encoderless dc motor performance

Hi,

Up to now I use my own electronics and mach3 cnc software for my 3d printer. Everything is nice. I want to try a dc geared extruder motor instead of my stepper. My question is;

Is it possible to use dc motor without encoder? If so how is the performance? What is the generic usage with an encoder or without an encoder?

Regards,
Nuri Erginer
[3dprint.gnexlab.com]

Nuri,
I have thought about switching the extruder to a geared, brushed DC motor myself. The main concern is that there must be a way to control exactly how much plastic comes out in a given time period. With the stepper motors, that is steps per second times gearing times cross section of the filament (plus second order effects). The important thing is that for fixed filament size and gearing, it is a constant time steps per second. Or, you get so many cubic mm of plastic per step pulse.

Since a brushed motor spins at a variable rate depending on the voltage (current) and load, this is not a fixed constant. The power supply is going to fluctuate a little, the filament not melt exactly, and so even with PWM, milliseconds of full power will not translate back to cubic mm of extruded plastic. An encoder of some kind in the gear chain puts you back in the stepper type of calculating, but with the advantages DC motor torque at low speed, or power at higher speeds. I have also wondered if using a pressure sensor right near the tip would be a useful feedback mechanism. Knowing the temperature from the heater thermocouple, and the static pressure inside the melt zone you could predict the rate at which plastic will flow out. Stop the motor, pressure drops to zero, and no more plastic oozes out. Crank up the speed on the motor, pressure climbs, and more cubic mm of plastic come streaming out. But this is only a guess. I still don't have my first working extruder yet.

Mike

The makerbot (popular rep-rap derivative) currently runs off of a geared DC motor without an encoder, so it is definitely possible, as long as the gearing is high enough. Basically, with the makerbot, you always run the motor at a constant voltage/PWM. This causes the speed to be constant enough that the filament generally comes out at a constant rate and a constant amount. Then, to change the thickness of a line of extrusion, the feed speed of the X, Y, and Z axis change.

This works... mostly. It has major issues, and therefore having stepper extruders on the makerbot are becoming more common. I think that in future version of the makerbot, the team is going to put an encoder on the motor output or something.

I started with a DC motor and a shaft encoder and that definitely gives better results than an open loop DC motor, but a geared stepper is much better because it can reverse at high speed and instantaneously. I.e. it can be going forward at normal pace and within one step be going backwards 10 times faster. A DC gear motor cannot do that because the motor is spinning at 1000's of RPM.

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[www.hydraraptor.blogspot.com]

Nophead
I know that you are using a worm gear stepper motor and reverse it in order to prevent ooze. I also know that your cnc did not use g-code. Does it work well? I mean reversing the motor in order to stop extruder immediately. Second question, Is it possible to do that with skeinforge or do I need to write my own script.

Regards,
Nuri Erginer

If you have sophisticated control electronics, it is possible to measure the back-EMF of the motor to determine its speed, which lets you get away without having sensors. But I think that technology might be somewhat out of our reach at the moment. At least, I don't know where to buy any off-the-shelf motor controllers with that capability - I just know of the technique.

Hi

I have a repstrap with stepper extruder. I am using skeinforge to generate 5D code which is activated by turning on the Dimension module. This module includes control over retract distance, retract speed and restart extra distance if you should need it.

Details here:-

[www.bitsfrombytes.com]

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Regards

Ian

nerginer,
Yes, reversing the extruder stops the filament and gets rid of strings between the parts.

[hydraraptor.blogspot.com]

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[www.hydraraptor.blogspot.com]