6.01, Spring Semester, 2008—Design Lab 9, Issued: Tuesday, April 8 1MASSACHVSETTS INSTITVTE OF TECHNOLOGYDepartment of Electrical Engineering and Computer Science6.01—Introduction to EECS ISpring Semester, 2008Design Lab 9, Issued: Tuesday, April 8All questions in this lab must be written up and handed in as part of problem set 9.Homework due In Design LabQuestion 9: Design and draw the circuit diagram for an op amp circuit to compute v1− v2,where v1and v2are the voltages coming out of the two potentiometers (refer to reviseddesign lab 8 handout for details).Design LabFinish your position controllerSee revised design lab 8 for a detailed explanation.Question 10: Construct your circuit for computing v1− v2. Turn the potentiometers and mea-sure the change in output voltage that is produced. Can you set the potentiometers so thatthe output is +6? So that it is -6?Question 11: Connect up the motor so that the feedback system drives the motor angle θ toequal θd. Does the motor turn to a stable position when you turn on power? Change theangle of potentiometer 1 (θd). Does the motor position change accordingly?Checkpoint: 60 minutes• Show the potentiometer controlling your motor. The head should move smoothly andhave a symmetric range of motion to the left and right.Explain how this system is different from the way you were controlling a motor witha pot in the previous checkpoints.The Eyes Have ItOur goal for the rest of this week’s lab is to design and build a system that causes the robot’s headto ’look’ in the direction of a strong light. We will do this by mounting two photo-resistors onthe top (“face”) board of the robot head, and then designing a circuit that will cause the head torotate so that the two photo-resistors are ’seeing’, or are exposed to, the same light level. It willalso be necessary to add some mechanism that puts one of them in a shadow when the board isnot pointed directly at the light source.Here is the pin-out diagram for the KA-334 package again:6.01, Spring Semester, 2008—Design Lab 9, Issued: Tuesday, April 8 2Question 12: Design a circuit for controlling the robot head based on the photo-resistors. Thereare many very complex ways to approach this, but there is also a simple solution, justinvolving your virtual ground and an amplifier. It should satisfy the constraint that you useonly three wires going to between the “face” (where you add only the photo-resistors) andthe board below. Draw the circuit.Question 13: Arrange for there to be a shadow on a photo-resistor so that the board can bemade to rotate toward the light. Ask an LA for ideas about how to do this.Question 14: Now, build up the photo-sensing circuit, and connect it to the motor. It shouldrotate the head toward the side of the photo-resistor measuring the highest light level.Demonstrate that the head does indeed turn toward a bright light. You might need toexperiment with the “nose” (device for shadowing the photo-sensors) or the placement ofthe photo-resistors to get it to work well.Checkpoint: 120 minutes• Show your robot head turning toward the light.Gainfully EmployedNow, we’re interested in understanding what happens when we change the gain in this controlsystem. We have some new potentiometers, which look just like our old potentiometers, exceptthat they have a red mark on them, and behave like our old potentiometers, except they have amaximum resistance of 50KΩ instead of 5KΩ.Question 15: Find a way to put one of these new potentiometers into your control circuit(without adding any additional op-amps), and add or change an existing resistor so that byturning the potentiometer, you can generate gains that span, at least, the range 0.2 to 10.Draw the schematic. Determine what resistance in the pot will generate an overall systemgain of 0.5. Determine what resistance in the pot will generate an overall system gain of 10.Question 16: Build the circuit. Test and describe its behavior for a gain of 0.5 and of 10.Question 17: At what gain does the motor stop responding? Intuitively, why does that happen?At what gain does the system become unstable? Intuitively, why does that happen? (Youmay need to go outside the range of gains suggested above in order to push the system tothese limits; if you have to do more than turn the potentiometer to do so, please describewhat changes you made to your circuit).6.01, Spring Semester, 2008—Design Lab 9, Issued: Tuesday, April 8 3Checkpoint: End of lab• Show your robot head behaving in three noticeably different ways, with three differentgains.Post-lab HomeworkQuestion 18: Show what resistors you would have to add or change in your answer to question 9in order to compute K(v1− v2) for K = 2. How about for K = 10?Question 19: There are two errors that would have been easy to make in your design for ques-tion 9. The first would have been to turn the head to the left when you turn the control potto the right (and vice versa). What error in your circuit would have caused that to happen?(Draw a circuit diagram showing a small change to your circuit that would have made itexhibit this problem).Question 20: The other error is to generate positive feedback, instead of negative feedback;that is, to send a larger correction to the motor when the error gets smaller. What error inyour circuit would have caused that to happen? (Draw a circuit diagram showing a smallchange to your circuit that would have made it exhibit this problem).Concepts covered in this lab• Design of simple buffers and differential amplifiers• Closed loop sensory-motor control of a
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