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Author Topic: Gaining steps...maybe? with Servo Drive  (Read 12632 times)

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Offline stirling

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Re: Gaining steps...maybe? with Servo Drive
« Reply #10 on: July 26, 2011, 08:39:09 AM »
Tweakie may well be right, but as a shot to nothing I may have an idea why noise could cause extra steps in one direction only AND why it might be worse the slower the feedrate. To check it out, try leaving your axis stationary a while and see if over time, it gently creeps upwards.

Cheers

Ian
« Last Edit: July 26, 2011, 08:43:24 AM by stirling »

Offline Hood

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Re: Gaining steps...maybe? with Servo Drive
« Reply #11 on: July 26, 2011, 08:54:00 AM »
I would do as I mentioned earlier and swap stuff around rather than replace a this time until you find the issue. You have already swapped the Gecko so that would seem to be in the clear, next I would swap the wiring on the BOB and assignment in Mach so that say X and Z are swapped.
Hood
Re: Gaining steps...maybe? with Servo Drive
« Reply #12 on: July 27, 2011, 12:58:52 PM »
Thanks for all the tips. I am working on this for the third day in a row, I am leaning toward a noise issue for now although a few questions remain.
I had been meaning to replace my Bob Board for a while since the speed control I was using was a little flaky (CNC4 PC patched in trough the Bob) and I had PMDX board sitting around waiting for a different project. I wanted to rule out any damage to the board so I went ahead and did the long task of swapping out the entire board. I also swapped the encoder from the Y and Z to rule out any encoder issues. I have already ruled out damage to the Gecko as stated before.
That seemed to sort it out pretty well at first. I was running tests, running the Z up and down at various speeds and it seemed to be holding tight. So I thought I would try and turn everything on, lights and Fans and the Spindle and run the test in a working environment. Then I started getting the same error again. all be it a much smaller error than I was getting before.
    So I started tying back wires and just making sure any higher voltage cables were as far from the encoder cable as possible. and continued to test. I have basically narrowed it down to the Spindle. If I run the spindle. I get errors. If I leave it off, seems fine.
    The encoder wires are shielded so I am not sure why I am having this problem all of a sudden. I have used this machine without issue for years. Is it possible for a lightning strike to cause shielding issues??? anyway, I am not really sure how I would go about shielding the encoder from the Spindle motor if that is what is generating the EMI. They are in reasonably close proximity (say 6" or so) Since the motor is on the Mill head and the encoder at the top of the Z column. But again, why all of a sudden. I am racking my brain to figure out what else is different in this picture.
Any thoughts?

 

Offline stirling

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Re: Gaining steps...maybe? with Servo Drive
« Reply #13 on: July 27, 2011, 01:32:46 PM »
I'm no servo expert but if it was noise on the encoder wires wouldn't that throw a fault? wheras noise on the step/dir wires... well I tried that one already - any joy - did you try it?

Cheers

Ian
Re: Gaining steps...maybe? with Servo Drive
« Reply #14 on: July 27, 2011, 01:45:33 PM »
I am not really sure how to test for noise on the Step Direction wires. nor what else I could do to remedy it.

CB

Offline stirling

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Re: Gaining steps...maybe? with Servo Drive
« Reply #15 on: July 27, 2011, 03:06:34 PM »
try leaving your axis stationary a while and see if over time, it gently creeps upwards.

Try the above with the spindle on for a while.

Let's find the problem first... then Hood will find the remedy  ;D

Cheers

Ian
Re: Gaining steps...maybe? with Servo Drive
« Reply #16 on: July 27, 2011, 03:16:01 PM »
ok, running your test now. I have been testing it all day and it seems that it does still creep up a bit even with the spindle off, But only a few thou after running it up and down over the course of about an hour. with the spindle on it creeps up much faster. If I run the spindle and move the z to -1 then back to 0 with a feed of .25 IPM it is about .01"-.015" higher at the end if the spindle is running. Is there any reason to worry that the PC itself is damaged? everything seems ok with the PC but I am really grasping at straws trying to hunt down this Gremlin in my machine.

CB
Re: Gaining steps...maybe? with Servo Drive
« Reply #17 on: July 27, 2011, 04:46:59 PM »
ok, well I let it sit for about an hour with the spindle running. No movement in the head as far as I can tell.

Offline stirling

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Re: Gaining steps...maybe? with Servo Drive
« Reply #18 on: July 28, 2011, 05:35:55 AM »
ok, well I let it sit for about an hour with the spindle running. No movement in the head as far as I can tell.

OK. Check your Z step setting in ports n pins. Is it set active LOW?. If it is, set it active HIGH, just for this test, (then set it back afterwards). Re-test as you've just done with the spindle on and see if there's any drift this time.

Ian
Re: Gaining steps...maybe? with Servo Drive
« Reply #19 on: July 28, 2011, 09:56:14 AM »
Ok well I tried it that way too. No movement either. Is this to rule out noise on the step signal?

Just trying to look at this with fresh eyes this morning. I am using the US digital E2 series optical encoders. from talking to people I know they can be buggy and subject to noise issues. One of the first things I though was at issue is that the O ring in my Z axis encoder cover had disintegrated and left pieces of itself all over the encoder. I cleaned it off real good and put it back on which lessened the issue. So I thought maybe the encoder was still mucked up so I swapped it for my Y axis encoder which was in good working order to rule out a bad encoder. Still same problem as before plus it really doesnt explain why the error is moving only in one direction.

My question is this. I have since lost the spacer tool that you use to position the slotted disk the appropriate distance on the shaft. I still have pretty good eyes so I just eye balled it exactly in the slot of the reader head by sight. Do these things have really fickle focal lengths or something? could that be the issue if it is not absolutely perfectly aligned?

I have two new encoders coming in the mail (with alignment tool) to help rule out damaged encoders (take them off and put em back on a few times and things can get less than perfect) as the old encoder from the Z, now on the Y is still kind of buggy. Also after talking to ta couple of people I also purchased US digital's PC4 cable drivers to snap on the encoders. They are not specifically designed for noise or anything but the have a resistor on them that helps quiet down noise on the encoder (or so I have been told) anyway. Thats where I am today. machine still down and I have work to do and can not do a thing about it  :'(

Still looking for things to try