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I am building a CNC-like machine using a microcontroller and 'FMD2740A Microstep Driver' modules to drive the stepper motors.

My problem is the vertical axis where, despite a counterweight, there is a load to lift. I want the driver to keep current flowing even after I've stopped sending it pulses. This particular power driver module (MFD2740A) turns off the power a fraction of a second after the last pulses were given. The motor can then free-wheel from the weight of the machine.

I imagine that this is a common problem. My question is two-fold, 1-has anyone used a stepper motor driver in a way to keep the power on, and 2-is the heating issue of having current on constantly a problem?

For the controller I'm using GRBL on an Arduino, an 36V motors.

(later edit) turns out the Gecko GM215 has a dip switch which turns off this behaviour. Looks like I need to just buy this module and not use my MFD2740A. See my answer below.

(another edit) the GM215 does indeed fix this problem, with the DIP switch setting for the standby mode set to OFF, it keeps the current flowing. And after keeping it on for 20 minutes I don't feel any appreciable heating, so that addresses my overheating concern.

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If your stepper motor driver does this, and there is no way to override it, you have a crappy stepper motor driver.

Most sane stepper drivers have that sort of behaviour as an option, if they have it at all.

Looking at what documentation I could find for that driver, it claims "In static, current automatically in half", so it's not disabling the motors, merely reducing the power when the axes are static.

Considering it's a Chinese no-name product, I think the only reasonable options are:

  1. Switch to a properly supported driver.
  2. Modify your driver to behave properly.

We can possibly help with 2, but you would need to be prepared to damage a driver or two, and possibly not succeed.

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It is due to a bad design of the board. You can fix this by removing R1, which will turn off the "current reduction at stop" feature.

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I have recently finished building my CNC router And I can assure you Z axis stability is not a common problem! You state that the motor free wheels when the power stops so don't stop the power to the motor. My router will sit with the motors locked in place for hours with no problems at all, after I sorted the heat problems. I'm using unipolar motors with PIC linear Stepper Motor Controllers which place most of the heat in the Darlington Transistors back at the controller so the motors stay very warm but you can hold your hand on them. I had to go up to a very large extraction fan to remove the heat from the controller. The Darlingtons are screwed to large heatsinks with temperature controlled fans, one per axis, which turn on at 50 degc with an over temp sensor set at 70 degc but could not get the hot air out of the cabinet fast enough to stop the over temp going off until I added the extraction fan. If you are using a bipolar stepper and chopper style power supply most of the heat will end up in the motor, burning hot! http://www.piclist.com/techref/io/stepper/linistep/index.htm

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  • \$\begingroup\$ Hello, thank you for your reply. I would have thought many people would have the problem of their stepper motor controller turning off the power, causing joints with a load to move. I looked at the controller you mention \$\endgroup\$ – Mark Jan 19 '15 at 3:46
  • \$\begingroup\$ I looked at the controller you mention, most likely my power driver FMD2740A is shutting off whereas yours does not. I was wondering if anyone has experience with this auto-shutoff problem. \$\endgroup\$ – Mark Jan 19 '15 at 4:32
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The GM215 module from Geckodrive (http://www.geckodrive.com/gm215-step-motor-motion-controller.html) has a dip switch setting SW2 (page 6 of their full GM215 datasheet) that is "Standby Current Reduction" which can be disabled with SW2=off. They even say "...the motor has a back-driving load such as holding up a weight against gravity, this selection may not be advisable...".

Update: I bought the GM215 and it solved my problem (temporarily until it stopped working -about to try find out why). It kept the current flowing in the absence of step pulses from the controller, and now my joint is nice and solid as expected.

So, unless I am missing something, the standby mode on the 'FMD2740A Microstep Driver' doesn't work, at least for the three modules I have.

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