One of the major challenges for vibration energy harvesters is to match their resonant frequency with ambient vibration frequency. The main limitation of the existing frequency-tuning techniques is the uncontrollable tuning process, i.e., the incapability of tuning the frequency to a desired value. In this article, we report a tunable electromagnetic vibration energy harvester (EVEH) capable of being tuned to a specific target frequency. The device adjusts the resonant frequency by changing the effective length of two polyimide springs through a set of smart tuning structures with scales. A maximum frequency-tuning range of 58 Hz (46–104 Hz, 126% relative tuning range) is realized with the proposed device. At a sinusoidal acceleration of 1 g, an output peak-to-peak voltage of 263.4 mV and an output power of $143.6~\mu \text{W}$ are obtained at the frequency of 56 Hz.

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.

This paper presents a low-frequency electromagnetic vibrational energy harvester (EVEH) with two degrees of freedom and two resonant modes. The proposed EVEH is based on a disc magnet suspended in a pendulum fashion by a polymeric spring between two sets of polymer coil stacks. The fabricated EVEH is capable of harvesting vibration energy on two directions with an extended bandwidth. With a sinusoidal acceleration of ±1 g on Z direction, a peak-to-peak closed-circuit output voltage of 0.51 V (open-circuit voltage: 1 V), and an output power of 35.1 μW are achieved at the resonant frequency of 16 Hz. With a sinusoidal acceleration of ±1.5 g on X direction, a peak-to-peak output voltage of 0.14 V and power of 2.56 μW are achieved, at the resonant frequency of 20 Hz.

A miniaturized reliability test system for microdevices with controlled environmental parameters is presented. The system is capable of measuring key electrical parameters of the microdevices while controlling the environmental conditions around the microdevices. The test system is compact and thus can be integrated with standard test equipment for microdevices. By using a feed-forward decoupling algorithm, the presented test system is capable of generating a temperature range of 0–120 °C and a humidity range of 20–90% RH (0–55 °C), within a small footprint and weight. The accuracy for temperature and humidity control is ±0.1 °C and ±1% RH (30 °C), respectively. The functionality of the proposed test system is verified by integrating it with a piezo shaker to test the environmental reliability of an electromagnetic vibration energy harvester. The proposed system can be used as a proof-of-technology platform for characterizing the performance of microdevices with controlled environmental parameters.

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 ±0.5 g (g = 9.8 m/s²) 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).

In this work, a packaging scheme for electromagnetic vibration energy harvester is designed and implemented, in order to achieve resonant frequency tuning and magnetic shielding simultaneously. The proposed packaging scheme is realized by the combination of a soft magnetic tuning component and housing. A proof-of-concept electromagnetic vibration energy harvester prototype is fabricated to verify the proposed functionality, capable of realizing targeted frequency tuning and shielding magnetic field. The fabricated energy harvester prototype achieves a tuning range of 112-192 Hz (71.4%). Compared to the traditional frequency-tuning method based on permanent magnet, the proposed approach realizes a quasi-linear, accurate, and targeted tuning process with optimal tuning range. In addition, the proposed packaging scheme also shields the magnetic interference between the energy harvester and external environment, which is crucial for its application in industrial scenarios.