M.J. Tudor

Energy harvesting vibration sources for microsystems applications

This paper reviews the state-of-the art in vibration energy harvesting for wireless, self-powered microsystems. Vibration-powered generators are typically, although not exclusively, inertial spring and mass systems. The characteristic equations for inertial-based generators are presented, along with the specific damping equations that relate to the three main transduction mechanisms employed to extract energy from the system. These transduction mechanisms are: piezoelectric, electromagnetic and electrostatic. Piezoelectric generators employ active materials that generate a charge when mechanically stressed. A comprehensive review of existing piezoelectric generators is presented, including impact coupled, resonant and human-based devices. Electromagnetic generators employ electromagnetic induction arising from the relative motion between a magnetic flux gradient and a conductor. Electromagnetic generators presented in the literature are reviewed including large scale discrete devices and wafer-scale integrated versions. Electrostatic generators utilize the relative movement between electrically isolated charged capacitor plates to generate energy. The work done against the electrostatic force between the plates provides the harvested energy. Electrostatic-based generators are reviewed under the classifications of in-plane overlap varying, in-plane gap closing and out-of-plane gap closing; the Coulomb force parametric generator and electret-based generators are also covered. The coupling factor of each transduction mechanism is discussed and all the devices presented in the literature are summarized in tables classified by transduction type; conclusions are drawn as to the suitability of the various techniques.

A micro electromagnetic generator for vibration energy harvesting

Vibration energy harvesting is receiving a considerable amount of interest as a means for powering wireless sensor nodes. This paper presents a small (component volume 0.1 cm3, practical volume 0.15 cm3) electromagnetic generator utilizing discrete components and optimized for a low ambient vibration level based upon real application data. The generator uses four magnets arranged on an etched cantilever with a wound coil located within the moving magnetic field. Magnet size and coil properties were optimized, with the final device producing 46 µW in a resistive load of 4 k? from just 0.59 m s-2 acceleration levels at its resonant frequency of 52 Hz. A voltage of 428 mVrms was obtained from the generator with a 2300 turn coil which has proved sufficient for subsequent rectification and voltage step-up circuitry. The generator delivers 30% of the power supplied from the environment to useful electrical power in the load. This generator compares very favourably with other demonstrated examples in the literature, both in terms of normalized power density and efficiency.

Strategies for increasing the operating frequency range of vibration energy harvesters: a review

This paper reviews possible strategies to increase the operational frequency range of vibration-based micro-generators. Most vibration-based micro-generators are spring-mass-damper systems which generate maximum power when the resonant frequency of the generator matches the frequency of the ambient vibration. Any difference between these two frequencies can result in a significant decrease in generated power. This is a fundamental limitation of resonant vibration generators which restricts their capability in real applications. Possible solutions include the periodic tuning of the resonant frequency of the generator so that it matches the frequency of the ambient vibration at all times or widening the bandwidth of the generator. Periodic tuning can be achieved using mechanical or electrical methods. Bandwidth widening can be achieved using a generator array, a mechanical stopper, non-linear (e.g. magnetic) springs or bi-stable structures. Tuning methods can be classified into intermittent tuning (power is consumed periodically to tune the device) and continuous tuning (the tuning mechanism is continuously powered). This paper presents a comprehensive review of the principles and operating strategies for increasing the operating frequency range of vibration-based micro-generators presented in the literature to date. The advantages and disadvantages of each strategy are evaluated and conclusions are drawn regarding the relevant merits of each approach.

Self-powered autonomous wireless sensor node using vibration energy harvesting

This paper reports the development and implementation of an energy aware autonomous wireless condition monitoring sensor system (ACMS) powered by ambient vibrations. An electromagnetic (EM) generator has been designed to harvest sufficient energy to power a radio-frequency (RF) linked accelerometer-based sensor system. The ACMS is energy aware and will adjust the measurement/transmit duty cycle according to the available energy; this is typically every 3 s at 0.6 m s?2 rms acceleration and can be as low as 0.2 m s?2 rms with a duty cycle around 12 min. The EM generator has a volume of only 150 mm3 producing an average power of 58 ?W at 0.6m s?2 rms acceleration at a frequency of 52 Hz. In addition, a voltage multiplier circuit is shown to increase the electrical damping compared to a purely resistive load; this allows for an average power of 120 ?W to be generated at 1.7 m s?2 rms acceleration. The ACMS has been successfully demonstrated on an industrial air compressor and an office air conditioning unit, continuously monitoring vibration levels and thereby simulating a typical condition monitoring application

Review of the application of energy harvesting in buildings

This review presents the state of the art of the application of energy harvesting in commercial and residential buildings. Electromagnetic (optical and radio frequency), kinetic, thermal and airflow-based energy sources are identified as potential energy sources within buildings and the available energy is measured in a range of buildings. Suitable energy harvesters are discussed and the available and the potential harvested energy calculated. Calculations based on these measurements, and the technical specifications of state-of-the-art harvesters, show that typical harvested powers are: (1) indoor solar cell (active area of 9 cm 2 , volume of 2.88 cm 3 ): â?

Autonomous Low Power Microsystem Powered by Vibration Energy Harvesting

This paper reports, for the first time, the implementation of a microsystem powered entirely from ambient vibrations. Sufficient electrical energy is harvested to power a radio-frequency (RF) linked accelerometer based microsystem. The microsystem is energy aware and will adjust the measurement/transmit duty cycle according to the available energy; this is typically every 50 seconds during normal operation. The system is fully powered from 45 muWrms scavenged by a miniature electromagnetic (EM) vibration energy harvester of volume ; level of 0.6ms-2.

Towards a nanostructured thermoelectric generator using ion-track lithography

This paper presents the process development towards a new generation of nanostructured thermoelectric generators for power harvesting from small temperature gradients by using a combination of traditional silicon microfabrication techniques, electroplating and submicron ion-track nanolithography. Polyimide nanotemplates with pore diameters ranging from 30 nm to 120 nm were fabricated. Preliminary results from the fabrication of poly(methyl methacrylate) (PMMA)-nanostructured templates are reported. Bi2Te3 nanowires (80 and 120 nm diameters) were electroplated into polyimide ion-track nanotemplates. Bi2Te3 nanowires of a R3 m structure, with preferential orientation in (1 1 0) crystallographic plans, were electroplated. The chemical composition of Bi2Te3 with nearly stoichiometric composition (Bi2.31Te3) was obtained. Homogeneity profiles of the chemical composition were obtained. A fine-grained observed microstructure (6–10 nm) and (1 1 0) crystalline orientation were obtained, which is extremely promising for improving the thermoelectric material properties. The thermoelectric properties of the Bi2Te3-electroplated thin films (Seebeck coefficient ?) and electrical resistivity of the Bi2Te3 bundle nanowires were found to be ?52 ?V K?1 and ?14 M cm, respectively. Polyimide (Kapton foil) and PMMA 950 photoresists were promising materials for the realization of a nanostructured thermoelectric generator on flexible and rigid substrates, respectively

Modelling and optimisation of micromachined silicon resonators

A vital requirement for a resonator-based sensor is a high degree of balance in the resonators chosen mode of vibration. A systematic study of the balance and stress sensitivity of a number of important resonator geometries has been performed. New methods have been developed both to evaluate and optimize the balance of a resonator structure. This paper presents the results of the study, details the methods used and provides some simple design rules for resonators. All the geometries evaluated are realizable as microsensors using silicon micromachining techniques.

A tunable kinetic energy harvester with dynamic over range protection

This paper describes the development and implementation of a self-powered control system that autonomously adapts the resonant frequency of an electromagnetic vibration-based energy harvester to ambient vibration frequency. The tuning mechanism adjusts the harvester’s spring stiffness by varying the axial tensile force between two permanent magnets. The system adjusts the resonant frequency of the harvester from 64 to 78 Hz, increasing the operational bandwidth of the harvester from 0.26 to 14 Hz, using a single structure. The same tuning principle is also applied to protect the harvester from over range acceleration which could cause physical damage to its structure. The closed loop control uses the phase difference between the harvester output signal and ambient vibration, measured by an accelerometer attached to the vibration source, to adjust the tuning mechanism.

Micromachined silicon resonant strain gauges fabricated using SOI wafer technology

The optimum mode of double-ended tuning-fork-style resonators is a lateral vibration in the plane of the wafer. Lateral vibrations are typically excited using the comb drive approach, but this requires modification to the resonator structure. This paper reports a simple method for exciting and detecting lateral vibrations without modifying the resonator, thereby enabling the optimum dynamically balanced structure to be used. This approach uses plane electrodes positioned parallel to the resonators tines to excite the vibrations while the change in resistance along the length of the resonator enables the vibrations to be detected. Test devices have been fabricated in single-crystal silicon using the buried oxide in silicon-on-insulator wafers as a sacrificial layer. The resonators are 340-/spl mu/m long, 3-/spl mu/m thick with tines 2-/spl mu/m wide. The gap between the tines and the electrode is 2 /spl mu/m. Visual inspection in a scanning electron microscope and electrical tests have confirmed the validity of this approach.