R. Lockhart

Design optimization of vibration energy harvesters fabricated by lamination of thinned bulk-PZT on polymeric substrates

The design optimization through modeling of a thinned bulk-PZT-based vibration energy harvester on a flexible polymeric substrate is presented. We also propose a simple foil-level fabrication process for their realization, by thinning the PZT down to 50 mu m and laminating it via dry film photoresist onto a PET substrate at low temperature (<85 degrees C). Two models, based on analytical and finite element modeling (FEM) methods, were developed and experimentally validated. The first, referred to as the hybrid model, is based mainly on analytical equations with the introduction of a correction factor derived from FEM simulations. The second, referred to as the numerical model, is fully based on COMSOL simulations. Both models have exhibited a very good agreement with the measured output power and resonance frequency. After their validation, a geometrical optimization through a parametric study was performed for the length, width, and thicknesses of the different layers comprising the device. As a result, an output power of 6.7 mu W at 49.8 Hz and 0.1 g, a normalized power density (NPD) of 11 683 mu W g(-2) cm(-3), and a figure of merit (FOM) of 227 mu W g(-2) cm(-3) were obtained for the optimized harvester.

Harvesting Energy From a Rotating Gear Using an AFM-Like MEMS Piezoelectric Frequency Up-Converting Energy Harvester

This paper presents an analytical and experimental study of a compact configuration to harvest energy from a rotating gear using piezoelectric microelectromechanical system harvesters. The reported configuration realizes a contact-type frequency up-conversion mechanism in order to generate useful electrical energy. The up-conversion mechanism was achieved using an atomic force microscope (AFM)-like piezoelectric cantilever plucked by the teeth of the rotating gear that could be eventually driven by an oscillating mass. This paper describes relevant design guidelines for harvesting energy from the low-frequency mechanical movement of a rotating gear through analytical modeling and finite element method (FEM) simulation followed by experimental validation. Different harvester configurations are investigated to identify the optimal configuration in terms of the output energy and energy conversion efficiency. The latter results are reported for the first time because of the implementation of an original concept based on the coupling of the harvester with a rotational flywheel. The experimental results reveal that free vibrations of the harvester after plucking contribute significantly to the output energy and efficiency. By adding a proof mass, the efficiency of the system can be greatly improved. For plucking speeds between 3 and 19 r/s, average output powers in the order of tens of microwatts were obtained for continuous plucking. By combining interaction energy, friction, and energy absorption, between the harvester and inertial mass, the maximum efficiency of the impact piezoelectric harvesters was found to be 1.4%. The efficiency results obtained were compared with the noncontact magnetic plucking approach further demonstrating the potential of our concept. Finally, different tip-gear materials combinations were evaluated showing the importance of their nature on the reliability of the presented configuration.

An Automatic Test Bench for Complete Characterization of Vibration-Energy Harvesters

This paper presents an automated test bench, based on a rigorous measurement procedure, used to fully characterize vibration energy harvesters including determination of the resonance frequency, impedance, optimal load, and output power as a function of both frequency and acceleration. The potential of this method and the performance of the automated test bench allows systematic data acquisition which is essential for a good comparison of all harvesters. A dedicated automation circuit was designed and fabricated. It uses stepper motors to mechanically control trimmers to vary the resistive load and reed relays to switch between the measurement sequences. With this, the setup is able to determine the optimal load of the device-under-test at its resonant frequency for a given acceleration. The test bench was used to fully characterize several types of vibration harvesters fabricated on both silicon and polymeric substrates. A comparison of the characterized devices is discussed using a figure of merit proposed here. A survey and compilation of current practices used to benchmark vibration harvesters is also reported.

On the experimental determination of the efficiency of piezoelectric impact-type energy harvesters using a rotational flywheel

This paper demonstrates a novel methodology using a rotational flywheel to determine the energy conversion efficiency of the impact based piezoelectric energy harvesters. The influence of the impact speed and additional proof mass on the efficiency is presented here. In order to convert low frequency mechanical oscillations into usable electrical energy, a piezoelectric harvester is coupled to a rotating gear wheel driven by flywheel. The efficiency is determined from the ratio of the electrical energy generated by the harvester to the mechanical energy dissipated by the flywheel. The experimental results reveal that free vibrations of the harvester after plucking contribute significantly to the efficiency. The efficiency and output energy can be greatly improved by adding a proof mass to the harvester. Under certain conditions, the piezoelectric harvesters have an impact energy conversion efficiency of 1.2%.

Vibration energy harvesters on plastic foil by lamination of PZT thick sheets

This paper presents a low-complexity and low temperature (85°C) fabrication process for vibration energy harvesters. The process employs lamination steps to transfer thinned PZT thick sheets onto flexible polymeric substrates, using dry film photoresist. The influence of geometrical parameters on the device performance were assessed by FEM simulations (using COMSOL) and supported by experiments. Optimization of the output power was performed by modifying the neutral plane within the device and by using a localized seismic mass at the tip, which has resulted in an output power of 30 μW at 52 Hz and an acceleration of 1g. Finally, a low-complexity and fully polymeric package is proposed, which together with the harvester process are compatible with large area fabrication methods.

On the optimization and performances of a compact piezoelectric impact MEMS energy harvester

This paper presents the development of a compact energy harvesting configuration to convert low frequency, mechanical oscillations into usable electrical energy using AFM-like MEMS piezoelectric cantilevers coupled to a rotating gear. In this approach, one or several piezoelectric harvesters can be positioned above a rotating gear driven by an oscillating mass. In order to analyze the motion and the electrical power output from the harvester, analytical and finite element models have been developed. The harvester, with an active device volume of 3.5 mm3 (3×5×0.23 mm3), is able to produce an average output power of 12 μW measured across an optimal resistive load of 4.7 kΩ at a rotational speed of 19 rps, demonstrating the potential of the compact MEMS piezoelectric micro-power generator.

A wearable system of micromachined piezoelectric cantilevers coupled to a rotational oscillating mass for on-body energy harvesting

In this paper, we present a compact, wearable piezoelectric on-body harvesting system that uses a small eccentric mass from a common watch movement to mechanically deflect a set of micromachined piezoelectric cantilevers when excited by the low frequency movements of the human body. The piezoelectric cantilevers are directly coupled to the rotating mass via a set of pins located near its rotational center. The energy produced by each pluck of a single cantilever is 545 nJ, corresponding to a maximum output power of 11 μW for continuous plucking; however, accounting for the periodic rest of typical human motion, the average output power over a full day cycle will be considerably less.

Flexible and Robust Multilayer Micro-Vibrational Harvesters for High Acceleration Environments

This paper presents the fabrication and characterization of multilayer PVDF resonant micro-vibrational energy harvesters designed to withstand environments in which high levels of acceleration are present. The multilayer cantilevers are fabricated by combining two folded PVDF stacks into a multilayered, bimorph structure. This acts to increase the overall capacitance of the harvester, a problem that plaques PVDF cantilevers as a result of its low dielectric constant. Moderate powers (7 mu W) are produced from the cantilevers even at high acceleration levels (20 g) due to the limited piezoelectric coefficient of PVDF; however, as a result of the high tensile strength and low elastic modulus of PVDF, the cantilevers are able to survive extremely high accelerations (> 4000 g) without breakage - a critical problem for harvesters based on brittle piezoelectric materials and substrates.

Multiphysics finite element model of a frequency-amplifying piezoelectric energy harvester with impact coupling for low-frequency vibrations

This paper presents experimentally-verified multiphysics finite element model of a wideband vibration energy harvester with impact coupling, which operates on the principle of frequency up-conversion: under low-frequency harmonic base excitation a cantilever-type resonator (with resonant frequency of 18.8 Hz) impacts a high-frequency piezoelectric cantilever, which starts freely vibrate at its resonant frequency of 374 Hz. Such input frequency amplification enables efficient power generation under low-frequency ambient excitations. The model was implemented in COMSOL and the contact between the cantilevers was formulated by using a nonlinear viscoelastic model. Reported results of dynamical and electrical testing of the fabricated vibration energy harvester confirm the accuracy of the model as well as reveal some operational characteristics of the device under varying impact and excitation conditions.

Silicon micromachined ultrasonic transducer with improved power transfer for cutting applications

This work presents a light and powerful silicon based ultrasonic micro-cutter. In order to achieve high cutting efficiency as well as good controllability when driven by commercially available control systems, important design parameters haven been identified. They have been verified by FEM-simulation as well as experiments via laser Doppler vibrometer measurements and cutting tests. The samples have been manufactured cost-effectively by microfabrication batch processing and their cutting ability has been successfully demonstrated on chicken tissue, while driven in a typical frequency range from 50 kHz to 100 kHz, generating tip displacements up to 36 μmpp.