Chee Kiong Soh

Toward Broadband Vibration-based Energy Harvesting

The dramatic reduction in power consumption of current integrated circuits has evoked great research interests in harvesting ambient energy, such as vibrations, as a potential power supply for electronic devices to avoid battery replacement. Currently, most vibration-based energy harvesters are designed as linear resonators to achieve optimal performance by matching their resonance frequencies with the ambient excitation frequencies a priori. However, a slight shift of the excitation frequency will cause a dramatic reduction in performance. Unfortunately, in the vast majority of practical cases, the ambient vibrations are frequency-varying or totally random with energy distributed over a wide frequency spectrum. Hence, developing techniques to increase the bandwidth of vibration-based energy harvesters has become the next important problem in energy harvesting. This article reviews the advances made in the past few years on this issue. The broadband vibration-based energy harvesting solutions, covering resonance tuning, multimodal energy harvesting, frequency up-conversion, and techniques exploiting non-linear oscillations, are summarized in detail with regard to their merits and applicability in different circumstances.

Performance of smart piezoceramic patches in health monitoring of a RC bridge

This paper presents the results of a health monitoring study, carried out during the destructive load testing of a prototype reinforced concrete (RC) bridge. The bridge was made up of cement-concrete reinforced with steel rods, and represented a popular class of road bridges in which regular health monitoring is a very important issue during the service life. The bridge was instrumented with piezoceramic transducer (PZT) patches, which were electrically excited at high frequencies, of the order of kHz, and the real part of admittance (conductance) was extracted as a function of the exciting frequency. The patches were scanned for the acquisition of this signature at various stages during the loading process. The signatures of the patches located in the vicinity of the damage were found to have undergone drastic changes, while those farther away were less affected. Damage was quantified in non-parametric terms using the root mean square of the deviation in signatures with respect to the baseline signature of the healthy state. This non-parametric index was found to correlate well with the damage progression in the structure.

Electromechanical Impedance Modeling for Adhesively Bonded Piezo-Transducers

The electromechanical impedance (EMI) technique for structural health monitoring (SHM) and nondestructive evaluation (NDE) employs piezoelectric-ceramic (PZT) patches, which are surface bonded to the monitored structures using adhesives. The adhesive forms a finitely thick, permanent interfacial layer between the host structure and the patch. Hence, the force transmission between the structure and the patch occurs through the bond layer, via shear mechanism, invariably causing shear lag. However, the impedance models developed so far ignore the associated shear lag and idealize the force transfer to occur at the ends of the patch. This paper analyses the mechanism of force transfer through the bond layer and presents a step-by-step derivation to integrate the shear lag effect into impedance formulations, both one-dimensional and two-dimensional. Further, using the integrated model, the influence of various parameters (associated with the bond layer) on the electromechanical admittance response is studied by means of a parametric study. It is found that the bond layer can significantly modify the measured electromechanical admittance if not carefully controlled during the installation of the PZT patch.

Application of Electromechanical Impedance Technique for Engineering Structures: Review and Future Issues

The recent advent of highly durable engineering materials and the advancement of latest structural design theories have made possible the fabrication of more efficient engineering structures. However, the safety and reliability of these structures remains the primary challenge and concern for engineers. Especially, for those structures which involve human traffic and huge investments such as the aerospace structures and bridges. Therefore, there is a compelling need to have high-quality online structural health monitoring (SHM) of such structures. The development of a real-time, in-service, and smart material-based SHM method has recently attracted the interest of a large number of academic and industrial researchers. In the recent past, piezoceramic (PZT) transducer has evolved as an efficient smart material, which is usually employed in electro mechanical impedance (EMI) and guided ultrasonic wave propagation techniques. In EMI technique, a PZT transducer interact with the host structure to result in unique health signature, as an inverse function of structural impedance, when it is subjected to high-frequency structural excitations in the presence of electric field. Using the self-actuating and sensing capabilities of PZT transducers, the EMI models attempted to detect loadings on, and damages in, the structures to be monitored. This article reviews some of the advancements in the field of PZT-based SHM made over the past two decades in engineering structures. This article also provides an insight into the possible future work and improvements required for PZT-based EMI technique.The recent advent of highly durable engineering materials and the advancement of latest structural design theories have made possible the fabrication of more efficient engineering structures. However, the safety and reliability of these structures remains the primary challenge and concern for engineers. Especially, for those structures which involve human traffic and huge investments such as the aerospace structures and bridges. Therefore, there is a compelling need to have high-quality online structural health monitoring (SHM) of such structures. The development of a real-time, in-service, and smart material-based SHM method has recently attracted the interest of a large number of academic and industrial researchers. In the recent past, piezoceramic (PZT) transducer has evolved as an efficient smart material, which is usually employed in electro mechanical impedance (EMI) and guided ultrasonic wave propagation techniques. In EMI technique, a PZT transducer interact with the host structure to result in un...

Improving functionality of vibration energy harvesters using magnets.

In recent years, several strategies have been proposed to improve the functionality of energy harvesters under broadband vibrations, but they only improve the efficiency of energy harvesting under limited conditions. In this work, a comprehensive experimental study is conducted to investigate the use of magnets for improving the functionality of energy harvesters under various vibration scenarios. First, the nonlinearities introduced by magnets are exploited to improve the performance of vibration energy harvesting. Both monostable and bistable configurations are investigated under sinusoidal and random vibrations with various excitation levels. The optimal nonlinear configuration (in terms of distance between magnets) is determined to be near the monostable-to-bistable transition region. Results show that both monostable and bistable nonlinear configurations can significantly outperform the linear harvester near this transition region. Second, for ultra-low-frequency vibration scenarios such as wave heave motions, a frequency up-conversion mechanism using magnets is proposed. By parametric study, the repulsive configuration of magnets is found preferable in the frequency up-conversion technique, which is efficient and insensitive to various wave conditions when the magnets are placed sufficiently close. These findings could serve as useful design guidelines when nonlinearity or frequency up-conversion techniques are employed to improve the functionality of vibration energy harvesters.

A novel two-degrees-of-freedom piezoelectric energy harvester

Energy harvesting from ambient vibrations using piezoelectric effect is a promising alternative solution for powering small electronics such as wireless sensors. A conventional piezoelectric energy harvester usually consists of a cantilevered beam with a proof mass at its free end. For such a device, the second resonance of the piezoelectric energy harvester is usually ignored because of its high frequency as well as low response level compared to the first resonance. Hence, only the first mode has been frequently exploited for energy harvesting in the reported literature. In this article, a novel compact piezoelectric energy harvester using two vibration modes has been developed. The harvester comprises one main cantilever beam and an inner secondary cantilever beam, each of which is bonded with piezoelectric transducers. By varying the proof masses, the first two resonant frequencies of the harvester can be tuned close enough to achieve useful wide bandwidth. Meanwhile, this compact design efficiently utilizes the cantilever beam by generating significant power output from both the main and secondary beams. An experiment and simulation were carried out to validate the design concept. The results show that the proposed novel piezoelectric energy harvester is more adaptive and functional in practical vibrational circumstances.

Practical issues related to the application of the electromechanical impedance technique in the structural health monitoring of civil structures: I. Experiment

The advent of smart materials such as the piezo-impedance transducer (lead zirconate titanate, PZT) and optical fiber (FBG) has ushered in a new era in the field of structural health monitoring (SHM) based on non-destructive evaluation (NDE). So far, successful research and investigations conducted on the electromechanical impedance (EMI) technique employing a piezo-impedance transducer are often laboratory based and mainly theoretical. Real-life application of the technique, especially under harsh environments, has frequently been questioned. In this research project, investigative studies were conducted to evaluate the problems involved in real-life applications of the EMI technique, attempting to reduce the gap between theory and application. This two-part paper presents a series of experimentation (part I) and numerical verification (part II) on various issues related to real-life application, including the durability of PZT transducers, and the effects of bonding and temperature under conceivable nominal construction site conditions. The repeatability of electrical admittance signatures acquired from the PZT patches surface bonded on aluminum structures was found to be excellent up to a period of one and a half years. Experimental investigations revealed that the bonding thickness should preferably be thinner than one-third of the patch to avoid any adverse effect caused by the PZT patchs resonance on the admittance signatures which reflect the host structural behavior. On the other hand, the effect of temperature on the admittance signatures was found to be closely related to the thickness of bonding, as an increase in temperature would reduce the stiffness of the bonding layer, thus affecting strain transfer. It was concluded that PZT patches with thick bonding thickness and high frequency of excitation are undesirable, especially at elevated temperatures.

Structural identification and damage diagnosis using self-sensing piezo-impedance transducers

The use of smart materials, such as lead zirconate titanate (PZT), has accelerated developments in the fields of structural identification and automated structural health monitoring (SHM). One such technique that has made much progress is the electro-mechanical impedance (EMI) technique, which employs self-sensing piezo-impedance transducers. In this technique, a PZT patch is surface bonded to the structure to be monitored and its corresponding electro-mechanical admittance signature is used for damage detection. This paper introduces a new method for identifying structures from the measured admittance signatures in terms of equivalent structural parameters, whereby the identified parameters are used for damage characterization. The new method has been applied to a truss, a beam and a concrete cube, and found to be able to successfully perform structural identification and damage diagnosis. In addition, several advantages have been ascertained in comparison with the conventional, non-parametric statistical methods.

Influence of loading on the electromechanical admittance of piezoceramic transducers

Damage detection using electromechanical (EM) impedance in structural health monitoring (SHM) of engineering structures is rapidly emerging as a useful technique. In the EM impedance method, piezoceramic (PZT) transducers are either surface bonded to or embedded inside the host structure and are subjected to electric actuation. The EM admittance signatures of the PZT transducers, which consist of real and imaginary parts, serve as indicators to predict the health/integrity of the host structure. However, in real life, structural components such as slabs, beams and columns are constantly subjected to some form of external loading. The EM admittance signature obtained for such a constantly loaded structure is different from that obtained when damages are present in the structure. This paper presents an experimental and statistical investigation to show the influence of loading on EM admittance signatures. It is also observed that the susceptance signature is a better indicator than the conductance signature for detecting in situ stress in the host structure. This observation is further supported by a statistical analysis. This paper is expected to be useful for the non-destructive evaluation of engineering structures with external loading.

Development of a broadband nonlinear two-degree-of-freedom piezoelectric energy harvester

Vibration energy harvesting using piezoelectric material has received great research interest in the recent years. One important concern for the development of piezoelectric energy harvesting is to broaden the operating bandwidth. Various techniques have been proposed for broadband energy harvesting, such as the resonance tuning approach, the frequency up-conversion technique, the multi-modal harvesting, and the nonlinear technique. A recently reported linear 2-degree-of-freedom harvester can achieve two close resonant frequencies both with significant power outputs, using its unique cantilever configuration. This article proposes to incorporate magnetic nonlinearity into the linear 2-degree-of-freedom system, aiming at further broadening its operating bandwidth. Experimental parametric study is carried out to investigate the behavior of such nonlinear 2-degree-of-freedom harvester. Among different configurations, an optimal configuration of the nonlinear 2-degree-of-freedom harvester is obtained to achieve significantly wider bandwidth. A lumped parameter model for such nonlinear 2-degree-of-freedom harvester is developed, and the results provide good validation for the experimental findings.