Monday, August 14, 2006


Under pressure

On Saturday, July 08, 2006 I blogged a start at a design for a peristaltic pump for driving pastes and the like through RepRap deposition heads. One of the adjuncts that this needs is a pressure sensor so the microcontroller can tell when to turn the pump on and off.

Here's a thing:

The little grey square on the left is a quantum tunnelling compound pill, which you can find out more about here, and buy here (product id: N18BU). But all you really need to know is that its resistance drops when you squash it, and it costs £0.50.

If you place it on some stripboard as above, then put a soft silicone tube over it, then clamp them together you get this arrangement:

The tube is blocked at the left, and the hypo will (in a moment) apply pressure. As you screw the clamp jaws together the resistance goes down - in the picture it's 3.36K.

Now depress the plunger:

The resistance drops gratifyingly smoothly to 590 ohms. Better yet, if you pull on the plunger to create a partial vacuum, the resistance rises above 3.36 K...

It should be very simple to design a little RP screw clamp to hold a small piece of stripboard, the QTC pill, and the soft tube. It might be better to use gold flash on the stripboard contacts, to avoid oxide contamination problems over time. The device could be placed in any pressure line, or attached to a blind tube as above.

Jette kul! :-D
I'm very tempted to see if it will work between 120 and 150 degrees. One of those in a recess in the inside of the extruder barrel wall would be just the thing, hey?
The silicone tube is definitely OK at quite high temperatures. We could just oil-fill a section of it, put one end in the extruder, and the other with the sensor on the outside.

And so to bed...
That's a pretty cool device. I was recently playing with strain guages for a similar purpose, but they're much harder to get good readings out of. Nice...
Unwanted gold-plated edge connectors might make a good platform-with-contacts for the pills.

Vik :v)
If the clamp was a polo with the pressure sensor on the ID (same size as the OD of the tube) that would be pretty sweet... and adjustable down the length of the tube.
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