Lihua Tang | The University of Auckland (original) (raw)

Papers by Lihua Tang

Proc. SPIE

Vibration energy harvesting using piezoelectric material is a promising solution for powering sma... more Vibration energy harvesting using piezoelectric material is a promising solution for powering small electric devices, which has attracted great research interest in recent years. Numerous efforts have been done by researchers to improve the efficiency of vibration energy harvesters and to broaden their bandwidths. In most reported literature, harvesters are designed to harvest energy from vibration source with a specific excitation direction. However, a practical environmental vibration source may include multiple components from different directions. Thus, it is an important concern to design a vibration energy harvester to be adaptive to multiple excitation directions. In this article, a novel piezoelectric energy harvester with frame configuration is proposed to address this issue. It can work either in its vertical vibration mode or horizontal vibration mode. Therefore, the harvester can capture vibration energy from arbitrary directions in a twodimensional plane. Experimental studies are carried out to prove the feasibility for multiple-direction energy harvesting using such harvester. The development of this two-dimensional energy harvester indicates its promising potential in practical vibration scenarios.

Proc. SPIE

Aeroelastic instabilities have been frequently exploited for energy harvesting purpose to power s... more Aeroelastic instabilities have been frequently exploited for energy harvesting purpose to power standalone electronic systems, such as wireless sensors. Meanwhile, various energy harvesting interface circuits, such as synchronized charge extraction (SCE) and synchronized switching harvesting on inductor (SSHI), have been widely pursued in the literature for efficiency enhancement of energy harvesting from existing base vibrations. These interfaces, however, have not been applied for aeroelastic energy harvesting. This paper investigates the feasibility of the SCE interface in galloping-based piezoelectric energy harvesting, with a focus on its benefit for performance improvement and influence on the galloping dynamics in different electromechanical coupling regimes. A galloping-based piezoelectric energy harvester (GPEH) is prototyped with an aluminum cantilever bonded with a piezoelectric sheet. Wind tunnel test is conducted with a simple electrical interface composed of a resistive load. Circuit simulation is performed with equivalent circuit representation of the GPEH system and confirmed by experimental results. Consequently, a self-powered SCE interface is implemented with the capability of self peak-detecting and switching. Circuit simulation for various electromechanical coupling cases shows that the harvested power with SCE interface for GPEH is independent of the electrical load, similar to that for a vibration-based piezoelectric energy harvester (VPEH). The SCE interface outperforms the standard interface if the electromechanical coupling is weak, and requires much less piezoelectric material to achieve the maximum power output. Moreover, influence of electromechanical coupling on the dynamics of GPEH with SCE is found sensitive to the wind speed. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Continuous electrode configuration (CEC) has been widely used in piezoelectric energy harvesters ... more Continuous electrode configuration (CEC) has been widely used in piezoelectric energy harvesters (PEHs). A PEH with CEC works around the first resonance efficiently but it suffers from low efficiency due to cancellation effect around higher modes. The use of segmented electrode configuration (SEC) can avoid the cancellation effect around higher modes. To achieve this, the output from each electrode pair on the opposite sides of the strain node needs to be rectified separately. In such a case, the theoretical formulation for power estimation becomes challenging because of some nonlinear electrical components included. In this paper, a method based on combining the equivalent circuit model (ECM) and the circuit simulation is proposed to estimate the power outputs of the cantilevered PEH with the SEC. First, the parameters in the ECM considering multiple modes of the PEH with the SEC are identified from the finite element analysis. The ECM is then established and simulated in the SPICE software. The optimal power outputs from the PEH with the SEC are compared with those from the PEH with the CEC. The results illustrate the advantage of the SEC to enhance the power outputs of a PEH at higher resonance frequencies. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Whereas a lot of efforts have been put on energy conservation in wireless sensor networks, the li... more Whereas a lot of efforts have been put on energy conservation in wireless sensor networks, the limited lifetime of these systems still hampers their practical deployments. This situation is further exacerbated indoors, as conventional energy harvesting (e.g., solar) ceases to work. To enable long-lived indoor sensing, we report in this paper a self-sustaining sensing system that draws energy from indoor environments, adapts its duty-cycle to the harvested energy, and pays back the environment by enhancing the awareness of the indoor microclimate through an "energy-free" sensing.

First of all, given the pervasive operation of heating, ven- tilation and air conditioning (HVAC) systems indoors, our system harvests energy from air ow introduced by the HVAC systems to power each sensor node. Secondly, as the harvested power is tiny (only of hundreds of uW), an extremely low but synchronous duty-cycle has to be applied whereas the system gets no energy surplus to support existing synchronization schemes. So we design two complementary synchronization schemes that cost virtually no energy. Finally, we exploit the feature of our harvester to sense the air ow speed (which can be used to infer the indoor microclimate) in an energy-free manner. To our knowledge, this is the rst indoor wireless sensing system that encapsulates energy harvesting, network operating, and sensing all together.

Harvesting flow energy by exploiting transverse galloping of a bluff body attached to a piezoelec... more Harvesting flow energy by exploiting transverse galloping of a bluff body attached to a piezoelectric cantilever is a prospective method to power wireless sensing systems. In order to better understand the electroaeroelastic behavior and further improve the galloping piezoelectric energy harvester (GPEH), an effective analytical model is required, which needs to incorporate both the electromechanical coupling and the aerodynamic force. Available electromechanical models for the GPEH include the lumped parameter single-degree-of-freedom (SDOF) model, the approximated distributed parameter model based on Rayleigh–Ritz discretization, and the distributed parameter model with Euler–Bernoulli beam representation. Each modeling method has its own advantages. The corresponding aerodynamic models are formulated using quasi-steady hypothesis (QSH). In this paper, the SDOF model, the Euler–Bernoulli distributed parameter model using single mode and the Euler–Bernoulli distributed parameter model using multi-modes are compared and validated with experimental results. Based on the comparison and validation, the most effective model is employed for the subsequent parametric study. The effects of load resistance, wind exposure area of the bluff body, mass of the bluff body and length of the piezoelectric sheets on the power output are investigated. These simulations can be exploited for designing and optimizing GPEHs for better performance.

Analytical and finite element electromechanical models that take into account the fact that the p... more Analytical and finite element electromechanical models that take into account the fact that the piezoelectric sheet does not cover the whole substrate beam are developed. A linear analysis of the analytical model is performed to determine the optimal load resistance. The analytical and finite element models are validated with experimental measurements. The results show that the analytical model that takes into account the fact that the piezoelectric patch does not cover the whole beam predicts accurately the experimental measurements. The finite element results yield a slight discrepancy in the global frequency and a slight overestimation in the value of the harvested power at resonance. On the contrary, using an approximate analytical model based on mode shapes of the full covered beam leads to erroneous results and overestimation of the global frequency as well as the level of harvested power. In order to design enhanced piezoelectric energy harvesters that can generate energy at low-frequency excitations, further analysis is performed to investigate the effects of varying the length of the piezoelectric material on the natural frequency and the performance of the harvester. The results show that there is a compromise between the length of the piezoelectric material, the electrical load resistance, and the available excitation frequency. By quantifying this compromise, we optimize the performance of beam–mass systems to efficiently harvest energy from a specified low frequency of the ambient vibrations.

Proc. SPIE, Dec 26, 2008

The decreasing energy consumption of today's portable electronics has invoked the possibility of ... more The decreasing energy consumption of today's portable electronics has invoked the possibility of energy harvesting from ambient environment for self power supply. One common and simple method for energy harvesting is to utilize the direct piezoelectric effect. Compared to traditional piezoelectric materials such as lead zirconate titanate (PZT), macro-fiber composites (MFC) are featured in their flexibility of large deformation. However, the energy generated by MFC is still far smaller than that required by electronics at present.

This letter presents a comparative study of different tip cross-sections for small scale wind ene... more This letter presents a comparative study of different tip cross-sections for small scale wind energy harvesting based on galloping phenomenon. A prototype device is fabricated with a piezoelectric cantilever and a tip body with various cross-section profiles (square, rectangle, triangle, and D-shape) and tested in a wind tunnel. Experimental results demonstrate the superiority of the square-sectioned tip for the low cut-in wind speed of 2.5 m/s and the high peak power of 8.4 mW. An analytical model is established and verified by the experimental results. It is recommended that the square section should be used for small wind galloping energy harvesters.

Energy harvesting from ambient vibrations using piezoelectric effect is a promising alternative s... more 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.

This letter proposes a magnetic coupled piezoelectric energy harvester (PEH), in which the magnet... more This letter proposes a magnetic coupled piezoelectric energy harvester (PEH), in which the magnetic interaction is introduced by a magnetic oscillator. For comparison purpose, lumped parameter models are established for the conventional linear PEH, the nonlinear PEH with a fixed magnet, and the proposed PEH with a magnetic oscillator. Both experiment and simulation show the benefits from the dynamics of the magnetic oscillator. In the experiment, nearly 100% increase in the operating bandwidth and 41% increase in the magnitude of the power output are achieved at an excitation level of 2 m/s^2.

Conventional vibration energy harvesters have been usually studied as single-degree-of-freedom mo... more Conventional vibration energy harvesters have been usually studied as single-degree-of-freedom models. The fact that such harvesters are only efficient near sole resonance limits their applicability in frequency-variant or random vibration scenarios. In this article, a novel multiple-degree-of-freedom piezoelectric energy harvesting model is presented. First, a two-degree-of-freedom model is analyzed, and its two configurations are characterized. In the first configuration, the piezoelectric element is placed between one mass and the base, and in the second configuration, it is placed between the two masses. It is shown that the former is advantageous over the latter since with a slight increase of overall weight to the single-degree-of-freedom model, we can achieve two close and effective peaks in power response or one effective peak with significantly enhanced magnitude. The first configuration is then generalized to an n-degree-of-freedom model, and its analytical solution is derived. This solution provides a convenient tool for parametric study and design of a multiple-degree-of-freedom piezoelectric energy harvesting model. Finally, the equivalent circuit model of the proposed n-degree-of-freedom piezoelectric energy harvesting model is developed via the analogy between the mechanical and electric domains. With the equivalent circuit model, system-level electric simulation can be performed to evaluate the system performance when sophisticated interface circuits are attached.

In recent years, several strategies have been proposed to improve the functionality of energy har... more 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.

In this work, a novel 2 degree-of-freedom (DOF) vibration energy harvester is proposed. The harve... more In this work, a novel 2 degree-of-freedom (DOF) vibration energy harvester is proposed. The harvester comprises one main cantilever beam and one secondary cantilever beam cut out within the main beam. By varying the proof masses, the first two resonances can be tuned close to each other, while maintaining significant magnitudes, thus providing a useful wide bandwidth for energy harvesting. Unlike previous 2-DOF harvesters, the proposed harvester is compact and utilizes the beam more efficiently by generating energy from both the main and secondary cantilevers. Therefore, the proposed harvester is more adaptive and functional in practical random or frequency-variant vibrational circumstances.

In the past few years, various power conditioning circuits have been proposed to improve the effi... more In the past few years, various power conditioning circuits have been proposed to improve the efficiency of piezoelectric energy harvesting, among which the synchronized charge extraction (SCE) technique has been enthusiastically pursued. In the literature, the SCE technique is investigated based on the uncoupled or in-phase assumptions. The uncoupled assumption is only valid for weak electromechanical coupling and the in-phase assumption is not applicable for energy harvesting at off-resonance. In this paper, we derive an accurate analytical solution for the piezoelectric energy harvesting systems with the SCE technique. Based on this solution, we investigate the applicability of the SCE technique for different cases, i.e. the piezoelectric energy harvester (PEH) with various degrees of electromechanical coupling and the PEH excited at various frequencies. Circuit simulation is also conducted with an accurate circuit model derived for PEHs and the results validate the analytical outcomes. Both the accurate analytical solution and the circuit simulation show that the SCE technique cannot improve or even reduces the power output at resonance if the coupling of the PEH is not negligible. The SCE technique is found capable of significantly boosting the efficiency of energy harvesting only for the PEH vibrating at off-resonance frequencies or with weak coupling.

ABSTRACT Vibration energy harvesters have been usually designed as single-degree-of-freedom (1DOF... more ABSTRACT Vibration energy harvesters have been usually designed as single-degree-of-freedom (1DOF) systems. The fact that such harvesters are only efficient near sole resonance limits their applicability in frequency-variant and random vibration scenarios. In this paper, a novel multiple-DOF piezoelectric energy harvester model (PEHM) is developed, which comprises a primary mass and n parasitic masses. The parasitic masses are independent of each other but attached to the primary mass.

Journal of Intelligent Material Systems …, Jan 1, 2010

The dramatic reduction in power consumption of current integrated circuits has evoked great resea... more 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.

This article develops one-dimensional and two-dimensional strain transfer models for macrofiber co... more This article develops one-dimensional and two-dimensional strain transfer models for macrofiber composite (MFC) actuators under quasi-static excitation. The adhesive layer (epoxy) between the actuator and the structure is considered in the Bernoulli–Euler formulation, and the results for various models are compared with the uniform strain model which includes the shear lag effect due to the adhesive layer. The reduction in actuation as a result of strain distribution inside the MFC actuator is considered to improve the prediction ability of the existing models based on the Bernoulli–Euler formulation. Finite element simulation is also carried out to study the strain transfer for both quasi-static and near natural frequency excitations. The effects of the thickness and modulus of elasticity of the adhesive layer on strain transfer are discussed. Finally, an experimental test is conducted to evaluate the accuracy and limitations of the developed models. Results show that the models are in good agreement with the experimental test.

Accurate modeling and computer aided simulation is advantageous during the design stage of a piez... more Accurate modeling and computer aided simulation is advantageous during the design stage of a piezoelectric energy harvesting system. In this paper, system-level finite element modeling (FEM) of a cantilevered piezoelectric energy harvester with a resistor is conducted using ANSYS. Considering that practical energy harvesting circuit includes nonlinear electrical elements, which is beyond the modeling capability of ANSYS, an equivalent circuit modeling (ECM) method is proposed to address the problem. After the parameters of equivalent circuit are identified, system-level simulation is conducted in SPICE software.

Abstract The success of a new material is usually dependent on its applications. When ionic polym... more Abstract The success of a new material is usually dependent on its applications. When ionic polymer-metal composites (IPMC) were first invented, the potential application areas were identified as biomedical and robotics, due to their resemblance to biological muscles in both properties and performances. Thus, IPMC is sometimes referred to as artificial muscles. Throughout the years, a large number of biomedical applications have been proposed, including the applications presented in Chapter 15.

Proc. SPIE, Mar 24, 2011

abstract In the past few years, various circuit techniques have been proposed to improve the effi... more abstract In the past few years, various circuit techniques have been proposed to improve the efficiency of piezoelectric energy harvesting, among which the synchronized charge extraction (SCE) circuit has been enthusiastically pursued. In the literature, the SCE technique is claimed to increase the power output of a piezoelectric energy harvester (PEH) by four times based on the assumption that the vibration of the harvester is not affected by the energy harvesting process.

Proc. SPIE

Vibration energy harvesting using piezoelectric material is a promising solution for powering sma... more Vibration energy harvesting using piezoelectric material is a promising solution for powering small electric devices, which has attracted great research interest in recent years. Numerous efforts have been done by researchers to improve the efficiency of vibration energy harvesters and to broaden their bandwidths. In most reported literature, harvesters are designed to harvest energy from vibration source with a specific excitation direction. However, a practical environmental vibration source may include multiple components from different directions. Thus, it is an important concern to design a vibration energy harvester to be adaptive to multiple excitation directions. In this article, a novel piezoelectric energy harvester with frame configuration is proposed to address this issue. It can work either in its vertical vibration mode or horizontal vibration mode. Therefore, the harvester can capture vibration energy from arbitrary directions in a twodimensional plane. Experimental studies are carried out to prove the feasibility for multiple-direction energy harvesting using such harvester. The development of this two-dimensional energy harvester indicates its promising potential in practical vibration scenarios.

Proc. SPIE

Aeroelastic instabilities have been frequently exploited for energy harvesting purpose to power s... more Aeroelastic instabilities have been frequently exploited for energy harvesting purpose to power standalone electronic systems, such as wireless sensors. Meanwhile, various energy harvesting interface circuits, such as synchronized charge extraction (SCE) and synchronized switching harvesting on inductor (SSHI), have been widely pursued in the literature for efficiency enhancement of energy harvesting from existing base vibrations. These interfaces, however, have not been applied for aeroelastic energy harvesting. This paper investigates the feasibility of the SCE interface in galloping-based piezoelectric energy harvesting, with a focus on its benefit for performance improvement and influence on the galloping dynamics in different electromechanical coupling regimes. A galloping-based piezoelectric energy harvester (GPEH) is prototyped with an aluminum cantilever bonded with a piezoelectric sheet. Wind tunnel test is conducted with a simple electrical interface composed of a resistive load. Circuit simulation is performed with equivalent circuit representation of the GPEH system and confirmed by experimental results. Consequently, a self-powered SCE interface is implemented with the capability of self peak-detecting and switching. Circuit simulation for various electromechanical coupling cases shows that the harvested power with SCE interface for GPEH is independent of the electrical load, similar to that for a vibration-based piezoelectric energy harvester (VPEH). The SCE interface outperforms the standard interface if the electromechanical coupling is weak, and requires much less piezoelectric material to achieve the maximum power output. Moreover, influence of electromechanical coupling on the dynamics of GPEH with SCE is found sensitive to the wind speed. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Continuous electrode configuration (CEC) has been widely used in piezoelectric energy harvesters ... more Continuous electrode configuration (CEC) has been widely used in piezoelectric energy harvesters (PEHs). A PEH with CEC works around the first resonance efficiently but it suffers from low efficiency due to cancellation effect around higher modes. The use of segmented electrode configuration (SEC) can avoid the cancellation effect around higher modes. To achieve this, the output from each electrode pair on the opposite sides of the strain node needs to be rectified separately. In such a case, the theoretical formulation for power estimation becomes challenging because of some nonlinear electrical components included. In this paper, a method based on combining the equivalent circuit model (ECM) and the circuit simulation is proposed to estimate the power outputs of the cantilevered PEH with the SEC. First, the parameters in the ECM considering multiple modes of the PEH with the SEC are identified from the finite element analysis. The ECM is then established and simulated in the SPICE software. The optimal power outputs from the PEH with the SEC are compared with those from the PEH with the CEC. The results illustrate the advantage of the SEC to enhance the power outputs of a PEH at higher resonance frequencies. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Whereas a lot of efforts have been put on energy conservation in wireless sensor networks, the li... more Whereas a lot of efforts have been put on energy conservation in wireless sensor networks, the limited lifetime of these systems still hampers their practical deployments. This situation is further exacerbated indoors, as conventional energy harvesting (e.g., solar) ceases to work. To enable long-lived indoor sensing, we report in this paper a self-sustaining sensing system that draws energy from indoor environments, adapts its duty-cycle to the harvested energy, and pays back the environment by enhancing the awareness of the indoor microclimate through an "energy-free" sensing.

First of all, given the pervasive operation of heating, ven- tilation and air conditioning (HVAC) systems indoors, our system harvests energy from air ow introduced by the HVAC systems to power each sensor node. Secondly, as the harvested power is tiny (only of hundreds of uW), an extremely low but synchronous duty-cycle has to be applied whereas the system gets no energy surplus to support existing synchronization schemes. So we design two complementary synchronization schemes that cost virtually no energy. Finally, we exploit the feature of our harvester to sense the air ow speed (which can be used to infer the indoor microclimate) in an energy-free manner. To our knowledge, this is the rst indoor wireless sensing system that encapsulates energy harvesting, network operating, and sensing all together.

Harvesting flow energy by exploiting transverse galloping of a bluff body attached to a piezoelec... more Harvesting flow energy by exploiting transverse galloping of a bluff body attached to a piezoelectric cantilever is a prospective method to power wireless sensing systems. In order to better understand the electroaeroelastic behavior and further improve the galloping piezoelectric energy harvester (GPEH), an effective analytical model is required, which needs to incorporate both the electromechanical coupling and the aerodynamic force. Available electromechanical models for the GPEH include the lumped parameter single-degree-of-freedom (SDOF) model, the approximated distributed parameter model based on Rayleigh–Ritz discretization, and the distributed parameter model with Euler–Bernoulli beam representation. Each modeling method has its own advantages. The corresponding aerodynamic models are formulated using quasi-steady hypothesis (QSH). In this paper, the SDOF model, the Euler–Bernoulli distributed parameter model using single mode and the Euler–Bernoulli distributed parameter model using multi-modes are compared and validated with experimental results. Based on the comparison and validation, the most effective model is employed for the subsequent parametric study. The effects of load resistance, wind exposure area of the bluff body, mass of the bluff body and length of the piezoelectric sheets on the power output are investigated. These simulations can be exploited for designing and optimizing GPEHs for better performance.

Analytical and finite element electromechanical models that take into account the fact that the p... more Analytical and finite element electromechanical models that take into account the fact that the piezoelectric sheet does not cover the whole substrate beam are developed. A linear analysis of the analytical model is performed to determine the optimal load resistance. The analytical and finite element models are validated with experimental measurements. The results show that the analytical model that takes into account the fact that the piezoelectric patch does not cover the whole beam predicts accurately the experimental measurements. The finite element results yield a slight discrepancy in the global frequency and a slight overestimation in the value of the harvested power at resonance. On the contrary, using an approximate analytical model based on mode shapes of the full covered beam leads to erroneous results and overestimation of the global frequency as well as the level of harvested power. In order to design enhanced piezoelectric energy harvesters that can generate energy at low-frequency excitations, further analysis is performed to investigate the effects of varying the length of the piezoelectric material on the natural frequency and the performance of the harvester. The results show that there is a compromise between the length of the piezoelectric material, the electrical load resistance, and the available excitation frequency. By quantifying this compromise, we optimize the performance of beam–mass systems to efficiently harvest energy from a specified low frequency of the ambient vibrations.

Proc. SPIE, Dec 26, 2008

The decreasing energy consumption of today's portable electronics has invoked the possibility of ... more The decreasing energy consumption of today's portable electronics has invoked the possibility of energy harvesting from ambient environment for self power supply. One common and simple method for energy harvesting is to utilize the direct piezoelectric effect. Compared to traditional piezoelectric materials such as lead zirconate titanate (PZT), macro-fiber composites (MFC) are featured in their flexibility of large deformation. However, the energy generated by MFC is still far smaller than that required by electronics at present.

This letter presents a comparative study of different tip cross-sections for small scale wind ene... more This letter presents a comparative study of different tip cross-sections for small scale wind energy harvesting based on galloping phenomenon. A prototype device is fabricated with a piezoelectric cantilever and a tip body with various cross-section profiles (square, rectangle, triangle, and D-shape) and tested in a wind tunnel. Experimental results demonstrate the superiority of the square-sectioned tip for the low cut-in wind speed of 2.5 m/s and the high peak power of 8.4 mW. An analytical model is established and verified by the experimental results. It is recommended that the square section should be used for small wind galloping energy harvesters.

Energy harvesting from ambient vibrations using piezoelectric effect is a promising alternative s... more 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.

This letter proposes a magnetic coupled piezoelectric energy harvester (PEH), in which the magnet... more This letter proposes a magnetic coupled piezoelectric energy harvester (PEH), in which the magnetic interaction is introduced by a magnetic oscillator. For comparison purpose, lumped parameter models are established for the conventional linear PEH, the nonlinear PEH with a fixed magnet, and the proposed PEH with a magnetic oscillator. Both experiment and simulation show the benefits from the dynamics of the magnetic oscillator. In the experiment, nearly 100% increase in the operating bandwidth and 41% increase in the magnitude of the power output are achieved at an excitation level of 2 m/s^2.

Conventional vibration energy harvesters have been usually studied as single-degree-of-freedom mo... more Conventional vibration energy harvesters have been usually studied as single-degree-of-freedom models. The fact that such harvesters are only efficient near sole resonance limits their applicability in frequency-variant or random vibration scenarios. In this article, a novel multiple-degree-of-freedom piezoelectric energy harvesting model is presented. First, a two-degree-of-freedom model is analyzed, and its two configurations are characterized. In the first configuration, the piezoelectric element is placed between one mass and the base, and in the second configuration, it is placed between the two masses. It is shown that the former is advantageous over the latter since with a slight increase of overall weight to the single-degree-of-freedom model, we can achieve two close and effective peaks in power response or one effective peak with significantly enhanced magnitude. The first configuration is then generalized to an n-degree-of-freedom model, and its analytical solution is derived. This solution provides a convenient tool for parametric study and design of a multiple-degree-of-freedom piezoelectric energy harvesting model. Finally, the equivalent circuit model of the proposed n-degree-of-freedom piezoelectric energy harvesting model is developed via the analogy between the mechanical and electric domains. With the equivalent circuit model, system-level electric simulation can be performed to evaluate the system performance when sophisticated interface circuits are attached.

In recent years, several strategies have been proposed to improve the functionality of energy har... more 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.

In this work, a novel 2 degree-of-freedom (DOF) vibration energy harvester is proposed. The harve... more In this work, a novel 2 degree-of-freedom (DOF) vibration energy harvester is proposed. The harvester comprises one main cantilever beam and one secondary cantilever beam cut out within the main beam. By varying the proof masses, the first two resonances can be tuned close to each other, while maintaining significant magnitudes, thus providing a useful wide bandwidth for energy harvesting. Unlike previous 2-DOF harvesters, the proposed harvester is compact and utilizes the beam more efficiently by generating energy from both the main and secondary cantilevers. Therefore, the proposed harvester is more adaptive and functional in practical random or frequency-variant vibrational circumstances.

In the past few years, various power conditioning circuits have been proposed to improve the effi... more In the past few years, various power conditioning circuits have been proposed to improve the efficiency of piezoelectric energy harvesting, among which the synchronized charge extraction (SCE) technique has been enthusiastically pursued. In the literature, the SCE technique is investigated based on the uncoupled or in-phase assumptions. The uncoupled assumption is only valid for weak electromechanical coupling and the in-phase assumption is not applicable for energy harvesting at off-resonance. In this paper, we derive an accurate analytical solution for the piezoelectric energy harvesting systems with the SCE technique. Based on this solution, we investigate the applicability of the SCE technique for different cases, i.e. the piezoelectric energy harvester (PEH) with various degrees of electromechanical coupling and the PEH excited at various frequencies. Circuit simulation is also conducted with an accurate circuit model derived for PEHs and the results validate the analytical outcomes. Both the accurate analytical solution and the circuit simulation show that the SCE technique cannot improve or even reduces the power output at resonance if the coupling of the PEH is not negligible. The SCE technique is found capable of significantly boosting the efficiency of energy harvesting only for the PEH vibrating at off-resonance frequencies or with weak coupling.

ABSTRACT Vibration energy harvesters have been usually designed as single-degree-of-freedom (1DOF... more ABSTRACT Vibration energy harvesters have been usually designed as single-degree-of-freedom (1DOF) systems. The fact that such harvesters are only efficient near sole resonance limits their applicability in frequency-variant and random vibration scenarios. In this paper, a novel multiple-DOF piezoelectric energy harvester model (PEHM) is developed, which comprises a primary mass and n parasitic masses. The parasitic masses are independent of each other but attached to the primary mass.

Journal of Intelligent Material Systems …, Jan 1, 2010

The dramatic reduction in power consumption of current integrated circuits has evoked great resea... more 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.

This article develops one-dimensional and two-dimensional strain transfer models for macrofiber co... more This article develops one-dimensional and two-dimensional strain transfer models for macrofiber composite (MFC) actuators under quasi-static excitation. The adhesive layer (epoxy) between the actuator and the structure is considered in the Bernoulli–Euler formulation, and the results for various models are compared with the uniform strain model which includes the shear lag effect due to the adhesive layer. The reduction in actuation as a result of strain distribution inside the MFC actuator is considered to improve the prediction ability of the existing models based on the Bernoulli–Euler formulation. Finite element simulation is also carried out to study the strain transfer for both quasi-static and near natural frequency excitations. The effects of the thickness and modulus of elasticity of the adhesive layer on strain transfer are discussed. Finally, an experimental test is conducted to evaluate the accuracy and limitations of the developed models. Results show that the models are in good agreement with the experimental test.

Accurate modeling and computer aided simulation is advantageous during the design stage of a piez... more Accurate modeling and computer aided simulation is advantageous during the design stage of a piezoelectric energy harvesting system. In this paper, system-level finite element modeling (FEM) of a cantilevered piezoelectric energy harvester with a resistor is conducted using ANSYS. Considering that practical energy harvesting circuit includes nonlinear electrical elements, which is beyond the modeling capability of ANSYS, an equivalent circuit modeling (ECM) method is proposed to address the problem. After the parameters of equivalent circuit are identified, system-level simulation is conducted in SPICE software.

Abstract The success of a new material is usually dependent on its applications. When ionic polym... more Abstract The success of a new material is usually dependent on its applications. When ionic polymer-metal composites (IPMC) were first invented, the potential application areas were identified as biomedical and robotics, due to their resemblance to biological muscles in both properties and performances. Thus, IPMC is sometimes referred to as artificial muscles. Throughout the years, a large number of biomedical applications have been proposed, including the applications presented in Chapter 15.

Proc. SPIE, Mar 24, 2011

abstract In the past few years, various circuit techniques have been proposed to improve the effi... more abstract In the past few years, various circuit techniques have been proposed to improve the efficiency of piezoelectric energy harvesting, among which the synchronized charge extraction (SCE) circuit has been enthusiastically pursued. In the literature, the SCE technique is claimed to increase the power output of a piezoelectric energy harvester (PEH) by four times based on the assumption that the vibration of the harvester is not affected by the energy harvesting process.