(625a) Distributed Fault Diagnosis for Networked Nonlinear Uncertain Systems | AIChE

(625a) Distributed Fault Diagnosis for Networked Nonlinear Uncertain Systems

Authors 

Shahnazari, H. - Presenter, McMaster University
Mhaskar, P., McMaster University
The last decades have witnessed increasing drive to economic efficiency leading to material and energy integration, and in turn, to increasing complexity. The complexity can be due to nonlinearities, uncertainties, strong interconnections between subsystems and high dimensionality (see e.g., [1]). Where possible, the resultant large scale process systems are often modeled as a set of interconnected subsystems and are termed networked systems. While advances in automation have led to process efficiencies, it has also driven the need to provide automated safeguards against actuator and sensor faults. Thus, monitoring schemes are required to avoid failures in the networked systems by diagnosing faulty equipments as soon as possible. Designing a centralized FDI scheme for this problem may not be the best solution due to computational limitations or reliability issues. In particular, a failure in a centralized FDI can lead to interruption in monitoring of all of the subsystems. If the interconnections are weak, a decentralized FDI scheme composed of an independent local FDI scheme for each subsystem can handle the situation. However, there are cases that interconnections between subsystems are not negligible, such as power systems or chemical plants, that results in poor performance of a decentralized FDI scheme. In this case, viable solution is designing an FDI scheme with distributed architecture. In the distributed architecture a local FDI scheme is designed for each subsystem while the local FDI schemes can communicate to exchange information. The exchanged information can be information about recently diagnosed faults in the other subsystems or any other information required by the other local FDI schemes.

These realizations have motivated the design of FDI schemes with distributed architecture for networked systems. There exist a plethora of research in designing fault detection and isolation framework for monitoring of engineering systems (see e.g., [2], [3], [4], [5], [6], [7], [8], [9], [10] and [11]). However, most of the existing results in the literature present a centralized FDI scheme that might not be applicable to networked systems due to computational capa- bilities, communication infrastructure and reliability issues as discussed above. There also exist several results for FDI design in networked systems. In [12], a distributed data based actuator fault identification scheme is presented for linear networked process systems. To this end, local FDI agents are designed for each subsystem using moving horizon estimator (MHE)s to identify the fault. In [13], a distributed actuators FDI scheme is proposed for a class of interconnected uncertain nonlinear systems. To this end, a local FDI framework is designed for each subsystem. In each local FDI scheme, first a fault detection scheme is designed which simply uses output estimation error as residual. Then, a bank of fault isolation estimators is designed using adaptive estimation techniques. In [14], a distributed actuators fault detection and isolation framework is presented for nonlinear uncertain large-scale discrete-time dynamical systems. To overcome the scalabil- ity issues, the large scale system under considerations is modeled as several interconnected subsystems. For each subsystem a local FDI scheme is designed using adaptive estimation techniques. Decisions made by the local fault diagnoser are gathered by a higher level global fault diagnoser to determine health of the overall system. A consensus-based estimator is designed to enhance the ability of FDI schemes in detection and isolation of faulty shared information between the subsystems. In [15], a distributed sensor fault diagnosis for a network of interconnected cyber-physicalsystem (CPS)s (see e.g., [16]) is presented. The FDI module consists of two layers. The first layer is built using local FDI schemes to isolate multiple local sensor faults. The detection is carried out using adaptive estimation techniques while the isolation is done using logic rules. The second layer is realized by applying a global decision logic designed to isolate multiple sensor faults that may propagate in the network. However, none of the existing results have addressed the problem of isolation of multiple actuator faults or simultaneous actuator and sensor faults in uncertain nonlinear networked systems. Also, the results addressing actuator faults are based on the assumption that full state measurements are available.

Motivated by the above considerations, in this work, we present a distributed fault diag- nosis framework for a generalized class of networked uncertain nonlinear systems. The idea is to design a bank of FDI schemes based on the robust FDI methodology presented in [11] in a distributed manner. In particular, the nature of the interconnection is first determined and appropriate FDI schemes are developed. To this end, at first boundedness of estima- tion error in the presence of uncertainty and exchange of information is established. Then a bank of residuals is designed for each subsystem using an appropriate subset of the available measurements (and associated state observers), to determine the expected behavior of the system and compare with the observed evolution, where each residual is sensitive to a subset of faults and insensitive to the rest, in the absence of uncertainty. To achieve FDI in the presence of uncertainty, thresholds are defined in a way that they account for the impact of the uncertainty on the estimation error and the prediction of the expected system behavior. In this way, each residual is still insensitive to a subset of faults in the presence of uncertainty and sensitive to the rest if the fault functionality satisfies a detectability condition. Then the detectability and isolability conditions for single and simultaneous faults are derived, when the detectability analysis establishes that the sensitive property of residuals is retained in the presence of uncertainty (see [11] for more on this). The local FDI schemes communi- cate to diagnose the faults in the shared interconnections between subsystems. Results are demonstrated using a networked CSTR example.

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