#
On Compositional Supervisor Synthesis for Discrete Event Systems

Licentiate thesis, 2012

Over the past decades, human dependability on technical devices has rapidly
increased. Many activities of such devices can be described by sequences of
events, where the occurrence of an event causes the system to go from one
state to another. This is elegantly modeled by automata. Systems that are
modeled in this way are referred to as discrete event systems. Many of these
systems appear in settings that are safety critical, and small failures may
result in huge financial and/or human losses. Having a control function is
one way to guarantee system correctness.
Supervisory control theory, proposed by Ramadge and Wonham, provides
a general framework to automatically calculate control functions for discrete
event systems. Given a model of the system, the plant, to be controlled,
and a specification of the desired behaviour, it is possible to automatically
compute, i.e. synthesise, a supervisor that ensures that the specification is
satisfied.
Usually, systems are modular and consist of several components interacting
with each other. Calculating a supervisor for such a system in the
standard way involves constructing the complete model of the considered
system which may lead to the inherent complexity problem known as the
state-space explosion problem. This problem occurs when composition of
the components results in a model with a huge number of states, as the
number of states grows exponentially with the number of components. This
problem makes it intractable to examine the states of a system due to lack
of memory and time.
This thesis uses a compositional approach to alleviate the state-space explosion
problem. A compositional approach exploits the modular structure
of a system to reduce the size of the model of the system. The thesis mainly
focuses on developing the methodology for abstracting a system in a way
that the final synthesis result is the same as it would have been for the nonabstracted
system. The algorithms have been implemented in the discrete
event system software tool Supremica and have been applied to compute
modular supervisors for several large industrial models.

abstraction

supervisory control theory.

synthesis

Finite-state automata

RoomEB, Hörsalsvägen11, Department of Signals and Systems, Chalmers University of Technology

Opponent: Prof. David Sands, Department of Computer Science and Engineering, Chalmers University of Technology