RE: [CAD_CAM_EDM_DRO] Bridgett update

>-----Original Message-----
>From: elson [mailto:elson]On Behalf Of Jon Elson
>Tim Goldstein wrote:
>> An interesting story on the E-bay servo motors. Then came with some real
>> nice 500 line encoders with ball bearing shafts and one of those real slick
>> factory mounts to the servos using a helical coupler. When I hooked them to
>> a scope they seemed to be working fine. I then hooked the motors to the G320
>> and fired it up and the servos would stabilize and then jump suddenly then
>> stabilize again. Motion was the same way, smooth, then some jumps and
>> lurches and then smooth.
>...
>More likely it is an electrical interference problem, from the PWM output

pulses

>of the Gecko drive getting into the encoder inputs.

I ran into the same effect when testing some motors on a G340. I had two
apparently identical motors with 1000-line encoders recovered from a Calcomp
plotter. One motor ran great; the other jumped around as described above. With
no step input, you could hear the whine from the servo dithering interrupted by
random thunks as the motor jumped to a new spot.

I could not accept the explanation of electrical noise in the encoder outputs:
The G340 was suddenly getting large positional errors, and to get this from
encoder noise would require the generation of a stream of alternating rising and
falling edges *in the correct phase* on *both* encoder lines.

It seemed much more likely that the encoder simply stopped producing pulses on
its output for a short time; the motor built up speed (due to the
ever-increasing motor drive generated by the PID network); eventually, the
encoder came online again, the G340 saw the motor whirring around, and braked it
to a halt, and backed it up to where it was when the encoder started working
again.

I had no way of testing this theory, however. To do so would require a way of
observing the shaft speed on the oscilloscope using some kind of linear encoder.
I thought of connecting the two motors shaft-to-shaft and frame-to-frame, and
testing one motor with the other's encoder, but I have more pressing things to
do...
-Kevin Martin