| Interest in the use of piezoelectric materials for electrical energy generation has increased over the years. The interface circuit between the piezoelectric device and electrical load plays an important role in the energy harvesting process. Most of the previous techniques are mainly passive, based on AC-DC diode-bridge rectifier circuits. However, since the diode-bridge rectifier circuit can only emulate a resistive load, while the internal impedance of a piezoelectric device is essentially capacitive, the impedance matching condition can not be satisfied, and so generally the power harvesting ability of the passive technique is very low. The semi-active techniques represented by the synchronized switch harvesting on inductor(SSHI) technique improves performance over the passive techniques by inverting the piezoelectric voltage in phase with the device velocity, but its performance drops dramatically at off-resonance frequencies because it does not guarantee impedance matching at off-resonance frequencies. The newly proposed active energy harvesting technique actively applies electrical boundary conditions to the piezoelectric device, and experimental results showed that the active technique can boost the harvested power dramatically under quasi-static conditions.;In previous work, the active energy harvesting experiments were all implemented under the quasi-static assumption, and the device displacement was assumed to be constant regardless of the applied voltage. However, in many practical applications, especially systems where the energy harvesting device possesses resonance frequency, the quasi-static assumption does not hold. Furthermore, the resonance frequency of a mechanical system is often changing and hard to estimate because of the variation of device parameters, and so previous techniques based on certain resonance frequencies may not be effective. Also, for a practical system, it is more likely that the excitation force instead of the displacement is fixed. Hence we seek to develop a more practical active energy harvesting technique for a real dynamic system application.;In this dissertation, the active energy harvesting technique for a spring-massdamper piezoelectric dynamic system under a constant-magnitude force excitation is investigated. First the model of a piezoelectric dynamic system is built, and from this model we derive a general expression for the harvested power. It is shown that the harvested power is proportional to the integral of the product of the device voltage and velocity, and the optimal control voltage that results in maximum energy harvesting is derived. It is also shown that for any excitation frequency, the optimal control voltage is always trying to shift the system's resonance frequency to the excitation frequency, and the equivalent load impedance matches the internal impedance of the electromechanical system. The harvested power can therefore always be maximized at any excitation frequency. Simulation results are shown to prove the theory.;In practical implementation, the circuit efficiency and change of device parameters make the situation much more complicated, and the theoretically predicted optimal control voltage may not be accurate. We propose an adaptive control algorithm that can find the optimal control voltage adaptively. The method is based on sequential quadratic interpolation, and only information that can be easily measured from the circuit is needed to implement the control. Simulation results show good behavior of the controller.;Circuits used to implement the active harvesting technique are proposed, including a full-bridge converter with a flyback converter, dead-time insertion circuit, current and voltage regulator, current and voltage sensing circuit, PWM and drive signal generation circuit, etc. All the circuits are built on a stand-alone PCB board. Experimental results show that the active technique can harvest five times more power than the passive technique at off-resonance frequencies. |
