*Article* **Vibration Energy Harvester Based on Torsionally Oscillating Magnet**

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

<sup>1</sup> School of Electrical Engineering, Xi'an Jiaotong University, Xi'an 710049, China;

xywang@stu.xjtu.edu.cn (X.W.); twinklestar2@stu.xjtu.edu.cn (J.L.); ZhouChenyuan@stu.xjtu.edu.cn (C.Z.) <sup>2</sup> Micro and Nano Electromechanical Systems Laboratory, Northwestern Polytechnical University,

Xi'an 710072, China; taokai@nwpu.edu.cn (K.T.); dyqiao@nwpu.edu.cn (D.Q.)

**\*** Correspondence: liyunjia@xjtu.edu.cn

**Abstract:** Most of the miniaturized electromagnetic vibrational energy harvesters (EVEHs) are based on oscillating proof mass suspended by several springs or a cantilever structure. Such structural feature limits the miniaturization of the device's footprint. This paper presents an EVEH device based on a torsional vibrating magnet over a stack of flexible planar coils. The torsional movement of the magnet is enabled by microfabricated silicon torsional springs, which effectively reduce the footprint of the device. With a size of 1 cm × 1 cm × 1.08 cm, the proposed EVEH is capable of generating an open-circuit peak-to-peak voltage of 169 mV and a power of 6.9 µW, under a sinusoidal excitation of <sup>±</sup>0.5 g (g = 9.8 m/s<sup>2</sup> ) and frequency of 96 Hz. At elevated acceleration levels, the maximum peak-to-peak output voltage is 222 mV under the acceleration of 7 g (±3.5 g).

**Keywords:** electromagnetic; energy harvester; MEMS; planar coil
