David N. Betts

Optimal configurations of bistable piezo-composites for energy harvesting

This paper presents an arrangement of bistable composites combined with piezoelectrics for broadband energy harvesting of ambient vibrations. These non-linear devices have improved power generation over conventional resonant systems and can be designed to occupy smaller volumes than magnetic cantilever systems. This paper presents results based on optimization of bistable composites that enables improved electrical power generation by discovering the optimal configurations for harvesting based on the statics of the device. The optimal device aspect ratio, thickness, stacking sequence, and piezoelectric area are considered. Increased electrical output is found for geometries and piezoelectric configurations, which have not been considered previously.

Shape memory alloy-piezoelectric active structures for reversible actuation of bistable composites

A study was conducted to introduce an actuation mechanism, called shape memory alloy-piezoelectric active structures (SMAPAS) that combined the advantages of the piezoelectric shape memory alloy (SMA) materials to achieve self-resetting bistable composites. The approach used piezoelectric actuation to provide a rapid snap-through with significant degree of control and a relatively slow, but high-strain SMA actuation to reverse the state change. A thin cantilever beam of carbon-fiber or epoxy material was used to demonstrate the two-way actuation. The composite lay-up procedure was a standard method for the manufacturing of carbon laminates through a standard cure cycle to a maximum cure temperature of 125°C and a pressure of 85 psi. A macrofiber composite piezoelectric actuator was used to conduct the investigations and it consisted of aligned piezoelectric fibers with an interdigitated electrode to direct the applied electric field along the fiber length.

Modeling and optimization of bistable composite laminates for piezoelectric actuation

Adaptive structures that allow large deformations under the application of a low and noncontinuous energy input are gaining increasing interest in the aerospace industry. One potential mechanism of realizing shape control is piezoelectric actuation of asymmetric composite laminates. This article presents an optimization study for the design of bistable laminates for a reversible snap-through enabled by two orthogonal piezoelectric layers. The formulation optimizes the load-carrying capability of the structure subject to deflection and actuation limits through a variation in ply orientations and laminate geometry. We find the problem to be multimodal with the multiple optima to be dependent on the loading and snap-through directions and the complex constraint boundary interactions. A reduction in the total actuation voltage is achieved through the simultaneous use of the positive and negative working ranges of the two piezoelectric layers.

Preliminary Study of Optimum Piezoelectric Cross-Ply Composites for Energy Harvesting

Energy harvesting devices based on a piezoelectric material attached to asymmetric bistable laminate plates have been shown to exhibit high levels of power extraction over a wide range of frequencies. This paper optimizes for the design of bistable composites combined with piezoelectrics for energy harvesting applications. The electrical energy generated during state-change, or “snap-through,” is maximized through variation in ply thicknesses and rectangular laminate edge lengths. The design is constrained by a bistability constraint and limits on both the magnitude of deflection and the force required for the reversible actuation. Optimum solutions are obtained for differing numbers of plies and the numerical investigation results are discussed.

Modelling the dynamic response of bistable composite plates for piezoelectric energy harvesting

This paper presents analytical modelling and experimental characterisation of an arrangement of bistable composite plates with bonded piezoelectric elements to perform broadband vibration-based energy harvesting from ambient mechanical vibrations. These bistable devices have the potential to exhibit improved power generation compared to conventional resonant systems by exploiting nonlinear modes of oscillation driven by a ‘snap-through’ mechanism. Snap-through behaviour is shown to lead to higher average power outputs over a much broader frequency bandwidth than a resonant device. The conditions which yield these snap-through modes are investigated in terms of drive frequency and amplitude of vibration, revealing the emergence of intermittent and continuous snap-through modes for higher amplitude oscillations. These modes are found to widen the half-power bandwidth from 7Hz for linear oscillations (106mW for 4g peak acceleration) to 22Hz for high amplitude snap-through behaviour (244mW for 10g peak acceleration).

Optimization of Stiffness Characteristics for the Design of Bistable Composite Laminates

Asymmetric composite laminates can have a bistable response to mechanical loading. The large deflections that can be achieved by snap-through from one stable state to another, along with a need for small and removable energy input,make themof interest for awide range of engineering applications. After 30 years of research efforts the shapes and response to applied strains of laminates of general layup have been well characterized. More recently, with the development of smart actuators, the design and application of bistable laminates has been considered. This paper presents an optimization technique for the design of bistable laminates enabled by an analytical solution for an asymmetric laminate design. The optimization formulationmaximizes the bending stiffness in the direction of known loading condition while the bending stiffness in the direction of snap-through is minimized. A minimum deflection requirement is applied as a constraint. The design problem has multiple local optima, with the global optimum not intuitively obvious from the problem definition, differing from the typical high-deflection cross-ply solution.

Static and dynamic analysis of bistable piezoelectric- composite plates for energy harvesting

This paper presents an arrangement of bistable composite plates with bonded piezoelectric patches to perform broadband vibration-based energy harvesting from ambient mechanical vibrations. These bistable nonlinear devices have the potential to exhibit improved power generation compared to conventional resonant systems and can be designed to occupy smaller volumes than bistable magnetic cantilever systems. In this paper we initially present the results of an optimization study to generate greater electrical power by discovering the correct geometric configuration for energy harvesting based on the static states of the device. The results consider the optimal choice of device aspect ratio, laminate thickness, laminate stacking sequence, and piezoelectric surface area. Increased electrical output is found for geometries and piezoelectric configurations which have not been considered previously. This study is then extended to include dynamic considerations of both the static shapes and the snap-through transition. Optimum designs are shown to be sensitive to the vibration pattern that is being harvested. The optimum geometric configurations based on the static analysis alone are not optimal under all dynamic conditions.

Optimization of piezoelectric bistable composite plates for broadband vibrational energy harvesting

This paper presents a unique arrangement of bistable composite plates with piezoelectric patches bonded to its surface to perform broadband vibration-based energy harvesting from ambient mechanical vibrations. These bistable nonlinear devices have been shown to have improved power generation compared to conventional resonant systems and can be designed to occupy smaller volumes than bistable magnetic cantilever systems. This paper presents the results of an optimization study of bistable composites that are capable of generating greater electrical power from a smaller space by discovering the correct geometric configuration for energy harvesting. Optimum solutions are investigated in a series of design parameter studies intended to reveal the complex interactions of the physical constraints and design requirements. The proposed approach considers the optimal choice of device aspect ratio, thickness, laminate stacking sequence, and piezoelectric surface area. Increased electrical output is found for geometries and piezoelectric configurations which have not been considered previously.

Piezoelectric fibres integrated into structural composites

This paper describes the manufacture of structural composites incorporating piezoelectric fibres which are finding interest in applications such as shape-changing applications, sensors to detect mechanical strain or vibration and energy harvesting. In this paper preliminary results are presented for a simple cantilever structure consisting of piezoelectric fibres with planar electrodes that are co-cured within a carbon fibre reinforced plastic (CFRP). Processing methods to embed functional fibres are described along with characterization of the piezoelectric and mechanical properties of the resulting material.

Design optimization of stiffness characteristics for bistable composite laminates

Asymmetric composite laminates can have a bistable response to loading. The potentially large deflections which can be achieved during snap-through from one stable state to another with small and removable energy input make them of interest for a wide range of engineering applications. After 30 years of research effort the shapes and response to applied strains of laminates of general layup can be quantitatively predicted. With attention switching to the incorporation of bistable laminates for real world applications this paper presents optimization of bistable composites enabled by an analytical solution for a simplified orthogonal laminate design. The optimization formulation maximizes the bending stiffness in the direction of known loading condition whilst the bending stiffness in the direction of snap-through is minimized. The deflection requirement is applied as a constraint. We find the design problem to have multiple local optima, with the global optimum differing from the traditional high deflection cross-ply solution.