servo motor moves on power up

[peterson76]

Hello,

I have a Cincinnati Milacron mill and as soon as I start the machine after boot up the Z axis servo runs away for a short time and the control gives me an error. Always in the same direction.

Only the Z axis is doing it and it is a 3 axis drive.

I verified that the signal going into the drive for the Z axis is null so the drive is not being commanded to run on that axis but runs anyways.

I swapped the Z axis servo with the Y axis servo and the Z axis is still the one doing it. Same servo different drive channel.

Considering that the Y axis servo works fine when connected to the Y axis channel but the Z axis servo has this error when connected to any channel, the problem is obviously not the drive.

What in the servo could be causing this? Would the servo run at all without encoder feedback to the drive for commutation?

I'm gonna start simple and check cabling but any help would be great.

Thanks.

[Al_The_Man]

If they are DC motors with Tach feedback to the drive, then look for a tach output problem, they usually consist of small DC generators built into the motor, output is relative to rpm.
Al.

[peterson76]

Al,

Thanks for the input.

Turns out, the drive was not "running away" it was just the brake turning off and the spindle falling.

I took the red cap off of the motor about a week before the problems started to epoxy a crack. When I put the cap back on the problem went away. I'm not sure why that caused the issue. The cap is plastic so it was not a noise/grounding issue and the encoder is under another red cap covered by the larger red cap. Possible ambient light interfering with the optical encoder? There is a small vent where ambient light could get into the encoder housing.

Thats just a stab in the dark, truth is I don't know what was causing the issue but it went away when I replaced the cap.

Thanks for your help.

Eric