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Re: Progressive Move Error with X and Y
« Reply #20 on: July 06, 2008, 03:39:45 PM »
Just completed some more testing and the Sherlin 1/2 Mode did not fix the problem.  I re-installed all of the software and went through the entire setup and tried both Sherlin and non Sherline mode and nothing seems to work.

I can perform single axis moves without any problem.  Always returns to starting point with no deviation using a dial indicator.  The Dial indicator also matches the DRO.

However, when I do simple xy axis moves combined, the deviation from the origin begins.

When I setup an oscilloscope, it was mostly square with some minor noise in the corners.  Not that I know much about what it should look like or how to describe it but that's the best I can do.  Does anyone have a picture or diagram of what it should look like?  Or some more information and how to interpret the output of the oscilloscope?

The thing I find most interesting is that I can do a complex milling operations milling out one quadrant of a gear spoke which reapeat 6 times just going deeper each time and it's spot on.  No deviation.  This makes no sense to me.  How can it do partial circles, complex xy moves, and other moves and still be right on for each of the 4 quadrant I'm milling?  It just seems to be simple xy move operations between the quadrants where the deviation is occurring.

I'm probably overlooking something obvious but I have not been able to fixure it out.  If anyone has suggestions, as always, it would be greatly appreciated.

Best Regards.

Carl

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Re: Progressive Move Error with X and Y
« Reply #21 on: July 06, 2008, 05:38:30 PM »
cmnewcomer , Have you checked your backlash? If you have backlash but have in a way compensated for it in your steps per, this could cause problems. Backlash will show up at axis changeing directions.

Brett
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Re: Progressive Move Error with X and Y
« Reply #22 on: July 06, 2008, 08:23:46 PM »
Brett,

Yes, I have 0.004 baklash on the x and 0.003 on the y.  I've set it up to compensate and to not compensate and the results are the same.

My problem is that the error is cummulative.  If I do single axis x moves or single axis y moves I can do them all day long and everthing remains in sync.  If I do combined xy moves, I progessively get off by about 0.010 per cycle.  In other words, if I do G1X2Y2 followed by X0Y0, I will not be back at X0Y0.  I am about 0.010  off.  If I do the same cycle again, it's about 0.020 and so on a and so forth.

What's more puzzling is that I can do complex arc moves combined with complex xy moves and it stays right on?!

At this point I don't know wether to suspect the computer, the electronics, or my software configuration.  Just looking for ideas and do appreciate the suggestion.

Best Regards.

Carl

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Re: Progressive Move Error with X and Y
« Reply #23 on: July 06, 2008, 08:28:47 PM »
Carl, I have sent you a personal message.

Brett
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Offline jimpinder

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Re: Progressive Move Error with X and Y
« Reply #24 on: July 07, 2008, 04:03:37 AM »
We are getting bogged down here.

I will return to the point I made in my first (and only) post - which has been , to a certain extent, picked up by Brett.

I have not seen yet where you have calculated your steps per inch movement for your axis. I know you say you have done thousands of tests til you are sure it is accurate, but any measurement is suspect.

I have said it before many times, and I will say this again - not because I am stubborn (which I am) - but becasue it is a mathmatical fact - steps per unit should be calculated not measured..

Can you please tell me what result you arrived at when you calculated your steps per inch.

I do not know how many steps per inch you arrived at - but if this is out, it will not show up on moving a table up and back. Your measurement could be in handspans - but as long as you went up three handspans, and then back, you would arrive at where you started.

If you have a relatively small number of steps per unit, your error, if you have made one, will be greater, although if these are clock parts I would have thought you would have a large steps per inch for accuracy.

When moving two axis together, the axis moving furthest takes the lead, and moves at what speed is set. The second axis has to move as per the tangent of the first, not often a precise number, at whatever speed is set, and the figures for the steps the computer puts out are necessarily truncated. Again, if you have made mistakes, then the calculation of tangent is being made on a mistake, which again, must produce a larger mistake.

I do not really want to get involved in this post, becasue I have laboured these points often in the past, but I would like to see some calculations for your steps per inch (for both axis, if not identical) and then perhaps we can pursue Bretts worry about your backlash.

In the post you quoted an error of 0.00035 - am I correct - are we talking about 1/3 of a thousanth of an inch. I do not have kit that can measure that accurately, and I do not think my hands and eyes could use it if I had.

They only problem  - going back to your steps per unit - if you are looking at errors that small, you are looking at a small number of steps
My steps per unit is 60,000 per inch, actually 6,000 if you ignore the Gecko microsteps - i.e. 6 per thousand - so 1/3 of a thousanth is two steps. If you have fewer steps per than that, one step could be 1/2 a thou.

Have you tried, on your motor driver lines, ticking "low active" for steps and direction (or vica versa if they are already on active low) . For your breakout board, it may be that the first step is missed if your output lines are artificially held in one state or another.



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Re: Progressive Move Error with X and Y
« Reply #25 on: July 07, 2008, 05:58:08 PM »
Jimpinder,

My apologies as I was under the assumption that if my 6" dial indictor registers 3" when I coded G1X3 then I assumed the steps were properly set correctly.  I was able to move back and forth and only off by 0.004 thousands comparing the dial indicator to theentered command and DRO for both the X and Y axis.  My equipment doesn't measure below ½ thousands unfortunately so if I entered that in one of my post, I fat fingered it by accident.  Sorry.

Let me go over the setup and calculations as I do agree that being a newbie it's probably human error on my part.

1.   Nema 34 Steppers on X and Y Axis.
2.   Nema 24 Stepper on Z Axis.
3.   Gecko 201s Driving Each Stepper
4.   5 TIP Ballscrews on the X and Y Axis.
5.   Using the Micro Feed on the Enco Table Top Mill for the Z Axis.
6.   CNC4PC C11G Breakout Board.
7.   48V Unregulated Power Supply.

I calculated the X and Y steps the same as follows:

Turns per Inch = 5
Motor Steps per Revolution = 200
Micro Steps = 10

5 * 200 * 10 = 10,000 Steps Per Inch

The Z axis required 10 turns per inch so the calculation was:

10 * 200 * 10 = 20,000 Steps Per Inch

These were the values I entered in Motor Tuning and Setup.

I am unable to get the Step Pulse and Dir Pulse to persist.  Each time I enter Motor Tuning and Setup and press OK, it decrements the values of each by 1 until they return to zero.  This I believe is also causing me problems as things change as I test and I'm starting to see a correlation. 

Not sure if the Sherlin ½ Pulse Mode uses these values but it did not work the same after I reinstalled the software and tried this option.  This was very puzzling as the first time I set it in Sherline ½ Pulse Mode, it appeared to be correct when doing combined xy moves.  But after the fresh installation, it was back to the original behavior no matter what mode I used.

The machine has always worked correctly doing single axis moves.  Once again, my assumption may be wrong but when I test with the dial indicator doing single axis moves, it matches the command entered and DRO.

I have Step Low Active for the X, Y, and Z axis.  I have Dir Low Active for the X and Z axis.  I do not have Dir Low Active for the Y axis.  I can't remember why but I believe it was due to the direction it was moving?

As you can see I have a steep learning curve and appreciate the time that your team has put into this.  Thanks again for all the help.

Best Regards.

Carl
Re: Progressive Move Error with X and Y
« Reply #26 on: July 07, 2008, 09:05:01 PM »
Brett,

Thanks again for the help.  I have attached the dirver test screen shot and the xml configuration files.  In addition, you can see the box I made at this link:  http://www.cnczone.com/forums/showthread.php?t=28973&page=3

If you need more details on the box let me know and I will get some more pictures.  I'm going to try and locate another PC and see how that works.  If that fails, I may wire the gecko drives direct as you recommended bypassing the breakout board.

Best Regards.

Carl

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Re: Progressive Move Error with X and Y
« Reply #27 on: July 08, 2008, 04:09:13 AM »
Thank you for such a clear explanation of your steps per. Your calculations are spot on, and should therefore produce accurate results. Errors are non-repetative so you are looking for a single error that causes this.

I look at your steps per inch and I would point out that with such a screw (5 tpi) and only 1000 steps per inch, a loss of a single motor step will cause a thousanth error. Yes - the Geckos do 10 microsteps - but this is an artificially held position for the motor, and amounts to one tenth of 1.8 degrees - and cannot be relied upon to be accurate. I have a 10tpi leadscrew and a 3 to 1 reduction, and therefore have 6 steps to the inch (or 60 with the Geckos) which is much more accurate. It also allows much more tolerance when rounding up or down in tangent calculations.

Your print out for your driver seems very good - probably better than mine - so I don't see that as a problem.

If you are in touch with Brett, I will leave it there. I would also try the Geckos wired directly to your LPT1 port - mine are, since my breakout board is non-powered, and this will eliminate any error on the board. The only problem will be that the Geckos require a +5volts feed - since they are opto isolated as well.  If you have twin opto-isolation this might be where the steps are going.
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Re: Progressive Move Error with X and Y
« Reply #28 on: July 08, 2008, 04:19:40 PM »
Jimpinder,

Thanks for the confirmation on the setting and driver output.  You mentioned that twin opto-isolation might be a problem.  I'm not sure what that means.  My breakout board is opto-isolated but how would I deterimne if it is twin opto-isolated?

Best Regards.

Carl

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Re: Progressive Move Error with X and Y
« Reply #29 on: July 08, 2008, 04:37:27 PM »
Hi, Carl

Your Gecko's are also opto-isolated (internally).

Chip