Operational Modal Identification of an Underwater Electro-Hydraulic Actuator Based on Variable-Speed Sweep Excitation

Underwater electro-hydraulic actuators (EHAs) constitute a key actuation technology for future deep-sea equipment, and their vibration behavior directly affects system reliability and stealth performance. In practice, the structural response of underwater EHAs is dominated by order-related excitations from rotating components such as the motor and pump. Consequently, modal characteristics are often obscured by forced vibrations, limiting the effectiveness of conventional operational modal analysis (OMA) under narrowband excitation. To address this issue, this paper proposes a modal identification method based on variable-speed sweep excitation. First, the mechanism by which excitation frequency components shift continuously with rotational speed and sweep through the resonant regions of the structural frequency response function (FRF) is investigated. Numerical simulations are then performed using a two-degree-of-freedom system and a multi-component sweep-frequency excitation signal. The simulation results demonstrate accurate identification of the natural frequencies at a signal-to-noise ratio of 10 dB, with frequency and amplitude errors below 3.5 Hz and 1.8 dB, respectively. ... mehrFinally, an oil-immersed run-up test is conducted on an underwater EHA. Multiple stable resonance bands are identified, thereby demonstrating the feasibility of extracting natural frequencies under realistic operating conditions. This paper provides a practical method for modal identification and low-vibration design of underwater rotating machinery and fluid-structure interaction systems.