Novel Self-capacitance Sensor to Measure Textile Moisture Content

Moisture content is an important property that is measured in textiles to regulate both the drying processes and the parameters of fabric manufacturing equipment. Currently commercialized sensors are bulky and too expensive for many industrial uses. The purpose of this study is to develop and test a novel low-cost and compact moisture sensor for textiles that is based, for the first time, on self-capacitance technology. The sensor is made up of a single-plate capacitor on a printed circuit board, connected to a capacitance-to-digital conversion circuit. The main challenge is to adapt a board, which was originally developed for proximity and touch applications, for moisture measurements. Three sensor layouts with different configurations of the electrodes are tested, coupled with diverse overlays and cables. The impedance analysis shows that electrodes designed with trace loops led to lower capacitance when compared to pad and hatch grids. Then, the performance of the sensor has been experimentally evaluated using four textiles: cotton, polycotton, wool and polyester. The results show good repeatability and resolution in the entire moisture range tested, from 5% till 70% using total-dry weight as reference point. Regression models are presented for each fabric type, with an excellent approximation confirmed by R-square values between 0.945 and 0.996. Finally, an analysis of both temperature and air humidity effects on the sensor measurement is discussed. The linear response at low and high humidity, with temperature as predominant factor, allows the usage of the sensor also in harsh environments.