Connection Configurations to Increase Operational Range and Output Power of Piezoelectric MEMS Vibration Energy Harvesters

Among the various methods of extracting energy harvested by a piezoelectric vibration energy harvester, full-bridge rectifiers (FBR) are widely employed due to its simplicity and stability. However, its efficiency and operational range are limited due to a threshold voltage that the open-circuit voltage generated from the piezoelectric transducer (PT) must attain prior to any energy extraction. This voltage linearly depends on the output voltage of the FBR and the forward voltage drop of diodes and the nature of the interface can significantly limit the amount of extracted energy under low excitation levels. In this paper, a passive scheme is proposed to split the electrode of a micromachined PT into multiple (n) equal regions, which are electrically connected in series. The power output from such a series connected MEMS PT allows for the generated voltage to readily overcome the threshold set by the FBR. Theoretical calculations have been performed in this paper to assess the performance for different series stages (n values) and the theory has been experimentally validated. The results show that a PT with more series stages (high n values) improves the efficiency of energy extraction relative to the case with fewer series-connected stages under weak excitation levels

Rectified Output Power Analysis of Piezoelectric Energy Harvester Arrays under Noisy Excitation

In the past decade, vibration energy harvesting has emerged as a potential alternative solution to power wireless sensor nodes. In real-world implementations, external excitation can be very noisy and includes noise signals in a wide frequency band. In order to accommodate operation under noisy excitation, arrays of energy harvesters with different resonance frequencies are often employed to improve responsibility. Due to the nature of noisy excitation and the difference in resonance frequencies, the response voltage signals from each harvester can be very different in amplitude, frequency and phase. In this paper, an array with two cantilevered energy harvesters is studied to analyze the rectified output power with different configurations using full-bridge rectifiers (FBR). The experiments show that connecting the two harvesters in parallel or in series before connecting with a FBR results in significant voltage cancellation due to phase mismatch. The most efficient way to extract energy is to use two FBRs for the two cantilevered energy harvesters, individually, and charge to one single storage capacitor connected at the outputs of the two FBRs

ELECTROMECHANICAL MODELING OF A HONEYCOMB CORE INTEGRATED VIBRATION ENERGY CONVERTER WITH INCREASED SPECIFIC POWER FOR ENERGY HARVESTING APPLICATIONS

Innovation in integrated circuit technology along with improved manufacturing processes has resulted in considerable reduction in power consumption of electromechanical devices. Majority of these devices are currently powered by batteries. However, the issues posed by batteries, including the need for frequent battery recharge/replacement has resulted in a compelling need for alternate energy to achieve self-sufficient device operation or to supplement battery power. Vibration based energy harvesting methods through piezoelectric transduction provides with a promising potential towards replacing or supplementing battery power source. However, current piezoelectric energy harvesters generate low specific power (power-to-weight ratio) when compared to batteries that the harvesters seek to replace or supplement. In this study, the potential of integrating lightweight cellular honeycomb structures with existing piezoelectric device configurations (bimorph) to achieve higher specific power is investigated. It is shown in this study that at low excitation frequency ranges, replacing the solid continuous substrate of a conventional piezoelectric bimorph with honeycomb structures of the same material results in a significant increase in power-to-weight ratio of the piezoelectric harvester. In order to maximize the electrical response of vibration based power harvesters, the natural frequency of these harvesters is designed to match the input driving frequency. The commonly used technique of adding a tip mass is employed to lower the natural frequency (to match driving frequency) of both, solid and honeycomb substrate bimorphs. At higher excitation frequency, the natural frequency of the traditional solid substrate bimorph can only be altered (to match driving frequency) through a change in global geometric design parameters, typically achieved by increasing the thickness of the harvester. As a result, the size of the harvester is increased and can be disadvantageous especially if the application imposes a space/size constraint. Moreover, the bimorph with increased thickness will now require a larger mechanical force to deform the structure which can fall outside the input ambient excitation amplitude range. In contrast, the honeycomb core bimorph offers an advantage in terms of preserving the global geometric dimensions. The natural frequency of the honeycomb core bimorph can be altered by manipulating honeycomb cell design parameters, such as cell angle, cell wall thickness, vertical cell height and inclined cell length. This results in a change in the mass and stiffness properties of the substrate and hence the bimorph, thereby altering the natural frequency of the harvester. Design flexibility of honeycomb core bimorphs is demonstrated by varying honeycomb cell parameters to alter mass and stiffness properties for power harvesting. The influence of honeycomb cell parameters on power generation is examined to evaluate optimum design to attain highest specific power. In addition, the more compliant nature of a honeycomb core bimorph decreases susceptibility towards fatigue and can increase the operating lifetime of the harvester. The second component of this dissertation analyses an uncoupled equivalent circuit model for piezoelectric energy harvesting. Open circuit voltage developed on the piezoelectric materials can be easily computed either through analytical or finite element models. The efficacy of a method to determine power developed across a resistive load, by representing the coupled piezoelectric electromechanical problem with an external load as an open circuit voltage driven equivalent circuit, is evaluated. The lack of backward feedback at finite resistive loads resulting from such an equivalent representation is examined by comparing the equivalent circuit model to the governing equations of a fully coupled circuit model for the electromechanical problem. It is found that the backward feedback is insignificant for weakly coupled systems typically seen in micro electromechanical systems and other energy harvesting device configurations with low coupling. For moderate to high coupling systems, a correction factor based on a calibrated resistance is presented which can be used to evaluate power generation at a specific resistive load

Analysis on One-Stage SSHC Rectifier for Piezoelectric Vibration Energy Harvesting

Conventional SSHI (synchronized switch harvesting on inductor) has been believed to be one of the most efficient interface circuits for piezoelectric vibration energy harvesting systems. It employs an inductor and the resulting RLC loop to synchronously invert the charge across the piezoelectric material to avoid charge and energy loss due to charging its internal capacitor ($C_P$). The performance of the SSHI circuit greatly depends on the inductor and a large inductor is often needed; hence significantly increases the volume of the system. An efficient interface circuit using a synchronous charge inversion technique, named as SSHC, was proposed recently. The SSHC rectifier utilizes capacitors, instead of inductors, to flip the voltage across the harvester. For a one-stage SSHC rectifier, one single intermediate capacitor ($C_T$) is employed to temporarily store charge flowed from $C_P$ and inversely charge $C_P$ to perform the charge inversion. In previous studies, the voltage flip efficiency achieves 1/3 when $C_T = C_P$. This paper presents that the voltage flip efficiency can be further increased to approach 1/2 if $C_T$ is chosen to be much larger than $C_P$

A Passive Design Scheme to Increase the Rectified Power of Piezoelectric Energy Harvesters

Piezoelectric vibration energy harvesting is becoming a promising solution to power wireless sensors and portable electronics. While miniaturizing energy harvesting systems, rectified power efficiencies from miniaturized piezoelectric transducers (PT) are usually decreased due to insufficient voltage levels generated by the PTs. In this paper, a monolithic PT is split into several regions connected in series. The raw electrical output power is kept constant for different connection configurations as theoretically predicted. However, the rectified power following a full-bridge rectifier (FBR), or a synchronized switch harvesting on inductor (SSHI) rectifier, is significantly increased due to the higher voltage/current ratio of series connections. This is an entirely passive design scheme without introducing any additional quiescent power consumption and it is compatible with most of state-of-the-art interface circuits. Detailed theoretical derivations are provided to support the theory and the results are experimentally evaluated using a custom MEMS PT and a CMOS rectification circuit. The results show that, while a PT is split into 8 regions connected in series, the performance while using a FBR and a SSHI circuit is increased by 2.3X and 5.8X, respectively, providing an entirely passive approach to improving energy conversion efficiency.UK Engineering and Physical Sciences Research Council (EPSRC) (Grant number: EP/L010917/1 and EP/N021614/1

Dynamic analysis and fabrication of a bi-stable structure designed for MEMS energy harvesting applications.

Thanks to the rapid growth in demand for power in remote locations, scientists’ attention has been drawn to vibration energy harvesting as an alternative to batteries. Over the past ten years, the energy harvesting community has focused on bistable structures as a means of broadening the working frequency range and, by extension, the effective efficiency of vibration-based power scavenging systems. In the current study, a new method is implemented to statically and dynamically analyze a bistable buckled, multi-component coupled structure designed specifically for low-frequency vibration energy harvesting systems in both macro and MEMS-scale sizes. Furthermore, several micro-fabrication steps using advanced manufacturing technology methods were applied to design and fabricate a micro-scale version of the energy harvester at the University of Louisville Micro/Nano Technology Center. First, previously efforts performed on different aspects of vibration energy harvesting systems are reviewed to show the current challenges associated with such devices. The coupled structure proposed in this project is then introduced and its equations of motion are developed based on nonlinear Euler-Bernoulli beam theory. These governing equations are discretized and solved using a Galerkin method in two different approaches: with some known shape functions which only satisfies the geometrical boundary conditions; with the exact shape functions obtained from solving the linearized coupled structure as a one single system. An experimental setup is also used to verify the advantages of designed structure in capturing bistable motion at low-frequency range. To validate the modeling approaches, the obtained results are compared with the ones captured from both FEA model and the experimental setup, which shows the superiority of the proposed approach in which exact shape functions of the system are used as the basis in the discretization process. After the validation of the proposed approach, it is applied on a micro-scale version of the system in which structural, piezoelectric, and electrode layers are all considered as they exist in an actual device. Furthermore, a different bistable system, which was previously studied by other researchers in the area, is analyzed by this method to show the reliability of the proposed model. For all these cases, the amplitude-frequency response of the system and snap-through regime with the variation of various parameters, including exciting frequency, base vibration, and buckling loads are investigated based on the developed model. It is shown that bisatble motion and other nonlinear phenomena such as super-harmonic behavior in the system can be captured under certain circumstances, which can significantly impact major system functionalities such as output voltage response and is crucial for the performance of energy harvesting devices. As mentioned above, various micro-fabrication techniques were also used to design and fabricate a micro-scale version of the proposed system, which eventually led to the successful fabrication of a MEMS device as a result of experimental efforts performed to overcome the challenges and issues associated with the designed manufacturing process

A Hybrid Technique of Energy Harvesting from Mechanical Vibration and Ambient Illumination

Hybrid energy harvesting is a concept applied for improving the performance of the conventional stand-alone energy harvesters. The thesis presents the analytical formulations and characterization of a hybrid energy harvester that incorporates photovoltaic, piezoelectric, electromagnetic, and electrostatic mechanisms. The initial voltage required for electrostatic mechanism is obtained by the photovoltaic technique. Other mechanisms are embedded into a bimorph piezoelectric cantilever beam having a tip magnet and two sets of comb electrodes on two sides of its substructure. All the segments are interconnected by an electric circuit to generate combined output when subjected to vibration and solar illumination. Results for power output have been obtained at resonance frequency using an optimum load resistance. As the power transduced by each of the mechanisms is combined, more power is generated than those obtained by stand-alone mechanisms. The synergistic feature of this research is further promoted by adding fatigue analysis using finite element method

DEVELOPMENT OF PIEZOELECTRIC ENERGY HARVESTING SYSTEM FOR LOW-FREQUENCY VIBRATIONS

Harvesting energy from vibration sources has attracted the interest of researchers for the past three decades. Researchers have been working on the potential of achieving self-powered MEMS scale devices. Piezoelectric cantilever harvesters have caught the attention in this field because of the excellent combination of high-power density and compact structure. The main objective of this thesis is to develop a novel and optimum piezoelectric harvester system using lumped parameter model (LPM) for given vibration sources. The finite element model (FEM) is used in this work as an original approach to be utilized for optimal design optimization. Three types of validations are accomplished to solidify the use of FEM in mimicking the distributed parameter model (DPM) for linearly tapered piezoelectric cantilevers. The first two validations are accomplished using beam deflection and relative transmissibility functions. Comparisons between the FEM and the DPM developed by the literature are performed. The third validation is carried for an electromechanical piezoelectric cantilever in FEM. Results confirmed the effectiveness of the developed FEM. A number of significant contributions are achieved while fulfilling the aim of this work. First, a dimensionless parameter, Power Factor (PF), is derived and used to understand the impact of the geometry on the piezoelectric harvester performance. The PF showed an optimum performance at a taper ratio of 0, taking the full length of the cantilever and thickness ratio of 0.7. Second, the accuracy of the LPM for linearly tapered piezoelectric harvesters and optimal design are investigated. Results indicated that the percentage of the deflection error between the LPM and the FEM reaches 9% when the taper ratio is zero. However, when tip-mass to cantilever ratios are larger than 2, the error decreases to less than 0.5% leading to more accurate results in the vibrational response of the beam. Further studies on the accuracy are accomplished using the relative transmissibility function. Results showed that as the taper ratio decreases towards zero, the percentage error of using the LPM to predict the vibration response increases significantly to 55%. These results lay the foundation for the third contribution of developing correction factors for tapered and optimal piezoelectric cantilever harvesters using FEM. Comparisons of the corrected LPM and FEM for different configurations are examined. Results indicated that as the taper ratio decreases, the surface power density increases. However, the developed optimal design exhibits the highest surface power density of 1.40×104 [(mW/g2)/ m2] which is 16.4% more than the best following shape of a taper ratio 0.2 and 58% more than the taper ratio 1. Furthermore, a parametric study of the optimal design is performed to scrutinize the effect of various parameters on the harvester performance. Finally, detailed criteria for designing the optimal piezoelectric harvester for different conditions are structured

DESIGN, FABRICATION AND CHARACTERISATION OF FREE-STANDING THICK-FILM PIEZOELECTRIC CANTILEVERS FOR ENERGY HARVESTING

Research into energy harvesting from ambient vibration sources has attracted great interest over the last few years, largely due to the rapid development in the areas of wireless technology and low power electronics. One of the mechanisms for converting mechanical vibration to electrical energy is the use of piezoelectric materials, typically operating as a cantilever in a bending mode, which generate a voltage across the electrodes when they are stressed. Traditionally, the piezoelectric materials are deposited on a non-electro-active substrate and are physically clamped at one end to a rigid base, which serves as a mechanical supporting platform. In this research, a three dimensional thick-film structure in the form of a free-standing cantilever incorporated with piezoelectric materials is proposed. The advantages of this structure include minimising the movement constraints on the piezoelectric, thereby maximising the electrical output and offering the ability for integration with other microelectronic devices. A series of free-standing composite cantilevers in the form of unimorphs were fabricated and characterised for their mechanical and electric properties. The unimorph structure consists of a pair of silver/palladium (Ag/Pd) electrodes sandwiching a laminar layer of lead zirconate titanate (PZT). An extended version of this unimorph, in the form of multimorph was fabricated to improve the electrical output performance, by increasing the distance of the piezoelectric layer from the neutral axis of the structure. This research also discusses the possibility of using an array of free-standing cantilevers in harvesting vibration energy in a broader bandwidth from an unpredictable ambient environment