Define a Curve marker (see documentation on 'Markers on Curves'). A constraint between this marker and your part so they move together (spherical is usually what you want).
Then create a body motion which uses the B1 (x-axis) of the curve marker (the x-axis will always point tangentially to the curve).
Although I am now closer to the solution, I have not yet managed to solve my problem. I will try to describe my model in detail. The curve representing the trajectory is on part A, which in turn is fixed with the ground. The curve marker is obviously also on part A; the solid that has to move along the curve is part B. I have bound B with the curve marker of A via a spherical joint. Now I should define the motion of the curve marker. Joint motion is not applicable in this case and if I use point motion on the curve marker it does not work. Does anyone have an example?
It should work if you create a point motion on the curve marker. But you have to think backwards.
The curve marker MUST be the j-marker for the motion, i.e. the one that defines the direction.
So you must have a marker on ground as i-marker and specify the motion as negative.
Or you can create a dummy part attached to ground (or other reference part )with a spherical joint and an orientation j-prim to the curve marker. Then a marker on the dummy part is always oriented the same as the curve marker.
Then you can use this marker on the dummy part as your j-marker for the point motion and a marker on part B as an i-marker.
