A mechanical-free designing method for tailoring nonlinearity in bi-stable piezoelectric energy harvesters

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.