The first task, once I decided to experiment with an orthotic elbow cuff, was to knock together a quick prototype. Kari and I sourced some cheap athletic elbow cuffs, I fabricated some silicone bladders, and she wrapped them in fabric and attached them to the cuff. Inflating the bladders created noticeable torque on our elbows while we were testing the device, so it seemed like time to gather some data.
I tend to prefer to fabricate my own testing equipment rather than sourcing specialty measurement tools. In this case I used an automotive torque meter at the hinge of a simple mock-up elbow. So far I've fabricated two versions of this device: one made of laser cut delrin supported by standoffs, the other a FDM printed elbow based on my own arm. Both could use improvements, especially in how they transmit force to the torque meter. There's enough force at the joint that the plastic deforms where it attaches to the metal meter and it skews the measurement. Still, it's fine for a rough and ready estimate of forces.
I'll be uploading files for both versions to Thingiverse in time, though I feel awkward about having to qualify them with: "Hey, these meters kind of work but be suspicious of what they tell you".
It's also crucial to understand where the design starts to fail. The first version of the interior bladder used a small luer-like printed fastener that was airtight up to 10psi. I redesigned it to beef everything up and added a flange to the bladder inlet, which has held up to 30psi without leaking. I'm a little nervous to take anything up beyond that to find its burst pressure without some kind of protective box. Fortunately I have some friends with a blast box in their lab, so it's something that I can tackle in the near future.
Here's the data from our first test with the delrin torque meter. This is on the first prototype which has a 9" bladder with a roughtly 1.5" diameter.
|Test||Peak PSI||Peak Torque (ft-lb)|