For Flexible Manufacturing Systems Research Articles

An energy-aware optimisation model to minimise energy consumption and carbon footprint in a flexible manufacturing system

This paper proposes an energy-aware production schedule model for flexible manufacturing systems (FMSs) that aims to minimise energy costs and carbon tax while considering rising energy costs and environmental policies motivated by climate change concerns. The model is based on a sequence-dependent five-machine FMS scheduling problem, which is flexible in levels of parts, tools, machines, and routings. The proposed model is solved as a multi-objective problem, with tardiness and energy consumption as primary goals and carbon footprint reduction policies incorporated into the framework as control strategies. The machine ON/OFF strategy is also introduced to reduce idle energy. The study analyses the trade-off between minimising tardiness and carbon emissions to achieve both service level and environmental sustainability of the FMS. The proposed model reduced total energy consumption and carbon emissions by 20% and 21%, respectively, without compromising manufacturing deadlines. Future research may explore the integration of renewable energy sources and storage systems, considering more complex manufacturing scenarios, such as stochastic demands, machine failures, and workforce constraints.

Synthesis of Liveness-Enforcing Petri Net Supervisors Based on a Think-Globally-Act-\Łocally 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.

An Improved Synthesis Method Based on ILPP and Colored Petri Net for Liveness Enforcing Controller of Flexible Manufacturing Systems

Petri nets are used to design deadlock control strategies for flexible manufacturing systems (FMS), which typically involve the addition of monitors and the associated arcs to the FMS. The addition of several monitors and associated arcs to first constructed Petri net model significantly complicates the Petri net controller. This paper develops a two-step method for preventing deadlocks based on a colored Petri net and a structurally minimal approach that significantly reduces the number of monitors. In the first step, a vector covering technique is applied to generate a minimal covered sets of first-met bad markings (FBMs) and legal markings that are respectively smaller than the sets of FBMs and legal markings. At one iteration, place invariants (PIs) corresponding to monitors are constructed by solving an integer linear programming problem (ILPP) to prohibit the maximum number of FBMs, while allowing all legal markings in the minimal covering set. The purpose of the ILPP is to maximize the number of FBMs forbidden by the PIs. Then, based on a colored Petri net, all generated monitors are combined into a global control place. Therefore, a supervisor with minimal structural complexity can be constructed. The obtained net model is controlled after the addition of the designed supervisor. Two instances from the literature are considered to illustrate the proposed approach.

A digital-twin visualized architecture for Flexible Manufacturing System

The new generation of industrial 4.0 intelligent manufacturing system consists of Human-Cyber-Physical System (HCPS), integrating human with cyber and physical systems. In manufacturing, a digital-twin visualization architecture is to solve the human-machine interaction problem that concerns digital-twin modeling on the Cyber-Physical (C-P) side and on the Human-Cyber side. Although there are many related research and applications, there lacks attention in terms of full life cycle functional services and lightweight architecture. This paper presents a general architecture of digital-twin visualization for flexible manufacturing systems (FMS). How the digital-twin C-P modeling of multi-source heterogeneous information can be described is investigated and how the 3D visualized human-machine interaction with digital-twin scenario information is explored in the proposed architecture. Besides, the visualization method of high-value information, relating to the life cycle planning, design, debugging and service stages, is studied and discussed thoroughly. Also, a digital-twin modeling concept of "Geometric information (G)-Historical samples (H)-Object attribute (O)-Snapshot collection (S)-Topology constraint (T)" (GHOST) is proposed, and methods for developing virtual digital-twin scenes architecture are presented. Based on the proposed modeling concept of GHOST for digital-twin, prototypes have been developed for the general platform of digital-twin RESTful services and the cross-platform general visual mock-up software. Experimental results show that this method is effective in the FMS lifecycle in various aspects.

Robust scheduling of flexible manufacturing systems with unreliable operations and resources

This paper addresses the topic of robust scheduling for flexible manufacturing systems (FMS) with operation interruptions and unreliable resources. The proposed approach uses timed Petri nets as a model of the FMS in uncertain environments. This model includes controllable and uncontrollable transitions. The unexpected firings of the uncontrollable transitions represent operation and resource failures and the risk to deviate from the scheduled trajectories. This paper proposes an anytime graph search algorithm with a new objective function that combines performance and risk. In addition, the graph search incorporates a new filtering mechanism that learns from previous runs and two node expansion strategies. The performance of the proposed algorithm is compared with that of another already existing algorithm. A running example and a case study from a real company illustrate the efficiency of the proposed scheduling approach.

Competitiveness of Manufacturing Industry in India: Need for Flexible Manufacturing Systems

The ongoing process of globalization has brought about radical changes in industry and business across the globe. Because of global competition, there is ever growing need for higher productivity as well as quality in production. So also, there is a imminent need for improving cost effectiveness in industrial operations to withstand the competitive pressures. So, every business constantly seeks innovative ways of production processes for improving their competitiveness. In this context, this study looks into the need for Flexible Manufacturing Systems (FMSs) in Indian, the effectiveness of FMS in raising the productivity of manufacturing operations, FMS adoption being still in its infancy in India. FMS concept, its suitability, its applications and main trends are discussed. Suitable suggestions for effective FMS implementation in Indian industry are made.

An Augmented Reality approach to factory layout design embedding operation simulation

The subject of this work is layout planning of machinery, especially for Flexible Manufacturing Systems (FMS), using Augmented Reality tools. The goal is for the user to evaluate suggested layouts by taking into account non-measurable factors, such as operator experience, empirical or non-tacit knowledge and on-site impression. By reference to existing machinery (in the case study: CNC lathe and an industrial robot) the missing elements of a complete FMS cell (conveyor belts, automatic storage and retrieval system, other robots and CNC machines etc.) are super-imposed. The functional connection is enabled between the collaborating real-physical and the virtual machinery of the layout in the most convincing way, i.e., by simulation of the full production process involving movement, manipulation and processing of parts by real and virtual equipment in parallel and serial co-existence. This involves predefined scenarios that can be experienced exploiting alternative equipment in the same application and alternative layouts assessed in analogous applications. Static and dynamic simulation of the manufacturing cell offers the possibility of extensive analysis of the selected layout by walk-through navigation. The application is implemented in ARKit™ API tool and Unity3D™.

Model-based routing in flexible manufacturing systems

Abstract In this paper, a model-based routing approach for flexible manufacturing systems (FMS) with alternative routes for the work pieces is proposed. For each work piece, an individual task has to be accomplished, which consists of several processing steps. Each processing step can be executed on alternative working stations of the FMS. The proposed routing method employs a model of the conveying system to find energy efficient and fast routes for the respective work pieces. The conveying system model is based on a directed graph, where the individual conveyors are modeled as weighted edges. It can be straightforwardly applied to several types of FMS by adjusting the application-dependent parameters. Efficient computation of the fastest route through the conveying system is accomplished by means of dynamic programming, i. e., by integration of Dijkstra’s algorithm in a dynamic programming framework, which is based on the proposed conveying system model. Additional consideration of energy efficiency aspects leads to a Mixed Integer Quadratically Constraint Program (MIQCP), which is solved by substitution of Dijkstra’s algorithm by a branch and bound method. Experimental results for an application scenario, where the energy efficient routing method is applied to route work pieces between the working stations of an FMS, lead to 20 % reduction of energy consumption on average.

On Computation Reduction of Liveness-Enforcing Supervisors

It is well recognized that the computation of an optimal liveness-enforcing supervisor (optimal supervisor for short) for a sequential resource allocation system is intractable due to the nature of the problem itself, since an integer linear programming model has to be formulated to find its solution. In this paper, a novel vector covering approach is developed to reduce the computational cost of designing maximally permissive supervisors for flexible manufacturing systems (FMSs), by reducing the number of operation places and legal markings (LMs) that need to be considered. A vector covering approach is used to find the minimal covered set of first-met bad markings (FBMs). Then, a novel vector covering approach is proposed to reduce the set of LMs to a smaller covering set. The proposed method can reduce the number of both variables and constraints in the integer linear programming problems (ILPPs). It raises the efficiency of designing optimal supervisors by solving ILPPs.

A Minimal Supervisory Structure to Optimally Enforce Liveness on Petri Net Models for Flexible Manufacturing Systems

This paper presents a new method of computing a minimal supervisory structure that optimally enforces liveness on the Petri net models for flexible manufacturing systems (FMSs). The proposed method utilizes the structural properties of a Petri net model to avoid the computation of its reachability graph, which in general leads to the state explosion problem. This paper aims to design a single control place for each concurrent process of a Petri net model or a sub-net model, which thus provides a constant number of control places in a supervisor regardless of the number of resource places in a Petri net or sub-net model. It is shown that the structural size of a supervisor is minimal as the number of control places depends on the number of concurrent processes in the Petri net model. Precisely, two algorithms are developed in this paper. The first aims to compute active uncontrolled transitions and the second is concerned with a method to compute the generalized mutual exclusion constraints (GMECs) for each process of the Petri net model of an FMS. Furthermore, it provides an approach to design control places for each computed GMEC without solving integer linear programming problems, which greatly reduces the computational costs. When the computed control places are coupled with the uncontrolled Petri net model for an FMS, it optimally enforces liveness behavior of the Petri net model, and hence ensures the high utilization of resources in a considered system.

Total energy consumption optimization via genetic algorithm in flexible manufacturing systems

In recent years, there has been growing interest in reducing energy consumption and emissions of manufacturing systems. Except for adopting new equipment or techniques, scheduling is crucial to reduce the total energy consumption of manufacturing systems. This paper focuses on the scheduling problem for flexible manufacturing systems (FMSs) with the objective of minimizing the total energy consumption, and proposes a novel scheduling algorithm for FMSs based on Petri net models and genetic algorithm. Considering that energy consumptions in different states of resources are different, this paper takes two ways for calculating total energy consumptions. In the proposed genetic algorithm, a potential schedule is represented by a chromosome consisting of route selection and operation sequence. Crossover and mutation operations are performed on the operation sequence to guarantee the population diversity. For deadlock-prone FMSs, not all chromosomes can be directly decoded to a feasible schedule. To check the feasibility of chromosomes and convert infeasible chromosomes into feasible ones, a repair algorithm is developed with the help of the deadlock avoidance policy. Experiment results on a typical FMS and an industrial stamping system are provided to show the effectiveness of our proposed scheduling algorithm.

Think globally act locally approach for the synthesis of a liveness-enforcing supervisor of FMSs based on Petri nets

This paper, by using Petri nets (PNs), reports a general approach, called a think globally, act locally (TGAL) method, to compute liveness-enforcing supervisors (LES) for flexible manufacturing systems (FMSs) prone to deadlocks. A place called global sink/source place (GP) is introduced provisionally help us to decide a set of monitors such that deadlock states can be removed. The TGAL method proceeds with liveness enforcement by an iterative way in which a complete state enumeration is computed at each step. The resulting LES is generally maximally permissive or suboptimal, without solving intractable integer linear programming (ILP) problems. Given a system, a sufficient condition is developed to decide whether the TGAL method can find maximally permissive, that is, optimal supervisors. Several typical FMSs popularly studied in the literature are used as the examples to demonstrate the proposed method.

Closed-form solution of controller synthesis for infinitely large systems of resource sharing systems of a subclass of Petri nets

Little literature has dealt with infinite systems where an arbitrarily large number of resources is shared between larger systems with many stages due to the associated state explosion problem. Current most advanced controlled techniques for flexible manufacturing systems (FMS) cannot handle very large systems. We proposed earlier a method without reachability analysis to find the closed-form solutions of all reachable, forbidden, and live markings for the so-called kth-order system. This is the first reported closed-form solution of arbitrarily large systems. This paper further extends the closed-form solutions to the controller synthesis of infinite kth-order systems, which is a subclass of FMS. The place-invariant-based deadlock prevention controls are adopted to reduce the computational burden by considering only the minimal set of all first-met bad markings (FBMs). No live states are lost by considering also only the minimal set of all live markings. The above requires solving integer linear programming problems (ILPPs), which is NP-hard and quite time-consuming. By merging several monitors into a single one while not losing live states, this paper is able to achieve the same best results in the literature while avoiding the time-consuming reachability analysis and complete siphon computation which does not scale well with the size of the nets. Furthermore, closed-form solution of arbitrarily large systems can be derived, which has never been attained so far. Application to large Gadara resource allocation systems is also mentioned. Further work on handling a very large system with resources shared between more than two processes is reported. Moreover, the initial markings of the two terminal resource places are allowed to vary above one.

Context-sensitive optimisation of the key performance indicators for FMS

This article presents a context-sensitive optimisation approach for flexible manufacturing systems (FMSs), considering dynamic machine utilisation rate and overall equipment effectiveness (OEE) as the key performance indicators (KPIs). Run-time contextual entities are used to monitor KPIs continuously to update an ontology-based context model and subsequently convert it into business-relevant information via context management. The delivered high-level knowledge is further utilised by an optimisation support system (OSS) to infer optimal job (re)scheduling and dispatching, keeping a higher machine utilisation rate at run-time. The reference architecture is presented as add-on functionality for FMS control, where a modular development of the overall approach provides the solution generic and extendable across other domains. The key components are functionally implemented to a practical FMS use-case within service-oriented architecture -based control architecture. Test runs are performed in a simulated environment provided by the use-case control software, and the results are analysed, which indicates an improvement of the dynamic machine utilisation rate and the enhancement of the OEE.

A divide-and-conquer-method for the synthesis of liveness enforcing supervisors for flexible manufacturing systems

In this paper a divide-and-conquer-method for the synthesis of liveness enforcing supervisors (LES) for flexible manufacturing systems (FMS) is proposed. Given the Petri net model (PNM) of an FMS prone to deadlocks, it aims to synthesize a live controlled Petri net system. For complex systems, the use of reachability graph (RG) based deadlock prevention methods is a challenging problem, as the RG of a PNM easily becomes unmanageable. To obtain the LESs from a large PNM is usually intractable. In this paper, to ease this problem the PNM of a system is divided into small connected subnets. Each connected subnet prone to deadlocks is then used to compute the LES for the original PNM. Starting from the simplest subnet prone to deadlocks to make the subnet live, monitors (control places) are computed. The RG of each subnet is considered and split into a dead-zone (DZ) and a live-zone. All states in the DZ are prevented from being reached by means of a well-established invariant-based control method. Next, the computation of monitors is followed for bigger subnets. Previously computed monitors are included within the bigger subnets based on a criterion. This process keeps the DZ of the bigger subnets smaller compared with the original uncontrolled subnets. When all subnets are live we obtain a set of monitors that are included within the PNM to obtain a partially controlled PNM (pCPNM). A new set of monitors is also computed for the pCPNM. Finally, a live controlled Petri net system is obtained. The proposed method is generally applicable, easy to use, effective and straightforward although its off-line computation is of exponential complexity in theory. Its use for FMS control guarantees deadlock-free operation and high performance in terms of resource utilization and system throughput. Two FMS deadlock problems from the literature are used to illustrate the applicability and the effectiveness of the proposed method.

Deadlock-free scheduling for flexible manufacturing systems using Petri nets and heuristic search

Deadlock-free control and scheduling are two different problems for flexible manufacturing systems (FMSs). They are significant for improving the behaviors of the systems. Based on the Petri net models of FMSs, this paper embeds deadlock control policies into heuristic search algorithm, and proposes a deadlock-free scheduling algorithm to minimize makespan for FMSs. Scheduling is performed as heuristic search in the reachability graph of the Petri net. The searching process is guided by a heuristic function based on firing count vectors of state equation for the Petri net. By using the one-step look-ahead method in the optimal deadlock control policy, the safety of a state is checked. Experimental results are provided to show effectiveness of the proposed heuristic search approach in deadlock-free scheduling for FMSs.

Necessary and sufficient liveness condition of GS3PR Petri nets

Structural analysis is one of the most important and efficient methods to investigate the behaviour of Petri nets. Liveness is a significant behavioural property of Petri nets. Siphons, as structural objects of a Petri net, are closely related to its liveness. Many deadlock control policies for flexible manufacturing systems (FMS) modelled by Petri nets are implemented via siphon control. Most of the existing methods design liveness-enforcing supervisors by adding control places for siphons based on their controllability conditions. To compute a liveness-enforcing supervisor with as much as permissive behaviour, it is both theoretically and practically significant to find an exact controllability condition for siphons. However, the existing conditions, max, max′, and max″-controllability of siphons are all overly restrictive and generally sufficient only. This paper develops a new condition called max*-controllability of the siphons in generalised systems of simple sequential processes with resources (GS3PR), which are a net subclass that can model many real-world automated manufacturing systems. We show that a GS3PR is live if all its strict minimal siphons (SMS) are max*-controlled. Compared with the existing conditions, i.e., max-, max′-, and max″-controllability of siphons, max*-controllability of the SMS is not only sufficient but also necessary. An example is used to illustrate the proposed method.

Recursive solution of number of reachable states of a simple subclass of FMS

This paper aims to compute the number of reachable (forbidden, live and deadlock) states for flexible manufacturing systems (FMS) without the construction of reachability graph. The problem is nontrivial and takes, in general, an exponential amount of time to solve. Hence, this paper focusses on a simple version of Systems of Simple Sequential Processes with Resources (S3PR), called kth-order system, where each resource place holds one token to be shared between two processes. The exact number of reachable (forbidden, live and deadlock) states can be computed recursively.

Controllability Conditions of Resultant Siphons in a Class of Petri Nets

Strict minimal siphons (SMSs) play a key role in the development of deadlock prevention policies by using Petri nets for flexible manufacturing systems (FMSs). In this paper, given two SMSs and their resultant siphon, the concept of key resource subsets is proposed which is shown to be the critical factor in deciding the controllability conditions of the latter. A necessary and sufficient condition is then proposed under which the resultant siphon can be always marked if its two SMSs are M-controlled, i.e., invariant controlled via the method proposed by Moody and Antsaklis. As for a resultant siphon that is composed by more than two composable SMSs, a sufficient condition is proposed under which it can be always marked if its SMSs are M-controlled. They are established by analyzing the structural characteristics and markings of the resource subnets in a class of Petri nets called L-S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> PR. When they are applied to some classes of S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> PR nets, i.e., those whose controlled ones are maximally permissive and live once their original SMSs are M-controlled, the number of monitors may be reduced. An FMS example is used to illustrate the application of the results.

Behaviorally Optimal and Structurally Simple Liveness-Enforcing Supervisors of Flexible Manufacturing Systems

This paper presents two iterative deadlock prevention policies for flexible manufacturing systems (FMSs). Both can find a maximally permissive liveness-enforcing supervisor with a small number of control places. A vector covering approach is used to obtain a minimal covering set of legal markings and a minimal covered set of first-met bad markings (FBMs), which are much smaller than the sets of legal markings and FBMs, respectively. At each iteration, by solving an integer linear programming problem (ILPP), a place invariant (PI) associated with a control place is designed to forbid as many FBMs as possible, and no markings in the minimal covering set of legal markings are forbidden. The objective function of the ILPP maximizes the number of FBMs that are forbidden by the PI. Then, the forbidden FBMs by the PI are removed from the minimal covered set of FBMs. This process is carried out until all FBMs are forbidden. Two ILPP design techniques are developed for the control policies. Though the proposed methods cannot in general guarantee the minimality of the supervisory structure, they reduce the overall computational time greatly, which is shown by numerical studies. Finally, a number of FMS examples from the literature are presented to illustrate the proposed methods.