I have a VP2500HP with 2 heads. The nozzle head is jittering - like it loses feedback.
It almost seems like the stepper is skipping (from lack of torque) but belt seems fine.

Honestly I think it's a feedback problem. The metal sensing bars (that zero the z axis) were loose so we tightened those but it didn't solve the problem.
What's on the bottom of the z axis head? Is that an encoder? How is the pulley attached to the shaft - I don't see a set screw but maybe it's hidden behind what I can see.

It there a repair document for the z axis head?
Can I order the whole assembly?

Here is a link to the video behavior
We can reset the machine - and it resets fine
It goes out of control when trying to pick up or place parts.
Here's a video on my google drive

https://drive.google.com/file/d/1EucmIOC...iew?usp=sharing

What parts are on that little analog board on the top of the placing heads? I was wondering about the sensors that read the absolute position.

I will check the stepper motor as you said - that's a good idea but I know I will find nothing as it would not reset.
Is there any kind of feedback on the stepper motors or is all placement open loop (meaning just count the stepper pulses)

Could the air pressure system cause this at all? I think I have it turned off on most parts.


Is there any kind of feedback on the placing heads at all? How about the air pressure sensor?

The motor seems fine - 6 ohms between windings on both sides.
I can control the z axis by hand without problem. We even put a load on it by pressing on it but could not cause it to
error out or for the stepper motor to skip.

It's something that is happening when under software control.

Bad stepper driver? But then why would it work manually?
No feedback in the Z system like an encoder?

Just those limit switches for feedback to zero the heads?

The cable conductor colors at the stepper motor and at the Z axis driver don't match. I unplugged the driver to make sure it was the correct stepper driver and the z-axis lost function so it's the right driver.
Is there a connector somewhere that the stepper wires go through - where the colors could change. It's all very strange.

I've decided that the symptoms do seem like maybe a flaky stepper cable. If one conductor were intermittent then it would miss pulses and be erratic

Zitat von mbruch im Beitrag #5

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The PCB on top has only pressure sensors. Chec the technicla manual please.
Your problem depend on cable or stepper driver nothing more.

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Fixed with new cable.
Old cable - conductors are 7 strands 22ga total - pretty standard - BUT NOT RECOMMENDED FOR ROBOTS
New cable - conductors are 25 strands 22ga total - this is what the factory sent me, I'm not sure if this is what they are using in production now.

But the real sin, in my opinion is using connectors in a section of the cable that has some flex. 3 different steppers are routed though this cable and the ends are broken out.
I didn't do the surgery to get ot the joint, but I'm guessing the wires were either crimped, or soldered, at this junction.
If I had to bet I'd say that the problem is this joint and crystalized wires on either side of this connection.

If you have the same problem I did - a broken stepper cable causing random stepper motion - and you have an early model 2500, you might try the surgery and see if you can find the failed connector.
Of course with 7 strand it could also be a wire.