Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Stepper Motor
Summary: More on servo applications...
Message-ID: <3242@kitty.UUCP>
Date: 18 Jun 89 16:14:21 GMT
References: <7137@cbnews.ATT.COM> <1137@cbnewsc.ATT.COM> <19588@cup.portal.com>
Distribution: usa
Organization: Recognition Research Corp., Clarence, NY
Lines: 84

In article <19588@cup.portal.com>, mmm@cup.portal.com (Mark Robert Thorson) writes:
> It says that
> most modern high-torque step motors are rated for case temperatures of
> 100 degrees C.  Mighty toasty, eh?

	Many of the newer high-torque stepping motors and servo motors are
rated for even higher temperatures.  I have used DC servo motors made by
Vernitron that are rated at 180 degrees C maximum winding temperature.
That is indeed toasty, and is also the reason why these motors all have
teflon wire leads (so do many stepper motors).

	Also, there are heat sinks available which surround stepping motors
to dissipate this heat.

> They also note that steppers generate
> excess heat even when the shaft is not moving (i.e. when they are exerting
> holding torque).

	Everything has a price.  An electromagnetic brake used with a DC
servo motor will dissipate about the same amount of heat.  While a stepping
motor keeps the heat in one place, it generally simplifies a mechanical
design since no external brake is required.

	While it is comparatively easy to implement dynamic braking of DC
servo motors while they are in motion, it is rather difficult to implement
an effective static brake without resulting in undesireable hunt and chatter
conditions.

> BTW.2, a friend of mine who is a machinist doesn't know how steppers and 
> servos work, but he sure knows which ones he likes.  He says steppers are
> crap.  Eventually they go bad, while servos last forever.  His exposure
> to motion control systems comes from CNC (computerized numerical control)
> systems used to run machine tools.

	I suspect your friend has not seen a large enough cross-section of
CNC equipment.  At worst, stepper motor versus DC servo drive motor systems
will show equal reliability; at best, stepper motors will show significantly
greater reliability.

	Why?  Because stepper motor systems are fundamentally simpler and
have vastly reduced analog circuitry requirements.  A stepper motor design
will typically just contain the stepper motor, plus a simple one-point
optical encoder for position reference.  The position and direction of the
stepper motor becomes a purely digital function, along with speed, which is
a simple function of step rate.

	Not so simple with a DC servo motor, which in exacting applications
not only requires a tachometer for speed control feedback, but requires a
resolver or optical shaft encoder for accurate position sensing and to
provide a step increment function.  While it is possible to obtain the speed
feedback from the position encoder, it is usually simpler to keep the motor
control circuit analog by using a voltage obtained from a tachometer.

> I suppose the mechanism of failure is
> the gradual de-magnetization of the magnets in the step motors.

	This problem is no less prevalent than in DC servo motors with
permanent magnet field. 

> BTW.4, an engineer at Allen-Bradley developed an interesting new use for
> PM step motors.  He uses them as transducers in control knobs on a console.
> When someone twists a knob, the passage of the magnets over the coils creates
> small magnetic pulses which can be amplified and converted into digital
> signals.  Advantages of this technique include the ability to set knobs
> to desired locations (by driving the motor as a motor) and a slight detent
> action when the knob is turned by hand.

	This technique has been used since before World War II in fire control
systems.  Instead of a stepping motor, however, the control knob is attached
to a "control transformer", which is a servo component that is AC-operated,
and which contains a 3-phase three-winding stator, and a single-winding
rotor.  A variation of this device is a "differential control transformer",
which also has a 3-phase three-winding rotor.  Control transformers (a _very_
misleading term for anyone without servo exeperience!) can be used as both
position sensors and motors in the same application.  Control transformers
are similar to "synchros" and "selsyns", but are generally designed for
lesser torque applications.  Control transformers are still used in current
fire control systems, and many solid-state support circuits are available
from the various analog "building block" manufacturers.

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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