Simple exoskeleton demo

[Gunflash]

Hello I want to create simple exoskeleton for elbow joint rehabilitation. I completed OpenSIM tutorials but I have feeling that OpenSIM creator might be easier to use (Im now on the tutorial 3) for this application. Im using Arm26 for obvious reasons and now my objective after finishing the tutorials is importing my 3D model from Autodesk Inventor to OpenSIM creator and attaching it to the Arm26 model.

Sadly there are no more tutorials for OpenSIM Creator, where there is shown how to operate with muscle or add constant torque to a joint (if thats even possible - to "simulate" my exoskeleton helping the muscles).

My questions are:

How do you program the activation of muscles in OpenSIM Creator?

How do you add torque to a joint?

Do I have to do static analysis in OpenSIM 4.5 or can it be done in Creator aswell?

I have experience in software like CADs, MatLab and even Ansys. If anyone is willing to guide me through this I would love to talk in real time via discord or microsoft teams. Thank you very much.

[adamkewley]

Sorry for the delayed reply, I'm a one-man show and I sometimes dissapear on holiday, or fighting fires somewhere else. Even if you've moved on, here's my replies to your questions:

How do you program the activation of muscles in OpenSIM Creator?

You can't do this in OpenSim Creator at the moment. It's a known shortfall. The reason why it isn't implemented yet is because OpenSim GUI offers a similar feature called the "Excitation Editor" (top-left menu, IIRC). The basic idea behind that editor is that you create an excitation pattern for the muscle, which is then applied to the muscle during simulation. The excitation --> activation relationship is handled by OpenSim (via your muscle parameters, etc.).

OpenSim's official documentation for that feature is here: https://opensimconfluence.atlassian.net/wiki/spaces/OpenSim/pages/53089924/Excitation+Editor . I'd like to create an OSC equivalent to it, because there's some design ideas I think would suit OSC better, but it isn't currently a priority.

How do you add torque to a joint?

This depends heavily on what exactly you are trying to do.

In OpenSim, joints are controlled by Coordinates (effectively, degrees of freedom). Some joints (e.g. PinJoint, FreeJoint) allow rotation, and those joints usually expose the rotation via a corresponding coordinate (e.g. PinJoint might expose a Coordinate for its rotation about Z as arm_flex or similar). Coordinates expose two parameters: a "value" (e.g. rotation) and a "speed" (e.g. the rate at which the rotation should change). Those can both be edited as initial states.

If you want to change a joint's degree of freedom (e.g. you want to apply a torque and for OpenSim to figure out what the corresponding speed/value should be at simulation-time). Then you're going to need to add a Force to the model that does what you want. OpenSim has a few forces that can play around with coordinates (e.g. ActivationCoordinateAcutator, CoordinateActuator, etc.).

As a guess, though, you are probably going to want to use something like an ExternalForce, ExternalLoads, or PrescribedForce for an exo project. Most tend to use these. They're forces that allow the designer to say something like "apply this force at this time to this location in this body", which is how those design experiments tend to go. The documentation for those forces is spotty, you can find echoes of the solution in OpenSim's documentation (e.g. https://opensimconfluence.atlassian.net/wiki/spaces/OpenSim/pages/53090636/Preparing+Force+Data). OSC provides a specialized tutorial that specifically addresses debugging ExternalForces etc. because it's a very common issue in our labs at TUD:

https://docs.opensimcreator.com/manual/en/latest/preview-experimental-data.html

Do I have to do static analysis in OpenSIM 4.5 or can it be done in Creator aswell?

Only in OpenSim 4.5. As usual, we plan on porting features like these to OSC over time but they're lower priority because a user pathway already exists (in OpenSim 4.5) - although I completely understand that it would be nice to just have one UI that does it all.