The main concern with these is sensitivity for small pull forces, and speed (sampling rate)Īnother option is a load cell with a circuit board with Serial or SPI out, or Bluetooth out. Some such scales have Bluetooth so you could just get data and be done with it, but I am still looking for cheaper ones in the low weight range. Then as before, another sensor could be looking at the thickness for sake of absolute mm control. So the pulling wheel assembly is on a guide shaft, etc, and can glide towards the extruder or away as the pull force changes, and we measure this displacement to derive the tension. Ok how about this, a pull sensor more akin to a digital fish scale, which tugs on the line length-wise? Then we are measuring the pull from the pulling wheel itself, but this wheel rides in a floating carriage on a shaft, and the pulling force stretches or compresses a string or moves a weight hung over a pulley, etc. The spool motor ran a PID routine with the potentiometer reading as the input, and ran at whatever speed was necessary to maintain a particular amount of tension. To keep the spool in sync, I ran the filament over a spring loaded arm which used a potentiometer shaft as its axle. Originally I was experimenting with using a separate puller to take up the dropped loop before I realized I could just make the spool do it. The tension control is useful for the other side of the puller, because the spool needs to stay in sync and adjust speed as the circumference of the spooled filament changes. When it increases, it doesn't reduce tension, it just bunches up, moving the excess plastic sideways, increasing diameter. So the tension at the puller doesn't change as the amount of plastic increases or decreases.
On top of this, there is the fact that while one end is rigid and resists bending, the other end is melted and offers almost no resistance to stretching at all. To get a meaningful tension reading, the outer wheels would need to be several inches apart before the middle wheel would be able to displace it. If you have a 12" straight piece and hold one end, the other end will only droop a little. The cooled filament is rigid, so it is more like a thin, flexible rod. Something that took me a bit to get my head around at the beginning, when thinking of filament handling ideas, is that it doesn't act like string.