PC/CP320 Physical Computing

Raspberry Pi Servo Motors

Overview

Servo motors are common.

Objectives

1. To introduce control of servo motors with the Raspberry Pi

Background

Preparation

Introduction to Servos

• What is a servo?
• How do servos work?

Equipment

• Raspberry Pi
• TTL-serial cable
• Oscilloscope
• Servo motor (SG-90)

Procedure

1. Datasheet Examination

   Look at the datasheet to determine the required supply voltage.
   Note: The Pi power pin may not provide enough power for the motor. If that is the case then use the bench supply, but make sure your bench ground and the Pi ground are connected.
   Since the control signal is going from the Pi to the motor, is there any danger to the Pi if the motor supply voltage is above 3.3V?
   Note that the servo motor internal electronics protect whatever is controlling it from EMF caused by the motor.
   Given that and the answer to the previous question, will an optoisolator be required to control this from the Pi?
   
   
2. Identify which wire has which function:
   • power
   • ground
   • control signal
   
   
   

3. To drive the motors, a pulse width modulated signal must be sent periodically. The width of the pulse will indicate direction and speed:

   • pulse width of 0, i.e. no pulse -- the motor will stop
   • pulse width < some value -- the motor will rotate in one direction
   • pulse width = some value -- the motor is in the neutral position
   • pulse width > some value -- the motor will rotate in the other direction
   • the drive signal is proportional, so the farther it is from the neutral position, the greater the rotation
   • NOTE: specified neutral is approximate, you must calibrate to get precise control
   From the data sheet, what is the required period for the pulses?
   What frequency does that correspond to for your PWM signal?
   
   From the data sheet, what is the approximate pulse width for the neutral position?
   What duty cycle does that correspond to for your PWM signal?
   
   From the data sheet, what is the approximate pulse width for the limit in one direction?
   What duty cycle does that correspond to for your PWM signal?
   
   From the data sheet, what is the approximate pulse width for the limit in the other direction?
   What duty cycle does that correspond to for your PWM signal?
   
   

4. Pulse Width Creation and Testing

   
   Start with the pwm_test_2018.py program that you used previously. Identify which PWM GPIO pin it uses.
   Connect the oscilloscope to the PWM GPIO pin.
   Don't connect the PWM pin to the servo motor yet.
   Modify the program to create a PWM signal of the required frequency and duty cycle for the neutral position as determined above.
   Now find the required duty cycles to give you the upper and lower limits as determined above.
   Demonstrate the program giving the correct signals on the oscilloscope.
   

5. Motor Calibration

   The values just determined will be approximate; each motor will vary slightly.
   Now connect the PWM pin to the motor.
   If you have done the previous part correctly, you should need no change to the frequency, and only small changes to the duty cycles.
   
   Now, use the program to find the duty cycle which keeps the servo motor in the neutral position; do not assume that the value in the datasheet is precise.
   If the motor goes to one end and buzzes, it means you are overdriving it. Stop the program and figure out what is wrong before procedding!
   Was it close to the expected value?
   Use the program to find the duty cycle which rotates the motor to the limit in one direction. Was it close to the expected value?
   Use the program to find the duty cycle which rotates the motor to the limit in the other direction. Was it close to the expected value?
   
   Determine the angular resolution limit of your program. In other words, how small a change in angle can you consistlently produce?
   
   Demonstrate the program putting the motor in each of the neutral, left, and right limits.
   

6. Program Refinement

   Modify your program so that you input the desired angle of rotation in degrees.
   Modify your program so that, when you quit, the motor will be returned to the neutral position.
   
   
7. Demonstrate your circuit and program to the lab supervisor.