Re: Simple Motor Control



CBFalconer wrote:
Bryan Hackney wrote:

CBFalconer wrote:

benn686@xxxxxxxxxxx wrote:


I have a motor that Im controlling using a micro with PWM. The
pulse width Im outputting is fixed and thats good enough to make
it move at a nice slow rate.

However, it would be nice to sense the speed. Is there a simple
way of doing that? Id rather not retrofit or mess too much with
the mechanical assembly but could a current sensing cicuit (or
IC) work for this ( to see if its moving faster/slower)? If not,
what would be another quick and dirty way of sensing speed to
adjust the pwm?

During the off period of the pulse, measure the back EMF generated
by the motor. You may have to filter out armature noise. That
voltage is proportional to the motor speed. It is acting as a
generator during the off period.

Assume this is a brush motor (seems likely). That armature "noise"
is from brushes making and breaking. I would rather use that as a
speedometer than back EMF.


That's a definite thought. However I have built analog systems
based on using full wave rectified and unfiltered AC through SCRs
to run model trains, and have achieved extremely slow speeds with
back EMF. I did that about 25 years ago, so the details have
slipped my mind, but it was cheap. A scope solidly synced to the
line was handy.


I don't know, but I have a guess that if the load varies significantly,
low speed control with back EMF may be lacking.

Finding more may be difficult, because fancy controls are generally
not used for brush motors. I have seen brush motors used as servo
motors, but the position (velocity) feedback was an encoder. Older
analog control schemes put a small generator inline with the motor, to
get a voltage proportional to speed that did not vary with the motor
load. The motor is a reactive device moving or not.

It would be nice to learn a little more from the OP. Is this a brush
motor? What is driven?



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