Sunday, January 28, 2007


Firmware for the Mk 2.1

I was able to code the basic control for the gearmotor on the 2.1 after breakfast with only one or two hilarious mishaps relating to specifying the wrong I/O channels. Right now I'm checking out the PC/controller board/Mk 2.1 interface, specifically the gearmotor control.

The Mk 2.1 has a number of changes in how it is operated. Probably the most important is the use of a channel on a 754410 dual Darlington chip rather than a transistor to run the gearmotor. What that means is that I can feed filament into the heated extruder barrel AND pull it back out. I've just tested that feature and it works properly.

I'm thinking that bidirectional control of the filament will make extracting tag ends of filament out of the Mk 2.1 easier and that it will let me avoid the dribbling that Vik has reported with CAPA by backing the filament up slightly when the extruder barrel is hot but we are not extruding.

The previously blogged low thermal inertia extruder barrel also makes heating response of the filament much more lively. I am also controlling the temperature of the melting filament indirectly rather than using a thermistor as per present practice. I'm controlling power input to the nichrome 80 on the extruder barrel as a function of filament feed speed rather than using a thermostat function.

Using the slow pluse "pseudo stepping" rather than PWM motor control also lets me maintain full torque down to zero rpm. We shouldn't be seeing stalling absent a complete extruder barrel jam with the Mk 2.1.

Thats very interesting. I love the backing up to eliminate dripping. I had always wondered if I might need some sort of pressure sensor to control the motor rotation. Is there a maximum extrusion rate based on the size of the orifice? I've seen some figures before on the extrusion rate and wondered if that was maximum or optimal in some way.
There are no optimals that we know of right now. That awaits somebody having a working Mk II and doing some careful tests.
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