Controlling a continuous servo with raspberry pi

Question

As the title states I want to be able to control a continuous rotation servo and use it as a wheel for my rc car. I have installed ServoBlaster but I can't find any tutorial or example on how to send pulses to the servo. Is there someone who can provide me with an example code for turning the servo back and forth? Any help is welcome

Answer 1

To answer your general rather than specific question.

My pigpio library will allow you to control servos.

Download and installation instructions

wget abyz.me.uk/rpi/pigpio/pigpio.zip
unzip pigpio.zip
cd PIGPIO
make
make install

The simplest way to control a servo is from the command line using the pigs utility.

sudo pigpiod # start the daemon

If you have a continuous rotation servo with its control wire connected to gpio 4 (pin 7 on the expansion header) use the following commands.

pigs s 4 1000 # spin fast anticlockwise
pigs s 4 1400 # spin slow anticlockwise
pigs s 4 1500 # stop
pigs s 4 1600 # spin slow clockwise
pigs s 4 2000 # spin fast clockwise

You can send the same commands from any programming language you want (slightly easier from C or Python).

Answer 2

First, start the servod daemon:

# sudo ./servod

Servo commands should be written to the device file "/dev/servoblaster". For example, to turn servo #0 (by default servo connected to board pin 7 is servo #0) to neutral position, you write "0=150\n" to /dev/servoblaster.

Here's my Python example that sends commands to servoblaster's device file to repeatedly sweep a servo from left extreme to right extreme position, and back. Go through comments to understand the logic.

# Start the servod daemon from ./servoblaster/PiBits/servoblaster/
# It sets up a named pipe device named /dev/servoblaster.
# Writing pulse width to /dev/servoblaster actuates the servo.
#
# Example: echo 0=150 > /dev/servoblaster
# The example means send pulse width of 150 steps (each step is 10us, so pulse width is 1500us or 1.5ms)
# to servo #0 (which by default should be connected to pin 7, unless the defaults are overridden from servod command line).
# Note that 'sudo' is not required because all users have write permission on /dev/servoblaster
#
# The list of default pins and servo numbers are shown below. What this means is if you connect
# servo to board pin 7, that servo can be addresses as '0'.
#      Servo number    GPIO number   Pin in P1 header
#          0               4             P1-7
#          1              17             P1-11
#          2              18             P1-12
#          3             21/27           P1-13
#          4              22             P1-15
#          5              23             P1-16
#          6              24             P1-18
#          7              25             P1-22

from time import sleep

hold_position_time = 0.1 # secs
step_size = 10 # ie, change pulse width by 10*10us = 100us or 0.1ms in each step

# Dont buffer the writes to the device file, to avoid explicit flush()es.
dont_buffer = 0

with open('/dev/servoblaster', "w", dont_buffer ) as servo_blaster_device:
    while True:
        for pulse_width in range(100,200,step_size):
            # The \n is required. Without it, it doesn't work.
            cmd = "0=" + str(pulse_width) + "\n"
            print cmd
            servo_blaster_device.write(cmd)
            # flush() is required if buffering is enabled, which by default is.
            #servo_blaster_device.flush()
            sleep(hold_position_time)

        for pulse_width in range(200,100,-step_size):
            cmd = "0=" + str(pulse_width) + "\n"
            print cmd
            servo_blaster_device.write(cmd)
            #servo_blaster_device.flush()
            sleep(hold_position_time)

Answer 3

if you can do an PWM in any of the GPIO then its possible. https://learn.adafruit.com/adafruits-raspberry-pi-lesson-8-using-a-servo-motor/overview

Make sure you power the servo seperately to prevent failure of the board