[How do I?] Looking for thoughts on diagnosing a CNC / Prototrak issue

[keeena]

I'm having an issue with my Prototrak where the Y-axis slows down when moving in a Y-minus direction only. My question is somewhat platform independent, however.

I confirmed the the DRO functions correctly and there isn't any problem moving the table manually in either Y direction. I looked for loose or suspect wiring in the controller plugs, within the end-cap of the servo, etc.. I then swapped all leads between X and Y axis (power, DRO, comms) in an attempt to isolate the problem to the Servo vs. Cables/brains. However, the Prototrak will not operate normally if you do this (the axis will start to move, but then immediately show a fault code; I assume because it's expecting a certain rate given the axis input and swapping everything results in the maths not working for that axis...just a guess? this is true of both X and Y). My next brute-force debug will be to physically swap the servos.

Anyway: I first just wanted to know if the slow-down behavior for only one direction tends to be generally be a symptom of the servo, wiring, or controller. My knowledge is elementary but I generally am thinking its most likely the servo. My first though is that the encoder in the servo may not be functioning correctly in only 1 direction. Is that likely? Are there other more likely culprits?

The symptom is easily reproduced with a jog in the Y-minus direction. The servo sound like it spools up to max feed rate for a fraction of a second (you can hear it), but then immediately reduces to a slower feed and maintains that slow feed during the jog. e.g. at 100% (100 IPM), the feed drops to something relatively high (60-70%)...it doesn't continue to drop during that 1 jog operation. If you keep re-trying new jog ops, the reduced speed eventually will get slower and slower. It may be worth nothing that if the the "slowed" speed is still relatively high (set 100%, but runs at 60%), if you drop the feed 50%, then the moves will work for a while (set 50%, runs at 50%). But eventually the problem starts to occur even if the set feed rate is 50% (e.g. it will run lower than 50%). It does eventually get as low as 10%. The numbers in my example are not exact...the slow speed will be all sorts of random, slower feed rates.

This problem will occur on immediate start-up of a cold mill and affects all operations (mill, circle, arc, etc…) whether its singular or a compound move. Y-plus moves work perfectly fine all the time.

 

[macardoso]

Hmmmm, I have a lot of servo and CNC controls experience, but not specifically with Prototrak. Let me ask some questions.

1) What kind of servo motors are they? Closed loop stepper, DC brushed, DC brushless, AC servo.

2) What kind of encoders? Incremental?

3) What are the servo drives?

4) Does this happen only during jogging or also under program operation?

Immediately off the bat it sounds like the load on the motor is too great and the servo drive enters a current foldback regime where it reduces motor output to maintain a constant heat load. I would expect this to cause a velocity and positioning error which would fault the controller, but maybe your control does not do that.

Can you get a wrench on the ballscrew and try to feel by hand if the screw is really hard to turn in the Y- direction? Can you get an amp clamp on the motor leads to determine current draw in both the Y positive and Y negative directions?

EDIT: The encoder malfunctioning would likely fault the servo drive, but it would at least definitely show up as positioning error (a substantial one).

 

[keeena]

1) These are DC brushed motors. SWI calls them motor/encoders. Here's a link to the replacement.
2) Not sure
3) Not sure
4) Any/all operations which involve any Y-minus direction are affected. Keep in mind that most milling ops are fairly slow, so unless/until the slow-down gets really bad: it won't be immediately noticeable. Initially you'll only notice it with moves at rapids (e.g. position drill, jog). But eventually the slow-down does get into the range of milling speeds.

I hear what you're saying about the load, but I've already ruled this out. I still have manual control (this is a retrofit 2-axis CNC on a knee mill...see intro post here). Y-plus and Y-minus feels perfectly fine by hand.

 

[macardoso]

Hmmmm, strange. I assume the part being machined gets misshapen during the process?

 

[strantor]

Have you done the dummy check for brushed motors (check the brushes & commutator)? I don't know specifically how bad brushes could cause what you describe, but bad brushes in a motor is like bad ground in a car - sometimes baffling symptoms.

 

[JimDawson]

A really noisy (failing) encoder could cause this, throwing extra pulses in only one direction. I've never seen this in a machine tool, but have seen it in other types of machines.

 

[keeena]

@strantor - yep! I did pull all 4 brushes when doing my cursory check of things; they looked perfectly fine

I did another test which had an interesting result: if I unplug the Y-axis scale, the Y-minus jog runs correctly (full rapid) for a second or two until the Prototrak realizes as a fault and shuts it down completely (fault ostensibly due to the system trying to reconcile the servo instruction with the actual table move). This period of time is much longer than when the Y-axis scale is plugged in. I'm not sure if this necessarily rules anything out other than proving the motor itself is operating OK (I didn't say encoder...just the motor).

The servo has 9 very small gauge comm lines and 2 power lines.

 

[keeena]

Update: I swapped the X and Y axis servos and kept their wiring (power, comm) with the servos. Problem stays with Y-minus. I'm pretty sure the scales (which aren't swapped) serve as the feedback loop. But the scale/DRO seems to work fine in both Y-directions, so its got to be a problem with the 'brains' or the Y-axis wiring inside the unit, yes? Any thoughts for diagnosing further? I'm not sure what I should look for on the comm lines inside the panel.

 

[keeena]

Welp, I might have this sorted. Looks like it was a shoddy servo signal connection at the pendant. There is a 25 pin parallel cable from the power unit to the pendant which carries the servo signal lines for both axis. During the course of probing the signal lines the issue fixed itself. It was either the DB25 panel mount on the pendant or the ribbon cable inside the pendant going to the PCB.

The parallel cable is new (unrelated...did that a while back because the other was junk). I'll be replacing both panel mounts with soldered connections and hopefully that will be the end of that.