1 ME 360: FUNDAMENTALS OF SIGNAL PROCESSING, INSTRUMENTATION AND CONTROL Speed Control of a DC Electric Motor 1. CREDITS Experiment Originated: Professors T-C. Tsao, October, 1995, and Norman Miller, January, 1997 Updated: D. Block, September 2009 2. OBJECTIVE The objective of this experiment is to study proportional-integral control of the speed of a DC motor. A secondary objective is to become familiar with the use of SIMULINK as a system simulation tool. 3. KEY CONCEPTS (a) As we know from previous experiments, a DC motor with voltage as the input and speed as the output behaves as a quasi-linear, first-order system with steady-state gain K and time constant τ. For the motor-generator system used in our laboratory, K is typically between 1.1 and 1.2 depending on the motor and various factors such as bearing wear. τ is typically between 40 and 70 ms. (b) A quasi-linear system is one that can be modeled as linear for the purpose of control but with dynamic parameters (K and τ) that vary with motor speed and motor age. (c) We can use the changes in K and τ as the motor ages as a means of diagnosing motor health (bearing condition). (d) Proportional-integral-derivative (PID) control is one of the most common control methods. A block diagram of PID control is given below. Special cases of PID control include: (i) P (only) control where KI and KD are both 0, (ii) PI control where KD is 0, and (iii) PD control where KI is 0. +–re u ycontrol blocksystem blockset point error state variablecontrol actionKD s2 + Kp s + KIs τ s + 1KController (e) In this experiment, we only consider P and PI control. Derivative (D) control is not considered. (f) Proportional control is the primary workhorse of PID control. The proportional gain Kp determines how quickly a system responds. (g) The system generally becomes more responsive (rise time is reduced) as proportional gain is increased. (h) A drawback of this increased responsiveness is that oscillations die away more slowly. Such oscillations may originate (i) in the system, (ii) in the input, or (iii) from other factors outside the system. (i) P-only control has nonzero steady-state error ess = r / (1 + K Kp). PI control has zero steady-state error. (j) The primary benefit of adding integral control to this system is elimination of the steady-state error. 4. SYNOPSIS OF PROCEDURE In this experiment, we add speed control to the motor-generator system used in the previous experiment. We investigate both proportional (P) and proportional-integral (PI) control. We first simulate the motor-generator2 system on the computer using SIMULINK. We then add speed control to the simulation, and evaluate the performance of the control system. Finally, we test the speed control on the actual motor-generator system and document the differences between simulated and actual system performance. More specifically, the MATLAB/SIMULINK software and PC-based hardware are used as follows. (a) The motor-generator system is simulated in software using the steady-state gain and time constant found experimentally in the previous two experiments. The SIMULINK extension of the MATLAB software is used for this purpose. (b) A proportional-integral-derivative controller is added to the simulation. The effect of proportional gain on the performance of a proportional-only control system is examined. The steady-state error and load sensitivity of proportional control are studied. (c) The ability of integral control to eliminate the steady-state error and load sensitivity of proportional control is then shown. The effect of integral gain on controller performance is also demonstrated. (d) Proportional-integral control is tested on the actual motor-generator system using the PC-based hardware to implement the controller digitally. The ability of the controller to maintain constant motor speed under varying load conditions is shown. The effect of the discrete time step on control system stability is also examined. 5. PROCEDURE Wire the System Figure 1 shows how the system should be wired. A brief summary of the connections is given below. DAC Output to Power Amplifier A gray shielded cable, with three banana plugs on each end, connects Analog Output Channel 0 to the input of the Power Amplifier (labeled "Amplifier" on the patch panel). Red is positive or high, black is negative or low, and white is the shield. At the amplifier end, connect red to red, black to black and white to white. At the DAC end, connect red to red and both black and white to black. Amplifier Output to Motor Input A second gray shielded cable connects the output of the Power Amplifier to the input of the motor. Red is positive or high, black is negative or low, and white is ground. Follow red-black-white color coding on each end of the cable.3 C H 1P o w e r S u p p l i e sredblkblublkblkbluLOO NO F F+ 5 V+ 1 0 V– 1 0 VH P0 – 2 0 VB e n c hG r o u n dblkredgrn8amp2ampR e f + S i g. G n d+ T a c h / G n d– T a c hO NO F FA m p I n h i b i tO u t p u t+–P o w e r G n dC u r r e n tM o n i t o r4 A / 1 VS i g. 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Wiring diagram and equipment layout for speed control experiment.org grywht blk redMotorGeneratorFly WheelCoupling CouplinCouplingpush button1X 2Y!1 MΩ- 13 pF400 V maxPosition Position15 V 2 mVV o l t s / D i vV o l t s