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Author Topic: Odd problem: Z axis stalls X or Y or both  (Read 6879 times)

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Odd problem: Z axis stalls X or Y or both
« on: March 16, 2013, 10:10:09 PM »
I've been trying to solve an odd problem for awhile and finally decided that I need some help from the forums as searching for this specific issue has come up dry.

Here is the problem:

When jogging the X or Y axis at 100% rapid (1500 mm/min), the axis I was jogging (X or Y or both) will stall the moment I begin jogging the Z axis. I can simultaneously jog the X and Y axis without any stalling, but I cannot jog X with Z or Y with Z without the X or the Y stalling. The Z axis never stalls, it only causes the X or Y or both to stall. The Z axis also only stalls the other axis if it is jogged after the X or Y is already jogging. If the Z axis is jogging first, then the X and Y will not stall when they are jogged. If the X or Y axis are jogging first, then they will stall the moment the Z axis is jogged.

The only solution I have found is to reduce the rapid speed to 500 mm/min. No stalling at all occurs at this speed.

I have done as extensive testing of the hardware that I am capable of and as I eliminated each piece of hardware I eventually arrived at Mach 3's handling the Z axis. This is where I am at now and am no longer able to troubleshoot this on my own.

Here is what I tested:
1. PSU and Mechanical axis. X and Y jog simultaneously without stalling. This means PSU at least has enough power to simultaneously jog 2 axis. It is a commerical 40V 12A PSU driving 130oz steppers. This remains true if I change the motors to drive any combination of the 3 mechanical axis of the machine. Stalling is not specific to mechanical axis. The stalling also happens when the motors are attached to nothing.
2. Motors. Swapped Z axis motor with X and Y, problem stayed on the Z axis output and did not follow the Z axis motor to the other axis. All motors caused the stalling when attached to Z axis output. None caused stalling when attached to X or Y axis outputs.
3. Drivers. Basically same as above. Problem followed the Z axis output. Whichever driver was attached to the Z axis output, caused the stalling. I've reduced the microstepping to 1/2 step, but still stalls.
4. Breakout board. This is also same as above. Problem followed the Z axis output. Which ever pins were programmed as the Z axis caused the stalling. For example, originally pins 8/9 where programmed as the Z axis output in Mach 3 and 6/7 were the Y axis. If I physically swap the Z pins with the Y pins on the breakout board - the problem stays on the Z axis output, meaning that when I jog the Z axis, which is now the mechanical Y axis, it causes the others to stall. I can jog Mach 3's configured X and Y axis, which are now the mechanical X and Z axis simultaneously without stalling. If I keep this setup and change the pin configuration in Mach 3 so that 6/7 are now the Z axis outputs (which were the Y axis outputs) and pins 8/9 are now the Y axis outputs (formerly Z axis outputs) - the problem follows the Z axis output. When I now jog the Z axis on pins 6/7 it will cause the other axis to stall, but I can jog the Y axis on pins 8/9 with the unchanged X axis simultaneously without stalling.

In summary, the Z axis output from Mach 3 was the cause of the stalling no matter what combination of breakout board pins, motors, drivers, cables, wires or mechanical axis were attached to it.

The part that confuses me is that I can solve the problem by reducing the rapid speed. I can rapid the X and Z axis at 2000 mm/min and the Y axis at 3000 mm/min with minimal stalling individually. 1500 mm/min is a conservative and very reliable rapid speed for the machine. To reduce it to 500 mm/min is painfully slow.

It appears to be caused by Mach 3's Z axis output somehow and I'm not sure how to go about troubleshooting this. I'm also ready to do further hardware testing, but I've exhausted all that I know of.

Does anyone know a solution or have suggestions?

Thanks,
Justin
Re: Odd problem: Z axis stalls X or Y or both
« Reply #1 on: March 16, 2013, 10:51:29 PM »
Hi Justin,
  Welcome to the forum. You did your homework very well, bravo.
Are your Z axis step and dir pulse settings the same as the other axis's ?
And, are the step and dir active lo settings the same as the others ?
Didn't see it mentioned.
Thanks,
Russ

 
Re: Odd problem: Z axis stalls X or Y or both
« Reply #2 on: March 17, 2013, 10:32:18 AM »
What are the specs on the motors?  Voltage, current and inductance? 

As steppers increase in speed, they lose torque.  Seeing as how you can fix it by slowing it down, you're probably just trying to go too fast with too small a motor and/or too little PS voltage.  Increasing voltage will increase torque at higher speeds but will cause excess heat build-up in the motors at slower speeds.  You always want to use as little voltage as you need to maintain desired performance.  Could also be that you need larger motors.  130oz/in are pretty small motors.  At higher speeds, they won't produce much torque.

The other thing that points to voltage is that 2 axes work and 3 do not.  When the third motor starts drawing current from the PS, there is less available to the other 2 axes.
Re: Odd problem: Z axis stalls X or Y or both
« Reply #3 on: March 17, 2013, 10:59:36 PM »
Thank you for the replies.

Overload, your questions got me thinking about how to troubleshoot the software and rrc1962, I appreciate your response, but I'm not sure you understood the problem correctly.

I spent sometime testing out my Mach 3 configuration today and managed to find the problem, the culprit - backlash compensation. More specifically, too quick of a Shuttle Accel time.

To begin testing the software, I detached the 3 stepper motors and placed them on my workbench and began playing with Mach 3's settings. I noticed that the Z axis output only caused the other axis to stall when I changed the Z direction - I hadn't noticed this detail before. When I slowed the rapid speeds to the point where they didn't stall, I noticed that the X or Y axis motor would "skip" on the workbench when I changed the direction of the Z output as the other axis was moving. This "skip" didn't happen when only the X and Y outputs were jogged simultaneously. This brought to mind the backlash compensation settings and when I called them up, sure enough, I had only set the Z axis. After disabling backlash compensation altogether, the problem was gone!

After testing the backlash compensation settings, I discovered that I needed to reduce the Shuttle Acceleration settings in the general config. This makes complete sense as the X and Y axis motors would jog along nicely until the Z direction was changed. This causes the X and Y axis to temporarily pause while the Z axis makes the adjustment. As I increased the rapid speeds, this pause became too abrupt and would stall the motors. Reducing the shuttle accel time to a more appropriate time for my rapids has solved this problem!

Thank you! It is nice to learn a bit more about these machines and the software through this.

Justin
« Last Edit: March 17, 2013, 11:06:07 PM by justinwol »
Re: Odd problem: Z axis stalls X or Y or both
« Reply #4 on: March 17, 2013, 11:04:07 PM »
That's good news Justin.
Thank you for posting your solution.
Great detective work !

Regards,
Russ