Explicit Dynamic Simulation - Prosthetic Foot

  • Last Post 03 May 2018
Laucen posted this 03 May 2018

Good day to you,

I hope to find a solution to my problem with your help. I'm an italian biomedical engineer and not very skilled in Ansys. Forgive bad grammar and typos!

I'd like to simulate the contact phase of a prosthetic foot from Heel Strike to Toe Off in Explicit Dynamic. I have the kinematic (both linear and rotational) of the foot (a human ankle) and I would like to force the foot to move like the human ankle, in order to understand what is the deformation with the terrain and the Ground Reaction Forces due to contact.

The problem is simple: I don't know how to move the foot, because I tried with Remote Displacement but of course the software sees it at a loading condition and it moves the faces causing a distortion and no movement of the foot at all.

Is something wrong with my approach or it is not possible to do?

What if I try to do a shorter quasi-static simulation of the foot from initial position to a certain time applying velocities instead, then export the deformed mesh, bring it to a second simulation with a different initial condition and so on?

Thank you for your time!!

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peteroznewman posted this 03 May 2018

Hello Laucen,

You don't have to use Explicit Dynamics to model a heel strike to toe-off motion. You can use Transient Dynamics.  You could also use Static Structural since the accelerations are low, so the inertia forces are small compared to the contact forces and tension in the part of the prosthetic that represents ligaments, etc.

I would start with Static Structural. Time is then not physical time, but just a way to divide the total motion up into convenient steps. You can enter the kinematic motion at some cut plane through the tibia/fibula. Configure the geometry so the prosthetic heel is tangent at first contact with the floor. The kinematic motion will lower and advance the displacement of the cut plane faces of the tibia/fibula causing the foot to flatten. You will have a lot of difficulty getting the model to converge, but once successful, you can convert this into a Transient Dynamics model and include the inertia forces.  If you can't get it to converge, this is when Explicit Dynamics can be useful. It doesn't need to converge, each tiny time step is correcting equilibrium, but it can have extremely long solve times (tens of hours).

If you are willing to share your model, please follow these directions to attach an archive to your reply. It will be much easier to offer detailed suggestions if you share your model.



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peteroznewman posted this 03 May 2018