What is the role of modal analysis in modern control and monitoring systems?

The popularity of modal analysis in terms of various monitoring and control systems is attributed to the fact, that the vibration control implies the need of structural dynamic behavior analysis. Structural dynamics and modal parameters control allows to identify such factors as natural frequencies, damping ratios, mode shapes. All these factors are of crucial importance for vibrational parameters monitoring of the control structure. It is necessary to control these vibrational parameters, since the vibrational impact affecting structures with complex dynamic behavior may eventually result into deterioration of the controlled object parameters or even destroy it.

Major types of modal analysis and their specific features

Even though the importance of modal analysis implementation is quite obvious for already several decades, it seems reasonable to differentiate between types of modal analysis which have certain distinctive features and application specifics.

In spite of the fact, that modern control and monitoring systems use a variety of modern analysis algorithms depending on the system scale, specifics of the controlled structure and intended functional range, it is still possible to outline two major types of modal analysis: experimental modal analysis and operational modal analysis.

In this article we shall consider the distinctive features of the Operational modal analysis.

This type of analysis is also sometimes referred to as output-only analysis or in-operation modal analysis. Thus, the main feature of this analysis is the possibility of using it in the real operational environment while obtaining the same modal parameters data as in the case of Classical modal analysis. Even from these assumptions, it is quite obvious that the Operational modal analysis (OMA) definitely has a number of advantages if compared to the classical methods of modal analysis in laboratory environment, where a lot of requirements to the instrumentation and measurement procedures have to be addressed.

Thus, there is no need to use a shaker (vibration exciter) or an impact hammer. This means, that there are no limitations in terms of test impact scale and duration.

Operational modal analysis also allows to use a variety of test rigs, which are not subject to any particular restrictions imposed by application of this research and analysis method.

There is also no need to control the input impact scale. Since OMA can be used for in-situ measurements, the test impact is actually represented whether by operational environment or by environmental impact.

As for the practical implementation of OMA, there are several scenarios of Operational modal analysis performance in terms of mounting and positioning of the measurement transducers used for modal parameters data acquisition.

Thus, in the case of Structural health monitoring systems (SHM systems) used for the control of civil facilities (e.g., wind turbines, bridges, foundations, high rise buildings, etc.) the measurement transducer are mounted on the controlled facility on a constant basis (i.e., the control system implementation implies the need for continuous data acquisition process and control of the measurement parameters based on the threshold levels assigned for each particular controlled object).

The second scenario implies relocation of the transducers along the controlled object in the course of measurements performance.

Another option of measurement transducers mounting is the use of high-accuracy reference transducers at the particular areas of the controlled object, while the other measurement transducers used for data acquisition are attached to the specimen in relation to the reference transducers.

Depending on the particular task to be addressed with the use of Operational modal analysis, it is possible to develop a different configuration of the measurement transducers mounting (depending on the number of measurement points, structural dynamics of the specimen, its dimensions, etc.).

Before we continue further discussion of OMA practical application, it is necessary to point out that Operational modal analysis should not be considered to be a substitution of the Experimental modal analysis. Besides, it is also wrong to think that the Experimental modal analysis is an outdated method for modern measurement systems and application scenarios. Actually, these two types of analysis are often used together with a lot of other methods intended for dynamic behavior control.

Operational modal analysis – scenarios of use and application spheres

1) Civil engineering. Operational modal analysis became increasingly popular about three decades ago, when large-scale civil and engineering structures became widely spread and there aroused the need to monitor uncontrolled long-term external impact applied to large structures. In this case, the use of Operational modal analysis is the best possible solution, since it is impossible to foresee all types of possible impacts, besides, monitoring of civil engineering structures implies the need of real-time monitoring with the possibility of alarm notification functions.

Operational modal analysis use in the civil engineering

2) Bridge structures monitoring systems. The systems used for bridge monitoring are becoming increasingly widely spread. It is attributed to a great number of factors, which include: complexity of bridge structures, combination of environmental and industrial impact applied to the bridge structures, expensive repair of bridge structural elements, safety issues, necessity of preventive maintenance implementation, increased highway traffic (and, hence, the dynamic load applied to the structural elements of the bridge, etc.). Another factor, that leads to increased popularity of the system, is its financial efficiency. Large-scale renovation of bridge structure or demolition of bridge and building a new one are quite expensive options. In addition to financial efficiency and cost reduction, implementation of bridge structure monitoring system also contributes to the enhanced safety of civil facilities operation. Nowadays there is a variety of bridge structures monitoring systems from various manufacturers, however, all these systems normally have common features. Bridge structures monitoring system usually consists of the following components: distributed network of the measurement transducers, power supply system, data communication system, system for real-time mode monitoring and alarm notifications. Even though these systems have a lot in common, each system is still unique both in terms of its components and operation algorithms used.

Use of Operational modal analysis in Bridge structures monitoring system

3) Use of operational modal analysis in wind energy systems. From the structural point of view, the wind turbines may seem to be quite simple. The wind turbines include blade, tower, nacelle and foundation. However, modal analysis of turbine blades is quite complex, since the dynamic behavior of turbine blade and interaction between system components are difficult to analyze. Besides, the wind turbine system also includes structural elements such as drive train, rotor and main frame, which also affect the dynamic behavior of the system. The purpose of implementing operational modal analysis is to develop optimal control system and to select the best possible elastic connections for a particular system in question. In order to achieve both goals, it is necessary to implement a two-stage survey.

The first stage implies identification of individual modes of the system components. At this stage, it is possible to use the standard experimental modal analysis in laboratory environment.

However, the second stage implies the need of long-term data acquisition in real operational environment. This is a more challenging task, since the structural components of the wind energy system begin to interact with each other in a complex way. Besides, the system assembly is also exposed to a variable long-term external impact, which eventually leads to a complex pattern of structural dynamics behavior. Thus, for the second part of this research, we have to use the operational modal analysis.

The results of these comprehensive surveys are useful in the course of wind energy system design (it becomes possible to get a clear notion of wind turbine system dynamic behavior in real operational environment and to introduce changes into structural elements and controller system parameters at the design stage, thus contributing to enhanced efficiency and durability of the future system). Besides, the accumulated data can also be used in the course of the turbine system operation (e.g., the information obtained allows to fine-tune the controller unit, to develop a schedule for preventive maintenance and repair, as well as to reveal normal operational parameters of the system).

Use of operational modal analysis in wind energy systems - task-specific software suite by ZETLAB

Thus, the Operational modal analysis should not be considered to be a substitution for the Experimental modal analysis. In fact, these two methods are often used together at different stages of the research and development process. However, the Operational modal analysis is a universal tool, which can be used for a variety of practical applications in terms of analysis and diagnostics. Besides, each particular practical implementation of Operational modal analysis requires individual selection of software and hardware configurations.

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