Deeper into mechanics

Lets take a closer look into the mechanics of the device. First of all: how to transfer moment from distant counterweight to arm module? Simple answer: by cable.

OK, it’s just the beginning of “The Cable Story”. I don’t mean electric wire – the device had to be mechanical only, no motors allowed! Other type of cable is Bowden cable that transmits mechanical force or energy. In some aspects similar to double rod design but flexible. Flexibility means total freedom of placing active device – on the back of wheelchair, on tripod, under the bed. And saves space around passive device at the same time, so helps in achieving second goal – opportunity of mounting directly to corset.

I was asking myself if it would work, it should, but thanks to rapid prototyping I could empirically test it. I speed-designed and 3d printed proof-of-concept prototype with 5mm bike derailleurs cables.

It works but bike metal cords are to stiff to be used in next prototypes. I didn’t need 100kg payload but frictionless operation. Happily there are plenty of synthetic lines to choose from.

Bowden cables are chosen as a link between active and passive devices. The active devices with counterweights “collects” gravity energy and transmits it through cables to the passive device on the other side where the energy is used to compensate gravity force on child’s arm. Both devices should be in perfect balance and provide true “weightless effect” hard to get in rubber powered devices.

To be continued...

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Assembling

Having all those pieces everything I had to do was put them together. The prototype contains 3D printed parts in three different technologies: SLA , SLS and FDM on a pair with standardized parts and even sewed padding. Assembling was a bit like jigsaw but done same way as simulated in Fusion 360 design software and took about 20 minutes. Passive module (mounted on hand) weights 350g and is way too heavy to be mounted directly on corset. Moreover, most users will be in half sitting position with head support right behind them. Following advice of physiotherapy experts, orthopedists and parents I added simple tripod mount to be used with Manfrotto Mini Arm and mounted at the back of any chair. After first try-on (without counterweights) the feedback was very positive. The device follows natural arm movement and doesn’t lock in any position. Looks as if it’s indeed more resistant to misalignment than Magic and Angel Arms. The prototype padding could be better, ladder locks and

Design choices

Recently I had opportunity to talk with two SMA child families and discuss issues in existing exoskeleton solutions. Before that, they mentioned rubber bands of Angel Arms and Magic Arms are difficult to setup, and change its characteristics over time. I came out with a new idea and wanted to verify it with them. I prepared some generic images of concepts and ask the parents to choose the most comfortable in their opinion. I tried not to bias their decisions, and restrict myself to raw description. A-B test used with parents They agreed elimination of rubber bands and separation active and passive modules were good starting point. I did several design choices to follow: the device should be fully mechanical the device should be counterweight driven the active (counterweight) module should be separated from passive (exoskeleton) module passive device (exoskeleton) could be mounted to corset forearm axis should be always

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