An electromagnetic, vibration-powered generator for intelligent sensor systems

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Abstract

This paper describes the design of miniature generators capable of converting ambient vibration energy into electrical energy for use in powering intelligent sensor systems. Such a device acts as the power supply of a microsystem which can be used in inaccessible areas where wires can not be practically attached to provide power or transmit sensor data. Two prototypes of miniature generator are described and experimental results presented. Prototype A is based around two magnets coupled to a coil attached to a cantilever; prototype B is based around four magnets.

For prototype A, experimental results are given for its resonant frequency and its open circuit and loaded output as a function of vibration amplitude. For prototype B, experimental results are given for the generator’s Q factor in air and vacuum, its output voltage as a function of vibration amplitude as well as its magnetic field strength. This generator has been tested on a car engine and shown to produce a peak power of 3.9 mW with an average power of 157 μW.

Introduction

There is an increasing level of research activity in the area of alternative power sources for MEMS devices [1] with the terms ‘energy harvesting’ and ‘parasitic power sources’ being adopted [2]. A recent review [3] of wireless sensors, covering the area of energy harvesting, indicated the following current areas of research:

  • solar,

  • vibration,

  • temperature difference,

  • electromagnetic fields,

  • chemical.

Typical application areas for ‘self powered’ intelligent sensor systems are:

  • inside the body (e.g. human, animal),

  • on rotating objects,

  • within liquids such as molten plastic or setting concrete,

  • structural monitoring such as within bridges, buildings, aircraft or roads,

  • environmental monitoring such as pollution monitoring in fields.

This paper describes research at the University of Southampton on an electromagnetic generator targeted at harvesting useful electrical power from ambient vibrations.

Studies performed at the University of Sheffield [4] indicated that power level of 1 μW were feasible for a simple spring mass system. Publications from the University of Hong Kong [5], [6] have described a single magnet on a laser machined copper spring moving within printed circuit board (PCB) based coil. This paper reports on two and four magnet generators produced at the University of Southampton.

For prototype A, a two magnet generator, experimental results are given for its resonant frequency, the open circuit coil voltage for a range of different amplitudes of base vibration, the load voltage and electrical power across a load resistor on the coil for a fixed base excitation and the load voltage across an optimum load resistance for a range of different amplitudes of base vibration.

For prototype B, a four magnet generator, experimental results are also given for the generator’s Q factor in air and vacuum, the output voltage as a function of vibration amplitude as well as its magnetic field strength. This generator was also tested in a real application on a car engine.

Section snippets

Microgenerator design and dimensions

A typical magnet-coil generator consists of a spring-mass combination attached to a magnet or coil in such a manner that when the system vibrates, a coil cuts through the flux formed by a magnetic core. The beam can either be connected to the magnetic core, with the coil fixed relative to the enclosure, or vice versa. Fig. 1, Fig. 2 show the magnet coil geometries investigated.

Design B has been chosen to create a magnetic field through a greater proportion of the length of each coil winding

Apparatus and experimental method

The devices were tested using the shaker and measurement apparatus shown in Fig. 5. A Goodman V.50 Mk.1 (Model 390) Vibration Generator (shaker) was used to supply mechanical vibrations to the samples under test. An accelerometer (Bruel & Kjaer Accelerometer Type 4369) was mounted to provide data on the amplitude of vibrations applied to the samples, which can not be determined solely from the electrical drive to the shaker, since the shaker has a non-linear response. The accelerometer has a

Practical applications

The following experiment was performed to demonstrate that the technology has the potential to be useful in a practical application. The generator was mounted on the top of the engine block of an, otherwise unmodified, 5-year-old Volkswagen Polo. Experiments showed that the power produced by the generator was largely determined by the engine speed with a resonant peak at around 3000 rev/s, perhaps relating to a resonance in the engine mounting. Fig. 11 shows data taken from a typical short drive

Conclusions

An electromagnetic generator based around a moving coil between two magnets is capable of generating useful level of power, however the output voltage is considered too low for practical application and the geometry requires coil winding around the magnets which is cumbersome. A second electromagnetic generator based around a coil between four moving magnets is capable of generating useful power and voltage levels from ambient vibrations. A device has been described which can produce an average

Acknowledgements

The authors wish to thank the Engineering and Physical Sciences Research Council (EPSRC) for their financial support under grant number GR/M35086. We also gratefully acknowledge the support and assistance given to us by Morgan Electroceramics Ltd.

P. Glynne-Jones graduated with first class honours from the School of Electronics and Computer Science at the University of Southampton. He was sponsored through his degree by the Defence Education and Science Group and was an IEE scholar. Peter was awarded a Ph.D. in vibration powered microsystems from the same University in 2001. He is currently in a Zen Buddhist Monastery in Luton.

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P. Glynne-Jones graduated with first class honours from the School of Electronics and Computer Science at the University of Southampton. He was sponsored through his degree by the Defence Education and Science Group and was an IEE scholar. Peter was awarded a Ph.D. in vibration powered microsystems from the same University in 2001. He is currently in a Zen Buddhist Monastery in Luton.

M.J. Tudor obtained a Ph.D. in Physics from Surrey University and a B.Sc. (Eng.) in Electronic and Electrical Engineering from University College London. In 1987, John joined Schlumberger Industries working first at their Transducer Division in Farnborough and then their Research Centre in Paris, France. In 1990, he joined Southampton University as a Lecturer. In 1994, John moved to ERA Technology becoming the Microsystems Programme Manager. In 2001, John returned to Southampton University as a Senior Research Fellow in the School of Electronics and Computer Science to pursue university based research in microsystems. John has 25 publications and 7 patents and served on the IEE Microengineering Committee for 4 years. He is both a Chartered Physicist and Engineer.

S.P. Beeby (EPSRC Advanced Fellow) has been a Research Fellow/Assistant at Southampton University for 9 years. His Advanced Fellowship is on the subject of piezoelectric thick-film materials for microelectromechanical systems (MEMS). His Ph.D., ‘Mechanical Isolation of Miniature Resonant Sensors and Stress Relieving Packages’, was on the subject of micromachined silicon resonators. His research interests include vibration powered microgenerators, microsystems, instrumentation and thick-film materials development. He has 75 publications in the area and is a Chartered Engineer and a Chartered Physicist.

N.M. White holds a personal chair in the School of Electronics and Computer Science, University of Southampton. He has been active in sensor development since 1985. In 1988, he was awarded a Ph.D. from the University of Southampton. He has considerable experience in the design and fabrication of a wide variety of sensors, formulation of novel thick-film sensing materials and intelligent sensor systems. In 1994, he co-authored a book entitled ‘Intelligent sensor systems’, he was the editor of the proceedings of ‘Eurosensors XII’, held at Southampton in September 1998 and the conference Chairman of ‘Sensors and their Applications X’, Cardiff, 1999. Professor White was the Chairman of the Instrument Science and Technology (ISAT) group of the Institute of Physics from 1997 to 1999. He has over 100 publications in the area of instrumentation and advanced sensor technology. His professional qualifications include Chartered Engineer, Fellow of the IEE, Fellow of the lOP, Chartered Physicist and Senior Member of the IEEE.

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