A device for moving a jet member on a carriage (6) between two selectable extreme positions along a guide (5) has means for detecting that the carriage has reached a selectable extreme position, said means comprising members (24, 25) adapted to form a mechanical resistance against continued movement of the carriage so as to influence the flow of hydraulic liquid in an hydraulic circuit (27) of a hydraulic drive arrangement for moving the carriage. Members (31 ) are arranged to sense a parameter of the hydraulic liquid, the magnitude of which is depending upon the flow of hydraulic liquid in the hydraulic circuit, and send information about the magnitude of this parameter to a control unit (14) adapted to control the turning of the carriage on the basis of this information.