Re: [CAD_CAM_EDM_DRO] Rutex Z axis problem--more info

tig415 wrote:

>Just to recap the problem I'm having. When ever I make a Z axis
>movement it travels farther than it should by about .015". It does
>it in the Z+ and Z- directions. It's not always .015" sometimes its
>less. Also it's intermittent. It seems to be good in the morning
>when I turn it on but over time it starts to happen and when it does
>it's intermittent. A move as small as .001 will cause it to happen.
>It's maintaining it's accuracy, .001" gives me .001" of movement on
>a dial indicator. I can cut for hours and not loose any steps. If I
>power up the servo amps alone there's no problem but when I turn on
>the drives the Z creeps down .015" then slowly back up.
>I don't think it's a tuning problem. I tried switching the drives
>and it made no difference. I then tried switching the servo amp
>itself and I still had the problem.
>I'm wondering if it may be an encoder issue.
>
>

OHHHH! You may be on to something, there! I've been trying to
figure out what was going on, but a loose encoder coupling could
provide EXACTLY this kind of error. It would include a backlash-like
effect. If you are not "losing steps", ie. a change in absolute
position, it
is not a classic encoder problem like a dirty wheel or weak LEDs. But
a shaft looseness could do things like this. Do you ever see the axis
twitch
suddenly?

One other problem could be a bad connection between the servo DAC output
and the velocity amp input. I have seen this on my system, too.
Reseating my
servo cards cures it for a year or so. Especially if the ground return
for the
servo DAC signal was making poor contact, you might see exactly this
symptom.

Anyway, there are two possible sources of the trouble. A sick tachometer
or tach cable might also be able to do something like this. One final
possibility
is too low gain on the servo loop. Try making very slow moves. If it
always
moves in jumps, it might just be that this axis needs more gain. This
could be
due to high friction or a weak motor.

Jon