Rethinking the Mk II

  • plaasjaapie

    plaasjaapie - 2006-10-20

    I've been idly wondering if you could use one of the channels on a 754410 chip to run the heater on a Mk II instead of those TIP chips.  You could use bang bang instead of PWM and then save the PWM for the little motor that runs the threaded screw rod that pumps the filament into the extruder.

    Mind, you could also run that gearmotor as a pseudostepper motor just as easily and avoid loss of torque that you get at low voltages.

    • Simon McAuliffe

      Simon McAuliffe - 2006-10-20

      The current extruder design uses PWM for both the motor and the heater.

      The TIP transistors (or similar) are used for the heater element because they drive a good current and only need to turn on and off.  The 754410 H-bridge is used to allow the motor to drive both forwards and backwards, although Vik uses a simplification where he uses just a transistor for driving the motor too.  It only goes forwards in that case, but that's mostly what you want to do.

      You generally use an H-bridge like the 754410 only where you need both a positive and negative voltage, and for a heater element you don't.  So it's more practical to use just a transistor for that.

      • plaasjaapie

        plaasjaapie - 2006-10-20

        I understand why under ordinary circumstances you'd use a transistor.  I was just wondering if there was any real obstacle to using a spare channel of a 75410 for the same purpose if one were available.  :-)

        • Simon McAuliffe

          Simon McAuliffe - 2006-10-22

          Oh, I see -- that's a good idea.  That would be a way of reducing the part count a bit.  It will only work if the heater element is fairly low current.  Mine is 3A IIRC, so it probably wouldn't work, but if somebody had a lower power heater it might be fine.

          On the other hand, a power transistor is a very cheap part and it gives the flexibility to use grunty heaters.

    • plaasjaapie

      plaasjaapie - 2006-10-22

      Vik details how the extruder barrel is constructed.

      ..3 or 4 layers of paint on the bolt, put down the wire, 2 more layers of paint (if you're using glass thermistors, put them on under some of the paint). Then I take a mixture of Plaster of Paris and fibreglass insulation, and smooth that down over the whole lot, ensuring that I can still put the removable nozzle on. When it's dry enough I tidy it up with a rasp and craft knife. This provides good electrical and thermal insulation.

    • plaasjaapie

      plaasjaapie - 2006-11-03

      I've been wondering if you could skip having a thermistor on the Mk II if you programmed a performance envelope into the PIC and used a shaft encoder to keep track of the rotational rate of the polymer pump.

    • plaasjaapie

      plaasjaapie - 2006-11-07

      I was idly thinking of using a pseudostepper motor with the Mk II instead of a PWM motor.  That got me to thinking about the Mk II in a pointillistic mode rather than using it to create a continuous thread of polymer.  You proceed to a spot and then put down a tiny button of polymer before proceeding to the next spot.

      Using a GM3 gear motor with a 5 msec step one could put down 0.04 mm^3 spots of polymer with each step on the Mk II.  Hmmm....


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