Friday, December 9, 2011

Battery pack for medical device

Any guesses as to what I've been up to?  It certainly looks a lot more dangerous than it really is.  The object on the left is an electronic ambulatory infusion pump used by medics.  The object on the right is a makeshift battery pack I put together to power the pump.

A little while ago a friend of mine came to me with a problem - he was running an in-vivo experiment using one of these pumps to provide a continuous nutrient supply; the problem was that he needed to run the pumps continuously throughout a two week experiment. The pumps only accepted AA batteries and would run out of juice after only 4 hours of use. Having to go to the lab and change batteries every four hours sounds a bit too much like experiencing the sleep debt of having a very young child... So, he asked me if there was a quick and easy way to improve the situation.

Connecting the pumps directly to the mains was out of the question; they needed to remain portable and untethered. So I thought we could add an external battery pack instead which would house larger, D size, batteries to increase the available power supply.

These pumps cost around $800 each, so I wanted to make the least invasive upgrade possible - yes, I'm a scaredy cat. Rather than ripping them open and soldering directly onto the circuit board, I decided to make some fake AA batteries to interface with the internal battery terminals. A couple of screws at the end of correctly sized dowels should do the trick. So, I measured an AA battery (length and diameter) and headed over to OSH to pick up some doweling (I know... I feel like I was cheating on home despot).

I cut two AA sized cylinders and decided on the right length of wire. I attached the wire to the ends of the dowel using a circular crimp terminal and a couple of wood screws. I had to drill a small hole in the pump casing in order to have the wire accessible, I threaded the wire through the hole and attached a mono jack plug to the end. For some reason I have a bag of these plugs and paired sockets sitting on my workbench... I figured they'd make life easier when replacing batteries by allowing the pump and battery pack to be separated and, if needed, I could make a few packs which would allow even quicker battery replacement.

If you've read any of my other posts you won't be surprised to hear that I opted for PVC pipe when making the main body of the battery packs. It turns out that D batteries fit snugly in 1 1/4" PVC piping, which is readily available in Lowe's/OSH/Home Depot.

 I bought some single D battery holders from RadioShack so I could cannibalize the springs (and wire). I dissected them using a proxxon (a dremmel variant gifted to Lin by my PhD buddy Claire) and glued them to a couple of PVC end caps.  If you look at the picture below you can see that one of the end caps consists of two pieces where one slides onto the main body and the other screws onto that piece. I glued the spring onto the inside lip of the larger piece, drilled a hole in the smaller cap to fit the jack plug and soldered it all together.  The connecting wires were passed through small holes drilled in the sides of the cap.  The black tape you can see is to secure the wire that connects the lower terminal with the jack socket.  I thought about passing the wire down the inside of the case, but I figured that having the wire on the outside would make it very obvious how far the wire can be stretched safely.  If it was on the inside someone might pull the cap off too hard/far and damage the battery pack - not good halfway through an experiment!

It turns out that the modified pumps and battery packs worked well.  The D batteries supplied enough juice to run the pumps at full pelt for over 16 hours;  Result!

Wednesday, December 7, 2011

CNC update

I just realised that I haven't posted about the CNC machine for over a year! Judging by the blog, it looks like I failed and let it die a quiet death... but in reality it's been pretty great; it is up and running and working better than I had originally hoped.

Admittedly, it took longer to get into a decent state than I had planned for, but then I never had a lot of time to dedicate to it and I am not "skilled in the art" (oh no, not by a long shot!). One of the main reasons for embarking on the CNC build was to push myself into learning a lot of new skills; with that in mind it's been a huge success :) I picked up more woodworking skills, some metal working skills, knowledge of CAD, CAM, gcode and learned a ton of other random facts and figures along the way. But, best of all, I have this amazing contraption in my garage that can craft things out of wood, plastic, foam and aluminum with speed and precision that I couldn't possibly hope to achieve by hand.

So, what have I done with it so far?  Well, a lot of the things I've made so far have been improvements and additions for the machine itself.  First off, I made some hold downs (thingyverse thing:776) and some knobs to go with them (gcode provided free by, which is where I bought the CNC plans from in the first place).

You can see the new knobs and a couple of the hold downs in use here, holding down some 1/2" MDF with some more hold downs being cut out.

At the start of the year I upgraded the leadscrews from all-thread to ACME rod (the cheap stuff from Enco) which meant that I needed new leadnuts (also from Enco) and leadnut holders - I designed these in CamBam and cut them out on the machine (after a few false starts mostly to do with my understanding of g-code).  I guess I should upload those designs to thingyverse really; though I'm not sure how useful they are to others.

The leadscrew upgrade removed a lot of backlash and resulted in faster and smoother performance.

I noticed that the cutting surface wasn't level so next up was to add some MDF planks and level them using the machine to mill the entire surface flat.  After spending a couple of hours following the router bit with a vacuum hose and still getting myself and everything in the garage covered in a fine layer of MDF dust (not nice) I decided the next upgrade should be a vacuum system - I went with the one described on the Solsylva site with one minor modification.  I had a lot of trouble getting the ring cut from a 2 liter plastic bottle to fit around the vacuum attachment, so I cut it open and attached it with duct tape...  frustrated, moi?  That was a bit unstable (and ugly) and I ended up replacing the tape with a #56 band clamp, which has been working fine so far.  I intend on cutting out a few more in order to have quick change vacuum feet with different length bristles on each of them.

It's not all been machine related production though.  Some of the first things I made were random carvings for the kids.  I carved their names into blocks of wood; made spirals, hearts and shapes for them to colour-in and mess around with; I made a few door signs for bathrooms; name plates for kids friends bedrooms; and a few other random objects.

I've also mocked up a small version of Carys' birthday present and cut it out on the machine.  I can't wait to be able to post about the completed full sized birthday project, but that's going to have to wait until it's finished. I love the fact that the kids birthday presents are always collaborative efforts with my lovely missus - all the projects are so much better when she's involved. Remember the colour conundrum?  That would just have been an ugly cardboard box with wires sticking out if it weren't for her creative talents and influence!

The current crop of unphotographed and unfinished projects include "kids name marble mazes" and rubber band guns.  I've been wondering how hard it would be to make simple versions of star wars ships like the millenium falcon and mill them out.  I guess I wouldn't be able to share the files though (or I'd risk getting sued).  The one thing I haven't really thought about is actually one of the reasons I thought the CNC machine would be ace - PCB milling.  It turns out that getting boards through DorkbotPDX is much simpler than milling your own (unless you're in a rush) and the results are fantastic - you can see a few of my completed boards here.