# A Novel Impact Rotary-Linear Motor Based on Decomposed Screw-Type Motion of Piezoelectric Actuator
## 1 Introduction  
The impact piezoelectric motor is known as the stick-slip motor or inertia driving mechanism [1]. The piezoelectric actuator (PA) works as a stator which is excited by a saw-shaped driving voltage to produce periodic asymmetric motion [2; 3]. According to the unequal value of driving force and friction force between the contact surfaces of the stator and shaft, the shaft will realize single-direction stepping motion [4]. The impact motor often has a simple structure compared to the inchworm motor and it is often easy to be controlled compared to the ultrasonic motor. The impact piezoelectric motor has higher positioning accuracy and smaller structure compared to the traditional electromagnetic motors, thus it has been widely used in scanning probe microscopes, micromanipulators, microrobots, precision positioning occasions, and so on [5; 6].  
The multi-degree-of-freedom (multi-DOF) motor has more flexible motion compared to the single-DOF motor [7; 8; 9]. Nowadays, many structures which can realize rotary and linear motion and screw-type motion have been studied. The screw-type motion is a special coupled motion of rotary motion and linear motion. The motor which consists of two or more actuators can realize rotary and linear motion [10; 11]. This kind of motor can realize independent rotary and linear motion, and the two motions usually cannot move at the same time [12; 13]. The ultrasonic motor which uses the two specific vibration modes can realize a coupled rotary-linear or screw-type motion [14; 15; 16]. This kind of motor often realizes the screw-type movement with screw output shaft [17; 18; 19; 20]. The motor which usesthe special helical structure can realize screw-type motion under voltage drive. This kind of motor often easily achieves screw-type motion. However, according to its helical structure, the screw-type motion is often difficult to decompose into single rotary and single linear motion [21; 22; 23; 24; 25].  
The helical structure can produce screw-type motion. [PERSON] proposed a novel screw-type motion which is generated by the PA with helical interdigitated electrodes [26]. This kind of PA has the advantages of small size and simple manufacturing process. Since converting this screw-type motion or coupled rotary-linear motion of the PA with helical interdigitated electrodes to 2-DOF motions (independent rotary and linear motion) is not easy, sometimes one direction of the motion is restricted by the structure design or structure parameter, such as [PERSON] proposed [27]. This way will restrict the space and reduce the DOF of the rotary-linear motion.  
In this study, a way of changing the screw-type motion into independent rotary and linear motions is proposed in order to solve the problem of converting the coupled rotary-linear motion which is produced by a PA with helical interdigitated electrodes to independent 2-DOF motions. By two different friction interfaces and two adjusting structures of friction force, the coupled rotary-linear motion of PA can be converted into two independent DOF motions of the output shaft. This method will provide an idea for decomposing the complex curvilinear motion into some simple motions. The prototype motor is fabricated and tested. The structure and working principle of decomposed independent rotary and linear motions are introduced and detailed in Section 2. The fabrication of the piezoelectric motor is detailed in Section 3. In Section 4, a series of experiments of the prototype motor are carried out and the discussions of the experimental results are given.