Global exponential synchronization of networked dynamical systems under eventtriggered control schemes
 Jie Gao^{1, 2}Email author and
 Peiyong Zhu^{2}
https://doi.org/10.1186/s1366201610060
© Gao and Zhu 2016
Received: 17 May 2016
Accepted: 19 October 2016
Published: 8 November 2016
Abstract
This paper investigates exponential synchronization of networked dynamical systems under eventtriggered control schemes. Two eventtriggered sampleddata transmission schemes, which only need the latest observations of their neighborhood and the virtual leader to predict the next observation time, are designed to realize exponential synchronization of networked dynamical systems. That is, the coupled information is updated only when the triggered conditions are violated. Hence, continuous communication can be avoided and the number of information transmission is reduced. A positive lower bound for interevent intervals is achieved to exclude Zeno behavior. Finally, two numerical simulation examples are provided to illustrate the effectiveness of the proposed results.
Keywords
1 Introduction
Networked control dynamical systems (NCDSs) have been widely studied over the past decades [1–4]. It is usually investigated in the NCDSs that all oscillators approach a uniform dynamical behavior, that is, all the nodes in the NCDSs reach synchronization eventually. In the process of synchronization, the couplings among nodes and/or external distributed and cooperative control between nodes exist inevitably and meanwhile conflicts may exist due to the limitations of network resources and traffics [5]. In most previous references [6–8], each node received their neighbors’ information continuously, which may cost much. As an important component in NCDSs, intermittent sampling has emerged as an interesting topic to avoid communication continuously [9–11]. Nevertheless, sampleddata systems are usually applied periodically in time [12, 13], that is, timetriggered sampling, which might be conservative in terms of the number of control updates.
In order to utilize the realtime information sufficiently and to reduce communication and computation load in NCDSs, aperiodic eventtriggered sampling, which is triggered only when measurement error signal violates a prescribed threshold [14–16], is proposed in the last few years. As pointed out in [17], eventtriggered sampling was proved to possess a better performance than timetriggered sampling. In eventtriggered sampling control, the control law is updated only when some specific significant events occur, other state changes or occurrences of realtime entity are considered insignificant and are neglected [18–22]. Eventtriggered sampling control could adjust task periods to variations in system states adaptively, which produces longer task periods than timetriggered sampling control. In [23], the authors studied the eventtriggered distributed averageconsensus of discretetime firstorder multiagent systems with limited communication data rate and general directed network topology. The authors of [24] studied the problem of average consensus over directed and timevarying digital networks of discretetime firstorder multiagent systems with limited communication data transmission rates. Each agent has a realvalued state but can only exchange binary symbolic sequence with its neighbors due to bandwidth constraints.
Recently, great efforts are still made on applying eventtriggered scheme (ETS) to cooperation of multiagent systems. A key issue of the eventtriggered scheme is how to design and optimize eventbased conditions and a big challenge is how to prove that innerevent time intervals are positive which can assure the absence of Zeno behavior. In [25], leaderfollowing consensus of general linear multiagent is investigated by the eventtriggered scheme. Three types of schemes, namely, distributed ETS, centralized ETS, and clustered ETS for different network topologies are proposed. All these schemes guarantee that all followers can track the leader eventually. In [26], eventtriggered coupling configurations are utilized to realize synchronization of linearly coupled dynamical systems. The diffusion couplings are set up from the latest observations of the nodes and their neighborhood and the next observation time is triggered by the proposed criteria based on the local neighborhood information as well. However, the graph in [26] is undirected and connected.
Motivated by above statement, the objective of this paper is to design two eventtriggered schemes for exponential synchronization of networked dynamical systems. The contributions of this paper are listed as follows. First of all, the network topology is directed and contains a directed spanning tree rooted at a virtual node. All the nodes are equipped with nonlinear dynamics. Thus, the model of this paper is more general than in [20, 25, 26]. Second, two distributed eventtriggered schemes are proposed to realize exponential synchronization of the networked dynamical systems. The main difficulty of this paper is to prove the Zeno behavior is excluded under the two eventtriggered schemes. In addition, to further reduce the number of updatings, two distributed selftriggered schemes are proposed. It is proved that the exponential synchronization can be achieved and the Zeno behavior can be excluded simultaneously under the two selftriggered schemes.
The remainder of this paper is outlined as follows. In Section 2, preliminaries including some necessary definitions and lemmas and the model description are stated. In Section 3, eventtriggered schemes are proposed to realize exponential synchronization and the Zeno behavior can be excluded under the proposed schemes. In Section 4, the distributed selftriggered schemes are presented according to the eventtriggered schemes. In Section 5, some numerical examples are given to show the effectiveness of the theoretical results. Finally, the conclusion is drawn in Section 6.
Notation
Throughout this study, \(\mathbb{R}^{n}\) and \(\mathbb{R}^{n\times n}\) represent the set of all n dimensional real column vectors and the set of all \(n\times n\) dimensional real matrices. The superscript T represents the transpose. \(\\cdot\\) denotes the Euclidean norm, that is, for any vector \(\xi\in\mathbb{R}^{n}\), \(\\xi\=(\xi_{1}^{2},\ldots,\xi_{n}^{2})^{\frac{1}{2}}\). \(\x\_{p}=(x^{T}Px)^{\frac{1}{2}}\) for some positive definite matrix \(P\in\mathbb{R}^{n\times n}\). \(\lambda_{\max}(A)\) and \(\lambda_{\min}(A)\) are respectively the maximum and minimum eigenvalues of matrix A. ⊗ represents the Kronecker product.
2 Preliminaries and problem formulation
For \(t\in[t_{k_{i}}^{i},t_{k_{i}+1}^{i})\), \(i=1,\ldots,N\), synchronization error and measurement error of node i are respectively defined as \(e_{i}(t)=x_{i}(t)s(t)\), and \(\delta_{i}(t)=x_{i}(t_{k_{i}}^{i})x_{i}(t)\), \(\delta_{j}(t)=x_{j}(t_{k_{i}}^{i})x_{j}(t)\), \(j\neq i\), \(\delta_{0}(t)=s(t_{k_{i}}^{i})s(t)\).
The objective of this paper is to design appropriate eventtriggered schemes such that (3) and (2) can reach exponential synchronization.
Throughout the rest of the paper, the following assumptions and lemma are needed.
Definition 1
Assumption 1
Assumption 2
The augmented graph contains a directed span tree rooted at the virtual node.
Lemma 1
 (1)
If \(\mathscr{G}\) is balanced, then \(\frac{{L}+{L}^{T}}{2}+{D}>0\) if and only if \(\overline{\mathscr{G}}\) is weakly connected.
 (2)If \(\overline{\mathscr{G}}\) has a directed spanning tree, then there exists a positive diagonal matrix \(\Xi=\operatorname{diag}\{\xi_{1},\ldots,\xi_{N}\}\), such that$$ \Xi({L}+{D})+({L}+{D})^{T}\Xi>0. $$(6)
Remark 1
In [20], the trajectories of all nodes commonly converge to a timevarying weighed average \(\bar{x}=\sum_{j=1}^{N}\xi_{j}x_{j}(t)\). All the nodes in this paper could synchronize with an arbitrary desired state \(s(t)\) which can be an equilibrium point, periodic orbit or chaotic attractor. In addition, the authors of [20] make use of the general algebraic connectivity to reach global synchronization, which is fit for a strongly connected network. This paper only requires the augmented graph contains a directed spanning tree rooted at the virtual node which does not require the coupling matrix to be symmetric.
3 Eventtriggered scheme for pinning synchronization
In this section, pinning synchronization of the considered network (3) with (2) is investigated under eventtriggered mechanism. The following algorithm [27] is proposed to determine at least how many and what kinds of nodes should be pinned such that Assumption 2 holds.
Algorithm 1
 Step 1::

Check whether there exists at least one node \(n_{k}\) belonging to \(G_{h}\), which is reachable from a node \(n_{g}\) belonging to \(G_{j}\), \(j=1,2,\ldots,{\omega}\), \(j\neq h\). If it holds, go to Step 2; if it does not hold, go to Step 3.
 Step 2::

Check whether the following condition holds: \(h<\omega\). If so, let \(h=h+1\) and return Step 1; else stop.
 Step 3::

Arbitrarily having selected one node in \(G_{h}\) and pinned, let \(m_{h}=1\). Check whether the following condition holds: \(h <\omega\). If so, let \(h=h+1\) and return Step 1; else stop.
Remark 2
Using Algorithm 1, we should select at least \(\delta=\sum_{i=1}^{\omega}m_{i}\) nodes in \(\mathscr{G}(A)\) to be pinned such that Assumption 2 holds. That is, Assumption 2 can never be ensured if there are only μ nodes to be pinned, where \(\mu<\delta\).
Based on the above analysis, one may obtain the following theorem, which summarizes the main result of this section.
Theorem 1
 (1)Set \(t_{k_{i}}^{i}\) as the triggering time point by the rule$$ t_{k_{i}+1}^{i}= \max_{t} \Biggl\{ t\geq t_{k_{i}}^{i}: \bigl\Vert \kappa_{i}(t) \bigr\Vert \leq\frac{\beta'}{\sqrt{N\xi_{\max}\lambda_{\max}(P)}} \sqrt{\sum _{i=1}^{N} \xi_{i}e_{i}^{T}(t)Pe_{i}(t)} \Biggr\} . $$(7)
 (2)Set \(t_{k_{i}}^{i}\) as the triggering time point by the rulewhere a is a positive constant and \(0< b<\beta\beta'\).$$ t_{k_{i}+1}^{i}= \max_{t} \bigl\{ t\geq t_{k_{i}}^{i}: \bigl\Vert \kappa_{i}(t) \bigr\Vert \leq a\exp(bt) \bigr\} , $$(8)
Proof
That is, the network (3) can synchronize with (2) exponentially fast with the rate of \(2(\beta\beta')\).
Note \(\max_{\varepsilon>0}\{\frac{\varepsilon(2\beta' \varepsilon\lambda_{\max}(P))}{N\xi_{\max}}\}=\frac{\beta^{\prime2}}{ N\xi_{\max}\lambda_{\max}(P)}\), when \(\varepsilon=\frac{\beta'}{\lambda_{\max}(P)}\).
Then (7) can be guaranteed when \(\varepsilon={\beta'}/{\lambda_{\max}(P)}\).
Remark 3
In Theorem 1, two eventtriggered conditions (7) and (8) are proposed. At the triggering time \(t_{k_{i}}^{i}\), the lefthand term \(\\kappa_{i}(t_{k_{i}}^{i})\=0\). In (7), if there exists at least one node which does not synchronize with \(s(t)\), the righthand term must be positive. Therefore, the next triggering time \(t_{k_{i}+1}^{i}\) must be greater than \(t_{k_{i}}^{i}\). In (8), the righthand term must be always positive for all nodes. Thus, the interevent intervals of all nodes are strictly positive. Although the two updating rules (7) and (8) are closely related to each other in some respects, the eventtriggered condition in (8) is verified more easily than the condition in (7). Moreover, according to (16) and (19), the convergence under the eventtriggered scheme (7) is better than under (8). The number of updating times under the eventtriggered scheme (7) is more than (8).
Next, it is proved in detail that under the two updating rules (7) and (8), the interevent sampling time instants \(t_{k_{i}+1}^{i}t_{k_{i}}^{i}\) for each node is strictly positive, that is, the coupled network can avoid the Zeno behavior.
Theorem 2
 (1)
Under the eventtriggered scheme (7), each node has positive interevent interval which is lower bounded by a constant \(\tau_{D}^{i}\).
 (2)
Under the eventtriggered scheme (8), the interevent interval of every node is strictly positive and is lower bounded by a common constant \(\tau_{D}^{i}\).
Proof
This completes the proof. □
4 Selftriggered scheme for pinning synchronization
Under the updating rules (7) and (8), one is required to verify the eventtriggered condition continuously. To avoid continuously communication among nodes, a selftriggered scheme based on Theorem 1 is proposed. Under the selftriggered scheme, each node in the network can predict next triggered time instant \(t_{k_{i}+1}^{i}\) only based on the received information at time \(t_{k_{i}}^{i}\). This scheme does not require one to verify the eventtriggered condition continuously and hence more energy can be saved for the network. Inspired by the work of [26, 29], a selftriggered scheme is investigated in the following.
Algorithm 2
Selftriggered algorithm
 Step 1::

Initialization: set \(t_{0}^{i}=0\), for all \(i=1,2,\ldots,N\).
 Step 2::

At time \(t_{k_{i}}^{i}\), \(k_{i}\geq1\), solve the following equation to find the next triggering time \(t_{k_{i}+1}^{i}=t_{k_{i}}^{i}+\tau_{D}^{i}\):$$\begin{aligned}& \sup \biggl\{ \tau_{D}^{i}\geq0: \frac{(\sum_{j=1}^{N}l_{ij}\varphi_{j}+d_{i}\varphi_{i})\\Gamma\}{k} \bigl(\exp \bigl(k\tau _{D}^{i} \bigr)1 \bigr) \\& \quad \leq \frac{\beta'\sqrt{V(t_{k_{i}}^{i})}}{ \sqrt{N\xi_{\max}\lambda_{\max}(P)}} \exp \biggl(\frac{\varpi}{2}\tau_{D}^{i} \biggr) \biggr\} . \end{aligned}$$(40)
 Step 3::

If node i does not receive the renewed information from any of its neighbors during \((t_{k_{i}}^{i},t_{k_{i}+1}^{i})\), node i is triggered on time instant \(t_{k_{i}+1}^{i}\).
 Step 4::

If node i receives the renewed information from its neighbor j at time \(t_{k_{i}'}^{i}< t_{k_{i}+1}^{i}\), compute the new value of \(\vartheta_{j}\) and go to Step 2.
Theorem 3
Proof
Then, according to Theorem 1, (3) can achieve synchronization with (2) exponentially.
Remark 5
In (41), if \(\tau_{D}^{i}=0\), the lefthand term equals zero, while the righthand term is nonzero. Therefore, the interevent interval of node i is strictly positive and has a lower bound \(\tau_{D}^{i}\), which is given as (41). To predict the next triggering time, each node only requires the states of itself and its neighbors at the last triggering time.
According to the eventtriggered scheme (8), one can get the following result.
Theorem 4
 (1)
Initialization: set \(t_{0}^{i}=0\), for all \(i=1,2,\ldots,N\).
 (2)At time \(t_{k_{i}}^{i}, k_{i}\geq1\), solve the following equation to find the next triggering time \(t_{k_{i}+1}^{i}\):$$ \frac{(\sum_{j=1}^{N}l_{ij}\varphi_{j}+d_{i}\varphi_{i})\ \Gamma\}{k} \bigl(\exp \bigl(k\tau_{D}^{i} \bigr)1 \bigr)\leq a\exp \bigl(b \bigl(t_{k_{i}}^{i}+ \tau_{D}^{i} \bigr) \bigr). $$(46)
 (3)
Trigger node i by changing \(t_{k_{i}}^{i}\) into \(t_{k_{i}+1}^{i}=t_{k_{i}}^{i}+\tau_{D}^{i}\).
The proof of this theorem is similar to Theorem 3.
5 An illustrative example
6 Conclusions
In this paper, eventtriggered schemes and selftriggered schemes are investigated to realize the exponential synchronization of the networked dynamical systems. The coupled information under these schemes is updated only when the triggering conditions are violated. The next observation time for these nodes is predicted only based on the latest observations of their neighborhood and the virtual leader. Thus, continuous communication can be avoided and the number of information transmission is largely reduced. Moreover, a positive lower bound for interevent intervals is achieved and the Zeno behavior can be excluded. Finally, two numerical simulation examples are provided to illustrate the effectiveness of the proposed results. In the future, we will focus on the related applications of the eventtriggered scheme in the coupled neural networks with timedelays and quantization.
Declarations
Acknowledgements
This work was supported by the National Natural Science Foundation of China (11501391, 11601449) and by the Key Program of Sichuan Provincial Department of Education (16ZA0066).
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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