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

Strict Minimal Siphon-Based Colored Petri Net Supervisor Synthesis for Automated Manufacturing Systems With Unreliable Resources

Various deadlock control policies for automated manufacturing systems with reliable and shared resources have been developed, based on Petri nets. In practical applications, a resource may be unreliable. Thus, the deadlock control policies proposed in previous studies are not applicable to such applications. This paper proposes a two-step robust deadlock control strategy for systems with unreliable and shared resources. In the first step, a live (deadlock-free) controlled system that does not consider the failure of resources is derived by using strict minimal siphon control. The second step deals with deadlock control issues caused by the failures of the resources. Considering all resource failures, a common recovery subnet based on colored Petri nets is proposed for all resource failures in the Petri net model. The recovery subnet is added to the derived system at the first step to make the system reliable. The proposed method has been tested using an automated manufacturing system deployed at King Saud University.publishedVersio

Comparison and Evaluation of Deadlock Prevention Methods for Different Size Automated Manufacturing Systems

In automated manufacturing systems (AMSs), deadlocks problems can arise due to limited shared resources. Petri nets are an effective tool to prevent deadlocks in AMSs. In this paper, a simulation based on existing deadlock prevention policies and different Petri net models are considered to explore whether a permissive liveness-enforcing Petri net supervisor can provide better time performance. The work of simulation is implemented as follows. (1) Assign the time to the controlled Petri net models, which leads to timed Petri nets. (2) Build the Petri net model using MATLAB software. (3) Run and simulate the model, and simulation results are analyzed to determine which existing policies are suitable for different systems. Siphons and iterative methods are used for deadlocks prevention. Finally, the computational results show that the selected deadlock policies may not imply high resource utilization and plant productivity, which have been shown theoretically in previous publications. However, for all selected AMSs, the iterative methods always lead to structurally and computationally complex liveness-enforcing net supervisors compared to the siphons methods. Moreover, they can provide better behavioral permissiveness than siphons methods for small systems. For large systems, a strict minimal siphon method leads to better behavioral permissiveness than the other methods

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

Algoritam za sprječavanje zastoja temeljen na uzastopnoj kontroli sifona Petrijeve mreže

This paper presents a formal calculation method of a deadlock prevention supervisor by the use of Petri nets. The proposed algorithm uses reachability tree to detect deadlock state and iterative siphon control method to synthesize the deadlock prevention supervisor. Such supervisor is maximally permissive and consists of minimal number of control places. The algorithm is intended for reversible or partially reversible P-T Petri net, but it can also be applied to Ordinary Petri nets. The calculation of the supervisor is illustrated by two examples. The first example shows the synthesis of deadlock prevention supervisor in a manufacturing system consisting of three conveyors and three robots, where the deadlock can occur due to concurrent requests of the conveyors for the robot engagements and unpredictable duration of those engagements. The second example shows the synthesis of deadlock prevention supervisor in a marine traffic system, where dangerous vessel deadlock situations may occur in case of vessels\u27 irregular motion through the system. To avoid this, the vessel traffic is supervised and controlled by traffic lights using the deadlock prevention supervisor, which is responsible for vessels\u27 stopping only in the case of dangerous situation and until this situation elapses.Članak opisuje formalnu metodu proračuna nadzornika za sprječavanje zastoja korištenjem Petrijevih mreža. Predloženi algoritam koristi stablo dostupnih stanja za detekciju stanja zastoja i metodu uzastopne kontrole sifona za sintezu nadzornika za sprječavanje zastoja. Nadzornik je najviše dopuštajući i sadrži najmanji broj kontrolnih mjesta. Algoritam je namijenjen za reverzibilne ili djelomično reverzibilne P-T Petrijeve mreže, ali se može koristiti i za obične Petrijeve mreže. Proračun nadzornika pokazan je na dva primjera. Prvi primjer prikazuje sintezu nadzornika za sprječavanje zastoja u fleksibilnom proizvodnom sustavu s tri robota i tri proizvodne trake, gdje se zastoj može dogoditi zbog međusobnog natjecanja transportnih traka za angažiranjem robota te zbog nepredvidljivosti trajanja tih angažmana. Drugi primjer prikazuje sintezu nadzornika u pomorskom prometnom sustavu, gdje se opasne situacije zastoja plovila mogu dogoditi poradi neodgovarajućeg pomicanja plovila kroz sustav. Da bi se to izbjeglo, promet plovila se nadzire i upravlja pomoću svjetlosne signalizacije korištenjem nadzornika za sprječavanje zastoja, koji je odgovoran za zaustavljanje plovila samo u slučaju opasnog stanja te dok to stanje ne nestane

Algoritam za sprječavanje zastoja temeljen na uzastopnoj kontroli sifona Petrijeve mreže

This paper presents a formal calculation method of a deadlock prevention supervisor by the use of Petri nets. The proposed algorithm uses reachability tree to detect deadlock state and iterative siphon control method to synthesize the deadlock prevention supervisor. Such supervisor is maximally permissive and consists of minimal number of control places. The algorithm is intended for reversible or partially reversible P-T Petri net, but it can also be applied to Ordinary Petri nets. The calculation of the supervisor is illustrated by two examples. The first example shows the synthesis of deadlock prevention supervisor in a manufacturing system consisting of three conveyors and three robots, where the deadlock can occur due to concurrent requests of the conveyors for the robot engagements and unpredictable duration of those engagements. The second example shows the synthesis of deadlock prevention supervisor in a marine traffic system, where dangerous vessel deadlock situations may occur in case of vessels\u27 irregular motion through the system. To avoid this, the vessel traffic is supervised and controlled by traffic lights using the deadlock prevention supervisor, which is responsible for vessels\u27 stopping only in the case of dangerous situation and until this situation elapses.Članak opisuje formalnu metodu proračuna nadzornika za sprječavanje zastoja korištenjem Petrijevih mreža. Predloženi algoritam koristi stablo dostupnih stanja za detekciju stanja zastoja i metodu uzastopne kontrole sifona za sintezu nadzornika za sprječavanje zastoja. Nadzornik je najviše dopuštajući i sadrži najmanji broj kontrolnih mjesta. Algoritam je namijenjen za reverzibilne ili djelomično reverzibilne P-T Petrijeve mreže, ali se može koristiti i za obične Petrijeve mreže. Proračun nadzornika pokazan je na dva primjera. Prvi primjer prikazuje sintezu nadzornika za sprječavanje zastoja u fleksibilnom proizvodnom sustavu s tri robota i tri proizvodne trake, gdje se zastoj može dogoditi zbog međusobnog natjecanja transportnih traka za angažiranjem robota te zbog nepredvidljivosti trajanja tih angažmana. Drugi primjer prikazuje sintezu nadzornika u pomorskom prometnom sustavu, gdje se opasne situacije zastoja plovila mogu dogoditi poradi neodgovarajućeg pomicanja plovila kroz sustav. Da bi se to izbjeglo, promet plovila se nadzire i upravlja pomoću svjetlosne signalizacije korištenjem nadzornika za sprječavanje zastoja, koji je odgovoran za zaustavljanje plovila samo u slučaju opasnog stanja te dok to stanje ne nestane