Piezoelectric vibration energy harvesting technologies have attracted a lot of attention in recent decades, and the harvesters have been applied successfully in various fields, such as buildings, biomechanical and human motions. One important challenge is that the narrow frequency bandwidth of linear energy harvesting is inadequate to adapt the ambient vibrations, which are often random and broadband. Therefore, researchers have concentrated on developing efficient energy harvesters to realize broadband energy harvesting and improve energy-harvesting efficiency. Particularly, among these approaches, different types of energy harvesters adopting magnetic force have been designed with nonlinear characteristics for effective energy harvesting. This paper aims to review the main piezoelectric vibration energy harvesting technologies with magnetic coupling, and determine the potential benefits of magnetic force on energy-harvesting techniques. They are classified into five categories according to their different structural characteristics: monostable, bistable, multistable, magnetic plucking, and hybrid piezoelectric–electromagnetic energy harvesters. The operating principles and representative designs of each type are provided. Finally, a summary of practical applications is also shown. This review contributes to the widespread understanding of the role of magnetic force on piezoelectric vibration energy harvesting. It also provides a meaningful perspective on designing piezoelectric harvesters for improving energy-harvesting efficiency.

The quasi-zero stiffness(QZS) nonlinear isolators have been paid much attention in recent literatures due to their excellent vibration isolation performance under low frequency excitation compared to traditional linear vibration isolators. However, passive QZS isolators are the most widely studied and they are incapable of dealing with varying conditions such as changing the excitation frequency or load in operating state. To promote the adaptability of QZS isolators, a novel approach to achieve real-time controllable QZS is proposed in this paper. An electromagnetic negative stiffness unit comprised of two electromagnet with MR elastomer is employed to produce negative stiffness, and the stiffness characteristic of the system can be accurately controlled by adjusting the driving current according to the displacement information in real time. Theoretical calculation and finite element analysis is conducted to establish the exact model of electromagnetic force followed by the general design of the system. Then a customized driver is developed for electromagnetic negative stiffness unit, and the dynamic model of the isolator is set up to study the effect of relative parameters on the transmissibility. Finally, a series of experiments are performed to evaluate the real-time controllable QZS characteristic and confirm the vibration isolation effect of the proposed electromagnetic QZS isolator.

In the article, a novel 2-degree-of-freedom hybrid piecewise-linear piezoelectric–electromagnetic vibration energy harvester is presented to achieve better energy harvesting efficiency. The harvester consists of a primary piezoelectric energy harvesting device to which an electromagnetic mechanism is coupled to improve the integral energy output, and a driven beam is mounted to broaden the operating bandwidth by inducing nonlinearity. Considering the piezoelectric–electromagnetic coupling characteristics and the nonlinear factors, dynamic equations of the system are established. Expressions of the output power are deduced though averaging method. Characteristic parameters are analyzed theoretically, including the piezoelectric parameters, electromagnetic parameters, and the piecewise-linearity. Frequency sweep excitation test is conducted on the setup at an excitation acceleration of 0.3 g and results demonstrate that two resonant regions are obtained with the peak output power of 5.4 and 6.49 mW, respectively, and the operating bandwidth is increased by 8 Hz. Moreover, though adjusting the stiffness of the driven beam k₃ and the gap between the primary beam and the driven beam d, the performance of the harvester can be further optimized.

The porous foam metal magnetorheological fluid damper has a broad application prospect in the field of vibration isolation of precision instruments with small damping force because it does not need complex dynamic seal structure. The traditional single-ring magnetic pole porous foam metal magnetorheological fluid damper has a small effective area for the magnetic core that affects the damper output range due to the geometric constraints of the coil and the low magnetic field utilization. Therefore, in order to increase the effective area ratio of the magnetic core, this article introduces the built-in multi-pole magnetic core into the porous metal magnetorheological fluid damper and integrates four axial wound fan-shaped magnetic poles on the magnetic core to improve the output performance of the damper. The magnetic circuit is analyzed based on Ohm’s law of magnetic circuit, and the mathematical model of damping force is established. Based on this, the important geometric parameters of the damper are determined. The finite element method is used to simulate the magnetic field of the damper, and the output performance of the damper is numerically simulated. The dynamic performance test system of the damper is set up to test the designed damper, and the test data and numerical simulation results are verified with each other. The results show that the damping force peak and dynamic regulation range of the damper designed in this article are higher than those of the traditional porous foam metal magnetorheological fluid damper with magnetic core, which effectively improves the mechanical properties of the magnetorheological fluid damper with porous foam metal.