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stepper motors have a mind of their own?!

Posted by ralphthompson 
stepper motors have a mind of their own?!
March 14, 2009 01:19PM
I'm using the single arduino setup, with everything built more or less.
I'm testing various individual parts of the electronics with all boards wired in to arduino and powered.
When I test something, e.g. opto endstop test code on the board build page, the stepper motors seem to move by themselves. Anyone else experienced this, or better still, know how to fix it?
they dont physically move the shaft, they seem to be taking one step forwards, then one back. all three motors do it, at regular timings but different from each other. The pattern seems to change at random - power off then on doesnt seem to have an effect, but power off and waiting a while will make them change timings.
they are all set on master at the moment, changing combinations of master/slave didnt seem to help.
any clues?
Re: stepper motors have a mind of their own?!
March 15, 2009 04:24AM
Could be noise getting into the step lines or poor ground connections between the stepper boards and the arduino.

People have reported similar effects when the L298 chips overheat. Have you got substantial heatsinks? Have you tried reducing the current setting?

Re: stepper motors have a mind of their own?!
March 15, 2009 06:19AM
Yep. I had this after a few minutes, using the original heatsinks. The x-axis would start to 'jiggle' back and forth.

I did three things and now mine's A-OK: I can't be sure which one it was...

I adjusted the current settings until the steppers have just enough current to move:
I added some bigger heatsinks:

Also, I seem to remember adding the 'sync' wires between the stepper boards and setting the z-board to master, the others to slave. (I'm unsure whether this made any difference). Most others seem to just have 3 unconnected master boards that work OK?

Hope this helps

Re: stepper motors have a mind of their own?!
March 15, 2009 08:42AM
One of the problems with the RRRF 1.x stepper boards is that the pot meter used to tune the maximum current should only be used in a very small part of its range. Basically the pot meter is a voltage divider that can set a voltage between 0V and 5V. This is the maximum voltage that the circuit will allow over the sense resistor. The current through this resistor is than V / Rsense, and with Rsense = 0.5 Ohm, that works out to 2*V. So, essentially you can regulate the current from 0A to 10A (!). You should however make sure you stay in the range below 2A, or at most 1V out of the pot meter.
Re: stepper motors have a mind of their own?!
March 15, 2009 02:00PM
Thanks for your suggestions.
I'm using the heatsinks that came with the board (RRRF SMD v1.1).
I did notice the heatsinks on those boards seemed to get very hot very fast. Overheating would explain why it works as expected for somewhere between 10-30 secs from cold before it starts behaving oddly.
It seems strange that any power at all is being disappated by the heatsinks, since the board has no instructions.
I'm at uni at the moment, I'll raid my PC odds and sods box for some meatier heatsinks when I get home, and have a fiddle with the dashpots.
Not too sure how to go about checking noise in the step wires but I've double checked all the connections are ok.
I'll let you know how I get on...
Re: stepper motors have a mind of their own?!
March 15, 2009 02:08PM
Yes it sounds like it is simply the current pots set too high.

The board will dissipate power, even if the motor is stationary, unless the enable line is driven low to turn the motor off.

Re: stepper motors have a mind of their own?!
March 19, 2009 01:58AM
I have all of my stepper controller pots set for the max current for the steppers type.
i.e. 1.6A for the BFB steppers and 1A for NEMA 17 steppers.

I fitted fairly large heat sinks on all 6 controlers with fan cooling.

- an old 486 cpu fan on Z an old PSU fan bridgeing X and Y heatsinks

I have used pin 15 of the Audrion & Sanguino this is connected to all 3 Axis stepper ctrl Enable pins on both repstraps.
This saves power reduces heatsink heat and motor heat in the case of the NEMA 17 steppers as they were sitting at 70C prior to using the enable pins.

Bodge It [reprap.org]

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BIQ Heater Block PCB BIQ Extruder Peek clamp replacement BIQ Huxley Seedling
BIQ Sanguinololu mounting BIQ standalone Sanguinololu or Ramps mounting Print It Stick It Cut it

My rep strap: [repstrapbertha.blogspot.com]

Buy the bits from B&Q pipestrap [diyrepstrap.blogspot.com]
How to Build a Darwin without any Rep Rap Parts [repstrapdarwin.blogspot.com]
Web Site [www.takeaway3dtech.com]
Re: stepper motors have a mind of their own?!
March 19, 2009 11:24PM
Yep, it seems much happier after trimming the current.
I note this step has already been added to the latest SMD board build instructions :-)
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