Gantry Troubles

I have setup a gantry using the attached configuration files. Everything appeared to be moving in sync until I ran a simple part. The part is diamond shaped and the toolpath works its way from the center out progressively getting larger and larger. As the toolpath progressed the gantry started to rack and I could see the two sides were moving out of sync. I combed through my ini file to find that I had two different values for acceleration on each of those joints. I was pretty confident that would solve the problem but it persists. Any ideas as to why this might be happening. I swapped the drives for the two motors and it seems to follow.

I am still having issues with this and I am noticing something which may be a clue. When I start cutting a pattern everything begins fine. I can stand inline with the gantry and visually confirm that both motors are moving at the same time. However, after a few minutes of running, the gantry then begins to rack. At this point I can see that when the axis comes to a stop and starts to move again that one motor does not for a period of time. The delay is quite obvious. On the order of a second. So, it seems as if something is accumulating. Might this mean something to someone here?

First I would try dramatically reducing your acceleration and see if the problem persists.

Rick G

Thanks! I tried lowering the acceleration from 10 to 1 (10x reduction) but the problem persists.

Hi

There are a 101 mechanical things that could be to blame, everything from backlash on one screw and not the other, to the bed not being square.

However on the electronic / HAL front, you are using the same step / dir pulse for Y axis for both motors.
How physically are you doing it?

If you run a cable to the Y driver and from there to the A driver, there is a danger that a weak signal will be 'sunk' by the first driver
and not always get through to the second driver at a strength sufficient to trigger.
If your PCI card is running at 3.3v, which it very likely is, there could be little latitude.

This would cause the A driver to miss steps and thus rack

The wiring should be 'star' fashion from the strongest signal point (eg. the BOB screw terminal) on separate wires to each driver,
preferably the same gauge wire and same distance.

Just a thought, may not apply

regards

Yes, sorry I should have elaborated on the physical setup. I am using a Gecko G540 connected to a PCI parallel port. There are 4 drivers within the G540 and each receives a step and direction signal pair from the parallel port.

As far as my HAL settings go, I am not certain I am doing this the best way. I have tried trivkins and gantrykins. Both of them seem to work the same but I prefer using trivkins at the moment (no homing needed). To me, the problem appears to be incorrect acceleration on one joint of the axis that develops after some time running. I've tried lowering my acceleration parameters but that doesn't help. I've also tried raising my steplen, stepdir timing parameters with no success as well. One thing I haven't done is watch the step signals on halscope to see if I can see a difference between the two when this occurs. I will do that now.

I think I can rule mechanical issues out because the problem follows the cable/driver. When I swap the cables (leaving them attached to the original drivers but changing the motor they attach to) the problem follows. So, the lazy joint becomes the opposite one on the gantry. Moreover, I can feel no slop mechanically. There is a vibration there which I can't find but I don't think the two are linked.

Ok, evaluating the parport pins in HALscope I can see that both drivers seem to be getting the same signal. This kind of points to the driver but I really doubt it is the driver. After thinking about your input regarding the signal voltage from the PCI card I tried the following:

1) I swapped the motherboard but used the same PCI parallel port card. The issue continued.
2) I used the onboard parallel port (had to go buy a ribbon to db25 adapter) for this new motherboard (previous one didn't have onboard LPT). So far so good.

So, I think the problem may have been the combination of that particular PCI card and the Geckos minimum input voltage on the step and direction pins. Fingers crossed! I'll do a lot more testing before I declare it good.

Thanks for the input so far!

Well... no joy. It's still misbehaving. I have one last idea to swap out the driver for the 'lazy' joint.

Do you have your parallel port in EPP mode?
Have you checked your parallel port cable?
Rick G

The parallel port mode is set to 'EPP and ECP v1.9' in the BIOS. There is options for 'EPP and ECP v1.7', 'STD', 'EPP and STD' as well as 'STD Print Driver'. Any of those sound like a better choice?

I haven't looked at the parallel port cable. I ruled this out when I swapped the drivers for the two joints and it followed the joint. I would expect the problem to stick with the joint if it were the parallel port cable.