*Article* **Visual Servo Control System of a Piezoelectric 2-Degree-of-Freedom Nano-Stepping Motor**

#### **Cheng-Lung Chen and Shao-Kang Hung \***

Department of Mechanical Engineering, National Chiao Tung University, No. 1001, University Road, Hsinchu 30010, Taiwan; bruce.me08g@nctu.edu.tw

**\*** Correspondence: skhung@nctu.edu.tw; Tel.: +886-3571-2121 (ext. 55115)

Received: 9 October 2019; Accepted: 22 November 2019; Published: 25 November 2019

**Abstract:** A nano-stepping motor can translate or rotate when its piezoelectric element pair is electrically driven in-phase or anti-phase. It offers millimeter-level stroke, sub-micron-level stepping size, and sub-nanometer-level scanning resolution. This article proposes a visual servo system to control the nano-stepping motor, since its stepping size is not consistent due to changing contact friction, using a custom built microscopic instrument and image recognition software. Three kinds of trajectories—straight lines, circles, and pentagrams—are performed successfully. The smallest straightness and roundness ever tested are 0.291 μm and 2.380 μm. Experimental results show that the proposed controller can effectively compensate for the error and precisely navigate the rotor along a desired trajectory.

**Keywords:** piezoelectricity; visual servo control; stepping motor; nano-positioner; stick-slip
