Pneumatic Straps for Ortesis and Exosceletons

inflating of waist strap
deflating of waist strap

One of my favorite projects as an engineer at ILT was the „Hybrid„. The aim of the project is to combine the advantages of a wheelchair with the flexibility an exosceleton can offer. During most every day situations a wheelchair is more reliable and convenient – at least with todays exosceleton technology. But there are also edge cases where being able to stand up can really make a difference. For example discussing something on the same eye level, walking stairs, getting over an obsticle or reachnig for someting higher up.

Silvia Rohner, the leader of the MedTech Lab, wanted to try a new approach to designing an exosceleton. Normally engineers go about designing the electronic and mechanical system first and then they tackle the challenge of securing a human in this contraption – not an easy task. Makeing a human body conform to a mechanical system or vise versa is hard and can be even problematic, when done in a improper way – joints can easily be damadged and injuries can happen quickly during testing.

So her idea was to start designing a suitable interface to transfer the loads to the body and then according to this, design the mechanical system around it. During the ideation phase and talking to our pilots that opperate our exosceletons, it became clear that standing up really would be an edge case. If they first need to strap in for 3 minutes just to quickly stand up, they would rather just stay in the wheelchair.

For makeing standing up spontaneously possible, It became clear that the strapping in should be automatic and fast. This is how the ide of pneumatic straps was born – slightly inspired by fighter jet G-suits that work in a similar fashion.

This is one of the first „quick and dirty“ prototypes for the strap attaching to the upper part of the leg. It consists simply out of a bicycle tire tube and two layers of non-flexible fabric.

Even this simple prototype quickly revealed the potential: the fastening is tight enough and transferes the loads comfortably.

In a similar fashion, the first prototypes for the waist and chest straps were built.

The bottom layer that contacts the skin consists out of a breathable distance inducing fabric. The top layer consists out of a more rigid mesh that makes the starp hold its intended shape. Also it proveokes that the bulging due to the air pressure mainly faces to the inside.

It has to be stressed that the working principle mainly relies on the contraction of the fabric itslef, which is responsible for the majority of the fasteing effect. There were tests conducted with a flexible inner layer that makes bulging of the tube more pominent and leads to less contraction of the fabric itslef. With the bulging, the limbs are „squeezed“ instead of „gripped“ which results in a far less comfortable feel and also the straps tend to slip away more from their intended position.

After some itterations the topology was improved with the possibility of creating destinct 3 dimentional shapes with different paths of the tubes.

Each strap has its own 2-2-valve and its own pressure sensor, so each strap can individually be monitored.

This makes the whole system easily powerable by a central pump with a simple 3-2-valve for creating a path for the air to escape if an individual valve on a strap is opened. For individual pressure adjustements all the other straps have to close their 2-2-valve.

This was one of the first test walking done by Cédric with the newest itteratrion of the straps – it really was a highlight seing it working.

This was me testing the inflation process for the first time. Seeing an idea working even better than anticipated after so many hours is really a rewarding process. I love engineering and moments like this make me really appreciate my job!

inflating and defalting of the upper leg strap

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