Force Sensing Resistors for Automatic Bed Leveling

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Force Sensing Resistors for Automatic Bed Leveling

Postby Jason Fisher » Sat Mar 01, 2014 7:36 am

Johann (Kossel) has successfully employed force sensing resistors under the bed, and they are relatively inexpensive at ~$8*3 .. a steal actually, if you consider the amount of time you've spent leveling beds. :)

Follow the original thread here:
https://groups.google.com/forum/#!topic ... fxnM20nYKc

Johann's initial announcement, with videos/details/Marlin source:

Instead of the retractable Z probe, now using Force Sensing Resistors
(FSR) sandwiched between the glass print surface and the triangle
frame, wired up to the unused heated bed thermistor input.

When the hotend pushes down on the print surface, their resistance
changes quickly from 500 kΩ to 50 Ω, depending on the force. The raw
ADC value on the Arduino changes from 1023 to under 200, and the bed
temperature display in Repetier-Host goes up from 0.0 to 240 Celsius.

This allows calibrating the print surface with 0.03 mm repeatability,
which is good enough for printing PLA on cold naked glass. This
prototype was created in one sleepless night. I think we can improve
the repeatability to better than 1 microstep using Newton's method.

At this point my retractable Z probe is completely obsolete, because
the manual calibration of the probe's X,Y,Z offset was way too
complicated, compared to touching the print surface with the actual
hotend.

Thanks to Steve Graber and others for showing that the hotend can be
used as the autolevel probe. The difference in my FSR design is that
the hotend mount is completely solid (no hinges and microswitches).

YouTube video: http://youtu.be/HcnhCNh2Ln4
Documentation: http://reprap.org/wiki/FSR
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

Cheers,
Johann
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Re: Force Sensing Resistors for Automatic Bed Leveling

Postby Nicholas Seward » Sat Mar 01, 2014 8:02 am

I love this idea. You could also use this to make a rough touch sensor. You can put a pause/resume button right on the bed. Using the ADC for the thermistor is pretty smart.

With the bolt through connection I have, it will be hard to do this. We need more of a floating plate. Thoughts?
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Re: Force Sensing Resistors for Automatic Bed Leveling

Postby NeoTheFox » Sat Mar 01, 2014 11:19 am

Nicholas Seward wrote:I love this idea. You could also use this to make a rough touch sensor. You can put a pause/resume button right on the bed. Using the ADC for the thermistor is pretty smart.

With the bolt through connection I have, it will be hard to do this. We need more of a floating plate. Thoughts?


You have already made something similar with lisa. I suggest longer screws to be inserted from the down side? with springs and terminal nuts:
Image
(As horrible as I could)

This way it would be easy to adjust the bed, and if basalt bed have a good center of mass it should be able to push springs evenly.
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Re: Force Sensing Resistors for Automatic Bed Leveling

Postby Jason Fisher » Sat Mar 01, 2014 5:52 pm

I think with the sandwich hub design that GUS uses, you might be able to work a FSR into a slightly compressible (sprung?) area of "HUB CENTER", so when the hot end hits the bed, the top arm acts as the compression backplane. This would let you get away with a single FSR, and keeps it away from the heated bed.

https://github.com/NicholasSeward/Conce ... CENTER.stl

I really like the idea of having it on the bed also though. I feel like a combination of the two would allow you to resume a print after power-failure, after printing one of something successfully. (find "TDC" of the print on the platform, compare with bed FSR scale vs projected bed FSR using a generated table of extruded volume vs resistance)
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Re: Force Sensing Resistors for Automatic Bed Leveling

Postby Jason Fisher » Sat Mar 01, 2014 6:13 pm

I could see an automated calibration process that does something like:

- find, calibrate the bed
- read resistance
- print ~5 layer height 10mm squares in a pattern around the bed (or maybe a single large geometric shape like a pentagon or venn diagram)
- read resistance
- allow time to cool
- read resistance
- find/calibrate against the test part

You would have a delta for evaporation loss for that material/shape, approximate printed layer height and a correction table for print geometries. You might need to repeat/average this process over a few runs.
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Re: Force Sensing Resistors for Automatic Bed Leveling

Postby Ralphxyz » Wed Mar 19, 2014 3:37 pm

Doesn't anyone see a problem with using the extruder nozzle as your probe?

What if you use hair spray or ABS glue, you will be gunking up your nozzle.

Then there is the chance you could deform you brass nozzle.
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Re: Force Sensing Resistors for Automatic Bed Leveling

Postby Nicholas Seward » Wed Mar 19, 2014 4:29 pm

Some nozzles are beefier than others. I would consider having a mic and just tapping on the glass with a Ubis nozzle. There is a large flat area around the nozzle. Others you could just look at wrong and the nozzle will get deformed.
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Re: Force Sensing Resistors for Automatic Bed Leveling

Postby mateusz.p » Thu Mar 20, 2014 6:12 pm

Hello,
Maybe it was already mentioned before in community before, but we could use IR sensors:
http://www.pololu.com/product/1132/pictures
based on Image
It can detect when sensor went into its known range. Also when it left minimum range.
That can give not only bed leveling details but also Z calibration (on cartesian printers).
On delta printers it can calibrate all towers when using different spots on build platform to detect min/max of the IR sensor.

There are different range sensors that could be accommodated.

Sensor would be mounted on hotend - carriage
Mat
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Re: Force Sensing Resistors for Automatic Bed Leveling

Postby Guizmo » Thu Mar 20, 2014 9:04 pm

I suggested that same Sharp sensor on the Reprap forum and they said: no :oops: . I do believe it can be used.
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Re: Force Sensing Resistors for Automatic Bed Leveling

Postby Nicholas Seward » Fri Mar 21, 2014 2:03 pm

@Guizmo: It would be a good but we do have an small offset problem. Nothing blocking.
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