University of Southampton Institute of Transducer Technology

Over the years there has been a growing interest in the field of miniature sensors and their wide range of applications such as medical implants, embedded sensors in buildings etc. One specific area that has received little attention is how to supply the required electrical power to such sensors. Conventional power supplies external to such sensors is one way. However, many applications do require such sensors to be completely embedded in the structure with no physical connection to the outside world. Supplying power to such systems is difficult and as a result they need to have their own power supply unit making them self-powered microsystems. Noise reductions, elimination of cross talk between power lines and signal lines, reduction of power delivery control system complexity are just some of the advantages associated with truly integrated and self-powered microsystems.

The three main aspects of this project are the power generation, energy storage and signal extraction and transmission.

The conventional solution to supply power is to use batteries. However, some of the drawbacks associated with batteries are that they contain a finite amount of energy, have a limited life, and contain chemicals that could cause a hazard, may be quite bulky and can fail at inconvenient times. An alternative solution is to design a microgenerator to convert energy from an existing ambient energy into electrical energy. Some possible ambient energy sources, which can be converted into electrical energy, include light energy, thermal energy, volume flow energy and mechanical energy. The focus here is on applications concerning mechanical vibrations as the ambient source of interest. The transformation of mechanical vibrations into electrical power is based on piezoelectric effect. The micro-mechanical generator (micromachined silicon/piezoelectric device) will basically be constructed by depositing thick-films of lead zirconate titanate (PZT) onto single crystal silicon. The thick-film PZT paste has been studied and employed in number of applications. Single crystal silicon possesses excellent mechanical properties for this application. Various techniques for construction of such micro-mechanical generator are at present being investigated in details. The result of these studies determines the characteristics of such generator and its applicability to certain applications.

An important aspect of the current work is that of energy srorage. This is necessary due to number of other reasons. For instants, the generated output may vary rapidly with time, the ambient energy may not be present at all times or a start-up power may normally be required. Ideally an energy storage mechanism that can exhibit high power and energy densities with the smallest size possible is required. For this purpose the following candidates as energy storage mechanisms are being investigated.

• Electrochemical capacitors or supercapacitors
• Rechargeable Lithium batteries

In general, battery technologies are energy dense but deficient in power density and capacitor technologies are power dense but limited by energy density. Therefore, a third option being investigated is a capacitor-battery system. Such approach can result in a more compact and efficient system.

The system must be able to communicate with the outside world and it must do so while maintaining the isolation of the sensor. The objective is to use as small an energy as possible to transmit each data so that the mean power can be reduced to a low level by limiting the data rate. Optical fibre communications using very low power are viable options for this purpose. Magnetic coupling using resonant inductive circuits, where the transmission range is reasonably short, is another technique. For longer transmission ranges, radio telemetry techniques are to be investigated.

Contact: usitt@soton.ac.uk © 2002 USITT & Department of Electronics and Computer Science