144 research outputs found
Feedback control logic synthesis for non safe Petri nets
This paper addresses the problem of forbidden states of non safe Petri Net
(PN) modelling discrete events systems. To prevent the forbidden states, it is
possible to use conditions or predicates associated with transitions.
Generally, there are many forbidden states, thus many complex conditions are
associated with the transitions. A new idea for computing predicates in non
safe Petri nets will be presented. Using this method, we can construct a
maximally permissive controller if it exists
Optimal Supervisory Control Synthesis
The place invariant method is well known as an elegant way to construct a
Petri net controller. It is possible to use the constraint for preventing
forbidden states. But in general case, the number forbidden states can be very
large giving a great number of control places. In this paper is presented a
systematic method to reduce the size and the number of constraints. This method
is applicable for safe and conservative Petri nets giving a maximally
permissive controller.Comment: Journ\'ee sur l'Instrumentation Industrielle J2I, ORAN : Alg\'erie
(2009
Continuous flow Systems and Control Methodology Using Hybrid Petri nets
International audienceIn this paper, we consider the controller synthesis for continuous flow systems. These lasts are a sub-class of hybrid dynamic systems. Their main characteristics are positiveness and linearity. Transport, manufacturing, communication and biological systems are examples of continuous flow systems. Numerous tools and techniques exist in the literature for modelling and analyzing such systems. As positiveness is a hard constraint, an appropriate tool integrating naturally this constraint is strongly needed. Hybrid Petri Nets are an elegant modeling tool of positive systems, while Hybrid Automata are a powerful tool giving formally the reachable dynamic space. Combining these two tools aim to a sound approach for control synthesis of continuous flow systems. We start by considering the process to control and compute its reachable state space using specialized software like PHAVer. Algebraic inequalities define this reachable state space. The constrained behaviour is obtained by restricting this state space into a smaller desired space. This reduction is expressed in term of linear constraints only over the continuous variables; while the control is given by the discrete transitions (occurrence dates of controllable events). The controller synthesis methodology is based on the control of a hybrid system modelled by a D-elementary hybrid Petri Net. The control consists in modifying the guard of the controllable transitions so as the reachable controlled state space is maximally permissive
Supervisor Synthesis for Discrete Event Systems under Partial Observation and Arbitrary Forbidden State Specifications
In this paper, we consider the forbidden state problem in discrete event
systems modeled by partially observed and partially controlled Petri nets. Assuming
that the reverse net of the uncontrollable subnet of the Petri net is structurally
bounded, we compute a set of weakly forbidden markings from which forbidden markings can be reached by firing a sequence of uncontrollable/unobservable transitions.
We then use reduced consistent markings to represent the set of consistent
markings for Petri nets with structurally bounded unobservable subnets. We determine
the control policy by checking if the firing of a certain controllable transition
will lead to a subsequent reduced consistent marking that belongs to the set of weakly
forbidden markings; if so, we disable the corresponding controllable transition. This
approach is shown to be minimally restrictive in the sense that it only disables
behavior that can potentially lead to a forbidden marking. The setting in this paper
generalizes previous work by studying supervisory control for partially observed and
partially controlled Petri nets with a general labeling function and a finite number of
arbitrary forbidden states. In contrast, most previous work focuses on either labeling
functions that assign a unique label to each observable transition or forbidden states
that are represented using linear inequalities. More importantly, we demonstrate
that, in general, the separation between observation and control (as considered in
previous work) may not hold in our setting
Control of Safe Ordinary Petri Nets Using Unfolding
International audienceIn this paper we deal with the problem of controlling a safe place/transition net so as to avoid a set of forbidden markings "F" . We say that a given set of markings has property REACH if it is closed under the reachability operator. We assume that all transitions of the net are controllable and that the set of forbidden markings "F" has the property REACH. The technique of unfolding is used to design a maximally permissive supervisor to solve this control problem. The supervisor takes the form of a set of control places to be added to the unfolding of the original net. The approach is also extended to the problem of preventing a larger set "F" of impending forbidden marking. This is a superset of the forbidden markings that also includes all those markings from which—unless the supervisor blocks the plant—a marking in "F" is inevitably reached in a finite number of steps. Finally, we consider the particular case in which the control objective is that of designing a maximally permissive supervisor for deadlock avoidance and we show that in this particular case our procedure can be efficiently implemented by means of linear algebraic techniques
Synthesis of Liveness-Enforcing Petri Net Supervisors Based on a Think-Globally-Act-Locally Approach and a Structurally Minimal Method for Flexible Manufacturing Systems
This paper proposes a deadlock prevention policy for flexible manufacturing systems (FMSs) based on a think-globally-act-locally approach and a structurally minimal method. First, by using the think-globally-act-locally approach, a global idle place is temporarily added to a Petri net model with deadlocks. Then, at each iteration, an integer linear programming problem is formulated to design a minimal number of maximally permissive control places. Therefore, a supervisor with a low structural complexity is obtained since the number of control places is greatly compressed. Finally, by adding the designed supervisor, the resulting net model is optimally or near-optimally controlled. Three examples from the literature are used to illustrate the proposed method
Deadlock Prevention Policy with Behavioral Optimality or Suboptimality Achieved by the Redundancy Identification of Constraints and the Rearrangement of Monitors
This work develops an iterative deadlock prevention method for a special class of Petri nets that can well model a variety of flexible manufacturing systems. A deadlock detection technique, called mixed integer programming (MIP), is used to find a strict minimal siphon (SMS) in a plant model without a complete enumeration of siphons. The policy consists of two phases. At the first phase, SMSs are obtained by MIP technique iteratively and monitors are added to the complementary sets of the SMSs. For the possible existence of new siphons generated after the first phase, we add monitors with their output arcs first pointed to source transitions at the second phase to avoid new siphons generating and then rearrange the output arcs step by step on condition that liveness is preserved. In addition, an algorithm is proposed to remove the redundant constraints of the MIP problem in this paper. The policy improves the behavioral permissiveness of the resulting net and greatly enhances the structural simplicity of the supervisor. Theoretical analysis and experimental results verify the effectiveness of the proposed method
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