Fighting with computers

Computers are not always friendly.

Saturday, May 06, 2017

Spark on a Printrbot Simple Metal

One dry winter day I noticed a spark jumped from my finger when toughing the metal plate of a Printrbot Simple metal printer. Next time I tried to use that printer the hot-end would ram into the print bed so I had to stop the printer immediately. I remembered the spark incident and I assumed damage was done due to that static discharge.

On to find a solution. Printrbot forums showed several entries about damaged sensors and how to replace them. Others had to replace both the inductive proximity sensor plus the Printrboad control electronics too. But one thing I noticed is that my inductive sensor was apparently working, as the LED will lit when approaching a metal part to it. After testing with M119 command, I realized that as odd as it might seem, now my printer thought the signal on Z-MIN end-stop was reversed. So it was triggered when the LED was off and open when the LED was lit. That was pretty odd but as the sensor seemed to be working I set my path to modify the firmware configuration.

Once I found a suitable source code of the firmware I just changed the polarity of the ZMIN input signal and my printer was able to print again. I could not really explain why that was happening but somehow it was working so it had to be a reason for that. Anyway, the joy was short-lived and two days later the hot-end will ram into the bed at the beginning of a printing, again!

Testing the sensor again shown LED was still working as expected, but this time the M119 command will always show the sensor as opened no matter the status of the LED. After taking out the sensor output signal I would not measure any voltage change on it when detecting metal. And the ZMIN input pin was now stuck to ground so it could not longer be used as an input.

My guess is that the electrostatic damage would eventually caused the pin structure to go through a transient period of inverted polarity but eventually that pin would end up stuck to ground. And that latter phase most likely killed the sensor output driver so sensor was now busted.

Repairing all that would require a new sensor and setting the firmware to use a new input pin (or replacing the printrboard entirely).

I bought a new inductive sensor on eBay and replaced it but this time I tried if the sensor would work powered at 5V instead of the 12V it was using before, which it did. This way the sensor output could be attached directly to any free input pin of the processor. I chose PE5 signal available in ETX1 connector as the new input for ZMIN in the firmware and after uploading the new version the printer is up and running again.

I would need to be more careful in the future touching ground before touching the printer :-)

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