Steppers/Axis jerky when moved by hand - willing to listen

Steppers/Axis jerky when moved by hand - willing to listen

Postby stargeezer » Wed Feb 25, 2015 4:50 pm

Hi Folks. I'd sure like to make my first post not be a problem, but life is not always like that, is it? :lol: I'm building my first CNC router but have built a number of printers. With all of my printers I could move the axis by hand and they always moved with no resistance. With this router I can freely move the gantry by hand with the steppers disengaged, but when I put the belts on (engaging the steppers) and move the gantry it moves a bit and jerks to a stop, then moves a bit further and jerks again.

I am using Nema 23 425 in. oz steppers (overkill but I had them) hooked to a 5 axis BOB and DQ542MA I got off Ebay. http://www.ebay.com/itm/400860909457?_t ... EBIDX%3AIT Everything is all wired up and I'm ready to apply power for the first time. I'm double checking each connection and see nothing amiss so far.

I'll be running Mach 3, but I'm not connected to a computer yet.

Thanks for any advice.
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Re: Steppers/Axis jerky when moved by hand - willing to list

Postby twforeman » Wed Feb 25, 2015 5:03 pm

If your steppers are hooked up to the controller then it's normal for there to be some resistance to movement.

The steppers are generating back EMF that is feeding into the controller which is damping the movement.
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Re: Steppers/Axis jerky when moved by hand - willing to list

Postby stargeezer » Wed Feb 25, 2015 5:13 pm

Got that, but in my limited experience it's been a constant amount of resistance. This is a sharp lock of the stepper after moving a couple inches which releases in just a moment. I suppose it could be due to my moving the gantry at a high enough rate to generate a bigger field than I'm used to seeing. This Shapeoko 2 is the first project I've built using these steppers.

Thanks for the help. I'm learning.
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Re: Steppers/Axis jerky when moved by hand - willing to list

Postby cvoinescu » Wed Feb 25, 2015 7:19 pm

Slow movement will causes increasing resistance, but at some point the motor suddenly becomes easier to turn. Depending on how your controller works, what you describe is also possible -- it's caused by the current induced in the motor windings being rectified by the protection diodes in the driver and charging the bulk capacitors on the motor power supply line. Once they've charged somewhat, the motor suddenly becomes easier to turn, but it will resist further acceleration briefly. It feels very weird.
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