controlled invariance for discrete time nonlinear systems with an application to the disturbance decoupling problems invariant distributions are defined for discrete time nonlinear control systems and necessary and sufficient conditions are given for their controlled invariance this extends to discrete time systems the basic tool which has been so important in solving the various synthesis problems for continuous time systems to indicate their utility in the discrete time setting they are used to locally solve the disturbance decoupling problem

feedback affinization of nonlinear control systems abstract in this paper conditions for the nonlinear control system x f x u x m u r m to have a nonlinear feedback control u x v v r m m m 0 such that the nonlinear system takes a form of an affine system x a x b x v are presented all results require algebraic operations and differentiation of functions only

aerodynamic shape optimization for natural laminar flow using a discrete adjoint approach a framework for the design of natural laminar flow airfoils is developed based on multipoint aerodynamic shape optimization capable of efficiently incorporating and exploiting laminar turbulent transition a two dimensional reynolds averaged navier stokes flow solver making use of the spalart allmaras turbulence model is extended to incorporate an iterative laminar turbulent transition prediction methodology the natural transition locations due to tollmien schlichting instabilities are predicted using a simplified en method or the compressible form of the arnal habiballah delcourt criterion the reynolds averaged navier stokes solver is subsequently used in a gradient based sequential quadratic programming shape optimization framework the transition criteria are tightly coupled into the objective and gradient evaluations the gradients are obtained using an augmented discrete adjoint formulation for nonlocal transition criteria robust design over a range of cruise flight conditions is demonstrated thro

a multilevel approach to single and multiobjective aerodynamic optimization an optimization platform with multilevel structure which is capable of efficiently solving design optimization problems in aerodynamics is proposed the multilevel structure relies on a two way regular exchange of information between successive search levels each level can be associated with a different evaluation software different problem parameterization and or different search tool for the minimization of the same objective function s the combination of some or all of the aforementioned strategies is possible although beyond the scope of this paper the basic optimization tool which is associated with at least one of the levels is a metamodel assisted evolutionary algorithm candidate solutions that have previously been examined are memorized and serve to train radial basis function networks operating as local surrogate evaluation models to handle multiobjective optimization problems with different search techniques at each level we hybridize an evolutionary algorithm that computes the front of pareto optimal solutions at the lower level s with a gradient based method at the upper level for the purpose of refinement through local search in aerodynamic optimization the adjoint method is used to compute the gradient of the objective function for the gradient method to apply to a front of solutions rather than a single individual an approximation of the spea 2 utility function gradient needs to be devised the multilevel platform is demonstrated on mathematical problems as well as the design of isolated and compressor cascade airfoils

feasibility study of e transition prediction in navier stokes methods for airfoils when navier stokes methods are applied in the design process for laminar airfoils the prediction of the transition location still represents an unresolved problem by the acceptence of the e n method as representing a convenient transition prediction tool the requirements for coupling e n to navier stokes methods will be demonstrated in particular the possibility to determine the laminar and turbulent viscous length scales is outlined based on the knowledge of the viscous layer thickness a mesh adaption procedure can be applied the navier stokes results produced using adapted meshes are shown to be identical to boundary layer results when the computed wall pressure from the navier stokes solution is used as input for the boundary layer calculation for the validation of the necessary steps in the coupling procedure the laminar airfoil doal3 was selected this airfoil was measured in the transonic wind tunnel braunschweig facility at the dlr german aerospace research establishment the limiting n factor for that wind tunnel was determined beforehand

the matching of a prescribed linear input output behavior in a nonlinear system in this paper we deal with the problem of finding a feedback under which the input output behavior of a nonlinear system becomes exactly the same as that of a specified linear model the solvability of this problem is shown to depend on a formal infinite zero structure associated with the system

nonlinear decoupling via feedback a differential geometric approach the paper deals with the nonlinear decoupling and noninteracting control problems a complete solution to those problems is made possible via a suitable nonlinear generalization of several powerful geometric concepts already introduced in studying linear multivariable control systems the paper also includes algorithms concerned with the actual construction of the appropriate control laws

predicting stall and post stall behavior of airfoils at low mach numbers an interactive boundary layer method together with the e approach to the calculation of transition is used to predict the stall and post stall behavior of airfoils in incompressible and compressible flows at low freestream mach numbers two separate inviscid methods a panel method with compressibility corrections and a full potential method are used to compute the external velocity distribution needed in the solution of the compressible boundary layer equations the turbulence model is based on the cebeci smith algebraic eddy viscosity formulation with improvements for strong pressure gradient effects comparison of calculated results with inviscid flow computed with a panel method indicate excellent agreement with experiment for a wide range of reynolds numbers in incompressible flows comparison of calculated results obtained with inviscid flow computed with a full potential method also indicate excellent agreement with experiment for a wide range of angles of attack including stall for compressible flows at low freestream mach numbers the study also shows that even though the compressibility corrections to the panel method are adequate at small to moderate angles of attack they are not satisfactory at higher angles of attack especially near stall

on minimal compensators for decoupling control abstract the general construction of minimal regular compensators for decoupling a nonlinear system is given by means of vector subspaces which are generated by differential vectors of outputs and states and the minimal dimension of a decoupling compensator is expressed in intrinsic characterization the equivalence relation of all minimal regular compensators is established though these compensators may have different dynamic behavior from each other

generalized multipoint inverse airfoil design in a rather general sense inverse airfoil design can be taken to mean the problem of specifying a desired set of airfoil characteristics such as the airfoil maximum thickness ratio pitching moment part of the velocity distribution or boundary layer development from thie information the corresponding airfoil shape is determined we present a method that approaches the design problem from this perspective in particular the airfoil is divided into segments along which together with the design conditions either the velocity distribution or boundary layer development may be prescribed

fully local formulation of a transition closure model for transitional flow simulations a new computational fluid dynamics compatible transition model for the natural transition and laminar separation bubble induced transition is established using two transport equations with consideration of analysis results through linear stability theory linear stability theory one transport equation is introduced to describe the growth of the laminar kinetic energy in the boundary layer in addition another transport equation of intermittency factor is built to trigger the transition by controlling the source term of turbulent kinetic energy equation finally applying the two transport equations to a shear stress transport turbulence model completes a four equation transition turbulence model comparisons of predictions using the new model with wind tunnel experiments of the schubauer and klebanoff flat plate s809 airfoil aerospatiale a airfoil and dlr f5 wing validate the predictive qualities of the new transition turbulence model

subgrid stabilized projection method for 2d unsteady flows at high reynolds numbers a subgrid stabilization technique is developed for solving the two dimensional incompressible navier stokes equations at high reynolds numbers the time marching algorithm is based on a well established fractional step pressure correction projection method the advection diffusion step is enriched by an implicit subgrid stabilizing term and by an explicit dissipative shock capturing term the former is calculated by means of a hierarchical finite element setting the latter is included to avoid gibbs phenomenon in the boundary layer convergence tests on prototypical two dimensional examples are reported and the method is used to simulate the viscous incompressible flows around the airfoil naca0012 at zero incidence and reynolds numbers ranging from 105 to 106

robust tracking in nonlinear systems a nonlinear feedback multivariable controller is used to implement multivariable tracking in a nonlinear system the tracking error is measured by a general function of system state and the input command the controller is robust in the sense that the tracking error is ultimately bounded in the presence of modeling errors free parameters which affect the form of the controller allow flexibility in determining such factors as the size of the ultimate bound the rate of error decay excursion of the control conditions on the class of modeling errors and the level of system gain restrictive assumptions on the structure of the model and the modeling errors are required but they are treated in a transformation framework which allows the generalization of similar conditions which appear in the prior literature these assumptions hold for robotic manipulators this application is investigated at some length and it appears that the resulting control scheme may have advantages over others which have been proposed in the robotics literature

two dimensional transonic aerodynamic design method this paper demonstrates the capabilities of a new transonic two dimensional design method based on the simultaneous solution of multiple streamtubes coupled through the position of and pressure at the streamline interfaces this allows the specification of either the airfoil shape direct analysis mode or the airfoil surface pressure distribution inverse design mode the nonlinear system of equations is formulated in a conservative manner which guarantees the correct treatment of shocks and is solved by a rapid newton solution method viscous effects can also be included through a coupled integral boundary layer analysis the first set of results shows the effect of different far field treatments demonstrating the improvement in accuracy obtained by including the second order doublet terms in addition to the usual first order vortex term the results are also compared to those obtained by specifying straight far field streamlines corresponding to solid wall wind tunnel experiments or constant far field pressure corresponding to freejet experiments to show the sensitivity to the farfield distance in the second set of results the design method is used to design a transonic airfoil with c 1 000 at a oo 0 70 it is shown that the off design performance is improved by specifying a surface pressure distribution with a very weak shock

a novel key management scheme for dynamic access control in a user hierarchy considering the solution of dynamic access problems in a user hierarchy a novel scheme based on one way hash function is proposed to manage the cryptographic keys in the paper the scheme attempts to achieve both efficiency and non iteration in deriving the successor s secret key besides the other issues in relation with dynamic access control problems such as adding deleting classes adding deleting relationships and changing secret keys can be held good to the scheme in view of security a competent central authority must provide the user a convenient way to change his her key at any time therefore the design toward the algorithm in the paper contains such a function what weight to mention especially among these characteristics of the scheme is the simplification of procedure in changing the private key and no other current keys need altering simultaneously

new matching conditions for output regulation of a class of uncertain nonlinear systems some sufficient conditions for the structure of modelling uncertainties for output regulation of uncertain systems are presented it turns out that these conditions include as special cases the ordinary matching conditions for state regulation of uncertain systems and the disturbance decoupling condition the previous restrictions on the structure of modelling uncertainties can be considerably relaxed in the case of output regulation the significance of the result is shown by a simple example

a constructive condition for dynamic feedback linearization the paper deals with dynamic feedback linearization of continuous time affine systems a constructive procedure based on prolongations is proposed based on necessary geometric conditions the algorithm computes a set of prolongation indices

measurement feedback disturbance decoupling in discrete time nonlinear systems the paper studies the disturbance decoupling problem by the dynamic measurement feedback for discrete time nonlinear control systems to address the problem the algebraic approach called the algebra of functions is applied which allows the system description also depend on non differentiable functions a necessary and sufficient condition is given in terms of controlled and h f invariant functions also algorithms are derived which find invariant functions and the required feedback the algorithms are implemented in mathematica software which is made available over the internet

adaptive regulation for a class of uncertain systems with partial knowledge of uncertainty bounds an adaptive regulation method is presented for a class of uncertain systems where the uncertainty bounds are partially known both the state and the parameter error of the closed loop system are shown to be ultimately bounded the proposed method improves on the adaptive control method of m j corless and g leitmann 1981 so that performance degradation and the destabilization of the control system can be effectively presented in practical situations

a geometric approach to the theory of 2 d systems the authors develop a geometric approach to the theory of 2 d two dimensional systems defining in a suitable way the notion of a sub 1 2 b sub 1 2 invariant subspaces and of controlled invariant subspaces such subspaces are shown to have good computational and feedback properties which make them useful in application in particular sufficient conditions and constructive procedures are obtained for the solutions of disturbance decoupling and model matching problems moreover it is shown that certain structural properties of a 2 d system can be described by means of a set of indexes defined in geometric terms and that such structural indexes can be used to reformulate the sufficient condition for model matching

globally stable robust tracking of nonlinear systems using variable structure control and with an application to a robotic manipulator the globally stable robust output tracking for a class of nonlinear systems is considered based only on the knowledge of the bounds on the uncertainties a variable structure control vsc law is developed under the structure matching assumption it is shown that the outputs of the closed loop system asymptotically track given output trajectories despite the uncertainties while maintaining the boundedness of all signals inside the loop all signals inside the loop are shown to be bounded for all time to illustrate the efficiency of the controller the approach is applied to the case of a two degree of freedom dof robotic manipulator with variable payload numerical simulation results are also provided

continuous state feedback guaranteeing uniform ultimate boundedness for uncertain dynamic systems we consider a dynamic system containing uncertain elements only the set of possible values of these uncertainties is known based on this information a class of state feedback controls is proposed in order to guarantee uniform ultimate boundedness of every system response within an arbitrarily small neighborhood of the zero state these feedback controls are continuous functions of the state

observability of autonomous discrete time non linear systems a geometric approach in this paper the observability of autonomous discrete time systems is studied from a purely differential geometric point of view as with continuous time systems this approach leads to a local canonical form for an observable system a proposal for the generalization of an invariant subspace is made

a nonregular solution of the nonlinear dynamic disturbance decoupling problem with an application to a complete solution of the nonlinear model matching problem the nonregular dynamic disturbance decoupling problem for nonlinear control systems is introduced a local solution is given by means of a constructive algorithm that is based on singh s algorithm and the clamped dynamics algorithm further studied is the nonlinear model matching problem that is defined as follows given a nonlinear control system to be referred to as the plant and another nonlinear control system to be referred to as the model can a compensator for the plant be found in such a way that the input output behavior of the compensated plant matches that of the model by proving that the solvability of the nonlinear model matching problem is equivalent to the solvability of an associated nonregular dynamic disturbance decoupling problem a complete local solution of this problem can be established

on decomposition of nonlinear control systems in this paper the problem is studied when the system x i k 1 u t g t x x 0 x 0 defined on an n dimensional analytic manifold can be decomposed into either independent subsystems or a cascade of subsystems necessary and sufficient conditions for the existence of such local decompositions are given in terms of lie algebras generated by the system

computation of unsteady supersonic quasi one dimensional viscous inviscid interacting internal flowfields the present steady and unsteady quasi 1d internal supersonic flowfield calculations employs a novel viscous inviscid interaction technique directly coupling and simultaneously solving the inviscid and viscous equations the coupled system is numerically solved using a two stage runge kutta scheme with first order one sided spatial differencing simple supersonic internal flow test cases are computed to demonstrate the capabilities of the computational procedure

one approach to the control of uncertain dynamical systems we present a controller design methodology for uncertain systems which is based on the constructive use of lyapunov stability theory the uncertainties which are deterministic are characterized by certain structural conditions and known as well as unknown bounds as a consequence of the lyapunov approach the methodology is not restricted to linear or time invariant systems the robustness of these controllers in the presence of singular perturbations is considered the situation in which the full state of the system is not available for measurement is also considered as are other generalizations

comments on controlled invariance for nonlinear systems the sufficient conditions given in theorem 4 12 of the above paper for controlled invariance are a special case of more general sufficient conditions reported earlier

simplified linear stability transition prediction method for separated boundary layers an existing transition prediction method for attached two dimensional incompressible boundary layers based on linear stability analysis is extended to separated two dimensional incompressible boundary layers such as those found in laminar transitional separation bubbles it is shown why the present method which tracks the growth of disturbances at many different frequencies is more accurate than the so called envelope methods for nonsimilar boundary layer developments reliance on a database of precalculated stability characteristics of known velocity profiles makes this method much faster than traditional stability calculations of similar accuracy the falkner skan self similar profiles are used for attached flow and a new very general family of profiles is used for separated flow comparisons with measured transition locations inside the bubble show good agreement over the range of chord reynolds numbers and airfoil angles of attack of interest

an efficient key assignment scheme for access control in a large leaf class hierarchy the employees of an organization are usually divided into different security classes to authorize the information retrieval and the number of leaf classes is substantially larger than the number of non leaf classes additionally the alternations in leaf classes are more frequent than in non leaf classes we proposed a new key assignment scheme for controlling the access right in a large poset partially ordered set hierarchy to reduce the required computation for key generation and derivation with the storage amount of data decreased

transition prediction for three dimensional boundary layers in computational fluid dynamics applications a method is presented for estimating the laminar turbulent transition location in three dimensional boundary layers for computational fluid dynamics cfd applications requiring numerous transition estimates with no user intervention given the reynolds number and the cp distribution the location of transition is estimated based on the attachment line state the potential for relaminarization the occurrence of laminar separation and the growth of instabilities transition caused by instability is estimated based on n factors calculated for tollmien schlichting waves and for stationary crossflow instabilities a neural network is used in place of solving the orr sommerfeld stability equation for determining the instability growth rates the current version of the method assumes incompressible flow the boundary layer flow and instabilities are calculated based on an infinite sweep strip boundary layer approximation the instability calculations also employ the parallel flow approximation

an adaptive control strategy for mechanical manipulators this paper focuses on the study of an adaptive perturbation control which tracks a desired time based trajectory as close as possible for all times over a wide range of manipulator motion and payloads the proposed adaptive control is based on the linearized perturbation equations in the vicinity of a nominal trajectory the controlled system is characterized by feedforward and feedback components which can be computed separately and simultaneously the feedforward component computes the nominal torques from the newton euler equations of motion to compensate all the interaction forces among the various joints the feedback component consisting of recursive least square identification and an optimal adaptive self tuning control algorithm for the linearized system computes the perturbation torques which reduce the position and velocity errors of the manipulator along the nominal trajectory a computer simulation study was conducted to evaluate the performance of the proposed adaptive control

output tracking control of nonlinear systems with mismatched uncertainties abstract this paper addresses the output tracking problem of a class of nonlinear systems given input output linearizability of the class of nonlinear systems a robust output tracking controller derived via a lyapunov based approach is proposed here the model uncertainties do not need to satisfy the conventional matching condition but states and tracking error remain bounded furthermore the magnitude of the tracking error will bear dominant dependence on the magnitude of the mismatched uncertainties

controlled invariance for nonlinear systems necessary and sufficient conditions are derived for a b invariance called here controlled invariance for nonlinear systems dot x f x u the obtained results generalize and elucidate already known results about systems dot x a x sum min i 1 max m u i b i x a new and direct differential geometric interpretation of the concept of controlled invariance and the derived conditions is given

condensed forms of linear control system under output feedback abstract we present two complementary condensed forms of linear control system with output x a x b u x r n u r m y c x y r r under transformation of the state coordinates from them we derive two complementary condensed forms under output feedback as an application of the condensed forms we show that the output feedback control of the linear control systems generically reduces to a matrix completion problem and we provide a partial solution of that problem based on existing methods

input output decoupling of hamiltonian systems the linear case in this note we give necessary and sufficient conditions for a linear hamiltonian system to be input output decouplable by hamiltonian feedback i e feedback that preserves the hamiltonian structure in a second paper we treat the same problem for nonlinear hamiltonian systems

a comparison principle for state constrained differential inequalities and its application to time optimal control in this paper we present a comparison principle that characterizes the maximal solutions of state constrained differential inequalities in terms of solutions of certain differential equations with discontinuous right hand sides for the sake of completeness we show through some set valued analysis that the differential equations determining the maximal solutions have the unique solutions in the carathe spl acute odory sense in spite of discontinuity of their right hand sides we apply our comparison principle to the explicit characterization of the solution to a time optimal control problem for a class of state constrained second order systems which includes the dynamic equations of robotic manipulators with geometric path constraints as well as single degree of freedom mechanical systems with friction specifically we show that the time optimal trajectory is uniquely determined by two curves that can be constructed by solving two scalar ordinary differential equations with continuous right hand sides hence the time optimal trajectory can be found in a computationally efficient way through the direct use of the well known euler or runge kutta methods another interesting feature is that our method to solve the time optimal control problem works even when there exist an infinite number of switching points finally some simulation results using a two degrees of freedom dof robotic manipulator are presented to demonstrate the practical use of our complete characterization of the time optimal solution

analytical approximation of two dimensional separated turbulent boundary layer velocity profiles abstract a single analytical expression is proposed to describe the velocity distribution in a two dimensional separated turbulent boundary layer on smooth impermeable adiabatic walls over the domain 0 less than or to y less than infinity the expression is an extension of one previously derived for attached flow which depends upon local values of skin friction shape factor and reynolds number based on momentum thickness boundary layer shape factor and local skin friction correlations applicable to separated flows are derived from fitting the proposed analytical expression for separated velocity profiles to available experimental data these correlations are then available for analytically describing separated velocity profiles without further fitting

viscous inviscid interaction using the navier stokes equations a numerical model for the calculation of incompressible viscous flows past airfoils and wings has been developed the approach is based on a strong viscous inviscid coupling of a boundary element method with the navier stokes equations in vorticity stream function formulation a semiadaptive or fully adaptive grid is used the interaction is achleved by iterative updating of the boundary conditions through the wall transpiration concept the navier stokes equations are discretized on a semistaggered grid space marching integration is performed starting from the stagnation streamline on two independent blocks results are presented for two different airfoils

controlled invariance by static output feedback for nonlinear systems the paper deals with the notion of static output feedback for nonlinear systems necessary and sufficient conditions are derived for c a b invariance here called measured controlled invariance for nonlinear control systems

an indirect adaptive robot controller abstract indirect adaptive control of robot manipulators represents an approach parallel to direct and composite adaptive methods this paper develops a new globally convergent indirect adaptive controller which requires neither measurements of joint acceleration nor inversion of the estimated inertia matrix several aspects of this paper such as new parameter estimators with desirable properties and a physically based technique for guaranteeing the uniform positive definiteness of the estimated inertia matrix along with the related convexity proof are also of interest in their own right

a lyapunov approach for robust nonlinear state feedback synthesis the authors apply the input output linearization approach for nonlinear state feedback synthesis the model uncertainty under consideration is a class of state model perturbations that satisfy appropriate matching conditions the controller design uses a lyapunov based approach to guarantee uniform ultimate boundedness of the states and the output

dynamic surface control for a class of nonlinear systems a method is proposed for designing controllers with arbitrarily small tracking error for uncertain mismatched nonlinear systems in the strict feedback form this method is another synthetic input technique similar to backstepping and multiple surface control methods but with an important addition spl tau 1 low pass filters are included in the design where spl tau is the relative degree of the output to be controlled it is shown that these low pass filters allow a design where the model is not differentiated thus ending the complexity arising due to the explosion of terms that has made other methods difficult to implement in practice the backstepping approach while suffering from the problem of explosion of terms guarantees boundedness of tracking errors globally however the proposed approach while being simpler to implement can only guarantee boundedness of tracking error semiglobally when the nonlinearities in the system are non lipschitz

characterization of allowable perturbations for robust decoupling of affine non linear systems for affine non linear systems we study the effects of uncertainties in systems vector fields on decoupled linear input output behaviour a characterization of an allowable class of perturbations which preserve the decoupling properties of the nominal system is given an immediate consequence of this characterization indicates that decoupling position control of rigid robotic manipulators is not robust under any perturbations in robot dynamics the result is applied to simple hamiltonian control systems to characterize the allowable perturbation of total energy function

generalized output regulation problem for a class of nonlinear systems with nonautonomous exosystem the problem of the generalized output regulation is considered here in contrast to the classical output regulation problem where the references to be tracked and disturbances to be rejected are generated by an autonomous exogeneous system for the generalized output regulation an exogenous system is driven by an unknown external reference signal in such a way the class of reference or disturbance signals is expanded presented result solves the full measurement feedback case and is based on the well known notion of the regulator equation combined with the classical concept of the invariant distribution illustrative example including matlab simulink simulations is presented to demonstrate the suggested approach

on solvability of several affine system problems we examine seven problems and properties of single input single output affine control systems and classify the properties topologically as being structural generic or residual these properties are accessibility observability invertibility preservation of a given distribution and partial linearizability the mathematics involved relies upon the thorn transversality theorem

cascade decomposition of nonlinear systems foliations and ideals of transitive lie algebras abstract we relate the cascade decomposition of nonlinear systems to the joint notions of foliations and ideals of transitive lie algebras

linearization and input output decoupling for general nonlinear systems necessary and sufficient conditions are obtained for the linearization and input decoupling by state feedback of general nonlinear systems it is shown how these conditions can be derived from the already known conditions for affine nonlinear systems thereby also elucidating the existing theory for affine systems

exact nonlinear control of large angle rotational maneuvers the rigid body attitude control problem with external torques is transformed into an equivalent linear form implementable by three double integrators the linearizing transformations themselves are formulated in vector algebra requiring no integrators for implementation it is thereby shown that optimal command generation for fast slewing maneuvers can be carried out exactly in the transformed system together with regulator design without gain scheduling for correction of unmodeled disturbances in particular a general rest to rest maneuver is computed in closed form and coupled with a previously obtained exact detumbling maneuver so that arbitrary initial conditions can be accommodated an illustrative simulation is appended

on normal forms of nonlinear systems affine in control the nonlinear equivalences of both finite and infinite zero structures of linear systems have been well understood for single input single output systems and have found many applications in nonlinear control theory the extensions of these notions to multiple input multiple output systems have proven to be highly sophisticated in this paper we propose constructive algorithms for decomposing a nonlinear system that is affine in control these algorithms require modest assumptions on the system and apply to general multiple input multiple output systems that do not necessarily have the same number of inputs and outputs they lead to various normal form representations and reveal the structure at infinity the zero dynamics and the invertibility properties all of which represent nonlinear equivalences of relevant linear system structural properties

invariant distributions for discrete time nonlinear systems the purpose of this paper is to introduce some notions of invariant distributions for the class of linear analytic discrete time systems and to give an algebraic characterization of them a brief final discussion motivates the usefulness of the notions introduced in the study of the geometric properties of the system

robust output tracking for nonlinear systems with weakly non minimum phase this paper is concerned with the problem of designing a robust output tracking controller for mimo nonlinear systems with weakly non minimum phase based on our system formulation control plants with uncertainties and or with actuator dynamics fall into the class under consideration the controller design here is divided into two phases fast feedback control and slow feedback control so that a final composite control is obtained the former is chosen to stabilize the boundary layer system whereas the latter essentially handles the mismatched uncertainties after the system is reformulated under some mild assumptions it is shown that the overall states are bounded and the tracking errors converge to a residual set whose size is a class k function of e as e 0 the residual set shrinks to the origin an interesting application to a simplified aircraft model with fast actuator dynamics which turns out to be weakly non minimum phase is given the computer simulation has verified the expected satisfac

a revisit of the tonal noise of small rotors in this study asymptotic analysis of the frequency domain formulation to compute the tonal noise of the small rotors in the now ubiquitously multi rotor powered drones is conducted simple scaling laws are proposed to evaluate the impacts of the influential parameters such as blade number flow speed rotation speed unsteady motion thrust and observer angle on the tonal noise the rate of noise increment with thrust or rotational speed is determined by orders of blade passing frequency harmonics and the unsteady motion the axial mean flow influence can be approximated by quadratic functions at given thrust the sound decreases rapidly with the radius and blade number as the surface pressure becomes less intensive the higher tonal harmonics are significantly increased if unsteady motions although of small amplitude are existed as indicated by the defined sensitivity function emphasizing that the unsteady motions should be avoided for quiet rotor designs the scaling laws are examined by comparing with the full computations of the rotor noise using the frequency domain method the implementation of which has been validated by comparing with experiments good data collapse is obtained when the proposed scaling laws which highlights the dominant influence of the design parameters are incorporated

geometric tools for solving the fdi problem for linear periodic discrete time systems this paper studies the problem of detecting and isolating faults in linear periodic discrete time systems the aim is to design an observer based residual generator where each residual is sensitive to one fault whilst remaining insensitive to the other faults that can affect the system making use of the geometric tools and in particular of the outer observable subspace notion the fault detection and isolation fdi problem is formulated and necessary and solvability conditions are given an algorithmic procedure is described to determine the solution of the fdi problem

an optimal algorithm for assigning cryptographic keys to control access in a hierarchy a cryptographic scheme for controlling access to information within a group of users organized in a hierarchy was proposed in 1 the scheme enables a user at some level to compute from his own cryptographic key the keys of the users below him in the organization

global transformations of nonlinear systems recent results have established necessary and sufficient conditions for a nonlinear system of the form dot x t f x t u t g x t with f 0 0 to be locally equivalent in a neighborhood of the origin in rnto a controllable linear system we combine these results with several versions of the global inverse function theorem to prove sufficient conditions for the transformation of a nonlinear system to a linear system in doing so we introduce a technique for constructing a transformation under the assumptions that g ldot f dotg ad n 1 f ldotg span an n dimensional space and that g ldot f ldot g ad n 2 f ldotg is an involutive set

extended lie brackets for nonlinear time delay systems in this note the extended lie bracket operator is introduced for the analysis and control of nonlinear time delay systems nltds this tool is used to characterize the integrability conditions of a given submodule the obtained results have two fundamental outcomes first they define the necessary and sufficient conditions under which a given set of nonlinear one forms in the n dimensional delayed variables x t x t sd with d constant but unknown are integrable thus generalizing the well known fundamental frobenius theorem to delay systems secondly they set the basis for the extension to this context of the geometric approach used for delay free systems the effectiveness of the results is shown by solving the problem of the equivalence of a nltds to an accessible linear time delay system ltds by bicausal change of coordinates

computational fluid dynamics compatible transition modeling using an amplification factor transport equation a new laminar turbulent transition model for low turbulence external aerodynamic applications is presented that incorporates linear stability theory in a manner compatible with modern computational fluid dynamics solvers the model uses a new transport equation that describes the growth of the maximum tollmien schlichting instability amplitude in the presence of a boundary layer to avoid the need for integration paths and nonlocal operations a locally defined nondimensional pressure gradient parameter is used that serves as an estimator of the integral boundary layer properties the model has been implemented into the overflow 2 2f solver comparisons of predictions using the new model with high quality wind tunnel measurements of airfoil section characteristics confirm the predictive qualities of the model as well as its improvement over the current state of the art in computational fluid dynamics transition modeling at approximately half the computational expense

transonic aeroelastic instability searches using sampling and aerodynamic model hierarchy doi 10 2514 1 j050509 a hierarchy of flow models is exploited for transonic aeroelastic stability analysis using the kriging interpolation technique applied within the schur complement eigenvalue framework in the schur framework a modified structural eigenvalue problem describes the coupled aeroelastic system with a precomputed interaction term depending on the response frequency the interaction term representing the influence of the high dimensional computational fluid dynamics system is approximated by reconstruction based on samples that can be computed using a frequency or time domain solver the computationally cheap approximation model is developed and discussed in this paper for two degree of freedom aerofoil cases the approximation model is used for both the parametric blind search of aeroelastic instability and for updating predictions based on aerodynamic models of different fidelities

cryptanalysis and improvement of an access control in user hierarchy based on elliptic curve cryptosystem in a key management scheme for hierarchy based access control each security class having higher clearance can derive the cryptographic secret keys of its other security classes having lower clearances in 2008 chung et al proposed an efficient scheme on access control in user hierarchy based on elliptic curve cryptosystem information sciences 178 1 2008 230 243 their scheme provides solution of key management efficiently for dynamic access problems however in this paper we propose an attack on chung et al s scheme to show that chung et al s scheme is insecure against the exterior root finding attack we show that under this attack an attacker adversary who is not a user in any security class in a user hierarchy attempts to derive the secret key of a security class by using the root finding algorithm in order to remedy this attack we further propose a simple improvement on chung et al s scheme overall the main theme of this paper is very simple a security flaw is presented on chung et al s scheme and then a fix is provided in order to remedy the security flaw found in chung et al s scheme

bisimilar control affine systems the notion of bisimulation plays a very important role in theoretical computer science where it provides several notions of equivalence between models of computation these equivalences are in turn used to simplify verification and synthesis for these models as well as to enable compositional reasoning in systems theory a similar notion is also of interest in order to develop modular verification and design tools for purely continuous or hybrid control systems in this paper we introduce two notions of bisimulation for nonlinear systems we present differential geometric characterizations of these notions and show that bisimilar systems of different dimensions are obtained by factoring out certain invariant distributions furthermore we also show that all bisimilar systems of different dimension are of this form

optimization of gaussian random fields many engineering systems are subject to spatially distributed uncertainty i e uncertainty that can be modeled as a random field altering the mean or covariance of this uncertainty will in general change the statistical distribution of the system outputs we present an approach for computing the sensitivity of the statistics of system outputs with respect to the parameters describing the mean and covariance of the distributed uncertainty this sensitivity information is then incorporated into a gradient based optimizer to optimize the structure of the distributed uncertainty to achieve desired output statistics this framework is applied to perform variance optimization for a model problem and to optimize the manufacturing tolerances of a gas turbine compressor blade

on nonlinear observers in this note we make some remarks about the general observer problem for nonlinear systems we show how the existence of a partial observer without stability requirements is locally equivalent to the existence of a conditioned invariant distribution we argue that by considering not only identity observers we can put the stability issue in a broader perspective

analysis of airfoil leading edge separation bubbles a local inviscid viscous interaction technique was developed for the analysis of low speed airfoil leading edge transitional separation bubbles in this analysis an inverse boundary layer finite difference analysis is solved iteratively with a cauchy integral representation of the inviscid flow which is assumed to be a linear perturbation to a known global viscous airfoil analysis favorable comparisons with data indicate the overall validity of the present localized interaction approach in addition numerical tests were performed to test the sensitivity of the computed results to the mesh size limits on the cauchy integral and the location of the transition region

new results on f g invariance abstract the existing result for a nonsingular and involutive distribution to be f g invariant has a priori assumption that g is nonsingular this paper treats the problem of f g invariance for the case where g is singular necessary and sufficient conditions for a nonsingular and involutive distribution to be f g invariant are presented the constructive proof provides an algorithm for the construction of the feedback law

viscous inviscid analysis of transonic and low reynolds number airfoils a method of accurately calculating transonic and low reynolds number airfoil flows implemented in the viscous inviscid design analysis code ises is presented the euler equations are discretized on a conservative streamline grid and are strongly coupled to a two equation integral boundary layer formulation using the displacement thickness concept a transition prediction formulation of the e type is derived and incorporated into the viscous formulation the entire discrete equation set including the viscous and transition formulations is solved as a fully coupled nonlinear system by a global newton method this is a rapid and reliable method for dealing with strong viscous inviscid interactions which invariably occur in transonic and low reynolds number airfoil flows the results presented demonstrate the ability of the ises code to predict transitioning separation bubbles and their associated losses the rapid airfoil performance degradation with decreasing reynolds number is thus accurately predicted also presented is a transonic airfoil calculation involving shock induced separation showing the robustness of the global newton solution procedure good agreement with experiment is obtained further demonstrating the performance of the present integral boundary layer formulation

new class of control laws for robotic manipulators part 1 non adaptive case a new class of exponentially stabilizing control laws for joint level control of robot arms is introduced it has recently been recognized that the non linear dynamics associated with robotic manip

on ultimate boundedness control of uncertain systems in the absence of matching assumptions this paper describes an approach to ultimate boundedness control under hypotheses less stringent than heretofore required instead of imposing the so called matching conditions of previous authors a certain decomposition of the dynamics is performed the resulting decomposed system has two parts a matched portion and a mismatched portion a certain measure of mismatch m is defined and it is shown that effective control is possible as long as m does not exceed some critical mismatch threshold m it is seen that this threshold depends on the choice of feedback hence it becomes possible to investigate the tradeoffs between the size of the mismatch threshold and the sizes of the feedback gains which achieve the desired ultimate boundedness of solutions