On Thu, 11 Jun 2009 22:15:27 -0400, 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"?

Thanks,
Bob

Servo motors are intended for use in closed-loop control systems. This
makes the motor's dynamic response important: how quickly its speed
can track applied voltage. Time constants of all elements, including
motors, determine how responsive a control system can be with
stability, acceptable damping and freedom from overshoot. Servo motors
are therefore characterized and specified in considerably more detail
than motors that need merely function as prime movers. The system
designer will need to know torque constant as fn of current, speed as
fn of voltage, inductance, moment of inertia, etc etc.

It was once part of the education of young EE candidates to see how
the abstractions of control theory play out in the lab. Seeing red
pencil marks on a paper when one has offended Mr. Nyquist and wandered
into the right half plane is one thing, but seeing a 50 HP DC motor
go building-shakingly berserk does get yer attention. It did mine,
anyway.