Fault accommodation

Several kinds of failures may compromise safety and productivity of industrial processes. Indeed, faults may affect the efficiency of the process (e.g., lower product quality) or, in the worst scenarios, could lead to fatal accidents (e.g., temperature runaway) with injuries to personnel, environmental pollution, and equipments damage. In the chemical process fault diagnosis area, the term fault is generally defined as a departure from an acceptable range of an observed variable or a parameter. Fault diagnosis (FD) consists of three main tasks fault detection, i.e., the detection of the occurrence of a fault, fault isolation, i.e., the determination of the type and/or the location of the fault, and fault identification, i.e., the determination of the fault profile. After a fault has been detected, controller reconfiguration for the self-correction of the fault effects (fault accommodation) can be achieved in some cases. 

Once a fault has been detected, in some applications a controller reconfiguration for the self-correction of the fault is required (fault accommodation). A fault-tolerant control system possesses this capability, and its adoption is of the utmost... 

It is obvious that for FDI a system must be observable and in order to allow for fault accommodation it must be controllable. Therefore, it is important to analyse the structural properties of a model. If it turns out that, for a given set of sensors, a system is structurally not observable then it is numerically not observable. In a hybrid system model, switches disconnect or reconnect model parts at the advent of discrete events. That is, the model structure is system mode dependent. Also, the set of model equations that holds for a system mode differs from the one for another system mode with regard to its form and its properties. 

Fault accommodation Means to assess the severity of a fault and to decide if and what actions can be taken. If a fault cannot be accommodated, a controlled shut-down may become necessary. If it can be accommodated, possible actions may be a change of controller parameters or a controller redesign, or a system reconfiguration, i.e. a malfunctioning component is replaced by one with similar functionality. 

The steps executed after fault detection are termed alarm interpretation which classify the actual fault, its characteristics (occurrence time, fault size, consequences, etc.), and the root cause. Fault characterization and quantification is required to determine the immediate process state and to determine whether the fault can be safely accommodated at that process state. Based on this input, fault accommodation may be performed through reconfiguration when standby devices in healthy condition are available or through fault-tolerant control (FTC) where the... 

On the other hand, when more than one fault can influence the system at the same time, advanced diagnostic methods are used. These methods are based on parameter estimation. Sensitivity bond graph formulation [12] allows real-time parameter estimation and thus it is possible not only to isolate multiple faults but also to quantify the fault severities. Parameter estimation in single fault [2] or multiple fault scenarios [12] are essential steps to be performed before fault accommodation. The parameter estimation scheme also gives the temporal evolution of system parameters. Thus, it is possible to identify and quantify different kinds of fault occurrences. A progressive fault shows gradual drift in estimated parameter values and intermittent fault shows spikes in the estimated parameter values. The advances made in the field of control theory have made it possible to develop state and parameter estimators for various classes of nonlinear systems. Analytical redundancy relations may also be used in optimization loop for parameter estimation because it avoids the need for state estimation. Interested readers may see Ref. [3] for further details and some solved examples. 

Samantaray, A.K., Ghoshal, S.K. (2008) Bicausal bond graphs for supervision Prom fault detection and isolation to fault accommodation. Journal of Franklin Institute 345(1) 1-28. 

Fault accommodation This is a common approach to achieve fault tolerance. Fault accommodation is limited to internal controller changes. In this method there will be changes in controller parameters or structure to avoid fault consequences. However, the input—output (I/O) between controller and plant remains unchanged, that is, the loop is not completely restructured. 

Most oil companies offer a planned lubrication maintenance (PLM) service that will meet these requirements with the minimum of effort on the part of the customer. These schemes provide logical routing for the lubrication operative, balanced workloads and clear instructions to those responsible for specific tasks associated with lubrication and fault-reporting facilities. Many schemes are now designed for computer operation, which also accommodate plant and grade changes, operation costs and manpower planning. It is essential that any such scheme should be adaptable to individual requirements. 

Similarly, the (111) GaAs substrate could be used to achieve epitaxial growth of zinc blende CdSe by electrodeposition from the standard acidic aqueous solution [7]. It was shown that the large lattice mismatch between CdSe and GaAs (7.4%) is accommodated mainly by interfacial dislocations and results in the formation of a high density of twins or stacking faults in the CdSe structure. Epitaxy declined rapidly on increasing the layer thickness or when the experimental parameters were not optimal. 

Jefferson s studies of the pyroxenoids has added greatly to our application of the way in which, through the intermediary of planar - or planar and Kinke - faults one structure is converted into another (45). And Audier, Jones and Bowen (46) have revealed how unit cell strips of Fe C may be accommodated as extended defects in the Fe C structure. Both these carbidic phases can be readily identified by HREM at the interface of iron catalysts used for the disproportionation of CO (to yield C j+CC ). 

M.M. Polycarpou and A.J. Helmicki. Automated fault detection and accommodation a learning systems approach. IEEE Transactions on Systems, Man, and Cybernetics, 25 1447-1458, 1995. 

All of these materials seem to be equally intolerant of point-defect populations as those described earlier, and others could readily be cited. Thus the conclusion to be drawn from this Section is that phases which accommodate changes in anion to cation stoicheiometry by way of planar fault or intergrowth behaviour comprise a substantial number of inorganic materials, and such phases are in no way the poor relations of defect chemistry compared to systems which are point-defect biased. 

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