A Telemetry Link Using Electromagnetic Induction

Abstract

The term 'telemetry' describes the process of taking measurements at remote locations and transmitting the information (in an appropriate form) to a central receiving point. Research has been carried out on the application of magnetic induction fields to the high security, short-range transmission of power and measurement information through electrically lossy media like soil and water. This differs from conventional telemetry in that remote units are powered by the link and require no extra remote energy source. Feasibility of the induction link has been studied both in the laboratory and at field trials involving soil and freshwater media. Prototype equipment has been designed, built and tested, and allows either manual or automatic, micro-computer control. The practical problems encountered in translating the design concept into a functional prototype are discussed. An overall scheme has been developed. This consists of a large, multi-turn surface induction loop for transmission and reception, communicating with a number of remote units - called transponders. Transponders store energy, derived exclusively from the induction field, and use it to perform remote measurement processes on demand from the surface. They require a very long working life without access or maintenance, and must consume the minimum of energy in fulfilling their role. Maximum performance is obtained by high tuning, and considerable effort has been devoted to the remote transducer problem. In many instances, integrated circuits are not suitable and low-power discrete components are necessary to meet specifications. Full circuit design and micro-computer details are included with a theoretical appraisal of the link, remote transducers and associated equipment. Various applications are discussed including the extraction of data from an isolated biochemical process vessel, and from the foundations of new buildings and elevated motorways. The evaluation of pollution in rivers and waterways is also possible.