Local derivatives on an edge

Hello Community,

I'm building a 3D model in which edges are randomly oriented. On these edges I'm trying to implement a weak formulation of a pde but I can't figure out how to compute the derivatives in the local coordinate system.
I tried to have a look at the beam physics model and there a derivative orthogonal to the edge is computed by the following expression: "mod1.umod1.vmod1.wts". But even with this information I wasn't able to get the derivatives. I'm really desperate to get the answer because I have been working on this problem for a long time now.

Thanks for your help.

kind regards

Jan Kaul