I've been having a recurring problem that seems to involve repeatability when runnung more than one axis at a time on my machine.  It seems that if i program it to do a diagonal move and return, after it completes a number of iterations it has offset in one or the other axis by the same number of "steps" as the number of iterations of the program.  The program is similar to the one in "Using Mach 3" manual.  Except that I'm moving to both X, and Y coordinates.  Hence diagonally.  If I use the program as shown in the manual, each axis works perfectly.  Help!  I'm losing my hair.

This is a strange one... you're only seeing it when both X and Y are moving...? Is it absolutely repeatable and consistent on both axes? Or it randomly appears on either one of them?

May be the breakout board is under powered? Try using a different power supply with higher current rating.

Dan

Hi, Dan, Here is some more detail as to the nature of my problem.  I tried origially to post a lengthy description but it choked on it so i'll try to be somewhat abbreviated...
X and Y motors are 1200 oz. in. steppers.
Z motor is a1780 oz. in. stepper
All axes use 5 thread per inch ball screws and 1:2 belt drives.
Mechanically the machine is VERY solid.
Motor drives are Gecko G212
Power supply is a 800VA transformer, rectified into a 27,000 uf capacitor yielding an output of 70VDC.
Breakout board is a PMDX 120.
     Now for the problem.
When running repeatability tests such as the program shown on page 5-15 of the Using Mach 3 manusl, each axis repeats and returns to zero perfectly.  However, when I changed the program to move 2 axes simultaneously(diagonal), while the DRO says everything is back to zero, one or more of the axes is offset .005” from zero!  This corresponds to 50 step increments.  The example program is below:
F20G20G90
M98P1234L50
M30
O1234
G1X.1Y.1
G1X0.Y.0
M99
     Starting at X 0.0000 and Y 0.0000, when the program as written above ends the X axis has offset exactly +.005”.  The Y axis returns to zero perfectly.  Does it every time regardless of feedrate, acceleration, etc.
     FYI:  Motor Tuning info:  X and Y Velocity is set to 40, Z is 30.  Steps per unit is 20000.  Acceleration is set to 4 for all axes.  Step pulse is set to 2 u/s per Gecko.  I've played with these settings to no avail.  The drives are set to 10 microstep per step mode using the jumpers on them.
    If I change line 5 of the program to read: G1X.1Y-.1 and run the program, the X axis offsets +.005” and the Y axis offsets -.005”.
     If I change line 5 to read: G1X-.1Y-.1 and run it, the X axis returns to zero perfectly, but the Y axis offsets -.005”.
     If I change line 5 to read: G1X-.1Y.1 and run it, both axes return to Zero perfectly.
Running any one of these programs repeatedly, continues the offsets: .005, then .005 more etc. 
This cumulative offsetting is bothersome.  I would suspect some electrical crosstalk but rearranging the wiring etc. hasn't made any difference.

My mill is a Bridgeport sized bed mill made by Siber Hegner (Tiawaneze).  The head runs up and down the column on ways.  All leadscrews are anti-backlash preloaded ballscrews.
  THINGS I HAVE TRIED, and things I have noticed.
1.The original computer was a PC Chips mobo with an AMD athlon 2500+ processor on it. 1 gig of RAM, an ATI agp video card, and a Moschips PCI parallel port card to support the MPG.  It ran with Windows XP Home Edition and a lockdown version of Mach3 from 2+ years ago.  I thought it could be a cheap computer issue, so I recently built another computer.  Its Specs follow:
            Motherboard:  ASUS P5QL/EPU
            CPU:  Intel E6500  2.93ghz.
   RAM:  Kingston KVR800D2N5/2G  one stick, 2 gigs.
   Video card:  ASUS EN8400GS Silent Video Card
   Hard Drive: Hitachi Travelstar 250 Gig SATA.
   Operating System is Windows XP Home Edition, and Mach3 R3.043.022 plus my lic.dat file.
     The new computer exhibits the same problem as the old one.
2.I have changed out the parallel cable twice, to no effect.
3.I set up and substituted a CNC4PC model C10 breakout board.  This had no effect, so I've reverted back to the PMDX 120.
4.I re set all the drives and breakout board to common ground instead of +5V common.  BAD choice!  The axes ran jumpy and jerky as hell.  I set everything back to +5V common.
5.I tried running the Geckos directly from the printer port with a home made cable, but I got no motion whatsoever.  I wonder why?
6.I also wrote another program that moves in a square:
   F20G20G90
   M98P1234L50
   M30
   O1234
   G1X.1
   G1Y.1
   G1X0
   G1Y0
   M99
   I got the same offset errors as with the diagonal program.  I noticed that the toolpath display    was rounding corners on the motion except at the X0Y0 coordinates...  So I went into General    Config. And changed from Constant Velocity to Exact Stop mode.  Running the Square program    now yields no offset error!  I ran the Diagonal program, and the offset problem is back.  It    seems that only if one axis is allowed to move at a time will the machine repeat correctly.  If 2    or more axes are moving at the same time, something is losing or injecting pulses into the step    signal. Huh?  I put the General Config back into Constant Velocity mode.
7.Setting Line 5 of the Diagonal Program to read G1X.00005Y.00005 and running it yields an interesting result.  The X axis offsets +.005” as with the original program, but now the Y axis also offsets -.005”.  Hmmm

Can you try running the program at a really slow feedrate, something like 1in/min? Does it still lose position? I think it is something related to the back EMF voltage produced by the motor while decelerating. So use a very slow feed rate and also reduce the acceleration. See it if it cures the problem.

You probably didn't have any motion when connected directly from PP to the Gecko drives, since your PP is outputting low voltages (3.3V is common) which is not enough for the drives.

Dan

Dan, Thankyou for the help.  The PP voltage being too low for the Geckos is my theory too. 
     Now to the real problem...
I'm using the program listed in the previous posting.  Only the feedrate inline one is changed.
     I set the feedrate to 1 inch per minute and the acceleration on each axis to .5.  Results:  X axis offset +.010, and Y axis offset -.005”.
     Curiosity satisfies, so I changed the feedrate to 20 and re-ran it and the results?  X axis offsets +.010” and Y axis offsets -.005”. 
     I changed acceleration back to 4 for all axes, Feedrate 10, ran it again.  Results as originally reported.  X offset +.005, and Y returning perfectly.
     Same results at feedrate set to 20 inch per minute.
     I tried it at feedrate 1” per minute and got the same results.
It seems that accelleration rates that are slow are making the problem worse.  Huh???  Feedrate didn't seem to matter. 

     I will try to post a picture of my setup so you can see it.  Picture worth a thousand words... 
Couls the close proximity of the Gecko motor outputs to each other be the problem?  Please make suggestions and I will do my best to implement them and test them.  Thanks for your help.  Don

Hi Don,

It's strange... What's interesting that usually the solution to such problems is a real simple thing. It can also be a combination of several factors and that would really make it hard to detect.

I don't believe it could be it, but we have seen weird things in the past, so just for the sake of it can you uninstall Mach completely and delete its directory including the XMLs. And do a fresh install and set it up from scratch.

Also, what do you have for a step pulse width under motor tuning? Try playing with it. I find that anything above 0 with my G540 doesn't work well. Also try playing with the Step "active low" setting under ports and pins. Also, can you temporarily use a different power supple for the Geckos? Something less than 70VDC. May be up to 60VDC. I still have a feeling it has something to do with the back EMF voltage.

Dan

Hi Dan,
   The Mach install is a completely new install New computer, new hard drive new installation of Windows XP, new download of Mach 3 from the internet to my empty USB drive, installed and set up using my notes regarding ports and pins, etc.  I'll try to put together a more detailed list of the settings and get back to you. 
   I've tried playing with the stepwidth but it didn't seem to make any difference.  I'll try again.
   I'll see if I can put together another power supply.  It might take some time. 
Thankyou for the help and suggestions.  Sooner or later we should hit pay dirt.   Don

When running repeatability tests such as the program shown on page 5-15 of the Using Mach 3 manusl, each axis repeats and returns to zero perfectly.  However, when I changed the program to move 2 axes simultaneously(diagonal), while the DRO says everything is back to zero, one or more of the axes is offset .005” from zero!  This corresponds to 50 step increments.

Clarify please. If you cycle 50 times the offset is 0.005" or 50 steps. OK, If you cycle 500 times is the offset still the same, or is it 0.05" ?

If the latter, it seems that someone somewhere is losing part of the last stepping pulse. In which case I don't think that it is electrical per se; rather it looks like either configuration or software. I think.
Mind you, switching the DIR line on each reversal might do something a bit strange, depending on the configuration.

Cheers

Problem is that Don has the problem only when two axes are moving simultaneously. When moving each axis separately there is no position lose. That's what makes it strange.

Dan