Newsgroups: comp.robotics
Path: brunix!cat.cis.Brown.EDU!agate!howland.reston.ans.net!swrinde!ihnp4.ucsd.edu!library.ucla.edu!csulb.edu!csus.edu!netcom.com!nagle
From: nagle@netcom.com (John Nagle)
Subject: Re: Servo motors
Message-ID: <nagleCrt8BK.8I@netcom.com>
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
References: <2u780d$id0@unix1.cc.uop.edu>
Date: Wed, 22 Jun 1994 17:52:30 GMT
Lines: 35

hughes@napa.eng.uop.edu (Ken Hughes) writes:
>As near as I can figure, a servo motor is a motor with an encoder or
>tachometer attached to the output shaft.  I pick up a copy of my convenient
>motor catalog and find listed (1) DC motors with encoders (2) DC motors with
>tachometers, (3) DC servomotors with tachometers.  Should I infer that 
>the difference between (2) and (3) is that the latter also includes some
>reference input and the motor then regulates its own speed?  

       No.  It's quite common to equip motors with separate position and
velocity sensors.  Why?  First, because there's a well-developed analog
speed servo technology based on generator-type tachometers.  You can
get control systems which use an analog servo loop to control motor
speed, and a slower digital servo loop to control position.

       But even for all digital control, it may be useful to have an
analog tachometer.  Speed sensing via shaft encoders presents problems.
At first glance, it seems easy enough to get speed by differencing the
shaft encoder values between sampling periods.  But this doesn't work as
well as you might like.  If the sampling interval is short, the number
of counts during the interval will be small, and the accuracy of the velocity
measurement will be low.  There will also be a variation of 1 count,
depending on when the sample period occurs.  Driving the speed control
servoloop off such data can result in speed variations.

       If you lengthen the sampling interval, now you get better velocity
data, but it's older.  So now the servoloop has more delay, and may
oscillate around the desired speed.

       Recently, HP has started selling encoders with very large numbers
of counts per rev to deal with this problem and give all-digital
controllers better speed control, but this is a recent development.

       So that's why.

					John Nagle
