Simscape - Variable Constraint between two Frames

Greetings,

i'm trying to write a custom block with 3 ports B, F and x.

Here, B and F correspond to Frames with the same Orientation, while the Origin of Frame F is translated along the z-Axis by x meters from B.

This basically is not a prismatic joint, but a distance constraint between two frames, where you can specify the offset.

Sadly, im struggling to find some code snippet on Coordinate Transformations that would tell me on how to define my nodes etc.

I would be grateful for any advice/explanation on how to achieve this.

Best Regards

BB