Thursday, 16 March 2017

3d printed peristaltic pump

While playing about with some ideas recently we pondered just how powerful a pump had to be to raise a column of water a set height (using, say, aquarium tube of 4.7mm diameter to a height of about 400mm).

We wondered if it would be possible to submerge a pump into a liquid and use inductive chargers (like those often used in Adafruit products to wireless charge lipo batteries) to power the pump. So the liquid could be in a container with the pump in it, which you then place on a separate base. All the clever stuff goes on in the base, and we just use inductive power to turn the pump on and off. It's probably not possible. But this was one of those "learn as you do it" type projects.

For a start, we've already discounted an impeller type design - while you can run an impeller pump off a small hobby motor, they usually have to run at quite high voltage, and they consume quite a bit of current (around half an amp or more, which is way more than any wireless system would be able to provide).

At this stage, we're not even convinced that wireless is even feasible. But we did get to thinking that some kind of peristaltic pump, driven by a small, geared, low current stepper motor might be worth investigating. Plus we've a spanky new 3d printer just sitting there waiting to be given it's first "proper" job!

The first thing was to design our peristaltic pump; for this we used Inkscape (originally with a view to making the pump from layers of laser cut acrylic).

Then after importing the svg shapes into Blender and quite a bit of buggering about (extrude, then alt+A or something to convert a spline-based extrusion into a 3d geometry mesh, applying a few boolean operations and so on) we had some pretty impressive looking models.

There was something about removing duplicates and triangulating the shapes (edit mode, control something, control + N) that we forgot to document, but then the whole thing was ready to export as an STL file. A few clicks later and our models appeared in UP! Studio

After the first export, the design appeared tiny. So we exported again from Blender, this time setting the scale setting in the export dialogue to 1000 (Blender's default units are metres but despite our shapes being described as 0.005 for 5mm for example, we still had to multiply up by 1000 to get them to the right size).

We're still learning how 3d printing works but it was still a bit of a surprise (and a  bit disappointing) to see our raft curling off the bed so early on in the first print. Stopping the print and trying again only caused the nozzle to block (which took about an hour to sort out in the end). So for the second print,  we left all the doors on the printer closed and set it running. As long as the printer was pulling filament off the spool, we were happy to leave it printing "blindly".

The final print was satisfyingly stinky when the UP! Mini beeped to tell us it had finished. The raft stayed stuck to the bed but there was still a tiny amount of warping in the print. The workshop was pretty cold (the UP! Studio said the nozzle started at 12deg C when we first booted the software up) so maybe that was it. Even so, the parts fitted together perfectly snugly.

We drew the shaft hole exactly 3mm high and 5mm wide as per the dimension drawing for the 28BYJ-48 stepper motor. The triangular part of our pump pushed over the shaft with a bit of force- it was a very snug fit; some might say perfectly scaled!

We connected up our stepper motor to a ULN2803A transistor array and threw together some simple code to make it power the coils in the correct sequence to get the motor to turn. We wrapped some 6mm aquarium pipe around the outside of the wheels and set the stepper motor spinning.

While everything appears to run fine, the 3d print holds up and it all looks good in principle, as soon as we added the (admittedly fairly stiff pipe) to the pump, it stalled the motor! Even at 12v, our little stepper just didn't have the grunt to squash the wheels against the sides of the tube. And that's without there being any fluid in the system!

So it looks like we're either going to have to use a bigger/better/stronger motor or softer/squashier tube before this goes anywhere near a bucket of water...

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