Supat Klinkhieo

Friction compensation as a fault-tolerant control problem

The control of systems that involve friction presents interesting challenges. Recent research has focused on detailed modelling of friction phenomena in order to use robust on-line friction compensation procedures, attempting to cancel out the friction force effect in the feedback control of a mechanical or mechatronic system. However, the friction modelling problem remains a very difficult challenge and this article proposes a new approach to friction compensation which is based on the theory of robust fault estimation. The friction forces acting in a dynamic system can be viewed as actuator faults with time-varying characteristics to be estimated and compensated within an output feedback fault-tolerant control (FTC) scheme, so that the limitations arising from the use of a friction model are obviated. The friction (fault) estimation problem is hence embedded inside a control system with required stability, and performance robustness. This can be a significant advantage over well-known model-based friction compensation methods in which detailed modelling of friction phenomena is essential and for which robustness with respect to friction characteristics is difficult to achieve using non-linear models.

Actuator fault estimation and compensation based on an augmented state observer approach

Faults or process failures may drastically change system behaviour leading to performance degradation and instability. The reliability and fault-tolerance of a control system can be achieved through the design of either an active or passive Fault Tolerant Control (FTC) scheme. This paper proposes a new approach to fault compensation for FTC using fault estimation by which the faults acting in a dynamical system are estimated and compensated within an adaptive control scheme with required stability and performance robustness. The FTC scheme has an augmented state observer (ASO) in the control system, which has an intrinsic robustness in terms of the stability and performance of the estimation error. The design concepts are illustrated using the notion that the friction forces in a mechanical system can be estimated and compensated to give good control performance and stability. The example given is that of a non-linear inverted pendulum with Stribeck friction.

LPV fault estimation and FTC of a two-link manipulator

This work is motivated by the challenge to develop an adaptive strategy for systems that are complex, have actuator faults and are difficult to control using linear methods. The novelty lies in combined use of LPV fault estimation and LPV fault compensation to meet active FTC performance requirements. The paper proposes a new design approach for systems which can be characterized via sets of LMIs and can be obtained using efficient interior-point algorithms. A polytopic LPV estimator is synthesized for generating actuator fault estimates used in an FTC scheme to schedule the nominal system state feedback gain, thereby maintaining the system performance over a wide operating range within a proposed polytopic model. The active FTC controller is a function of fault effect factors derived on-line. The effectiveness of the proposed method is demonstrated through a nonlinear two-link manipulator system with torque input faults at each joint.

Fault Tolerant Control in NCS Medium Access Constraints

This paper deals with the problem of fault-tolerant control of a Network Control System (NCS) for the case in which the sensors, actuators and controller are inter-connected via various Medium Access Control protocols which define the access scheduling and collision arbitration policies in the network and employing the so-called periodic communication sequence. A new procedure for controlling a system over a network using the concept of an NCS-Information-Packet is described which comprises an augmented vector consisting of control moves and fault flags. The size of this packet is used to define a Completely Fault Tolerant NCS. The fault-tolerant behaviour and control performance of this scheme is illustrated through the use of a process model and controller. The plant is controlled over a network using Model-based Predictive Control and implemented via MATLABcopy and LABVIEWcopy software.

A Two-Level Approach to Fault-Tolerant Control of Distributed Systems based on the Sliding Mode

Abstract A new approach to Fault-Tolerant Control (FTC) of distributed and interconnected systems is proposed based on applying sliding mode control (SMC) to the subsystems of a level de-centralised and hierarchical scheme. The SMC approach involves a new optimal control strategy for the design of local sliding functions for the subsystem controllers, replacing the conventional approach based on constrained locally linear LQ receding horizon control. The linear SMC gains handle the reachability for the sliding surfaces and the interaction effects from subsystem interconnections as well as small fault effects (i.e. giving passive fault-tolerance), whilst the non-linear (discontinuous) SMC gains facilitate a powerful way of accounting for larger but bounded system non-linearities and faults and can fulfill the role of an active FTC scheme. The local and global performance constraints are retained and implemented under autonomous learning supervision via the interaction-prediction principle. The scheme for an Autonomous Control and Supervision System (ACSS) is described that is capable of learning its coordination function and carrying out fault-tolerant balancing of the distributed system. The paper describes how the two-level learning strategy offers advantages over single-level FTC distributed SMC. The design concepts are illustrated using a non-linear 3-tank liquid level and heating control system with component faults

Robust FDI for FTC Coordination in a Distributed Network System

Abstract This paper focuses on the development of a suitable Fault Detection and Isolation (FDI) strategy for application to a system of inter-connected and distributed systems, as a basis for a fault-tolerant Network Control System (NCS) problem. The work follows a recent study showing that a hierarchical decentralized control system architecture may be suitable for fault-tolerant control (FTC) of a network of distributed and interacting subsystems. The main idea is to use robust FDI methods to facilitate the discrimination between faults acting within one subsystem and faults acting in other areas of the network, so that a powerful form of active FTC of the NCS can be implemented, through an autonomous network coordinator. By using a robust form of the Unknown Input Observer (UIO), fault effects in each subsystem are de-coupled from the other subsystems, thus facilitating a powerful way to achieve local FDI in each subsystem under autonomous system coordination. Whilst the autonomous distributed control system provides active FTC under learning control, the FDI-based Reconfiguration Task enhances the network fault-tolerance, so that more significant subsystem faults can be accommodated in order to achieve a suitable standard of Quality of Performance (QoP) of the NCS.

An Adaptive Approach to Active Fault-Tolerant Control

In this Faults or process failures may drastically change system behaviour leading to performance degradation and instability. The reliability and fault-tolerance of a control system can be achieved through the design of either an ac- tive or passive Fault Tolerant Control (FTC) scheme. This paper proposes a new approach to fault compensation for FTC using fault estimation by which the faults acting in a dynamical system are estimated and compensated within an adaptive control scheme with required stability and performance robustness. The FTC scheme has an augmented state observer (ASO) in the control system, which has an intrinsic robustness in terms of the stability and performance of the estimation error. The design concepts are illustrated using the notion that the friction forces in a mechanical system can be estimated and compensated to give good control performance and stability. The example given is that of a non-linear inverted pen- dulum with Stribeck friction.

Information Packets and MPC Enable Fault-tolerance in Network Control

This paper deals with fault-tolerant control of a network controlled systems (NCS) problem, where the sensors, actuator and controller are inter-connected via a communication network. A procedure is proposed for controlling a system over a network using the concept of an NCS-information-packet which is an augmented vector comprising control moves and fault flags. The size of this packet is used to define a completely fault tolerant NCS. The behavior and control of this scheme is illustrated by way of an example, where the plant is being controlled over a network. Implicit in this paper is that appropriate FDI schemes exist within the set up. The software environment used is MATLABcopy and LABVIEWcopy. The results illustrate that the scheme is tolerant to faults.

LPV approach to friction estimation as a fault diagnosis problem

The control of systems that involve friction presents interesting challenges. Recent research has focused on detailed modelling of friction phenomena as a very complex and difficult modelling challenge. However, the friction effects acting in a dynamic system can be viewed as actuator faults with time-varying characteristics to be estimated and compensated within a Fault Detection and Diagnosis (FDD) scheme, so that the limitations arising from the use of a friction model are obviated. This work is motivated by the utilisation of robust Linear Parameter Varying (LPV) estimation approach providing effective and robust fault estimation. The approach is illustrated using a two-link manipulator system with Stribeck friction. Results show that the time-varying friction forces on each joint can be simultaneously and robustly estimated through the online measurement of the varying parameters.

An LPV pole-placement approach to friction compensation as an FTC problem

The concept of combining robust fault estimation within a controller system to achieve active Fault Tolerant Control (FTC) has been the subject of considerable interest in the recent literature. The current study is motivated by the need to develop model-based FTC schemes for systems that have no unique equilbria and are therefore difficult to linearise. Linear Parameter Varying (LPV) strategies are well suited to model-based control and fault estimation for such systems. This contribution involves pole-placement within suitable LMI regions, guaranteeing both stability and performance of a multi-fault LPV estimator employed within an FTC structure. The proposed design strategy is illustrated using a non-linear two-link manipulator system with friction forces acting simultaneously at each joint. The friction forces, considered as a special case of actuator faults, are estimated and their effect compensated within a polytope controller system, yielding a robust form of active FTC that is easy to apply to real robot systems.