Now that we have calibrated and characterized all of the pieces of our system, we are ready to begin to attempt to accurately control the motor. Our system is designed to control the speed of the motor. The goal is to maintain a set speed no matter what the load is on the motor. A closed loop control system uses feedback from the motor to the controller. This feedback informs the controller about the status of the motor. If the motor is turning at the wrong speed the controller can adjust its output to change the motor speed. With the characteristic data we created in the sensors and calibration experiments, we could theoretically obtain any desired speed without feedback. This works if the system works exactly as I have characterized it. What happens if the motor is suddenly loaded? Our information doesn't account for that. If this happens in an open loop system the controller continues to output the same information but the motor turns slower. A closed loop system will make an adjustment to keep the speed at the desired set point. In our experiment we are going to use the blue knobbed pot as a way to load the motor. We will insert it between the PLC and the PWM board. When set normally, the output of the PLC will arrive at the PWM board and the motor will turn at full speed. However, if I adjust the pot, I can reduce the voltage applied to the PWM board. This will have the desired affect of slowing the motor down. If the control system is acting properly, the controller will adjust the output of the PLC to compensate for this. In theory the PLC output will increase until the voltage on the input of the PWM board is the original desired voltage. In this lab we will investigate 3 different methods of control: Open Loop, On/OFF and PID. These three methods range from least complicated to most complicated. They all have different limitations and uses. PID will give the most accurate control, but is also the most complicated. ON/OFF is good if feedback is required but accuracy is not paramount. A home heater system is a great example of ON/OFF control. Open loop control provides no feedback. This is best for digital applications where something should be in an ON or OFF state. Turning on street lights at night is typical open loop application. Objectives Compare accuracy of motor speed for all three types of controllers Be able to describe different control responses for each controller Recognize control system blocks in a system. Equipment Motor control trainer ** You should do these tasks in order. They will make more sense that way. Task 1: Open Loop This lab requires you to hook up the schematic shown in Schematic 1. This is similar to the schematic from the previous lab. In this lab, the 5K blue knob pot that we previously used for speed control is used as a method for disturbing our system. You should connect it between the PLC and the PWM board. Turn the pot fully CW for normal operation. When turned fully CW the voltage output from the PLC is applied to the PWM board. Turning the knob CCW will reduce the voltage to the PWM board, this will slow the motor down. We will use this to mimic Farrell 1 03/13/16
placing a mechanical load on our motor or any other process variation that causes our motor to slow down. For open loop we will use the same code as the calibration lab. You should use your modified code. If you don't have the code from that lab you will have to reload Calibration.ckp and modify it for your system. 1. Your code should be the same as Calibration.ckp except that 0% output produces the voltage needed to spin the motor at 3rpm. 100% output should spin the motor at 24rpm. 2. Speed will be controlled by modifying the values in the Data View Window. Before starting this experiment, you should double check the calibration of the tachboard. You can do this by setting the SP to 0 and 100% and verifying the proper response from the tachboard. Make sure Disturbance Pot, the blue knob, is fully CW. 1. Once again DF8 is our Setpoint, DF1 is the speed reported by the Tachboard, DF3 is the PLC output, and C5 is our start/stop. 2. 0% in DF1 should be 3rpm and 100% should be 24rpm. *** If you do not do this, you will have very bad results that will likely result in you doing all of the work again. For measurement on this task, connect the Oscope to the Encoder A or B output. Use the DMM to monitor the PLC control output voltage. This should be measured on the output of the PLC, not the input to the PWM board. Based on the data you calculated in the previous labs, Set the SP (DF8) of the PLC to produce the voltage needed to make your motor turn at 15rpm 1. Adjust this voltage until you get 15rpm's on the motor. 2. You will need the data from the switch and sensor lab to verify that the motor is turning at 15rpm. Measure the output voltage of the PLC, the frequency of the encoder, and input reading of the PLC (0-100%). If the output voltage is not correct adjust your PLC settings until it is correct. 1. How accurate was your original speed setting? Did you have to adjust DF8 after the initial calculation? (%Error is a good way to explain this) Turn the blue knob approximately one complete revolution CCW. This should happen very quickly. It doesn't need to be accurate, but needs to happen as instantly as possible. Measure the data again. 1. What happened to the motor and your speed error? Task 2: ON/OFF This task uses the same circuit wiring. Make sure the 'disturbance pot' is fully CW. For the PLC program use ON_OFF.ckp Use CH1 of the Oscope to measure the voltage signal from the PLC to the PWM board. Use CH2 to measure the voltage/current being return from Tachboard. 1. CH1 should be the trigger voltage Farrell 2 03/13/16
2. Set the measure function on the Oscope to measure the RMS value for both channels. 3. Make sure that you set the scale to produce several periods of the waveforms, this will place the scope in Scan mode. You will need to put the scope in normal mode, not auto mode. You cannot measure RMS in auto mode. 4. In order to get a good image in scan mode, set the scope to single trigger. When the scope triggers, the display at the top center will change from Armed or Ready to Triggered. Once this happens it takes a minute for it to collect all the data to display, be patient. 5. On the screen, set CH1 and CH2 so that the 0V indicator is at the same location for both channels and near the bottom of the screen. Use the smallest V/Div setting possible. Set the system to run at 15RPM. 1. What happens? 2. Create a screen capture of the data. 3. For each channel, record Vrms, Vmax, Vmin, and period. For the PLC output record the duty cycle. You can approximate all but the Vrms value. Vrms should use the measure function. Turn the disturbance pot approximately one turn CCW. This should happen very quickly. It doesn't need to be accurate, but needs to happen as instantly as possible. 1. Create a screen capture of the new data. 2. Record Vrms, Vmax, Vmin, and period for both channels. For the PLC output record the duty cycle. Make sure you print your screen captures and place them in your notebooks. Task 3: PID This task uses the same circuit wiring. Make sure the 'disturbance pot' is fully CW. For the PLC program use PID.ckp. Kp (DF12) should be set to 7, Ki (DF13)to 5, Kd (DF14) to 1, and Offset (DF9) to 0. If this is not the case change them in the data view. The setpoint (SP) (DF8) should be the same set point that you used for the previous labs. The controller should be set for Reverse Action (C2=0). It can be started and stopped with C5 as in the previous labs. All of these should be found in the Data View on the software. CH1 and CH2 of the Oscope should be set up the same as in the previous task. Make sure you line up the 0V marks for both channels. For each channel use the smallest V/Div that is possible. 1. Set the Oscope for Single Trigger 2. Set the trigger point of CH1 to just above the value of the current output. 3. Adjust the horizontal position so that the trigger point is near the left side of the screen. It should be about 2 divisions in from the left. Quickly turn the disturbance pot one turn CCW. You should see the scope change from Armed or Ready to Triggered 1. Once you have the waveform, make sure you see the MV change from initial value to its final value. Farrell 3 03/13/16
2. Record initial and final values of both the MV and PV. 3. Save the waveform for your notebook. Make sure you take your circuit apart, so that other groups don't need to do it for you. Discussion Questions Make sure you can fully explain and defend your answers. Explain where each of these variables is in our system 1. SP 2. PV 3. MV 4. Final Control Element 5. Process What SP setting did you use to achieve 15rpm? How accurate is the open loop control? What happens to the open loop control when you disturb the motor? How accurate is the ON/OFF controller? 1. Is the RMS MV and PV value correct for 15rpm? 2. What happens to the MV and PV signals when you disturb the system, turn the pot? Discuss these in terms of RMS, Vmax, Vmin, and period. What happens to the PID controller when it is disturbed? 1. What happens to the MV and PV values? 2. Do you observe any change in the PV values? (look carefully at your waveforms) Which controller has the least error at all times? Describe the differences between these three controllers. Farrell 4 03/13/16
Schematic 1: Control Loop Schematic Farrell 5 03/13/16