Auto Bed Leveling using piezo electric discs for very high sensitivity.

Update v 0.2!

Now using a single piezo disc glued (temporarily at least) to the x-carriage (see image). With the piezo element glued between the two bearings gliding on the x-axis shafts, and tiny amount of twisting (like the hotend touching the bed) will ever so slightly bend the plastic in the x-carriage. Since the piezo is glued on top, this will bend as well creating a relatively large signal. Thus even a minuscule force acting upon the hot-end can be detected using some electronics. This solves the problem of the previous setup with differing sensitivity of the three piezo discs (due to the discs them self and/or the mechanical setup) in v 0.1. Auto leveling is now even better, giving a perfect first layer every time (as long as the nozzle has been wiped clean of plastic, that is).

New video:
https://www.youtube.com/watch?v=LWRq2t9Pr-E
https://www.youtube.com/watch?v=0oBzWWZYKgE

Still to do: Electronics is rather sensitive to EMI, so I'm waiting for a metal case for it. Right now, it can trigger when probing too close to the x-axis stepper if it's moving, so a quick fix right now has been to add a pause (in firmware) after moving to the right x-y position before starting the z movement.

Old v 0.1 info:
Mounted between the bed springs and the z-stage, any force down on the bed can be registered and used as a probe signal. Hence one can measure when the hot-end touches the bed surface. No modification to the hot-end or carriage is therefore needed, and the probe is insensitive to temperature.

This is still a work in progress, so although it is working for me in it's current form, it needs constant tuning as the circuit is not yet perfected, and the mechanical mounts have some known issues. A new circuit and new mounts have been designed but not yet tested.

So far, I've tried to only use parts available from sparkfun. I'm guessing a total cost for all parts of less than $20 USD.

Video of version 0.1 doing bed height mapping : https://www.youtube.com/watch?v=Zn9X-WGBuvQ

Discussion at soliforum:
http://www.soliforum.com/post/88147