Analysis of DiscreteTime Systems


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1 TU Berlin DiscreteTime Control Systems TU Berlin DiscreteTime Control Systems 2 Stability Definitions We define stability first with respect to changes in the initial conditions Analysis of DiscreteTime Systems Overview Stability Sensitivity and Robustness Controllability, Reachability, Observability, and Detectabiliy Consider x[k + ] = fx[k], k Let x [k] and x[k] be solutions when the initial conditions are x [k ] and x[k ], respectively Definition Stability: The solution x [k] is stable if for a given ε, there exists a δε, k > such that all solutions with x[k ] x [k ] < δ are such that x[k] x [k] < ε for all k > k Definition Asymptotic Stability: The solution x [k] is asymptotically stable if it is stable and if δ can be chosen such that x[k ] x [k ] < δ implies that x[k] x [k] when k Stability, in general, is a local concept System is asymptotically stable if the trajectories do not change much if the initial condition is changed by a small amount TU Berlin DiscreteTime Control Systems 3 TU Berlin DiscreteTime Control Systems 4 Stability of Linear Discrete Time Systems System x [k + ] = Φx [k] x [] = a System with perturbed initial value x[k + ] = Φx[k] x[] = a The difference x = x x satisfies the equation x[k + ] = Φ x[k] x[] = a a If the solution x [k] is stable every other solution is also stable For linear, timeinvariant systems, stability is a property of the system and not of a special trajectory! Solution for the last system: x[k] = Φ k x[] If it is possible to diagonalize Φ then the solution is a combination of λ k i terms, where λ k i, i =,, n are the eigenvalues of Φ If it is not possible to diagonalize Φ then the solution is a linear combination of the terms p ikλ k i where pik are polynomials in k of the order one less the multiplicity of the corresponding eigenvalue To get asymptotic stability, all solution must go to zero as k increases to infinity Eigenvalues must have to property λ i < i =,, n Theorem Asymptotic Stability of Linear Systems: A discretetime linear timeinvariant system is asymptotically stable if and only if all eigenvalues of Φ are strictly inside the unit disk TU Berlin DiscreteTime Control Systems 5 TU Berlin DiscreteTime Control Systems 6 Stability Tests InputOutput Stability Definition BoundedInput BoundedOutput Stability: A linear timeinvariant system is defined boundedinput boundedoutput BIBO stable if a bounded input gives a bounded output for every initial value Theorem Relation between Stability Concepts: Asymptotic stability implies stability and BIBO stability Stability does not imply BIBO stability, and vice versa! Numerical computation of the eigenvalues of Φ or the roots of the characteristic equation detzi Φ = a z n + a z n + + a n = Scilab commands spec and roots Algebraic or graphical methods help to understand how parameters of the system or controller will influence the stability: Direct algebraic computation of the eigenvalues Methods based on the properties of characteristic polynomials Root locus method used to determine closedloop stability for known openloop system The Nyquist criterion used to determine closedloop stability for known openloop system Lyapunov s method
2 TU Berlin DiscreteTime Control Systems 7 Jury s Stability Criterion Characteristic polynomial Az = a z n + a z n + + a n = See blackboard: TU Berlin DiscreteTime Control Systems 8 Remark: If all a k are positive for k >, then the condition a can be shown to be equivalent to the conditions A > n A > These are necessary conditions for stability and hence can be used before forming the table above Example for Jury s Stability Criterion: See blackboard Theorem Jury s Stability Test: If a >, then Eq has all roots inside the unit disk if and only if all a k, k =,,, n are positive If no a k is zero, then the number of negative ak is equal to the number of roots outside the unit disk TU Berlin DiscreteTime Control Systems 9 TU Berlin DiscreteTime Control Systems Nyquist and Bode Diagrams for DiscreteTime Systems Continuoustime system Gs: The Nyquist curve or frequency response of the system is the Nyquist plot: Gjω dashed, He jω solid map Gjω for ω [, This curve is drawn in polar coordinates Nyquist diagram or as amplitude and phase curves as a function of frequency Bode diagram Gjω interpretation: Stationary amplitude and phase of system output when a sinusoidal input signal with frequency ω is applied Discretetime system with pulsetransfer function Hz: The Nyquist curve or frequency curves is given by the map He jω for ω [, π] up to the Nyquist frequency He jω is periodic with period 2π/ Higher harmonics are created Example: Gs = 66z + 55 s 2 ZOH & = 4s Hz = + 4s + z 2 45z + 57 TU Berlin DiscreteTime Control Systems Bode plot: Gjω dashed, He jω solid TU Berlin DiscreteTime Control Systems 2 Nyquist Criterion Wellknown stability test for continuoustime systems To determine the stability of the closedloop system when the openloop system is given Can be reformulated to handle discretetime systems Consider the discretetime system: H clz = Y z U cz = Hz + Hz with the characteristic polynomial + Hz =
3 TU Berlin DiscreteTime Control Systems 3 TU Berlin DiscreteTime Control Systems 4 Nyquist contour Γ c that encircles the area outside the unit disc instability area: Stability is determined by analysing how Γ c is mapped by Hz The number of encirclements N in the positive direction around , by the map Γ c is equal to N = Z P where Z and P are the numbers of zeros and poles, respectively, of + Hz outside the unit disk Zeros Z of + H are unstable closedloop poles Poles P of + H are unstable openloop poles Indentation at z = : to exclude the integrators in the openloop system infinitesimal semicircle with decresing arguments from π/2 to π/2 is mapped into the Hzplane as an infinitely large circle from nπ/2 to nπ/2 n  number of integrators in the openloop system If the openloop system is stable P = then N = Z The stability of the system is then ensured if the map of Γ c does not encircle the point , If Hz when z, the parallel lines III and V do not influence the stability test, and it is sufficient to find the map of the unit circle and the small semicircle at z = The map of the unit circle is He jω for ω, 2π TU Berlin DiscreteTime Control Systems 5 TU Berlin DiscreteTime Control Systems 6 Simplified Nyquist Criterium: If the openloop system and its inverse are stable then the stability of the closedloop system is ensured if the point , in the Hzplane is to the left of the map of He jω for ω = to π that is, to the left of the Nyquist curve Example: Consider the system 25K Hz = z z 5 with = The black line on the right graphic shows He jω for ω = to π, the Nyquist curve Relative Stability Amplitude and phase margins can be defined for discretetime systems analogously to continuoustime systems Definition Amplitude Margin: Let the openloop system have the pulsetransfer function Hz and let ω the smallest frequency such that arg He jω = π The amplitude or gain margin is then defined as A marg = He jω The amplitude margin is how much the gain can be increased before the closedloop system becomes unstable TU Berlin DiscreteTime Control Systems 7 TU Berlin DiscreteTime Control Systems 8 Sensitivity and Complementary Sensitivity Function Consider the closedloop system with the feedforward filter H ff and the feedback controller H fb: Definition Phase Margin: Let the openloop system have the pulsetransfer function Hz and further let the crossover frequency ω c be the smallest frequency such that uc Hff v H x e y He jωc = The phase margin is then defined as Hfb n φ marg = π + arg He jωc The phase margin is how much extra phase lag is allowed before the closedloop system becomes unstable Pulsetransfer operator from the inputs to y: Openloop transfer function: y = Hff H + L uc + H + L v + + L e L + L n L = H fbh
4 TU Berlin DiscreteTime Control Systems 9 TU Berlin DiscreteTime Control Systems 2 Closedloop transfer function: Sensitivity function S: H cl = Hff H + L Sensitivity of H cl with respect to variations in H is given by dh cl dh = Hff + L 2 The relative sensitivity of H cl with respect to H thus can be written as dh cl = dh H cl + L H = S dh H S = + L Pulsetransfer function from e to y, should be small at low frequencies Complementary sensitivity function T : T = S = L + L negative Pulsetransfer function from n to y or e to x, should be small at higher frequencies Remark: Due to T + S =, we can not make T and S for all frequencies and must make a tradeoff as outlined above! TU Berlin DiscreteTime Control Systems 2 Robustness The distance r from the Nyquist curve Le jω to the critical point , is a good measure for the robustness and is given by r = + L Note that the complex number + Le jω can be represented as the vector from the point to the point Le jω on the Nyquist curve Reciprocal of the smallest distance r min from the Nyquist curve Le jω to the critical point: = max r min + Le jω = max Se jω 2 Guaranteed bounds on the margins: A marg / r min, 3 φ marg 2 arcsinr min/2 4 By requiring max Se jω < 2, the system will have at least the robustness margins A marg 2 and φ marg 29 TU Berlin DiscreteTime Control Systems 22 Controllability, Reachability Question: Is it possible to steer a system from a given initial state to any other state? Consider the system x[k + ] = Φx[k] + Γ u[k] 5 y[k] = Cx[k] 6 Assume that the initial state x[] is given At state the sample instant n, where n is the order of the system, is given by where x[n] = Φ n x[] + Φ n Γ u[] + + Γ u[n ] = Φ n x[] + W cu W c = Γ ΦΓ Φ n Γ U = u T [n ] u T [] TU Berlin DiscreteTime Control Systems 23 TU Berlin DiscreteTime Control Systems 24 The matrix W c is referred to as the controllability matrix If W c has rank, then it is possible to find n equations from which the control signal can be found such that the initial state is transferred to the desired state x[n] The solution is not unique if there is more than one input! Definition Controllability: The system 5 is controllable if it is possible to find a control sequence such that the origin can be reached from any initial state in finite time Definition Reachability: The system 5 is reachable if it is possible to find a control sequence such that an arbitrary state can be reached from any initial state in finite time Controllability does not imply reachability: If Φ n x[] =, then the origin will be reached with zero input, but the system is not necessarily reachable See example Reachability is independent of the coordinates: W c = Γ Φ Γ Φn Γ = T Γ T ΦT T Γ T Φ n T T Γ = T W c If W c has rank n, than W c has also rank n for a nonsingular transformation matrix T Theorem Reachability: The system 5 is reachable if and only if the matrix W c has rand n The reachable states belong to the linear subspace spanned by the columns of W c Example
5 TU Berlin DiscreteTime Control Systems 25 TU Berlin DiscreteTime Control Systems 26 Assume a SISO system, a nonsingular W c and detλi Φ = λ n + a λ n + + a n = 7 Then there exists a transformation on controllable canonical form a a 2 a n a n z[k + ] = z[k] + u[k] y[k] = b b n z[k] State Trajectory Following Question: Does reachability also imply that is possible to follow a given trajectory in the state space? In order to drive a system from x[k] to a desired x[k + ] the matrix Γ must have rank n For a reachable SISO system it is, in general possible, to reach desired states only at each nth sample instant, provided that the desired points are known n steps ahead Good for computation of I/O model and for the design of a statefeedbackcontrol law TU Berlin DiscreteTime Control Systems 27 TU Berlin DiscreteTime Control Systems 28 Observability and Detectability Output Trajectory Following Assume that a desired trajectory u c[k] is given, the control signal should satisfy or y[k] = Bq u[k] = uc[k] Aq u[k] = Aq Bq uc[k] For a timedelay of d steps the generation of u[k] is only causal if the trajectory is known d steps ahead The signal u[k] is bounded if u c[k] is bounded and if the system has a stable inverse To solve the problem of finding the state of a system from observations of the output, the concept of unobservable states is introduced at first: Definition  Unobservable States: The state x is unobservable if there exists a finite k n such that y[k] = for k k when x = x o and u[k] = for k k The system 5 is observable if there is a finite k such that knowledge of the inputs u[],, u[k ] and the outputs y[],, y[k ] is sufficient to determine the initial state of the system Effect of u[k] always can be determined Without loss of generality we assume u[k] = y[] = Cx[] y[] = Cx[] = CΦx y[n ] = CΦ n x[] TU Berlin DiscreteTime Control Systems 29 Using vector notations: C y[] CΦ y[] x[] = CΦ n y[n ] The initial state x[] can be obtained if and only if the observability matrix has rank n W o = C CΦ CΦ n Theorem  Observability The system 5 is observable if and only if W has rank n TU Berlin DiscreteTime Control Systems 3 Theorem  Detectability A system is detectable if the only unobservable states are such that they decay to the origin That is, the corresponding eigenvalues are stable Observable Canonical Form Assume a SISO system, a nonsingular matrix W and detλi Φ = λ n + a λ n + + a n = 8 then there exists a transformation such that the transformed system is a b a 2 b 2 z[k + ] = z[k] + u[k] a n b n a n b n y[k] = z[k] which is called observable canonical form
6 TU Berlin DiscreteTime Control Systems 3 TU Berlin DiscreteTime Control Systems 32 Kalman s Decomposition The reachable and unobservable parts of a system are two linear subspaces of the state space This form has the advantage that it is easy to find the I/O model and to determine a suitable observer The transformation is given by T = W o W o where W o is the observable matrix of the system in observable canonical form Kalman showed that it is possible to introduce coordinates such that a system can be partitioned in the following way: Φ Φ 2 Φ 22 x[k + ] = Φ 3 Φ 32 Φ 33 Φ 34 Φ 42 Φ 44 y[k] = C C 2 x[k] Γ x[k] + u[k] Γ 3 TU Berlin DiscreteTime Control Systems 33 TU Berlin DiscreteTime Control Systems 34 The state space is partitioned into four parts yielding for subsystems: S or Observable and reachable S or Observable but not reachable S or Not observable but reachable S or Neither observable nor reachable The pulsetransfer operator for the observable and reachable subsystem is given by: Hq = C qi Φ Γ See picture Loss of Reachability and Observability Trough Sampling To get a reachable discretetime system, it is necessary that the continuoustime system is also reachable However it may happen that reachability is lost for some sampling periods Conditions for unobservability are more restricted in the continuoustime case: output has to be zero over a time interval for discretetime system only at sampling instants Continuoustime system may oscillate between sampling instants and remain zero at sampling instants hidden oscillations The sampleddata system thus can be unobservable even if the corresponding continuoustime system is observable
Analysis of DiscreteTime Systems
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