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 paper reviews the advances made in the past few years on this issue. The broadband vibration-based energy harvesting solutions, covering resonance tuning, 2 multimodal energy harvesting, frequency up-conversion and techniques exploiting nonlinear oscillations, are summarized in detail with regard to their merits and applicability in different circumstances.

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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.

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.

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.