POWER GENERATOR (INTRODUCTION)

Over recent years, an interest has developed in micro electromechanical systems (MEMS) and the subject has matured to the point where its applications to a wide range of areas are now clearly feasible. Applications such as medical implants and embedded sensors in buildings and similar structures, are just a few of many examples. The supply of power to such systems has so far been through batteries. However, in long-lived systems where battery replacement is difficult and in applications consisting of completely embedded structures with no physical links to the outside world, generating power from ambient sources becomes imperative. Systems that depend on batteries have a limited operating life, while systems having their own self-powered supply unit have a potentially much longer life. A potential and promising alternative solution to batteries is the use of miniature renewable power supply units. Such devices convert energy from existing sources energy within their environment into electrical energy.

Ambient energy may be available within the environment of a system and is not stored explicitly. The source of such energies, however, depends on the application. The most familiar ambient energy source is solar power (light energy from ambient light such as sunlight). Thermal energy is another ambient energy source (thermoelectric generators generate electricity when placed across a temperature gradient) . Flow of liquids or gases, energy produced by the human body and the action of gravitational fields are other ambient energy source possibilities. Other examples which depend on injected energy rather than naturally occurring ambient energy fields include electromagnetic fields used in RF powered tags , inductively powered smart cards and non-invasive pacemaker battery recharging . Our approach uses mechanical vibration as the ambient energy source for generation of electrical power . Therefore, in this paper a vibration-based magnet-coil power generator is described.

The most important parameters influencing the design of such a system are its physical size and conversion efficiencies. The size is dependent on the energy requirement and must be as small as possible, to be compatible with the general design objectives of MEMS. However as the size of the device is reduced, mechanical resonances tend to increase in frequency and it is the challenge of generating power from comparatively low vibrational frequencies (hundreds of Hz rather than kHz) that is addressed in this work. The ambient energy may be at a premium in a particular environment so the conversion efficiency must be as high as possible. To analyse the transformation efficiency and to assess the input-output relationship of such a generator, full electromechanical and magnetic analyses have been carried out. Finite element (FE) techniques for the magnetic field distribution solution have been employed. Fabrication and test results of a first prototype based on simulation and modelling results are fully discussed. Practical amounts of power within reasonable space (quarter of a cubic centimetre) have been achieved.