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Best filament motion sensor? Design or buy?

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--- Quote from: mushroom on July 06, 2022, 12:32:08 pm ---You mean using some sensor that measures the filament length that goes througth the device after straightening it between two pieces of tubing ?

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one piece of tube is enough..

--- Quote from: mushroom on July 06, 2022, 12:32:08 pm ---The BTT Smart Filament Sensor works this way. It is a very old design (mouse wheels, etc.). Recommended settings (from BTT's user manual): FILAMENT_RUNOUT_DISTANCE_MM 7 ; 7mm is 16.84 mm3 for a 1.75mm filament ; or a 0.4 x 0.2 x 210.5mm printed line.

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looking for BTT sensor keyword, yes thats what i'm talking about. now i dont have to reinvent the wheel.. it should work, if it isnt, i believe it can be fixed. ymmv.

It "should" but it doesn't. This thing detects failed prints. Not prints that are about to fail. Except if you cut the filament.

It is a *VERY OLD* design, the Internet is full of variations around this idea. They just payed some youtubers, created the hype, and make money. The filament sensor is probably the very first accessory a beginner will add to his brand new printer. We all did it.

An example from 2016 (6 years old, just one among many others) :

Remember that the OP was asking about spool tangle detection. Not about detecting a filament that someone had the brilliant idea to cut using pliers. A solution that can save he's print. Not some gadget that phushes a button saying "you're f*cked").

Again, do the math. The head will move 210mm (1.75 filament 0.4mm line width, 0.2mm layer) until it reacts, according to BTT recommendations (triggers after 7mm). Too late. It is a failed print. Wasted money, wasted plastic, wasted time.

[EDIT] DOES MARLIN TAKES THIS DISTANCE INTO ACCOUNT BEFORE IT RESUMES ? I doubt it can do it as it would require to rewind the gcodes...

Have a look to the 3D printers these youtubers have all around, watching their next videos. Do you see this device on their machines ? No. Why ? Because it is useless. They took the money and throwed this crap away. Otherwise all their machines would be equiped. Some are 3D printing for a living. And you will never see these sensors demonstrated in real world situation. Just the cutting pliers trick.

I'd love to hear Thunderf00t talk about this device. I doubt he's interested in such obvious debunking.

The only real well engineered device would be something that compares the length that enters the extruder against what goes out, and is fully integrated. Extruding and retracting. Maybe it exist in the industry world...

Look at this video. I'm sure the author is trying not to laught while mounting the sensor the BTT way :


--- Quote from: mushroom

The answer :

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--- Quote from: mushroom on July 05, 2022, 10:48:39 pm ---I'm sorry, I don't see what you mean. Could you post some drawing ?

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Here is what I'm going to try:

Swinging arm will need to be tensioned down lightly with a spring or heavily weighted, when filament runs out it drops. This is the same as your design.
Normally the tension on the spool is very low, and freely turns. So if any resistance is encountered it should bump the arm up, and activate the adjustable upper switch.

Since the spool size changes, the upper switch may not work reliably, and be falsely triggered easily.
But worst case I get a working runout sensor, which I don't have yet.

So I understood what you were meaning !
It wouldn't work on mine because it's a bed slinger.
Yours is a CoreXY, isn't it ?
Definitely worth testing on a CoreXY (or similar).

Maybe you could make the design even simpler using one switch and a cam.

On mine, when moving up (Z axis), the arm is also moving up (I had to find the sweet spot for its axis and the max angle as it influences the force that pulls the rocker arm). The arm is also moving up and down when the filament is near the end : it unrolls like an helix.


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