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Servomotor disaster

Servomotor disaster
March 29, 2021 03:07PM
I was forced to reduce the size of the sand table mechanism I've been powering with iHSV servomotors and was running tests after reassembling the mechanism. The machine homed OK, so i selected one of the old pattern files to run, forgetting that those patterns were for a slightly bigger table. The machine did what it was supposed to and when the carriage hit the end of one of the axes, there was a snap, a spark and the whole thing stopped dead. The death toll included the Duet 2 WiFi controller board (the source of the snap and spark), and the 200W power supply that was powering both the motor and controller board, about $180 worth of damage. Fortunately, the motors are OK.

I think that when the motor was physically forced to stop, the current spiked, probably causing the power supply to protect itself by shutting down, creating a back EMF voltage spike that was sufficient to destroy the controller and power supply.

Lesson to learn from this: don't share a power supply between a servomotor and an expensive controller board!

The new controller board is going to get its own power supply...

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Servomotor disaster
March 29, 2021 04:38PM
... ouch! eye popping smiley

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Re: Servomotor disaster
April 01, 2021 11:33AM
The problem I ran into is apparently well known among those who work with servomotors on a regular basis. There is a pretty cheap and easy solution described in this document from the Gecko Drives website.
The document illustrates a snubber that shunts the current from the motor to ground as long as the voltage from the motor (the back EMF) is higher than the supply voltage. That will protect the power supply, the motor driver, and anything else connected to the same power bus.

I'll be installing that circuit and using a separate power supply for the controller board in future servomotor projects. I suggest you do the same.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Servomotor disaster
May 08, 2022 05:04PM
I prepared a PCB and gathered most of the parts for the returned energy dump. I ran a simple test and it appears to work properly.

Details, including a link to the KiCAD PCB design files is here.

Video or it didn't happen...

Note: I wired it without the power in/out connector because they have been back-ordered for 4 months (so far).

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Servomotor disaster
May 09, 2022 01:25AM
Bear in mind that the current that can be absorbed by the 33 ohm resistor will be limited. For example, if you use a 24V supply then it's only about 700mA.

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Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Servomotor disaster
May 09, 2022 10:06AM
The diode is normally forward biased which holds the transistor off. When there's a voltage spike at the motor that exceeds the supply voltage (24V in this case), the diode gets reverse biased and that turns on the transistor. The current through the 33 Ohm resistor will depend on the magnitude of the voltage at the output, not the input supply voltage. If the spike goes to 80V, there will be about 2.4A peak through the resistor until the output voltage drops to about 23.5V, which forward biases the diode and turns off the transistor.

I have no idea how to estimate the magnitude of the spike at the output when the motor is stopped suddenly. I guess you could calculate the total kinetic energy of the moving parts and equate that to electrical energy to get an estimate. My simple test in the video shows a 7V spike (31V or so) but that's captured by my DMM at who knows what sampling rate. An O-scope might be a better way to see what is going on.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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