Synchronization of Fractional-order Chaotic Systems with Gaussian fluctuation by Sliding Mode Control

This paper is devoted to the problem of synchronization between fractional-order chaotic systems with Gaussian fluctuation by the method of fractional-order sliding mode control. A fractional integral (FI) sliding surface is proposed for synchronizing the uncertain fractional-order system, and then the sliding mode control technique is carried out to realize the synchronization of the given systems. One theorem about sliding mode controller is presented to prove the proposed controller can make the system synchronize. As a case study, the presented method is applied to the fractional-order Chen-L\"u system as the drive-response dynamical system. Simulation results show a good performance of the proposed control approach in synchronizing the chaotic systems in presence of Gaussian noise.


Introduction
Synchronization, which means "things occur at the same time or operate in unison", has received a great deal of interest among scientists from various fields in the last few years [1][2][3][4], especially in fractional-order chaotic systems. It has been recognized that many systems in interdisciplinary fields can be elegantly described by fractional-order differential equations, such as viscoelastic materials [5], electrical circuits [6], population models [7], financial systems [8], and so on. Meanwhile, most of precious studies have been shown that some fractional-order systems exhibit chaotic behavior [9][10][11][12][13][14]. In particular, fractional-order chaotic behavior has wide promising applications in information encryption, image processing, secure communication, etc [15][16][17][18]. Therefore, synchronization of fractional chaotic systems starts to attract increasing attentions due to its great importance in applications ranging from physics to engineering, from computer science to biology, and even from economics to brain science.
A basic configuration for chaos synchronization is the drive-response pattern, where the response of chaotic system must track the drive chaotic trajectory. Some approaches based on this configuration have been attained to achieve chaos synchronization in fractional-order chaotic systems, such as PC control [19], active control [20], adaptive control [21,22], sliding mode control (SMC) [23], and a scalar transmitted signal method [24], etc. In which, the sliding mode controller has some attractive advantages, including: (i) fast dynamic responses and good transient performance; (ii) external disturbance rejection; and (iii) insensitivity to parameter variations and model uncertainties [25][26]. In addition, SMC method plays an important role in the application to practical problems. For example, in [27], Tavazoei MS and his co-operator proposed a controller based on active sliding mode theory to synchronize chaotic fractional-order systems in master-slave structure. In [2], the problem of modified projective synchronization of fractional-order chaotic system was considered, and finite-time synchronization of non-autonomous fractional-order uncertain chaotic systems was investigated by Aghababa MP in [28].
All of the methods mentioned above have been used to synchronize the deterministic fractional-order chaotic systems. However, the synchronization of two different chaotic systems in uncertain environment plays a significant role in practical applications, and this problem will be more challenging and difficult if the fractional-order dynamic systems are influenced by some uncertainty. In this investigation, our aim is to synchronize two fractional-order chaotic systems with uncertain environment. To achieve this goal, we have proposed fractional integral (FI) sliding mode surface which combines the property of fractional-order equation with sliding mode control method. One theorem about sliding mode controller is presented and proved that the proposed controller can make the system synchronize well. Then a numerical example is given to verify the effectiveness of the mentioned method, and the good agreements are also found between the theoretical and the numerical results. This paper is organized as follows. In section 2, two fractional-order chaotic systems with uncertainty and problem formulation are presented. In section 3, we investigate the design method of sliding mode controller, and one theorem is obtained to prove the effectiveness of proposed controller. One example is presented to carry out the numerical simulations in section 4. Finally, conclusions are presented to close this paper.

System description and problem formulation
Consider the following class of fractional-order chaotic system with noise, which is described by Gaussian white noise vector, in which every two noises are statistical independent. And D  is the Caputo derivative, is the order of fractional derivative, which is defined as the Euler's Gamma function.
Let system (1) be the driving system, then response system with a controller   1 2 , ,...,   e e e e y x . Then, from system (1) and (3), one has the error dynamics Thus, the control problem considered in this study is that for chaotic driving system (1) and response system (3), the two fractional order systems are to be synchronized by designing an appropriate controller ( ) t u such that where . is defined as is the expected value function.

Sliding mode controller design and analysis
In the following context, we shall design sliding mode controller to establish the synchronization between driving system (1) and response system (3).

Sliding mode controller design process
Now, the control input vector ( ) t u is defined to eliminate the nonlinear part of the error dynamics: S S e is a switching surface to prescribe the desired sliding mode dynamics.

So the error system (4) is then rewritten as
Here, a new fractional integral (FI) switching surface is given as follows: As we all know, when the system is controllable in the sliding mode, the switching surface should satisfy the following conditions: together with: Substitute equations (7) and (8) into (9b), one obtains Therefore, the equivalent control law is obtained by In real-world applications, the Gaussian white noise W  is uncertain. Therefore, the equivalent control input is modified to To design the sliding mode controller, we consider the constant plus proportional rate reaching law [29][30] that is x represents sign function, that is Further, according to the control law and the updated law the controller are given by And the error system can be rewritten in the following differential form:

Synchronization analysis
Theorem 1 If the controller is selected as equation (15), with suitably selected r and  , then the synchronization of fractional-order chaotic systems between driving system (1) and response system (3) can be achieved (i.e., the synchronization error converge to zero in the mean square norm).
Proof Consider a Lyapunov function constructed by the mean square norm of ( ) t S and its differential form: According to the definition of derivative, it is obtained that: While, from equations (10) and (14), one gets Taking expectations to equation (21) and using the properties of Brownian motion mentioned, we have Therefore: Equation (23) implies that as long as suitable r and  which satisfies , according to barbalat's lemma [31], system (1) and system (3) can be achieved synchronization under the controller law in (15).
This completes the proof. □

Simulation
In this part, to confirm the validity of proposed method, we numerically examine the synchronization between fractional-order Chen and fractional-order Lü systems.
In the simulation, step-by-step method is performed to receive numerical solution, and the detailed descriptions of this algorithm are available in [32][33].
Here, we assume that the Chen system drives the Lü system. Hence, the driving system (fractional-order Lü system) is described as .
which can also be written as which has been shown that the fractional-order Lü system can demonstrate chaotic behavior [25] when 1 The response system (fractional-order Chen system) is given as follows: .

D y a y y D y c a y y y c y D y y y b y
which can be written in the following form

 
. According to (3), the response system with controller can be described as follows: where   is the control vector.
Now we apply the proposed sliding control approach to finish synchronization between fractional-order Lü system driven by Gaussian white noise and fractional-order Chen system. Here, we define the error states as and the sliding mode surface as The control law is given by In the numerical simulations, the initial conditions are set as 1 2 3 ( (0), (0), (0)) (7,4,4), ( (0), (0), (0)) (1,3, 1 . The time step size employed in this simulation is 0.0005 h  . Then the simulation results are summarized in Fig.1-Fig.4. The state trajectories of the system (25) and system (28) under the sliding mode control method are shown in Fig. 1(a) (signals 1 ; x 1 y ), Fig. 1(b) (signals 2 ; x 2 y ), Fig.   1(c) (signals 3 ; x 3 y ), respectively. Note that the driving system is shown by red lines where as response system is shown by blue lines. As one can see, the designed controller is effectively capable to achieve the synchronization of fractional-order Chen chaotic system, that is the state variables     Fig. 1(a). The state trajectories of the system (25) and system (28) with the sliding mode control method. (signals 1 ; x 1 y ).  Fig. 1(b). The state trajectories of the system (25) and system (28) with the sliding mode control method. (signals 2 ; x 2 y ).  Fig. 1(c). The state trajectories of the system (25) and system (28) with the sliding mode control method. (signals 3 ; x 3 y ).   Fig. 2(b). The time evolution of synchronization error 2 e of the drive system (25) and response system (28).  Fig. 2(c). The time evolution of synchronization error 3 e of the drive system (25) and response system (28).

Conclusions
In this paper, a new method for synchronizing two different fractional-order chaotic systems is introduced, and the driving chaotic system is excited by Gaussian white noise. A fractional sliding surface is introduced, and the sliding mode controller is proposed for synchronization. Furthermore, convergence property has been analyzed for the error dynamics after adding proposed controllers. It has been shown that by proper choice of the control parameters ( r and  ), the fractional-order chaotic systems in uncertain environment are synchronized. Finally, to further illustrate the effectiveness of the proposed controllers, one applies the presented algorithm to the fractional-order Chen and fractional-order Lü systems through numerical simulations. From the simulation results, it is obvious that a satisfying control performance can be achieved by using the proposed method.