Best Practices for Using Order-Based Modal Analysis for Industrial Applications

The Order-Based Modal Analysis (OBMA) technique shows to be very powerful for identifying the modal parameters in operational conditions in case of rotating machineries during transient operations. The main idea behind the method is that instead of estimating the spectra and apply Operational Modal Analysis (OMA) by using them, the so-called orders can be extracted and used as input for the OMA technique. It can be assumed that the measured responses are mainly caused by the rotational excitation. In this case, run-up and coast-down events can be assimilated to multi-sine sweep excitation in the frequency band of interest. Several studies have been performed to identify the best practice for OBMA both in terms of Order Tracking (OT) techniques and OMA techniques. Based on the boundary conditions, on the structure under-test and on the effective operational conditions a technique can be more powerful than another one. Basically there are two fundamental steps: a very good measurement of the tachometer signal and the correct extraction of the orders, both in amplitude and phase. For the first step several alternatives are possible. The sensors to be used for measuring the rotational speed are depending from both the application and the objective of the study. For this reason, several sensors can be found in the market with a huge variety of costs and performances. The best sensor can then be selected for each individual application based on the type of analysis, the accessibility of the shaft, the ease of instrumentation and the required accuracy or level of detail. For the second step, several techniques are available in commercial software and some others have been implemented in a research environment. Each of them has its own advantages and drawbacks. The final aim of the work is to provide guidelines for the correct use of the OBMA technique in an industrial context. Several cases will be shown: a locomotive cabin and a car during engine run-ups and a 3.2 MW wind turbine gearbox during controlled run-ups on a test rig.