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Rumba at 24 V motor skipping

Posted by Coherent 
Rumba at 24 V motor skipping
April 21, 2013 04:22PM
Hi everyone,
I know Rumba technically isn't ramps but since the design is similar I thought this would be the right forum. What are the possible reasons for my X axis to make reproducible pauses when it is not supposed to during prints? I am suspecting that it is possibly the pots being wrong on the stepper motors. I am running a rumba board at 24 V, does anyone know how running at the higher voltage affects where you pots should be compared to 12 V? I can tell when I back down the pot that the printer won't be able to keep up with an acceleration so the pauses I am getting at the higher setting instead happen when the X-axis is moving at a constant speed. Thanks for your help!
Re: Rumba at 24 V motor skipping
April 22, 2013 07:07AM
sounds to me (Im often wrong) like the driver is over heating and shutting down for a moment. As you say you cant reduce the current without it missing steps you are likely just trying to print faster than you are able to.

Solution, reduce current and speed.

EDIT: just one last thing. Do you have heat sinks on your drivers? and a fan blowing on them too? If not that will increase the ammount of current they can handle without overheating.

Edited 1 time(s). Last edit at 04/22/2013 07:09AM by lazzymonk.
Re: Rumba at 24 V motor skipping
April 22, 2013 01:46PM
Thanks for the response Lazzymonk. I was reading about how you can configure the current limit by measuring the voltage on the potentiometer of the Pololu driver itself:
By setting the set current at about 1 A, they claim that it should be low enough so that you don't need external fans on the Pololu's but I do have a fan just to be safe. I have the stepper motors from ultimachine.com which are these very high torque nema 17s. When I checked the voltage at the potentiometer it was maybe a little off ideal but definitely in the right ball park. I found it a little suspicious that the motors were pausing within the first 30 seconds of the print where it definitely is not over heated so these signs tell me that perhaps it is a different issue.

I am suspecting that I have a bum USB cable so I switched that and I am interested to see what happens. I can also try using an SD card or lowering the baud rate because I heard a few people say that the highest setting can sometimes be a problem. Any other suggestions are very welcome. I am using a mendelmax 2.0 with the rumba so the mechanics seem pretty smooth and I do not suspect binding at this point.

Thanks again!
Re: Rumba at 24 V motor skipping
April 24, 2013 06:16PM
I agree that it sounds like overheating.

When it pauses, does it actually miss steps? What I mean is when it unpauses is it now printing in the wrong spot? This is most likely overheating, especially if it is happening repeatedly. To diagnose this what you can do is:
1. move your x axis -- this will activate the motor
2. After the move finishes, try to push the print head... you should feel resistance (because the motor is powered). Keep a bit of force on the print head with your hand, and see if the resistance drops out -- if so then it is overheating.

If it pauses and then resumes where it left off (so your parts are fine, they just take a long time to print) then this is probably a communication issue and you're on track with a new usb cable (and also you could try tweaking settings in your operating system to give a higher priority to the usb serial interface, deactivate interactive features of your host software, etc)...but I put my money on overheating
Re: Rumba at 24 V motor skipping
April 24, 2013 07:32PM
I switched the USB cable and the baud rate to lower and that did not fix it. I also tuned the pot like I mentioned before based on the suggestions of pololu and that did not help. I will check the resistance when moving that is a great idea. What I observe is that the pause is in the x while the y moves as normal so sometimes it can be so dramatic that the x stops for a few seconds while the y moves. The motors are identical as are my pot settings on them so it would be strange if it is overheating even though that seems to be the symptoms. The axis behaves like it hit something but just in the x. The x motor and heatsink are not even warm since this will start happening right off the bat.
Thanks again!

PS- I think I might have it! The vibrations of the printer apparently loosened the set screw on the X axis pulley. The test where I pushed on it while printing totally made the problem obvious, thanks!

Edited 2 time(s). Last edit at 04/24/2013 11:20PM by Coherent.
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