**2. Overview of piezoelectric actuators**

Direct-driving piezoelectric actuators mainly utilize piezoelectric elements (such as piezoelectric stack, piezoelectric bimorph) to directly push the output mechanism, which could be easy to achieve significant advantages of high output accuracy, large output load, and compact size structure [7, 8]. The development of flexure hinge technology has greatly expanded the application field of piezoelectric actuators. Since the end of last century, researchers from the United States, Japan, Australia, Germany, China, and other countries have competed to develop various types of direct-driving piezoelectric actuators based on flexure hinge mechanism, which can be further divided into piezoelectric actuators without amplification mechanism and piezoelectric actuators with amplification mechanism.

Ultrasonic piezoelectric actuator, also known as ultrasonic motor, is one stepping piezoelectric actuator developed rapidly in the 1980s [9, 10]. It is based on inverse piezoelectric effect and the principle of ultrasonic vibration. In the working process, the micro-elliptical resonance of the elastomer in the ultrasonic frequency range is excited by the inverse piezoelectric effect of piezoelectric materials and transformed into the rotation or linear motion of the mover through the action of friction, so as to achieve the required output displacement and load. According to different wave propagations, they can be divided into standing wave ultrasonic piezoelectric actuators and traveling wave ultrasonic piezoelectric actuators. Commercial applications have been obtained for ultrasonic piezoelectric actuators.

The motion principle of friction-inertia piezoelectric actuator is based on the law of conservation of momentum [5, 6, 11]. By applying voltage signals, such as sawtooth wave to piezoelectric elements, the relative displacement between stator and mover or two mass blocks with different mass is generated, so as to realize stepby-step large stroke motion. The structure of friction-inertia piezoelectric actuator is relatively simple, and sometimes, the displacement output with high accuracy can be realized only with two moving parts (stator and mover), and the control strategy is also relatively simple. Therefore, it has attracted the continuous attention *Introductory Chapter: Piezoelectric Actuators DOI: http://dx.doi.org/10.5772/intechopen.104232*

of researchers all over the world. According to different motion principles, it can be further divided into impact-inertia piezoelectric actuator and stick-slip piezoelectric actuator.

Bionic piezoelectric actuator is one novel piezoelectric actuator, which mimics the motion style of different creatures in the nature to overcome the limitation of traditional piezoelectric actuators [2, 12, 13]. Bionic piezoelectric actuators are able to achieve large working stroke or large output force, which is of great significance for the development of piezoelectric actuators. Based on different motions of creatures, bionic piezoelectric actuator can be further divided into inchworm-type piezoelectric actuators, walking-type piezoelectric actuators, walrus-type piezoelectric actuators, and so on.

Up to now, many kinds of piezoelectric actuators have been proposed and investigated, and some kinds of piezoelectric actuators have been applied into real applications. However, due to the friction and wear of materials, how to achieve the high accuracy and long-term reliability for piezoelectric actuators is still a problem. This book introduces some basic foundations of piezoelectric actuators, including the piezoelectric phenomenon, the modeling and control of piezoelectric actuators, different kinds of piezoelectric actuators, and some applications in different fields of piezoelectric actuators. We hope this book could give an overview of piezoelectric actuators for the new researchers to get a basic introduction.
