z-axis seems to lose its height position and hovers above pick up and drop

[trev]

Hi

I have an RV4s that I have just started using. It has a weird fault to it that sometimes it just loses its z position. It does not drop like I have read elsewhere but does the opposite. It seems to lose steps and ends up thinking it is lower than it is.

So it will be picking parts up fine and dropping them down fine and then somewhat randomly it will stop dropping down enough to pick are part up from a feeder. I will stop the job, start again and it finds the position no problem.

Any thoughts?

Trev

[sheaton]

Asuming the mechanical movement is free, could it be an intermittant connection to the stepper motor ?

I had a similar problem with a different make of machine.  The z axis would occasionally move up and down wildly crashing the head, then it would go back to normal but be offset from its expected position. 

I found the fault in the end when it acted up whilst the machine was parked and I wiggled a few connectors and it fixed it.  I presume the stepper coils were energised to hold the z position, but the break in the feed to a coil caused the stepper to move 1/8th of a turn back and forth.

You might investigate a stepper coil becoming open circuit when hot, connectors dirty, or abrasion at the point the wiring passes through the arm joints.

In the PCB image, I am not certain the connectors on the right are for the steppers, so DYOR there.

[trev]

Appreciate the reply.

The thing is the head never crashes into a board. It is always loses position and hovers above. So it will be placing nicely and then it just misses a part and I check and see it move to the correct position in x and y but then when it drops down to pick the part up it is a few mm above the part.

If it was a loose wire I think the up and down would be, as you observed with your machine very erratic at times. Mine seems to be very repeatable when it does happen.

I have high pressure on the parts so that the reistors and capacitors sit nicely in the paste. Mabye they hit the board too hard on occassion and it thinks it had a collission and backs off a bit or the motor slips? I did notice it though when I drop a part down and just genlty place it. I believe the stepper motor is used to measure distance but maybe I need to actually look at the part of the circuit that detects when it presses down too hard? Maybe there is a loose wire there....

[Mike]

Have you checked the condition of the rack? - I've seen reports of problems due to wear of this

[sheaton]

Mentioning you are placing hard made me think that if the stepper pinion was loose on its shaft, then it could slip when placing resulting in the offset that you see. The offset would be cleared if the machine was re-homed.  I can't see how the pinion is fixed (I couldn't see a grub screw) so it may be press fit.  Just a thought.

[trev]

Appreciate the responses.

I will have a good look at it hopefully tomorrow.

Trev

[trev]

I stripped the rack and pinion down to examine for wear. It was all good. The gear was fine on the stepper motor. I cleaned the opto sensor in case that was causing issues.

I put it all back together, tried it a few times and it seemed to work. Then all hell broke loose! It suddenly started crashing! I wiped out 2 nozzles! It is something to do with the z axis motion. I think it might be wiring related.

The good news is I figured out how to force the machine to use the static camera for all components and it has worked perfectly for a few hours. So there is something screwy happening when it has to move up enough to use the OTF camera. I am happy enough to use the static camera for now, it is a lot less violent sounding not having the mirror mechanism slamming up and down.

Trev