Piezoelectric Motor Using In-Plane Orthogonal Resonance Modes of an Octagonal Plate
Piezoelectric motors use the inverse piezoelectric effect, where microscopically small periodical displacements are transferred to continuous or stepping rotary or linear movements through frictional coupling between a displacement generator (stator) and a moving (slider) element. Although many piezoelectric motor designs have various drive and operating principles, microscopic displacements at the interface of a stator and a slider can have two components: tangential and normal. The
... mal. The displacement in the tangential direction has a corresponding force working against the friction force. The function of the displacement in the normal direction is to increase or decrease friction force between a stator and a slider. Simply, the generated force alters the friction force due to a displacement in the normal direction, and the force creates movement due to a displacement in the tangential direction. In this paper, we first describe how the two types of microscopic tangential and normal displacements at the interface are combined in the structures of different piezoelectric motors. We then present a new resonance-drive type piezoelectric motor, where an octagonal plate, with two eyelets in the middle of the two main surfaces, is used as the stator. Metallization electrodes divide top and bottom surfaces into two equal regions orthogonally, and the two driving signals are applied between the surfaces of the top and the bottom electrodes. By controlling the magnitude, frequency and phase shift of the driving signals, microscopic tangential and normal displacements in almost any form can be generated. Independently controlled microscopic tangential and normal displacements at the interface of the stator and the slider make the motor have lower speed-control input (driving voltage) nonlinearity. A test linear motor was built by using an octagonal piezoelectric plate. It has a length of 25.0 mm (the distance between any of two parallel side surfaces) and a thickness of 3.0 mm, which can produce an output force of 20 N. In one way, there are normal and tangential microscopic displacements to the moving direction of a sliding element. Each actuator in a motor that generates a microscopic displacement has one single task, which is either to perform a "clamp" or "move" action. If an actuator has the task of performing a "clamp" action, the displacement is normal to the slider moving direction and the ultimate function of these actuators is to increase or decrease normal force and thus, to hold friction force. If an actuator is required to perform a "move" action, the displacement generated by the actuator is tangential to the moving direction of the sliding element. Expansion and shrinkage of this actuator creates a microscopic movement of the slider. After the structure proposed by Brisbane in 1965 , some other structures operated on the basis of the piezo-walk-drive principal     . Typically, these structures consist of three actuators, where two of them are responsible for the clamping and one is responsible for the moving action. In these motors, the required displacements in the normal and tangential directions, with respect to a sliding element, can be generated by actuators that use longitudinal, shear, transverse, and planar coupling of piezoelectric materials. In the early structures, the piezoelectric elements used in the piezo-drive type motors were in bulk forms, but in many of the commercialized structures, the actuators are manufactured in multilayer forms to generate sufficient displacement at a relatively low-driving voltage. Assuming that the activation makes the length, or the diameter, decrease and release, which causes an actuator to return to its rest position, the motion sequence as seen in Figure 1 can be started by activating one clamping actuator (A). When the moving actuator (C) is also activated, shrinkage of the moving actuator generates a half step. At this moment, the clamping actuator (A) is released so that it can clamp and maintain the holding force. In the following step, the second clamping actuator (B) is activated and the moving actuator (C) is released. When the second clamping actuator (B) is released, one motion sequence is finished.