> The motor is "geared down" considerably so that the output of the gearbox
> runs slower, but has more torque.  Attached to this gearbox output is a
> "pinon" gear which in turn drives a "rack" gear, thereby translating the
> rotary motion to a linear motion. The idea is to generate a precise force
at
> the end of the linearly-moving rack.  A calibrated load cell monitors the
> force, and when it has reached a prescribed level, the motor is turned off
> (by a PIC.)

Ah, so it seems you really care about the output force (motor torque) and
not so much its position.  You've already got feedback of the actual force.
Just run a PID control loop on the PIC to control the force to the desired
value.  The PIC output will drive the motor via PWM.  The motor voltage and
speed will end up being whatever it has to be to result in the desired
force.  But this is too obvious, what am I missing here?

> What I would like to have is more nearly a
> "proportional-servo", where the motor slows down as it approaches the
> set-point value.

Just like a real PID controller.

> If speed is a function of voltage, and torque is a function of current,

Torque is proportional to current, but the rest isn't that simple.  There
are issues like back EMF, inertia, and load characteristics.

> It does seem that electric cars have a similar problem, as it would seem
> that they need maximum torque at minimum speed, in order to get the thing
> going.  Anyone know how they do that?

They control one parameter (usually speed), and the motor voltage and torque
end up being what they need to be to achieve the desired speed profile.
Kindof like a cruise control where the throttle setting ends up being
whatever it has to be to maintain the desired speed.  What you are asking
for with the motor is like asking for a car to go half the speed at the same
throttle setting.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.