Feeding force measurement for bowden feeder

Hi!
While the positioning precision of my new um2 seems to be way oversized for normal 3d-printing, one aspect where it looks like they had to put lots of black magic inside to make it work seems to be the hysteresis of the bowden feeder. Since i started to develop my own dual feeder printer, i thought it might be a clever idea to measure the force that pushes the bowden tube (up) away from the print head (e.g. with a pin pushing on a force-sensitive resistor). The printer could then make a calibration print where clearance under the nozzle, head speed and feed rate are held constant for a while (to stabilize the pressure inside the nozzle) and then read the force sensor to build a calibration table f(measured force,head speed, clearance situation under the nozzle tip) -> mm³/s for the current material and temperature setup. This table could then be used to control the feeder stepper instead of the voodoo stuff, unfortunately the gcode might need to be extended as well to communicate overhangs and bridges :-/

Anyway, the above looks like tons of work so my question is if anybody maybe already implemented this obvious idea and if yes what are the results and findings.

Cheers
Claus

Interesting idea! Another approach would be to use a geared DC motor for the extruder drive instead of a stepper motor, with an optical rotary encoder to provide feedback, and measure the motor current to determine the force being provided.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I agree with DC42 about that....well done

---

Graduated from Soran University with First Class Degree with Honours in Computer Science.

... I've developed some force sensing applications with strain gauges - most of them were based on bending/twisting elements.

For measuring the entry force into the bowden feeder you can insert a (stiff) bending element between the motor and feeder and measure its sub-micrometer distortion ...

---

Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

I think I prefer to measure at the print head side of the bowden to avoid errors induced by friction inside the bowden tube.

Quote
dc42
Interesting idea! Another approach would be to use a geared DC motor for the extruder drive instead of a stepper motor, with an optical rotary encoder to provide feedback, and measure the motor current to determine the force being provided.

You want to use a dc motor to do the work of a stepper? Good luck with stopping the motor precisely. Even if you had a 3600 steps/rev Encoder, it produces more Problems, than it solves.

Better use a current shunt on the 12V line of the stepper driver to measure torque?
Olaf

Quote
o_lampe

Quote
dc42
Interesting idea! Another approach would be to use a geared DC motor for the extruder drive instead of a stepper motor, with an optical rotary encoder to provide feedback, and measure the motor current to determine the force being provided.

You want to use a dc motor to do the work of a stepper? Good luck with stopping the motor precisely. Even if you had a 3600 steps/rev Encoder, it produces more Problems, than it solves.

Better use a current shunt on the 12V line of the stepper driver to measure torque?
Olaf

The stepper motor current does not give an indication of torque, although the voltage might.

There are already people using DC motors with encoders in place of stepper motors. Stopping the motor accurately is not a problem, using a dedicated 8 bit microcontroller to control each motor or a pair of motors. There would of course be quite a high gear ratio. Another advantage of using a DC motor for the extruder drive is that it can be much lighter than a stepper motor for the same torque, making a non-Bowden extruder more practical.

Edited 1 time(s). Last edit at 05/10/2015 05:52AM by dc42.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
o_lampe

Quote
dc42
Interesting idea! Another approach would be to use a geared DC motor for the extruder drive instead of a stepper motor, with an optical rotary encoder to provide feedback, and measure the motor current to determine the force being provided.

You want to use a dc motor to do the work of a stepper? Good luck with stopping the motor precisely. Even if you had a 3600 steps/rev Encoder, it produces more Problems, than it solves.

Better use a current shunt on the 12V line of the stepper driver to measure torque?
Olaf

well....

not to sound like an expert here... for an extruder, you don't need to have precision. You can reverse, stop a dc motor especially if it's not really going that fast in the first place (which extruders aren't). I've built a testing stage with arduino code to operate a DC motor with an optical linear encoder strip from an inkjet printer. It very accurately goes from point A to B in a hurry, and can change direction easily. So, it's not impossible. It goes against the commonly accepted wisdom and most people think it is just harder than its worth in contrast to easy to use steppers.

Also, keep in mind that commercial CNC machines and robots dont use stepper motors, the use DC servos.

Cheers,
Mark

Edited 1 time(s). Last edit at 05/15/2015 04:59PM by thetazzbot.

I like the idea of dc motors or dc servos, but aren't they prone to electrical noise glitches? I know that you can protect them, but can you totally eliminate the possibilities with current boards?

Edited 1 time(s). Last edit at 05/16/2015 07:14PM by ggherbaz.