Active sensing of industrial fluid degradation via electro-mechanical impedance of a submerged piezoelectric wafer

Industrial fluids, such as lubricants and cutting compounds, are ubiquitous in diverse mechanical transmission systems and manufacturing scenarios. Throughout the operational lifespan, these fluids are vulnerable to degradation and contamination, rendering change of mechanical properties and loss of performance, therefore causing detrimental effects on subordinate machineries and product quality. To address such a concern, this article proposes an electro-mechanical impedance spectroscopy for the real-time monitoring of industrial fluids via a structure-fluid interactive piezoelectric sensor. To unravel the intricacies of the interactive mechanism between industrial fluids and the piezoelectric sensory device, an analytical model was established, with the influence of the fluid processed as an additional inertial load and dissipative viscosity. Meanwhile, finite element analysis was performed to scrutinize the nuanced influence of the fluid with fluid-structural interaction boundary condition and spring–damper dissipative elements. Parametric studies were conducted to evaluate the impedance spectral features arising from the fluid properties alterations. A damage index based on the amplitude and frequency shift of the impedance spectra can readily serve as a robust quantifier of fluid degradation severity. Ultimately, experimental tests were performed to validate against the analytical and finite element models. The glycerol–water-mixed fluid was utilized to verify the sensor’s sensitivity on density and viscosity alteration of the fluid. Two industrial fluids, gearbox lubricant and cutting fluid under various service periods, were employed to demonstrate the practical monitoring capability. This article culminates with summary, concluding remarks, and suggestions for future work.