input signal is applied to start the process. If the vat is not full, the transducer will sense it and send a signal to the error detector that says the liquid level is low. The error detector generates an output signal that is processed and used to start the pump motor.

8. Cont.. The motor rotates the pump, which puts liquid in the tank. When the vat is full, the sensor will generate a signal that is sent to the error detector. The error is now zero, so the pump motor is turned off. The primary benefit of a closed-loop control system is that its operation is automatic. In the open-loop system, an operator has to turn the system off and on manually to fill the tank. In the closed-loop control system, the operator gives an initial input, called the set point, to which the error detector will compare the feedback signal from the sensor. He or she turns the system on. From that point on, operation is automatic. The pump turns itself off when the vat is full. If the liquid level drops below the desired level due to usage, the sensor indicates a low level and automatically turns on the motor so that the pump will again fill the tank. When the tank is full, the unit shuts off by itself.

9. MID 2 ANSWER ALL QUESTIONS,EACH CARRY ONE MARK What is the frequency range Human Beings can hear? What the four things needed in audio electronics? What is a Video system? Define Aspect Ratio? What are the functions of a cable box?

10. MID 2 ANSWER ANY ONE QUESTION,EACH CARRY 10 MARKS. 1 A) Describe special sound applications. B) Write down the summary of screen technologies. OR 2 A) Draw and explain cable box with DVR. B) How a standard DVD player works, explain with block diagram.

11. SENSORS An essential element of most control systems is the inputs from sensors. Sensors are the components that detect physical changes or events and convert them into electrical signals to be processed. Sometimes the term transducer is used to refer to the sensor. A transducer is a device that converts one type of energy into another such as mechanical energy to electrical. There are literally hundreds of different types of sensors. A few examples of the most common types follow.

12. Temperature Sensors Temperature is probably the most often sensed physical characteristic. Temperature sensors convert temperature to a resistance change or voltage change. The most commonly used temperature sensors are RTDs, thermistors, and thermocouples.

13. RTD An RTD is a resistive temperature device. It is essentially just a platinum wire whose resistance varies with temperature. Most of them have a resistance of 100Ω at zero degrees Celsius (0°C). As the temperature changes, the resistance changes to reflect the change. RTDs have what we call a linear positive temperature coefficient. That is, as the temperature goes up, the resistance goes up and vice versa. To convert the resistance variation into a voltage, the RTD is usually put into a bridge circuit as shown in Fig. 12.6. The bridge is initially balanced with the variable calibrate control (CAL) to produce zero output at a given temperature. Then if the temperature varies, the bridge becomes unbalanced and a voltage is produced. This is then amplified into a larger voltage that is a measure of the temperature. The output can be measured with a meter or it can be digitized for use as input to a digital system for display or control.

14. Thermistor A thermistor is a resistor whose resistance has a negative temperature coefficient. As temperature goes up, the resistance goes down and vice versa (see Fig. 12.7A). The resistance change is much greater than that of an RTD. The thermistor can be used in a simple voltage divider to develop an output voltage or in a bridge circuit like the RTD (see Fig. 12.7B).

15. Thermocouple A thermocouple is a unique type of temperature sensor, as it develops a voltage rather than a resistance change. It is formed with two dissimilar metals. If the junction of the metals is heated, a voltage is developed. The voltage is usually in the millivolt range so that it usually has to be amplified before it becomes useful. The thermocouple’s main advantage is its accuracy at very high temperatures.

16. Pressure Sensors A pressure sensor responds to force or pressure. There are many different types. A widely used kind is called a strain gauge. A strain gauge is essentially a thin pattern of metal deposited on a plastic base like that shown in Fig. 12.8. It has a specific value of resistance. A value of 120Ω is common. The strain gauge is then glued or cemented with an adhesive to the object to which the pressure is to be applied.

17. Cont.. An example is a steel beam. When pressure is applied to the beam, it will bend. As it bends, it stretches or compresses the strain gauge resistance element. If the resistive element is stretched, its resistance increases. If it is compressed, the resistance decreases. Knowing the specific characteristics of the resistance change per pound of pressure, an exact measure of the pressure can be determined. Mostly strain gauges are used in a bridge circuit like that shown in Fig. 12.6. Since the resistance variation is small and the resulting bridge circuit output is small, a special amplifier called an instrumentation amplifier is used. It has differential inputs and very high gain.

18. Photoelectric Sensors One of the most widely used industrial transducers is a photoelectric sensor. A photoelectric sensor is a component that responds to light and produces an electrical signal that can initiate some operation. There are three basic types of photoelectric sensors. These are the photoresistive or photoconductive cell, the photovoltaic cell, and the phototransistor. The photoconductive sensor is a light-sensitive resistor. Its schematic symbol is shown in Fig. 12.9A.

19. Cont.. The resistance of the device varies with the amount of light falling on it. With no light falling on the sensor, its resistance will be very high, 100,000Ω or more. When bright light shines on the sensor, however, its resistance will drop to a very low value, usually several hundred ohms or less. Of course, at a light level between bright and dark, the resistance of the device will be somewhere between 100Ω and several hundred thousand ohms. The resistance is inversely proportional to the light level.

20. Cont.. Another type of photoelectric device is the photovoltaic cell, whose schematic symbol is illustrated in Fig. 12.9B. This is the solar cell described in an earlier chapter. Whenever bright light falls on the cell, it will generate a small DC voltage. The maximum output of a typical photovoltaic cell is approximately 0.45–0.5V. At lower light levels, the output voltage will be less. Photovoltaic cells are used primarily in power generation systems for producing charging voltage for batteries in satellites and other remote systems.

21. Cont.. A silicon diode can also be used as a light detector. Special diodes in clear housings are often used to detect light. These are called photodiodes. Photodiodes are operated in the reverse-biased direction, as shown in Fig. 12.10. With no light falling on the diode, it is cut off, so there is no voltage across R1. When light strikes the diode, its reverse leakage current increases dramatically, causing a voltage drop to occur across R1.

22. Cont.. A phototransistor is a light-sensitive transistor. Its symbol is given in Fig. 12.9C. It only has two terminals, the emitter and the collector. The device has a base, but typically it has no lead. Instead, the device is built so that when the base is exposed to light, it will cause the emitter–base junction to conduct. When light strikes the base, it causes ionization, which simulates base current flow. Therefore, the transistor conducts between emitter and collector. The primary advantage of a phototransistor is its very high sensitivity over a photodiode. It will conduct even with a small amount of light applied to the base. The transistor essentially provides amplification so that small light levels can control a large current.

23. THYRISTORS Many industrial switching circuits are implemented with MOSFETs designed for high voltage and high power. You will sometimes hear these devices called insulated gate field-effect transistors (IGFETs). Other power-switching devices are also widely used, especially thyristors. A thyristor is a semiconductor device used for switching purposes in industrial control. Like the relay, it is used to apply and control electrical power to motors, heating elements, lights, and other loads in industrial applications. In many applications, thyristors replace relays. A thyristor acts as a switch, but because of its solid-state nature, is far faster in switching than a relay. There are three basic types of thyristors: silicon-controlled rectifiers (SCRs), triacs , and diacs .

24. Silicon-Controlled Rectifier The SCR is a three-terminal thyristor that acts like a silicon rectifier diode whose conductor is controlled by an input current. The schematic symbol for an SCR is shown in Fig. 12.21. The symbol is similar to that of a diode with a cathode and an anode. Note that the third element of the SCR is known as the gate. The SCR will conduct current between cathode and anode, but only if the proper control current is applied to the gate. The gate must be made positive with respect to the cathode for the SCR to conduct. When conducting, the SCR acts like a closed switch. The voltage drop across the cathode and anode will be approximately 0.7–1.8V, depending on the size of the SCR and how much current is flowing through it. When the cathode and anode are reverse biased, current will not flow through the device.

25. Diac A diac is a two-terminal semiconductor device that conducts in either direction when a voltage of a specific level is exceeded. This voltage is called the triggering voltage. Typical diac triggering voltages are in the 20- to 45-V range. The diac is used as a triggering device for SCR and triacs as it determines when the device will conduct or cut off.

26. Triac A triac is another three-terminal thyristor. Its operation is similar to that of an SCR, but the triac will conduct in either direction. The schematic symbol for the triac is shown in Fig. 12.24. Current will flow from main terminal 1 (MT1) to main terminal 2 (MT2), or from main terminal 2 (MT2) to main terminal 1 (MT1), depending on the polarity of the voltage applied, and if the gate triggers the device on. An approximate, equivalent circuit of the triac is shown in Fig. 12.25. Here, two SCRs are connected back to back with their gates connected together. Current can flow in either direction, depending on whether D1 or D2 conducts. Of course, SCR conduction depends on whether a gate-triggering current is applied. Triacs , like SCRs, are used primarily to switch AC voltage to a load. If gate current is applied continuously, the triac will conduct in both forward and backward directions, causing both positive and negative half-cycles of the AC to be applied to a load.

27. PROGRAMMABLE LOGIC CONTROLLERS A very popular type of controller used in industry and process control is a specialized computer called a programmable logic controller (PLC), or programmable controller. It uses sensors to monitor physical variables and generates output control signals to operate heating elements, motors, pumps, solenoids, and other devices. The PLC, however, contains a microprocessor that can be programmed in a special language that permits the PLC to be customized to the particular control function. In this way, a standard PLC can be programmed and configured for virtually any application. The PLC is by far one of the most popular and versatile control devices in use in industry.

28. Cont … The main function of a PLC is factory automation. It is a versatile special purpose computer that can be quickly and easily reconfigured to meet changes and additions in factories, plants, and other operations that routinely need to control and sequence various machines and processes. While an embedded controller or personal computer could be used in some operations, they are harder to program and apply and are less flexible when rapid changes are needed. Furthermore, these common microcomputers are not designed for the factory environment. But PLCs are rugged and hardened against the harsh environment of most factories and plants. They can withstand the temperature ranges, vibration, and dirty conditions usually existing in a manufacturing or process control setting.

29. PLC Organization A general block diagram of a PLC is shown in Fig. 12.28. It is like most other microcomputer diagrams but there are some differences. A microprocessor is at the heart of the design. This CPU is coupled to a ROM where a special control program is stored. This program implements the basic scan operational mode of the PLC. RAM is also available to store the application program. Flash memory may also be used in place of RAM.

30. CONT.. The key to the PLC’s versatility is its extensive I/O section. It has a variety of input and output modules. These are fixed in small, single-function PLCs, but in the larger more flexible units, the PLC is designed to accept a wide variety of separate I/O modules. These usually plug into a rack containing the main PLC unit and power supply. The different I/O modules let the user customize the PLC to the specific application.

31. PLC Operation The general operation of a PLC is shown in Fig. 12.29. When the PLC is started, the internal operational program looks at the application program and interprets what is to be done. The sequence of operations is to first scan the inputs. The PLC looks at what the various sensors are doing. It gets a status update from the application itself. The program then interprets what to do next based on the application program. It processes the input to determine what the outputs should be. Then it updates the outputs to carry out the application. Then the cycle repeats itself at a high rate of speed. The inputs are scanned and outputs are updated every 1ms or so, slower or faster as the application requires.