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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 positioned (like in Magic Arms)

  The device should by only mechanical to simplify construction and reduce costs. Honestly I don’t believe in electronic orthotics in less developed countries. One day, but not now, and I planned create device achievable for everyone, everywhere.
  I decided on counterweights wanting the device to be balanced and frictionless like a turntable tonearm. I was looking for truly weightless effect and nothing seems to compensate gravity force better than… gravity itself. It should be also easy to setup. Tonearm design is unfortunately fragile and has very limited movement.
  The counterweights can’t be mounted directly to exoskeleton from obvious reasons. If we take a look on SMA children wheelchair we clearly see there is no space for counterweights. Whats more they are heavy and would make attaching device to corset impossible.
  Continuous positioning of forearm axis renders device less prone to misalignment and locking.


  Although all of it sounds reasonably, it’s still long way to make this happen.

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