Newsgroups: comp.robotics
Path: brunix!sgiblab!swrinde!cs.utexas.edu!csc.ti.com!tilde.csc.ti.com!mksol!strohm
From: strohm@mksol.dseg.ti.com (john r strohm)
Subject: Re: Controlling stepping motors
Message-ID: <1994Jan22.002514.19116@mksol.dseg.ti.com>
Organization: Texas Instruments, Inc
References: <CJzwzu.78G.3@cs.cmu.edu>
Date: Sat, 22 Jan 1994 00:25:14 GMT
Lines: 38

In article <CJzwzu.78G.3@cs.cmu.edu> mkb@cs.cmu.edu writes:
>
>I'm looking for references on designing drivers and control algorithms
>for stepper motors. Right now I'm using big current limiting resistors
>and FET switches in a unipolar configuration, but in order to get the
>speed I need, I have to use a high voltage, so gobs of power is lost
>through the resistors. I'm thinking that getting rid of the resistors
>and PWMing the FETs will allow me to get current in to the coils
>faster and more efficiently, but I'm not sure how to pick a chopping
>frequency and other details like that. Any pointers will be greatly
>appreciated.

Your best source for this kind of thing is the applications engineers at the
stepping motor manufacturers.

There is a small British monograph on the subject of stepping motors, but I
am certain that it is long since out-of-print.  It points out that the
series resistors are there to provide control over the RL time constant of
the stepper motor winding; this is where you get increased stepping speed
over the natural limit.  Going to a high-voltage feed with a FET chopper
will BASICALLY require you to match the average current through the winding
to the rated maximum current and then worry about derating from there for the
higher instantaneous currents.  *AND* you get to worry more about the time
constants.

*AND* you *ALSO* get to figure out how you are going to drive your MOSFETs
to switch them on and off that fast; MOSFET gates are fairly big capacitive
loads, as such things go, and really don't like to be switched quickly.

Depending on what it is you are trying to do, it may be that you are
approaching the outer limits of reasonable applications for steppers, and
should consider going to torquers and closed-loop servo control.
>
>		Mike Blackwell		mkb@cs.cmu.edu
>
>


