Experimental Generation of High-Frequency Oscillatory Flow in Hydraulic Systems

Abstract

Volumetric flow rate sensors are used in various technical applications. Therefore, it is interesting to use volumetric flow sensors that neither obstruct nor manipulate the flow to be measured nor are restricted to certain flow types and profiles. For this reason, the virtual volumetric flow sensor was developed. A test rig was constructed to validate this soft sensor, which can generate laminar, turbulent, steady, and unsteady flow rates. The dynamic part of the flow is generated by coupling three cylinders and operating a servo valve. In this work, an experimental hydraulic test platform capable of generating reproducible high-frequency oscillatory flow rates is investigated as an enabling system for validating a pressure-based virtual volumetric flow sensor. Two gain-scheduled PID control strategies are implemented to realize the required excitation profiles. Both controllers were intensively investigated on the test rig for various high-frequency scenarios, including pulsations up to 80 Hz. At 80 Hz, the direct controller achieves a normalized mean absolute error of 36.3% (including phase delay). After phase alignment, waveform fidelity corresponds to an nMAE of 10.1%, demonstrating suitability for high-frequency soft-sensor validation. The comparative results show that direct velocity control remains effective up to excitation frequencies of 80 Hz, while indirect position-based control becomes ineffective above approximately 40 Hz due to inherent phase delay. Eventually, the generated dynamic flow rate is utilized to demonstrate the high accuracy of the soft sensor for an oscillation of 20 Hz.