Yaskawa Encoder-DC Servo Motor trouble

[nlh]

Hi folks,

I've recently been having trouble with a Yaskawa UGCMEM-04-MU11 Cup motor. It has an attatched TFUE-05ZC7 feedback unit. This servo would "jump" at a certain spot when jogging, sounded similar to a bad bearing. Through trial and error I found that the tach unit armature on the end of the feedback unit had a dead spot in it. I found a used UGHMED servo with the same feedback unit and swapped the armatures but not the entire feedback unit. This solved the jumping problem, or so it seemed. Do to another problem elsewhere on the machine(binding in the Z axis screw which I found later) The control/drive would throw an alarm 35 when at zero return. I talked to a tech and he thought it could be a problem in the servo motor itself. I took the motor apart, careful to mark the motor so I could get the main case back together the way it came apart. I found that the armature(similiar to a fiberglass substrate w/ inlaid copper) had small cracks in it. A guy offered to recoat it and I let him do so. It turns out he used a urethane clear coat. I put the motor back together only to find out that it would jump erractically when in rapid mode and wouldn't hold position. Even in jog mode it was not smooth. Another tech told me I had to get the urethane off and have it coated with Glyptol (sp.) which I did but it still exhibits the sames problems, although less in effect. Is it possible that the encoder is not aligned to the servo correctly? What about the fact I took the tach unit from the other encoder but not the entire unit, is it possible these are a matched unit and should not be swapped? I have a feeling I'll either have to find a used one or have it repaired. Unfortunately Yaskawa doesn't carry any accessible information on thier website for either the motor or feedback unit. I would really appreciate it if the experts on here could give me some advice and direction. Thanks!

[Al_The_Man]

If it is a DC servo then the encoder position should have no bearing on motor performance, I assume it is a DC brushed tach, these tend to require cleaning after a while, and sounds like part of your problem.
Usually if the encoder is bad, the controller will pick it up and it will go into error.
What part of the tach did you take from the other unit, as generally tach's are mounted on the same shaft as the main armature.
Al.

[nlh]

Al,

Thank you for the response. The motor is a DC brushed unit(2 brushes). The feedback unit is a combination of an optical encoder and a brushed tachometer. The brushes in the tach part are new and were installed several weeks back, possibly there could be some residue in between the commutor lands from the old brushes. As a matter of fact there are only 2 ways I can rapid the motor, either with the motor out(not installed in the machine) or with it in the machine and by pressing the button quickly. When the servo is installed and you press on the on the axis button while in rapid mode it throws and alarm 134 which according to my book is a servo error on x axis. With the motor installed I can jog it but it is not smooth at and has almost a lopey sound to it. Instead of the nice constant "Whrrrrrrr" of the other motors, this one give a "Whrrr-rrr-rrr-rrr-rrr". With the servo removed from the gearbox and the rapid button is pressed the servo will rotate and then when the button is released the servo will attempt to stop but then throws itself in reverse for a very short period, almost like it has gone to far or is searching for position. This servo used to be the Z axis servo but I have since swapped it with the X due to the relative ease of getting at the X axis location.

The tach portion that was removed is just the armature(looks like a little dc motor armature, about 5/8" dia). The armature has a hollow shaft which slips onto the feedback unit shaft and then a small allen head cap screw threads into the middle shaft to hold it in place. The feedback unit itself is connected directly to the motor shaft through the use of a flexible coupling.

EDIT: If it would help I can get some pictures later today of the feedback unit and motor.

Thanks,
nlh