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current controlled DC motor by e-steps?

Posted by o_lampe 
current controlled DC motor by e-steps?
January 25, 2018 05:57AM
Short story: could I use the extruder step signals to PWM-drive a dc motor? ( Mosfet driven of course )

Long story:
I made a few assumptions that led me to this question.
1. Slic3r is able to vary printspeed based on constant extruder feed rate
2. A current controlled DC motor would produce a steady amount of chamber pressure ( amount of extrusion= const. )
3. extruder temp and filament diameter are stable

The problem with dc driven extruders was the encoder. A closed loop control circuit was necessary.

A torque curve of a current controlled motor always looks the same: upto 50% of the no_load RPM the torque is constant, then it falls off more or less steep.
If we could find the sweet spot where the torque is just right to feed a steady amount of filament and the slicer is able to control printspeed accordingly, we wouldn't need an encoder anymore.
Acceleration and decceleration are reflected by e-steps, so the e-steps is the parameter that controls the dc-motors current/torque.
I know, that RRF is able to produce e-steps with a defined length, would that be enough to drive a MosFet the way we need?

Edited 1 time(s). Last edit at 01/25/2018 05:59AM by o_lampe.
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