An Open Source Powered Leg

I read an article a couple of weeks back about an Open Source Bionic Leg, which was reporting on research began as a NSF funded project at the University of Michigan (UoM), with collaboration from Northwestern, University of Texas at Dallas and CMU. UofM has a website that provides everything you need to build your own open source leg (OSL) leg at OpenSourceLeg.com.

The challenge in human prosthetics these days is that all research is done in silos. Much of it is proprietary and only available within corporations but even university research has been hampered by the lack of a standard platform that could be used to develop new components and ideas on.

The real difficulty is defining the control logic (code). The OSL project is intended to resolve this lack of a platform by providing everything a researcher (hobbyist, or amputee) needs to build their own, at home or in the lab.

The website includes a parts lists and STEP files as well as an estimated cost ($28.5K) to build your own powered prosthetic leg. They also have a Excel spread sheet with all the parts listed, including part numbers and links to where they can be ordered (McMaster-Carr, SolidWorks, & Dephy)

They also show how to build a leg with a short youtube video of how to assemble the whole leg as well as details for each subassembly with separate how-to videos for each.

The open source leg makes use of code from FlexSEA (Flexible Scaleable Electronics Architecture) and Dephy. FlexSea was originally developed by Jean-Francois (JF) Duval while he was at MIT for his doctoral thesis. He has since joined Dephy a robotics design firm. The open source leg project uses FlexSea/Dephy code for its servo control mechanisms.

There is a GitHub Python, MatLab and C control library repo with all the code. The open source leg website also includes instructions, scripts and an image file which can be used to build your own RaspberryPi (4) controller for the leg.

The two (ankle and knee) servos are USB connected to the RPi. There are also other sensors such as the joint (servo-motor) encoders and a six axis load sensor I2C connected to the RPi. Each servo has its own 950mAh battery.

On the OSL website’s control page one can see these servos in action (with short youtube segments). They also provide instructions on how to use the open source control library to take the servo mechanisms through their paces.

Although on the OSL website’s control page I didn’t see anything which put the whole leg together to make use of it in a real world application. They did show on the Data page a youtube video with the OSL attached to a person and being used to walk up and down stairs, inclines and walking across a floor.

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Seeing as how the OSL website included STEP and PDF files for all the (machined) parts which represent $15.6K of the $28.5K, if one really wanted to do this on the cheap, one could just 3D print these parts in plastic. It would obviously not suffice mechanically for real use, but it could provide a platform for testing and developing control logic. At some point one could upgrade some or all of the plastic 3D printed parts to something more durable for use in human trials.

Another option is to purchase multiple sets of parts. The OSL website also showed price estimates for purchasing two sets of ankle and knee parts. But I’d imagine if one was so inclined, a number of researchers (hobbyists or amputees) could get together and order multiple sets of parts for reduced prices.

It’s also possible, with a lot of work, that the open source leg could be redesigned to support an open source arm-hand mechanism. This is where having 3D printed plastic parts could be extremely useful in helping to redesign the leg into an arm-hand.

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