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Control Theory & Applications, IET

Issue 6 • Date November 2007

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Displaying Results 1 - 19 of 19
  • Stability and stabilisation of stochastic differential delay equations

    Page(s): 1551 - 1566
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (251 KB)  

    The current developments and new trends in the study of stochastic differential delay equations are reviewed, with emphasis on stability and stabilisation. View full abstract»

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  • Robust model predictive control for LPV systems using relaxation matrices

    Page(s): 1567 - 1573
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (239 KB)  

    A method of computing a new model predictive control (MPC) law for linear parameter varying systems with input constraints is proposed. The proposed method improves feasibility and system performance by deriving a new sufficient condition for the cost monotonicity. The control problem is formulated as a minimisation of the upper bound of finite horizon cost function satisfying the sufficient conditions. The relaxation matrices yield less conservative sufficient condition in terms of linear matrix inequalities so that it allows to design a more robust MPC. A numerical example is included to illustrate the effectiveness of the proposed method. View full abstract»

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  • High-gain nonlinear observer design using the observer canonical form

    Page(s): 1574 - 1579
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (165 KB)  

    Exact error linearisation is a well-established nonlinear observer design method which is difficult to apply as it requires the solution of a system of partial differential equations. To avoid this difficulty, two observers and sufficient conditions for their global convergence are proposed. The methods combine the high-gain and exact error linearisation approaches. Assuming both the observer canonical form exists and the system's output injection is globally Lipschitz, both observer gain vectors can be computed efficiently without knowledge of either the observer canonical form or the associated transformation. The proposed observers are particularly well suited to systems which are globally Lipschitz in observer canonical form but not in the observable coordinates. This last requirement is sufficient for a traditional high-gain design. An example is provided which demonstrates the methods. View full abstract»

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  • Improved stabilisation method for networked control systems

    Page(s): 1580 - 1585
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (121 KB)  

    The controller design problem for networked control systems (NCSs) is investigated. By considering the relationship between the network-reduced delay and its upper bound, an improved stability criterion for networked closed-loop system is proposed without ignoring any terms in the derivative of Lyapunov-Krasovskii function. As a result, a state feedback controller design method is established using a modified cone complementary linearisation (CCL) algorithm with a new stopping condition. A numerical example is given to demonstrate the effectiveness and the benefits of the proposed method. View full abstract»

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  • Global stabilisation of multiple integrators via saturated controls

    Page(s): 1586 - 1593
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (366 KB)  

    The global stabilisation problem for an nth-order integrator system with saturated input is considered. A new class of nested-type nonlinear feedback law with new and useful characteristics is proposed. First, this approach allows the designer to pick some parameters that facilitate the placement of the closed-loop pole set consisting of pairs of conjugate complex numbers having negative real parts when none of the saturation elements in the control laws is saturated. Only real numbers are allowed in the other existing results. Second, there are more free parameters in this class of nonlinear feedback laws that can be used to improve the performance further. Simulations confirm the convergence of the closed-loop system with other existing techniques. View full abstract»

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  • Deadlock control policy for a class of petri nets without complete siphon enumeration

    Page(s): 1594 - 1605
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (425 KB)  

    Siphons are special structures of a Petri net. Their number grows exponentially with the net size. Hence, the traditional siphon-based deadlock control policies have two problems, that is, generating very structurally complex supervisory controllers and requiring intractable computation efforts. This paper intends to use the newly proposed concept, elementary siphons, and a mixed integer programming (MIP) method to design structurally simple supervisory controllers and reduce the computational burden. This method is applicable to a class of Petri nets, system of simple sequential processes with resources that can well model a wide class of discrete manufacturing systems. Siphons are divided into elementary and dependent ones. The proposed policy consists of three stages: siphon control, control-induced siphon control, and the elimination of control-redundant monitors. First, a monitor (control place) is added for each elementary siphon such that it is invariant-controlled. Because of the addition of monitors to the plant model, control-induced siphons are possibly generated in the augmented net. Next, monitors are added to make control-induced siphons in the augmented net always marked sufficiently without generating new problematic siphons. A MIP technique is used to guarantee that no siphon is insufficiently marked. Finally, we systematically remove control-redundant monitors. Compared with previous work in the literature, the deadlock prevention policy developed in this paper can lead to a structurally simple liveness-enforcing Petri net supervisor with more permissive behaviour by adding only a small number of monitors and arcs. Moreover, complete siphon enumeration is avoided. A manufacturing system example is utilised to illustrate the proposed methods. View full abstract»

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  • Control reconfiguration of a thermofluid process by means of a virtual actuator

    Page(s): 1606 - 1620
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1124 KB)  

    The application of a virtual actuator to the control reconfiguration of a thermofluid process after actuator faults is described. The virtual actuator is a model-based scheme for reconfiguring the control loop, after actuator faults have occurred. It works by hiding the fault from the controller, thus allowing the nominal controller to remain in the loop. Several design approaches proposed in the literature are tested experimentally for three actuator failure scenarios on a physical thermofluid process. The obtained experimental results are interpreted to evaluate the applicability of the virtual actuator and to motivate future research directions. View full abstract»

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  • Experimental validation of an autonomous control system on a mobile robot platform

    Page(s): 1621 - 1629
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (743 KB)  

    An autonomous control system designed for a non-holonomic wheeled mobile robot that is programmed to emulate a fixed-wing unmanned air vehicle (UAV) flying at constant altitude is experimentally validated. The overall system is capable of waypoint navigation, threat avoidance, real-time trajectory generation and trajectory tracking. Both the wheeled mobile robot experimental platform and the hierarchical autonomous control software architecture are introduced. Programmed to emulate a fixed-wing UAV flying at constant altitude, a non-holonomic mobile robot is assigned to follow a desired time-parameterised trajectory generated by a real-time trajectory generator to transition through a sequence of targets in the presence of static and popup threats. Hardware results of the autonomous control system where the trajectory tracker applies two velocity controllers accounting for fixed-wing UAV-like input constraints, are compared to simulation results of dynamic controllers that are based on non-smooth backstepping to demonstrate the effectiveness of the overall system. View full abstract»

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  • Improved robust absolute stability criteria for uncertain time-delay systems

    Page(s): 1630 - 1637
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (126 KB)  

    Improved delay-dependent robust absolute stability conditions for a class of uncertain time-delay systems with nonlinearities subject to sector-bounded constraints are proposed. These conditions are in terms of linear matrix inequalities, which are obtained by choosing new Lyapunov-Krasovskii functionals. Both the cases with time-varying and time-invariant nonlinearities are considered. Numerical examples are provided to show that the robust absolute stability conditions are less conservative than some existing ones in the literature. View full abstract»

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  • Nonlinear control design of an airfoil with active flutter suppression in the presence of disturbance

    Page(s): 1638 - 1649
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (580 KB)  

    The nonlinear flutter suppression of a typical wing section is investigated. A structural nonlinearity is considered in the pitch direction. Integral-input-to-state stability (iISS) concept is utilised for the construction of a feedback controller. One of the advantages of this design is its simplicity and straightforwardness. A backstepping method is used to compare the results for the typical section, which is a multi-input system. The iISS controller has an outstanding performance in comparison to the backstepping method, especially with regards to the disturbance attenuation problem. A Lyapunov-based controller was also introduced for the system. View full abstract»

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  • Model-following neuro-adaptive control design for non-square, non-affine nonlinear systems

    Page(s): 1650 - 1661
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (466 KB)  

    A new model-following adaptive control design technique for a class of non-affine and non-square nonlinear systems using neural networks is proposed. An appropriate stabilising controller is assumed available for a nominal system model. This nominal controller may not be able to guarantee stability/satisfactory performance in the presence of unmodelled dynamics (neglected algebraic terms in the mathematical model) and/or parameter uncertainties present in the system model. In order to ensure stable behaviour, an online control adaptation procedure is proposed. The controller design is carried out in two steps: (i) synthesis of a set of neural networks which capture matched unmodelled (neglected) dynamics or model uncertainties because of parametric variations and (ii) synthesis of a controller that drives the state of the actual plant to that of a desired nominal model. The neural network weight update rule is derived using Lyapunov theory, which guarantees both stability of the error dynamics (in a practical stability sense) and boundedness of the weights of the neural networks. The proposed adaptation procedure is independent of the technique used to design the nominal controller, and hence can be used in conjunction with any known control design technique. Numerical results for two challenging illustrative problems are presented, which demonstrate these features and clearly bring out the potential of the proposed approach. View full abstract»

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  • Optimal dynamic inversion control design for a class of nonlinear distributed parameter systems with continuous and discrete actuators

    Page(s): 1662 - 1671
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (480 KB)  

    Combining the principles of dynamic inversion and optimisation theory, two stabilising state-feedback control design approaches are presented for a class of nonlinear distributed parameter systems. One approach combines the dynamic inversion with variational optimisation theory and it can be applied when there is a continuous actuator in the spatial domain. This approach has more theoretical significance in the sense that it does not lead to any singularity in the control computation and the convergence of the control action can be proved. The other approach, which can be applied when there are a number of discrete actuators located at distinct places in the spatial domain, combines dynamic inversion with static optimisation theory. This approach has more relevance in practice, since such a scenario appears naturally in many practical problems because of implementation concern. These new techniques can be classified as 'design-then-approximate' techniques, which are in general more elegant than the 'approximate-then-design' techniques. However, unlike the existing design-then-approximate techniques, the new techniques presented here do not demand involved mathematics (like infinite-dimensional operator theory, inertial manifold theory and so on). To demonstrate the potential of the proposed techniques, a real-life temperature control problem for a heat transfer application is solved, first assuming a continuous actuator and then assuming a set of discrete actuators. View full abstract»

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  • Observer-based strategies for actuator fault detection, isolation and estimation for certain class of uncertain nonlinear systems

    Page(s): 1672 - 1680
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (196 KB)  

    Two observer-based actuator fault isolation schemes for a class of uncertain nonlinear systems have been presented. To deal with a broader class of uncertain nonlinearities than previously considered, novel diagnostic observers are proposed, which combine two nonlinear observer design strategies, namely Thau's observer with sliding-mode observer concepts. The proposed observers are primarily designed for actuator fault diagnostic purposes. The nonlinearities that can be attacked may include both Lipschitz nonlinearities and those uncertain nonlinearities that are not Lipschitz but satisfy certain matching conditions. Design of the proposed observer boils down to solving a set of linear matrix inequalities (LMIs), which can easily be accomplished using the Matlab's LMI toolbox. Using the proposed observers, two actuator fault isolation schemes are designed. Unlike the existing techniques, using m observers (where m is the number of actuators) in the first approach, and only one observer in the second approach, the proposed schemes can isolate any number of actuator faults occurring at the same time. In addition, both the proposed schemes are capable of estimating the shape of the faults which is useful for fault accommodation purposes. A numerical example is provided to show the effectiveness of the proposed model-based actuator fault isolation strategies. The simulation results confirm that the two proposed techniques are effective in dealing with robust actuator fault detection, isolation and estimation in the studied class of nonlinear systems. View full abstract»

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  • Speed sensorless field-oriented control of induction motor with interconnected observers: experimental tests on low frequencies benchmark

    Page(s): 1681 - 1692
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (408 KB)  

    Field-oriented speed control of induction motors (IMs) without mechanical sensors (speed sensor and load torque sensor) are considered. The methodology is divided into two parts. First, interconnected high-gain observers are designed to estimate the mechanical and magnetic variables from the only measurement of stator current. Secondly, the speed and flux estimation are used by a controller to achieve the speed/flux tracking. The flux regulation problem is simple and the traditional approach is followed by using proportional integral (PI) controller. For the speed-regulation problem, it is stated that flux regulation quickly happens by using a high- gain PI controller to regulate the g-axis current to its reference. Stability analysis based on Lyapunov theory is proved to guarantee the 'observer + controller' stability. To test and validate the controller-observer by considering the sensorless control problem of IM at low frequency, a significant benchmark is implemented. The trajectories of this benchmark are designed to validate the controller and observer under three operating conditions: low speed, high speed and very low speed. Furthermore, robustness tests with respect to parameter variations are given in order to show the performance of the proposed observer-controller scheme. View full abstract»

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  • Quantised control design for networked control systems

    Page(s): 1693 - 1699
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (160 KB)  

    Problems of the quantised stabilisation and quantised Hinfin control design for networked control systems (NCSs) are considered here. First, an NCS system model is proposed which considers the effects of both network-induced delay and quantisation levels. Then, sufficient conditions for the stabilisation and Hinfin control design are derived using the Lyapunov functional approach and shown in terms of linear matrix inequalities (LMIs). By solving LMIs, the feedback gain matrix, the Hinfin performance level and the quantisation parameters can be obtained. Finally, an illustrative example is given to show the effectiveness of the proposed method. View full abstract»

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  • On guaranteed cost fuzzy control for nonlinear systems with interval time-varying delay

    Page(s): 1700 - 1710
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (220 KB)  

    A guaranteed cost fuzzy control for a class of nonlinear time-delay systems has been reported. The time-delay is assumed to be a time-varying continuous function belonging to a given interval, which means that the lower and upper bounds of the time-varying delay are available. And no restriction on the derivative of the time-varying delay is needed, which allows the time-delay to be a fast time-varying function. The nonlinear time-delay systems are approximated by uncertain Takagi-Sugeno (T-S) fuzzy models with interval time-varying delay. Delay-dependent sufficient conditions on the existence of a guaranteed cost fuzzy controller are derived in terms of matrix inequalities. No model transformation is needed and no slack matrix variable is introduced. A non-convex minimisation problem is formulated for finding the least upper bound of guaranteed cost function under matrix inequality constraints. In order to solve this non-convex minimisation problem, a linearisation iterative algorithm is provided to design a controller achieving a suboptimal guaranteed cost for the considered systems. No parameter needs to be selected in advance. A numerical example is also given to show the effectiveness of the proposed design method. View full abstract»

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  • Transient performance enhancement of direct adaptive control of nonlinear systems using multiple models and switching

    Page(s): 1711 - 1725
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (802 KB)  

    Adaptive control of nonlinear systems with parametric uncertainties is considered. A novel approach is presented which makes use of multiple identification models and switching based on direct adaptive control scheme. The use of multiple fixed models and switching provides better transient response under sudden changes in the parameter values. The parameter estimates are reset to the fixed values whenever one of these fixed models yields smaller identification error. The restrictive matching conditions are not required, but the nonlinearities are assumed to be Lipschitz. This work is the generalisation of the methodology proposed by Ciliz and Cezayirli. View full abstract»

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  • Linear matrix inequality characterisation for H and H2 guaranteed cost gain-scheduling quadratic stabilisation of linear time-varying polytopic systems

    Page(s): 1726 - 1735
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (177 KB)  

    Necessary and sufficient linear matrix inequality (LMI) conditions are provided to compute parameter-dependent state feedback control gains that ensure closed-loop quadratic stability for linear systems affected by arbitrarily fast time-varying parameters inside a polytope. The proposed conditions, based on an extension of Polya's theorem and on the systematic construction of homogeneous polynomial solutions for parameter-dependent LMIs, are written as a sequence of progressively less and less conservative LMI conditions. Necessity is attained as the level of relaxation increases, providing a parameter-dependent state feedback gain that quadratically stabilises the system whenever such a gain exists. Moreover, parameter-dependent gains of arbitrary degree assuring quadratic stability with Hinfin and H2 guaranteed costs are also provided. The convergence to the minimum values of the attainable Hinfin and H2guaranteed costs under closed-loop quadratic stability occurs as the degree of the polynomially parameter-dependent gain increases. Numerical results illustrate the efficiency of the proposed conditions. View full abstract»

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