Ned,
That's the whole point of my reply. There is no classical sense of the term.
You may have different or favorite definitions, but others do as well. That
is the dilemma. Every one of my described devices provide a servo function
and I guess, can be legitimately called a servo. Even Fanuc calls any device
that drives a machine axis a servo, regardless of the technology used.
Steve

"Ned Simmons" wrote in message
...
On Fri, 12 Jun 2009 15:44:11 +0200, "Steve Lusardi"
wrote:

Bob,
The answer you are looking for is difficult to formulate, because people
misuse the term. A true servo motor provides drive torque, radial
positioning and position hold torque. These are analogue, multi phase
motors
and they are expensive to make, inefficient and consume lots of energy. In
use, there is a transmitter that provides all the power for all the
receivers. They are effectively selsyns and they phase lock together in
use,
sometimes used with a differential selsyn for small phase angle
adjustment.
Today, this technology is seldom used because although accurate, they are
much more expensive and can do very limited work. They were primarily used
for compass drive and radar screen/antenna synchronization. Today that
role
is being performed with stepper motors, with and without position
encoders.
However, these are also inefficient and are low torque for the same
reason,
using a large portion of the applied power for hold torque, but they are
less expensive. The high torque applications are usually satisfied with
permanent magnet (PM) motors, pulse width modulation (PWM), encoders,
tachometers and brakes because electronics are less expensive than copper
and they are very efficient. Unfortunately, they are all called servo
motors.

It sounds to me that you're confusing synchros/selsyns with servos. A
synchro, or one of its cousins, may be used for position sensing in
the feedback loop of a servo system. But a pair of synchros does not
constitute a servo, in the classical sense of the term.

--
Ned Simmons