Belt Slip detection/bit break detection

Would it be possible to use current detection on the motors, and possibly the spindle, to detect sudden drops/spikes in current. My thought being that as the motor binds up and releases like in an instance of hitting a pocket of harder material, and a built slipping. The current should reflect that to some extent. The hope being that the carve could be paused, possibly stopping the spindle, and reducing the risk of completely ruined material/projects. I would think this could be done in line with the current system in place, but am not sure what sort of internal wiring would need to be altered to allow for the controller to read the current going to the motors.

my understanding is that this is not possible you need to have at a minimum digital servo’s on your machine

You’d need to add current shunt resistors for each stepper driver on the VDC input, a way to measure the voltage of those resistors then using some sort of ADC to get a digital value and then add in some monitoring code to detect a change and then be able to tap into the feed hold of GRBL to do a stop if such an event occurs.

  1. The arduino with GRBL is full so it’d need an additional chip
  2. The sudden current change might be a real change and so it’d be a false positive
  3. How much of a change is enough to detect such events?
  4. I don’t know if a belt slip/lost steps would necessarily be detectable and once you’ve lost steps, you can’t just regain the steps and the material would have already been cut.

It’s not impossible to do but is it worth the effort? Good maintenance, reasonable and achievable feed rates and depths should be enough to avoid bit breaks and belt slips for most of the time.

@JustinBusby That is a lot of good information, and valid points. I think the main benefit would be peace of mind. My thoughts on potential false positives would largely depend on the predictability of the current going to the motors. If a command to move slower or faster were to trigger, then the expected voltage/current would not trigger, but even in a worse case scenario a false positive would only pause the machine. As you said it cant undo the fouled material though it may help prevent some mistakes from a built slip as that usually doesn’t show up until the bit hits the opposite side of the cut.Granted that it is a lot of additional work for what could be avoided in most cases. I was mostly curious as to if anyone else had thought in that direction!

What you want is positional encoding — it’s a feature on some CNC machines and was used to good effect on the Lobo CNC.