IEEE TRANSACTIONS ON SYSTEMS, MAN, AND CYBERNETICS: SYSTEMS, VOL. 45, NO. 6, JUNE 2015 967
Synthesis of Monitor-Based Liveness-Enforcing
Supervisors for S
3
PR With ξ-Resources
Dan You, Shouguang Wang, Senior Member, IEEE, and Mengchu Zhou, Fellow, IEEE
Abstract—Deadlocks are a rather undesirable phenomenon in flexible
manufacturing systems (FMSs). This work, by adding monitors, devel-
ops a deadlock prevention policy for FMSs that can be modeled by a
class of Petri nets called α-S
3
PR with ξ -resources. First, an algorithm is
presented to reduce an S
3
PR via a ξ -resource. Based on the algorithm,
ξ-resources in α-S
3
PRs are classified into two types: 1) A-ξ-resources
and 2) B-ξ-resources. Next, for an α-S
3
PR with only B-ξ -resources, it is
proved that a maximally permissive liveness-enforcing supervisor can be
designed by controlling all emptied strict minimal siphons. For an α-S
3
PR
containing A-ξ-resources, a liveness-enforcing supervisor can be designed
by iteratively reducing the net via A-ξ -resources and adding the corre-
sponding monitors. Finally, a deadlock prevention algorithm for α-S
3
PRs
is presented. Two FMS examples are used to illustrate its application. Its
comparison results with other state-of-the-art deadlock prevention poli-
cies validate its overall advantages in terms of computational complexity,
structural complexity, and behavior permissiveness.
Index Terms—
Deadlock prevention policy, flexible manufactur-
ing system (FMS), monitor-based, Petri nets.
I. INTRODUCTION
A flexible manufacturing system (FMS) is a system that contains
computer numerically-controlled machine tools, robots, automatic
guided vehicles, buffers, and fixtures, served by a material-handling
system. It usually exhibits a high degree of resource sharing to
increase its flexibility, which, however, may lead to deadlocks [24].
At a deadlock, two or more jobs keep waiting indefinitely for the
other jobs in the production sequence to release resources. Obviously,
deadlocks are a rather undesirable situation in an FMS since their
occurrences often deteriorate the utilization of resources and may
lead to catastrophic results in safety-critical systems. What is worse,
once deadlocks occur, they persist until an external agency resolves
them. Therefore, developing effective control methods to ensure that
deadlocks never occur in such systems is necessary.
Over the last two decades, deadlock problems in FMSs received
much attention from both academic and industrial communities.
Petri nets [24] have been viewed as a powerful, graphical, and
Manuscript received March 14, 2014; revised June 30, 2014 and
September 10, 2014; accepted October 21, 2014. Date of publication
February 16, 2015; date of current version May 13, 2015. This work was
supported in part by the National Natural Science Foundation of China
under Grant 61472361, Grant 61100056, and Grant 61374148, in part by
the Zhejiang Provincial Natural Science Foundation for Distinguished Young
Scholars under Grant LR14F020001, in part by the Zhejiang Science and
Technology Project under Grant 2013C31111, in part by the Zhejiang New
Network Standard and Technology Key Laboratory under Grant 2013E10012,
and in part by the Zhejiang Gongshang University Innovation Project
under Grant 3080XJ2513233 and Grant 3080XJ2513236. This paper was
recommended by Associate Editor L. Fang. (Corresponding author:
Shouguang Wang.)
D. You and S. G. Wang are with the School of Information and Electronic
Engineering, Zhejiang Gongshang University, Hangzhou 310018, China
(e-mail: wsg5000@hotmail.com).
M. C. Zhou is with the Department of Electrical and Computer Engineering,
New Jersey Institute of Technology, Newark, NJ 07102 USA.
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TSMC.2014.2376476
mathematical tool well suited to represent FMS characteristics due to
its inherent characteristics [42]–[46]. Thus, many researchers adopt
Petri net models as a formalism to handle deadlock problems in
FMSs, resulting in a wide variety of approaches [5], [21], [25], [45].
Generally speaking, these approaches can be classified into
four categories: 1) deadlock ignorance; 2) deadlock preven-
tion [1], [7]–[20], [27], [29]–[31], [36]–[38], [42]; 3) deadlock
avoidance [32], [33], [39], [40]; and 4) deadlock detection and
recovery [34].
Ezpeleta et al. [4] propose a special class of Petri nets that is
called systems of simple sequential processes with resources (S
3
PR).
To handle deadlock problems in S
3
PR, they present a policy that
prevents siphons from being emptied by adding monitors. They suc-
cessfully separate a plant and its supervisor but their policy suffers
from such problems as high computational complexity, high structural
complexity, and low behavior permissiveness.
Xing et al. [40] pioneered in the concept of maximal perfect
resource-transition circuit (MPRT-circuit). Based on MPRT-circuits,
an optimal (i.e., maximally permissive) deadlock avoidance pol-
icy with polynomial complexity is given for an S
3
PR net without
ξ-resources. As for an S
3
PR net with ξ -resources, a suboptimal dead-
lock avoidance policy with polynomial complexity is obtained. The
deadlock avoidance problem is thus polynomially solved in the frame-
work of Petri nets. Note that the supervisors derived from these
deadlock avoidance policies all belong to logic-based supervisors
since a one-step look-ahead method is adopted. Compared with logic-
based supervisors, monitor-based supervisors, as an alternative kind
of supervisors, have many advantages: 1) computation of control
actions is faster; 2) the same Petri net execution algorithm can be
used for both the original and controlled systems; and 3) a closed-loop
model of the system under control can be built with the standard net
composition technique. Hence, efforts are made to design monitor-
based supervisors. In this paper, we propose a deadlock prevention
policy that can synthesize a monitor-based supervisor.
Xing et al. [41] prove that a maximally permissive monitor-based
liveness-enforcing supervisor can be designed for an S
3
PR with-
out ξ-resources by M-controlling all strict minimal siphons (SMS)
(there exists a one-to-one correspondence between SMSs and MPRT-
circuits) where M-control is equivalent to the invariant control method
proposed by Moody and Antsaklis [26]. It is clear that complete
siphon enumeration is required if the supervisor is designed according
to the method in [41]. To reduce the computational and struc-
tural complexity of the supervisor, Wang et al. [38] propose a
new siphon-based deadlock prevention policy, which can obtain a
maximally permissive monitor-based liveness-enforcing supervisor
with reduced complexity. An algorithm extracting a desired emptied
SMS from a given emptied siphon based on loop resource subsets
contributes to such reduction. However, how to design a monitor-
based liveness-enforcing supervisor for an S
3
PR with ξ -resources
remains open. This work aims to answer it and makes the following
contributions:
1) An algorithm is given to reduce an S
3
PR via a ξ -resource,
which plays an important role in the classification of a
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