Applications exist for sensors without physical galvanic connection to their surrounding environment. Examples include self-powered micro-systems (explained in the paper) located on roads, bridges, fields, in the home, machinery and maybe even inside the human body. At present, communication with such sensors is often powered using a lithium battery. Such batteries have a finite operational life, they can often be larger than the sensor and recent advances in integrated circuit technology of the type described by Moore's law have not been matched by similar advances in battery technology. Batteries can therefore be seen as a significant obstacle when producing small micro-systems, based for example on MEMS technology. A SPMS that does not require a battery, thus enjoying a greatly increased operational life would suit applications where the sensor is embedded or where a battery can not be easily replaced. Such systems having their own power source that makes use of ambient energy have a potentially infinite life time. The source of such ambient energy (that is the energy available in the SPMS surroundings, but is not stored explicitly) will inevitably depend on the given application. The most familiar ambient energy source is solar power from ambient light. Other possible sources of ambient energy are the flow of liquids or gases, energy produced by the human body, thermal energy and the action of gravitational fields. Other similar approaches to non-galvanic energy transfer include electromagnetic fields used in RF powered ID tags, inductively powered smart cards and non-invasive pacemaker battery recharging. Such approaches all aim to increase the amount of energy that is available per unit volume in the sensors immediate surroundings. This paper introduces the concept of a SPMS where a planar coil is used for both power and data transfer. MicrochipTM have developed a RF-ID tag (MCRF202) that is limited to a single logic-level input. Power transfer to the tag is based on planar technology and signal extraction is achieved via backscattering, however this paper develops the general concept of a planar based SPMS specifically for sensor applications
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