Shengxi Zhou

Enhanced broadband piezoelectric energy harvesting using rotatable magnets

We investigate a magnetically coupled nonlinear piezoelectric energy harvester by altering the angular orientation of its external magnets for enhanced broadband frequency response. Electromechanical equations describing the nonlinear dynamic behavior include an experimentally identified polynomial for the transverse magnetic force that depends on magnet angle. Up- and down-sweep harmonic excitation tests are performed at constant acceleration over the range of 0–25 Hz. Very good agreement is observed between the numerical and experimental open-circuit voltage output frequency response curves. The nonlinear energy harvester proposed in this work can cover the broad low-frequency range of 4–22 Hz by changing the magnet orientation.

Influence of potential well depth on nonlinear tristable energy harvesting

Numerical and experimental investigation into the influence of potential well depth on tristable energy harvesting performance is provided. The potential well depth depending on the polynomial coefficients of nonlinear restoring force is analyzed along with its effect on the numerical energy harvesting performance. Experiment results reveal that the geometry parameters of the multi-stable configuration can alter the potential function of tristable energy harvesters. Moreover, the shallower potential well depth will enhance the broadband performance and the capability of harvesting energy from low frequency ambient vibration.

Nonlinear time-varying potential bistable energy harvesting from human motion

A theoretical and experimental investigation into nonlinear bistable energy harvesting with time-varying potential energy is presented. The motivation for examining time-varying potentials comes from the desire to harvest energy from human motion. Time-varying potential energy function of bistable oscillator with respect to the swing angle are established to derive the governing electromechanical model for harvesting vibration energy from the swaying motion during human walking or running. Numerical simulations show good agreement with the experimental potential energy function under different swing angles. Various motion speed treadmill tests are performed to demonstrate the advantage of time-varying bistable harvesters over linear and monostable ones in harvesting energy from human motion.

Impact-induced high-energy orbits of nonlinear energy harvesters

This letter presents an impact-induced method for nonlinear energy harvesters to obtain high-energy orbits over a wide frequency range under low excitation levels. Based on the impact principle and conservation of momentum, nonlinear electromechanical equations are derived to describe the system response due to initial impacts. Numerical and experimental results show that nonlinear bistable and tristable harvesters can sustain large-amplitude interwell oscillations over a wide range of frequencies, by achieving high-energy orbits in the beginning induced by an initial impact. The proposed impact-induced method could facilitate to efficient energy harvesting from low level ambient vibrations.

Genetic Algorithm-Based Identification of Fractional-Order Systems

Fractional calculus has become an increasingly popular tool for modeling the complex behaviors of physical systems from diverse domains. One of the key issues to apply fractional calculus to engineering problems is to achieve the parameter identification of fractional-order systems. A time-domain identification algorithm based on a genetic algorithm (GA) is proposed in this paper. The multi-variable parameter identification is converted into a parameter optimization by applying GA to the identification of fractional-order systems. To evaluate the identification accuracy and stability, the time-domain output error considering the condition variation is designed as the fitness function for parameter optimization. The identification process is established under various noise levels and excitation levels. The effects of external excitation and the noise level on the identification accuracy are analyzed in detail. The simulation results show that the proposed method could identify the parameters of both commensurate rate and non-commensurate rate fractional-order systems from the data with noise. It is also observed that excitation signal is an important factor influencing the identification accuracy of fractional-order systems.

Modeling and experimental verification of doubly nonlinear magnet-coupled piezoelectric energy harvesting from ambient vibration

This paper presents a nonlinear doubly magnet-coupled energy harvesting system (DMEHS) which could exhibit co-bistable and monostable dynamic characteristics. Its various characteristic responses induced by the magnetic force can be conveniently obtained using the adjustable horizontal distance between two coupled harvesters in the DMEHS. In the case of appropriate relative positions, the DMEHS appears in a co-bistable structure which is different from the traditional bistable structure. Additionally, both the inclination angle of endmost magnets and the displacement perpendicular to the vibration direction are taken into account to calculate the nonlinear magnetic force in the nonlinear electromechanical equations. The numerical investigations show good agreement with experimental results with respect to the output voltage response. Each harvester without magnetic coupling is tested independently to compare with the DMEHS. Both numerical and experimental results also demonstrate the frequency bandwidth and performance enhancements by changing the horizontal distance between the two coupled harvesters.

Design and modeling of a flexible longitudinal zigzag structure for enhanced vibration energy harvesting

The use of piezoelectric materials for vibration energy harvesting at low frequencies is challenging and requires innovative structural design. Here, a flexible longitudinal zigzag structure is developed to enhance energy harvesting at low-frequency ambient vibrations. The proposed structure is composed of orthogonal beams which enable vibration energy harvesting in two directions. A theoretical model based on Euler–Bernoulli beam theory is formulated to study the dynamic response of the structure under free vibrations. The free vibration analysis demonstrates that low operating frequencies can be obtained by increasing the number of, and/or the length of, beams in the proposed structure. To validate the accuracy of the developed theoretical model, finite element analysis is performed using ANSYS. On verification of the model’s accuracy, the piezoelectric effect of the active beams is considered in the model to evaluate the energy harvesting performance of the proposed flexible longitudinal zigzag structure. Numerical results demonstrate that the output voltage and the working frequency of these energy harvesting structures can be tailored through simply altering the number of beams. Overall, the results indicate that the proposed structure is capable of efficient energy conversion at low frequencies, which makes them suitable for a wide range of working conditions.

Analytical and experimental investigation of flexible longitudinal zigzag structures for enhanced multi-directional energy harvesting

This paper makes a complete investigation of flexible longitudinal zigzag (FLZ) energy harvesters for the purpose of enhancing energy harvesting from low-frequency and low-amplitude excitation. A general theoretical model of the FLZ energy harvesters with large joint block mass is proposed. In order to verify the accuracy of the theoretical model, both experimental results and finite element analysis via ANSYS software are presented. Results show that the theoretical model can successfully predict the dynamic response and the output power of the FLZ energy harvesters. Both theoretical and experimental results demonstrate that the proposed energy harvesters can effectively harvest vibration energy even when the direction of excitation relative to the harvester varies from 0° to 90°. Under the low excitation level of 0.18 m s−2, the experimental maximum output power of a FLZ energy harvester with five beams was found to be 1.016 mW. Finally, the results indicate that the proposed structure is capable of effective energy conversion across a large range of excitation angles at low-frequency and low-amplitude excitations, which makes it suitable for a wide range of working conditions.

Nonlinear dynamic characteristics of variable inclination magnetically coupled piezoelectric energy harvesters

This paper investigates the nonlinear dynamic characteristics of a magnetically coupled piezoelectric energy harvester under low frequency excitation where the angle of the external magnetic field is adjustable. The nonlinear dynamic equation with the identified nonlinear magnetic force is derived to describe the electromechanical interaction of variable inclination angle harvesters. The effect of excitation amplitude and frequency on dynamic behavior is proposed by using the phase trajectory, power spectrum, and bifurcation diagram. The numerical analysis shows that a rotating magnetically coupled energy harvesting system exhibits rich nonlinear characteristics with the change of external magnet inclination angle. The nonlinear route to and from large amplitude high-energy motion can be clearly observed. It is demonstrated numerically and experimentally that lumped parameters equations with an identified polynomials for magnetic force could adequately describe the characteristics of nonlinear energy harvester. The rotating magnetically coupled energy harvester possesses the usable frequency bandwidth over a wide range of low frequency excitation by adjusting the angular orientation.

Numerical analysis and experimental verification of broadband tristable energy harvesters

Abstract This paper analyzes the dynamic characteristics of broadband tristable energy harvesters to reveal their response mechanism via a bifurcation diagram, the corresponding frequency spectral analysis and the phase portrait topology. The bifurcation diagram of response voltages shows that tristable energy harvesters orderly undergoes singly periodic intrawell oscillation, singly periodic interwell oscillation, triply periodic interwell oscillation, singly periodic interwell oscillation, double-periodic interwell oscillation, chaotic oscillation, singly periodic interwell oscillation, multi-period oscillation, and finally enters into chaotic oscillation range, as the increase of the excitation amplitude. The frequency spectral analysis demonstrates that sub-harmonics and super-harmonics numerically and experimentally exist in the response voltages of tristable energy harvesters. In addition, it is found that both the first harmonic and the third harmonic are main frequency components in the response voltages.