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.

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A broadband active self-tuning frequency piezoelectric energy harvester (AST-PEH) is proposed in this paper. It consists of a piezoelectric cantilever beam, an energy harvesting and control circuit (EHCC), a miniature stepper motor, a screw rod and a T-section mass. It can actively adjust the position of the T-section mass according to the ambient vibration frequency. It matches the AST-PEH's resonant frequency with the ambient vibration excitation frequency, and it widens the operating frequency range of the AST-PEH. The expression of the AST-PEH's resonant frequency function is constructed based on the dynamic and static theoretical analysis. The theoretical model of the AST-PEH under different position of the T-section mass is analyzed by ANSYS15.0 APDL, which is verified by two different materials and sizes. The tested two type substrate materials are steel and polylactic acid (PLA). The experimental results show that the frequency range of the two type harvesters are 4.8 Hz-6.3 Hz and 8.17 Hz-14.3 Hz respectively, and the widening rates of the working frequency are 31.25% and 75% respectively. The tested maximum output power of steel type AST-PEH is 0.32 mW at 6.3 Hz under 0.32 g acceleration and with a 100 KΩ load resistance.

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