Saturday, February 7, 2015

Fun with DIY Unlicensed Medical Devices


I'm not a doctor.

Nor do I play one on TV.  But who needs medical credentials when there is good making to be done!

Recently, I had the opportunity to build something different--a quick project that was simple but rewarding.

A few months ago a good friend mangled her arm in ATV rollover.  After several surgeries, her parts are back together (more or less), but her recovery entails daily hours of physical therapy to regain flexibility and strength.  The official routine is done with simple weights and manual devices, is repetitive and monotonous, and requires constant focus to maintain proper form and pressure.
As far as I know, being shirtless conjoined twins is not a requirement for the therapy shown.

I decided to I might be able to help with a particular exercise, one to extend the wrist range of motion in rotation.  My first thought was that there ought to be plenty of commercial mechanical devices on the market to help automate the process.  I looked, but except for a few vague references to mostly description-free items that could be purchased overseas in bulk, they didn't exist... yet.

It could be useful, I thought, to use an electric motor to perform the rotation, as it could rotate back and forth, performing a perfectly repeatable motion with a user-adjustable torque limit in each direction.
The "Wondermotor"

The first order of business was to find a suitable motor.  By lifting a one-gallon jug of water at the end of a 1-foot stick, we estimated that we needed up to 8 foot pounds of torque (since one gallon weighs 8.3 lbs).  That's pretty tall order for a low-voltage motor, but I found exactly one readily-available candidate--a 12-Volt Wondermotor ($69) on Amazon--that fit the bill.

It has a built-in worm drive to slow the motion and some impressive specs, and I'll have to remember this one when it's time to build powered go carts for the little Monkeys.
It used to be that you had to make stuff like this yourself

For power control, I got a small PWM motor speed control board ($10), also from Amazon, along with some switches, connectors, and miscellaneous parts.  Some pipe from the hardware store, and a cheap angle finder gauge from Harbor Freight completed the parts list.

I mounted all the components on some scrap lumber, elevating the motor tied to a block of wood with a pipe support strap.  I built a handle out of a bicycle grip, a galvanized pipe nipple, and PVC pipe fittings, and mounted it to the motor through an oval hole drilled in the pipe nipple.

For some reason this reminds me of that Simpsons episode where Homer bulks up just one arm and uses it to hustle folks arm wrestling.

To power the motor, I used a spare 12-Volt wall wart wired as input to the motor speed control.  I wired the output of the control to the motor, passing through a two-way momentary DPDT toggle switch to allow the motor to change directions.

The idea was to have motor stall when the user's wrist reached its end of motion, suddenly increasing the load.  By adjusting the power/speed control, one can get the unit to exert the same force for every rotation.

Here is a video of the device in operation during initial trial tests...  
I got lots of funny looks from the security guards that morning.
and upon delivery.