Abstract: A system for positioning with respect to a patient's body an observation and/or intervention device comprising at least a mechanical part and a system for actuating the mechanical part, the actuation system comprising a piston capable of sliding a rod and connected to means for supplying a fluid under pressure; a drive element connected to the mechanical part and comprising dissymmetrical teeth; and a bearing element assembled on the rod and capable, on displacement of the rod along a determined direction, of cooperating with a tooth of the drive element to displace the drive element by a determined distance and/or angle, the bearing element, on displacement of the rod along the direction opposite to the determined direction, exerting a substantially zero force on the drive element to avoid displacing the drive element.