A fluid-dynamic actuator having a detection system for determining the position of a piston along its path of travel within a cylinder. The piston includes a head and rod extending therefrom. First grooves are machined on respective sections of the rod. Projections of the first grooves onto a first plane parallel to the longitudinal axis of the rod define segments arranged obliquely with respect to the axis. Second grooves extend along the axis for predetermined lengths such that a respective number of second grooves run across each section. Optical sensors detect the presence of second grooves through a second plane transverse to the axis and a displacement of segments along a line coplanar with the second plane due to movement of the piston. A processor counts detected second grooves to determine the position of the piston as a function of the count of second grooves and the displacement of segments.