Question: Definition of frame with symbolic parameters

Hello,

I would like to define my model with symbolic parameters. Consequently, i have to define the symbolic parameters and theirs values in main folder.

To illustrate, i define my parameters a1, a2 like this.

After, i need to define a frame by using the parameter a1. This frame is define like this :

My problem is that in the 3D visualisation, the definition of my frame seems not to be good because it seems that the parameters are not evaluated. Indeed, if i put numeric values to define my frame and not symbolic, i have the frame i want but not with the symbolic parameters.

How is it possible to define a model with symbolic parameters (the numerical values are defined in main folder) which can be visualized that is to say in the position defined by the numerical values ?

I hope i'm enough clear.

Thnk you for your help !