Non-Invasive Parameter Identification in Rotordynamics via Fluid Film Bearings: Linking Active Lubrication and Operational Modal Analysis

In recent years, theoretical and experimental efforts have transformed the conventional tilting pad journal bearing (TPJB) into a smart mechatronic machine element. The application of electromechanical elements into rotating systems makes feasible the generation of controllable forces over the rotor as a function of a suitable control signal.

The servovalve input signal and the radial injection pressure are the two main parameters responsible for dynamically modifying the journal oil film pressure and generating active fluid film forces in controllable fluid film bearings. Such fluid film forces, resulting from a strong coupling between hydrodynamic, hydrostatic and controllable lubrication regimes, can be used either to control or to excite rotor lateral vibrations.

If non-invasive forces are generated via lubricant fluid film, in situ parameter identification can be carried out, enabling evaluation of the mechanical condition of the rotating machine.Using the lubricant fluid film as a non-invasive calibrated shaker is troublesome, once several transfer functions among mechanical, hydraulic and electronic components become necessary.

In this framework the main original contribution of this paper is to show experimentally that the knowledge about the several transfer functions can be bypassed by using output-only identification techniques. The manuscript links controllable (active) lubrication techniques with operational modal analysis, allowing for in-situ parameter identification in rotordynamics, i.e. estimation of damping ratio and natural frequencies.The experimental analysis is carried out on a rigid rotor level system supported by one single pair of pads.

The estimation of damping and natural frequencies is performed using classical experimental modal analysis (EMA) and operational modal analysis (OMA). Very good agreements between the two experimental approaches are found. Maximum values of the main input parameters, namely servovalve voltage and radial injection pressure, are experimentally found with the objective of defining ranges of non-invasive perturbation forces.