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Experiment 4: Measurement and Equipment Control Freshman Lab GENG 250 Spring 2014, Prepared by Ali Abu Odeh 1 Introduction Control systems are used to command or regulate a process. The process may be anything from the acceleration of a car to the output of a factory. In this experiment, the speed of a DC electric motor is controlled in real time. The speed is controlled using an open loop control system and two different closed loop control schemes. Spring 2014 2 Background Information Control systems are classified into two types: open loop and closed loop. An open loop system controls a process independent of the output. In other words, the input affects the output, but the conditions of the output have no effect on the system. For example, adjusting the desired speed changes the power supply voltage, which changes the speed of the DC motor as seen in figure 1. Spring 2014 3 Figure 1: Open loop control of a DC motor. The higher the voltage, the faster the motor spins. However, if the motor speed suddenly changes, the controller does not have the ability to self-correct the error. In this case a correction must come from the human operator. Spring 2014 4 Closed loop systems are characterized by feedback. In control systems using feedback, a transducer is used to measure the variable being controlled (in this case motor speed). Spring 2014 5 This measurement is then sent back to the controller to be used as part of the input. Feedback allows the control system to compare the desired value to the measured value and make corrections to the output. Closed loop control has the ability to respond to unexpected changes in the variable being controlled without the help of a human operator. Spring 2014 6 Figure 2 displays the block diagram of a closed loop control system for controlling the speed of a DC motor. The user inputs the desired speed. The feedback is the measured motor speed, which is sensed by an opto-switch transducer. The transducer converts the motor speed (Revolutions Per Second or RPS) to a voltage waveform with a corresponding frequency. The error signal, e(t), is defined as the difference between the desired motor speed and measured speed. Spring 2014 7 The control scheme uses the error signal in order to compute the control signal, c(t), which adjusts the power supply, and thus, the speed of the motor. Figure 2: Closed loop control of a DC motor. Spring 2014 8 There are several types of closed loop control schemes. The appropriate scheme depends on the type of process to be controlled. In order to determine the best scheme, the response time, overshoot and stability are analyzed. The response time is defined as how fast the system responds to a change in the desired input or output load. Overshoot is defined as the maximum error in a closed loop system that is responding to a change in the desired input or load. Spring 2014 9 Open-Loop Control Scheme There are many types of open-loop control schemes. Examples include direct command of the control signal, as in the engine speed of a car, or a timed system, like in most water sprinklers for plants and gardens. As displayed in figure 1, the open-loop control scheme does not use any measurements of the output to control the system. The control signal is described mathematically by the following equation: c(t) = co Spring 2014 10 Closed-Loop On-Off Control Scheme The control signal of this scheme has two possible values. The sign of the error signal (positive or negative) is used to determine which value to use. The most common application of an on/off control scheme is an air conditioner. If the room temperature is more than the desired temperature, the controller turns on the air conditioner (applies 240VAC). Spring 2014 11 If the room temperature is cooler than the desired temperature, the air conditioner is turned off (0VAC applied). While on/off control works well for air conditioning, you will observe that it does not work well for motor speed control. The control signal is described mathematically by the following equation: c1 c(t ) c2 for e(t ) 0 for e(t ) 0 Spring 2014 12 Closed-Loop Proportional Control Scheme Another type of control scheme computes the control signal as a proportion of the error signal. In other words, the control signal, c(t), equals the error signal, e(t), multiplied by a constant of proportionality, KP, called the proportional gain. c(t) = KP e(t) + co Spring 2014 13 Thus, the larger the error, the larger the response of the controller. While the proportional control is superior to on/off control for many applications, care must be taken in the choice of the value, KP. A value too small causes the controller to perform virtually the same as the open loop control system. A value too large causes the system to become unstable. The best KP value is between these two limits and chosen to give the smallest error when the motor is loaded while still being stable as shown in figure 3. Spring 2014 14 Figure 3: Proportional, closed-loop control with large KP value (unstable) and appropriate KP value (stable). Spring 2014 15 Procedures The speed of a DC motor is to be controlled. The motor is used as an actuator in this experiment. The motor speed changes according to the voltage of the power supply. LabVIEW VIs are written to control the power supply, and thus, the motor speed. Feedback is provided via an opto-switch transducer. This transducer produces a number of pulses for each motor revolution. Spring 2014 16 The oscilloscope measure driver is used to measure the frequency of the pulses, which is then used to determine the motor speed in revolutions per second. Figure 4. DC Motor Disk and Equipment Connections for Measuring and Controlling the Motor Speed. Spring 2014 17