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This paper presents a mechanical-free method for providing and tailoring the nonlinear force in bistable piezoelectric energy harvesters (BPEHs). The nonlinear force can be tailored to obtain a lower threshold for inter-well motions, or for the harvester to operate at various excitation levels and frequencies without changing the mechanical structure or the overall assembly. In BPEHs, the nonlinear force is tailored to match a specific excitation level and frequency, and the mechanical structure is designed to achieve higher strain (and thus higher output power). The design of nonlinearity can be separated from the design of the mechanical structure by using magnetic interactions. Hence, the design of nonlinearity is the arrangement of the external magnetic field of the harvester. In this paper, arranging the external magnetic field is achieved by arranging the magnetization distribution of one external magnet. With the locally demagnetizing technique, a uniformly magnetized permanent magnet can be locally demagnetized with desired patterns. The external magnetic field is provided by a locally demagnetized permanent magnet (LDPM). The nonlinear force can be tailored by simply altering the properties of the LDPM. This method converts the design of providing and tailoring the nonlinear force into the design of the LDPMs. For demonstration, we show that without increasing the distance between magnets, the potential barrier of the bistable system is dramatically reduced by using LDPMs. Melnikov’s method is utilized to show that the energy harvesters with LDPMs possess a lower threshold for homoclinic tangency than energy harvesters with a normal magnet. The influence of the parameters of the LDPMs on the energy harvesting performance is studied via simulations and experiments. Results show that without violating the mechanical part, changing the locally demagnetizing patterns can effectively change the harvester's working frequency and excitation threshold.

PurposeThe existing Nonlinear Dynamic Vibration Absorbers (NLDVAs) have the disadvantages of complex structure, high cost, high installation space requirements and difficulty in miniaturization. And most of the NLDVAs have not been applied to reality. To address the above issues, a novel Triple-magnet Magnetic Dynamic Vibration Absorber (TMDVA) with tunable stiffness, only composed of triple cylindrical permanent magnets and an acrylic tube, is designed, modeled and tested in this paper.Design/methodology/approach(1) A novel TMDVA is designed. (2) Theoretical and experimental methods. (3) Equivalent dynamics model.FindingsIt is found that adjusting the magnet distance can effectively optimize the vibration reduction effect of the TMDVA under different resonance conditions. When the resonance frequency of the cantilever changes, the magnet distance of the TMDVA with a high vibration reduction effect shows an approximately linear relationship with the resonance frequency of the cantilever which is convenient for the design optimization of the TMDVA.Originality/valueBoth the simulation and experimental results prove that the TMDVA can effectively reduce the vibration of the cantilever even if the resonance frequency of the cantilever changes, which shows the strong robustness of the TMDVA. Given all that, the TMDVA has potential application value in the passive vibration reduction of engineering structures.

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