Printing smart coating of piezoelectric composite for application in condition monitoring of bearings

This paper reports on a novel technique of a bearing load monitoring based on the use of smart sensor coating. For easier process and integration, screen printing is carried out to achieve multilayered thin film deposited on an outer bearing or a simple flat steel substrate. The formulation of piezoelectric ink is relied on the development of a UV curable barium titanate/polyurethane acrylate (BaTiO3/PUA) composite. A new design is proposed to enhance the sensitivity of the smart coating, which consists of three stacked layers: piezocomposite layer, dielectric layer (if needed), and conductive layer including electrodes and conductive tracks (CT). On one hand, the dimensions of the coating electrodes are revealed to be critical to the sensing performance. On the other hand, the CT has small impact on the piezoelectric measurement. As a result, no dielectric treatment is required between the composite and the conductive layers, leading to simplified printing process. Full characterizations of dielectric and mechanical properties, together with direct sensing measurement through electromechanical coupling are investigated on the home-made structure. Analytical and finite element models are developed to predict the mechanical properties of the tested substrate as well as the sensor sensitivity under different applied loads. Experiments are conducted on a 4-point bending (4 PB) setup, allowing to validate the analytical and numerical solutions. Good agreement between the model-predicted sensor outputs and the empirical measurements are observed, confirming high reliability of the proposed approach. It is eventually pointed out that the piezoelectric smart coating has higher sensitivity and easier integration than classical piezoresistive technology. Indeed, the printed sensor is capable to provide a direct voltage signal instead of a traditional strain gage where signal conditioning is needed. Accordingly, piezoelectric sensing together with printing technology could offer an efficient method for on-line and in situ monitoring of bearing.