On/off for custom air paste nozzle Printrboard

[suiiie22]

I'm trying to figure out a way to (with minimal hardware) and using a printrboard change the stepper motor of the extruder in to a simple on off for a valve that will deliver air pressurised paste/ceramic.

Ive had a look in to arduino firmware settings but havent found anything useful.

Any ideas?

[RetireeJay]

The Printrboard outputs its control signal for the Extruder stepper through a stepper-motor driver chip. Unfortunately, there's no simple and clean way to modify the firmware to make that chip control a simple on/off output because the chip itself has an internal state control that is not fed back to the CPU.

So just off the top of my head, you have several choices:
(1) You could modify the firmware to turn the stepper "enable" on and off only when "stepping" - which might be fairly easy to do, since that function is already present in the firmware, just not timed the way you want it. Then you would have to tap into the "enable" signal and use it to drive your valve.
(2) You could modify the firmware to send a signal to an unused output pin on one of the expansion ports
(3) You could design a circuit that looks for pulses on the motor-control wires and turns on the valve when it sees pulses. This has the advantage of no firmware mods at all.
(4) If you are not using a heated bed, you could revise the firmware to use the heated bed output to drive your valve. This has the advantage of needing no external circuitry because the heated bed driver transistor is capable of significant current (assuming your valve runs off 12V DC).

In the first three scenarios, you'll have to have some external circuitry to boost the logic signal up to sufficient strength to run the valve.

However, you need to be aware that the existing extruder control modulates how fast to extrude material based on how fast the head is moving, combined with knowledge of the nozzle size and layer thickness. A simple on/off control will not give you as good a control. And particularly there could be difficulties due to the compressibility of air: there will be a lag between when you call for extrusion and when it starts, and another lag between when you tell it to stop and when it actually stops.

[Mooselake]

Use M42 and wire the valve to a free pin. Coaxing the correct code out of the slicer is left as an exercise, although I suggest that a postprocessor might be the place to start. I have no idea if Cura supports these, but slic3r does.

I suspect that on/off air pressure isn't a particularly precise method of extrusion control so you'll have problems with getting the extrusion width and volume to work well. You'll also need to minimize travel moves outside the printed area; slic3r's avoid crossing perimeter option would probably be useful here.

Paste extruders have been done for other bots, including using air pressure. Some time trolling the depths of the reprap.org forums, and working on your googlian search-fu can find out what they did to solve the same problem. Printrbot's past extruder has sunk into the depths and disappeared so I don't know if anybody ever successfully used it.

Kirk

[lairdb]

What about using the extruder stepper as a peri pump motor? That would seem to offer all the control advantages.

(https://en.wikipedia.org/wiki/Peristaltic_pump)

Positive drive of the paste through the stepper action, keep the positive pressure feed (though lower; just enough to make sure the peri mechanism is supplied.

(A quick Google tells me I am not the first to think of this: http://lmgtfy.com/?q=peri+pump+extruder)