ReviewSmart structure dynamics
Introduction
Passive measures for reducing noise and vibration or for ensuring optimal structural performance have reached certain limits. For this reason, smart structures are becoming increasingly important. As funds have become available to pursue research in this area, terminologies have been introduced to define the field of study. The terms smart structures, intelligent structures, adaptive structures, active structures, adaptronics, and structronics all belong to the same field of study [1]. All these terms refer to the integration of actuators, sensors in structural components, and the usage of some kind of control unit or enhanced signal processing with a material or structural component (Fig. 1). The goal of this integration is the creation of a material system having enhanced structural performance, but without adding too much mass or consuming too much power. Due to its nature, the field of smart structures depends on inter-disciplinary research since numerous disciplines (e.g. material science, applied mechanics, control theory, etc.) are involved in the design of a smart structure system solution.
The materials used in smart structures often have interesting and unusual properties. Electrostrictive materials, magnetostrictive materials, shape memory alloys, magneto- or electrorheological fluids, polymer gels, and piezoelectric materials, for example, can all be used to design and develop structures that can be called smart. However, the materials themselves are not smart. “Smartness” refers to the exploitation of material properties to better serve a design function than would be possible through conventional structural design.
Section snippets
Modelling of smart materials and structures
The field of literature related to the modelling of smart structures is vast. Tzou et al. [3] recently presented an overview of smart materials and their modelling. In the same spirit, this paper presents a general summary of the more intensively researched smart materials.
Semi-passive damping
Piezoelectrics have the ability to efficiently transform mechanical energy to electrical energy and vice versa. It is this dual transformation ability which makes them useful as structural dampers. Hagood and von Flotow [23] presented a passive damping mechanism for structural systems in which piezoelectric materials are bonded to the structure of interest. Their work is based on the papers of Forward [24] and Edwards and Miyakawa [25] who first presented this type of passive piezoelectric
Energy harvesting
Energy harvesting research has been driven by the need for remote electrical power supplies for applications ranging from structural health monitoring to walking-powered electronics. Portable systems which make use of power harvesting techniques do not have to depend on traditional methods for providing power, such as the battery, which has a limited operating life. The general idea underlying energy harvesting research is the extraction of electrical energy from the operating environment.
Semi-active concepts
The concept of semi-active damping was formally proposed by Karnopp et al. [42]. The concept involves the use of control theory to augment the damping properties of a passive element in real time. Sometimes referred to as active–passive damping, the technique offers considerable advantages in performance over passive damping elements, and with only a slight increase in system cost/complexity. On the other hand, semi-active damping cannot deliver the level of performance of a fully active
Active vibration control
One of the earliest studies of active vibration control was completed by Swigert and Forward [76]. They conducted a theoretical and experimental study that involved electronic dampers. In that study, a system of electromechanical transducers made from lead zirconate titanate (PZT) were implemented to control the mechanical vibration of an end-supported mast. The output signals from the sensors were amplified and appropriately shifted in time to provide control inputs for actuators positioned
Active noise control/active structural-acoustic control
The concept of active noise control is not new. Lueg [85] received a patent for a system implementing active control of sound in a duct. The sound field is first detected using a microphone. The microphone signal is then used to produce a cancelling wave which is emitted from a loudspeaker in the downstream duct. Superposition of the two waves results in destructive interference at a reference location. In 1953, Olson and May [86] developed a different active noise control system. In this
Active vibration isolation
Isolating a piece of delicate equipment from the vibration of a base structure is of practical importance in a number of engineering fields. Passive anti-vibration mounts are widely used to support the equipment and to protect it from severe base vibration. Although conventional passive mounts offer good isolation at high frequencies, they suffer from vibration amplification at the mounted resonance frequency. Generally, the best isolation performance is achieved by using an active system in
Structural health monitoring (SHM)
Nearly all in-service structures require some form of maintenance for monitoring their integrity and health condition. Appropriate maintenance prolongs the lifespan of a structure and can be used to prevent catastrophic failure. Current schedule-driven inspection and maintenance techniques can be time consuming, labour-intensive, and expensive. SHM on the other hand involves autonomous, in-service inspection of a structure. The first instances of SHM date back to the late 1970s and early 1980s.
Conclusions
This paper addresses several fields of application of smart structure dynamics technologies. After discussing a variety of multifunctional materials, some examples for the application of these technologies were given. First, semi-passive concepts used to enhance the damping behaviour of structures were summarised. Then energy-harvesting technologies are discussed, and their combination with the shunted piezoelectric concept is highlighted. Semi-active concepts are also considered, particularly
Acknowledgements
This review article is based in part upon responses to a questionaire directed at leading scientists in the field of smart structures. The authors gratefully wish to acknowledge the helpful input received from these conscientious contributors.
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