Contents I. Introduction
Inductive sensor is widely used for displacement or position measurement because of its simple structure and convenient operation. A large number of inductive sensors in single, differential or ratio modes have been developed [1]–[7]. In most applications, such a sensor often consists of a fixed thin long coil and a movable iron rod. When the rod moves in the coil along the axis direction, the displacement can be measured by detecting the variation of the equivalent inductance of the coil. Since the manufacture of large length coil is complicated and the movement of the iron rod with large length and weight is difficult, the measuring range of such a sensor is only several centimeters or around. This shortage limits its applications in many of long displacement measurements. In some special applications such as underwater displacement measurement, it is also difficult to use. To enlarge the measuring range of inductive displacement sensor, a novel design of inductive sensor for long displacement measurement is presented. In it, the heavy and long iron rod is fixed as a static part whose shape is no longer a column but a stair or cone with variable cross-section area. The light and small-size coil is used as a moving part and is packaged in an iron cover. The role of the iron cover is to constitute the external magnetic path of coil and also the magnetic shield of the sensor. All of these improvements expand the measuring range and make the manufacture, installation and operation of the sensor more convenient.
