7.4 IVHM Framework for Locating Faults in Rotating Machinery
7.4.4 Summary Development Chart
In consolidating the information discussed throughout this chapter (and drawing upon the information gathered from the thesis), a graphical summary has been constructed. This can be seen in Error! Reference source not found.. The
three ‘tracts’ can be seen, indicating the Simulation and Data-Driven approaches along with the core ‘IVHM’ chain. The chart can be seen to demonstrate a workflow for the creation of a system for diagnosing, localising and prognosing common faults in rotating machinery.
In addition to the three main ‘tracts’, some additional notes have been made on the chart. This includes the addition of the Rolls Royce ‘IVHM’ tracts – Sense, Acquire, Transfer, Analyse, Act. This has been displayed such that it fits in with the developed framework. The importance of the application (e.g. aerospace, marine, power) has been described to influence the business case, as well as an indication of whether the system will be next generation or legacy (retrofitted). Partitioning considerations also feed into the design for IVHM stage, and data fusion techniques may form a useful aspect of feature extraction and diagnosis. The OSA-CBM framework (described by Sreenuch et
al., (2012)), has also been included for the relevant stages.
Whilst it is not possible to incorporate all considerations into a single diagram, this flow provides a general guide and methodology for designing and incorporating new IVHM systems for rotating machinery, with specific relevance to imbalance localisation. If this information had been available at the beginning of the project in such concise form, the development of the described system could have enjoyed significant streamlining. Therefore, the development of such an outline enables future studies and developments to benefit from the findings from this study (both novel and summarised from existing literature). In this way, the chart describes a development methodology for localising common faults in rotating machinery, and as such presents a final novel aspect to the project, in summarising the findings for use in future work.
7.5 ‘Case Study’ – IVHM Implementation of Novel Imbalance
Localisation System
The benefits of rotating machinery diagnostics and prognostics in next generation IVHM systems are therefore clear. In order to study potential implementation of a system such as that highlighted in this research, a ‘case study’ has been considered. In this study, implementation in an off-board, off- line aerospace application is discussed. This scenario enables reduced certification requirements and cost – with minimal (or ideally no) modification to existing equipment. In this case, implementation is discussed in terms of application to legacy (already existing) aircraft. Through the discussion of a practical application, advantages and limitations of the proposed system can be highlighted. This discussion has been broken down into five key aspects, as follows.
7.5.1 Sense
Figure 7-2 Location of EHM Sensors on a Rolls Royce Trent 900 Engine
Existing aircraft engine turbines provide a good example of complex rotating machinery commonly fitted with a single accelerometer. In such cases, the single fitted accelerometer has a ‘remote’ position relative to the actual rotating parts. In the case of such machines, temperature issues prevent placement upon bearing housings, or other optimal places for detecting vibrational features. In the case of the research described for this project, a single accelerometer placed in a relatively remote position still has the potential to detect required nonlinearities. As such, for this case minimal modification would be required.
7.5.2 Acquire
One potential limitation of implementing such fault localisation technology into legacy systems is the inability to access data from integrated sensors. The recent generations of aircraft are beginning to incorporate the facility to share data across systems and store larger amounts of data. In aircraft such as the Boeing 777 and 787, this integrated facility enables the potential for data to be collected which is sufficient for identifying and localising potential faults through a system such as that presented here. In older generation aircraft, such as the highly successful Boeing 737, the layout of the architecture is such that it is not possible to obtain data at a sufficient sampling rate for such a period of time, and easily store this data. As such, the latest generation of aircraft possess a much improved capacity for aftermarket modification for IVHM purposes. The acquisition system used for the novel research in this paper required small ‘snapshots’ of data, however tested at a high sample rate. This indicates that optimum development would be for next-generation systems, unless a large enough benefit could be determined which would cover the high retrofitting costs.
7.5.3 Transfer
Another key limitation for the implementation of IVHM systems for rotating machinery is the issue of data transfer. In order to process the data off-board, a sufficient amount of data has to be transferred to a suitable ground station for processing and analysis (commonly ‘operations centres’ in the case of Rolls
Royce). This process typically takes the three forms. Firstly, the ACARS system – whereby a small (typically 3kb) amount of data is transferred via VHF radio and at key points of operation (e.g. take off, ascent and cruise). This system enables essential engine parameters, including basic vibration data to be assessed and trended quickly for any sign of deterioration. The limitations in bandwidth of this system, however, mean that the complex analysis required for implementation of the proposed imbalance localisation system would likely be impractical given currently implemented technology (this may however change in future - Sudolsky, (2007))
A second method is the wireless transfer of data upon arrival at certain airports. These features include Gatelink, GSM and WiFi (Brady Jr, et al. (2011)). These systems are not currently implemented in all airports (however it is gaining in popularity). It does, however allow for sufficient transfer of data from the aircraft to the ground for processing post-flight. The limitations of this system are, as mentioned, that it has yet to be fitted to all large commercial airports and that information cannot be obtained during flight, only after landing. However, given that the main advantages of localising imbalance faults can be found in maintenance and operations (as opposed to in flight safety) this requirement becomes less of an issue.
Finally, where the platform/infrastructure enables it, data ‘snapshots’ may be stored on board, for manual downloading by personnel only during maintenance procedures. Whilst this prevents any form of early-detection (aside from already incorporated means), it still has the potential to improve maintenance operations and, to a letter extent, operations.
The latter two options currently appear most viable for a system of fault diagnosis and localisation, if such a system were to be developed based upon the foundations within this study.
7.5.4 Analyse
In considering the case of an off-board system, limitations in processing become negligible as long as sufficient data can be provided. If data is
transferred to external monitoring and operations centres, the application of an ANN (or alternate logic system), along with trending can be performed with relative ease and speed. In the case of Rolls Royce, ANNs are already used to interpret basic vibration data for the purposes of monitoring trends and potential degradation. In this way, fleet wide monitoring and trending can take place from a central hub, with information collated, analysed and dispatched for the purposes of maintenance. It is also important to note that only faults with a long P-F interval curve can be used for this purpose. Whilst imbalance faults may fall into this category (if, for example, a small developing crack is causing the imbalance), imbalance faults may be such that immediate attention is required. Re-training an ANN (or similar system) is likely to require flight data (as opposed to test-bed). Therefore a short period of training during which the imbalance localisation system is inactive may result after maintenance has been performed on a system.
7.5.5 Act
Acting upon knowledge the type, underlying cause and location of a fault has already stated benefits. In the case of an off-board EHM system, the benefits are slightly more limited. The pre-defined points at which data is obtained (e.g. only upon arrival at certain airports), enables longer-term maintenance operations to be planned with distinct accuracy. However, short term decisions, e.g. preparation for maintenance/inspection upon arrival of a plane at an airport, are not possible. Despite this, the ability to collect, store and trend data from a wide variety of aircraft to be collated centrally does provide several advantages. One example scenario may involve certain environmental conditions affecting engines operating at a certain global location. If this has been observed to cause imbalance issues for several engines in, for example, the high pressure compressor stage, then during the next scheduled maintenance operation for any engine operating in these conditions, this aspect can be directly investigated. This option has obvious advantages over investigating a large number of engines for any sign of imbalance across the machinery.