Raspberry Pi Servo Jitter

Here’s the final solution I came up with to finally get a servo motor to behave on a Pi. It may not seem like much but it took a lot of doing to gather all the right bits. This was tested with an SG90 and an SG92R on a Pi Zero.

Scroll straight to the end for the solution.

The most common way for controlling a servo motor on a Pi with is through RPi.GPIO as such:

#!/usr/bin/python3
import RPi.GPIO as GPIO
import time

servo = 23

GPIO.setmode( GPIO.BCM )
GPIO.setup( servo, GPIO.OUT )

# info on frequency and PWM formula at https://rpi.science.uoit.ca/lab/servo/
pwm = GPIO.PWM( servo, 50 )
pwm.start( 2.5 )

print( "0 deg" )
pwm.ChangeDutyCycle( 2.5 )  # turn towards 0 degree
time.sleep( 3 )

print( "90 deg" )
pwm.ChangeDutyCycle( 7.5 )  # turn towards 90 degree
time.sleep( 3 )

print( "180 deg" )
pwm.ChangeDutyCycle( 12.5 ) # turn towards 180 degree
time.sleep( 3 )

pwm.stop()
GPIO.cleanup()

Stuff you might need for this to run:

sudo apt-get update && sudo apt-get install python3-rpi.gpio

It results in super jitter which is unacceptable for the holy mission of pen plotting.

As far as I understand, the jitter comes from the wave form RPi.GPIO produces for Pulse Width Modulation, which is made in software and so it’s not super stable (no dedicated resources to build it). From what I gather, pigpio is programmed to tap into the one hardware PWM that Pis have.

The solution thus is as such:

#!/usr/bin/python3
import RPi.GPIO as GPIO
import pigpio
import time

servo = 23

# more info at http://abyz.me.uk/rpi/pigpio/python.html#set_servo_pulsewidth

pwm = pigpio.pi() 
pwm.set_mode(servo, pigpio.OUTPUT)

pwm.set_PWM_frequency( servo, 50 )

print( "0 deg" )
pwm.set_servo_pulsewidth( servo, 500 ) ;
time.sleep( 3 )

print( "90 deg" )
pwm.set_servo_pulsewidth( servo, 1500 ) ;
time.sleep( 3 )

print( "180 deg" )
pwm.set_servo_pulsewidth( servo, 2500 ) ;
time.sleep( 3 )

# turning off servo
pwm.set_PWM_dutycycle(servo, 0)
pwm.set_PWM_frequency( servo, 0 )

Stuff you might need for this to run:

sudo apt-get update && sudo apt-get install python3-pigpio
sudo pigpiod

And the resulting super smooth motion and holds:

Nosy Monster alive and well

The Nosy Monster has been sitting on my desk idle for a while now, I’ve always wanted to work on it a bit and make it more reactive to web input. Finally, I bit the bullet and it’s now a lot smoother to operate. Instead of clicking for pre-timed commands, the start of a key press actuates and the end stops. With a websocket communication layer and a socket python command server, the commands find their way from keyboard to motors super fast.

To account for wireless imperfections and AP hopping, I also wrote the logic such that there isn’t a “start” and a “stop” command. Instead there is only a “start” command which gets sent every 200ms and if it isn’t heard every 300ms, Nosy Monster stops. This ensures it won’t be locked in a state of actuation which could lead to perilous situations.

I intend on polishing the code & publish a tutorial that hopefully a 7 year old could handle.

The Nosy Monster was deployed at the Dartmouth Thayer School of Engineering’s open house. I didn’t know how robust it was going to be under sustained kid use.

It turned out to be a huge hit, I let the kids drive it around a bit and then I’d throw the Nosy Monster in another room where they would have to find their way around with no visibility other than the camera. It performed flawlessly through the evening.

It’s always an enormous point of satisfaction to see kids get into something you made. Once I sent the RC car “to Mars” (the other room), the kids were really focused.¬† All but one got stuck and never came home. I should have had a prize for the one, but what’s next, participation trophies?

With a much more usable and reliable toy, I decided to setup the same kind of maze in a house room under construction.

I moved the camera up on a stick for a better angle of vision, the next model will have a fisheye camera.

Without a reverse, it takes a few tries to get through the maze without getting stuck. He had to learn to be careful.

I’m bubbling up with ideas¬† of cool things to do with the concept and acquired techniques. From a solar powered exterior land rover, to battle bots.