http://www.ifac-papersonline.net/ en Copyright 05:38 PM Tuesday 07, 2012 IFAC-PapersOnline http://www.ifacpapersonline.com 05:38 PM Tuesday 07, 2012 05:38 PM Tuesday 07, 2012 ET webmaster@ifac-papersonline.net webmaster@ifac-papersonline.net http://www.ifac-papersonline.net/Detailed/39882.html 04:00 PM Wednesday 31, 1969 In this paper we develop a convex optimization approach to stability analysis of linear systems with interval time-varying delay, by using the standard and the delay-partitioning-based Lyapunov-Krasovskii Functionals (LKFs). We introduce novel LKFs with matrices that depend on the time-delays. These functionals allow the derivation of the condition for stability, which depend on both, the upper and the lower bounds on delay derivatives. A numerical example is given which illustrates the efficiency of the new stability criteria. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39890.html 04:00 PM Wednesday 31, 1969 The paper presents a new method for the design of output regulators for a class of nonlinear systems characterized by a possibly unstable zero dynamics. In the case of linear systems, the method is always applicable if the controlled plant has a multiple zero at the origin and all other zeros with negative real part. If the plant has zeros with positive real part, the method is applicable if the frequencies which characterize the harmonic components of the exogenous input exceed a minimal value determined by the gain needed to solve an auxiliary stabilization problem. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39892.html 04:00 PM Wednesday 31, 1969 In this paper, we develop a combined sliding mode - model reference adaptive control (MRAC) scheme for a class of delayed nonlinear dynamic systems with actuator failure which is robust with respect to actuator failures, an unknown plant delay, to a nonlinear perturbation, and to an external disturbance with unknown bounds. An appropriate Lyapunov-Krasovskii type functional with ``virtual'' adaptation gains is introduced to design the adaptation algorithms, and to prove stability. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39847.html 04:00 PM Wednesday 31, 1969 The hypothalamic-pituitary-adrenal (HPA) axis is a neuroendocrine control system regulating the stress response of the human body as well as affecting mood and cognition. HPA disfunction has been associated with a variety of disorders including Fibromyalgia, and chronic fatigue syndrome (CFS). Recently, a lumped parameter model of the HPA axis which includes the effect of pharmacological agents has been proposed. In this work, an analysis of the HPA axis dynamics near rest points is used to develop an approach for robust control of the HPA system. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39859.html 04:00 PM Wednesday 31, 1969 A controller for a spacecraft mounted flexible antenna is designed. The antenna plant model has a certain degree of uncertainty and disturbances from the host spacecraft are transmitted to the antenna and need to be attenuated, while a high degree of pointing accuracy must be achieved. The design concept explored herein entails feedforward control to slew the antenna. Feedback control is then exclusively used to compensate for plant uncertainty and to reject the disturbance signals. A tight feedback control loop is designed to meet performance specifications while minimizing the control gains. It is shown that the integration of feedforward control action and feedback compensation produces better responses than the implementation of either individual control system. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39866.html 04:00 PM Wednesday 31, 1969 In this paper continuous piecewise linear (CPWL) approximations of non-linear affine control systems are considered. An optimal control context allows to determine asymptotic error bounds between real and approximate control systems. Then it is possible to apply the controller designed using the CPWL approximation to the real vector field with guarantee of stability. An example is presented to show the potential applications of these ideas. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39846.html 04:00 PM Wednesday 31, 1969 http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39854.html 04:00 PM Wednesday 31, 1969 The paper presents a method for the automated design of robust PID controllers in continuous and discrete time. In the continuous case it systematically improves the settling time, i.e. the real parts of all closed eigenvalues are smaller than σ0 . In the discrete case also the damping is improved. For fixed KP the stable region in the (KD,KI)-plane consists of convex polygons. For several representive operation conditions of the plant the intersection of such polygons represents the simultaneously stabilizing controllers. For a grid on KP the centers of gravity of the polynomials and the corresponding sigma0 -values are computed. The best sigma0 -value from this list is a feasible specification for the next design iteration in a transformed coordinate system. Application to a complex track-guided bus problem shows good performance after a few automatic iterations. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39889.html 04:00 PM Wednesday 31, 1969 Backlash and friction reciprocal effect on limit cycle existence is preliminarily investigated for the basic 2nd order servo system with simplified models of Coulomb friction and backlash. By the describing function method,it is shown for this case that the lower boundary for limit cycle existence, in the backlash gap [rad] - dry friction moment [Nm] plane, is a straight line starting in the origin. Besides, a novel condition is formulated for this system to avoid limit cycle excitation for any combinations of dry friction and backlash gap. This condition can be valuable for control systems design with fully unknown or uncertain values of backlash gap and dry friction. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39864.html 04:00 PM Wednesday 31, 1969 The paper considers high purity distillation control of a binary mixture in a tray column. The approach is based on a reduced model of the column in descriptor form. Model reduction is archived by considering dynamics of concentration waves along the tray coordinate. The resulting descriptor model is augmented by weighting systems in a mixed-sensitivity set-up. Specialization of a recent general quadratic performance approach for descriptor systems to the singular H-infinity setting is the basis for a LMI based controller computation. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39905.html 04:00 PM Wednesday 31, 1969 Approaches for clearance of flight control laws for time varying pilot inputs, using optimisation methods are presented. These approaches are based on simulation of nonlinear aircraft models, parameterisation of input pilot signals and solving the resulting nonlinear optimisation problem. The purpose is to find the pilot signals that result in the worst possible behaviour of the aircraft. The studied pilot signals are lower order piecewise polynomials, and the resulting parameter set is a real valued finite dimensional set on which global and local optimisation methods are studied. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39904.html 04:00 PM Wednesday 31, 1969 Lyapunov-based analysis is a widely used tool to assess robust stability of dynamic systems affected by parametric uncertainties. When the uncertain parameters are constant, it is well known that parameter-dependent Lyapunov functions allow one to reduce conservatism of sufficient conditions for robust stability. In this paper, we consider several different techniques for robustness analysis of systems with LFR uncertainty structure, based on parameter-dependent Lyapunov functions. Such techniques are applied to LFR systems derived from real-world flight control schemes, with the aim of finding the largest region in the uncertain parameter space and/or in the flight envelope, for which robust stability is guaranteed. The trade off between conservatism and computational burden is analyzed for each considered technique. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39894.html 04:00 PM Wednesday 31, 1969 In this paper, a robust control strategy based on the uncertainty and disturbance estimator (UDE) is extended and applied to uncertain linear continuous-time systems with state delays and disturbances. It does not require the knowledge of the bounds of uncertainties and disturbances. Both known and unknown delay cases are considered. In the case of an unknown time delay, the term with the delay is treated as an additional disturbance to the system. The proposed algorithm shows excellent tracking and disturbance rejection capabilities. The robust stability of LTI-SISO systems is analysed and simulations are given to show the effectiveness of the UDE-based control. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39897.html 04:00 PM Wednesday 31, 1969 We reconsider and advance the analysis of structural properties (controllability and observability) of a class of linear Networked Control Systems (NCSs). We model the NCS as a periodic system with limited communication where the non updated signals can either be held constant (the zero-order-hold case) or reset to zero. Periodicity is dealt using the lifting technique. We prove that a communication sequence that avoids particularly defined pathological sampling rates and updates each actuator signal only once is sufficient to preserve controllability (and observability for the dual problem of sensor scheduling). These sequences can be shorter than previously established and we set a tight lower bound to them. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39851.html 04:00 PM Wednesday 31, 1969 Often, when a controlled plant is modified, e.g. if a new sensor or actuator becomes available, it is desirable to retain the existing controllers and apply the new control capabilities in a gradual, online fashion rather than decommissioning the entire existing system and replacing it with the new system. This paper combines recently developed methods for gain scheduling based on closed loop identification and controller reconfiguration. The resulting scheme allows for introducing new control components in a smooth manner, providing stability guarantees during the transition phase, while retaining the original control structure. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39885.html 04:00 PM Wednesday 31, 1969 This paper is dedicated to the stability analysis of a class of uncertain distributed delay systems, the kernel of which can be modeled as a polynomial function. The results are constructed by rewriting the system as an uncertain interconnected model. Appropriate robust control tools, i.e. quadratic separation, are then used to address the stability issue. To this end, some relations that highlight relevant characteristics of the delayed term are added to the interconnected model leading then to the conservatism reduction. Finally, numerical examples show the effectiveness of the proposed method. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39896.html 04:00 PM Wednesday 31, 1969 We consider coupled systems consisting of an infinite-dimensional part (a PDE system) and a finite-dimensional part (an ODE system) connected in feedback, such as for example the well-known SCOLE system (a beam with a rigid body attached at one end). The external world interacts with the coupled system via the finite-dimensional part, which receives the external input and sends out the output. The infinite-dimensional part is assumed to be a system node. Under several assumptions, we derive well-posedness and exact controllability results for such coupled systems. The first main result concerns the case when the input signal of the finite-dimensional part is the difference between the external input and the feedback signal. The second main result allows a more general structure for the finite-dimensional part. We also prove a result for the approximate controllability of the coupled system. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39848.html 04:00 PM Wednesday 31, 1969 Recently it was shown how one can reformulate the MPC problem for LPV systems to a series of mpLPs by a closed-loop minimax MPC algorithm based on dynamic programming. A relaxation technique is employed to reformulate constraints which are polynomial in the scheduling parameters to parameter-independent constraints. The algorithm allows the computation of explicit control laws for LPV systems and enables the controller to exploit information about the scheduling parameter. This improves the control performance compared to a standard robust approach where no uncertainty knowledge is used, while keeping the benefits of fast online computations. In this paper the explicit LPV-MPC scheme is extended to incorporate limits on the rate of parameter variation in the controller design. Taking these limits into account can further improve control performance since impossible trajectories of the scheduling parameter do not have to be considered during control law computations. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39888.html 04:00 PM Wednesday 31, 1969 Hysteresis compliance arises from elasto-plastic properties in mechanical systems and represents a strict nonlinearity with memory effect, which handicaps the predictability of state trajectories and the system controllability in general. This paper presents an inverse model approach for feed-forward compensation of hysteresis compliance. The hysteresis compliance of an electro-mechanical system with elasticities is modeled by means of a well-known Preisach formalism. The model specific Preisach density function is identified from an appropriate drive experiment, which provides a set of major and minor hysteresis loops. The inverse Preisach model is attained using the inverse function derivative and the first order Taylor expansion at working point. The efficiency of proposed inverse model approach is shown in the simulation and affirmed with experiments. The proposed control strategy is well suitable for a broad class of hysteresis phenomena independent on their cause-effect relationship. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39903.html 04:00 PM Wednesday 31, 1969 The flight control law clearance problem can be formulated as a robustness analysis problem, where a set of suitably defined clearance criteria must be checked to lie within certain limits for all admissible variations of aircraft parameters, pilot inputs and all flight conditions. The idea of optimisation based worst case clearance is to use available and efficient optimisation methods to find those parameters/inputs/flight conditions for which the criteria are violated or poorly satisfied. The potentials of this approach are its general usability for both frequency-domain and time-domain analysis, and for both linear and non-linear models and control laws including protections. Moreover, there is no limitation on the number of parametric uncertainties that can be investigated and the method does not itself add conservatism to the clearance problem as many other alternatives do, mainly those based on approximations or simplifications. The most challenging problem with the optimisation based approach is not to find hidden weaknesses if they exist, but to confidently assert that no violation exists for all parameters/inputs/flight conditions. To assert this a global optimum solution has to be found usually resulting in a huge amount of computational work. For highly nonlinear clearance problems regarding manoeuvrability in the peripheral flight envelope of a civil transportation aircraft the suitability of the optimisation based approach is demonstrated. Different global optimisation algorithms and strategies are investigated with respect to its reliability in finding worst cases, computational efficiency and its potential for distributed computation in order to reduce time to clearance. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39902.html 04:00 PM Wednesday 31, 1969 This paper presents the formulation of a flight clearance criterion as a convex optimization problem. The criterion is the stability margins criterion which is specified as an allowable phase and gain margin of a certain loop transfer function. The satisfaction of the criterion amounts to the Nichols plot of the loop transfer function being outside a specified trapezoidal region. It was shown previously that the exclusion condition from an ellipsoidal region is implied by using the generalized stability margin b_{PC} and its calculation was performed frequency-wise by solving a sequence of convex optimization problems. In this paper we formulate the calculation of a lower bound on b_{PC} as a convex optimization problem using Integral Quadratic Constraints (IQCs) and avoid the gridding procedure in the frequency domain. Furthermore, we formulate the problem of obtaining a lower bound on the perturbed stability margin, which is defined as the worst-case b_{PC} over variations in real uncertain parameters. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39843.html 04:00 PM Wednesday 31, 1969 http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39891.html 04:00 PM Wednesday 31, 1969 For nonlinear systems described by differential-algebraic equations a new solution is proposed to the problem of constructing the control law which provides full decoupling with respect to unknown inputs. This solution involves nonlinear transformation of available equations to ordinary differential equations. A way is considered for the transformation within the scope of algebraic approach. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39850.html 04:00 PM Wednesday 31, 1969 This paper presents Hoo performance analysis and control synthesis for discrete-time linear systems with time-varying parameters. The parameters are assumed to vary inside a polytope and have known bounds on their rate of variation. The geometric properties of the polytopic domain are exploited to derive parameter-dependent linear matrix inequality conditions that consider the bounds on the rate of variation of the parameters. A systematic procedure is proposed to construct a family of finite-dimensional relaxations based on Polya's Theorem and a homogeneous polynomially parameter-dependent parameterization of arbitrary degree for the Lyapunov matrix. A numerical example illustrates the proposed approach. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/39906.html 04:00 PM Wednesday 31, 1969 In the aeronautical industry, the robustness properties of an aircraft are usually assessed using intensive and time-consuming simulations. Fortunately, several optimization techniques can be implemented and applied for clearance of flight control laws, in order to improve the efficiency and reliability of the certification process. Some of them, such as Lyapunov-based analysis and mu-analysis, require the considered models to be written as Linear Fractional Representations (LFR), which is unfortunately not the case of the aeroelastic models available in an industrial context. A whole methodology is thus proposed in this paper to convert a set of numerical flexible aircraft models into a suitable LFR which depends on the aircraft configuration and the flight conditions. This is a challenging issue, since the size of the initial models is very large and the state vector does not have the same physical meaning for the whole model set. Nevertheless, an LFR is obtained, which is representative of the aircraft behavior in the sense that its eigenvalues and frequency responses almost exactly match those of the initial flexible models. Moreover, its complexity proves compatible with the use of robustness analysis tools. Numerical results indeed show that the considered clearance problem is computationally tractable. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif