Control Of Flexible Manufacturing Systems Research Articles

Development of Cyber-Physical Production System based Manufacturing Control System for Aircraft Parts Plant

To enhance the effectiveness of the FMS (flexible manufacturing system), it is necessary for the manufacturing control system to be upgraded by integrating the cyber and the physical manufacturing systems. Using the CPPS (Cyber-Physical Production System) concept, this study proposes a 4-stage vertical integration and control framework for an aircraft parts manufacturing plant. In the proposed framework, the process controller prepares the operations schedule for processing work orders generated from the APS (advanced planning & scheduling) system. The scheduled operations and the related control commands are assigned to equipments by the dispatcher of the line controller. The line monitor is responsible for monitoring the overall status of the FMS including work orders and equipments. Finally the process monitor uses the simulation model to check the performance of the production plan using real time plant status data. The W-FMCS (Wing rib-Flexible Manufacturing Control & Simulation) are developed to implement the proposed 4-stage CPPS based FMS control architecture. The effectiveness of the proposed control architecture is examined by the real plant’s operational data such as utilization and throughput. The performance improvement examined shows the usefulness of the framework in managing the smart factory’s operation by providing a practical approach to integrate cyber and physical production systems.

Modelling and Control of Flexible Manufacturing Systems by Means of Interpreted Petri Nets

Modelling and Control of Flexible Manufacturing Systems by Means of Interpreted Petri Nets

PLC-based implementation of supervisory control for flexible manufacturing systems using theory of regions

The supervisory control is a common theory for the synthesis of Petri net (PN) supervisors for discrete event systems given a PN model and a control specification for the maximum permissive behaviour. The theory of regions as one of control synthesis method generates a PN controller to satisfy the control specification. Though the theory of regions has for over a decade received a considerable attention in academy, still very few applications exist. The real cause of this seems to be a contradiction between the abstract controller and its physical implementation. This is evident in particular when the implementation is supposed to be based on a programmable logic controller (PLC), as is the case for flexible manufacturing systems. Indeed, since the synchronous PLC is based on signals, the PN supervisor remains asynchronous; this explains its implementation difficulty. In this work, a control synthesis method using the theory of regions is implemented with a Java application on PLCs of a flexible manufacturing system (FMS) installed in our research laboratory in the University of Lorraine in Metz, France.

A Petri Net Modeling Framework for the Control of Flexible Manufacturing Systems

Abstract The changes in manufacturing practice towards increasingly flexible structures demand novel strategies for production scheduling and control. To this end, we introduce an automated model generation scheme that builds upon a coarse description of the production process and the factory. The generated model is based on a Petri Net representation of the production process, which we slightly enhance to separate automatically evolving processes from consciously taken manufacturing decisions. From the Petri Net model we deduce a state space description which can be used for the synthesis of feedback control for the manufacturing system. The model generation is illustrated with an example and further possibilities to utilize and enhance the model are discussed.

PLC-based implementation of supervisory control for flexible manufacturing systems using theory of regions

PLC-based implementation of supervisory control for flexible manufacturing systems using theory of regions

Component-wise Supervisory Controller Synthesis in a Client/Server Architecture

The manual design of monolithic controllers for flexible manufacturing systems is no longer feasible due to the sheer size of the problem. A well-known approach to tackle this scalability problem is to create a set of smaller controllers and orchestrate their interaction in an architecture. Another approach is to use synthesis techniques to generate a controller model from models of the uncontrolled system and the formalized requirements. In this paper we describe a pragmatic approach that combines the complementary advantages of these two approaches, where we decompose the design problem of the controller into a number of sub-controllers by introducing intermediate interfaces and use supervisory controller synthesis to synthesize the sub-controllers. We have evaluated this approach on an industrial case study, where we examined a large controller in a lithography machine. We found that the approach can successfully be used to generate a large portion of the needed sub-controllers.

A collaborative CPN−Fuzzy modelling strategy for conflict solution in flexible manufacturing systems

The increasing usage of Flexible Manufacturing Systems (FMS) poses new challenges, such as conflict situations in manufacturing processes. Therefore, there is a need to implement modelling and control of FMS in order to prevent or deal with such occurrences. However, modelling and controlling an FMS is itself a complex task due to the many subsystems and elements that comprise an FMS. Moreover, the existing high coupling between these subsystems also makes modelling and control tasks difficult to implement. This paper proposes a modelling strategy of FMS, which the decomposes the system into smaller and cohesive subsystems. In such a strategy, a part of the system should be designed to address interlocking issue, based on Petri Nets (PN) model and, collaboratively, the other part of the system should be designed to address sequencing of operations, based on Fuzzy Logic. It enables to perform more complex calculus to decide issues regarding FMSs, considering, optimisation criteria like makespan and idleness system resources. This strategy was used for modelling a simulated factory with 6 machines, composed by input/output buffers, 3 AGVs (Automated Guided Vehicles), a load/unload station and AGVs park.

Distributed Computation of Liveness-Enforcing Supervisory Control of Flexible Manufacturing Systems

This paper presents a modular deadlock prevention policy for large-scale flexible manufacturing systems. It can find maximally permissive liveness-enforcing supervisory control in an efficient computational complexity manner. A vector covering approach is used to minimize essential set of legal markings and first-met bad markings. Providing that modules of system interact using choice, we will prove that monitor-based supervisory control methods can be applied distributedly. Then, choice operator of Petri net will be used to synthesis subsystems. Even if such assumption limits the application of modular monitor-based supervisory control, the proposed results constitute a primary step towards a synthesis procedure that overcomes time and space complexity of monitor-based deadlock prevention policies.

Efficient optimal deadlock control of flexible manufacturing systems

The development of an efficient deadlock avoidance policy (DAP) is a critical step in the design and operation of a flexible manufacturing system (FMS). However, even for a simple FMS, the computation of an optimal DAP is intractable. This work addresses the optimal deadlock control problem of FMSs. Based on their Petri net models, it introduces the concept of κ-resources and proves that an FMS containing no κ-resources has only two types of reachable states: safe ones and deadlocks. It then can obtain an optimal DAP with polynomial computational complexity for a broader class of FMSs, which was never seen before. At the same time, for these FMSs, new structurally simpler optimal Petri net controllers are proposed.

Near-optimal supervisory control of flexible manufacturing systems using divide-and-conquer iterative method

This article proposes an iterative deadlock resolution method for flexible manufacturing systems modeled with G-systems. To design a non-blocking controlled system with maximally permissive behavior in a G-system ( GS), a reachability graph-based analysis technology is utilized. Since the reachability graph of a large-scale GS easily becomes unmanageable, an optimal non-blocking supervisor becomes a challenging problem in a GS. To facilitate this problem, the Divide-and-Conquer approach is a good choice for complex G-systems. First, an uncontrolled GS resolves into a number of associated subnets. Then, every subnet suffering from deadlocks is utilized to design the liveness-enforcing supervisor for the original GS. Thus, additional monitors can be obtained if the liveness of all subnets is achieved. Subsequently, a partially controlled GS is derived by including all monitors within the GS, and its liveness can be ensured by designing a new set of monitors. Consequently, a non-blocking GS is derived. The major advantage of the proposed method is that a non-blocking supervisor with near-optimal behavioral permissiveness can be obtained in general. Finally, a typical GS example popularly studied in the literature is applied to demonstrate the validity and the availability of the method in this article.

A new method for the redundancy analysis of Petri net-based liveness enforcing supervisors

The deadlock control of flexible manufacturing systems (FMSs) has been widely studied in the literature. Petri nets (PNs) are extensively used as a tool for modelling, analysis and controller synthesis of such systems. In general, Petri-net-based liveness enforcing supervisors (LESs) include control places ( CPs) together with their input/output arcs. It is well known that the methods proposed for computing CPs may provide redundant and necessary CPs. In this paper, a new method is proposed for redundancy test of CPs by means of supervisory control theory (SCT). The proposed method is based on the idea that after the removal of a CP from an LES, if the controlled model is still live, then the removed CP is redundant. The proposed method makes use of the TCT implementation tool of SCT. It is applicable to a PN-based LES consisting of a set of CPs. The applicability of proposed method is demonstrated by means of examples from the relevant literature. For some examples, the redundancy test provides more permissive behaviour with structurally simpler supervisors.

Control of Flexible Manufacturing Systems under model uncertainty using Supervisory Control Theory and evolutionary computation schedule synthesis

A new approach for the problem of optimal task scheduling in flexible manufacturing systems is proposed in this work, as a combination of metaheuristic optimization techniques with the supervisory control theory of discrete-event systems. A specific encoding, the word-shuffling encoding, which avoids the generation of a large number of infeasible sequences, is employed. A metaheuristic method based on a Variable Neighborhood Search is then built using such an encoding. The optimization algorithm performs the search for the optimal schedules, while the supervisory control has the role of codifying all the problem constraints, allowing an efficient feasibility correction procedure, and avoiding schedules that are sensitive to uncertainties in the execution times associated with the plant operation. In this way, the proposed methodology achieves a system performance which is typical from model-predictive scheduling, combined with the robustness which is required from a structural control.

On Further Reduction of Constraints in “Nonpure Petri Net Supervisors for Optimal Deadlock Control of Flexible Manufacturing Systems”

The above paper proposes a method to design optimal control places with self-loops for flexible manufacturing systems by solving an integer linear programming problem (ILPP) at each iteration. However, some constraints in the ILPP are redundant. This technical correspondence shows that they can be removed without changing the feasible region of the ILPP.

Programación y Control de Sistemas de Fabricación Flexibles: un Enfoque Holónico

Holonic Manufacturing Systems are a new manufacturing paradigm. They are included into distributed methodologies and propose new manufacturing management schemes that combine self-organization, dynamic hierarchies and horizontal relationships. This approach is closer to manufacturing system nature than centralized or strictly traditional hierarchical approaches. It allows modeling manufacturing systems in a more realistic way. So, it make easier to implement more flexible, scalable, responsiveness, reconfigurable, and robust production management systems. In this paper we propose a holonic system for scheduling and control of flexible manufacturing systems. The system is based on a new architecture that integrates two decision levels: a level of auction-based scheduling, where each holon performs its own local schedule, and another level of dispatching and control that coordinate holons activities. Unlike other studies in which local schedules are integrated into a global schedule that centralizes the system, the proposed system is not so. The dispatching and control mechanism is responsible to integrate the local programs. In addition, the developed prototype can operate with different degrees of decentralization, which allows evaluating the advantages that our approach provides by means of simulation.

Petri net based decision system modeling in real-time scheduling and control of flexible automotive manufacturing systems

This paper presents the design and the implementation of a Petri net (PN) model for the control of a flexible manufacturing system (FMS). A flexible automotive manufacturing system used in this environment enables quick cell configuration, and the efficient operation of cells. In this paper, we attempt to propose a flexible automotive manufacturing approach for modeling and analysis of shop floor scheduling problem of FMSs using high-level PNs. Since PNs have emerged as the principal performance modeling tools for FMS, this paper provides an object-oriented Petri nets (OOPNs) approach to performance modeling and to implement efficient production control. In this study, we modeled the system as a timed marked graph (TMG), a well-known subclass of PNs, and we showed that the problem of performance evaluation can be reduced to a simple linear programming (LP) problem with m−n+1 variables and n constraints, where m and n represent the number of places and transitions in the marked graph, respectively. The presented PN based method is illustrated by modeling a real-time scheduling and control for flexible automotive manufacturing system (FAMS) in Valeo Turkey.

Corrections to “Nonpure Petri Net Supervisors for Optimal Deadlock Control of Flexible Manufacturing Systems” [Mar 13 252-265

Some numerals in are found to be incorrect. Also, Definition 7 in is unclear and should be stated as follows.

Optimal Supervisory Control of Flexible Manufacturing Systems by Petri Nets: A Set Classification Approach

Supervisory control is usually considered as an external control mechanism to a system by controlling the occurrences of its controllable events. There exist Petri net models whose legal reachability spaces are nonconvex. In this case, they cannot be optimally controlled by the conjunctions of linear constraints. For Petri net models of flexible manufacturing systems, this work proposes a method to classify the legal markings into several subsets. Each subset is associated with a linear constraint that can forbid all first-met bad markings. Then, the disjunctions of the obtained constraints can make all legal markings reachable and forbid all first-met bad markings, i.e., the controlled net is live and maximally permissive. An integer linear programming model is formulated to minimize the number of the constraints. A supervisory structure is also proposed to implement the disjunctions of the constraints. Finally, examples are provided to illustrate the proposed method.

The effect of responsiveness of the control-decision system to the performance of FMS

In Flexible Manufacturing System (FMS), decision-making process is one of the key aspects for its performance enhancement, particularly for shop-floor control, where operation managers need to make a large number of control decisions. A term called ‘response-time’ in decision-making process has been defined in this study, which refers to a lead time in decision-making and its implementation. This paper contributes a methodology for decision-maker to study the decision-making process and identify a suitable decision-making approach, while considering critical factors such as decision automation levels, routing flexibility levels, and control strategies. Considering the complexity in modelling an FMS with routing flexibility levels, control strategies (sequencing and dispatching rules), and decision-making process with information system, computer simulation modelling has been employed to study the makespan performance. The results show that decision-making process with response-time for FMS control performs as good as with the real-time control when routing flexibility level is low. Furthermore, under some specific situations, it even outperforms the real-time control. This research gives insight to decision-maker to identify whether a decision system with response-time will be more suitable and economically justified, or real-time decision-making system is more appropriate.

Reducing myopic behavior in FMS control: A semi-heterarchical simulation–optimization approach

Heterarchical FMS control architectures localize decisional capabilities in each entity, resulting in highly reactive, low complexity control architectures. Unfortunately, these architectures present myopic behavior since decisional entities have limited visibility of other decisional entities’ behavior and the alignment of an entity’s decision with the system’s global objective. In this paper, we propose a semi-heterarchical architecture in which a supervisor tackles different kinds of myopic decisions using simulation–optimization mechanisms and the current conditions of a flexible manufacturing system (FMS). The supervisor uses simulation results to calculate local and global performances and to evolve the solutions proposed by the optimization mechanisms. The approach proposed was configured to control a real assembly cell with highly heterarchical approaches. The completion time variance was used as the performance measure for myopic behavior reduction. The simulation results showed that the semi-heterarchical architecture can reduce myopic behavior whereby it strikes a balance between the ability to react to disturbances and maintaining low complexity, thus making it suitable for production control.

Reaching most states via refining controller regions for supervisors of two well-known S3PRs

Deadlock control of flexible manufacturing systems (FMS) using Petri nets has been a very active research topic for the past two decades. Siphon-based deadlock control of FMS suffers from reaching fewer states than those of optimal control. This paper proposes three sets of siphon-based liveness-enforcing supervisors (LES) for a well-known FMS to obtain near optimal solutions. The first set includes 11 control places (monitors) providing 21,363 good states. The second set consists of 14 monitors providing 21,562 good states, the same as that using the method of the first-met bad marking (FBM) (based on an iterative method using reachability graph analysis) but with fewer monitors and control arcs. This result is achieved by refining some monitors into several monitors with smaller controller regions, causing fewer disturbances to the original uncontrolled model. The third set contains 15 monitors providing 21,573 good states – the best suboptimal solution so far in the literature. This result is obtained by further shrinking the controller region of the second set. Formal proof of the correctness is provided. Although reaching eight states fewer than that by the set-covering approach, it does not employ weighted control arcs and runs more efficiently. It also computes all lost states based on invariant without reachability analysis. Application to another well-known example is also illustrated.