A torsional actuator has been developed at NRL utilizing the high piezoelectric shear coefficient, d15. This torsional actuator uses an even number of alternately poled segments of electroactive PZT. Under an applied electric field, the torsional actuator produces large angular displacement and a high torque. The solid freeform fabrication technique of the laminated object manufacturing (LOM) is used for rapid prototyping of torsional actuator with potential cost and time saving. First step to demonstrate the feasibility of the LOM technique for the torsional actuator device fabrication is to make near net shape segments. We report a prototype PZT torsional actuator using LOM prepared PZT-5A segments. Fabrication processes and test results are described. The torsional actuator PZT-5A tube has dimensions of 13 cm long, 2.54 cm OD and 1.9 cm ID. Although the piezoelectric strain is small, it may be converted into large displacement via accumulation of the small single cycle displacements over many cycles using AC driving voltage such as with a rotary 'inchworm' actuator or an ultrasonic rotary motor. A working prototype of a full-cycle motor driven by the piezoelectric torsional actuator has been achieved. The rotational speed is 1,200 rpm under 200 V/cm field at the resonant frequency of 4.5 kHz.
The displacement output of the telescoping actuators, as developed in the NRL, depend on the material properties. Recently developed relaxor ferroelectric materials, with compositions near the morphotropic phase boundary and in single crystal forms, show high strain output capability and high coupling coefficients. These materials, when design into devices, would provide high performance, in terms of displacement output, and high transduction efficiency. We have utilized one of these single crystal materials, namely, lead zinc niobate-lead titanate, PZN-PT, for the construction of the NRL telescoping actuators. The evaluation of the piezoelectric performance of these single crystal-based telescoping actuators showed much higher displacement outputs than that for the ones previously made with polycrystalline PZT ceramics.
A family of high authority actuators was developed at the Naval Research Laboratory. These actuators are based on displacement amplification within a compact, solid state, monolithic piezoelectric actuator, by using a telescoping tube design. In this design, concentric tubes are mechanically connected in series. This gives an effective actuator length that is equal to the sum of the lengths of the individual concentric elements. The high displacement output of this actuator permits efficient coupling of the actuator output into a load of similar impedance, and thereby much greater effective actuator output. Initial prototypes were made of commercially available PZT tubes of three different diameters and wall thickness. These tubes were pulsed through the thickness of the walls and the change in their lengths were used for actuation. Their actuation is therefore making use of the d31 piezoelectric coefficient. Alternatively, electrodes can be applied to the ends of the individual concentric tubes and their lengthwise displacement will subsequently be proportional to the d33 parameter of the material. The tubes were bonded at their ends to alumina plates using epoxy-based adhesive. The displacement obtained from the assembly is close to the sum of those of the three individual tubes at the same applied field. Other parameters such as blocking force and energy densities are also reported. These actuators have applications where high force and simultaneously large displacement are required and space is limited. Potential uses include high end aerospace as well as low tech commercial applications.
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