Showing papers in "Iet Control Theory and Applications in 2011"

Non-linear disturbance observer-based robust control for systems with mismatched disturbances/uncertainties

Abstract: Robust control of non-linear systems with disturbances and uncertainties is addressed in this study using disturbance observer-based control (DOBC) technique. In this framework, the `disturbance` is a generalised concept, which may include external disturbances, unmodelled dynamics and system parameter perturbations. The existing DOBC methods were only applicable for the case where disturbances and uncertainties satisfy so-called matching condition, that is, they enter the system in the same channel as the control inputs. By appropriately designing a disturbance compensation gain vector in the composite control law, a non-linear disturbance observer-based robust control method is proposed in this study to attenuate the mismatched disturbances and the influence of parameter variations from system output channels. The proposed method is applied to a missile system with non-linear dynamics in the presence of various uncertainties and external disturbances. Simulation shows that, compared with the widely used non-linear dynamic inversion control (NDIC) and NDIC plus integral action methods, the proposed method provides much better disturbance attenuation ability and stronger robustness against various parameter variations.

Consensus of heterogeneous multi-agent systems

Abstract: In this study, the consensus problem of heterogeneous multi-agent system is considered. First, the heterogeneous multi-agent system is proposed which is composed of first-order and second-order integrator agents in two aspects. Then, the consensus problem of heterogeneous multi-agent system is discussed with the linear consensus protocol and the saturated consensus protocol, respectively. By applying the graph theory and Lyapunov direct method, some sufficient conditions for consensus are established when the communication topologies are undirected connected graphs and leader-following networks. Finally, some examples are presented to illustrate the theoretical results.

Dynamic consensus of linear multi-agent systems

Abstract: This study concerns the consensus of a network of agents with general linear or linearised dynamics, whose communication topology contains a directed spanning tree. An observer-type consensus protocol based on the relative outputs of the neighbouring agents is adopted. The notion of consensus region is introduced, as a measure for the robustness of the protocol and as a basis for the protocol design. For neutrally stable agents, it is shown that there exists a protocol achieving consensus together with a consensus region that is the entire open right-half plane if and only if each agent is stabilisable and detectable. An algorithm is further presented for constructing such a protocol. For consensus with a prescribed convergence speed, a multi-step protocol design procedure is given, which yields an unbounded consensus region and at the same time maintains a favourable decoupling property. Finally, the consensus algorithms are extended to solve the formation control problems.

Consensus problems for linear time-invariant multi-agent systems with saturation constraints

Abstract: A class of networked multi-agent systems is studied in this study where each agent has an identical dynamics of a simple integrator and the topology of the connections is fixed. It is proved that, when there are saturation constraints, a general consensus protocol widely used in the literatures for this class of multi-agent systems remains valid. As an extension, a 'bang-bang' type of consensus protocol is proposed to achieve the finite-time consensus, which relaxes the previous undirected connection assumption.

Hierarchical platoon control with heterogeneous information feedback

Abstract: The problem of autonomous platoon control via wireless communication network is studied in this study. Firstly, a novel hybrid model is established for the platoon's longitudinal movement, where disturbances of lead vehicle acceleration and wind gust, parameter uncertainties and intermediate uncertainties induced by communication network (e.g. time delay, quantisation and packet dropout) are given full considerations and involved in the model for the first time. Then, the authors establish a hierarchical platoon controller design framework comprising a feedback linearisation controller at the first layer and a guaranteed cost H ∞ controller at the second layer. By reducing the non-linear system to a linear model using the top layer feedback linearisation controller, a robust H ∞ controller, the kernel controller, is designed utilising novel techniques in robust control of time-delay systems. For the general objective of disturbance attenuation, string stability and robust platoon control to be achieved simultaneously, the robust H ∞ controller is complemented by additional conditions established for guaranteeing string stability and zero steady-state spacing errors. Simulations are given to show the efficiency of the proposed results.

Back-stepping sliding mode control for missile systems based on an extended state observer

Abstract: A novel approach combining the sliding mode control and extended state observer (ESO) is proposed for attitude control of a missile model which is non-linear in aerodynamics. Combining the back-stepping technique, the corresponding sliding mode controller is designed to guarantee the state variables of the closed-loop system to converge to the reference state with the help of the ESO by estimating the unknown variable. Also, simulation results are presented to illustrate the effectiveness of the control strategy.

Consensus control for high-order multi-agent systems [Brief Paper]

Abstract: In this study, the lth order (l ≥ 2) consensus problem for multi-agent systems is considered, which generalises the existing second-order consensus algorithm. A linear consensus protocol is proposed for solving such a consensus problem, which includes two parts: a feedback controller and interactions from the neighbours. A sufficient and necessary condition for consensus in high-order systems is obtained. As special cases, criteria for second- and third-order systems are given, in which the exact relationship between feedback gain and system parameters is established. Finally, numerical simulations are reported to illustrate the effectiveness of this protocol.

Attitude control of a quadrotor aircraft subject to a class of time-varying disturbances

Abstract: Here, the attitude control of a quadrotor aircraft subject to a class of disturbances is studied. Unlike disturbances mentioned in most of the existing literature, the disturbance considered here is time varying and non-vanished. An extended observer is designed to estimate the disturbance by treating it as a new unknown state. Based on the estimation, a feedback controller with a sliding mode term is designed to stabilise the attitude of the quadrotor. Furthermore, to avoid the discontinuity of the control law caused by the sliding mode term, a modified sliding mode term is designed. The resulting continuous feedback controller makes the attitude error uniformly ultimate bounded. Theoretical results are confirmed by numerical simulations.

Brief paper: Lleast squares-based recursive and iterative estimation for output error moving average systems using data filtering

Abstract: For parameter estimation of output error moving average (OEMA) systems, this study combines the auxiliary model identification idea with the filtering theory, transforms an OEMA system into two identification models and presents a filtering and auxiliary model-based recursive least squares (F-AM-RLS) identification algorithm. Compared with the auxiliary model-based recursive extended least squares algorithm, the proposed F-AM-RLS algorithm has a high computational efficiency. Moreover, a filtering and auxiliary model-based least squares iterative (F-AM-LSI) identification algorithm is derived for OEMA systems with finite measurement input-output data. Compared with the F-AM-RLS approach, the proposed F-AM-LSI algorithm updates the parameter estimation using all the available data at each iteration, and thus can generate highly accurate parameter estimates.

Robust adaptive formation control of underactuated autonomous surface vehicles with uncertain dynamics

Abstract: In this study, a neural network-based control design is developed for underactuated autonomous surface vehicles moving in a leader–follower formation, in the presence of uncertain leader dynamics as well as uncertain local dynamics caused by coriolis and centripetal force, hydrodynamic damping, unmodelled hydrodynamics and disturbances from environment. By online approximating the uncertain leader dynamics, the proposed method only uses the measurements of line-of-sight range and angle by local sensors, no other information about leader is required for control implementation. The proposed control law is model-independent that does not rely on the accurate model of the vehicle, which is difficult to obtain in practice. Based on the Lyapunov synthesis, it is proved that with the developed neural formation controller, all the tracking errors converge to a small neighbourhood of zero. Simulation results demonstrate the effectiveness of the method.

Note on fractional-order proportional–integral–differential controller design

Abstract: This study deals with the design of fractional-order proportional-integral-differential (PID) controllers. Two design techniques are presented for tuning the parameters of the controller. The first method uses the idea of the Ziegler-Nichols and the A-stro-m-Ha-gglund methods. In order to achieve required performances, two non-linear equations are derived and solved to obtain the fractional orders of the integral term and the derivative term of the fractional-order PID controller. Then, an optimisation strategy is applied to obtain new values of the controller parameters, which give improved step response. The second method is related with the robust fractional-order PID controllers. A design procedure is given using the Bode envelopes of the control systems with parametric uncertainty. Five non-linear equations are derived using the worst-case values obtained from the Bode envelopes. Robust fractional-order PID controller is designed from the solution of these equations. Simulation examples are provided to show the benefits of the methods presented.

Introduction to single-input, single-output fractional control

Abstract: This is a tutorial study to introduce fractional control to a reader with a background in control theory. Fractional controllers are those making use of fractional-order derivatives and integrals, and have been receiving increased attention over the last few years because of the robust performance (in the face of plant gain variations and even plant uncertainties in general) they can achieve. The study covers the fundamentals of the theory of derivatives and integrals with arbitrary real or complex orders, fractional transfer functions and their approximations, identification of fractional transfer function models from experimental data, first- and second-generation Crone controller, fractional proportional-integral-derivative (PID) control and third-generation Crone control.

Robust fault-tolerant control for spacecraft attitude stabilisation subject to input saturation

Abstract: This study investigates the robust fault-tolerant attitude control of an orbiting spacecraft with a combination of unknown actuator failure, input saturation and external disturbances. A fault-tolerant control scheme based on variable structure control is developed that is robust to the partial loss of actuator effectiveness, where the actuators experience a reduced actuation but are still active. The results are then extended to the case in which some of the actuators fail completely, although some redundancy in actuation is assumed. In contrast to traditional fault-tolerant control methods, the proposed controller does not require knowledge of the actuator faults and is implemented without explicit fault detection, separation and accommodation processes. Moreover, the designed controller rigorously enforces actuator saturation constraints. The associated stability proof is constructive and develops a candidate Lyapunov function that shows the attitude and the angular velocities converge asymptotically to zero. Simulation studies are used to evaluate the closed-loop performance of the proposed control solution and illustrate its robustness to external disturbances, unknown actuator faults and even input saturation.

Distributed adaptive consensus algorithm for networked Euler-Lagrange systems

Abstract: Most consensus protocols developed in the past are for linear-integrator systems or deterministic non-linear systems. Here, the authors study the state consensus for non-point, non-linear networked Euler-Lagrange systems with unknown parameters. Specifically, state consensus problems with both coupling time delay and switching topology are investigated. By establishing a unified architecture based on the passivity property, adaptive consensus protocols are developed. It is shown that state consensus is reachable despite the unknown parameters, and the estimation errors of these parameters converge to zero. Furthermore, by introducing the leader-follower architecture, the authors show that each agent will converge its origin. Finally, a numerical example is given to illustrate the effectiveness of the proposed algorithms.

Spectrum-based stability analysis and stabilisation of systems described by delay differential algebraic equations

Abstract: An eigenvalue-based framework is developed for the stability analysis and stabilisation of coupled systems with time-delays, which are naturally described by delay differential algebraic equations. The spectral properties of these equations are analysed and a numerical method for computing characteristic roots and stability assessment is presented, thereby taking into account the effect of small delay perturbations on stability. Subsequently, the design of stabilising controllers with a prescribed structure or order is addressed, based on a direct optimisation approach. The effectiveness of the approach is illustrated with numerical examples. All algorithms have been implemented in publicly available software.

Continuous-time model identification of fractional-order models with time delays

Abstract: Modelling of real physical systems having long memory transients and infinite dimensional structures using fractional-order dynamic models has significantly attracted interest over the last few years For this reason, many identification techniques both in the frequency domain and time domain have been developed to model these fractional-order systems However, in many processes time delays are also present and estimation of time delays along with continuous-time fractional-order model parameters have not been addressed anywhere This study deals with the continuous-time model identification of fractional-order system models with time delays In this study, a new linear filter is introduced for simultaneous estimation of all model parameters for commensurate fractional-order system models with time delays The proposed method simultaneously estimates time delays along with other model parameters in an iterative manner by solving simple linear regression equations For the case when the fractional order is unknown, we also propose a nested loop optimisation method where the time delay along with other model parameters are estimated iteratively in the inner loop and the fractional order is estimated in the non-linear outer loop The applicability of the developed procedure is demonstrated by simulations on a fractional-order system model by doing Monte Carlo simulation analysis in the presence of white noise The proposed algorithm has also been applied to identify a process of thermal diffusion in a wall in simulation, which are characterised by fractional-order behaviour

Brief paper swarm stability of high-order linear time-invariant swarm systems

Abstract: In this study the swarm stability problem of high-order linear time-invariant (LTI) swarm systems with directed graph topology is dealt with. Consensus can be regarded as a specific type of swarm stability problem. Necessary and sufficient conditions for both swarm stability and consensus are presented. These conditions depend on the graph topology, the dynamics of agents and the interactions among the neighbours. Simulation instances are shown to illustrate the theoretical results.

Lateral directional fractional order (PI) π control of a small fixed-wing unmanned aerial vehicles: controller designs and flight tests

Abstract: In this study, a fractional order (PI)λ controller is developed and implemented to improve the flight control performance and robustness of a small fixed-wing unmanned aerial vehicle (UAV). The decoupled roll-channel control is realised under certain conditions and tested using the designed controllers in this study. The inner closed-loop system of the roll-channel is approximately identified as a first-order plus time delay model using the flight test data. For comparison purpose, an integer-order PI controller is designed following the modified Ziegler–Nichols (MZNs) tuning rule, based on this identified roll-channel control model. According to three design pre-specifications, the integer-order proportional integral derivative (PID), fractional-order PIλ and (PI)λ controllers are designed for the roll-channel flight control system of a small fixed-wing UAV. These three designed controllers share the same gain crossover frequency and phase margin settings for fair comparisons. From both simulation and real flight experiments, the two designed fractionalorder controllers outperform the MZNs PI and the designed integer-order PID controllers. The designed (PI)λ controller can achieve even better performance than the designed PIλ controller.

Optimal instrumental variable method for closed-loop identification

Abstract: This study presents in a new unified way, optimal instrumental variable methods for identifying discrete-time transfer function models when the system operates in a closed loop. The conditions for the optimal design of prefilters and instruments depending on common model structures are analysed and different approaches are developed according to whether the controller is known or not. The performance of the proposed approaches is evaluated by Monte-Carlo analysis in comparison with other alternative closed-loop estimation methods.

Hybrid impulsive control for switched singular systems

Abstract: In this study, the authors investigate the control problem of switched singular systems aiming to compress their inconsistent state jumps when switch occurs between two different singular subsystems. The proposed hybrid impulsive controller consists of a feedback controller and an impulsive controller. With introduction of the impulsive controller, the state at each switching instant for the closed-loop system can be changed. Based on the given controller structure, some sufficient conditions are derived under which the closed-loop system is admissible (regular, impulse free and stable) and such a controller has the capability of eliminating or minimising the instantaneous state jumps at switching instants. The validity and advantage of the proposed hybrid impulsive controller are illustrated using two examples.

Leader-following consensus for multi-agent systems via sampled-data control

Abstract: In this article, the authors study a leader-following consensus problem for multi-agent systems in a sampled-data setting. A distributed coordination algorithm based on sampled-data control is proposed to track the considered leader. By employing M -matrix theory, the authors derive sufficient conditions on the sampling period and control parameters to ensure that the tracking errors are bounded. Numerical simulations are presented to illustrate the effectiveness of the theoretical results. Moreover, some previous results concerning the leader-following problem with switched coupling topology are improved.

Disturbance observer-based control of non-linear haptic teleoperation systems

Abstract: Teleoperation systems are subject to different types of disturbances. Such disturbances, when unaccounted for, may cause poor performance and even instability of the teleoperation system. This study presents a novel non-linear bilateral control scheme using the concept of `disturbance observer-based control` for non-linear teleoperation systems. Lumping the effects of dynamic uncertainties and external disturbances into a single disturbance term enables us to design a disturbance observer to suppress these disturbances and alleviate their adverse effects on the teleoperation system. A disturbance observer-based control law is proposed for non-linear teleoperation systems which will guarantee global asymptotic force tracking and global exponential position and disturbance tracking when the bilateral teleoperation system is experiencing slow-varying disturbances. In the case of fast-varying disturbances, the tracking errors are shown to be globally uniformly ultimately bounded, with an ultimate bound that can be made as small as desired using the design parameters. Simulations are presented to show the effectiveness of the proposed approach.

Robust Kalman filtering for uncertain state delay systems with random observation delays and missing measurements

Abstract: The robust Kalman filtering problem is investigated for uncertain stochastic systems with time-invariant state delay d 0 , bounded random observation delays and missing measurements. The described model is generalised to the case that d 0 ≠ d 1 , where d 1 denotes the upper bound of random observation delays. The random delays and missing measurements are described by multiple Bernoulli random processes and their probabilities are assumed to be known. For robust performance, stochastic parameter perturbations are considered. Unlike the system augmentation approach, the robust Kalman filtering is derived in the linear minimum variance sense by using the innovation analysis approach, and the dimension of the designed filter is the same as the original systems. Moreover, the performance of the designed filter is dependent on the probabilities of delays and missing measurements at each step. An illustrative example is presented to demonstrate the effectiveness of the proposed design method.

Cooperative control strategy for multiple photovoltaic generators in distribution networks

Abstract: A cooperative control strategy was provided to regulate the active and reactive power outputs of multiple photovoltaic (PV) generators installed on a distribution network. The proposed control strategy not only makes a group of PVs converge and operate at the same ratio of available power, but also regulates the total active and reactive power outputs of the PVs such that the active power across a concerned line and the voltage of a critical bus are kept to a referenced value. The stability of the closed-loop dynamical system was analysed by considering some special properties of classical distribution networks, and the minimal requirement of the communication topology among the PVs was provided. Simulations on a radial distribution power system network were provided to verify the validness of the proposed control strategy.

Adaptive discrete-time iterative learning control for non-linear multiple input multiple output systems with iteration-varying initial error and reference trajectory

Abstract: Most of the available results in iterative learning control (ILC) hitherto have considered the ILC systems with fixed initial error and iteration-invariant reference trajectory. An adaptive discrete-time ILC scheme is presented to deal with the ILC problem of non-linear multiple input multiple output systems with iteration-varying initial error and reference trajectory. The designed adaptive ILC law learns parametric system dynamics and pursues the iteration-varying reference trajectory tracking from iteration to iteration. It can drive the ILC tracking error to zero asymptotically beyond the initial time step, and keep all adjustable parameters and system signals bounded as the number of iteration approaches infinity. Numerical example is given to illustrate the effectiveness of the adaptive ILC scheme.

Identification of a thermal system using continuous linear parameter-varying fractional modelling

Abstract: In this study, the authors present an identification procedure of a non-linear thermal system that exhibits a diffusive interface. Indeed, the system deals with heat conduction in a homogeneous material through a wall. Heating front face temperature dynamics when random heat flux is applied to the wall's front face area are considered. If the thermophysical properties of the material depend on the temperature, the heat equation is not linear anymore. Hence, in order to explicitly take into account the dependence of the system dynamics on the temperature, a fractional continuous linear parameter-varying model is used and determined thanks to a local approach composed of two steps. First, the authors show that for small heating temperature variations, the heat diffusion can be modelled with the help of a local fractional model based on a fractional integrator operator of order (1/2), which acts only over a limited spectral band. The transfer function parameters of this model are estimated thanks to an output-error technique at different initial temperature-operating points. Secondly, the transfer function parameters dependence with respect to the initial temperature of the material is obtained by direct polynomial interpolation. Simulations using ARMCO iron are used to demonstrate the performances of the proposed local approach.

Observer-based adaptive fuzzy backstepping dynamic surface control for a class of non-linear systems with unknown time delays

Abstract: An adaptive fuzzy backstepping control approach is considered for a class of non-linear systems with unknown time delays and immeasurable states. Fuzzy logic systems are used to approximate the unknown non-linear functions, and a fuzzy state observer is designed for estimating the immeasurable states. By combining the adaptive backstepping technique and dynamic surface control technique, an adaptive fuzzy output-feedback backstepping control approach is developed. The proposed control method not only overcomes the problem of ‘explosion of complexity’ inherent in typical backstepping design approaches but also overcomes the problem of unavailable state measurements. It is proved that all the signals of the closed-loop adaptive control system are semi-globally uniformly ultimately bounded, and the observer and tracking errors converge to a small neighbourhood of the origin with appropriate choice of design parameters. Simulation results are provided to show the effectiveness of the proposed approach.

Time-constant robust analysis of a fractional order [proportional derivative] controller

Abstract: This study discusses fractional order [proportional derivative] (FO[PD]) controller tuning rules for robustness motion control systems. According to the proposed method, the controller is designed simultaneously satisfying the robustness property with respect to time-constant variation and the desired phase margin criteria. In this study, the authors focus on the first-order plus time delay model with an integrator. A systematic tuning rule is developed for the FO[PD] controller. Numerical computation of the tuning formulae and the relationship between design specifications and design parameters are both discussed. For simplifying the computation and achieving online tuning, the crossover frequency has been discussed. Experimental results are included to validate the proposed tuning method.

Intelligent double integral sliding-mode control for five-degree-of-freedom active magnetic bearing system

Abstract: This study presents a decentralised intelligent double integral sliding-mode control (IDISMC) system, which consists of five IDISMCs, to regulate and stabilise a fully suspended five-degree-of-freedom (DOF) active magnetic bearing (AMB) system The system structure and drive system with differential driving mode (DDM) are introduced first Then, the decoupled dynamic model of the five-DOF AMB is analysed for the design of the decentralised control Moreover, a decentralised integral sliding-mode control (ISMC) system is designed based on the decoupled dynamic model to control the five-DOF AMB considering the existences of the uncertainties Furthermore, since the control characteristics of the five-DOF AMB are highly non-linear and time varying, the decentralised IDISMC system is proposed to further improve the control performance of the five-DOF AMB In each IDISMC, the adopted double integral sliding surface reinforces the control law with the integral (I) control feature In addition, the control gains of the IDISMC can be adjusted on-line and the system uncertainty can also be observed simultaneously by using of a modified proportional–integral–derivative neural network (MPIDNN) observer Thus, the proposed IDISMC combines the merits of the ISMC, adaptive control and neural network (NN) Finally, the experimental results illustrate the validities of the proposed control systems using various operating conditions

Stability analysis of polynomial fuzzy-model-based control systems under perfect/imperfect premise matching

Abstract: This study presents an improved sum-of-squares (SOS)-based stability analysis result for the polynomial fuzzy-model-based control system, formed by a polynomial fuzzy model and a polynomial fuzzy controller connected in a closed loop. Two cases, namely perfect and imperfect premise matching, are considered. Under the perfect premise matching, the polynomial fuzzy model and polynomial fuzzy controller share the same premise membership functions. While different sets of membership functions are employed, it falls into the case of imperfect premise matching. Based on the Lyapunov stability theory, improved SOS-based stability conditions are derived to determine the system stability and facilitate the controller synthesis. Simulation examples are given to verify the stability analysis results and demonstrate the effectiveness of the proposed approach.