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3D Mendel FSR implementation
This new way of measuring the Z axis is great for providing perfect prints every time but there are a few things you need to keep in mind. First at the moment the base Marlin firmware although configurable does not really work right. Use https://github.com/jcrocholl/Marlin/tree/fsr at least thats what got it working for me. I used 5 of FORCE SENSING RESISTOR,0.5 INCH ,CIRCLE,1oz-22LB,FLEXIBLE from Jameco.
Another thing I learned is you do need to secure them with two sided tape (You know the stuff white and kind of thick), it’s not just to secure the resistors but also it helps to distribute the force, trust me on this you need the tape!
Now I used a pretty simple way of wiring it all up. I just took 5 FSR sensors and put them in parallel and wired them all up through an old contact switch sensor I had from an earlier printer build. That way I had a little sensor board with an led to work with. I did try during my testing to wire the thing right to the Ramps 1.4 Z switch plugs as well that that worked too. The resistor pretty much works as a simple switch, off with no pressure and no real resistance when pressure applied.
Last but not least keep in mind that you have to update your slicer to put the new scripting in pre-Gcode, you need this or the slicer default firmware pretty much overrides all the work you did. Here is the Code,
G28 X0 Y0 ;home X and Y
G29 ;probe bed
G90; set absolute coordinates
G92 E0; reset extruder distance
G1 Z5 F300 ;move platform down 5mm
G1 X145 Y145 F3000 ; move to back right corner