Newsgroups: comp.robotics
Path: brunix!cat.cis.Brown.EDU!agate!howland.reston.ans.net!news.intercon.com!udel!news2.sprintlink.net!news.sprintlink.net!crash!newshub.nosc.mil!avalon.chinalake.navy.mil!c3941fp.chinalake.navy.mil!Barry.Niesen
From: Barry Niesen <Barry.Niesen@chinalake.navy.mil>
Subject: Re: Servo motors
Message-ID: <CrvLp1.G9o@avalon.chinalake.navy.mil>
X-Xxmessage-Id: <AA2F786C2101035B@c3941fp.chinalake.navy.mil>
X-Xxdate: Thu, 23 Jun 1994 17:35:40 GMT
Sender: usenet@avalon.chinalake.navy.mil (NAWS news admin)
Organization: Naval Air Warfare Center Weapons Div.
X-Newsreader: Nuntius Version 1.2
References: <2uc854$1rv@unix1.cc.uop.edu>
Date: Fri, 24 Jun 1994 00:36:37 GMT
Lines: 38

Re: Servo motors

In article <2uc854$1rv@unix1.cc.uop.edu> Ken Hughes,
hughes@napa.eng.uop.edu writes:
>Ok, so let me rephrase the question:  what IS the difference then between
>a DC motor with a tachometer and a DC servomotor with a tachometer?

Pick up a DC motor catalog from a company such as Inland Motor or
Magnetic Technology and you will find two different types of motors,
torque motors and servo motors. Now both types are DC motors but the
difference is as follows.
	
A torque motor is typically described as a pancake configuration, i.e. a
large diameter and a narrow width. This configuration generally has a
large number of poles to increase the torque available in a given volume
(also really helps with maintaining a torque output that is independent
of motor position...excellent linearity!). This large number of poles ,
however, also causes more commutation arcing as speed increases than for
a motor with few poles. Torque motors are commonly used in positioning
and slow-speed rate applications where commutation is not a limitation. 

A servo motor is characterized by a long, small diameter configuration.
Lengthening a motor while maintaining a small diameter allows a
significant increase in torque while minimizing the increase in rotor
inertia. The end result is an improved mechanical time constant, and
therefore, motor response. Servo motors are most commonly used in running
applications where good high-speed commutation is demanded and operation
at or near stall is not required.

The above information was taken from the Inland motor manual. I should
point out that these motors quite exotic devices using rare earth magnets
and are mainly used in industrial and military application. (Precision
positioning equipment and airplane servo-actuators to name a few.)

Sorry to be long winded but I hope this helps. If there is more interest
I can easily drag up more info.

barry
