Exoboots (3D Printed Concept)

As a kid first and a mech engineering student after, I of course had always the Iron Man dream.

Now that finally I have some free time during work and I can play around with my 3D printer, I decided to put all my old ideas together and start to build something, actually a whole exoskeleton. Currently is just a concept, fully 3D printable but the future plans are others of course.

As part of the suit of course, there are legs and particularly boots which I am presenting here as a simple but effective application of math/geometry.

Here is a render/preview of almost the full lower portion of the leg, but in this section I am going to discuss only the boot piece.

All components are showed in the video with an exploded view

• 3D printed components:
• Front piece
• Back piece
• Side pieced (Left + Right)
• 2 x 608RS roller bearings
• 2 x M8 bolts + nuts
• 14 x M4 bolts + nuts
• 2 x 150N gas springs (furniture type) with hinges

The boots kinematic was the first step of the whole process, before even thinking to the boot shape.

And here is where specifically simple geometry/math came into help.

What did I want exactly?

A boot piece to be worn as a shoe by the user, providing a certain amount of force at the heel in order to support plantarflexion of the user. This could have multiple use, such as simply supporting the weight of the overall exoskeleton of supporting the walking movement of the user.

This concept was built using passive elements (gas springs) but of course in future, the idea would be to use an active system in order to apply the force at the stepping instant and then remove it when not needed.

In any case, this is not the main point of the discussion, but rather it is the kinematic designed, for which I filed a patent so I can freely discuss it here.

Due to its configuration I define it as triangular kinematic. This because, as you notice from the picture, the main moving elements of it create a triangle whose vertices are represented by the hinges of the kinematic itself, which has 1 degree of freedom.

In particular, the front piece of the boot (from toes to metatarsus) is connected to the back piece (metatarsus to heel) by means of a mechanical hinge (in this case bolts and roller bearings).

In this way, the two pieces can rotate relatively around this hinger (center of rotation of the system).

Additionally, on the front piece, there are two side extensions on which are connected the lower hinges of the gas springs. In this way, the distance "A" of the picture represents the first segment of the triangle (fixed length).

The upper hinges of the gas spring, instead, are connected to the back piece (on two vertical extensions), constituting in this way the segment B of the triangle.

In such a configuration, as shown in the scheme picture, the distance between the center of rotation and the upper hinges of the gas springs is constant and represents the segment R of the triangle.

So the variable segments of the triangle is B and in fact it is constituted by the gas springs, which for their nature can change their length begin compressed or released and therefore applying more or less force.

So where does come math into the game?

Well, remembering Carnot theorem, the length of the segment of a triangle, can be determined knowing the other two segments and the angle between them.

Basically we have that:

• A is fixed and known for what said above (distance between two hinges)
• R is fixed and known (radius of the circumference which has the center of rotation of the system as center)
• T is variable and known (considering a typical angle of the foot during plantarflexion)

So the length of the gas spring (B) can be determine as:

B = sqrt (A^2 + R^2 - 2ARcos(T))

And by taking the vertical projection of the force excerpted by the gas spring, it is basically possible to write down the pushing force at the heel as a function of the distance A or R.

Such a function can be used for optimization of the exoboot, setting as constraints the wear-ability for example (limiting the length of the gas spring and therefore R) or the walk-ability (limiting the length A in order to don't complicate the walking movement).

I will not go into details and formulas but I guess the idea behind the kinematic is clear.

Not I was left with designing a piece that would allow me to have such a kinematic physically and test it.

Once all the parts where designed and printed (material used was PLA+ and the print orientation, settings and color were chosen in such a way to guarantee no failure), I proceeded assembling them as shown in the video of the previous section.

This concept was built using passive gas springs so, as you can see from the picture, the released boot would stay in the flexed position but in future the plant is to make it fully active so actuation would be performed only when necessary.

In any case time for test!

And the video will speak on its own. It's about 25kg push force at the heel and yes it feels nice :)

Hope you guys enjoyed the description and if something is not clear, feel free to ask.

I already said this, but this is only a piece of a bigger project a I am developing which will include more and more geometry/math and not only.

I you would like to follow the journey, I am posting most of the updates on my IG channel @nozzle_torino, so feel free to check it out.

Thanks for reading and cheers!