**1. Introduction**

Piezoelectric stick–slip actuators have obtained considerable significance and have become the focus area of research for camera focusing mechanisms, cell phones, scanning probe micro-scopes, zoom lens systems and blue-ray devices, because of compact structure, low cost, theoretically unlimited displacement and convenient control [1–3]. Based on the configuration designs, the piezoelectric stick–slip actuators are divided into parallel type [4–6] and non-parallel type [7–9], as illustrated in **Figure 1**.

The parallel type stick–slip actuator means the motion direction of the stator is parallel to that of the slider. For example, Lee et al. presented a stick–slip actuator

**Figure 1.**

*Schematic diagram of parallel and non-parallel structures. (a) Parallel structure. (b) Non-parallel structure with an angle. (c) Non-parallel structure with asymmetrical flexure hinge.*

with the parallel structure using a piezoelectric element and two sliders; and the friction force could be controlled by the spring and the screw [4]. Hunstig et al. proposed a stick–slip actuator with parallel structure, and the friction force was controlled by means of magnets, which could also be predicted by the established friction model [6]. However, the friction force of the parallel type stick–slip actuators cannot be adjusted during the operation, which results in an obvious backward motion. In addition, the velocity and load of such actuators are also seriously restrained.

The piezoelectric stick–slip actuator with non-parallel structure means that there is a movement angle between the stator and the slider. For instance, Li et al. proposed a linear stick–slip actuator with a non-parallel structure based on the lateral motion, which could be generated by a parallelogram-type flexure hinge mechanism and a piezoelectric element [7]. Wen et al. presented a rotary stick–slip actuator with non-parallel structure, and the friction force could be regulated by changing the positive pressure between the slider and stator [9]. Although the velocity and load of the non-parallel type stick–slip actuators are improved relative to the parallel type stick–slip actuators, it is still difficult to meet the actual application requirements. Meanwhile, the backward motions are also generated during the operation. Besides, such actuators are generally structurally asymmetric and perform poor consistency in bidirectional performance.

In this regard, our team focused deeply on the fields of the piezoelectric stick–slip actuators in both structural designs and driving methods. In terms of structural designs, the axial stiffness of the stator is unevenly distributed, and the parasitic motion is generated by introducing the flexure hinge mechanism into the design of the stator, including trapezoid-type piezoelectric stick–slip actuator [10], piezoelectric stick–slip actuators with asymmetrical flexure hinges [11, 12] and mode conversion piezoelectric stick–slip actuator [13]; these structural designs can comprehensively adjust the friction force during the movement of the stick–slip actuator, thereby significantly improving the velocity and load; besides, a coupled asymmetrical flexure hinge mechanism is also developed to achieve the bidirectional motion of the non-parallel type stick–slip actuators [14]. In terms of driving methods, a nonresonant mode smooth driving method (SDM) based on ultrasonic friction reduction is first proposed for the parallel type stick–slip actuator, and the backward motion is restrained during the rapid contraction stage [15]. According to ultrasonic friction reduction, the smaller kinetic friction between the frictional part and the slider can be realized at the resonant frequency. A resonant mode SDM is further developed to improve the performance of the stick–slip actuator [16]. On this basis, the low voltage characteristics and symmetry of the SDM are researched [17, 18]. Finally, a directionguidance hybrid method (DGHM) is proposed for the non-parallel type stick–slip

*The Asymmetric Flexure Hinge Structures and the Hybrid Excitation Methods for Piezoelectric… DOI: http://dx.doi.org/10.5772/intechopen.95536*

actuator with flexure hinge mechanism [19], and the superior performance of this excitation method is achieved, especially for high speed.
