learning the ropes

things I made at ITP and after: sketches, prototypes, and other documentation

Sunday, October 6, 2013

Troubleshooting Pololu A4988 Failure

Pololu A4988 Troubleshooting After receiving my replacement A4988 driver boards, I wanted to verify that the driver board I took out of my printer had actually failed.  After wiring up the minimum required connections, I measured the voltage at 1A, 1B, 2A, and 2B on the suspected driver board.  VMOT is 16.6V (from a spare Linksys 12V power supply)

Regardless of the voltage applied to the STEP pin,

V1B = 12.96V

V1A = 0.75V

V2B = 11.13V

V2A = 12.97V

A forum post at Pololu’s site indicates that these outputs should be stepping in response to pulses on the STEP pin.

Let’s try the new board.

V1B = alternates between 0V and 16.65V on successive pulses of the STEP pin.

V1A = alternates between 0V and 16.65V  (opposite of V1B)

V2B =  alternates between 0V and 16.65V on successive pulses of the STEP pin.

V2A = alternates between 0V and 16.65V  (opposite of V2B)

This seems to confirm that the previous stepper board that was driving the extruder motor is not functioning normally.

This does not give me further confirmation that plugging the new driver board into RAMPS is going to work normally though — and it doesn’t confirm that there hasn’t been any damage to the RAMPS board. I don’t really want to sacrifice a A4988 to figure this out.  I reviewed the RAMPS 1.4 schematic; the stepper motor outputs of the Pololu board are wired directly to the stepper motor connectors on the RAMPS board.

Maybe the Makergear IRC channel is the next stop…

posted by Michael at 10:18 pm  

Wednesday, September 18, 2013

Failure Log

I have had a Makergear version of the Prusa Mendel for just over a year now. Eventually, I may get around to posting documentation of some of the things I’ve done with it… but for now I want to document the process of troubleshooting a problem I’ve had.

The original stepper plastruder that came with the Prusa Mendel has been pretty reliable.  The most consistent problem I’ve had with it is ABS filament jamming between the direct gear drive and the idler wheel.

For a while, I’ve had my eye on replacing plastruder with the more elegant and functional Birdstruder.  The advantage: a single cam-tensioned bolt to release instead of four spring-loaded hex-headed bolts any time filament gets jammed or needs to be changed.

So, over the past few days, I’ve finished printing out the Birdstruder and replaced the standard stepper plastruder. One of the nuts disappeared during dissassembly. During the initial print run, the Mendel stopped printing.  Usually when this happens, cycling power on both external power supply and USB connection to RAMPS gets communication going again. No dice.

So — troubleshooting began.

Is it a USB communication problem?

  • Checked Windows Device Manager — board didn’t show up there.
  • Tried plugging in another Arduino Mega board. Detected without a problem.

Did something happen to the power?

  • Is Arduino attached to Prusa getting power?
  • No lights seemed to be lit anywhere on the board. Watched while plugging connection in; one power LED lit up briefly and then went out.
  • Found the nut I lost — it was sitting on the Pololu stepper driver circuit board for the extruder. Cut all power, removed nut, tried to power up again. Still not working.  Note to self: another reason to put the electronics in a case.
Removing the suspicious extruder stepper board caused the Arduino Mega lights to come back on.  When the suspicious Pololu board is inserted, the Arduino lights do not come on.  I removed the Pololu board again and tried connecting to Mendel printer via Pronterface. Success. I’m able to jog each of the axes — and heat up the build platform and extruder.

I hope the damage is isolated to the stepper board, though it is possible that the nut laying on top of the board was not really the problem — and that the Birdstruder (because the idler wheel is not turning smoothly) caused the driver to overheat. I’m not sure how I would be able to test that theory with the failed board. I could contact Pololu for assistance.

posted by Michael at 6:40 pm  

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