Accelerometer Z probe

crc1337 · 7 Oct 2016, 00:02

Does anyone know if it's possible to use an accelerometer for z-probing? I was thinking something like the one SeeMeCNC uses on the Rostock MAX v3:

https://www.seemecnc.com/collections/parts-accessories/products/hotend-accelerometer-probe-pcb-board-rev-5c

zzing · 7 Oct 2016, 03:45

The only use I could see this thing being would detecting when you crash your hot end into the bed. I have considered the use of an IMU as a kind of feedback system. But more likely would be useful as a kind of fail detector (you think it is moving, is it really moving?)

T3P3Tony · 7 Oct 2016, 07:21

@cchaffee I looked at the SeeMeCNC design and support could be added (the i2c commands to setup the accelerometer) the trigger is a changing level on the interrupt pin. What would be interesting to find out is how much force the bed impact was in triggering the probe, might it damage expensive PEI build surfaces?

dc42 · 7 Oct 2016, 07:49

One thing g I don't like about the SeeMeCNC design is that it uses I2C for communications between the hot end and the electronics. I2C is very prone to interference because it uses passive pull up. It was only ever intended for connecting devices on the same PCB. Apparently you can't use the SeeMeCNC probe with the heater on and I am told that this is because of interference. It would be better to include a microcontroller on the accelerometer board to produce a robust output signal.

crc1337 · 17 Oct 2016, 01:39

I happen to have one of the SeeMeCNC probes on-hand and have been able to interface with the accelerometer using an ATtiny85 micro. It's possible to adjust the amount of force required to trip the probe, but by default it seems fairly sensitive. I don't have experience with anything other than a glass build surface, but it certainly seems like this could work for others.

If I get some free time I may try to hack this arrangement onto my hotend and give it a try. Any suggestions on how to best interface with the Duet? Right now I've got the attiny setting an output high when the sensor is tripped, the output is held high for an arbitrary period of time (currently 100ms) and then reset. I had also though of using the Deploy Probe feature to arm/disarm the sensor, although I haven't looked into how that works yet or if it'd even be necessary.

jmjcoke · 17 Oct 2016, 02:01

So are you using the ATtiny85 just for the I2C data that sets the sensitivity and controls the output duration and the output is being produced to the blue wire from the accelo board?

crc1337 · 17 Oct 2016, 02:54

That is correct. Of course, this is just for testing. If it works I may design a custom board with just the sensor and micro. The SeeMeCNC board has a bunch of stuff that is, for me, unnecessary and its footprint isn't compatible with my current setup. I do like the idea though, and I'm curious to see how well it works.

jmjcoke · 17 Oct 2016, 13:57

I want to get this working with duet wifi, right now I am using the rambo original. I guess the other option is to wait until the 32 bit rambo comes out.

peridot · 17 Oct 2016, 22:54

Would it be feasible - this is a firmware question I guess - to respond to probe triggers when not probing? With the accelerometer set right you might be able to detect head crashes and stop the print. Obviously this doesn't make sense for all sensor designs.

dc42 · 18 Oct 2016, 06:59

@peridot:

Would it be feasible - this is a firmware question I guess - to respond to probe triggers when not probing? With the accelerometer set right you might be able to detect head crashes and stop the print. Obviously this doesn't make sense for all sensor designs.

Yes, if the acceleration in fast travel moves can be distinguished well enough from the abnormal situations you wan to detect.