Harvesting vibration energy to power wearable devices has become a hot research topic, while the output power and conversion efficiency of a vibration energy harvester with a single electromechanical conversion mechanism is low and the working frequency band and load range are narrow. In this paper, a new structure of piezoelectric electromagnetic coupling up-conversion multi-directional vibration energy harvester is proposed. Four piezoelectric electromagnetic coupling cantilever beams are installed on the axis of the base along the circumferential direction. Piezoelectric plates are set on the surface of each cantilever beam to harvest energy. The permanent magnet on the beam is placed on the free end of the cantilever beam as a mass block. Four coils for collecting energy are arranged on the base under the permanent magnets on the cantilever beams. A bearing is installed on the central shaft of the base and a rotating mass block is arranged on the outer ring of the bearing. Four permanent magnets are arranged on the rotating mass block and their positions correspond to the permanent magnets on the cantilever beams. The piezoelectric cantilever is induced to vibrate at its natural frequency by the interaction between the magnet on cantilever and the magnets on the rotating mass block. It can collect the nonlinear impact vibration energy of low-frequency motion to meet the energy harvesting of human motion.

This article presents an efficient piezoelectric (PE) energy harvesting (EH) circuit based on the techniques of series synchronized switch harvesting on inductor (Series-SSHI) and maximum power point tracking (MPPT). The MPPT is achieved using the proposed fractional normal-operation voltage (FNOV) method, which is based on the Series-SSHI operating principle. The FNOV method no longer disconnects the PE harvester from the load circuit to measure its open-circuit (OC) voltage. Instead, it measures the peak PE voltage during the normal Series-SSHI operation to enable the continuous EH process. The FNOV-MPPT circuit is implemented based on a variable width hysteresis controller for saving energy. In addition, the prototyped circuit can achieve the figure of merit factor of 2.68 and the maximum MPPT efficiency of 98.1% with the PE OC voltage ranging from 2 to 5 V. The measurements confirm the validity of the proposed FNOV-MPPT solution for the weakly coupled PE harvesters.

At present, most piezoelectric energy harvesters are developed based on a single piezoelectric transducer (PZT), but in practical applications, a vibration source can be equipped with piezoelectric transducer array in order to harvest more power. The time-sharing multiplexing scheme is mainly employed for multi-input piezoelectric energy harvesters. There is less study on the synchronized multi-input piezoelectric energy harvester. In this paper, a multi-input synchronous electric charge extraction (MI-SECE) interface based on buck structure for piezoelectric energy harvesting is proposed. Both theoretical analysis and experimental test results demonstrate the effectiveness of the proposed interface. The results show that the MI-SECE interface can extract energy from multi-PZTs simultaneously with any phase difference (0-2π).

Harvesting ambient vibration energy using piezoelectric elements is a popular energy harvesting technique. Energy harvesting efficiency is the research focus. Using synchronous electric charge extraction technology in piezoelectric energy harvesting systems can greatly improve the energy harvesting efficiency. This article presents a self-powered efficient synchronous electric charge extraction circuit for piezoelectric energy harvesting systems, in which four self-powered switch circuits are used to optimize the time sequence of charge extraction so that the rectifier bridge circuit used in traditional synchronous electric charge extraction can be saved. The effect of phase lag on extraction efficiency, system energy, and loss of overall circuit is analyzed. A piezoelectric vibration experimental platform is built for testing the power generation performance of the self-powered efficient synchronous electric charge extraction and those published energy harvesting circuits. The experimental results accord with the theoretical analysis. Moreover, the harvesting energy of the proposed self-powered efficient synchronous electric charge extraction is about three times more than those of the standard energy harvesting circuit under its maximum power point and the self-powered synchronized switch harvesting on inductor in most cases. The energy harvesting efficiency of self-powered efficient synchronous electric charge extraction remains at a high level (>80%) in most cases, and the maximum energy harvesting efficiency is up to 85.1%.

A self-powered hybrid rectifier based on the association of Series Synchronized Switch Harvesting on Inductor (S-SSHI) and Synchronous Electric Charge Extraction (SECE) is proposed for Piezo-Electric (PE) energy harvesters, aiming to achieve a good balance between the Rectifier Peak Output Power (RPOP) and Optimal Rectified Voltage Range (ORVR). The theory and simulation of the hybrid rectifier are presented with a good agreement. The measurements demonstrate that the prototyped hybrid rectifier can output the peak power of 0.28 mW and obtain the maximum end-to-end efficiency of 72.8% under PE open-circuit voltage of 6 V. It can achieve 1.55 times the RPOP of the SECE rectifier and 4.58 times the ORVR of the S-SSHI rectifier. The balance between the two indexes grasped in the hybrid rectifier is better than the others in the test. According to the estimation of ${FoM, BW_{0.9}}$ and ${RoI_{0.9}} factors, the overall performance of the hybrid rectifier is at the same level compared to state-of-the-art rectifiers. Additionally, the hybrid rectifier is easy to implement with some simple discrete components, which can be an alternative rectifier solution for PE harvesters.

A self-powered multi-input serial synchronized switch harvesting on inductor (MIS-SSHI) interface circuit with arbitrary phase difference for piezoelectric energy harvesting is proposed in this article. The proposed MIS-SSHI can extract energy from multiple PZTs based on S-SSHI technique and handle inductor access conflict to avoid mutual influence between the PZTs, without using extra inductor access controlling circuit. Four cases of different phases between PZTs are discussed to explain how to handle the inductor access conflict. Besides, the diodes in traditional S-SSHI circuit are removed to improve the conversion efficiency. An experimental platform is built for testing the power generation performance of the proposed circuit. The experimental results show that the proposed MIS-SSHI circuit can harvest energy from two PZTs with arbitrary phase difference (0-2π) based on single inductor. The maximum harvested power of the proposed MIS-SSHI circuit for single PZT can reach 3.7 times that of the standard energy harvesting circuit.