*Article* **Electromagnetic Vibration Energy Harvester with Tunable Resonance Frequency Based on Stress Modulation of Flexible Springs**

**Yunjia Li 1,\* , Chenyuan Zhou <sup>1</sup> , Qi Cao <sup>1</sup> , Xinyi Wang <sup>1</sup> , Dayong Qiao <sup>2</sup> and Kai Tao 2,\***

> <sup>1</sup> School of Electrical Engineering, Xi'an Jiaotong University, Xi'an 710049, China; zhouchenyuan@stu.xjtu.edu.cn (C.Z.); caoqisky@126.com (Q.C.); xywang@stu.xjtu.edu.cn (X.W.)

> <sup>2</sup> Micro and Nano Electromechanical Systems Laboratory, Northwestern Polytechnical University, Xi'an 710072, China; dyqiao@nwpu.edu.cn

**\*** Correspondence: liyunjia@xjtu.edu.cn (Y.L.); taokai@nwpu.edu.cn (K.T.)

**Abstract:** This paper presents a compact electromagnetic vibrational energy harvester (EVEH) with tunable resonance frequency. The resonance frequency of the EVEH is tuned by adjusting the axial stress in the flexible polymeric springs, which is realized by physically pulling and pushing the springs. The stress tuning functionality is realized with a compact structure with small volume. The total frequency tuning range of the proposed EVEH is 56 Hz (74–130 Hz), which is 64% of the natural resonance frequency of the EVEH (88 Hz). It is found that the tensile stress increases the resonance frequency of the EVEH, while the compressive stress firstly reduces the resonance frequency and then increases the resonance frequency due to buckling.

**Keywords:** polymer beam; vibration energy harvesting; tunable resonant frequency
