1.
Introduction
As a kind of important model, Hamiltonian system is widely used in many fields [1,2,3,4,5,6,7]. And the chaotic behaviors of Hamiltonian systems have attracted many scientists and many remarkable research results are obtained. Based on energy analysis of a Sprott-A system, H. Jia and his cooperators formulate a new four-dimension chaotic Hamiltonian system. Then the chaotic characteristic of the 4-D system and the existence of coexisting hidden attractors are studied by numerical analysis and field programmable gate array (FPGA) implementation [8]. The Sprott-A system is reported by Sprott in 1994, which is a algebraically simple three-dimensional chaotic system [9]. The chaotic behaviors of a paradigmatic low-dimensional Hamiltonian system subjected to different scenarios of parameter drifts of non-negligible rates are investigated in [10]. In addition, similar studies were carried out in [11,12,13]. Compared to chaos, hyperchaos is more complex and has stronger randomness and unpredictability. Thus, hyperchaos has great potential applications. For example, hyperchaos could help us build better quantum computer [14]. Therefore, we constructed a new hyperchaotic Hamiltonian system. The rich dynamical behaviors of the system with holonomic constraint and nonholonomic constraint, and hyperchaos control are investigated.
The organization of this paper is as follows. In Section 2, we formulate a six-dimension hyperchaotic Hamiltonian system. The invariance, equilibrium points and their linear stability, and hyperchaotic behaviors of the system are analyzed. In Section 3, we present a holonomic constrained system and a nonholonomic constrained system, respectively. By using the method of [15], the explicit equations for constrained systems are obtained. The influences of constrained condition on the hyperchaotic behaviors of the Hamiltonian system are investigated. In Section 4, based on feedback control, how to control the hyperchaotic behaviors of the Hamiltonian system and holonomic system are studied. The conclusions are summarized in Section 5.
2.
Hyperchaotic Hamiltonian system
In this section, we assume that the Hamiltonian system is described by a function
here q=(q1,q2,q3) is the generalized coordinate, p=(p1,p2,p3) is the canonical momentum and t denotes time. Then, we obtain the Hamiltonian system as follows:
where, θ, β, η are positive parameters. The dot expresses the derivative with respect to t.
Obviously, the system (2.2) is invariant for the coordinate transformation
thus it is symmetric with respect to the origin. For arbitrary equilibrium point E=(q1,q2,q3,p1,p2,p3) of (2.2), the Jacobian matrix is
where I3 denotes third order identity matrix and
Correspondingly, the characteristic equation at E is as follows:
where
It is easy to visualize that the system (2.2) always has the equilibrium point E0=(0,0,0,0,0,0), the characteristic equation of Jacobian matrix is
In the following, similar to [16], we investigate the stability of E0 by discussing the coefficients of characteristic polynomial at different conditions. When η2θ+θ2≥2η2, the roots of the characteristic equation are
When η2θ+θ2≤2η2, λ5,6 becomes
the other characteristic roots above remain unchanged. It shows that E0 is an unstable equilibrium point.
Next, we illustrate that the system (2.2) exist non-zero equilibrium point. Let ˙p2=0, ˙p3=0, we have βq23−θq2=βq33−θq3, (q2−q3)(βq22+βq2q3+βq32−θ)=0, then
When q2=q3, substitute q1=q2(βq22−θ)η into ˙p1=0, direct calculation shows that
Since
it follows that f(q2) has non-zero roots. Furthermore, if q2=0, we have q1=0 based on ˙p2=0, then we have q3=0 based on ˙p1=0. And we also obtain q1=0, q2=0 when q3=0. Therefore, if q2 and q3 satisfy q22+q2q3+q32−θβ=0, we obtain q2q3≠0. In conclusion, the system (2.2) exist non-zero equilibrium point.
To further illustrate the equilibrium points and dynamical behaviors of the system (2.2), we fix the parameters η=0.1, θ=0.5, β=2.5. The calculations show that the system (2.2) has twenty-seven equilibrium points, the values of equilibrium points and their corresponding characteristic root results are given in Table 1. It shows that the system (2.2) has hyperbolic equilibrium points and non-hyperbolic equilibrium points, and there exist unstable manifold and stable manifold around the equilibrium points from Table 1. Here, all the calculation results are done by Maple software. Because it does not affect the description and result of the problem, for the convenience of expression, only approximate results are given in this paper. For the other calculation results, the same process is done based on the same consideration. It has been long supposed that the existence of chaotic behavior in the microscopic motions is responsible for their equilibrium and non-equilibrium properties [17]. In addition, Lyapunov characteristic exponents have been widely employed in studying dynamical systems, especially for measuring the exponential divergence of nearby orbits along certain directions in phase space [18,19]. A. Wolf and his cooperators have presented a trajectory algorithm to calculate the Lyapunov exponents [20]. The basic idea of the method is to keep track of perturbations away from the trajectory in linearized phase space. And the Wolf algorithm is comparatively suitable for the analysis of experimental data [21]. So in this paper, we calculate the Lyapunov exponents using Wolf method, where the system is integrated using a fourth-order Runge Kutta method with a fixed step size equal to 0.01 [21]. And any system containing more than one positive Lyapunov exponent is defined to be hyperchaotic [22]. By computations, the Lyapunov exponents are obtained as follows:
thus the system (2.2) is hyperchaotic. The spectra of Lyapunov exponents of (2.2) is given in Figure 1. It indicates that the values of Lyapunov exponents tend to stable after 40000 steps.
The Poincaré map of a system is defined by crossings of orbits with one plane [23]. And the evolution of a dynamical system can be studied by using surfaces of section, in which color is used to visualize the fourth dimension [24,25,26,27,28,29]. Therefore, we consider the projection of 4D space on a 3D subspace, where color is used to indicate the 4th dimension. The (q1,p1,p3,q2) 4D surface of section is depicted in Figure 2, the location of the consequents is given in the (q1,p1,p3) subspace and are colored according to their q2 value. The corresponding Poincaré map on the section hyperplane p2=0 is shown in Figure 3. Here, initial value is (0.01,0.01,0.01,0.001,0.001,0.001). Figure 4 indicates that the Hamiltonian system is sensitive to initial values, where the blue trajectories' initial value is (0.01,0.01,0.01,0.001,0.001,0.001), the red trajectories' initial value is (0.01,0.01,0.001,0.001,0.001,0.001) and the green trajectories' initial value is (0,0,0,0.43,0.09,0.09), respectively.
It should be noted that the system has different dynamical characteristics under different initial values, such as initial value is (0.01,0.01,0.001,0.001,0.001,0.001), the Lyapunov exponents are:
The spectra of Lyapunov exponents with the initial value is given in Figure 5. The results show that different initial conditions lead to different spectra of Lyapunov exponents and occur different hyperchaotic behaviors. Figure 6 shows the (q1,p1,p3,q2) 4D surface of section and the location of the consequents is given in the (q1,p1,p3) subspace and are colored according to their q2 value.
Obviously, (2.2) is still a Hamiltonian system when the parameters of the Hamiltonian system are all non-positive. There are still many equilibrium points in the system under certain parameters, such as η=−0.2, θ=−0.5, β=−1. The values of equilibrium points and their corresponding characteristic root results are given in Table 2. In this section, since we mainly focus on the dynamical behaviors of the Hamiltonian system with positive parameters, the detailed analysis of (2.2) with non-positive parameters is omitted.
3.
The system under constraint conditions
Constraints exist in a wide range of systems, such as fluid particle systems [30], multi-qubit systems [31] and robotic system [32], etc. In this section, we mainly study the hyperchaotic phenomena in system (2.2) under the holonomic constraint condition and nonholonomic constraint condition.
3.1. Holonomic constraint
Assume the constraint on Hamiltonian system (2.2) is
Differentiating Eq (3.1) with respect to time, we obtain
where
Differentiating Eq (3.2) with respect to time, we have
where
The matrix
[15] presented a method for getting the explicit equations of constrained Hamiltonian system through the development of the connection between the Lagrangian concept of virtual displacements and Hamiltonian dynamics. By using the three-step approach in [15], the holonomic Hamiltonian system [(2.2), (3.1)] becomes
where
Then,
where
Thus the holonomic Hamiltonian system is transformed into
where
Obviously, the system (3.5) is invariant for the coordinate transformation
In order to intuitively reflect the complex dynamical behaviors of the constraint system, we fix the parameters β=2.5, η=1 and L=0.01. The calculations show that the system (3.5) has six equilibrium points, the values of equilibrium points and their corresponding characteristic root results are given in Table 3.
Similarly, it indicate that the system (3.5) has unstable manifold and stable manifold at the equilibrium points. The Lyapunov exponents are obtained as follows:
the initial value is (0,0,0.01,0.011,0.051,0.051). Therefore, the holonomic system (3.5) is hyperchaotic. Figure 7 indicates that the holonomic Hamiltonian system is sensitive to initial values, where the blue trajectories' initial value is (0,0,0.01,0.011,0.051,0.051), the magenta trajectories' initial value is (0,0.01,0,0.011,0.051,0.051) and the green trajectories' initial value is (0.01,0,0,0.011,0.051,0.051), respectively. The (q3,p1,p3,q2) 4D surface of section of Poincaré section on the section hyperplane p2=0 for the first condition is depicted in Figure 8, the location of the consequents is given in the (q3,p1,p3) subspace and are colored according to their q2 value.
3.2. Nonholonomic constraint
Assume the constraint on Hamiltonian system (2.2) is
Differentiating Eq (3.6) with respect to time, we have ¨q1+¨q2=˙q3, then
By using the method in [15], the nonholonomic constraint system [(2.2), (3.6)] is equivalent to
where
Similarly, we fix the parameters θ=0.5, β=2.5, η=0.1, the nonholonomic system (3.7) has three unstable equilibrium point
the corresponding eigenvalues of Jacobian matrix of (3.7) at these equilibria are
The Lyapunov exponents are
here initial value is (0,0.02,0.02,0.01,0.01,0.2). Thus, the nonholonomic constrained Hamiltonian system is hyperchaotic. Figure 9 shows the (q1,q3,p1,p2) 4D surface of section and the location of the consequents is given in the (q1,q3,p1) subspace and are colored according to their p2 value. The corresponding Poincaré map on the section hyperplane p3=0 is given in Figure 10.
4.
Hyperchaos control
From the above discussion, we can see that the constraints (3.1) and (3.6) have changed the status of the hyperchaotic system. It indicates that the hyperchaotic system can be generated by introducing constraint into higher-dimension Hamiltonian system. In some cases, the hyperchaotic behaviors will cause serious harm, for example, may lead to catastrophic voltage collapse or even blackout in actual power system [33], thus, these hyperchaotic systems need to be controlled by appropriated methods. In this section, we will investigate the control problem of the hyperchaotic system. Chaos control has been widely concerned by scholars, and many valuable chaotic controller have been designed, such as optimal control [34,35], impulse control [36], feedback control [37], etc. To achieve hyperchaos control, the linear feedback control [37] is used to suppress hyperchaos to stable equilibrium. Suppose that the controlled system is the following form:
where u is feedback coefficient. We select the same values of the parameters in Section 2, the Jacobian matrix of (4.1) is
When u=−1, there is only one zero-equilibrium point (0,0,0,0,0,0) in the system (4.1). By computations, the eigenvalues of Jc is as follows:
Therefore, the controlled hyperchaotic system (4.1) is asymptotically stable at equilibrium (0,0,0,0,0,0). The behaviors of the state q1,q2,q3,p1,p2,p3 of (2.2) and (4.1) with time are shown in Figures 11 and 12, respectively. Here, the horizontal coordinate data denote time, initial value is (0.01,0.01,0.01,0.001,0.001,0.001).
It should be noted that the linear feedback control method is also applicable in system (3.5). In this case, the controlled system is as follows:
Similarly, we obtain the unique asymptotic stable equilibrium point E1(0,0,0,0,0,0), the corresponding eigenvalues at E1 is as follows:
here k=−5. The controlled hyperchaotic system (4.2) is asymptotically stable at E1.
The behaviors of the state q1,q2,q3,p1,p2,p3 of (3.5) and (4.2) with time are shown in Figures 13 and 14, respectively. Here, the horizontal coordinate data denote time, initial value is (0,0,0.01,0.011,0.051,0.051). In conclusion, the hyperchaos control of the hyperchaotic system (2.2) and system (3.5) can be achieved by selecting appropriate feedback coefficients.
5.
Conclusions
In this paper, a new hyperchaotic Hamiltonian system is formulated and analysis the complex dynamic behaviors including the multi-equilibrium points their characteristics, Poincaré section, Lyapunov exponents. The explicit equations of the Hamiltonian system under holonomic constraint and nonholonomic constraint are obtained. The results show that the constrained systems have different dynamic behaviors from the unconstrained system and the new hyperchaotic systems are generated by introducing holonomic constraint and nonholonomic constraint. Finally, the hyperchaos control is achieved by using linear feedback which suppress hyperchaotic system to asymptotic stable zero-equilibrium.
Acknowledgments
Project supported by the Doctoral Scientific Research Foundation of Hanshan Normal University (No. QD202130).
Conflict of interest
The authors declare that they have no competing interests in this paper.