accoustic detection Z limit switch

Hey,
I've thought about this while I was operating my printer and its hotend fan. What I heard is that the frequency of the sound of the vibrations of the fan changed when the hotend contacted the bed.
I've read about using force resistance pads to readout when the hotend touches the bed in order to do automatic leveling, but could we also use a system where accoustic power is generated on one side (vibrating motor on the carriage; or even just using the vibration of the hotend fan, IDK), propagates through the hotend, and by putting a piezo sensor under the bed, the sensor could trigger when enough vibration is transmitted, i.e. when the hotend touches, not even with a lot of downward forces.
I'll try and make it but I just wanted to get opinions.

I quite like this idea, especially if you can just use the vibration generated by the fan. I'd imagine you should be able to use a simple op amp filter circuit to pick up the vibration.

... some +20 years ago I've done this "acousto-detecting" for a pen-digitizer with a sound generating piezo in the pen and three piezos as sensors in 3 edges of the glass front of a PC-monitor.

The detecting accuracy was pretty good for a 'mechanical' setup! - could detect the position of the pen per triangulation with roughly 1mm accuracy - for longer lasting I've inserted a plastic tip into the pen.

For simply detecting the contact one sensor is enough -- but think about 3 sensors and time/position-detection ...

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

I tried this with a vibrator motor such as is used on mobile phones and pagers. Although I settled on pursuing a piezoelectric pressure detecting method, the vibration transmission method looked well worth trying. What I did find is that there were "blind spots" on the build plate where detection was poor - this may be avoided by careful positioning of the sensor and getting the frequency right.

Mike

... this "blind spots" could be resonancies or "wave-interferencies", where the reflected 'echoes' from the borders comes back with inverted amplitude and so 'kill' the signal.

To avoid this you can use much higher frequenzies or better single pulses ...

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]