Design of planar capacitive type sensor for water content monitoring in a production line

Capacitive type sensors for the measurement of water content of various materials exhibit the serious handicap of the sensitivity dependence on the spacing between sensor and material. The results of theoretical investigation show that is possible under certain conditions, related to the relative scaling of the sensor structure, to minimize the aforementioned problem and thus to construct an inexpensive robust and reliable sensor for industrial use. An optimized system was constructed consisting of two modules the sensing bead and an electronics unit connected through coaxial cables. The sensor head contains driving and sensing plates. The electronics unit accompanying the sensor outputs two voltage signals bearing information about the product's relative permitivity and resistivity coefficient. Many factors affecting the sensor head design are discussed. The response of the sensor is investigated in two cases: with and without presence of a ground plane and for various materials with different dielectric constants. Experimental results are also presented that confirm to a great extent the simulation results. Slight discrepancies observed are explained. (C) 2004 Elsevier B.V. All rights reserved.