ME461_F11_Prelab7_10_28_11 - ME 461 Homework #7 Fall 2011...

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Unformatted text preview: ME 461 Homework #7 Fall 2011 Due at the beginning of Lab #7 Suggested Reading: Allegro A3953 Full‐bridge motor controller datasheet. Lab 7 manual. 1. A simplified schematic of the motor controller that is used on your robot is shown below. In your own words (i.e., do not simply copy words from the datasheet), briefly explain the purpose and use of the REF, BRAKE, MODE, and SENSE pins on the A3953. Why are they connected as shown? V CC V+ (Battery) VCC A3953 V REF BRAKE OUTA BB VBB 47uF A3953 V 47uF MODE SENSE MODE SENSE Motor2 CC REF BRAKE OUTA Motor1 OUTB RC MCU P2.5 PHASE 24.9k 680pF VCC V+ (Battery) GND P4.0 PHASE P4.1/TB1 P4.2/TB2 ENABLE OUTB RC 24.9k 680pF ENABLE GND VCC 2.2k Amp Enable 2. To turn the motor amplifier on, should the Amp Enable switch be connected to Vcc or GND? Why? Note that the output of the Amp Enable switch feeds one of the two inputs of a standard OR gate. The OR gate is a combinational logic gate (rather than a sequential logic element such as those discussed in class). A combinational logic gate performs standard binary logic operations on multiple inputs to produce a binary output. The truth table for the OR gate is: A ME 461 1 1 1 1 0 0 C=A+B C=A+B 0 0 1 1 A B B 0 1 1 Prelab #7 3. The diagram below is a simplified schematic of a typical H‐bridge. Determine the states (HIGH or LOW) of the four pins (A, B, C, D) for the following modes: Forward, Reverse, Brake, and Coast. A C B D 4. As you know from the Lab 7 manual, a 20 kHz PWM signal on P4.2 and P4.1 will be used to control motor speed, and P2.5 and P4.0 will be used to control direction. Based on the motor control schematic in question 1, briefly explain how and why this works. Relate your answer also to the H‐ bridge diagram in question 3. The truth table contained in the datasheet for the A3953 might be helpful in answering this question. Ignore the details of fast/slow current decay modes. 5. An alternative means of controlling the motor speed is to PWM the PHASE pin. Briefly explain how and why this would work, and do not neglect the ENABLE pin. Given our application (battery‐ powered DC motor control), why do you think it is a better idea to PWM the ENABLE pin on the motor bridges? 6. If you sample the motor encoder (using the LS7266) every 1 ms and the motor is spinning at 1 rev/s, what is the speed in encoder counts/sample? (Assume that you have configured the LS7266 to count in normal x4 mode.) What is wrong with this? List 2 possible solutions to this problem. (Hint: think about what you can change in the equation you use to calculate counts/sample.) 7. Suppose you are sampling the encoders in 5 ms intervals. At a given time instant, the reading of one of the encoders is 15896. At the next sample, the reading is 15873. What is the angular speed (in rad/s) of the motor shaft? ME 461 2 Prelab #7 ...
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