Junyi Cao

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.

Design of Fractional Order Controllers Based on Particle Swarm Optimization

An intelligent optimization method for designing fractional order PID (FOPID) controllers based on particle swarm optimization (PSO) is presented in this paper. Fractional calculus can provide novel and higher performance extension for FOPID controllers. However, the difficulties of designing FOPID controllers increase, because FOPID controllers append derivative order and integral order in comparison with traditional PID controllers. To design the parameters of FOPID controllers, the enhanced PSO algorithm is adopted, which guarantee the particle position inside the defined search spaces. The optimization performance target is the weighted combination of ITAE and control input. The numerical realization of FOPID controllers uses the methods of Tustin operator and continued fraction expansion. Experimental results show the proposed method is highly effective

Improved quality GaN by growth on compliant silicon-on-insulator substrates using metalorganic chemical vapor deposition

The use of compliant silicon-on-insulator (SOI) substrates instead of Si substrates is shown to improve the quality of epitaxial GaN layers by releasing the strain and absorbing the generated threading dislocations in the thin Si overlay of the SOI substrate. GaN layers have been grown on SOI substrates by low-pressure metalorganic chemical vapor deposition and various growth conditions and compared with GaN layers grown on Si substrates. Crystal uniformity, surface morphology, and number of threading dislocations of GaN layers grown on SOI substrates are improved compared to layers grown directly on Si substrates as evidenced by x-ray diffraction spectroscopy (XRD) and transmission electron microscopy. Full width at half maximum XRD values improved from 672 to 378 arcsec by growth on SOI instead of Si substrates. The GaN layers grown directly on Si substrates are highly resistive while all as-grown GaN layers on SOI substrates are unintentionally n type. For a 1–2 μm thick GaN layer grown on SOI, the ele...

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.

NONLINEAR DYNAMICS OF DUFFING SYSTEM WITH FRACTIONAL ORDER DAMPING

In this paper, nonlinear dynamics of Duffing system with fractional order damping is investigated. The four order Runge-Kutta method and ten order CFE-Euler methods are introduced to simulate the fractional order Duffing equations. The effect of taking fractional order on the system dynamics is investigated using phase diagrams, bifurcation diagrams and Poincare map. The bifurcation diagram is also used to exam the effects of excitation amplitude and frequency on Duffing system with fractional order damping. The analysis results show that the fractional order damped Duffing system exhibits period motion, chaos, period motion, chaos, period motion in turn when the fractional order changes from 0.1 to 2.0. A period doubling route to chaos is clearly observed.Copyright

Photoluminescence properties of GaN grown on compliant silicon-on-insulator substrates

A compliant substrate approach has been employed to release lattice-mismatch caused strain in GaN epilayers through stress absorption in the substrate. GaN layers have been grown on silicon-on-insulator (SOI) substrates by low-pressure metalorganic chemical vapor deposition. Photoluminescence measurements at 4 K show the spectrum of grown GaN being dominated by UV emission around 3.47 eV related to neutral-donor bound excitons. The much weaker yellow luminescence shows a broad spectrum around 2.16 eV. Peak position of the UV emission changes both with measurement temperature and strain. At room temperature, the UV peak is red shifted by 64 meV corresponding well to the band-gap temperature dependence. Strain-induced blue shift of the peak, compared to unstrained GaN, is much less than for growth on sapphire, indicating strain relief in the GaN by growth on SOI. Further reduction of the blue shift is consistent with increased electron mobility.

Artificial neural network maximum power point tracker for solar electric vehicle

Abstract This paper proposes an artificial neural network maximum power point tracker (MPPT) for solar electric vehicles. The MPPT is based on a highly efficient boost converter with insulated gate bipolar transistor (IGBT) power switch. The reference voltage for MPPT is obtained by artificial neural network (ANN) with gradient descent momentum algorithm. The tracking algorithm changes the duty-cycle of the converter so that the PV-module voltage equals the voltage corresponding to the MPPT at any given insolation, temperature, and load conditions. For fast response, the system is implemented using digital signal processor (DSP). The overall system stability is improved by including a proportional-integral-derivative (PID) controller, which is also used to match the reference and battery voltage levels. The controller, based on the information supplied by the ANN, generates the boost converter duty-cycle. The energy obtained is used to charge the lithium ion battery stack for the solar vehicle. The experimental and simulation results show that the proposed scheme is highly efficient.

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.