Fringing Electric and Magnetic Field Sensors

Fringing electric and magnetic field sensors are widely used for non-destructive measurements of material properties. A simple conventional fringing field sensor consists of a sensor head, a current or voltage source, an impedance measurement circuit, and data processing/data acquisition capability. The sensor head is usually a patterned array of electrodes or windings on an electrically insulating substrate. The voltage or current signal is applied to these electrodes and creates, respectively, electric or magnetic fields in the space around the electrodes or windings. These fields penetrate into the materials near the electrodes or windings. Changes of dielectric and magnetic properties of materials affect the distribution of these fields. The impedance measured between the electrical terminals of the sensor head is a function of the material electrical and geometric properties and the electric and magnetic fields. This general principle allows design of sensors for a very broad spectrum of applications including relating changes in electrical properties to physical properties such as temperature, density, defects, moisture, etc. This chapter provides theoretical background, principles, and examples of design of sensor elements, and description of the most common applications for two types of fringing field sensors, namely, the fringing electric field (FEF) sensors and fringing magnetic field (FMF) sensors. These two types of sensors share many common features and are treated in parallel in the subsequent sections. 2. Theoretical Background