Re: [Gumstix-users] Simple feedback by brushed motor

[J.p. L. <jl...@ne...>]

 <wbig <at> fastwebnet.it> writes:

> 
> Hi all,
> I'm working to a quadrotor (draganflyer V) project, using a gumstix with a 
> robostix. I'd like to estimate rotors speed avoiding quite expensive and hard 
> to install optical encoder or similar.
> Connecting an oscilloscope, I noted that my brushed motors generate little 
> picks every rounds, and I thought that maybe I can read this signal using a2d 
> pins on robostix, but the signal is really noisy...
> Someone already tried to do something like this? Or do you have other ideas?
> 
> Thanks
> Claudio

Claudio,

I don't know if this will help you, but I have done something similar with a
brushless motor on a recent project.  I used the input capture (ICP) unit on the
Robostix to measure the width of the drive pulses.  I think the a2d is probably
too slow to catch a dynamic signal like that (although perhaps the analog
compare unit would work for you).

The noise you are seeing is probably the PWM that's applied by the speed
controller to throttle the motor.  If the PWM frequency is higher (or lower)
than the pick you're looking for, you could maybe get rid of it with an RC
filter.  I haven't done much with brushed motors, so I don't know if this will
be the case.  If not, the idea of using a hall effect sensor is probably the way
to go.

Hope this helps,
J.p. Lien