Abstract: A device for displacing a member in a graduated and motor-driven manner inside a tube that is external to the device, the member being displaced from one end of the tube, the displacement being evaluated by at least one position encoder delivering information on the position of the member placed at one end of a tubular element which is mounted to move in translation on a support body for supporting the device. The tubular element is in the form of a member-carrying worm which is provided with longitudinal toothed racks on its periphery. The worm is displaced in translation relative to the body which carries it under the action of a first toothed belt which is mounted on the support body, which is driven by a motor fixed to the body, and which is meshed with one of the racks. Another of the racks meshes with a second toothed belt which entrains a position encoder fixed to the body.