Linear variable displacement transformers (LVDTs) are used extensively for displacement measurement with ranges of 1 mm to over 50 cm. They were originally described in a patent by G.B.Hoadley in 1940 (US Patent 2,196,809) and became popular in military and industrial applications due to their ruggedness and high resolution [21].The operating principle of an LVDT is illustrated in Fig. 27. The stationary part of the sensor consists of a single driving coil and two sensing coils wound onto a thermally stable bobbin. The movable component of the transducer is a permeable material such as Nickel-Iron (Permalloy), and is placed inside the bobbin. The core is long enough to fully cover the length of at least two coils. Thus,at either extreme, the central coil always has a complete core at its center.