http://www.ifac-papersonline.net/ en Copyright 07:57 PM Wednesday 22, 2012 IFAC-PapersOnline http://www.ifacpapersonline.com 07:57 PM Wednesday 22, 2012 07:57 PM Wednesday 22, 2012 ET webmaster@ifac-papersonline.net webmaster@ifac-papersonline.net http://www.ifac-papersonline.net/Detailed/43921.html 04:00 PM Wednesday 31, 1969 The fields of control and robotics are contributing to the development of bipedal robots that can realize walking motions with the stability and agility of a human being. Dynamic models for bipeds are hybrid in nature. They contain both continuous and discrete elements, with switching events that are spatially driven by unilateral constraints at ground contact and impulse-like forces that occur at foot touchdown. Control laws for these machines must be hybrid as well. The goals of this paper are threefold: highlight certain properties of the models which greatly influence the control law design; present two control design approaches; and indicate some of the many open problems. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44201.html 04:00 PM Wednesday 31, 1969 This paper provides a converse Liapunov theorem for uniformly locally exponentially stable, locally Lipschitz, non-linear, time-varying, possibly non-smooth systems that admit Crthdy solutions. The main result proves that a critical point of such a system is uniformly locally exponentially stable if and only if the system admits a local (possibly non-smooth, time-varying) Liapunov function. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43933.html 04:00 PM Wednesday 31, 1969 This work deals with a networked model predictive coordinated control scheme for discrete-time constrained nonlinear systems characterized by mutual physical interconnections and subject to local state-feedback control policies. A centralized controller, communicating with the distributed agents through a packet-based network, is designed to guarantee the uniform boundedness of the trajectories of the overall closed-loop system. In order to cope with timevarying transmission delays, typically affecting networked control systems, a network delay compensation strategy based on the time-stamping of data packets is adopted. With respect to the existent model-based approaches for nonlinear networked control system, the proposed method eliminates the need for acknowledgment messages, by suitably modifying the finite horizon optimization problem associated to the MPC. The proposed coordinated networked policy guarantees the robust enforcement of state and input constraints even in presence of bounded model uncertainty. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43867.html 04:00 PM Wednesday 31, 1969 In this paper we check a priori global identifiability of a nonlinear HIV models taken from the recent literature by using a recently proposed differential algebra approach. The HIV model is rather complex and has not yet been successfully analyzed by traditional techniques. Here, we demonstrate that identifiability of this complex model can be tested by a differential algebra algorithm based on previous work of the author. The algorithm actually provides a general and reliable tool to test global identifiability of several HIV models proposed in the literature. We have recently developed a computer algebra software tool, called DAISY (Differential Algebra for Identifiability of SYstems), based on differential algebra, which can be used to automatically check global identifiability of (linear and) nonlinear models described by polynomial or rational differential equations. The software can be used by researchers with a minimum of mathematical background and is available on the web site http://www.dei.unipd.it/~pia. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43835.html 04:00 PM Wednesday 31, 1969 A rather natural way to address the stabilizability problem for switched systems is to make use of periodic switching laws. The success is guaranteed, provided that the discrete dynamical system associated to the periodic law is stable. In this paper we discuss a more general type of stabilizing switching laws, called near-periodic, which can be implemented when the associated discrete dynamical system has a non-trivial stable submanifold, and a suitable controllability condition is fulfilled. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43967.html 04:00 PM Wednesday 31, 1969 The extension to nonlinear systems of adaptive feedforward control (AFC) techniques for harmonic disturbance rejection is an area of ongoing research. For nonlinear and linear systems alike, one of the longstanding open problems concerns the issue of robustness of the adaptive scheme with respect to the sign of the coefficients of the steady-state response of the plant. For linear systems, for instance, a standard assumption entails a positive-realness condition on the plant transfer function in the range of frequency of interest. In this paper, we take a first step towards the definition of a novel scheme for AFC that bears the promise of removing the aforementioned obstruction, without resorting to harsh techniques such as Nussbaum gains. Our attention shall be focused here to presenting a dynamic update law that may be used in combination with an existing AFC scheme in a restrictive class of nonlinear systems. The stability analysis of the closed-loop system is deferred to forthcoming works, while the extension to more general plant models is currently under investigation. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44085.html 04:00 PM Wednesday 31, 1969 This paper proposes a novel class of Command Governor (CG) strategies for input/state constrained linear systems subject to bounded disturbances in the absence of explicit state measurement or estimation. While in traditional CG schemes, the set-point manipulation is undertaken on the basis of either the actual measure of the state or of its suitable estimation, it is shown here that the CG design problem can be solved with limited performance degradation and with similar properties also in the case that such an explicit measure is not available, by forcing the state evolutions to stay “not too far” from the manifold of feasible steady-states. This approach, which will be referred to as FeedForward CG (FF-CG), may be a convenient alternative CG solution in all situations whereby the cost of measuring the state may be a severe limitation, e.g. in distributed or decentralized applications. In order to evaluate the method here proposed, numerical simulations on a physical plant have been performed and comparisons with the standard state-feedback CG solution reported. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44095.html 04:00 PM Wednesday 31, 1969 Is it possible to synthesize an easy-to-implement predictive force controller for electropneumatic cylinders? In this paper, this problem is treated in details. As an electropneumatic cylinder is a highly nonlinear actuator, our strategy is based on the precise nonlinear modeling of the actuator and the application of a feedback linearization strategy. This enables to have an equivalent linearized model and therefore, to find an explicit solution of the predictive optimization problem. The obtained controller is then an easy-to-implement one and the number of control parameters is very reduced: the weighting coefficient and the prediction horizon. Experimental results prove the availability of this control approach and good performances in terms of capacity of tracking long duration static forces of high amplitudes. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43975.html 04:00 PM Wednesday 31, 1969 We propose a kinematic model of a system moving in $mathbb{R}^{m+1}$ and consisting of $n$ rigid bars attached successively to each other and subject to the nonholonomic constraints that the velocity of the source point of each bar is parallel to that bar. We prove that the associated control system is controllable and feedback equivalent to the $m$-chained form around any regular configuration. Hence we deduce that the $n$-bar system is flat and the cartesian position of the source point of the last bar is a flat output. The $n$-bar system is a natural generalization of the $n$-trailer system and we provide a comparison of flatness properties of both systems. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44185.html 04:00 PM Wednesday 31, 1969 We consider an infinite horizon optimal impulsive control problems for which a given cost function is minimized by choosing control strategies driving the state to a point in a given closed set. We present necessary conditions of optimality in the form of a maximum principle for which the boundary condition of the adjoint variable is such that non-degeneracy due to the fact that the time horizon is infinite is ensured. These conditions are given for conventional systems in a first instance and then for impulsive control problems. They are proved by considering a family of approximating auxiliary interval conventional (without impulses) optimal control problems defined on an increasing sequence of finite time intervals. As far as we know, results of this kind have not been derived previously. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43993.html 04:00 PM Wednesday 31, 1969 The paper provides an overview of the neighboring extremal approach to nonlinear Model Predictive Control (MPC) of discrete time systems with state and control constraints. A closed form neighboring extremal control solution is presented for the general case when both control sequence and parameters are adjustable by a MPC controller and a sufficient condition for existence of this solution is specified. Computational details are addressed. An experimental application to a DC/DC converter is summarized to illustrate the utility of the algorithm. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44037.html 04:00 PM Wednesday 31, 1969 In this paper, a new cascaded wheel-slip control strategy based on wheel slip and wheel acceleration measurements is presented. It is shown that the proposed algorithm is able to stabilize globally and asymptotically the wheel slip around any prescribed setpoint, both in the stable and unstable regions of the tyre. Compared to other approches, the main original points of our algorithm are : a) the outputs that are assumed to be measured (both wheel slip and wheel acceleration), b) the proof of global stability and the bounds obtained for the feedback gains, c) the filtered reference (that allows an analysis of the system in a new time-scale) and the feedforward terms. An other original point is the model used for the control synthesis (with the assumption of a constant acceleration), which is an important step for the obtention of our results. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43815.html 04:00 PM Wednesday 31, 1969 In this paper we consider the stabilization problem of unstable periodic orbits of discrete time chaotic systems. For simplicity we consider only one dimensional case. We propose a novel periodic feedback controller law and present some stability results. This scheme may be considered as a novel generalization of the classical delayed feedback scheme, which is also known as Pyragas scheme. The stability results show that all hyperbolic periodic orbits can be stabilized with the proposed method. The stability proofs also give the possible feedback gains which achieve stabilization. We will also present some simulation results. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43889.html 04:00 PM Wednesday 31, 1969 This paper presents a nonlinear canonical form which is used for the design of a reduced order observer. Sufficient and necessary geometric conditions are given in order to transform a special class of nonlinear systems to the proposed nonlinear canonical form and the corresponding reduced order observer is analyzed. After that, the main results were generalized to nonlinear systems with inputs. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43829.html 04:00 PM Wednesday 31, 1969 In this paper, a hierarchical multirate control scheme for nonlinear discrete-time systems is proposed, composed of a robust model predictive controller (MPC) and a multirate integral sliding mode (MISM) controller. In particular, the MISM controller acts at a faster sampling time than the MPC controller, and reduces the effect of model uncertainties and external disturbances, in order to obtain, at the next sampling instant of the MPC controller, a value of the system state that is as close as possible to the nominal one. To obtain this result, the control variable is composed of two parts: one generated by the MPC controller, and the other by the MISM controller. The a-priori reduction of the disturbance terms turns out to be very useful in order to improve the convergence properties of the MPC controller. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44157.html 04:00 PM Wednesday 31, 1969 The model reduction problem by moment matching for (single-input, single-output) nonlinear systems is study. A new family of reduced order models, achieving moment matching, is presented. It is shown that this new family is a natural nonlinear enhancement of the family of models obtained, in the linear case, using Krylov projections. Finally, connections between this novel family of models and the family proposed in Astolfi [2010] are discussed. The theory is illustrated by means of a simple example. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43773.html 04:00 PM Wednesday 31, 1969 This note presents a result on stabilization for a class of minimum phase, relative degree one hybrid systems. The note focuses on the situation where the derivative of the output is the sum of the input and a term that vanishes everywhere on the attractor of the zero dynamics except at finite number of (nonequilibrium) points. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43925.html 04:00 PM Wednesday 31, 1969 In this work we present a novel multi-parametric nonlinear programming (mp-NLP) algorithm for explicit multi-parametric nonlinear model predictive control (mp-NMPC). The algorithm is based on (i) local sensitivity analysis of nonlinear programs (NLP), and (ii) an exploration procedure which makes use of successive linearizations of the dynamic system and the nonlinear constraints. The algorithm is illustrated with an example problem drawn from the open literature. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44075.html 04:00 PM Wednesday 31, 1969 A simple numerical algorithm that allows to find three-dimensional projections of reachability sets is discussed that is aimed at a particular class of nonlinear control systems rather than those of general kind. The algorithm is based on using of the Cauchy characteristics to construct numerically the boundary of projection. Besides, numerical examples are given, applying the algorithm to find several three-dimensional projections for control systems of the selected class. These examples illustrate some possible situations that may occur which the algorithm may process successfully. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44139.html 04:00 PM Wednesday 31, 1969 This paper presents a path-following feedback control law with a variable velocity for a new type of undulatory locomotor, a trident steering walker. The feedback control law previously presented by one of the authors guarantees asymptotic stability of all the state variables of the locomotor by feedback control, except its position on a path under the precondition that its moving velocity on the path controlled by feedforward is a non-zero constant. In order to make a smooth start and stop, it is necessary for the locomotor to vary its moving velocity continuously. Especially, the slippage between the passive wheels of the locomotor and the floor causes its moving velocity to be reduced so that it cannot move at a desired moving velocity by feedforward control. Therefore, we modified the feedback control law previously presented. In the modified feedback control law, all the state variables of the locomotor are controlled by feedback and their asymptotic stability is guaranteed. Furthermore, the deviation of its moving velocity from the desired moving velocity because of the slippage between the passive wheels of the locomotor and the floor is compensated and converges to zero by a new feedback control loop in which its moving velocity on the path is variable. We developed an experimental apparatus of the locomotor and verified the effectiveness of the feedback control law with a variable velocity. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43991.html 04:00 PM Wednesday 31, 1969 In this paper a Sum-of-Squares based Model Predictive Control (MPC) strategy is proposed for a particular class of non-polynomial systems subject to input/state constraints. The key idea is to recast, when possible, the non-polynomial flow into a polynomial vector field by adding fictitious state variables and equality/inequality constraints. The sum of squares (SOS) decomposition techniques can then be applied and suitable control prescriptions be ensured to the original non-polynomial system. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43917.html 04:00 PM Wednesday 31, 1969 In this paper, the problem of feedback stabilization of systems with delays is addressed. The systems under consideration are nonlinear and nonaffine in control. We first consider the case of system with discrete delay and then proceed to develop our results in the case of system with both discrete and distributed delays. By using the Lyapunov approach, we deduce general conditions for stabilizing the closed-loop system and derive stabilizing state feedback control laws for the latter. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43843.html 04:00 PM Wednesday 31, 1969 A powerful approach for analyzing the stability of continuous-time switched systems is based on using tools from optimal control theory to characterize the "most unstable" switching law. This reduces the problem of determining stability under arbitrary switching to analyzing stability for the specific "most unstable" switching law. For discrete-time (DT) switched systems, the variational approach received considerably less attention. It is based on using a first-order necessary optimality condition in the form of a maximum principle (MP), and typically this is not enough to completely characterize the "most unstable" switching law. In this paper, we provide a simple and self-contained derivation of a second-order necessary optimality condition for DT bilinear control systems. This provides new information on the optimal controls that is not included in the first-order MP. We demonstrate several applications of this second-order MP to the stability analysis of DT linear switched systems. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/43853.html 04:00 PM Wednesday 31, 1969 This paper presents a novel Long Baseline (LBL) position and velocity navigation filter for underwater vehicles based directly on the sensor measurements. The solution departs from previous approaches as the range measurements are explicitly embedded in the filter design, therefore avoiding inversion algorithms. Moreover, the nonlinear system dynamics are considered to their full extent and no linearizations are carried out whatsoever. The filter error dynamics are globally asymptotically stable (GAS) and it is shown, under simulation environment, that the filter achieves similar performance to the Extended Kalman Filter (EKF) and outperforms linear position and velocity filters based on algebraic estimates of the position obtained from the range measurements. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44167.html 04:00 PM Wednesday 31, 1969 It is well known that arbitrary small delays in the sliding mode control lead to oscillations and the systems may even become unstable for a greater values of the delay. Output feedback sliding mode control of systems with bounded matched disturbances under small time-varying input delay is considered. The design objective is to achieve ultimate boundedness of the closed-loop system with the bound proportional to the delay and to the disturbance bounds. Sliding mode controller with a high linear gain is proposed, which leads to the closed-loop singularly perturbed system. An LMI-based solution for evaluation of the design parameters and of the resulting ultimate bound is derived by using different Lyapunov-based methods. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif