Heath Hofmann

Adaptive piezoelectric energy harvesting circuit for wireless remote power supply

This paper describes an approach to harvesting electrical energy from a mechanically excited piezoelectric element. A vibrating piezoelectric device differs from a typical electrical power source in that it has a capacitive rather than inductive source impedance, and may be driven by mechanical vibrations of varying amplitude. An analytical expression for the optimal power flow from a rectified piezoelectric device is derived, and an energy harvesting circuit is proposed which can achieve this optimal power flow. The harvesting circuit consists of an AC-DC rectifier with an output capacitor, an electrochemical battery, and a switch-mode DC-DC converter that controls the energy flow into the battery. An adaptive control technique for the DC-DC converter is used to continuously implement the optimal power transfer theory and maximize the power stored by the battery. Experimental results reveal that use of the adaptive DC-DC converter increases power transfer by over 400% as compared to when the DC-DC converter is not used.

Optimized piezoelectric energy harvesting circuit using step-down converter in discontinuous conduction mode

An optimized method of harvesting vibrational energy with a piezoelectric element using a step-down DC-DC converter is presented. In this configuration, the converter regulates the power flow from the piezoelectric element to the desired electronic load. Analysis of the converter in discontinuous current conduction mode results in an expression for the duty cycle-power relationship. Using parameters of the mechanical system, the piezoelectric element, and the converter; the optimal duty cycle can be determined where the harvested power is maximized for the level of mechanical excitation. It is shown that, as the magnitude of the mechanical excitation increases, the optimal duty cycle becomes essentially constant, greatly simplifying the control of the step-down converter. The expression is validated with experimental data showing that the optimal duty cycle can be accurately determined and maximum energy harvesting attained. A circuit is proposed which implements this relationship, and experimental results show that the converter increases the harvested power by approximately 325%.

Damping as a result of piezoelectric energy harvesting

Abstract Systems that harvest or scavenge energy from their environments are of considerable interest for use in remote power supplies. A class of such systems exploits the motion or deformation associated with vibration, converting the mechanical energy to electrical, and storing it for later use; some of these systems use piezoelectric materials for the direct conversion of strain energy to electrical energy. The removal of mechanical energy from a vibrating structure necessarily results in damping. This research addresses the damping associated with a piezoelectric energy harvesting system that consists of a full-bridge rectifier, a filter capacitor, a switching DC–DC step-down converter, and a battery. Under conditions of harmonic forcing, the effective modal loss factor depends on: (1) the electromechanical coupling coefficient of the piezoelectric system; and (2) the ratio of the rectifier output voltage during operation to its maximum open-circuit value. When the DC–DC converter is maximizing power flow to the battery, this voltage ratio is very nearly 1/2, and the loss factor depends only on the coupling coefficient. Experiments on a base-driven piezoelectric cantilever, having a system coupling coefficient of 26%, yielded an effective loss factor for the fundamental vibration mode of 2.2%, in excellent agreement with theory.

Energy harvesting using a piezoelectric "cymbal" transducer in dynamic environment

In this study, we investigated the capability of harvesting the electrical energy from mechanical vibrations in a dynamic environment through a cymbal piezoelectric transducer. Targeted mechanical vibrations lie in the range of 50–150 Hz with force amplitude in the order of 1 kN (automobile engine vibration level). It was found that under such severe stress conditions the metal–ceramic composite transducer cymbal is a promising structure. The metal cap enhances the endurance of the ceramic to sustain high loads along with stress amplification. In this preliminary study, the experiments were performed at the frequency of 100 Hz on a cymbal with 29 mm diameter and 1 mm thickness under a force of 7.8 N. At this frequency and force level, 39 mW power was generated from a cymbal measured across a 400 kΩ resistor. A DC–DC converter was designed which allowed the transfer of 30 mW power to a low impedance load of 5 kΩ with a 2% duty cycle and at a switching frequency of 1 kHz.

Single crystal PMN-PT/Epoxy 1-3 composite for energy-harvesting application

One key parameter in using electroactive materials to harvest electric energy from mechanical sources is the energy conversion efficiency. Recently, it was shown that, in the relaxor ferroelectric PMN-PT single crystals, a very high longitudinal electromechanical coupling factor (>90%) can be obtained. This paper investigates energy harvesting using 1-3 composites of PMN-PT single crystals in a soft epoxy matrix. It is shown that 1-3 composites enable the single crystals operating in the longitudinal mode to achieve high efficiency for energy harvesting, and the soft-polymer, matrix-supported single-crystal rods maintain high mechanical integrity under different external loads. For comparison, 1-3 composites with piezoceramic PZT also are investigated in energy-harvesting applications, and the results show that the high coupling factor of single crystal PMN-PT 1-3 composites leads to much higher electric energy output for similar mechanical energy input. The harvested energy density of 1-3 composite with single crystal (22.1 mW/cm/sup 3/ under a stress of 40.4 MPa) is about twice of that harvested with PZT ceramic 1-3 composite (12 mW/cm/sup 3/ under a stress of 39 MPa). At a higher stress level, the harvested-energy density of 1-3 PMN-PT single crystal composite can reach 96 mW/cm/sup 3/.

Speed sensorless vector torque control of induction machines using a two-time-scale approach

Conditions for observability of a linearized smooth-airgap induction machine model are examined, and limitations of any speed-sensorless observer scheme based on such a model are discussed. An approach for speed-sensorless flux estimation based on singular perturbation theory is developed. This approach relies on the natural time-scale separation between the electrical and mechanical dynamics of the induction machine. A full-order observer of an induction machine is presented, incorporating a correction term which has an intuitive explanation when one considers steady-state stator currents. Using singular perturbation theory, convergence of the observer is shown for all open-loop stable operating points of the induction machine, with the exception of DC excitation. Sensitivity of the observer to parameter deviations is discussed. Experimental results are presented confirming the validity of the above approach.

Active Piezoelectric Energy Harvesting: General Principle and Experimental Demonstration

In piezoelectric energy harvesting systems, the energy harvesting circuit is the interface between a piezoelectric device and an electrical load. A conventional view of this interface is based on impedance matching concepts. In fact, an energy harvesting circuit can also apply electrical boundary conditions, such as voltage and charge, to the piezoelectric device for each energy conversion cycle. An optimized electrical boundary condition can therefore increase the mechanical energy flow into the device and the energy conversion efficiency of the device. We present a study of active energy harvesting, a type of energy harvesting approach which uses switch-mode power electronics to control the voltage and/or charge on a piezoelectric device relative to the mechanical input for optimized energy conversion. Under quasi-static assumptions, a model based on the electromechanical boundary conditions is established. Some practical limiting factors of active energy harvesting, due to device limitations and the efficiency of the power electronic circuitry, are discussed. In the experimental part of the article, active energy harvesting is demonstrated with a multilayer PVDF polymer device. In these experiments, the active energy harvesting approach increased the harvested energy by a factor of five for the same mechanical displacement compared to an optimized diode rectifier-based circuit.

An active energy harvesting scheme with an electroactive polymer

We investigate the energy harvesting with an electrostrictive polymer, possessing high electromechanical response and elastic energy density, which make it possible to generate high electric energy density and attractive for the active energy harvesting scheme. It is shown that combining the active energy harvesting scheme and high electromechanical response of the polymer yields a harvested electric energy density of ∼40mJ∕cm3 with a 10% efficiency.

Stator-flux-oriented vector control of synchronous reluctance Machines with maximized efficiency

This paper presents a position-sensorless vector torque controller designed to achieve maximum efficiency over a range of power and rotational speed for a synchronous reluctance machine. A model of the synchronous reluctance machine is presented which incorporates both winding and core losses. It is then shown that a stator-flux-oriented control scheme can achieve synchronous operation of the machine without a position sensor at medium and high electrical frequencies. For a given speed and torque, power losses in the machine are shown to be a function of only the stator flux magnitude. As the power losses are a convex function of the stator flux level, the optimal flux value can be found using a one-dimensional optimization algorithm, such as the Method of Sequential Quadratic Interpolations. Optimal flux values for a synchronous reluctance machine are determined using an experimental setup that accurately determines losses in the motor/drive system. Experimental results obtained from the test setup confirm the validity of the controller and the optimization algorithm.

Control of High-Speed Solid-Rotor Synchronous Reluctance Motor/Generator for Flywheel-Based Uninterruptible Power Supplies

A hybrid controller, consisting of a model-based feedforward controller and a proportional-integral feedback compensator, for a solid-rotor synchronous reluctance motor/generator in a high-speed flywheel-based uninterruptible power supply application is proposed in this paper. The feedforward controller takes most of the control output of the current regulator based on the machine model, and the PI controllers compensate the possible inaccuracies of the model to improve the performance and robustness of the complete control system. The machine current tracking error caused by parameter inaccuracy in the model-based controller is mathematically analyzed and utilized to dynamically compensate the estimated flux linkage to eliminate the steady-state error in current regulation. Stability analysis is also presented, and it can be seen that the regulation performance and robustness of the system are improved by the proposed hybrid controller. Simulation and experimental results consisting of a flywheel energy storage system validates the performance of the controller.