Decentralised control for complex systems - an invited survey <s> Applications <s> The dynamics of water quality constituents in the River Nile is expressed by a linear continuous-time model. Two control actions are employed: the first is through the stream velocity, which is implemented by regulating the water discharge into the basin and the second is through the adjustment of the concentration of water quality constituents discharged from polluter stations. A decentralized control structure is used to adjust the concentration of water quality constituents. The unknown gains are computed by a three-level algorithm. Simulation results have demonstrated clearly the effectiveness of the analytical approach. <s> BIB001 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> Abstract A new approach to decoupled control of large-scale non-linear systems is applied to a dynamic flight control. Control synthesis is performed in two steps. First the nominal, programmed control is synthesized using the complete model of flight dynamics. This nominal control should realize the nominal trajectory under ideal conditions with no perturbations. In the second step the tracking of the nominal trajectory is realized. The system is viewed as a set of decoupled subsystems and for each subsystem local control is synthesized. Then, the stability of the overall system is analyzed and the global control is introduced to compensate coupling among some of the subsystems. A particular choice of subsystems in a case of flight control and of local and global control synthesis is proposed. A simulation of flight control with the proposed control law is also presented. <s> BIB002 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> Large scale control systems results of multivariable feedback systems models and structure of interconnected systems decentralized stabilizability the decentralized stabilization and pole assignment optimal decentralized control stability analysis of interconnected systems decentralized design for strongly coupled subsystems decentralized design for weakly coupled systems decentralized PI controllers strongly coupled symmetric composite systems survey of the results and open problems in feedback control of large-scale systems. <s> BIB003 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> This paper describes a theoretical and experimental investigation into an active four-mount vibration isolation system, in which electromagnetic actuators are installed in parallel with each of the four passive mounts placed between a three-dimensional piece of equipment and a vibrating base structure. Decentralized velocity feedback control is applied, where each actuator is operated independently by feeding back the corresponding equipment vibration velocity at the same location. Although one end of the actuator acts at the sensor position on the equipment, the system is not collocated because of the reactive force at the other end acting on the flexible base structure, whose dynamics may be strongly coupled with the mounted equipment. The investigation of this actuator installation and its practical implementation are the motivation of this research. Isolation of low-frequency vibration is considered where the equipment can be modeled as a rigid body and the mounts as lumped-parameter springs and dampers. A general theoretical formulation for analysing multiple-mount vibration isolation systems using the impedance method is presented and is used to investigate the control mechanisms involved. Experimental results show that up to 14 dB reduction in the kinetic energy of the equipment can be achieved in practice. If very high gains are used in the experiments, however, instability occurs at low frequencies due to phase shifts in the transducer conditioning electronics. <s> BIB004 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> This paper presents two novel decentralized nonlinear adaptive controllers (DNAC) for large-scale interconnected power systems, via state-feedback and output-feedback strategies respectively. In the both controllers, system perturbation, which includes all subsystem nonlinearities and interactions between subsystems, is estimated by a high-gain observer and then involved the decentralized adaptive feedback linearizing control law. For the first DNAC, when all subsystem states are available, a second-order high-gain perturbation observer is designed to estimate the system perturbation, which leads to a decentralized nonlinear adaptive state-feedback controller. For the second, a decentralized nonlinear adaptive output-feedback controller is designed using a high-gain states and perturbation observer, when only one subsystem state is measured. The stability of the closed-loop controller/observer system is analyzed by the Lyapunov direct method. Both the controllers have been evaluated in a simulation study based on a three-machine power system. The results show that with a simple structure, both the controllers have robust performance of improving the transient stability and damping of multimode oscillations, under different power-system operation and fault conditions. <s> BIB005 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> In this paper, we develop a fully adaptive decentralized controller of robot manipulators for trajectory tracking. With high-order and adaptive variable-structure compensations, the proposed scheme makes both position and velocity tracking errors of robot manipulators globally converge to zero asymptotically while allowing all signals in closed-loop systems to be bounded, even without any prior knowledge of robot manipulators. Thus this control scheme is claimed to be fully adaptive. Even when the proposed scheme is modified to avoid the possible chattering in actual implementations, the overall performance will remain appealing. Finally, numerical results are provided to verify the effectiveness of the proposed schemes at the end. <s> BIB006 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> In this paper, the trajectory tracking and obstacle avoidance of a car-like mobile robot (CLMR) within an intelligent space via mixed H2/Hinfin decentralized control is developed. To obtain obstacle avoidance and trajectory tracking, two distributed charge-coupled device (CCD) cameras are established to realize the pose of the CLMR and the position of the obstacle. Based on the authority of these two CCD cameras, a suitable reference command for the proposed controller of the CLMR is planned online by the information of the CCD camera with higher authority. Many of the problems encountered by classic mobile robots are solved. Because an arbitrarily smooth trajectory can be approximated by a set of piecewise lines, the trajectory tracking of the piecewise line is addressed. The features of the proposed control are smaller energy consumption with bounded tracking error, attenuation of output disturbance, and improvement of control performance. The overall system contains two processors (i.e., personal computer and digital signal processor) with multiple sampling rates. Finally, a sequence of experiments including the comparison among the proposed control, PID control, different microprocessor control system, and different initial poses are carried out to confirm the usefulness of the suggested control system. <s> BIB007 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> In this paper, a novel nonlinear control design method for the wind turbine (WT) with doubly fed induction generator (DFIG) using differential geometry theory is proposed. The WT with DFIG is represented by a third-order model where electromagnetic transients of the stator are neglected. Then the model is exactly linearized using the coordinate transformation in differential geometry theory. Since the relative degree of the model of the WT with DFIG is equal to the order of the original system model, the linearized model can be transformed into the Bronovsky norm form. The linear quadratic regulator (LQR) design method is introduced to design the optimal control for the linearized system, and the nonlinear control for the WT with DFIG obtained by the inverse coordinate transformation only depends on the parameters of the WT with DFIG. Hence, the nonlinear control can be considered as a decentralized control, which is a desirable feature for a WT with DFIG. To illustrate the effectiveness of the nonlinear controller proposed, simulations on a single machine infinite bus (SMIB) system and a four-machine system are performed. Compared to the conventional PI control, the proposed nonlinear control can improve the transient stability of the power systems and enhance the system damping. <s> BIB008 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> In this paper, robust decentralized actuator fault detection and estimation is considered for a class of non-linear large-scale systems. A sliding mode observer is proposed together with an appropriate coordinate transformation to find the sliding mode dynamics. Then, based on the features of the observer, a decentralized fault estimation strategy is proposed using an equivalent output error injection, and a decentralized reconstruction scheme follows by further exploiting the structure of the uncertainty which is allowed to have non-linear bounds. The estimation and reconstruction signals only depend on the available measured information and thus the proposed strategy can work on-line. The theoretical results which have been obtained are applied to an automated highway system. Simulation shows the feasibility and effectiveness of the proposed scheme. <s> BIB009 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> The decentralized adaptive stabilization method is proposed for uncertain interconnected nonlinear systems with unknown non-symmetric dead-zone inputs. The class of systems considered in this paper consists of strict-feedback nonlinear subsystems with unknown non-symmetric dead-zone inputs which interact through their outputs. The unknown nonlinear interaction terms are assumed to be bounded by nonlinear functions with unknown parameters. For the simple controller design, the local controller for each subsystem is systematically derived based on the dynamic surface design technique, without constructing the dead-zone inverse and requiring the bound information of dead-zone parameters (slopes and break-points). All unknown parameters of interconnected nonlinear systems are compensated by the adaptive technique. From Lyapunov stability theorem, it is proved that all signals in the interconnected closed-loop system with decentralized adaptive controllers are semi-globally bounded. Simulation results for tripled inverted pendulums demonstrate the effectiveness of the proposed approach. <s> BIB010 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> This paper presents a variable structure adaptive fuzzy logic control approach for designing a decentralised controller for load frequency control of interconnected power areas. The proposed variable structure adaptive fuzzy logic load frequency controller (AFLFC) is designed to improve the dynamic performance of the frequency and tie-line power flow under a sudden load change in the power areas. The AFLFC provides a stabilising signal to work in conjunction with the original integral controller. The effect of generation rate constraint (GRC) for both areas has been considered in the controller design. Time-domain simulations are performed to demonstrate the effectiveness of the proposed AFLFC. The simulation results show that the proposed AFLFC provides good damping and reduces the overshoot of the different area frequency deviations and also reduces the interchanged tie power. <s> BIB011 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> In this paper, an effective integrated failure detection and identification (FDI) and fault-tolerant control (FTC) technique is developed for a class of nonlinear systems actuated by actuators that may undergo several different types of failures. Assuming that the actuator dynamics are fast, a baseline controller is designed and, using the singular perturbation arguments, shown to achieve the control objective. Typical failures in flight control actuators are considered next, and online FDI algorithms are derived for second-order actuator dynamics with non-measurable actuator rates, and third-order actuator dynamics when only output of the actuator is measurable. The FDI subsystem is decentralized in that an observer is run at each of the actuators, and the parameter estimates are adjusted using only the local information. The major issue of how to use this information to reconfigure the control law and assure the stability of the resulting closed-loop control system is addressed. An adaptive fault-tolerant controller that uses the parameter estimates from the FDI subsystem at every instant is designed next. It is demonstrated that all the signals in the system are bounded and that the tracking error converges to zero asymptotically despite multiple simultaneous actuator failures even in the case of second or third order actuator dynamics. The properties of the proposed FDI-FTC algorithms are evaluated through piloted simulations of the F/A-18 aircraft. <s> BIB012 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> Adaptive trajectory-tracking control of double inverted pendulums (DIPs) connected by a spring is considered in this paper. By incorporating ‘dynamic surface control (DSC)’ approach and ‘minimal learning parameters (MLP)’ algorithm, a systematic procedure for the synthesis of a novel decentralised robust adaptive neural control scheme is developed. Two main advantages of the developed scheme are that: ::: ::: The computational burden has thus been greatly reduced. In addition, the stability in the sense of semi-globally uniform ultimate boundedness (SGUUB) of the closed-loop system is established via Lyapunov stability analysis, and the tracking error can be made arbitrarily small. Simulation results for the trajectory tracking of the DIPs are presented to demonstrate the effectiveness and good transient performance of the proposed scheme. <s> BIB013 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> This paper proposes a novel Lyapunov-based excitation control (LEC) technique to deal with both global asymptotic stability and voltage regulation for the multi-machine power systems. Lyapunov function is built as a quadratic form of the control objectives consisted of active power offset, rotor speed offset and terminal voltage offset. A completely controllable linear system is constructed to design the trajectories of the time-derivatives of the control objectives. Furthermore, time-derivative of Lyapunov function (TLF) is also expressed as a quadratic form of the control objectives. Feedback gains are chosen to guarantee the negative definition of the TLF for any real number except for the equilibrium point. Therefore, the global asymptotic stability is achieved. The design flexibility of the proposed method provides convenience to introduce the voltage feedback signal for maintaining acceptable voltage level. The system voltage is forced to converge to its reference accompanying with the energy attenuation of Lyapunov function. As a result, voltage deviation is eliminated when Lyapunov function is equal to zero. Finally, a Lyapunov-based decentralized excitation controller is developed for the multi-machine power systems, and it is only related to local measurements. Simulations on a six-machine power system have illustrated better performances in comparison to the existing controllers. <s> BIB014 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> In this paper, we propose a decentralized adaptive robust controller for trajectory tracking of robot manipulators. In each local controller, a disturbance observer (DOB) is introduced to compensate for the low-passed coupled uncertainties, and an adaptive sliding mode control term is employed to handle the fast-changing components of the uncertainties beyond the pass-band of the DOB. In contrast to most of the local controllers using DOB for robot manipulators that are based on linear control theory, in this study, by some special nonlinear damping terms, the boundedness of the signals of the overall nonlinear system is first ensured. This paves the way to analyze how the DOB and adaptive sliding mode control play in a cooperative way in each local subsystem to achieve an excellent control performance. Simulation results are provided to support the theoretical results. <s> BIB015 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> Abstract This paper presents an approach, based on dissipative systems theory, to the analysis and control design of interconnected nonlinear processes. The objective is to design distributed feedback controllers to achieve plant-wide stability. Extensions of classical results on the stability of large-scale interconnected systems lead to input–output dissipativity constraints for each subsystems, encoded as supply rates from input to output interconnecting ports. For each subsystem, a parameterized nonlinear feedback controller is designed using nonlinear dissipative inequalities to ensure that the aforementioned dissipativity constraints are met in closed-loop. One focus of this paper is the design of domination-based nonlinear feedback controllers to meet the above interconnection constraints. This paper also presents new results on the construction of storage functions for control affine systems, as a generalization of physics-based approaches to dissipative systems theory. Control of interconnected chemical reactors with a recycle stream is presented throughout the paper to demonstrate the proposed construction. <s> BIB016 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> Satellite formations offer more powerful and robust architectures than a single, traditional large satellite. Investigations into the relative attitude control of satellite formations are important for the applications of future space missions. This article addresses two issues concerning the coordinated attitude control of satellite formation flying: (1) decentralized coordination strategies of multiple satellites flying in formation and (2) coordinated controller design. Under the assumption that the relative position and attitude are known, a fully decentralized variable uncertainties and environmental disturbance torques and achieve the attitude coordination among the multiple satellites flying in formation. <s> BIB017 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> Autonomous decentralized control has attracted considerable attention because it enables us to understand the adaptive and versatile locomotion of animals and facilitates the construction of truly intelligent artificial agents. Thus far, we have developed a snake-like robot (HAUBOT I) that is driven by a decentralized control scheme based on a discrepancy function, which incorporates phasic control. In this paper, we investigate a decentralized control scheme in which phasic and tonic control are well coordinated, as an extension of our previous study. To verify the validity of the proposed control scheme, we apply it to a snake-like robot (HAUBOT II) that can adjust both the phase relationship between its body segments and the stiffness at each joint. The results indicate that the proposed control scheme enables the robot to exhibit remarkable real-time adaptability over various frictional and inclined terrains. These findings can potentially enable us to gain a deeper insight into the autonomous decentralized control mechanism underlying the adaptive and resilient locomotion of animals. <s> BIB018 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> A major challenge to developing functional electrical stimulation (FES) systems for paraplegic walking and widespread acceptance of these systems is the design of a robust control strategy that provides satisfactory tracking performance. The systems need to be robust against time-varying properties of neuromusculoskeletal dynamics, day-to-day variations, subject-to-subject variations, external disturbances, and must be easily applied without requiring offline identification during different experimental sessions. Another major problem related to walker-assisted FES-activated walking concerns the high metabolic rate and upper body effort that limit the clinical applications of FES systems. In this paper, we present a novel decentralized modular control framework for robust control of walker-assisted FES-activated walking. For each muscle-joint dynamics, an independent module control is designed, and the dynamics of the plant are identified online. This process requires no prior knowledge about the dynamics of the plant to be controlled and no offline learning phase. The module is based on adaptive fuzzy terminal sliding mode control and fuzzy logic control. The module control adjusts both pulse-amplitude and pulsewidth of the stimulation signal in such a way that upper body effort is minimized and the lower extremity walking pattern lies within a defined boundary of the reference trajectory. The proposed control strategy has been evaluated on three paraplegic subjects. The results showed that accurate tracking performance and smooth walking pattern were achieved. This favorable performance was obtained without requiring offline identification, manual adjustments, and predefined ON/OFF timing of the muscles. <s> BIB019 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> The problem of controlling a string of vehicles moving in one dimension is considered so that they all follow a lead vehicle with constant spacing between successive vehicles. Due to practical design and implementation, the negative effect of the tracking lag parameter is taken into account. A hierarchical platoon controller design framework is established comprising a feedback linearization controller at the first layer and a decentralized bidirectional control controller at the second layer. The stability criterion is examined by using a partial differential equation approximation in the limit of the number of vehicles subjected to unequal asymmetry in position and velocity feedback. For disturbance attenuation, string stability analysis is also examined. At the end of the paper, simulations are given to show the efficiency of the proposed results. <s> BIB020 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> This paper is concerned with the stabilization problem for a class of large-scale nonlinear time-delay systems in lower triangular form. The uncertain nonlinearities are assumed to be bounded by continuous functions of the outputs or delayed outputs multiplied by unmeasured states or delayed states. An observer based output feedback control scheme is proposed using the dynamic gain control design approach. Based on Lyapunov stability theory, global asymptotic stability of the closed-loop control system is proved. Contrary to many existing control designs for lower triangular nonlinear systems, the celebrated backstepping method is not utilized here. An example is finally given to demonstrate the effectiveness of the proposed design procedure. <s> BIB021 </s> Decentralised control for complex systems - an invited survey <s> Applications <s> The problem of formation control of a team of mobile robots based on the virtual and behavioral structures is considered in this paper. In the virtual structure, each mobile robot is modeled by an electric charge. The mobile robots move toward a circle, and due to repulsive forces between the identical charges, regular polygon formations of the mobile robots will be realized. For swarm formation, a virtual mobile robot is located at the center of the circle, and other mobile robots follow it. In the introduced approach, each mobile robot finds its position in the formation autonomously, and the formation can change automatically in the case of change in the number of the mobile robots. This paper also proposes a technique for avoiding obstacles based on the behavioral structure. In this technique, when a mobile robot gets close to an obstacle, while moving toward its target, a rotational potential field is applied to lead the mobile robot to avoid the obstacle, without locating in local minimum positions. <s> BIB022
Decentralised control is powerful in dealing with large scale interconnected systems and thus has been widely used in various application areas. As a typical interconnected system, multi-machine power systems have received much attention and many decentralised control strategies have been developed including state feedback, static output feedback and dynamical output feedback control approaches BIB005 BIB014 . A multi-link robot can be modelled as an interconnected system, and various decentralised control schemes have been developed to study the control of robot manipulators BIB006 BIB015 . Decentralised H 2 /H ∞ tracking control has been applied to car-like mobile robots in BIB007 and a decentralised cooperative control scheme has been developed for a team of mobile robots in BIB022 . A variable structure adaptive fuzzy logic control approach for designing a decentralised controller for load frequency control of interconnected power areas in BIB011 to improve the dynamic performance of the frequency and tie-line power flow under a sudden load change in the power areas. In addition, decentralised control has been applied to other practical systems such as a coupled inverted pendulum system BIB010 BIB013 , wind turbine BIB008 , vibration isolation system BIB004 ), a platooning vehicles system BIB009 BIB020 , river pollution control BIB001 , chemical process control BIB016 BIB021 BIB003 , satellite attitude control BIB017 and flight control BIB002 BIB012 to name a few. In recent years, decentralised control strategies have been employed in bio-engineering systems BIB018 BIB019 . Widening application of decentralised control will stimulate new developments in the area.