6 01 Spring Semester 2008 Assignment 8 Revised Issued Thursday April 3 1 MASSACHVSETTS INSTITVTE OF TECHNOLOGY Department of Electrical Engineering and Computer Science 6 01 Introduction to EECS I Spring Semester 2008 Assignment 8 Revised Issued Thursday April 3 Revised Design Lab Replaces only the design lab section of the original handout There are post lab homework problems in the original handout that you need to do Design Lab The goal of today s design lab is to build a circuit to control the position of a motor In later labs we will use this motor to turn the head of our robot At a high level we can think of our motor controller as follows d K V motor We wish to command a desired angle d that is the input to a feedback loop that compares the desired angle to the measured angle The difference is multiplied by a gain K to generate an output voltage V that will cause the motor to turn in a direction to make the measured and desired angles equal We cannot conveniently manipulate angles so we will use potentiometers to convert the angle of their input shaft to a voltage A potentiometer is a three terminal device The resistance between the middle and bottom terminals increases in proportion to the angle of the input shaft and the resistance between the middle and top terminals decreases so that the sum of the top and bottom resistors is constant here it is 5K By connecting a potentiometer as shown below we can create a device that converts into a voltage Vo measured relative to GND 12 V 1K 5K 1K 12 V Model Vo pot Vo 6 01 Spring Semester 2008 Assignment 8 Revised Issued Thursday April 3 2 Question 11 Construct the circuit shown above be sure to adjust the power supplies to 12V before connecting the circuit Mathematically determine the range of V0 that the device can generate Measure the range relative to GND and compare the measured and calculated ranges Question 12 Use the potentiometer from the previous question to drive the voltage input to the motor Use a Lego motor with adaptor cable and breadboard adapter 6 pins The motor is connected between pins 5 and 6 of the adaptor Connect pin 5 to the output of the potentiometer and connect pin 6 to GND How fast can you get the motor to turn Explain Question 13 Use an op amp to buffer the output of the potentiometer this is similar to what we did last week when we used an op amp as a buffer between two voltage dividers Use one of the two op amps in the KA 334 package see schematic below Let Vcc 12 V pin 2 and Vss 12 V pin 4 Compare the behavior of the motor now to its behavior in the previous circuit Can you make it go in both directions Checkpoint 30 minutes Show the potentiometer controlling your motor Be sure it can go in both directions When we use the motor to position the robot head we will only have access to a 12 V supply the robot does not have a 12 V supply One way to get bidirectional turning from a single 12 V supply is by generating a virtual ground that is midway between 0 V and 12 V Question 14 Reconnect power to the op amp so that it uses a single 12 V supply i e connect Vcc 12 V pin 2 and Vss GND pin 4 Use a second op amp to generate a virtual ground signal Modify your previous op amp circuit so that it uses the virtual ground to allow the motor to turn in both directions Be careful about the following Your virtual ground should be halfway between the power supply rails So that the power supply rails become 6 with respect to the virtual ground Henceforth think of this as ground think of what was formerly 12 as 6 it is 6 volts higher than ground and think of what was formerly 0 as 6 it is 6 volts lower than ground Remember that what matters is the relative voltages 6 01 Spring Semester 2008 Assignment 8 Revised Issued Thursday April 3 3 Checkpoint 60 minutes Show the potentiometer controlling your motor with a 6 and 6 supply Be sure it can go in both directions For the remainder of today s lab you will need a head assembly which you can obtain from the staff These head assemblies have circuitry that is pre built on the bottom board The top board which is mounted so that it can be swiveled by a Lego gear motor has no circuitry on it right now Examine and familiarize yourself with the circuitry on the bottom board shown below You will note that there are two KA334 op amps wired for power but not for input or output The photograph of the bottom board above shows the following 1 Potentiometer 1 This pot is connected between the power supply and ground rails by two 1k resistors The potentiometer has a total resistance of 5k just like the example you started this lab with 2 Potentiometer 2 When you get the head assembly this potentiometer will be connected to the bottom of the Lego motor It is connected to the power and ground rails directly not through resistors 6 01 Spring Semester 2008 Assignment 8 Revised Issued Thursday April 3 4 3 Two KA334 op amps As you can see they are both connected to the power and ground rails Two of the input pins numbered 5 and 8 are connected by short red wires to rows of holes beyond the ends of the op amps This is just for your convenience in wiring The other two input pins are unconnected The two op amp outputs pins 1 and 3 are also connected to outboard rows of pins Note that pin 1 of one of the op amps is connected to two 47 ohm resistors in parallel You should use that connection to the motor when it comes time to connect to the motor DO NOT DO THIS YET 4 There are two connectors from pins to cables One of these is an RJ 11 like the one you have already used to connect to the motor As you already know pins five and six are the motor connections The other cable connection accepts the RJ 45 cable that is on the robot and is wired so that this board will get power and ground connections from the robot battery For the moment you can ignore this connector and derive 12V and 0V from the same power supply you have been using 5 Note that on some but not all protoboards the left and right power rails must be connected by jumpers The jumpers for the bottom two power rails are shown in the photograph as VCC VSS Jumpers There must be similar jumpers for the top two power rails The upper jumpers cannot be seen in the photograph because they are beneath the RJ 11 Motor Jack Please make sure that there …
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