Investigation of Fluid-Mechanical Interfaces (FMIs) for the Transmission of Oscillations in Hydraulic Fluid

This study investigates fluid-mechanical interfaces (FMIs) for transmitting pressure oscillations in a hydraulic fluid to a piezoelectric stack, with the aim of supporting self-powered sensing and energy harvesting in hydraulic systems. A hydraulic test rig was employed in which a pump, switching valve, pressure sensors, and a piezoelectric stack were coupled through stainless-steel diaphragms of varying thickness (0.10 to 0.80 mm, 48 mm diameter). The effect of maximum pump pressure (20–50 bar) and valve switching frequency (1–50 Hz) on force transmission was evaluated, and diaphragm integrity was assessed after testing. The results demonstrate that diaphragm thickness strongly influences transmissivity under cyclic hydraulic loading. For the stainless-steel diaphragms tested, a low thickness of 0.25 mm provided a favourable balance between safety and force transmission, performing essentially as well as the thinnest 0.10 mm diaphragm while offering improved mechanical robustness. Thicker diaphragms up to 0.80 mm remained viable for sensing applications where maximal transmissivity is less critical. The findings suggest that FMI geometry can be tuned to improve energy transfer to embedded transducers while maintaining hydraulic integrity. Further work is needed to assess long-term durability under extended cyclic loading and to adapt the design for higher-pressure applications.