Analytical Evaluation of a Coplanar Interdigitated Sensor Capacitance for 1-N-1 Multilayered Structure

In this article, an analytical expression (model) for the capacitance of coplanar interdigitated (IDT) capacitive sensors of four 1- n -1 patterns (such as 1-1-1, 1-3-1, 1-5-1, and 1-11-1) based on conformal mapping (CM) techniques has been obtained. An important feature of the proposed model is that it is universal and independent of any specific application. The proposed model can be used for any number of layers (variable permittivity and thickness), finger width, and the gap between the fingers. In general, the capacitance of a particular IDT sensor depends on finger (electrode) length, the number of fingers, potential distribution of the fingers, dielectric permittivity (substrate and sensitive layers), and geometric parameters (nondimensional): 1) the ratio between the finger widths and gap and 2) the ratio between the top sensitive layer thickness and the sensor wavelength (spatial). Keeping these parameters in mind, comparisons with finite element analysis and experimental results have been made. Also, to confirm the effectiveness of the proposed model, different patterns of 1- n -1 structured IDT sensors have been developed to measure the water droplets on the leaf canopy by a noncontact method. The proposed sensor structure can be said to be a noncontact leaf wetness sensor (LWS), generally employed to quantify the leaf wetness duration (LWD).