596 IEEE CONTROL SYSTEMS LETTERS, VOL. 4, NO. 3, JULY 2020
Structural Controllability of Undirected Diffusive
Networks With Vector-Weighted Edges
Yuan Zhang , Yuanqing Xia , Han Gao, and Guangchen Zhang
Abstract—In this letter, controllability of undirected net-
worked systems with diffusively coupled subsystems is
considered, where each subsystem is of identically fixed
general high-order single-input-multi-output dynamics. The
underlying graph of the network topology is vector-
weighted, rather than scalar-weighted. The aim is to find
conditions under which the networked system is struc-
turally controllable, i.e., for almost all vector values for
interaction links of the network topology, the correspond-
ing system is controllable. It is proven that, the networked
system is structurally controllable, if and only if each sub-
system is controllable and observable, and the network
topology is globally input-reachable. These conditions are
further extended to the cases with multi-input-multi-output
subsystem dynamics and matrix-weighted edges, or where
both directed and undirected interaction links exist.
Index Terms—Undirected diffusive network, structural
controllability, network analysis and control, vector-
weighted Laplacian.
I. INTRODUCTION
A
NALYSIS and synthesize of networked systems with
diffusively coupled subsystems, also known as diffusive
networks, have received much attention in the fields of syn-
chronization, consensus, stability, as well as controllability and
observability [1]–[3]. This is because the diffusive coupling
mechanism frequently arises naturally in thermal systems,
power systems, car-following traffic systems, etc. [2], [4]. As is
known to all, controllability is a fundamental system property.
Particularly, controllability of a leader-follower multi-agent
system (MAS) running the consensus protocol guarantees that
the system can reach agreement subspace arbitrarily fast [5].
Many works have focused on controllability of leader-
follower MASs [3], [5]–[7]. For example, [6] gave conditions
for controllability of such networked systems in terms of
eigenvectors of the Laplacian matrices. The works [3], [5], [7]
studied the same problem by means of graph-theoretic
tools. However, except [3], all the above works assume
that each subsystem is a single-integrator. On the other
hand, controllability of networks with high-order subsystems
Manuscript received January 9, 2020; revised March 9, 2020;
accepted March 27, 2020. Date of publication April 8, 2020; date of cur-
rent version April 28, 2020. Recommended by Senior Editor M. Arcak.
(Corresponding author: Yuan Zhang.)
The authors are with the School of Automation, Beijing Institute of
Technology, Beijing 100811, China (e-mail: zhangyuan14@bit.edu.cn;
xia_yuanqing@bit.edu.cn; gaohhit@163.com; guangchen_123@
126.com).
Digital Object Identifier 10.1109/LCSYS.2020.2986250
has also attracted much research interests in [8]–[13]. To
be specific, [8], [11], [13] focused on networked systems
with identical subsystems (called homogeneous networks),
whiles [9], [10], [12] on networked systems with general
heterogeneous subsystems (called heterogeneous networks).
Particularly, controllability as a generic property for a net-
worked system is studied in [12], [13].
However, except [9], [10], [12] which focus on hetero-
geneous networks, almost all results on controllability of
homogeneous networks are built on the condition that all
weights of edges in the network topology belong to {0, 1} or
some given scalars [3], [5]–[8], [11], [13]. Notice that, when
each subsystem is not of single-input-single-output (SISO),
there is a typical situation that different interaction chan-
nels between two subsystems are weighted differently, either
because of differences in the nature of physical variables they
convey, or the variants of the channels themselves. For exam-
ple, in some networks consisting of both physical couplings
and cyber couplings, the physical channels and the cyber ones
between two subsystems can have different weights. See more
examples in [14]–[16]. If we use a graph to denote the sub-
system interaction topology, then each edge of the graph may
have a vector-valued or matrix-valued weight as introduced
in [14], [15]. In such case, some existing approaches for con-
trollability analysis for networks with scalar-weighted edges
may not be applicable (such as the spectrum-based approaches
in [3], [11]).
In this letter, we study structural controllability of an undi-
rected diffusive networked system with high-order subsystems,
where the underlying graph of the network topology has
symmetric vector-weighted edges. Our purpose is to find
conditions under which the networked system is structurally
controllable, i.e., for almost all vector values for edges of
the network topology, the whole system is controllable. The
main contributions of this letter are three-fold. First, we
prove that, an undirected diffusive networked system with
identical single-input-multi-output (SIMO) subsystems (lead-
ing to vector-weighted edges) is structurally controllable,
if and only if each subsystem is controllable and observ-
able, and the network topology is globally input-reachable.
Second, we show that our conditions are still valid even
when both directed edges and undirected ones exist. Third,
we extend our results to the case with arbitrarily dimensional
matrix-weighted edges.
It is remarkable that some relations between connectivity
and observability have been revealed in [15] for a net-
worked system where subsystems are decoupled whereas
their outputs are relatively measured by sensor networks, and
each interconnection edge defined therein has a semi-definite
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