Z axis steppers stuck, not enough current?

Hi everyone!


My z axis stepper motors are getting stuck, what could this be?: (video)


[www.youtube.com]


Situation:

Reprap prusa v2. Gen6 electronics. Nema 17 stepper motors.

Using a PC PSU of 500w. 12v = 30A. (see video for full specs)


The potentiometer for the z motors in the circuit board is at full.


I connected many 12V cables from the psu to the Gen6 power cable. I saw in several videos that this may increase current. It actually improved movement in the other motors, and z axis motors themselves, athough not enough. (see video for close up)


Threaded rods rotate smoothly by hand, they're not stuck.


The rest of the motors, the heater and the thermistor seem to be running fine.


My insight:


When I had the potentiometer at half, the symptoms were similar but more pronounced. Not only were the motors getting stuck, but sometimes they moved in different directions, and sometimes only one moved. This automatically improved as I increased the potentiometer, and as I connected more 12v cables from the psu to the gen6 power cord.


Note: as you saw in the video, they (the motors) run very fine in all increments when not attached to the threaded rods. So perhaps they aren't getting enough current to move the threaded rods themselves?


What do you think this may be?


Thanks in advance

Edited 1 time(s). Last edit at 12/20/2012 02:16PM by Pixbae.

Your loosing steps, reduce your speed and/or acceleration.

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Please check the alignment of the Thread rods with the Motor (double check the coupler whether it have fitted correctly)

The thread rod should be parallel to the cylindrical rod so strucks can be reduced !

With the pot turned up on full the stepper driver could be overheating.

(I'm hoping you figured it out by now, but I'll answer anyway and we'll see if I was right.)

When the driver (Pololu's at least) overheats, it stops driving the motor... in the video, they're making a loud noise when they stall. I'm pretty sure I only got that noise when the motor was operating fine but the load on the motor was too high.

When I crank the current up all the way on my Pololus driving Kysan NEMA 17's, they don't operate smoothly... I seem to recall that they made excessive noise and jittered. I don't think it was the noise in your video, but more of a stuttering noise.

Hm. In your video, you note that the steppers operate fine when not under load (detatched from the screws) and when moving 10mm or less. I'm thinking you may have your Z travel rate too high (not the steps/mm, but the max axis speed). Your Z axis jogs a lot faster than mine, and I have a hundred dollar pair of precision leadscrews and not threaded rod that's *designed* to bind the nut when under tension.

Are you sure you have no binding issues the length of the Z axis? Misalignment between the screws, or between the screws and the gantry rods, can result in smooth operation at one end of the axis, but bind more and more as you approach the other end. Is the screw lubricated and free of debris? (Seriously... a little bit of fuzz can be enough to jam a nut on a screw enough to stall the motor.)

Assuming the mechanicals are all in order and there's still a problem, I don't think "more current" is the right solution. Try approaching the problem the other direction... tweak the current down as low as you can get away with (to avoid overheating of the driver and motors) and drop to a low travel rate and adjust that rate up until you get stalling.

Turn your driver current pots down to the lowest point that they will still turn the motors smoothly over short travel, maybe 5mm if 10mm is the limit at which you're experiencing failure now. Doing it with the screws detached is easiest (because then you can just run them as "far" as you want), but you might need to give the current a little extra tweak to account for load.

Then turn your Z travel rate way down... I'm thinking a tenth of what it's at now. See how far you can travel at that rate without error. If it'll go 30mm or more, double the speed and try again. Keep doing that until you get a speed you're happy with or it stalls. If it stalls, back off half of the last doubling and try again, repeating the halving (cut the "distance" between the last good speed and the current speed in half) until the stall goes away.

If the problem is pushing the motors to move the X gantry too fast, that should find an appropriate speed without running the drivers/motors too hot.

i got kind off the same problem, but if happened when i updated my Marlin software,

TRoager,

Is it happening when homing the Z axis? It happened to me when I updated my Marlin firmware too. I had to revert back to an old Marlin build because I couldnt figure out what the issue was...

Hi, look at DEFAULT_MAX_FEEDRATE in Configuration.h

My settings {X, Y, Z, E}:
#define DEFAULT_MAX_FEEDRATE {500, 500, 2, 25} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {3000,3000,20,10000}
...

I had this too until I got a clean compile. I had harddrive problems.
New laptop, Arduino 1.6.6, current Marlin.
Things are fine now. I had a few ranting posts because of the fustration.

I also wiped all rods with a light coat of lithium grease.