Niimi, N., Yoshida, T., and Isogai, T., "Capacitive Humidity Sensors Using Highly Durable Polyimide Membrane," SAE Int. J. Passeng. Cars – Electron. Electr. Syst. 6(1):328-334, 2013, doi:10.4271/2013-01-1337.
Humidity sensors used in automatic windshield defogging controls contribute to the improvement of fuel consumption. The optimum control of air conditioning systems can be realized by adding humidity information to conventional systems which have used only temperature information.While resistive humidity sensors have been widely used, their sensing range and responsiveness are observed as issues. Resistive sensors cannot function at a humidity range of around 100% RH as well as at a low temperature range, and have a low response rate to sudden changes in humidity. It is considered that resistive humidity sensors will be replaced with capacitive ones which have a wide sensing range and high responsiveness. However, since capacitive sensors use macromolecule organic materials for their humidity sensing membranes, characteristic changes caused by two factors in a high temperature and high humidity environment are concerns: the deterioration of the membrane itself, and the decrease of the adhesive strength between the membrane and the underlying layers.This paper describes a newly developed capacitive humidity sensor with high durability by suppressing hydrolysis and swelling of membranes which cause characteristic changes in a high temperature and high humidity environment. The main development processes are selection of polyimide as a basic organic material, the addition of fluorine atoms of hydrophobic molecules, the extension of the molecular repeat units and the formation of the cross-linked structure to molecular ends. In addition, silane coupling agents is used to increase the adhesive strength of the membrane, thus realizing capacitive humidity sensors for automobile use.