Help with Simulating Laparoscopic Fulcrum Effect in Unity

Hi everyone,

I’m working on a laparoscopic simulation in Unity and need help accurately replicating the fulcrum effect. I am controlling the virtual tool with MoCap tracking in real time. Essentially, I need to constrain the surgical tool so that it always passes through a fixed point (the trocar, acting as the fulcrum), while still allowing:

  • Free rotation around this point in all directions.
  • In-and-out movement along the tool’s axis.

I first attempted using constraints, but the tool’s behavior became inconsistent between the Scene view and Game view. Then I tried a Configurable Joint, but despite locking movement in the X-Z plane, the tool still moves freely in unintended ways.

Has anyone successfully implemented a similar constraint system?
Any guidance or suggestions would be greatly appreciated!

Thanks!

Maybe I’m misinterpreting something here, but if it has to pass through a fixed point couldn’t you just use a parent game object that remains at a fixed position and then move the scope along its forward axis as a child?

The structure i have is this. As you see they are already parent and son. The problem is that the Tube movement is directly controlled by my movements tracked with a motion capture. since i am controlling the handle of the tool i am trying to project the movement on the Tube. It works fine but to be 100% realistic i need to make the tube pass through the trocar point.

If i use an additional position contraint it works, but then i am not able to slide in and out that point, and if i free the Y direction or whatever direction i need to slide, the constraint unlocks the movement based on the orientation of the trocar frame (which is static so when i change the orientation of the tube the sliding is on the wrong axis).

then i tried to insert a rotation constraint to the trocar, so that it could follow the orientation of an empty object at the final edge of the tube, but if i do that everything is perfect, except that the position i see in the game view is totally mismatched with the one in the Edit view