# Nonlinear Control Lecture 2:Phase Plane Analysis

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1 Nonlinear Control Lecture 2:Phase Plane Analysis Farzaneh Abdollahi Department of Electrical Engineering Amirkabir University of Technology Fall 2010 r. Farzaneh Abdollahi Nonlinear Control Lecture 2 1/53

2 Phase Plane Analysis: is a graphical method for studying second-order systems by providing motion trajectories corresponding to various initial conditions. then examine the qualitative features of the trajectories. finally obtaining information regarding the stability and other motion patterns of the system. It was introduced by mathematicians such as Henri Poincare in 19th century. Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 2/53

3 Motivations Importance of Knowing Phase Plane Analysis: Since it is on second-order, the solution trajectories can be represented by carves in plane provides easy visualization of the system qualitative behavior. Without solving the nonlinear equations analytically, one can study the behavior of the nonlinear system from various initial conditions. It is not restricted to small or smooth nonlinearities and applies equally well to strong and hard nonlinearities. There are lots of practical systems which can be approximated by second-order systems, and apply phase plane analysis. Disadvantage of Phase Plane Method: It is restricted to at most second-order and graphical study of higher-order is computationally and geometrically complex. Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 3/53

4 Introduction Motivation Phase Plane Vector Field Diagram Isocline Method Qualitative Behavior of Linear Systems Case 1: λ 1 λ 2 0 Case 2: Complex Eigenvalues, λ 1,2 = α ± jβ Case 3: Nonzero Multiple Eigenvalues λ 1 = λ 2 = λ 0 Case 4: Zero Eigenvalues Local Behavior of Nonlinear Systems Limit Cycle Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 4/53

5 Concept of Phase Plane Phase plane method is applied to autonomous 2nd order system described as follows: f 1, f 2 : R 2 R. ẋ 1 = f 1 (x 1, x 2 ) (1) ẋ 2 = f 2 (x 1, x 2 ) (2) System response (x(t) = (x 1 (t), x 2 (t))) to initial condition x 0 = (x 10, x 20 ) is a mapping from R to R 2. The x 1 x 2 plane is called State plane or Phase plane The locus in the x 1 x 2 plane of the solution x(t) for all t 0 is a curve named trajectory or orbit that passes through the point x 0 The family of phase plane trajectories corresponding to various initial conditions is called Phase protrait of the system. Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 5/53

6 How to Construct Phase Plane Trajectories? Despite of exiting several routines to generate the phase portraits by computer, it is useful to learn roughly sketch the portraits or quickly verify the computer outputs. Some methods named: Isocline, Vector field diagram, delta method, Pell s method, etc Vector Field Diagram: Revisiting (1) [ and (2): ] f1 (x) ẋ = f (x) =, ẋ = (ẋ f 2 (x) 1, ẋ 2 ) To each vector (x1, x 2 ), a corresponding vector (f 1 (x 1, x 2 ), f 2 (x 1, x 2 )) known as a vector field is associated. Example: If f (x) = (2x1 2, x 2), for x = (1, 1), next point is (1, 1) + (2, 1) = (3, 2) Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 6/53

7 Vector Field Diagram By repeating this for sufficient point in the state space, a vector field diagram is obtained. Noting that dx 2 dx 1 = f 2 f1 vector field at a point is tangent to trajectory through that point. starting from x0 and by using the vector field with sufficient points, the trajectory can be constructed. Example: Pendulum without friction x 1 = x 2 x 2 = 10 sin x 1 r. Farzaneh Abdollahi Nonlinear Control Lecture 2 7/53

8 Isocline Method The term isocline derives from the Greek words for same slope. Consider again Eqs (1) and (2), the slope of the trajectory at point x: S(x) = dx 2 = f 2(x 1, x 2 ) dx 1 f 1 (x 1, x 2 ) An isocline with slope α is defined as S(x) = α all the points on the curve f 2 (x 1, x 2 ) = αf 1 (x 1, x 2 ) have the same tangent slope α. Note that the time is eliminated here The responses x 1 (t) and x 2 (t) cannot be obtained directly. Only qualitative behavior can be concluded, such as stable or oscillatory response. Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 8/53

9 Isocline Method The algorithm of constructing the phase portrait by isocline method: 1. Plot the curve S(x) = α in state-space (phase plane) 2. Draw small line with slope α. Note that the direction of the line depends on the sign of f 1 and f 2 at that point. 3. Repeat the process for sufficient number of α s.t. the phase plane is full of isoclines. Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 9/53

10 Example: Pendulum without Friction Consider the dynamics ẋ 1 = x 2, ẋ 2 = sinx 1 S(x) = sinx 1 x 2 = c Isoclines: x 2 = 1 c sinx 1 Trajectories for different init. conditions can be obtained by using the given algorithm The response for x 0 = ( π 2, 0) is depicted in Fig. The closed curve trajectory confirms marginal stability of the system. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 10/53

11 Example: Pendulum with Friction Dynamics of pendulum with friction: ẋ 1 = x 2, ẋ 2 = 0.5x 2 sinx 1 S(x) = 0.5 sinx 1 x 2 Isoclines: x 2 = c sinx 1 Similar Isoclines but with different slopes Trajectory is drawn for x 0 = ( π 2, 0) = c The trajectory shrinks like an spiral converging to the origin r. Farzaneh Abdollahi Nonlinear Control Lecture 2 11/53

12 Qualitative Behavior of Linear Systems First we analyze the phase plane of linear systems since the behavior of nonlinear systems around equilibrium points is similar of linear ones For LTI system: ẋ = Ax, A R 2 2, x 0 : initial state x(t) = Me Jr t M 1 x 0 J r : Jordan block of A, M : Matrix of eigenvectors M 1 AM = J r Depending on the eigenvalues of A, J r has one of the following forms: [ ] λ1 0 λ i : real & distinct J r = 0 λ 2 [ ] λ k λ i : real & multiple J r =, k = 0, 1, 0 λ [ ] α β λ i : complex J r = β α The system behavior is different at each case Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 12/53

13 Case 1: λ 1 λ 2 0 In this case M = [v 1 v 2 ] where v 1 and v 2 are real eigenvectors associated with λ 1 and λ 2 To transform the system into two decoupled first-order diff equations, let z = M 1 x: ż 1 = λ 1 z 1 ż 2 = λ 2 z 2 The solution for initial states (z 01, z 02 ): z 1 (t) = z 10 e λ 1t, z 2 (t) = z 20 e λ 2t eliminating t z 2 = Cz λ 2/λ 1 1, C = z 20 /(z 10 ) λ 2/λ 1 (3) Phase portrait is obtained by changing C R and plotting (3). The phase portrait depends on the sign of λ 1 and λ 2. Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 13/53

14 Case 1.1: λ 2 < λ 1 < 0 t the terms e λ1t and e λ2t tend to zero Trajectories from entire state-space tend to origin the equilibrium point x = 0 is stable node. e λ2t 0 faster λ 2 is fast eignevalue and v 2 is fast eigenvector. Slope of the curves: dz 2 dz 1 = C λ 2 λ 1 z (λ 2/λ 1 1) 1 λ 2 < λ 1 < 0 λ 2 /λ 1 > 1, so slope is zero as z 1 0 infinity as z 1. The trajectories are tangent to z 1 axis, as they approach to origin parallel to z 2 axis, as they are far from origin. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 14/53

15 Case 1.1: λ 2 < λ 1 < 0 Since z 2 approaches to zero faster than z 1, trajectories are sliding along z 1 axis In X plane also trajectories are: tangent to the slow eigenvector v 1 for near origin parallel to the fast eigenvector v2 for far from origin r. Farzaneh Abdollahi Nonlinear Control Lecture 2 15/53

16 Case 1.2: λ 2 > λ 1 > 0 t the terms e λ1t and e λ2t grow exponentially, so The shape of the trajectories are the same, with opposite directions The equilibrium point is socalled unstable node r. Farzaneh Abdollahi Nonlinear Control Lecture 2 16/53

17 Case 1.3: λ 2 < 0 < λ 1 t e λ2t 0, but e λ1t,so λ 2 : stable eigenvalue, v 2 : stable eigenvector λ 1 : unstable eigenvalue, v 1 : unstable eigenvector Trajectories are negative exponentials since λ 2 λ 1 is negative. Trajectories are decreasing in z 2 direction, but increasing in z 1 direction tangent to z1 as z 1 and tangent to z 2 as z 1 0 r. Farzaneh Abdollahi Nonlinear Control Lecture 2 17/53

18 Case 1.3: λ 2 < 0 < λ 1 The exceptions of this hyperbolic shape: two trajectories along z 2 -axis 0 as t 0, called stable trajectories two trajectories along z 1 -axis as t 0, called unstable trajectories This equilibrium point is called saddle point Similarly in X plane, stable trajectories are along v 2, but unstable trajectories are along the v 1 For λ 1 < 0 < λ 2 the direction of the trajectories are changed. Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 18/53

19 Case 2: Complex Eigenvalues, λ 1,2 = α ± jβ z 1 = αz 1 βz 2 z 2 = βz 1 + αz 2 The solution is oscillatory = polar coordinates (r = z1 2 + z2 2, θ = tan 1 ( z 2 z 1 )) ṙ = αr r(t) = r 0 e αt θ = β θ(t) = θ 0 + βt This results in Z plane is a logarithmic spiral where α determines the form of the trajectories: α < 0 : as t r 0 and angle θ is rotating. The spiral converges to origin = Stable Focus. α > 0: as t r and angle θ is rotating. The spiral diverges away from origin = Unstable Focus. α = 0: Trajectories are circles with radius r 0 = Center Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 19/53

20 Case 2: Complex Eigenvalues, λ 1,2 = α ± jβ r. Farzaneh Abdollahi Nonlinear Control Lecture 2 20/53

21 Case 3: Nonzero Multiple Eigenvalues λ 1 = λ 2 = λ 0 Let z = M 1 x: ż 1 = λz 1 + kz 2, ż 2 = λz 2 the solution is :z 1 (t) = e λt (z 10 + kz 20 t), z 2 (t) = z 20 e λt [ z10 z 1 = z 2 + k ( )] z 20 λ ln z2 Phase portrait are depicted for k = 0 and k = 1. When the eignevectors are different k = 0: similar to Case 1, for λ < 0 is stable, λ > 0 is unstable. Decaying rate is the same for both modes (λ 1 = λ 2 ) trajectories are lines z 20 r. Farzaneh Abdollahi Nonlinear Control Lecture 2 21/53

22 Case 3: Nonzero Multiple Eigenvalues λ 1 = λ 2 = λ 0 There is no fast-slow asymptote. k = 1 is more complex, but it is still similar to Case 1: Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 22/53

23 Case 4.1: One eigenvalue is zero λ 1 = 0, λ 2 0 A is singular in this case Every vector in null space of A is an equilibrium point There is a line (subspace) of equilibrium points M = [v 1 v 2 ], v 1, v 2 : corresponding eigenvectors, v 1 N (A). ż 1 = 0, ż 2 = λ 2 z 2 solution: z 1 (t) = z 10, z 2 (t) = z 20 e λ 2t Phase portrait depends on sign of λ 2 : λ 2 < 0: Trajectories converge to equilibrium line λ2 > 0: Trajectories diverge from equilibrium line r. Farzaneh Abdollahi Nonlinear Control Lecture 2 23/53

24 Case 4.2: Both eigenvalues zero λ 1 = λ 2 = 0 Let z = M 1 x ż 1 = z 2, ż 2 = 0 solution: z 1 (t) = z 10 + z 20 t, z 2 (t) = z 20 z 1 linearly increases/decreases base on the sign of z 20 z 2 axis is equilibrium subspace in Z-plane Dotted line is equilibrium subspace The difference between Case 4.1 and 4.2: all trajectories start off the equilibrium set move parallel to it. Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 24/53

25 As Summary: Six types of equilibrium points can be identified: stable/unstable node saddle point stable/ unstable focus center Type of equilibrium point depends on sign of the eigenvalues If real part of eignevalues are Positive unstability If real part of eignevalues are Negative stability All properties for linear systems hold globally Properties for nonlinear systems only hold locally Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 25/53

26 Local Behavior of Nonlinear Systems Qualitative behavior of nonlinear systems is obtained locally by linearization around the equilibrium points Type of the perturbations and reaction of the system to them determines the degree of validity of this analysis A simple example: Consider the linear perturbation case A A + A, where A R 2 2 : small perturbation Eigenvalues of a matrix continuously depend on its parameters Positive (Negative) eigenvalues of A remain positive (negative) under small perturbations. For eigenvalues on the jω axis no matter how small perturbation is, it changes the sign of eigenvalue. Therefore node or saddle point or focus equilibrium point remains the same under small perturbations This analysis is not valid for a center equilibrium point Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 26/53

27 Multiple Equilibria Linear systems can have an isolated equilibrium point or a continuum of equilibrium points (When deta = 0) Unlike linear systems, nonlinear systems can have multiple isolated equilibria. Qualitative behavior of second-order nonlinear system can be investigated by generating phase portrait of system globally by computer programs linearize the system around equilibria and study the system behavior near them without drawing the phase portrait Let (x 10, x 20) are equilibrium points of ẋ 1 = f 1(x 1, x 2) ẋ 2 = f 2(x 1, x 2) (4) f 1, f 2 are continuously differentiable about (x 10, x 20) Since we are interested in trajectories near (x 10, x 20), define x 1 = y 1 + x 10, x 2 = y 2 + x 20 y 1, y 2 are small perturbations form equilibrium point. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 27/53

28 Qualitative Behavior Near Equilibrium Points Expanding (4) into its Taylor series ẋ 1 = ẋ 10 + ẏ 1 = f 1 (x 10, x 20 ) + f 1 }{{} x 1 0 ẋ 2 = ẋ 20 + ẏ 2 = f 2 (x 10, x 20 ) + f 2 }{{} x 1 0 (x10,x 20 ) (x10,x 20 ) y 1 + f 1 x 2 y 2 + H.O.T. (x10,x 20 ) y 1 + f 2 x 2 y 2 + H.O.T. (x10,x 20 ) For sufficiently small neighborhood of equilibrium points, H.O.T. are negligible { ẏ1 = a 11 y 1 + a 12 y 2, a ẏ 2 = a 21 y 1 + a 22 y ij = f i 2 x, i = 1, 2 x0 The equilibrium point of the linear system is (y 1 = y 2 = 0) [ ] [ ] a11 a f1 f 1 ẏ = Ay, A = 12 x = 1 x 2 = f a 21 a 22 x f 2 x 1 f 2 x 2 Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 28/53 x0

29 Qualitative Behavior Near Equilibrium Points Matrix f x is called Jacobian Matrix. The trajectories of the nonlinear system in a small neighborhood of an equilibrium point are close to the trajectories of its linearization about that point: if the origin of the linearized state equation is a stable (unstable) node, or a stable (unstable) focus or a saddle point, then in a small neighborhood of the equilibrium point, the trajectory of the nonlinear system will behave like a stable (unstable) node, or a stable (unstable) focus or a saddle point. Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 29/53

30 Example: Tunnel Diode Circuit ẋ 1 = 1 C [ h(x 1) + x 2 ] ẋ 2 = 1 L [ x 1 Rx 2 + u] u = 1.2v, R = 1.5KΩ, C = 2pF, L = 5µH, time in nanosecond, current in ma ẋ 1 = 0.5[ h(x 1 ) + x 2 ] ẋ 2 = 0.2[ x 1 1.5x ] Suppose h(x 1 ) = 17.76x x x x x 1 5 equilibrium points (ẋ 1 = ẋ 2 = 0): Q 1 = (0.063, 0.758), Q 2 = (0.285, 0.61), Q 3 = (0.884, 0.21) Dr. Farzaneh Abdollahi Nonlinear Control Lecture 2 30/53

31 Example: Tunnel Diode Circuit The global phase portrait is generated by a computer program is shown in Fig. Except for two special trajectories which approach Q 2, all trajectories approach either Q 1 or Q 3. Near equilibrium points Q 1 and Q 3 are stable nodes,q 2 is like saddle point. The two special trajectories from a curve that divides the plane into two halves with different behavior (separatrix curves). All trajectories originating from left side of the curve approach to Q 1 All trajectories originating from left side of the curve approach to Q 3 r. Farzaneh Abdollahi Nonlinear Control Lecture 2 31/53

32 Tunnel Diode:Qualitative Behavior Near Equilibrium Points Jacobian matrix f x = [ 0.5 h(x 1 ) ] h(x 1 ) = dh dx 1 = x x x x 4 1 Evaluate the Jacobian matrix at the equilibriums Q 1, Q 2, Q 3 : [ ] = (0.063, 0.758), A 1 =, λ = 3.57, λ 2 = 0.33 stable node [ ] Q 2 = (0.285, 0.61), A 2 =, λ = 1.77, λ 2 = 0.25 saddle point [ ] Q 3 = (0.884, 0.21), A 3 =, λ = 1.33, λ 2 = 0.4 stable node similar results given from global phase portrait. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 32/53

33 Tunnel Diode Circuit In practice, There are only two stable equilibrium points: Q 1 or Q 3. Equilibrium point at Q 2 in never observed, Even if set up the exact initial conditions corresponding t Q2, the ever-present physical noise causes the trajectory to diverge from Q 2 Such circuit is called bistable, since it has two steady-state operating points. Triggering formq 1 to Q 3 or vice versa is achieved by changing the load line r. Farzaneh Abdollahi Nonlinear Control Lecture 2 33/53

34 Special case: If the Jacobian matrix has eigenvalues on jω, then the qualitative behavior of nonlinear system near the equilibrium point could be quite distinct from the linearized one. Example: ẋ 1 = x 2 µx 1 (x x 2 2 ) ẋ 2 = x 1 µx 2 (x x 2 2 ) It has equilibrium point at origin x = 0. [ ] 0 1 A = λ 1 0 1,2 = ±j center Now consider the nonlinear system x 1 = r cos θ, x 2 = r sin θ ṙ = µr 3, θ = 1 nonlinear system is stable if µ > 0 and is unstable if µ < 0 the qualitative behavior of nonlinear and linearized one are different. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 34/53

35 Limit Cycle A system oscillates when it has a nontrivial periodic solution x(t + T ) = x(t), t 0, for somet > 0 The word nontrivial is used to exclude the constant solutions. The image of a periodic solution in the phase portrait is a closed trajectory, calling periodic orbit or closed orbit. We have already seen oscillation of linear system with eigenvalues ±jβ. The origin of the system is a center, and the trajectories are closed the solution in Jordan form: z 1 (t) = r 0 cos(βt + θ 0 ), z 2 = r 0 sin(βt + θ 0 ) r 0 = z z2 20, θ 0 = tan 1 z 20 z 10 r0 : amplitude of oscillation Such oscillation where there is a continuum of closed orbits is referred to harmonic oscillator. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 35/53

36 Limit Cycle The physical mechanism leading to these oscillations is a periodic exchange of energy stored in the capacitor (electric field) and the inductor (magnetic field). We have seen that such oscillation is not robust any small perturbations destroy the oscillation. The linear oscillator is not structurally stable The amplitude of the oscillation depends on the initial conditions. These problems can be eliminated in nonlinear oscillators. A practical nonlinear oscillator can be build such that The nonlinear oscillator is structurally stable The amplitude of oscillation (at steady state) is independent of initial conditions. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 36/53

37 Limit Cycle On phase plane, a limit cycle is defined as an isolated closed orbit. For limit cycle the trajectory should be 1. closed: indicating the periodic nature of the motion 2. isolated: indicating limiting nature of the cycle with nearby trajectories converging to/ diverging from it. The mass spring damper does not have limit cycle; they are not isolated. Depends on trajectories motion pattern in vicinity of limit cycles, there are three type of limit cycle: Stable Limit Cycles: as t all trajectories in the vicinity converge to the limit cycle. Unstable Limit Cycles: as t all trajectories in the vicinity diverge from the limit cycle. Semi-stable Limit Cycles: as t some trajectories in the vicinity converge to/ and some diverge from the limit cycle. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 37/53

38 Limit Cycle r. Farzaneh Abdollahi Nonlinear Control Lecture 2 38/53

39 Example1.a: stable limit cycle ẋ 1 = x 2 x 1 (x x 2 2 1) ẋ 2 = x 1 x 2 (x x 2 2 1) Polar coordinates (x 1 := rcos(θ), x 2 =: rsin(θ)) ṙ = r(r 2 1) θ = 1 If trajectories start on the unit circle (x 2 1 (0) + x 2 2 (0) = r 2 = 1), then ṙ = 0 = The trajectory will circle the origin of the phase plane with period of 1 2π. r < 1 = ṙ > 0 = trajectories converges to the unit circle from inside. r > 1 = ṙ < 0 = trajectories converges to the unit circle from outside. Unit circle is a stable limit cycle for this system. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 39/53

40 Example1.b: unstable limit cycle ẋ 1 = x 2 + x 1 (x x 2 2 1) ẋ 2 = x 1 + x 2 (x x 2 2 1) Polar coordinates (x 1 := rcos(θ), x 2 =: rsin(θ)) ṙ = r(r 2 1) θ = 1 If trajectories start on the unit circle (x 2 1 (0) + x 2 2 (0) = r 2 = 1), then ṙ = 0 = The trajectory will circle the origin of the phase plane with period of 1 2π. r < 1 = ṙ < 0 = trajectories diverges from the unit circle from inside. r > 1 = ṙ > 0 = trajectories diverges from the unit circle from outside. Unit circle is an unstable limit cycle for this system. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 40/53

41 Example1.c: semi stable limit cycle ẋ 1 = x 2 x 1 (x x 2 2 1) 2 ẋ 2 = x 1 x 2 (x x 2 2 1) 2 Polar coordinates (x 1 := rcos(θ), x 2 =: rsin(θ)) ṙ = r(r 2 1) 2 θ = 1 If trajectories start on the unit circle (x 2 1 (0) + x 2 2 (0) = r 2 = 1), then ṙ = 0 = The trajectory will circle the origin of the phase plane with period of 1 2π. r < 1 = ṙ < 0 = trajectories diverges from the unit circle from inside. r > 1 = ṙ < 0 = trajectories converges to the unit circle from outside. Unit circle is a semi-stable limit cycle for this system. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 41/53

42 Bendixson s Criterion: Nonexistence Theorem of Limit Cycle Gives a sufficient condition for nonexistence of a periodic solution: Suppose Ω is simply connected region in 2-dimention space in this region we define f = f 1 x 1 + f 2 x 2. If f is not identically zero over any subregion of Ω and does not change sign in Ω, then Ω contain no limit cycle for the nonlinear systeṁ x1 = f 1 (x 1, x 2 ) ẋ 2 = f 2 (x 1, x 2 ) Simply connected set: the boundary of the set is connected + the set is connected Connected set: for connecting any two points belong to the set, there is a line which remains in the set. The boundary of the set is connected if for connecting any two points belong to boundary of the set there is a line which does not cross the set exmaple of connected but not simply connected set r. Farzaneh Abdollahi Nonlinear Control Lecture 2 42/53

43 Bendixson s Criterion Proof by contradiction: Recall that dx 2 dx 1 = f2 f 1 = f 2 dx 1 f 1 dx 2 = 0, Along a closed curve L of a limit cycle: (f 2 dx 1 f 1 dx 2 ) = 0 L Using Stoke s Theorem: ( L f.ndl = fds = 0 S is enclosed by L) S ( f1 + f ) 2 dx 1 dx 2 = 0 x 1 x 2 S This is true if f = 0 x S or f changes sign in S This is in contradiction with the assumption that f1 x 1 and does not change sign there is no closed trajectory. + f2 x 2 does not vanish r. Farzaneh Abdollahi Nonlinear Control Lecture 2 43/53

44 Nonexistence Theorem of Periodic Solutions for Linear Systems Sufficient condition for nonexistence of a periodic solution in linear systems: ẋ 1 = a 11 x 1 + a 12 x 2 ẋ 2 = a 21 x 1 + a 22 x 2 f 1 x 1 + f 2 x 2 = a 11 + a 22 0= no periodic sol. This is consistent with eigenvalue analysis form of center point which is obtained for periodic solutions: λ 2 (a 11 + a 22 ) λ + (a 11 a 22 a 12 a 21 ) = 0 center a 11 + a 22 = 0, a 11 a 22 a 12 a 21 > 0 r. Farzaneh Abdollahi Nonlinear Control Lecture 2 44/53

45 Limit Cycle Example for nonexistence of limit cycle ẋ 1 = g(x 2 ) + 4x 1 x 2 2 ẋ 2 = h(x 1 ) + 4x 2 1 x 2 f 1 x 1 + f 2 x 2 = 4(x1 2 + x 2 2 ) > 0 x R2 No limit cycle exist in R 2 for this system. Note that: there is no equivalent theorem for higher order systems. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 45/53

46 Poincare-Bendixson Criterion: Existence Theorem of Limit Cycle If there exists a closed and bounded set M s.t. 1. M contains no equilibrium point or contains only one equilibrium point such that the Jacobian matrix f x at this point has eigenvalues with positive real parts (unstable focus or node). 2. Every trajectory starting in M stays in M for all future time = M contains a periodic solution The idea behind the theorem is that all possible shape of limit points in a plane (R 2 ) are either equilibrium points or periodic solutions. Hence, if the positive limit set contains no equilibrium point, it must have a periodic solution. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 46/53

47 Poincare-Bendixson Criterion: Existence Theorem of Limit Cycle If M has unstable node/focus, in vicinity of that equilibrium point all trajectories move away By excluding the vicinity of unstable node/focus, the set M is free of equilibrium and all trajectories are trapped in it. No equivalent theorem for R n, n 3. A solution could be bounded in R 3, but neither it is periodic nor it tends to a periodic solution. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 47/53

48 Example for Existence Theorem of Limit Cycle ẋ 1 = x 2 x 1 (x x 2 2 1) ẋ 2 = x 1 x 2 (x x 2 2 1) Polar coordinate: ṙ = (1 r 2 )r θ = 1 ṙ 0 for r 1 + η 1, η 1 > 0 ṙ 0for r 1 η 2, 1 > η 2 > 0 The area found by the circles with radius 1 η 2 and 1 + η 1 satisfies the condition of the P.B. theorem a periodic solution exists. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 48/53

49 Existence Theorem of Limit Cycle A method to investigate whether or not trajectories remain inside M: Consider a simple closed curve V (x) = c, where V (x) is a continuously differentiable function The vector f at a point x on the curve points inward if the inner product of f and the gradient vector V (x) is negative: f (x). V (x) = V x 1 f 1(x) + V x 2 f 2(x) < 0 outward if f (x). V (x) > 0 tangent to the curve if f (x). V (x) = 0. Trajectories can leave a set only if the vector filed points outward at some points on the boundary. For a set of the form M = {V (x) c}, for some c > 0, trajectories trapped inside M if f (x). V (x) 0 on the boundary of M. For an annular region of the form M = {W (x) c 1 and V (x) c 2 } for some c 1, c 2 > 0, trajectories remain inside M if f (x). V (x) 0 on V (x) = c 2 and f (x). W (x) 0 on W (x) = c 1. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 49/53

50 Example for Existence Theorem of Limit Cycle Consider the system ẋ 1 = x 1 + x 2 x 1 (x x 2 2 ) ẋ 2 = 2x 1 + x 2 x 2 (x x 2 2 ) The system has a unique equilibrium point at the origin. The Jacobian matrix [ f 1 3x 2 x = 1 x 2 ] 2 1 2x 1 x 2 x=0 2 2x 1 x 2 1 x1 2 3x 2 2 has eigenvalues at 1 ± j 2 Let M = V (x) c, where V (x) = x x 2 2. x=0 = [ M is bounded and contains eigenvalues with positive real parts ] r. Farzaneh Abdollahi Nonlinear Control Lecture 2 50/53

51 Example for Existence Theorem of Limit Cycle On the surface V (x) = c, we have V f 1 (x) + V f 2 (x) = 2x 1 [x 1 + x 2 x 1 (x1 2 + x2 2 )] x 1 x 2 +2x 2 [ 2x 1 + x 2 x 2 (x1 2 + x2 2 )] = 2(x x 2 2 ) 2(x x 2 2 ) 2 2x 1 x 2 2(x x 2 2 ) 2(x x 2 2 ) 2 + (x x 2 2 ) = 3c 2c 2 Choosing c > 1.5 ensures that all trajectories trapped inside M. by PB criterion, there exits at least one periodic orbit. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 51/53

52 Index Theorem Let C be a simple closed curve not passing through any equilibrium point Consider the orientation of the vector field f (x) at a point p C. Let p traverse C counterclockwise the vector f (x) rotates continuously with an angel of 2πk The angle is measured counterclockwise. If C is chosen to encircle a single isolated Equ. point X The integer k is called the index of X k = +1 for a node or focus, k = 1 for saddle. Inside any periodic orbit γ, there must be at least one equilibrium point. Suppose the equilibrium points inside γ are hyperbolic, then if N is the number of nodes and foci and S is the number of saddles, it must be that N S = 1. An equilibrium point is hyperbolic if the jacobian at that point has no eigenvalue on the imaginary axis. If the Equ point is not hyperbolic, then k may differ from ±1 It is useful in ruling out the existence of periodic orbits r. Farzaneh Abdollahi Nonlinear Control Lecture 2 52/53

53 Index Theorem Example: The system ẋ 1 = x 1 + x 1 x 2 ẋ 2 = x 1 + x 2 2x 1 x 2 has two equilibrium points at (0, 0) and (1, 1). The Jacobian: [ ] [ ] [ ] [ f 1 0 f 0 1 = ; = x (0,0) 1 1 x (1,1) 1 1 ] (0, 0) is a saddle point and (1, 1) is a stable focus. The only combination of Equ. points that can be encircled by a periodic orbit is a single focus. Periodic orbit in other region such as that encircling both Equ. points are ruled out. r. Farzaneh Abdollahi Nonlinear Control Lecture 2 53/53

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