Joint start and end position

[Jayson Mintz]

Hello,

I am attempting to predict an elbow flexion movement using an upper extremity OpenSim model with Scone. From my understanding the best way to do this would be to use a feedforward controller on just the elbow joint. I am a little confused on how to utilize it to have the actuator for that joint go from 0 to 130 degrees using the feedforward controller, is there any advice you can give on how to achieve this motion?

-Jayson Mintz

[Thomas Geijtenbeek]

Are you using a torque actuator, or muscle actuators? With the FeedForward controller, there is no feedback loop to detect the current elbow angle, but you can use a measure to evaluate the motion, and optimize the feedforward terms based on that. If you are using muscle actuators, then perhaps the ReflexController might be a good option.

[Jayson Mintz]

I believe the model we are looking at utilizes muscle actuators, do you have an example that utilizes the ReflexController?

-Jayson Mintz

[Thomas Geijtenbeek]

The gait and the balance examples use reflex controllers, you could have a look at those (the balance example is easiest to understand). But you can also start with a FeedForward controller to make sure your model simulates ok. Just make sure you define a Measure so the optimizer knows what to optimize for. Perhaps a DofMeasure for your elbow angle is a good starting point.

[Jayson Mintz]

Thanks so much Thomas! I will take a look at those examples

-Jayson Mintz

[Jayson Mintz]

I'm definitely having more progress than previously, the main error I am getting now is when I try to optimize the simulation I am getting a bunch of "fiber length cannot be zero" errors. The attached image shows the exact error, any clue as to what may be causing the issue?

-Jayson Mintz

[Thomas Geijtenbeek]

My apologies for the late response.

The errors you show indicate an issue with the OpenSim model you're using. It appears to be entering an invalid state during the simulation.