Monday, March 29, 2021

Oops! A Sand Table Disaster

Progress on the smaller sand table, Arrakis, was recently brought to a standstill. There was an incident...

The mechanism was working fine so I was working on the sand box. I got it mostly assembled and discovered that I had underestimated the protrusion of the rivets holding the box corners together and as a result, I had to take the mechanism apart and reduce it's size slightly to get the sandbox to fit properly. The disassembly, modification, and reassembly went fine, and then I powered up the machine for testing. 

The machine homed as it should, so I selected a pattern file forgetting that the dimensions of the table are now smaller than the pattern. The magnet took off and when it hit the end of motion because the table was now slightly smaller, I heard a pop, saw a spark and the whole thing shut down.

I found the Duet board had let the smoke out of a voltage regulator chip. I ordered a replacement regulator chip and installed it, but the board was still dead. Apparently more than just the regulator chip blew. I decided not to expend any more effort trying to revive the board- it was toast. I ordered a replacement.

I also checked the 200W power supply. The output on indicator LED was pulsating instead of being on continuously. Voltage out read around 12V that bounced up and down a volt or so. It was supposed to be a 24V supply.  Hmmm. I ordered a replacement.

I tested both motors and they appear to be OK, as does the smaller 150W power supply that was powering the other motor.

What happened?

DC42 at Duet3D forum has what is probably the best explanation for what happened:

"My guess is that when the servo motor hit the hard stop, it first increased the current to maximum to try to overcome the resistance. When that failed, it turned the current off, at which time all the energy stored in the motor inductance was dumped into the power supply rails, causing a voltage spike that blew both the PSU and the Duet. The stepper driver chips and the fan mosfets on the Duet are rated at 30V, although if the drivers are not energised then in theory they can take 60V. The capacitors on the VIN line are rated at either 35V or 50V. Your PSU most likely had output capacitors rated at 35V."

Now what?

I ordered replacements for the Duet controller and the power supply. Since back EMF from the motors can cause a problem if you exceed their rpm specs or suddenly block the motion (apparently), I decided to add a third, small power supply exclusively for the Duet board. That way, any mishap in the motion system won't end up killing the controller board again. I'll add some high voltage capacitance to the motor power rails, too.

I will still power the LEDs from the new 200W motor power supply as they use buck converters that are able to withstand much higher voltage input so the LEDs and buck converters aren't likely to be damaged in another "incident".

Engineering Solution

yngndrw at the Duet3D forum posted this link for a circuit designed to protect the motor driver and power supply in the event of an unexpected sudden stop. I will be adding a couple of these to protect the power supplies and motor drivers.

Essentially, this circuit shunts current from the motor to ground whenever the back EMF from the motor exceeds the power supply voltage. That will protect anything sharing the power bus with the motor.

Update 4/12/21

The mechanism is back up and running with the new controller board on its own power supply. I haven't put the protection circuits together yet, but that's coming up as I gather the parts. More to come...

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