Fault-tolerant Control System Design Research Articles

Accepting performance degradation in fault-tolerant control system design

A novel fault-tolerant control system design technique has been proposed in this paper, which blends the multiple-model principle with the unavoidable performance degradation due to faults in actuators, sensors or system dynamics. The number of models employed depends on the characteristics of the system, the nature of the failures considered, and the physical limits of system variables. The achievable performance under various component failures are represented in the form of reference models, known as performance reduced reference models. These models are used to synthesize a set of controllers. Under a specific fault condition, proper controller and revised control system command input are selected automatically to achieve desired performance. A simulation example of an aircraft subject to different type of failures has been used to illustrate the design process and to demonstrate the effectiveness of the method.

FUZZY FAULT-TOLERANT CONTROL SYSTEM DESIGN WITH MULTI-INDICES CONSTRAINTS

This note address the problem of fuzzy fault-tolerant control systems design for nonlinear systems with multi performance indices constraint. Based on the decomposition and equivalence principle, a sufficient and necessary condition for stabilization of original actuator failure fuzzy system is presented. And then, the design of fuzzy fault-tolerant control system is decomposed into design of a set of linear fault-tolerant control extreme subsystems. In succession, a new fault-tolerant controller design strategy is set up for linear system with regional pole index, steady state-variance index and H-infinity index constraints, and the consistency theory on the three kinds of performance indices is also discussed.

Fault-tolerant control against stuck actuator faults

A fault-tolerant control system (FTCS) design technique against stuck actuators is investigated using an iterative learning observer (ILO). The principle of the proposed fault-tolerant control (FTC) is to design a reconfigurable controller using estimated system states, relying on control input adjustments on the redundant actuators to compensate for the effects of stuck actuators. The amount of adjustment is updated based on the transient of fault compensation. The ILO provides both the estimates of the system states and the information on such transients. Multiple faults can also be dealt with. The fault compensation can be carried out swiftly due to the rapid convergence of the ILO. It is shown that the proposed FTCS ensures that the system follows the reference model under both normal conditions and with some stuck actuators. The closed-loop stability of the system is established, and the performance is evaluated using the lateral dynamics of an F-8 aircraft model.

MANAGING PERFORMANCE DEGRADATION IN FAULT TOLERANT CONTROL SYSTEMS

A fault tolerant control system design technique has been proposed and analyzed for managing performance degradation in the presence of multiple faults in actuators. The method is based on a control structure with model reference reconfigurable control design in an inner loop and command input adjustment in an outer loop. The reduced dynamic performance requirements in the presence of different actuator faults are accounted for through different performance reduced (degraded) reference models. The degraded steady-state performance are governed by the reduced levels of command inputs. The reconfigurable controller is designed on-line and automatically in an explicit model reference control framework so that the dynamics of the closed-loop system follows that of the performance reduced reference model under each fault condition. The reduced command input level is determined to prevent potential actuator saturation. The proposed method has been evaluated and analyzied using an aircraft example against actuator faults subject to constraints on the magnitude and slew-rate of actuators.

Fault-Tolerant Control of Multi-Unit Process Systems Using Communication Networks

This work proposes a methodology for the design of fault-tolerant control systems for chemical plants with distributed interconnected processing units. Bringing together tools from Lyapunov-based non linear control and hybrid systems theory, the approach is based on a hierarchical architecture that integrates lower-level feedback control of the individual units with upper-level logic-based supervisory control over communication networks. The local control systems consist each of a family of control configurations connected, via a local communication network, to a local supervisor that orchestrates switching between them on the basis of the stability regions in the event of failures. The local supervisors communicate, through a plant-wide communication network, with a plant supervisor responsible for monitoring the different units and coordinating their responses in a way that minimizes the propagation of failure effects. The communication logic is designed to ensure efficient transmission of information between the units while respecting the inherent limitations in network resources. The proposed approach provides explicit guidelines for managing the various interplays between the tasks of feedback control, switching and communication. The efficacy of the proposed approach is demonstrated through a chemical process example.

A stability guaranteed active fault‐tolerant control system against actuator failures

AbstractIn this paper, a new strategy for fault‐tolerant control system design has been proposed using multiple controllers. The design of such controllers is shown to be unique in the sense that the resulting control system neither suffers from the problem of conservativeness of conventional passive fault‐tolerant control nor from the risk of instability associated with active fault‐tolerant control in case that an incorrect fault detection and isolation decision is made. In other words, the stability of the closed‐loop system is always ensured regardless of the decision made by the fault detection and isolation scheme. A correct decision will further lead to optimal performance of the closed‐loop system. This paper deals with the conflicting requirements among stability, redundancy, and graceful degradation in performance for fault‐tolerant control systems by using robust control techniques. A detailed design procedure has been presented with consideration of parameter uncertainties. Both total and partial actuator failures have been considered. This new control strategy has been demonstrated by controlling a McDonnell F‐4C airplane in the lateral‐direction through simulation. Copyright © 2004 John Wiley & Sons, Ltd.

Fault-Tolerant Control of Process Systems: Integrating Supervisory and Feedback Control Over Networks

This work proposes a methodology for the design of fault-tolerant control systems for nonlinear processes with actuator constraints. The proposed approach is predicated upon the idea of integrating supervisory and feedback control over networks. Initially, a family of candidate control configurations, characterized by different manipulated inputs, are identified. For each control configuration, a bounded nonlinear feedback controller, that enforces asymptotic closed-loop stability in the presence of constraints, is designed, and the constrained stability region associated with it is explicitly characterized. A switching policy is then derived, on the basis of the stability regions, to orchestrate the activation/deactivation of the constituent control configurations in a way that guarantees closed-loop stability in the event of control system failures. The switching laws are implemented by a higher-level supervisor that constantly monitors the process and communicates with the various control configurations over a network. The effects of delays in fault-detection, network communication and actuator activation are taken explicitly into account in executing the switching logic. The efficacy and implementation of the proposed approach are demonstrated through a chemical process example.

Implicit fault-tolerant control: application to induction motors

In this paper we propose an innovative way of dealing with the design of fault-tolerant control systems. We show how the nonlinear output regulation theory can be successfully adopted in order to design a regulator able to offset the effect of all possible faults which can occur and, in doing so, also to detect and isolate the occurred fault. The regulator is designed by embedding the (possible nonlinear) internal model of the fault. This idea is applied to the design of a fault-tolerant controller for induction motors in presence of both rotor and stator mechanical faults.

Decentralized fault-tolerant control system design for unstable processes

Fault-tolerant control is an important issue in control of mission critical processes. In this paper, a new approach to fault-tolerant control of unstable processes is proposed based on the Passivity Theorem. The control system is designed in two sequential steps: A multi-loop proportional controller is used to stabilize the unstable process; a passivity-based decentralized unconditionally stabilizing (DUS) controller is then applied to the stabilized process. While the multi-loop stabilizing controllers need to be built with redundancy, the DUS controller is inherently fault tolerant and can maintain closed-loop stability when any of its loops fail. By using a stabilizing proportional controller with the fewest loops, control redundancy can be reduced to the minimum level.

Fault tolerant control system design with explicit consideration of performance degradation

A new approach is proposed for active fault tolerant control systems (FTCS), which allows one to explicitly incorporate allowable system performance degradation in the event of partial actuator fault in the design process. The method is based on model-following and command input management techniques. The degradation in dynamic performance is accounted for through a degraded reference model. A novel method for,selecting such a model is also presented. The degradation in steady-state performance is dealt with using a command input adjustment technique. When a fault is detected by the fault detection and diagnosis (FDD) scheme, the reconfigurable controller is designed automatically using an eigenstructure assignment algorithm in an explicit model-following framework so that the dynamics of the closed-loop system follow that of the degraded reference model. In the mean time, the command input is also adjusted automatically to prevent the actuators from saturation. The proposed method has been evaluated using the lateral dynamics of an F-8 aircraft against actuator faults subject to constraints on the magnitude of actuator inputs. Very encouraging results have been obtained.

Instrument fault detection and compensation scheme for direct torque controlled induction motor drives

The effects of instrument faults in direct-torque-control-based induction motor drives are analysed and an instrument fault detection isolation scheme (IFDIS) for the drives is proposed. The IFDIS detects and isolates the incipient fault(s) of a speed sensor and current sensors in real-time. The scheme consists of an adaptive gain scheduling observer as a residual generator and a special sequential test logic unit. Although the IFDIS is a single observer scheme, it has the function of fault isolation that normally only a multiple-estimator-based IFDIS possesses. Simulation results show the detection and isolation performance of the IFDIS and the applicability of the scheme to fault tolerant control system design.

Fault Diagnosis and Reconfigurable Control of a Pressurizer in a Nuclear Power Plant

In this paper, an integrated approach to the design of fault diagnosis and reconfigurable fault-tolerant control system against actuator and sensor faults in a nuclear power plant pressurizer is proposed. The scheme uses a reconfigurable proportional-integral (PI) control structure and a setpoint adjustment technique to achieve required performance at both the dynamic and steady-state conditions. A singular value decomposition (SVD) based eigenstructure assignment (EA) technique is used to achieve on-line automatic redesign of the controller. Fault diagnosis and controller reconfiguration mechanisms are carried out using statistical hypothesis tests based on the information from a two stage adaptive Kalman filter. The proposed approach has been evaluated under different type of Unanticipated actuator faults: e.g. abrupt and incipient faults; total and partial faults; single, multiple and consecutive faults under both set-point regulation and set-point tracking. Through set-point tracking, specified performance degradation in the presence of actuator fault. can be eexplicitly considered. Detection and isolation of sensor faults are achieved by voting among outputs from several hardware redundant sensors. The effectiveness of the approach is demonstrated via simulation using a pressurizer model.

Actuator Fault Estimation Scheme for Flight Applications

The paper presents a new actuator fault estimation scheme based on the augmented error method that estimates the actuator effectiveness on-line. The proposed scheme is applied to a fault-tolerant flight control system design for an F4-E fighter aircraft. The output of a nominal H∞ controller is continuously reconfigured to compensate for any loss of actuator effectiveness. Simulation results demonstrate good fault estimation and flying qualities obtained using the proposed scheme under the actuator fault conditions.

Integrated sensor/actuator FDI and reconfigurable control for fault-tolerant flight control system design

AbstractIn this paper an approach to fault-tolerant flight control system design based on the integration of sensor/actuator fault detection and isolation (FDI) and reconfigurable control is presented. The effects of the sensor and actuator faults in the innovation process of the Kalman filter are investigated, and a decision approach to isolate the sensor and actuator faults is proposed.The reconfigurable control algorithm based on the Extended Kalman Filter (EKF) is presented. The control reconfiguration procedure is executed by considering the identified control distribution matrix. In the simulations, the longitudinal dynamics of an aircraft control system are considered, and control reconfiguration is examined. In the paper, the principal block diagram of a fault tolerant aircraft control system is offered.

Active fault-tolerant flight control systems design using the linear matrix inequality method

This paper discusses the issues of robust control law design for fault-tolerant systems. Based on the assumption that the effects of faults can be expressed in linear-fractional-transformation (LFT) forms, a fault-tolerant control systems design problem is formulated and solved via a linear matrix inequality (LMI)-based synthesis approach. In order to recover the convexity of the design problem whilst considering the robust performance and robust stability against faults and uncertainties simultaneously, a constrained optimisation approach is used. The simulation results of a design example (a longitudinal motion flight control problem for an unmanned aircraft in the case of suffering battle damage on its wing ) show that robust stability and satisfactory performance have been achieved.

Fault tolerant structural control systems for civil engineering applications

AbstractThe objective of this work is to experimentally examine some practical issues relating to the reliability and performance of active structural control systems. Using the newly constructed shaking table in the Department of Structural Mechanics at the University of Pavia, experiments were conducted employing an actively controlled, three story test structure. A DC‐motor based active mass driver system was used as the active control device. An H2/LQG controller based on acceleration feedback is applied to the system. The first objective of this work is to demonstrate the efficacy of the acceleration feedback control strategy. Next, two experiments are conducted with the goal of investigating fault tolerant control system design. In the first case, the occurence of a sensor failure is simulated experimentally, and the resulting controlled performance and robustness characteristics are assessed. The second set of tests is to evaluate the ability of the controller to reduce the structural responses when reduced information is available for control action determination. Both of these issues are important practical concerns in the design and implementation of a structural control system.

Design of fault-tolerant distributed control systems

Real-time control systems, involving continuous measuring and monitoring of physical quantities, need often to take into account fault-tolerance capabilities to guarantee a correct behavior (even in the presence of a given amount of hardware faults and software errors) or, at least, to provide gracefully degradable performances. In this paper, hardware dimensioning, the optimum allocation of the computation, and the fault-tolerance issues are afforded contemporaneously, with specific attention to the design of dedicated distributed control systems. A single optimization frame is defined to identify a globally optimum solution with respect to these conflicting goals.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Distributed flight control system using fiber distributed data interface (FDDI)

The design of fault-tolerant distributed control systems is discussed. The application described is a generic flight control system (FCS) for a fighter aircraft. The system is designed for high redundancy and expandability to meet various requirements. The communication network is based on the fiber distributed data interface (FDDI), which provides a high level of electromagnetic interference resistance and reliability. The choice of fiber optic media also provides an extremely high bandwidth and tremendous growth capability for future communications needs. This study is not specific to FDDI or flight control systems. The methods and configurations presented should be applicable to many real-time control implementations. >

Shiryayev sequential probability ratio test for redundancy management

An essential aspect in the design of fault-tolerant digital flight control systems is the design of failure detection and redundancy management systems. A decision rule, the Shiryayev sequential probability ratio test (SPRT), is derived from a dynamic programming approach and is used to detect failures between similar instruments, as well as between dissimilar instruments through analytic redundancy. Unlike the Wald SPRT, which tests for the presence of failure or no failure in all of the data sequence, the Shiryayev SPRT detects the occurrence of a fault in the data sequence in minimum time if certain conditions are met. The performance of the Shiryayev SPRT in detecting a failure between two rate gyros as compared to standard fixed interval schemes is presented, as well as the performance of a single accelerometer failure using translational kinematic equations to form a parity relation for analytic redundancy.