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Position creep in all three axis / csmio/ip-a
« on: October 13, 2015, 10:17:59 PM »
Hello again Guys,

       I can't seem to catch a break with this retrofit. Today I actually got around to cutting the first real part on the Matsuura MC500V retrofit with CSMIO/IP-A. It was a fixture plate for some parts I plan on making. About halfway through the program I noticed two things that must be addressed.

This first was a jerkiness in arc movements when I tried running faster than 30 ipm. I think this either has to do with rough servo tuning or I need to uncheck constant velocity mode in configurations. I would have tried this today, but I didn't have internet at the shop so I couldn't  research and trouble shoot the problem. If its a servo tuning issue, I might need a bit of help. These are the first servos I've tuned and although I'm using the auto tune application in the CS plugin, I'm still showing slight errors. (I think I get 20 for a max error when oscilating at full speed and acceleration over an inch) When I try to adjust the PID parameters to 'tighten' up the tune, The servodrive makes a high pitched oscillation sound. When I think about all the fiddling I've done with the tuning of these servos, I realize that I should probably start a CS labs servo-tuning thread.

The second problem is a bit more scary. It involves position creep. As the machine worked its way through the program, I noticed that my z depth seemed to be steadily increasing. At the end of the program I confirmed my suspicions. Sure enough, not only was the Z axis creeping, but the X and Y were creeping as well. Its like the machine was missing steps but only in the positive directions for all three axis. After the program was finished (the fixture is useless) I re-homed the machine and suddenly my zero point was back were it started. This leads me to believe its a missing steps problem, but why would it only miss steps in one direction? In the case of the X axis, it was missing steps and thus over-traveling while under load and moving at 20ipm rather than at rapid speed.

Thanks for any and all help!

Jonathon
Re: Position creep in all three axis / csmio/ip-a
« Reply #1 on: October 15, 2015, 11:15:43 PM »
Hi Jonathon,

I wonder if your issue may be servo tuning for both problems? I too used the auto-tune mode in the CSMIO plugin and it arrived at max errors on each axis of around 2 or 3. The auto-tune process took quite a long time on each axis... I remember being surprised at how long it took and how each time a parameter would start changing how the errors would slowly approach zero an than start to rise before arriving at a chosen value. After the auto-tune my machine worked so well I never did any manual tuning. I did check all three axis for distance traveled and backlash with a dial indicator and it was spot on. Maybe I got lucky or perhaps my machine is not tuned as well as it could be?

Depending on the resolution of your encoders, if your machine is off by 20ppr that could really add up over the distance of the part you were making. When you home do you mean running to a limit switch or "goto zero" in Mach? If your trying to return to your zero position based on DRO origin when your started the part then I would expect the error to be evident. If your homing to a limit switch then maybe Mach is NOT going to a position but is looking for the switch?

Good luck!

Mike

  
Re: Position creep in all three axis / csmio/ip-a
« Reply #2 on: October 15, 2015, 11:57:49 PM »
Mike,

     Thanks for your response.

      I found solutions to both problems yesterday with a bit more investigating. The jerkiness was solved (mostly) by setting the look ahead in mach3 to 200 lines instead of 10. Now it doesn't slam to a stop in the middle of an arch movement for no reason. It will stop though on a direction change so abruptly that I can feel it in my feet. An almighty jerk that makes me worry for my ballscrews.  I lowered my acceleration parameters for the servos from 10 to 7 in the mach motor tuning window and it helps minimally. Though, it may have more to do with the tool path and less with the machine.

As far as the second problem goes, it turned out to be an incorrect steps per unit of distance in the motor tuning window. I had recently set the machine to inches rather than mm. The machine is a metric machine so naturally, in the beginning, I set it up that way. But, mach is buggy when it comes to running a standard program on a metric configured machine. When I switched to inches I calculated the steps in inches but I was off by about 10 pulses per rev. I don't know how I screwed the math up but I did. When I put in the correct value of 12700 pulses per rev the drifting problem went away. I ran the program again and it was spot on.

As far as tuning goes, I think I still have work to do. Each servo drive has three knobs to adjust the gain, zero point and speed I think. Can't remember off the top of my head. But I notice that when I turn these knobs ever so slightly the autotuning gives me different numbers. And running auto tuning on each axis comes up with wildly different.  To top it off, when I run autotune, I usually end up with a final max error of 50 to 100. Thats way too much. Even when I manually adjust the pid parameters I still can't seem to get lower than 20. Ive read the instructions on autotuning the IPA but I'm not real sure about tuning the analog drives themselves on the machine.
Re: Position creep in all three axis / csmio/ip-a
« Reply #3 on: October 16, 2015, 02:44:56 PM »
Happy news your machine is able to make parts... If you can find the tuning procedure for your servo amps that will likely solve your problems with CSMIO tuning. The CSMIO docs stressed that the servo amps must be in good tune in order to get good results from CSMIO tuning. Perhaps if you tune your amps according to manufacturer procedure you'll get a better auto-tune from CSMIO? You can likely tune with nothing more than a VOM and a non metallic screwdriver for turning the pots but I think you can get even better results with a oscilloscope if you have one.

Good luck!

Mike
Re: Position creep in all three axis / csmio/ip-a
« Reply #4 on: October 16, 2015, 06:58:30 PM »
Thanks Mike!

    I'll have to read into the tuning procedure. I have an osciliscope, but I'm not sure how to use it to tune the servo drives. I'll have to do some research. What is a VOM?


Jonathon
Position creep in all three axis / csmio/ip-a
« Reply #5 on: October 16, 2015, 09:25:41 PM »
Volt ohm meter. Multimeter.


Alan