Kudos for out of the box thinking. It is perfectly acceptable for this application for the circle to move as long as it still contains the rectangle and does not grow beyond a certain diameter. I still have not figured out the constraint you used to keep the corners of the rectangle coincident with the perimeter of the circle. A clue to that trick would be appreciated. I modified your drawing a bit to change the controls to be 2 polar stretches with distance limits. They also have angle limits that keep the center of the circle within the rectangle. The grip in the top right seems to work correctly but the one in the bottom left does not stay on the corner of the rectangle. Maybe the constraints are preventing that? I case the "real world" application of this will help with the solution, the circle represents the coverage area of a smoke detector (15 foot radius). The rectangle represents the walls of a room served by the smoke detector. As long as the room resides inside the circle, it has smoke protection. So if someone can figure out what I did wrong with the lower left grip and can find a way to keep it on the lower left corner of the rectangle I will be all set.