Steve Beeby

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.

Design and fabrication of a new vibration-based electromechanical power generator

A device is described for generating electrical power from mechanical energy in a vibrating environment. The design utilises an electromagnetic transducer and its operating principle is based on the relative movement of a magnet pole with respect to a coil. The approach is suitable for embedded remote microsystems structures with no physical links to the outside world. Simulation, modelling and test results following fabrication of a first prototype have demonstrated that generation of practical amounts of power within a reasonable space is possible. Power generation of more than 1 mW within a volume of 240 mm3 at a vibration frequency of 320 Hz has been obtained.

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

A novel thick-film piezoelectric micro-generator

The use of alternative electrical energy sources to batteries is of particular significance to remote sensor systems. A vibration-powered micro-generator, based on a screen printed piezoelectric material, is proposed for this purpose. Theoretical and experimental results show that 2 µW can be generated for a vibration frequency of only 80 Hz. The device is not optimized and significant improvements are envisaged in the future.

A Silicon Microfluidic Ultrasonic Separator

Ultrasonic standing waves can be used to generate forces on particles within a fluid. Such forces have a number of potential applications in microfluidic devices. This paper describes a device that provides filtration on a microfluidic scale. It is microfabricated and uses ultrasound in the megahertz frequency range to concentrate particles at a node within the flow. It offers the possibility of a functional equivalent of a centrifugal separator for microfluidic systems. It is constructed using silicon and Pyrex, and hence is compatible with established microfabrication techniques. The modelling, design, fabrication and control of the device are discussed.

Optimization of an Electromagnetic Energy Harvesting Device

This paper presents the modeling and optimization of an electromagnetic-based generator for generating power from ambient vibrations. Basic equations describing such generators are presented and the conditions for maximum power generation are described. Two-centimeter scale prototype generators, which consist of magnets suspended on a beam vibrating relative to a coil, have been built and tested. The measured power and modeled results are compared. It is shown that the experimental results confirm the optimization theory

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?

Processing of PZT piezoelectric thick films on silicon for microelectromechancial systems

Thick-film printed lead zirconate titanate (PZT) structures can be combined with micromachined silicon structures and offer relatively large actuation forces compared to alternative techniques. This paper describes the initial investigation into the compatibility issues of micromachining silicon wafers with PZT layers printed on the surface. It assesses the effect of many standard photolithography and micromachining upon the printed PZT layer. In particular the adhesion of the printed layer to the substrate and its internal structure have been studied after exposure to each process. Standard photolithography using positive resists has been found to destroy the adhesion of the platinum electrode and an alternative masking technique using a thick-film printed dielectric polymer has been developed. Aluminium top electrodes have been fabricated using this masking technique combined with ion beam milling. Finally many standard micromachining etching processes have been carried out on a range of silicon substrates incorporating platinum electrode/thick-film PZT structures. Wet silicon and silicon dioxide etches were found to be unsuitable since they also attacked the adhesion of the bottom electrode. Plasma etching processes appear to be well suited for the combination of materials since there is a wide range of etches available that do not affect the PZT. These can therefore be used for the micromachining of the silicon substrate after thick-film processing greatly expanding the range of applications suited to this combination of technologies.