FSR

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FSR

Release status: working

Fsr.jpeg
Description
Force Sensing Resistors
License
GPL
Author
Contributors
Based-on
Categories
autolevel
CAD Models
External Link


Instead of the retractable Z probe, now using Force Sensing Resistors (FSR) sandwiched between the glass print surface and the triangle frame. We use three FSR wired in parallel, connected to the Z_MIN endstop pin. This allows calibrating the print surface with 0.02 mm repeatability, which is good enough for printing PLA on cold naked glass.

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When the hotend pushes down on the print surface, the FSR changes quickly from 5MΩ to <50Ω, depending on the force. 3 Newton (300g weight) is sufficient to trigger the Z_MIN endstop digital input pin, because it's using a 20kΩ internal pull-up resistor inside the Arduino or similar microcontroller.

Johann's first prototype used the heated bed thermistor (analog) input pin, but that's actually not necessary. The endstop (digital) input pin works just fine, without any additional parts.

Configuration

Because Force Sensing Resistors are normally open (NO) make sure that you have the following in Configuration.h:

const bool Z_MIN_ENDSTOP_INVERTING = true; // set to true to invert the logic of the endstop.

Testing

When none of your endstops are touched, the result of M119 should look like this:

SENDING:M119
PC>Reporting endstop status
PC>x_max: open
PC>y_max: open
PC>z_min: open
PC>z_max: open

When you put a weight of about 300g (e.g. a roll of blue tape) on the print surface, it should look like this:

SENDING:M119
PC>Reporting endstop status
PC>x_max: open
PC>y_max: open
PC>z_min: TRIGGERED
PC>z_max: open

Links

FSR product details: http://www.phidgets.com/products.php?product_id=3104

Source code: https://github.com/jcrocholl/Marlin/tree/fsr

Project history: http://deltabot.tumblr.com