Does anyone have experience in using encoders on servo motors that are NOT attached directly to the servo's shaft?

The reason that I'm asking is that, because of the gear reduction necessary with a servo, the step pulse from either the Shopbot controller or from Mach 3 (non-G100) is way too slow for a servo. Even an ID3003 servo from MCG turns at 2,200 RPM (or faster) when connected to a 70VDC Gecko G320 driver. If a 10:1 gearbox were used with that motor, and a 500 CPR (2,000 pulses per revolution) were connected directly to the motor, and the spur gear gave a feed of 3.14-inches per revolution, the pulse generator would have to put out more than 70,000 pulses per second to give adequate speed.

On the other hand, if the encoder were either connected to the spur gear shaft, or to an intermediate shaft in a multi-stage belt-driven gearbox, the pulse rate required would be proportionally less. The big concern that I have is that by using an encoder mounted somewhere besides the motor, that the motor would oscillate (or hunt).

Mike
I ran across a post on Gecko's forum that recomended putting encoder on motor only. You might try a search to get more details.
Dirk

At a cut resolution 0f .002 inches wouldn't a 300 pulse encoder on the motor shaft be more than enough? (you need 150 pulses but to be safe you double it)

To get adequate speed you would only need 10500 pulses.

Dick, your thinking is the same as mine - if there is a very high gear ratio, then a lower resolution encoder at the motor is needed. I would think that one could have a really cheap&nasty encoder in this case....?