Defining Joint Motion using Generalized Cordinates

[Rahul Agrawal]

Hello Everyone,

I am using 6DOF custom joint between scapula and humerus to stimulate dislocation. I have generated trajectory in Matlab i.e. translation XYZ and rotation Euler angles. Opensim requires generalized coordinate as an input for prescribing joint motion. Does anyone know how I can implement it.

Please see the attached file from "Minimal formulation of joint motion for biomechanisms"- Ajay seth
I have Theta1,Theta2,Theta3 and P1,P2,P3 wrt. parent body . How can I calculate generalized coordinate q?

Thanks!

[Dimitar Stanev]

If theta and p are a function of one independent variable then the easiest way is to use a function in the TransformAxis (e.g. see the example for knee translation as a function of knee_angle_r):

https://github.com/opensim-org/opensim- ... .osim#L590

Unfortunately, I am not sure if OpenSim supports a multivariate function. In this case you would have to implement your own custom function and export it as an OpenSim plugin.

[Rahul Agrawal]

Hi Dimitar,

Thank you for your reply.

In my case theta and p are independent of each other as I am prescribing motion from located position to dislocated position for humerus.

Do you think is there any way to implement this eg (modelling 6 DOF with 6 different joints at the same location and prescribing thetha and p accordingly).

Thanks!

Regards,
Rahul

[Dimitar Stanev]

Yes as I told you, you will have to implement your own Function as a plugin that can have multiple dependent variables, because OpenSim does not have such a function. Then you can add this function to the TransformAxis providing the dependent coordinates and the coefficients for evaluating the function value, if we assume that it is a polynomial.

You can look at the source code to see how different function are defined (e.g. check the inheritance diagram https://simtk.org/api_docs/opensim/api_ ... ction.html and particularly the PolynomialFunction) and create your own function by extending OpenSim::Function. Then create a plugin:

https://simtk-confluence.stanford.edu/d ... ng+Plugins