Newsgroups: comp.robotics
Path: brunix!sgiblab!swrinde!cs.utexas.edu!howland.reston.ans.net!agate!library.ucla.edu!csulb.edu!csus.edu!netcom.com!nagle
From: nagle@netcom.com (John Nagle)
Subject: Re: Synchro Versus Stepper
Message-ID: <nagleCLJHJ1.I1J@netcom.com>
Organization: NETCOM On-line Communication Services (408 241-9760 guest)
References: <tallentCLGAA7.6FH@netcom.com>
Date: Sun, 20 Feb 1994 20:13:48 GMT
Lines: 32

tallent@netcom.com (Mike Tallent) writes:
>What are the advantages of Syncro versus stepper motors in a robot 
>platform. When will stepper motors work?

      Synchros, or selsyns, are not much used as actuators at present,
although they were widely used for that purpose from 1930 to 1960 or so.
They're still used as position sensing devices, and there are ICs for
converting synchro outputs to digital data.  Synchros are rugged and
usable in dirty environments, so they're still used in industrial and
military applications.   A synchro is basically a special kind of AC
motor, typically with one rotor winding and three stator windings.  If
you hook two synchros together (AC power to the rotor windings, and each 
of the three stator leads to the corresponding lead of the other synchro)
the two devices will rotate together; turn one and the other will turn.
It's possible to build drivers for these things and control them with a
computer, but since you need to generate AC waveforms at high power, it's
a pain.

      Steppers are cheap and simple, but they draw power even when
stationary and provide no position or force feedback.  Still, they
can be stepped an arbitrary number of steps, and unless overloaded,
will move that number of steps.  Useful for low-end arms.  Not
good for battery-powered applications.

      DC servomotors (which are motors with shaft encoders attached)
are the main output device used today.  Both force and position feedback
are possible.  "Brushless DC servomotors" (which are actually AC motors)
are becoming more popular; drive is more complicated, but the motors are
simpler and more reliable.  Almost all serious robotics work
today uses servomotors.

					John Nagle
