The medical appts. has a positioning device (2) for an object under investigation. A positioning plate (3) is movable about a horizontal axis (4). A signal generator (8) generates a signal dependent on the movement of the positioning plate. A controlled (6) electromechanical drive (7) moves the plate longitudinally. The control arrangement (6) includes a signal converter (9) which converts input signals corresp. to a predetermined characteristic line (10,11,14,16,17) into an output signal. The control arrangement receives the signal from the signal generator.
