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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Constant Frequency at LOW temperature
References: <3DC53B22.email@example.com> <3DC543F3.8C692E48@webaccess.net> <3DC54ABE.firstname.lastname@example.org> <3DC553C3.D48D866E@webaccess.net> <3DCB4E2A.D3198E12@webaccess.net>
Date: Sat, 09 Nov 2002 01:11:50 GMT
NNTP-Posting-Date: Fri, 08 Nov 2002 17:11:50 PST
Tomas Modeer wrote:
> Chuck Simmons wrote in message news:<3DCB4E2A.D3198E12@webaccess.net>...
> > SNIP <
> > Thus the stepper drive motor will
> > experience varying torque and its rotational angle from the ideal
> > unloaded angle will vary during tracking. This turns out to be a
> > nonrepeatable error of significant magnitude with practical gear ratios.
> > Thus a conventional DC torque motor with sufficiently fine encoder will
> > give about an order of magnitude better tracking accuracy than a
> > stepper. This has been tested in retrofitting microstep drives with high
> > quality DC servo drives. For the same gears, the improvement is
> > dramatic.
> Thanks! Learning each day. I can see the use of a position feedback if
> load varies a lot, especially if the rest of the tracking system is
> In my mind this does not necessarily rule out any motor type. The
> steppers main
> advantage as I see it (open loop positioning) is gone. A high
> resolution synchronous motor (i.e. a stepper) should not do any worse
> than a (brushed) DC motor.
Once you put an encoder on the shaft of a stepper motor, it becomes
interesting to look at commutating a stepper as a brushless DC motor
thus converting it to a DC torque motor. This is possible because
stepper motors and brushless DC motors are absolutely identical
magnetically and functionally. This means, of course, you garbage can
the microstep driver and replace it with something simpler and much more
efficient. However, stepper motors are not optimized for low torque
ripple when driven this way. Torque ripple is bad because loop gain is
always varying. The torque ripple can be measured from BEMF and
compensated but this is getting a bit ridiculous given that low torque
ripple motors are very easily obtained in either brush type or
brushless. So the stepper goes into the garbage. Commercially available
are DC torque motors and controllers that act like steppers with
thousands of steps per turn. The controller accepts step and direction
commands. These are inherently better than steppers because within their
torque limits, they position to zero error according to the encoder.
A major disadvantage in stepper motors as steppers is their horrible low
speed efficiency. Any stepper running at low speed is inherently less
efficient than a DC torque motor with similar torque ratings running at
the same speed.
Stepper motor telescope drives are cheaper than DC torque motor drives
and that seems to be the only advantage.
The telescope I currently work on from time to time has a high quality
torque motor drive. If the telescope project had not been a rebuild of a
telescope damaged in an accident (the entire optical system was smashed
when the base tipped over five years ago), I would have used torque
motor preload of the worms. During the rebuild there was no room to add
large spur gears for preload so the final decision was the old wrap the
axes with cables and hang weights on the cables. These preloads are a
pain to disengage when the final drive safety clutches are released for
balancing. Not that the safety clutches really protect anything. The
owner of the telescope has a step ladder that was slightly crushed by
the drive running the telescope down on the ladder. You really need some
minimal torque to drive that size telescope.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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