Hi Dan and rcaffin. 

   If I up the number of repetitions, the amount of the offset seems to go up proportionally.  As a result, I've been able to get away with ignoring it for one-off and simple parts.  But its not supposed to do this.

Dan, I rewired the transformer inputs so my power supply voltage is half, or 35V, measured.  While the motors sound a little softer than they do at 70V, the results of running the program were the same...
     Test results this time:
Setting to “active Hi”  had no effect.  Didn't even sound different.
I tried setting the step pulse to 0 and to 5, and it made no change.
These tests were conducted with the original power supply setup.
After rewiring to 35V, The results are the same.  I slowed the acceleration down to .5 again and the offsets were X +.010, and Y -.005”.  As before.
I tried something else...  Acceleration 10 on both axes, and the result is that the X-axis offsets +.010” and the Y-axis offsets -.005”.  just like when the acceleration is very slow!!!
Setting the acceleration to 6 yields the result that X-axis offsets +.005 and Y-axis offsets -.005.   Hmmmm.
I then returned the acceleration to 4 and as before, the X axis offset +.005 while the Y-axis returned perfectly to zero. 
     I'm not sure what this means,  but I'm with you that back emf or crosstalk may be involved.  Do you think re-arranging the motor cables coming out of the Geckos might be worth a try?
    Thanks for your help.  Don

Yes, Don. Do try rearranging the wires, but still a better thing to try would be to use shielded cables for the motor cables. It's got to be something in the particular way you have it all connected and setup. Lots of people are using exactly the same hardware (and of course software ) with no issues, so it's something in the way you have it all arranged. Power supply doesn't seem to do any difference any more, so do try the wiring.

Dan

    If I up the number of repetitions, the amount of the offset seems to go up proportionally.

OK, so it would seem possible that you are gaining or losing one pulse with each cycle. My suspicion would be that this is happening at the turn-around.

  Acceleration 10 on both axes, and the result is that the X-axis offsets +.010” and the Y-axis offsets -.005”.  just like when the acceleration is very slow!!!
Setting the acceleration to 6 yields the result that X-axis offsets +.005 and Y-axis offsets -.005.

This almost suggests that the effect is not really dependent on the value for acceleration?

but I'm with you that back emf or crosstalk may be involved.

Somehow I can't see this. It might be the case with a DC servo motor, but I think you said you are running large steppers? I am not aware of any significant back emf problems with steppers.

Two thoughts come to mind.
* You are often getting slightly different results for the X & Y axes. Which one has the greater mass? Could the momentum of the carriages have anything to do with it?
* Have you checked the quality of the drive signals between the SimpleStepper 9assumption?) and the BOB and the driver amplifiers. Could switching the DIR line be causing any problems to the STEP lines?

Cheers

I would also check the Ports and Pins configuration, to see whether you have the right sort of pulses for the Geckos. A mis-config here could give you that funny result.

Cheers

Somehow I can't see this. It might be the case with a DC servo motor, but I think you said you are running large steppers? I am not aware of any significant back emf problems with steppers.

When a stepper motor decelerates large mass from high speed it will act as a current source and send current back to the drive instead of drawing voltage from it. If the power supply voltage is near the drive's max it could damage the drive if proper means of dissipating this energy are not implemented. I was assuming that Don was having some moderate variant of this phenomenon, since the problem was only occurring when two of the motors where moving in the same time.


Dan

Hum, yeah, but as the result was the same at 35 V as at70 V, it does not seem very likely in this case - unfortunately!

Puzzles ...
Cheers

Yep

Dan

Hi, again.  I'll try rearranging the wires but I would like to find some suitable 4 conductor shielded cable to use... 

I too am suspecting that the error is occurring at the turnaround point, either when the direction signal is changing, or when the currents to the motors change in their sequence...

Acceleration values are affecting the result, but not in a linear fashion.  Too high or too low and the errors get larger?  This is strange.

I'm not aware of any back EMF issues with steppers either but with 6 billion people and thousands of possible configurations of these machines in the world, why is it me that has the weird one?

As per my original posts describing the problem, The only differences are that the Y axis moves – and the X axis moves + when it gets these offset errors.  The direction in which the program starts off ( which quadrant it goes into) seems to determine which axis/axes will drift.  When they drift, it is always the same direction.

The X and Y axes have exactly the same motors and drive components.  1200 oz.in. Motors, 1:2 timing belts, 5TPI Rockford Ballscrews.  The cross slid and table are exactly the same as Tiawanese Bridgeport copies.  9” by 42” table with the same saddle used on the knee mills.

I don't have the knowledge or equipment to test the signal qualities...  I would probably smoke the parallel port, knowing my luck...

For information on stepper motor electrical characteristics search www.geckodrive.com for an article called Step Motor Basics.  It is VERY informative.

I'll be going to the Oregon GEARS model exhibition in Portland Saturday,  but I hope to get some re cabling done soon. 
Thanks for all the help.  Don

As per my original posts describing the problem, The only differences are that the Y axis moves – and the X axis moves + when it gets these offset errors.  The direction in which the program starts off ( which quadrant it goes into) seems to determine which axis/axes will drift.  When they drift, it is always the same direction.

I find that very consistent with the idea that it is a change in the DIR line at the end of a traverse which may be causing the single step. I don't know exactly how Mach handles reversals, but see the next comment.

I don't have the knowledge or equipment to test the signal qualities...  I would probably smoke the parallel port, knowing my luck...

Actually, you can do some testing with no gear at all. First, check the specs on the Gecko drives: do they want positive-going pulses or negative-going? Then look at the Pins&Ports page for outputs: are the lines to the motors drives configured to be normal or inverted? ie, what polarity signals are being created? If there is a mismatch here it could cause the problem. Try altering the polarity of the signals and see whether that makes ANY changes in the behaviour.

What i am suggesting is that IF you have the polarity of the pulses reversed, switching the DIR line may cause a small pulse on a STEP line as an artefact of the software. I have no idea whether this can happen with Mach, but it is not unknown in some device interfaces.

And do check the earthing yet again... Funny stuff.

Cheers

Gib play will can cause what you are seeing. You are not llikely to see it until XY move is done but it will be some min/max range varying within the range on return to zero. Backlash could also play into it all. Make sure the mechanics are all correct before looking for lost pulses.
RICH