1. Raspberry Pi Hands-on Seminars
Interfacing to Servo Motors
Gregory Griffes
Gregory Griffes
8/16/15

2. Overview Types of servo motors and operation
Electronic connections to a servo
Python code to control a servo
Wiring and operating your own servo motor

3. Hardware High power charging hub Custom servo cable Servo motor
RPi GPIO
Breakout
breadboard
3.3v to 5v
Level shifter
(I2C safe version)

4. Parts Needed
Canakit Ultimate Starter kit with breadboard, breakout board, ribbon cable, jumper wires, and electronic parts
one Tek Republic TUH-3700 USB Port Hub charging hub ($30)
one HiTec HS-5055MG digital servo motor ($18)
one Parallax continuous rotation servo motor ($10)
one Adafruit 4-channel I2C-safe Bi- directional Logic Level Converter ($8)

5. Step 1: Avoid Damage to your Rpi hardware
ESD – ElectroStatic Discharge Use a wrist strap, mat, & ESD bags

6. Step 2: Breadboard Remove the Rpi from the static bag
Connect breadboard circuit (see video)
Make sure the connector is correctly aligned

7. Step 2: Hardware Warnings
DANGER!
Step 2: Hardware Warnings
+5V
Damage!
Voltage and current can damage Rpi
GPIOs are 3.3 volt and NOT protected from other voltages
Applying 5 volts to a GPIO can cause permanent damage to the Rpi
When wired correctly – no damage occurs
Rpi GPIO max voltage = 3.3v
Rpi GPIO max current = 16ma (0.016)
Rpi all GPIOs max current = 50ma
/ /-
Rpi
Correct
+3.3V
Rpi
GPIO

8. Pulse Width Modulator
PWM used for controlling the servo motor

9. Servo Motors
PID (Proportional Integral Differentiator)

10. Servo Motor Specs Parameter Tower Pro SG51r Tower Pro SG-5010
Stall Torque
@4.8v, 8.3 oz-in1
@4.8v, 76.4 oz-in
Speed
@4.8v, 0.08sec/60°2
@4.8v, 0.19sec/60°
Price $5
Availability
good
Surge current3
5.2v v
Voltage Droop4
3.9v
3.3v
Voltage Droop5
4.8v
4.6v
1” away from center line 8.3 ounces of force. The closer to the shaft the more force is applied but the travel distance is less.
Takes 80 milliseconds to move 60 degrees at 4.8v power source
Amps required to move to a new position
Long wires (>12”); small gauge (breadboard wires); no filter cap
Short wires (6”); large gauge (USB charging cable); 1000uf filter cap

11. 180 degree servo (aka. Standard Servo)
Type A:
Wire colors: Brown (GND), Orange (+5v power), Yellow (PWM signal)
500us -> 2500us = counter-clockwise rotation
Type B:
Wire colors: Black (GND), Red (+5v power), Yellow (PWM signal)
500us -> 2500us = clockwise rotation

12. Continuous Rotation servo
Wire colors: Black (GND), Red (+5v power), White (PWM signal)
1000us = maximum speed clockwise rotation
1500us = stopped
2000us = maximum speed counter- clockwise rotation

13. Custom Servo Motor cable
PWM signal
connection
1000uf
capacitor
USB Charging
cable
Servo motor

14. Wiring up the servo Power off the Raspberry Pi Wire breadboard
Connect servo
Power on servo and Rpi

15. Three approaches to controlling the servo
Direct from 3.3v GPIO
Using the Adafruit I2C safe level shifter
Using the Adafruit 8 channel level shifter
3.3V GPIO
I2C Safe
8 Channel
Advantage
Simple
Cheap
No soldering
Can be used for both servo and sensors
Fits on GPIO breakout breadboard
8 channels
No pullups needed
Smaller footprint than two I2C safe devices
Disadvantage
Unreliable long term
Does not protect the RPi output from higher voltages
Only 4 channels
Requires pull ups
Does not support I2C interface
Additional breadboard needed
Cost
$0.05 $4 $8

16. Servo Motor Schematic (I2C safe)
Use a separate power source for the servo
Reason: when the motor moves, it causes a spike on the +5v power line

17. Using 3.3 volt GPIO output Custom Servo cable Servo motor GPIO 18
Series resistor
(3.9k approx)
RPi GPIO
Breakout
breadboard

18. Using Adafruit 8 channel level shifter
Servo motor
Servo cable
3.3v to 5v
Level shifter
(8ch version)
RPi GPIO
Breakout
breadboard

19. Verify
Have someone else verify your wiring to make sure you don’t damage your Rpi when you turn it on

20. Turn On Rpi Connect monitor, mouse, and keyboard
Turn on power and boot the Rpi
Login and startx, open file mgr
Create a projects directory
Connect to internet or use thumb drive
Hover over the “projects” directory and “open terminal window”
Enter “git clone

21. Run the program Enter “cd servo_motors”
Enter “chmod +x servo-slider.py” to make it executable.
Enter “sudo ./servo-slider.py”
Use mouse to move slider, observe servo motor moving

22. DMA Controller Direct Memory Access (DMA)
Hardware used to perform memory transfers to/from memory or peripherals independent from CPU operation
Very fast data transfers which are not interrupted by the CPU
16 DMA channels

23. Using the DMA engine to control the servo
joan2937 commented a day servoblaster and pi-blaster (and my pigpio) have been updated to work on the Pi2. The servoblaster github repository has not been updated. You can find a link to the updated software by searching for servoblaster on the rasperrypi.org forums.

24. Using the DMA engine This program does not use the DMA engine
Observe how the servo motor jitters
Use Servoblaster, pi-blaster, or pigpio to implement the DMA servo controller

25. Switch to Continuous Rotation servo
Unplug the 180 degree servo and plug in the continuous rotation servo
Observe the different behavior

26. Questions ?
Done!