Walking simulations with feedback control

[Ross Miller]

Hi all,

I think the answer to my question is "yes" generally speaking, but wanted to check with anyone who has actually done it in practice.

A student and I are interested in generating muscle-actuated simulations of walking that have feedback controls, i.e. the muscle excitations u are u = f(x,t) instead of u = f(t), where x is the state of the model. Is this possible?

More specifically, we'd like to first generate a set of excitations without feedback, u = f(t), that produces a well-coordinated walking gait, using standard tracking methods, e.g. CMC or Moco if possible. We would then like to have the user specify an arbitrary form of the feedback control functions, u = f(x,t), and through some sort of least-squares minimization or just hand-tuning, adjust the feedback function parameters so that u = f(x,t) resembles the originally well-coordinated controls closely. I'm imaging this latter step would be done in the Matlab interface.

Has anyone successfully done something like this with OpenSim/Matlab in the past?

Thanks much!
Ross

[Dimitar Stanev]

Hi Ross,

Yes, it is possible to some extend. You can control the model by providing the muscle excitations and stepping the integrator manually. It has been done with the osim-rl project in Python:

https://github.com/stanfordnmbl/osim-rl ... sim.py#L19

However, keep in mind that there is a small drawback of this approach compared to the native C++ implementation. With C++ you can create a new controller that is evaluated each time the numerical integration takes a step. With the osim-rl approach one provides u(t) given x(t) at time = t and then predicts x(t + dt) keeping u(t) constant between t and t + dt.

Alternatively, you can do function approximation given that you know u, x for each t without OpenSim. Although, the last might not necessarily mean that f(x, t) produces stable solutions. How do you plant to ensure that your feedback controller is stable?

Regards,
Dimitar

[Ross Miller]

Thanks Dimitar!