The first steps are taken: the MARCH II walks! Achieving this was quite the challenge, especially since in the hip and knee joints of the MARCH II exoskeleton, high precision and great forces come together. To ensure that these forces do not deform the joints too much, we performed Finite Element Analyses. For this, we brought in the expertise of engineering firm Code Product Solutions, who has had experience with these kinds of analyses for decades. Code provided us with useful tips and design improvements. These FEA-based adjustments have led to our final joint design. 

Image 1. The final design of the knee joint.

Image 1. The final design of the knee joint.

One of the requirements for the joint design was that the distance between the magnetic encoder ring and the sensor should not exceed the boundaries of 0.1 mm and 0.3 mm. The sensor reads out the angle of the joint, so when big forces are acting on the joint, only minimal deformations can be allowed. Soon we experienced that the weight of our pilot in combination with the force acting on the bolts that are keeping the housing together would cause the joint of the exoskeleton to twist. That made it impossible for the sensor to conduct good measurements. The deformation acts the same way as a hamburger that is squeezed on one side (depicted in image 2). Therefore, we named the observed deformation phenomenon the ‘hamburger effect’.

 
Image 2. The joint of the exoskeleton bends the same way as a hamburger that is squeezed on one side.

Image 2. The joint of the exoskeleton bends the same way as a hamburger that is squeezed on one side.

 

With the help of Code Product Solutions in making design improvements, we adjusted the housing to make the joint stiffer. Because of these improvements, we are certain that the joint will not deform and that it will therefore be able to measure the angle of the joint in all circumstances.

Another point of focus during the FE analysis was the mechanical end stop. This is a safety system that makes sure the knee will never be able to rotate more than the knee of the pilot. Together with Code Product Solutions we looked into the shape of this part to make sure the pilot or the exoskeleton won’t be damaged, even during unexpected situations.

In the end, we were able to reach a much better division of forces and we could also accomplish a broader, flat end stop. With these improvements, we are fully confident that our pilot, Ruben de Sain, and the MARCH II will overcome all obstacles during the Cybathlon competition at the RehaCare fair in Düsseldorf!

Image 3. The initial housing. Because the screw holes are not completely supported, the ‘hamburger effect’ could appear.

Image 3. The initial housing. Because the screw holes are not completely supported, the ‘hamburger effect’ could appear.

Image 4. The final, improved housing. With the new end stops we are able to guarantee the safety of the pilot and the exoskeleton.

Image 4. The final, improved housing. With the new end stops we are able to guarantee the safety of the pilot and the exoskeleton.