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Offline Hood

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Re: calibration question for the experts
« Reply #10 on: March 11, 2009, 03:48:41 AM »
Check that the Step and Dir signals have the active state set correctly for your drives,
Have you checked for backlash?
Hood
Re: calibration question for the experts
« Reply #11 on: March 11, 2009, 11:20:23 PM »
Spent quite a bit more time with it tonight and I'm not exactly sure what to make of things.  I had been trying to check everything out with my digital calipers.  I tossed those and hooked up the magnetic slide scale I have, it's reads to .001 mm.  I fixed it to the x axis itself and bolted a stop to the linear rail. 

I set the steps to 409.6 and started checking things.  I noticed that the X axis was off about the .4 mm I was seeing, but found out that it only happens when it's moving in the plus direction.  When I move it in the minus direction it was fairly accurate but any move positive would be around .4mm off.  Checking the backlash showed no serious backlash, certainly not .4 mm.  I hope I"m not just being obtuse and missing something easy.  I decided to try dumping .4 mm in the backlash compensator just to see what would happen and it's much better, probably around .02 - .05 mm.  What would you consider good?

Thanks,

Bob

Offline Hood

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Re: calibration question for the experts
« Reply #12 on: March 12, 2009, 03:14:39 AM »
What I would consider good is if you command G0X100  when the axis was at zero then it would move exactly 100mm. It will of course depend on the rest of your hardware such as how accurate the ballscrews are , if the machine is worn etc.
Did you check to see you have the Step and Dir signals active state set to suit your drives?
Hood
Re: calibration question for the experts
« Reply #13 on: March 12, 2009, 08:13:31 AM »
Yes, I checked those and they are fine.  One thing I failed to mention about the x axis being off .4mm is that it was always off.  Even after consecutive positive moves they would all be off the about the same amount.  Moving 0 to 25 then 25 to 50, and so on.  So it's not backlash but I have no idea what could cause that. 

Offline Hood

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Re: calibration question for the experts
« Reply #14 on: March 12, 2009, 09:05:41 AM »
so its just 0.04 off even if you move 0 to 100? if so then your steps per unit have to be correct, it must be the direction change that is causing the problem. Try the active states the opposite way from what you have, you will have top reverse the axis from the homing and limits so it still travels the correct direction..
Hood
Re: calibration question for the experts
« Reply #15 on: March 12, 2009, 10:09:40 AM »
I'll try again to change them this evening.  Best I remember though the states I have them set to in Mach were the only way I could get them to move.  I did try changing the pulse widths but setting them to 1 or 5 seem to make no difference at all. 
Re: calibration question for the experts
« Reply #16 on: March 12, 2009, 04:27:49 PM »
This is almost getting funny now.  I changed the step and direction from low active to high active and the problem just changed directions with it.  Now I get the .4mm offset when I go in the minus direction. 

Offline Hood

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Re: calibration question for the experts
« Reply #17 on: March 12, 2009, 05:06:10 PM »
Are you using a Breakout Board?
If so is it optically isolated?
Hood
Re: calibration question for the experts
« Reply #18 on: March 12, 2009, 05:16:13 PM »
I'm using the pmdx-122 board.

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Re: calibration question for the experts
« Reply #19 on: March 12, 2009, 05:23:32 PM »
Should be fine with that, is there any way you can easily take the Step/Dir signals straight from parallel port and miss out the BOB?
Hood