Following a question raised in response to a recent post on PWM, I hooked up a small servo motor to a Pi.
The motor in question is a 3 wire jobbie that needs a short pulse to control its position.
This short post explains how to drive the servo motor using wiringPi with a touch of Gambas.
Popular "closed loop" servo motors include a control circuit, allowing precise setting/positioning of the rotor.
I have a few Tower Pro SG90 servo motors which are supposed to operate over a +/-90 degree range.
In fact the range seems to vary from motor-to-motor, with some units exceeding 180 while others can only manage 170 degrees.
My servo motors can be controlled from a Raspberry Pi using a PWM output, where the frequency is set to approximately 50Hz and the positive going pulse width is as follows:-
- 1.5ms = motor mid-way
- 1ms = 45 deg clockwise
- 2ms = 45 deg counter clockwise
Using wiringPi and Gambas
Follow my earlier post to install wiringPi and create the Gambas test program.
The pwm frequency = 19200000Hz / pwm clock / pwm range.
The recommended frequency for my motors is 50Hz, so I set the clock divisor to 200 and the range to 1920.
50Hz has a period of 20ms. So to generate a 2ms pulse we set an output value of 192 (i.e. 20ms x 192/1920 = 2ms).
And for 1ms, the output value should be 96 (i.e. 20ms x 96/1920 = 1ms).
Modify the Gambas test program:-
- set the initial values: Clock= 200, pwm range=1920, output=144
- set Mark/space mode: pwmSetMode(0)
- disable the timer
- add a button which sets the pwm values (i.e. replaces the timer function)
For small, low power motors:-
- connect gnd wire to gpio connector physical pin 6
- connect +V wire to gpio connector physical pin 4
- connect pwm wire to gpio connector physical pin 12
Now you can run the test program and vary the output value as follows:-
- 144 = mid position
- 96 = 45 deg clockwise
- 192 = 45 deg counter clockwise
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