Motor lock on powerdown?

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DRobs86
Posts: 148
Joined: Fri Jun 05, 2015 4:19 am

Motor lock on powerdown?

Post by DRobs86 » Sat Nov 28, 2015 3:05 pm

I have a custom built machine running grbl, grbl controller, protoneer CNC shield, and DRV8825s with dual NEMA 17s on each axis. When I power down the machine the motors are in a "hold" state making it hard to move by hand. When I power it back on (even before I connect the arduino to the computer) the motors unlock.

This doesnt seem to happen on my 3d printer running marlin and a4988s.

I am building another custom machine now with a belt driven Z. I want to make sure that it locks on shutoff to prevent crash. What specifically is it that makes the motors lock on power down? I know that shorting particular leads with an unconnected motor does it, but I am looking for a way to purposefully make this happen.

Thanks

AnonymousPerson
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Re: Motor lock on powerdown?

Post by AnonymousPerson » Sat Nov 28, 2015 4:27 pm

The NEMA 17's you mention, do you have their model number handy?

Asking because people have mentioned some motors having brakes or similar which apply at power off. Looking at the specs for your motors might help see if that's what's happening. :)
Shapeoko 3 #516

aam
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Re: Motor lock on powerdown?

Post by aam » Sat Nov 28, 2015 8:37 pm

I'm experiencing the same issue since i switched from A4988 drivers to DRV8825s.
I found no sign of regularity for this behavior, it's 1 in 6-7 times that the motors are locked, and yes, it feels like as if you shorted both motor coils on an unconnected motor.
For your purpose, getting a motor with a built-in electromagnetical brake system seems best.

DRobs86
Posts: 148
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Re: Motor lock on powerdown?

Post by DRobs86 » Sun Nov 29, 2015 2:59 am

Both my printer and my mill are using automation technology model KL17H248-15-4A. The printer doesn't seem to lock.

This does sound much like a driver feature (or bug). The only reasonable explanation that I can put forth is that the drv8825s are shorting phases when they aren't powered. I'm open to all other explanations. I'd love to know exactly What I could do to lock it.

I Have considered a spring but would love for the motors to remove the need for extra hardware

cvoinescu
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Re: Motor lock on powerdown?

Post by cvoinescu » Sun Nov 29, 2015 9:25 pm

I'm not sure it's a difference between the DRV8825 and the A4988. Both have MOSFETs in the same configuration, with reverse diodes and all. These diodes will feed the current produced by the motors into the power supply, effectively braking them when the supply is off. However, I don't see much braking with either A4988 or DRV8825, so I guess it depends on the power supply and on what else is powered from it.
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