Tech Briefs

Linearity is not assumed and the physical plant is not modeled.

Aconcept of sensor validation using nonlinear minor-component analysis (NLMCA) has been proposed as a theoretical basis of a sensor fault-detection-and isolation (FDI) module in a fault-tolerant control system of an aircraft jet engine or other complex physical plant. As used here, "sensor validation" signifies, loosely, analysis of the readouts of all the sensors in such a system for the purpose of identifying which (if any) sensors are faulty and, if possible, the magnitudes of the faults. Once a fault in a sensor or any other component was detected and isolated, the fault-tolerant control system would automatically reconfigure itself to compensate for the effect of that and any other faults so as to maintain acceptable (or as nearly acceptable as possible) control performance of the plant even in the presence of the faults. In the case of a faulty sensor, the system would utilize any available analytical redundancy among all sensor signals to estimate the value of the physical quantity desired to be measured by that sensor, and that value would then be used for feedback control.

Figure 1. An NLMCA Structure can be represented as an assembly of concatenated NLPCA structures.
The present sensor-validation concept can be characterized as a data-driven (in contradistinction to a model-driven) FDI concept: the concept involves the use of sensor output data only, and a model of the physical behavior of the physical plant is not needed or used. Moreover, unlike in some prior sensorvalidation concepts, training data in the form of output data from faulty sensors is not needed or used: instead, only training data acquired during normal operation are used.

NLMCA is an extension of nonlinear principal-component analysis (NLPCA), in which a neural network or other suitable nonlinear signal-data-processing structure is used to extract the principal components, defined as the components corresponding to the largest eigenvalues of an input vector, X. The first residual (e1 C the residue left after extraction of the first principal component, PC1) is fed as input into the same NLPCA structure to obtain the second principal component (PC2), the second residual (e2) is fed into the same NLPCA structure to obtain the third principal component, and so forth. In NLPCA (see Figure 1), the process as described thus far is truncated once the principal components have been obtained. In NLMCA, the process is not so truncated and, instead, is continued to obtain the minor components (MC1...MCk), defined as those corresponding to the smallest eigenvalues.