# Motion Characteristics of Self-Sensing Piezoelectric Actuator for Yarn Micro-Gripper
## 1 Introduction  
The micro-gripper is a device that has been widely used in the field of micromanipulation and micro-assembly, such as MEMS [1; 2; 3], bio-engineering [4; 5; 6], advanced optics [7; 8; 9], precision instruments [10], and other fields. As an end-effector that performs micro-assembly and micro-manipulation tasks, it directly interacts with the manipulated object. The actuator provides force and displacement to the clamp and is the power source to ensure the smooth progress of the work. In addition, the clamping stroke and resolution of the micro-clamp mainly depend on the performance of the actuator, and an excellent driving method is conducive to improving its performance. Different micro-grippers have been investigated according to different actuation principles. They can be driven by vacuum [11; 12; 13], electrostatic [14; 15], electrothermal [16; 17; 18], shape memory alloy (SMA) [19; 20; 21], or piezoelectric [22; 23; 24; 25; 26; 27; 28; 29; 30] activity. Compared with other micro-grippers, piezo-electric materials exhibit advantages such as simple structure, high resolution, and fast response capabilities. Depending on the structure, piezoelectric micro-grippers can be classified into the compliant mechanism type [22; 23; 24] or the bimorph type [25; 26; 27; 28; 29; 30]. The compliant mechanism type uses a stacked piezoelectric actuator to drive a lever amplification mechanism to achieve clamping of micro-objects by the gripper. The finger clamping force of the compliant-mechanism-type micro-gripper is large. Nevertheless, greater finger displacement must be achieved by amplifying the compliant mechanism, making it complex and extensive. This gives it no advantage in compact applications. Each finger of the bimorph micro-gripper is a cantilever beam consisting of two piezoelectric elements glued to either side of the substrate. One is used for expansion andone for contraction in response to the driving voltage. The two fingers produce opposite bending micro-displacements for clamping the object. The bimorph micro-gripper has less clamping force but more displacement. It can be fabricated using IC technology and can be miniaturized for greater advantage in applications. To meet the demands of micromanipulation tasks with very accurate and fast response times, a variety of sensors can be used. However, these external sensors are not ideal for the micrometer scale. This is due to the limitations of size, performance, and degrees of freedom of piezoelectric ceramic actuators. Therefore, self-sensing methods have subsequently become a hot research topic [31]. Meanwhile, self-sensing of piezoelectric ceramics can also be used to significantly reduce the cost of control systems by eliminating expensive sensors. There have been related studies on self-sensing of piezoelectric ceramics, such as voltage detection [25; 32; 33], charge detection [28; 29; 34; 35; 36; 37], permittivity detection [38; 39; 40], and impedance detection [41; 42]. In addition, it can be seen from previous studies that the resolution of piezoelectric ceramic self-sensing can also be sub-micron, comparable to that of external sensors. Therefore, the piezoelectric microgrippers with self-sensing can be better applied in practical production. Piezoelectric micro-grippers have been used in various fields, but as existing work has focused on micron- or even nanometer-scale clamping targets, in the textile field, their application is not widespread [43]. The weft accumulator is used for temporary storage of weft yarns in rapier and piece-shuttle weaving machines, which creates good conditions for weft rewinding at high speed for the weft draw-in, and allows the rewinding of weft yarns to obtain a more uniform tension. The yarn gripper is one of the important components in the control unit of the weft accumulator due to its lifting and dropping of the pin to complete the weft of the fixed-length rewinding and control the rewind time. Its performance directly affects the weaving machine performance and yarn quality.