In the latest generation of astronomical telescopes the increase of the primary mirror diameter has placed ever increasing demands on the technical performance of the mirror support systems. In this paper the authors discuss the mechanical and the electronic active control system design and subsequent tests of the position actuator prototype that mechanically link the 8.4 m honeycomb mirrors of the large binocular telescope to six rigidly reinforced locations in each primary mirror cell structure. During telescope operation, these adjustable length actuators precisely control the six degrees of freedom of motion of the mirror. Each actuator has a high mechanical axial stiffness and, as new feature, an active control system, based on piezoelectric elements, in order to increase its axial stiffness, with a bandwidth from dc up to 30 Hz, assuring that the natural frequencies of the mirror do not degrade the optical performance of the telescope. Moreover, other requirements have been satisfied in the mechanic of the actuators: flexures are provided on each end to minimize any moments applied to the attachment of the actuator to the mirror; one axial load cell for each actuator provides a precise measurement of the external forces applied to the mirror, such as wind loads, for the control of the pneumatic force system that supports the weight of the mirror; a very sensitive and precise capacitive sensor measures the total length of the actuator to sub-micron resolution upon request. Each actuator has a reliable fail- safe system that limits the compressive and tensile forces that can be applied to the mirror. The mechanical and the electronic design, and the later experimental tests, of this actuator prototype have been performed in the Arcetri Laboratories under the supervision of the authors of this paper.