Perhaps more detail is necessary to accurately illustrate the problem:

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.)

Everything works fine, EXCEPT:  With the crude ON/OFF control of the motor,
the inertia of the system causes the mechanism to "coast" beyond the
set-point, with the result that the terminal force considerably exceeds the
specified (set-point) value.  What I would like to have is more nearly a
"proportional-servo", where the motor slows down as it approaches the
set-point value.  A straightforward version of this works well for low
values of terminal force, but if higher values are called for, slowing the
motor (by reducing the voltage) causes the motor to stall before it reaches
the desired force value.

Maybe I asked the question backwards, but what I am after is for the motor
to be able to run at a lower RPM without stalling at a lower load value than
it would if it were running at full speed.

If speed is a function of voltage, and torque is a function of current, it
would seem that what is needed is a way to provide a relatively high
(average?) current at a relatively low (average?) voltage.  I guess what my
question boils down to is whether there is a way to accomplish this with PWM
techniques, or by some other magical means I can't presently imagine.

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?

Thanks to all for your interest.

Foster


> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Olin Lathrop
> Sent: Monday, August 06, 2001 4:47 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: DC-PM motor, speed vs torque?
>
>
> > We are working on a (PIC driven) application which uses a 90 Volt DC-PM
> > motor. ...
> > Does anyone know of a control technique (PWM maybe?) that
> > allows lowering the speed while maintaining the torque?
> > ...
> > Am I trying to break the laws of physics here?
>
> Yes.
>
> For a DC electric motor, torque is proportional to current.  Speed is a
> function of torque and load.  You can't dictate both the torque
> and speed of
> the motor without controlling the load.  If the torque is contant
> the speed
> will be constant if the load doesn't change.  Asking for the same
> torque at
> a lower speed makes no sense.
>
> So you need to decide, do you really care about the speed or the
> torque?  If
> you could magically make the motor run at half the speed, why do you care
> what torque is required to maintain that speed?  If you really care about
> speed, then you probably need to do some speed feedback.  Not knowing the
> situation, my generic answer is to control the motor via PWM, which is in
> turn controlled by a PID control loop in the PIC that works with a speed
> feedback signal.  I know this is doable because I've done it before.
>
>
> ********************************************************************
> Olin Lathrop, embedded systems consultant in Littleton Massachusetts
> (978) 742-9014, olin@embedinc.com, http://www.embedinc.com
>
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