Messages

1

General Mach Discussion / Re: calibration question for the experts

« on: March 12, 2009, 07:37:57 PM »

Well I found the problem, although I'm not sure why it was acting exactly like it did.  I got to thinking that I was still running with the default parameters in the drives and I started looking through them to see if I could change anything that would help.  The first one I changed fixed the problem, it was the position loop gain.  The range was 0-200 and the default was 40.  I kept moving it up and things kept getting better.  I maxed it out and didn't see anything detrimental to leaving it there and it got me within .002 mm everytime. 

Weird that it showed up almost exclusively in one direction only, but there's no denying that it has fixed the problem.

Thanks to everyone for your help.

Regards,

Bob

2

General Mach Discussion / Re: calibration question for the experts

« on: March 12, 2009, 05:16:13 PM »

I'm using the pmdx-122 board.

3

General Mach Discussion / Re: calibration question for the experts

« 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. 

4

General Mach Discussion / Re: calibration question for the experts

« 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. 

5

General Mach Discussion / Re: calibration question for the experts

« 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. 

6

General Mach Discussion / Re: calibration question for the experts

« 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

7

General Mach Discussion / Re: calibration question for the experts

« on: March 10, 2009, 08:24:02 PM »

I did some tests and one thing I noticed is that I told you wrong earlier.  With the default params and 409.6 pulses I was getting a pocket of 149.40 by 9.90 instead of 9.10 as I said before.   

 I did an outside cut as you suggested with the same default parameters and the measurements were nearly identical with the pocketing dimensions.  Then I removed the elec gearing and changed Mach to 1638.4 pulses per and the measurements that way were 149.2 by 9.6.

I'm as confused as a baby in a strip bar. 

Thanks,

Bob
 

8

General Mach Discussion / Re: calibration question for the experts

« on: March 10, 2009, 06:18:07 PM »

Apologies for not being more specific in my first post.  I set up the pulses according to Mach's setup.  Which led me to put 409.6 pulse per mm in the parameter originally.  When I do this I end up with a pocket that is 149.40 by 9.10.  Using Mach's calibration setup gets me closer but still not where I think I should be. 

Is there something else I'm missing?  And what sort of accuracy should I be seeing with this arrangement?  I realize that the ballscrews are rather coarse, but I was hoping for much better than what I am getting.  This is why I was wondering if perhaps more pulses per mm would help with my accuracy.

thanks,

Bob

9

General Mach Discussion / Re: calibration question for the experts

« on: March 09, 2009, 11:01:54 PM »

You are correct, I went back and doublechecked and the default is 8192 pulses per rev with a default 4 to 1 elec gear ratio getting down to the 409 pulses per mm.  I eliminated the gearing one time, but it killed the travel speeds in Mach.

10

General Mach Discussion / Re: calibration question for the experts

« on: March 09, 2009, 10:14:53 PM »

If I understand the terminology correctly they default to 2048 pulses per rev.