This paper offers a model-based fault detection and isolation (FDI) scheme for a rudder servo system (RSS) that manages ship navigation. Inherent nonlinearities and unknown external disturbances present great challenges in applying FDI technologies to RSS in practice. This paper presents the derivation of the state equations and commonly encountered fault types of RSS from physical laws, and the design of five nonlinear unknown input observers (NUIOs) capable of eliminating the influences of unknown disturbance while detecting and isolating faults. In the NUIOs, no boundary assumption is made on the disturbance, and their parameter matrices are obtained using a linear-matrix-inequality method. Along with an algorithm that programs the logic rules for FDI, the model-based FDI scheme has been implemented on an actual RSS test rig for validating the RSS model and experimentally evaluating it in a real-world environment; both actuator and sensor faults are considered. Based on experimental tests, an adaptive threshold method is introduced to improve decision making and hence effectively eliminate false alarms. Experimental results show that the model-based scheme is efficient and can be used for online FDI.Index Terms-Adaptive threshold, fault detection, fault isolation, nonlinear unknown input observer (NUIO), rudder servo system (RSS). NOMENCLATURE Capital letters ASystem matrix of RSS model. B, CInput and output matrices of RSS model, respectively. C iOutput matrix of RSS model obtained by deleting ith row of C. DDisturbance matrix of RSS model. DDisturbance matrix of RSS model obtained by adding e i to D. E Matrix of the observer for RSS output term.

The important parameters to describe waves are their amplitude and length. In order to make it easier to improve wave amplitude and facilitate wave experiment, a simple push-type wave generating method using digital rotary valve control was proposed and different wave amplitudes were generated by the new method. After the mathematical model of the new method had been established, numerical analysis based on the linear wave theory was carried out by means of Matlab/Simulink software tools, and experiments were conducted on the push-type wave maker to ascertain the validity of the established model and the numerical simulation results. It shows that both experimental and theoretical results agree relatively well, and the plate motion frequency and amplitude of the push-type wave maker can be continuously adjusted and the various required regular waves can be obtained. Although the wave amplitude and length descends with the increasing of working frequency, the wave amplitude can be improved conveniently by setting the axial opening width of the valve and the oil supply pressure of system. The wave length remains unchanged with the axial opening width and the oil supply pressure change. The research indicates that different regular waves can be easily generated by the new method and the wave amplitude can be further improved in a certain plate motion frequency range.

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