Hi,

I am playing with PWM for speed control of a small DC motor.  The motor is
from a Tamiya "Universal Gearbox" kit.  Actually the motor is a small 3V
motor from Mabuchi, and Tamiya have put it together with a neat little
gearbox that gears it all down to a useful speed.  There are three gear
ratios you can choose, and three output shaft orientations, but I digress.

The thing that piqued my interest was that the first driven wheel in the
gearbox kit has a little hole in it.  This immediately conjured up an image
of a slotted opto-switch, speed measurement and closed loop control.  It has
taken me a long while to find the time to actually put together what I
visualised, but I am having fun doing it.

Anyway, I am using a Zetex ZHB6718 H-bridge to drive the motor, and a
PIC12C508A to measure speed and adjust the PWM.  The PWM code is Scott
Dattalo's, from www.dattalo.com, which works nicely on the 12xx devices.  If
you take a look at that code you can see that he has left various timeslots
for your own code, so that's where I put my speed measurement and PWM
adjustment.

How it works.
The opto switch output is wired to T0CKI, so the LSB of TMR0 toggles for
every revolution.  I time how long it stays high, which gives me the period.
Scott's code is basically a big loop which repeats every 256us, so my time
is in units of 256us.

I then have some simple math that works out whether the period is too long
or too short for the desired speed (which is expressed in terms of
percentage of full speed) and I nudge the PWM up or down accordingly.  The
output is PWM and a direction pin, so you can drive an H-bridge or other
driver circuit.

It seems to work quite well, but I need to implement "proper" P-control as
it is not very responsive to changes in load.  Right now I only nudge the
PWM up or down by one unit per loop, which means that it takes a while to
settle.  I do have stall detection though, that boosts the PWM in an attempt
to 'unstick' the motor, and a 2400bd serial interface to set the desired
speed.

For three motors it would probably be cheap to have a PIC12xx for each
motor, although the speed detector would have to be a long way up the gear
train to give a significant number of pulses per second, so the code can do
a significant number of adjustments per second.  Also, the code could be
easily modified to operate for a certain number of revolutions and then
stop, but it would require a lot more math to compensate for acceleration
and deceleration of the motor.

---
Andrew  Errington
http://www.piclist.com/member/AME-SYNA-FA8a
PIC/PICList FAQ: http://www.piclist.com

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