This actuator was developed out of a need for a cryogenic actuator that can operate effectively in spite of the thermal mismatch involved with construction materials that have different expansion coefficients. Also, there is a need for a cryogenic motor that can drive infrared systems and produce minimal thermal energy that can interfere with their operation.
The classic ultrasonic motor uses an annular piezoelectric thin plate bonded to a metallic ring with conductive adhesive. In order to generate a traveling wave on the surface, the piezoelectric element electrodes must be segmented with the polarization direction of each of the sectors opposite from their neighbors. When piezoelectric plates with forward-and reverse-poled sections are driven by two-phase electrical power sources, a traveling wave is generated and a rotor pressed onto the stator surface is propelled into motion by frictional forces. Unfortunately, manufacturing this type of motor is very complex, and the production cost for construction is high.
The solution is based on using effective cryogenic piezoelectric stacks that are operational at extreme temperatures, and particularly at cryogenic temperatures. The proposed ultrasonic horn type rotary motor uses piezoelectric multilayer stacks, where a number of thin, alternately poled piezoelectric layers are connected mechanically in series and the electrodes are connected in parallel with two — a positive and a negative — terminals. The poling directions in the piezoelectric layers are from the electrodes of the positive terminal to the negative. The presence of the horn further amplifies the stroke of the piezoelectric stack with a proper design. Thus, this construction produces a large-output torque with minimal drive voltage and power compared to the conventional one.
In addition, the fabrication process is much simpler and production cost is relatively inexpensive. One unique advantage of these motors is that they can be designed in a monolithic structure, so it does not require an adhesive layer between the piezoelectric material and the stator, or a bolt/spring clamping structure to provide pre-stress on the piezoelectric materials.
This structure allows for not only a mechanical simplification of the structure, but also a low-mechanical-loss, high-efficiency electromechanical system. The contact wear problem of this motor is less severe compared to a standing wave-type motor, which requires impacts periodically at the driving frequency.
This motor configuration consists of eight piezoelectric stacks and ultrasonic horns with flexures that drive a stator as well as a rotor. The piezoelectric stacks are pre-stressed using flexures to provide compressive force, and are positioned perpendicular to each other (see figure). The motor works by applying two alternating voltages whose phases differ by 90° to each other both in time and in space, which produces a traveling wave on the surface of a metallic ring stator. This ultrasonic mechanical wave on the stator surface drives the rotor forward or backward by friction, where the direction of movement is changed by reversing the phase of the excitation signals.