On 2009-06-12, Bob Engelhardt wrote:
Is a servo motor simply a DC motor with an encoder or tach? Or is there
something about the motor itself that makes it a "servo motor"?

The DC servo motors which I have have several features distinct
from the average DC motor:

1) The number of poles and commutator segments is much higher to
minimize torque "cogging".

2) The rotor is wound on air, and potted in epoxy to hold its
shape, so the inertia is much less than a rotor wound on
laminated steel poles. There are likely to be stationary poles
inside the rotor to help concentrate the flux lines for maximum
torque for a given current.

This lightweight rotor construction allows much quicker
reactions to changes in command current. (However, it probably
does a lot to set the maximum RPM to be allowed. :-)

3) The motor and the tach generator share a common shaft, instead
of being coupled between housings. The tach generator has
a similarly large number of commutator segments, and is also
wound on air.

4) If there is an encoder as well (sometimes the encoder is
a liner one on the ways instead of on the motor), it also shares
a common shaft, and lives in a housing on the back of the motor.

All in all -- a true DC servo motor is designed for the task,
and will give much better performance at the high end than a motor,
generator, and encoder cobbled together.

Enjoy,
DoN.

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