A new method for on-line monitoring of brake fluid condition using an enclosed reference probe

Abstract

This paper presents a new method for on-line monitoring of the liquid level and water content of brake fluid using an enclosed reference probe as the capacitive sensing part. The probe has an enclosed cavity at the end which is designed to hold fresh brake fluid as an on-line reference. Three capacitances formed by four electrodes are used for the liquid level, water content and reference measurement and form the mutual calibrating output functions of the sensing probe. The liquid level measurement is calibrated to the permittivity changes by the capacitance for water content measurement. At the same time, the water content measurement is calibrated to temperature changes and variety of fluids by the capacitance of the reference measurement. Therefore, once the permittivity characteristics of brake fluids are experimentally modeled, the proposed method has a self-calibration ability to influence factors including temperature, water content (to liquid level measurement) and variety of brake fluids without an additional sensor supported by database as in conventional intelligent sensor systems. The design and implementation method are discussed with a prototype probe developed and tested. The permittivity characteristics of brake fluid samples are discussed. The calibration method and errors analysis are presented. The method presents a different way to construct a smart sensor which is useful in brake fluid condition monitoring and also other liquid measurement applications.