ELECTRICAL ENGINEERING TECHNOLOGY PROGRAM EET 433 CONTROL SYSTEMS ANALYSIS AND DESIGN LABORATORY EXPERIENCES EXPERIMENT 4: ERROR SIGNAL CHARACTERIZATION In this laboratory experience we will use the two rotary potentiometers to analyze the error signal that is normally used to drive the system under control. We will also analyze how an open system responds to the several inputs. SECTION 4.1 ROTATY POTENTIOMENTER CALIBRATION For this and following experiments, the rotary potentiometers from the Feedback system will be used to generate the signals that will be used at different stages in the building of the control system in the upcoming experiments. It is then critical that we understand how they work and they are correctly adjusted. Make the following connections: EET 433 EXPERIMENTS ON CONTROL SYSTEMS LAB #4 page 1
Measure the voltage between the movable terminal (3) for each potentiometer and ground for different angles. The voltage should vary smoothly as the knobs are rotated. Rotate the cursor until the voltage reads 0 Volts. This should correspond with the mark at 0, indicating that the rotary potentiometers are calibrated correctly. To ensure the success of this and next experiments it is very important that both rotary potentiometers are correctly calibrated. If the 0 in the dial does not correspond to 0 Volts or the voltages do not change smoothly, contact your instructor for assistance in calibrating the rotary potentiometers. Question 1: What are the ranges of voltage for both potentiometers? Question 2: Are there any differences between both rotary potentiometers? If there are, describe them SECTION 4.2: ROTARY POTENTIOMETER CHARACTERIZATION Connect now the Operational Amplifier Unit, what will result in the circuit below. Make sure that the feedback selector in this unit is switched to the 100 ko resistor. EET 433 EXPERIMENTS ON CONTROL SYSTEMS LAB #4 page 2
Disconnect the leads from the output of both rotary potentiometers (terminal (3)) and the input to the operational amplifiers (terminals (1) and (2)). Connect the three input terminals in the Op Amplifier unit to ground (0 V)\ Modify the Zero adjust knob in the Op. Amp. unit until the output voltage in the unit reads 0 V. Question 3: What is the reason for this adjustment? Question 4: Explain in your own words what have we adjusted and how Disconnect the leads to ground, and reconnect the original leads, making sure that the connections are as shown in the previous figure. Question 5: Draw the schematic diagram for the circuit above Question 6: Analyze the circuit. What type of circuit is this? What is the purpose of this circuit? EET 433 EXPERIMENTS ON CONTROL SYSTEMS LAB #4 page 3
Rotate the two cursors in the two Rotary Potentiometers and measure the voltage at the output of the Op. Amp. Repeat this process for different degrees of rotation, completing the following table: Input potent. rotation Output Pot. Rotation Output Voltage Input potent. rotation Output Pot. Rotation 30 30-30 -30 60 60-60 -60 90 90-90 -90 120 120-120 -120 150 150-150 -150 Question 7: What are the expected voltages in the previous cases? Output Voltage Question 8: Are all the measured voltages the same? Explain Set the output potentiometer at 0 and rotate the cursors of the input potentiometer, measuring the output voltage for the Op Amp Unit, completing the following table: Output potentiometer at 0 Output potentiometer at 0-150 30-120 60-90 90-60 120-30 150 0 Question 9: Plot the previous graph, configuring the parameters of the graph so the relationship between the two variables shows more clearly Question 10: What type of relationship appears to be between these two variables? Question 11: Comment on the possible causes for any possible discrepancies between the measured values shown in the graph and an "idealized" version. EET 433 EXPERIMENTS ON CONTROL SYSTEMS LAB #4 page 4
Question 12: What is the slope of the curve? Is it constant? What are the implications? Now, set the output potentiometer at -60, and repeat the readings completing the following table: Output potentiometer at -60 Output potentiometer at -60-150 30-120 60-90 90-60 120-30 150 0 Finally, set the output potentiometer at 60 and complete the following table: Output potentiometer at 60 Output potentiometer at 60-150 30-120 60-90 90-60 120-30 150 0 Question 13: Plot the three graphs (output pot, at 0, -60 and 60 simultaneously Question 14: What do you notice about the three graphs? Comment on similarities and differences, and give reasons to explain them. Question 15: What is the most appearing non-linearity that shows in the previous graphs? How could we modify in the circuit to minimize its effect? Question 16: What is the "error factor" constant (in V/ ) for the circuit that we have characterized? EET 433 EXPERIMENTS ON CONTROL SYSTEMS LAB #4 page 5
SECTION 4.3: ERROR SIGNAL CHARACTERIZATION Before we build an automatic control system driven by the error signal, we will analyze what happens manually. This will help the students to understand how the control system reacts given a disturbance. For the first part of this analysis, one of the students in the group will turn the output potentiometer and will set it in any range so the output voltage is within ±11 Volts. Without looking at the input or output potentiometers, only at the voltage at the output of the Op. Amp unit, the other student will try to replicate the output potentiometer's by moving the knob in the input potentiometer until Vout reaches 0 V. Repeat for several for the output potentiometer, for all the students in the group interchanging s. Question 17: How easy/difficult is it to match the s for the potentiometers? Question 18: What type of input (impulse, step, ramp ) signal does the previous experience try to replicate? Now, one of the students will slowly and continuously change the output potentiometer, while the other student will try to keep the output voltage at 0 Volts by rotating the input potentiometer. Repeat the procedure for all the students in the group interchanging s. Question 19: What are the similarities and differences between this and the previous experience? Question 20: Is it easier or more difficult than in the previous experience? Question 21: What type of input signal does this experience try to replicate? Finally, repeat the procedure while the student at the controls of the output potentiometer suddenly changes its EET 433 EXPERIMENTS ON CONTROL SYSTEMS LAB #4 page 6
Question 22: Comment on the results The previous experience can be seen as a control system, with actuators and sensors (fingers, eyes ), preamplifiers (brain), controller (muscles), etc Question 23: Draw the block diagram with these components that describe the experience of trying to "track" the output potentiometer. Comment on the blocks used and the signals at the output of each one of them. Question 24: Is this an open-loop system or a closed-loop system? Why? EET 433 EXPERIMENTS ON CONTROL SYSTEMS LAB #4 page 7
SECTION 4.4: A MOTOR-DRIVEN POSITION CONTROL SYSTEM Connect the following elements in the Feedback system as shown: Before turning the power on, make sure that the potentiometer in the attenuation unit is in 0. EET 433 EXPERIMENTS ON CONTROL SYSTEMS LAB #4 page 8
Use the push-on coupling to link the low-speed shaft of the motor gear-box to the output potentiometer shaft. From the attenuator potentiometer in 0, gradually rotate it until the motor just starts to rotate. Question 25: What is the setting in the attenuator scale that makes the motor to start rotating? Question 26: What is the direction of rotation? Set the input potentiometer at 30. Move the attenuator control above the setting found in Question 25. After the motor has been rotating for about 30 seconds, try to stop the scale of the output rotating potentiometer at 30 using only the attenuator potentiometer. Try it several times, interchanging the students. Record the values of output voltage in the Op Amp unit for the different tries Question 27: What are the lower and higher values of Vout (error signal)?. Are all they zero? Why? Question 28: What could you change in the system to minimize the value of the error signal? Question 29: Is this system operating in open-loop or in closed-loop? Explain it. Find the lead that connects the central terminal (2) in the attenuator unit to the input (2) of the servo amplifier, and connect to the input (1) of the servo amplifier instead. Question 30: What happens? Question 31: How could we do this automatically? What we would need to modify? EET 433 EXPERIMENTS ON CONTROL SYSTEMS LAB #4 page 9
ELECTRICAL ENGINEERING TECHNOLOGY PROGRAM EET 433 CONTROL SYSTEMS ANALYSIS AND DESIGN EXPERIMENT 4: ERROR SIGNAL CHARACTERIZATION SUMMARY SHEET To be completed during the laboratory period Submit it to the instructor at the end of the laboratory period Student Name:. 1.- Zone of linearity for the measurements in Section 4.2: 2.- Gain in linear zone for measurements of Section 4.2: 3.- Range of values of error signal (Question #27): Instructor s Signature and Date: