Mogens Blanke

Fault-tolerant control systems — A holistic view

Abstract Fault-tolerant control is used in systems that need to be able to detect faults and prevent simple faults related to control loops from developing into production stoppages or failures at a plant level. This is obtained by combining fault detection with supervisory control and re-configuration to accommodate faults. Much attention has been focused on fault detection in its own right. This paper deals with fault tolerant control from a much wider point of view, covering the entire design process from the engineering of the interface to structural implementation. Experience ranging from a simple temperature control to a complex satellite control system demonstrates significant improvements in plant availability using simple means.

Concepts and methods in fault-tolerant control

Faults in automated processes will often cause undesired reactions and shut-down of a controlled plant, and the consequences could be damage to technical parts of the plant, to personnel or the environment. Fault-tolerant control combines diagnosis with control methods to handle faults in an intelligent way. The aim is to prevent that simple faults develop into serious failure and hence increase plant availability and reduce the risk of safety hazards. Fault-tolerant control merges several disciplines into a common framework to achieve these goals. The desired features are obtained through online fault diagnosis, automatic condition assessment and calculation of appropriate remedial actions to avoid certain consequences of a fault. The envelope of the possible remedial actions is very wide. Sometimes, simple re-tuning can suffice. In other cases, accommodation of the fault could be achieved by replacing a measurement from a faulty sensor by an estimate. In yet other situations, complex reconfiguration or online controller redesign is required. This paper gives an overview of recent tools to analyze and explore structure and other fundamental properties of an automated system such that any inherent redundancy in the controlled process can be fully utilized to maintain availability, even though faults may occur.

Fully magnetic attitude control for spacecraft subject to gravity gradient

This paper presents stability and control analysis of a satellite on a near polar orbit subject to the gravity torque. The satellite is actuated by a set of mutually perpendicular coils. The concept is that interaction between the Earths magnetic field and a magnetic field generated by the coils results in a mechanical torque used for attitude corrections. Magnetic actuation due to its simplicity and power efficiency is attractive on small, inexpensive satellites. This principle is however inherently nonlinear, and difficult to use since the control torque can only be generated perpendicular to the geomagnetic field vector. This paper shows that three-axis control can be achieved with magnetorquers as sole actuators in a low Earth near polar orbit. It considers the problem from a time-varying, nonlinear system point of view and suggests controllers for three-axis stabilization. A stability analysis is presented, and detailed simulation results show convincing performance over the entire envelope of operation of the Danish Orsted satellite.

What is Fault-tolerant Control

Abstract Faults in automated processes will often cause undesired reactions and shut-down of a controlled plant, and the consequences could be damage to the plant, to personnel or the environment. Fault-tolerant control is the synonym for a set of recent techniques that were developed to increase plant availability and reduce the risk of safety hazards. Its aim is to prevent that simple faults develop into serious failure. Fault-tolerant control merges several disciplines to achieve this goal, including on-line fault diagnosis, automatic condition assessment and calculation of remedial actions when a fault is detected. The envelope of the possible remedial actions is wide. This paper introduces tools to analyze and explore structure and other fundamental properties of an automated system such that any redundancy in the process can be fully utilized to enhance safety and availability.

Diagnosis and Fault-tolerant Control, 2nd edition

Fault-tolerant control aims at a gradual shutdown response in automated systems when faults occur. It satisfies the industrial demand for enhanced availability and safety, in contrast to traditional reactions to faults, which bring about sudden shutdowns and loss of availability. The book presents effective model-based analysis and design methods for fault diagnosis and fault-tolerant control. Architectural and structural models are used to analyse the propagation of the fault through the process, to test the fault detectability and to find the redundancies in the process that can be used to ensure fault tolerance. It also introduces design methods suitable for diagnostic systems and fault-tolerant controllers for continuous processes that are described by analytical models of discrete-event systems represented by automata. The book is suitable for engineering students, engineers in industry and researchers who wish to get an overview of the variety of approaches to process diagnosis and fault-tolerant control. The authors have extensive teaching experience with graduate and PhD students, as well as with industrial experts. Parts of this book have been used in courses for this audience. The authors give a comprehensive introduction to the main ideas of diagnosis and fault-tolerant control and present some of their most recent research achievements obtained together with their research groups in a close cooperatio n with European research projects. The third edition resulted from a major re-structuring and re-writing of the former edition, which has been used for a decade by numerous research groups. New material includes distributed diagnosis of continuous and discrete-event systems, methods for reconfigurability analysis, and extensions of the structural methods towards fault-tolerant control. The bibliographical notes at the end of all chapters have been up-dated. The chapters end with exercises to be used in lectures.

Nonlinear output feedback control of underwater vehicle propellers using feedback form estimated axial flow velocity

Accurate propeller shaft speed controllers can be designed by using nonlinear control theory and feedback from the axial water velocity in the propeller disc. In this paper, an output feedback controller is derived, reconstructing the axial flow velocity from vehicle speed measurements, using a three-state model of propeller shaft speed, forward (surge) speed of the vehicle, and the axial flow velocity. Lyapunov stability theory is used to prove that a nonlinear observer combined with an output feedback integral controller provide exponential stability. The output feedback controller compensates for variations in thrust due to time variations in advance speed. This is a major problem when applying conventional vehicle-propeller control systems. The proposed controller is simulated for an underwater vehicle equipped with a single propeller. The simulations demonstrate that the axial water velocity can be estimated with good accuracy. In addition, the output feedback integral controller shows superior performance and robustness compared to a conventional shaft speed controller.

Dynamic Model for Thrust Generation of Marine Propellers

Abstract Mathematical models of propeller thrust and torque are traditionally based on steady state thrust and torque characteristics obtained in model basin or cavitation tunnel tests. Experimental results showed that these quasi steady state models do not accurately describe the transient phenomena in a thruster. A recently published dynamic model was based on the experimental observations. Describing zero advance speed conditions accurately, this model, however, does not work for a vessel at nonzero relative water speed. This paper derives a large signal dynamic model of propeller that includes the effects of transients in the flow over a wide range of operation. The results are essential for accurate thrust control in dynamic positioning and in underwater robotics.

Consistent design of dependable control systems

Abstract Design of fault handling in control systems is discussed, and a method for consistent design is presented. The method is based on an analysis of component failure modes and their effects. Automated analysis provides decision tables for fault handling. Mathematical models for fault detection and isolation are obtained from bond-graph models of components and subsystems. Automated analysis helps present the propagation of component faults, and shows where fault handling can be applied to stop the migration of a fault. The result is the means of obtaining significantly improved dependability of control systems with a limited implementation effort.

Identification of a class of nonlinear state-space models using RPE techniques

The recursive prediction error methods in state-space form have been efficiently used as parameter identifiers for linear systems, and especially Ljungs innovations filter using a Newton search direction has proved to be quite ideal. In this paper, the RPE method in state-space form is developed to the nonlinear case and extended to include the exact form of a nonlinearity, thus enabling structure preservation for certain classes of nonlinear systems. Both the discrete and the continuous-discrete versions of the algorithm in an innovations model are investigated, and a nonlinear simulation example shows a quite convincing performance of the filter as combined parameter and state estimator.

Diagnosis of Airspeed Measurement Faults for Unmanned Aerial Vehicles

Airspeed sensor faults are common causes for incidents with unmanned aerial vehicles (UAV) with pitot tube clogging or icing being the most common causes. Timely diagnosis of such faults or other artifacts in signals from airspeed sensing systems could potentially prevent crashes. This paper employs parameter adaptive estimators to provide analytical redundancies and a dedicated diagnosis scheme is designed. Robustness is investigated on sets of flight data to estimate distributions of test statistics. The result is robust diagnosis with adequate balance between false alarm rate and fault detectability.