Hi,
That will be a little more involving actually.
Basically you need the following:
-- a center position acting as the reference of your constraint
-- the radius
with this, everyframe you check of the position of your gameobject, and create a vector 3 of the position minus the center position, this will give you two indications,
-- the magnitude and the direction.
if the magnitude of greated than the radius, normalize this vector, and multiply it by the radius, that will be the delta position from the center position of your gameobject.
If you do this sequence, then you can still control the gameobject around, it will move within this constraint.
bye,
Jean