Tobbens Adding Inductive Proximity Sensor Log

Introduction

This page documents adding a Research_On_Inductive_Proximity_Sensors#LJ12A3-4-Z.2FBX LJ12A3-4-Z/BX inductive proximity sensor to a Folgertech 2020 Prusa i3. Printer assembly up until this point is documented in Tobbens_Folgertech_Kit_Unboxing_Log and Tobbens_Folgertech_Kit_Assembly_Log. Not documented there is only the addition of an aluminum plate on top of the heat bed, like shown in the pictures below:

Note that the sensor also triggers on the heat bed, so I'm not sure the top aluminum plate is necessary for bed probing yet. Will be investigated closer...

Theory

The sensor if of NPN type and normally open. It is not drivable with 5 V, so we will feed it with 12 V. We expect the following two states if we wire it in directly:

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The Z_min pin is part of the RAMPS' logical circuitry and will be damaged if it is exposed to 12 V, so we need to protect it. For that, we use a diode and internal Z_min pullup:

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The diode stops current to flowing into the Z_min pin with harmful 12 V pressure, but allows the pullup voltage of 5V to flow down to ground.

Implementation

I used an IN4004 MIC. Any diode would work .

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  Click to magnify. The black wire is connected to the positive Z_min pin through a diode.

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12 V directly from power supply. I did a quick and dirty solder directly to 12 V connectors, just to test if it works.

Updated Marlin in repo to auto bed compensate.

Usage

The printer will do the probing procedure upon receiving the G29 command. Bed compensation will be performed on every normal G1 or G0 move after that.

Posted video of homing followed by a nine-point probing procedure:

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Here's another video showing probing directly on the MK2A heat bed: