A methodology for designing flexible cellular manufacturing systems

Cell formation in cellular manufacturing deals with the identification of machines that can be grouped to create manufacturing cells and the identification of part families to be processed within each cell. Dynamic and random variations in part demands can negatively impact cell performance by creating unstable machine utilizations. The purpose of this paper is to introduce and illustrate an interactive cell formation method that can be used to design 'flexible' cells. Flexibility in this context refers to routing flexibility (i.e., the ability for the cellular system to process parts within multiple cells) and demand flexibility (i.e., the ability of the cell system to respond quickly to changes in part demand and part mix). Through an experimental analysis using multiple data sets, we also validate the procedure and provide guidelines for parameter settings depending upon the type of flexibility of interest to the user. Finally, trade-offs and interdependences between alternative types of flexibility in the context of cellular systems are illustrated.     

An object-oriented knowledge representation for hierarchical real-time control of flexible manufacturing

In this paper we demonstrate how a knowledge base can be constructed and used for real-time control of tool loading in a flexible manufacturing system (FMS). A procedure for dynamic tool loading is delineated to support variable production requirements expressed as a master production schedule (MPS). The main emphasis of this work is to propose a framework for real-time control to provide insights into the use of object-oriented structures for representing FMS entities.     

A flexible costing system for flexible manufacturing systems using activity based costing

Flexible manufacturing systems (FMSs) are designed to integrate the flexibility of job shops and the efficiency of mass production systems. Product costing methods have to adapt to this new technological environment. On one hand, the high production overhead cost of these systems requires a special attention to overhead allocation. On the other hand, the constantly changing setup configuration and production plans require a constant recalculation of overhead allocation and an a priori estimation of the expected production cost. This paper introduces the concept of flexible costing in FMSs, and proposes a method that modifies the overhead allocation based on the results of the production plan and on the simulated performance of the process. This approach is illustrated with some numerical examples.     

A due date-based dispatching rule for flexible manufacturing systems

In the paper a new dispatching rule is proposed, which extends the CEXSPT rule for dispatching jobs in a general cell's environment, i.e., a multiple-machine cell, where each machine type is assigned to a priority value. Each job is represented as a finite sequence of operations, ordered according to the type priorities of the cell's machines they are performed by. The algebraic background of the dispatching rule is described. The dispatching rule is implemented in the GOLDEN COMMON LISP programming language for IBM PC AT and compatible computers. A computer study of the performance of the proposed dispatching rule is conducted based on a four-machine cell, and the results are reported and analysed. The conclusions obtained are that the proposed dispatching rule produces cell's makespans, better than the CEXSPT dispatching rule, because it reduces idle time on cell's machines.     

A framework for the design of cellular manufacturing systems

A two-stage procedure for the design of a cellular manufacturing system is proposed. The first stage forms the part families. The use of clustering techniques with a new proximity measure is advocated for this stage. The proximity measure uses the manufacturing operations and the operations' sequences. The second stage forms the machine cells. An integer programming model is proposed for this stage. The solution of this model will specify the type and the number of machines in each cell and the assignment of the part families to the cells. The relevance of this approach in the design of flexible manufacturing systems is discussed.     

Data-driven generic simulators for flexible manufacturing systems

Despite the apparent move toward using data-driven simulators in manufacturing modelling, as opposed to simulation languages and packages that require programming, there have been few rational efforts to evaluate the development and use of these tools. As the number of tools continues to grow, such evaluation is necessary if simulation users are going to make sensible informed choices. This paper presents two specialized data-driven simulators developed to model Flexible Manufacturing Systems called RENSAM (Rensselaer Simulator for Automated Manufacturing) and RENVIS (Rensselaer Visual Interactive Simulator). Experience with these packages leads to consideration of the benefits of using such tools. Advantages include the ease with which models can be developed and the rapid pace of that development, and the enforcement of proper statistics collection; disadvantages include misplaced perception of how easy the tool is to use, weaknesses in implementation and the limitations of the simulator. It is shown that where a tool is deployed in the modelling process is of paramount importance, and guidance on deployment is provided. Other guidelines for developers and users of data-driven simulators are also developed.     

Integrated analytical models for flexible manufacturing systems

Product form queueing networks (pfqn) and generalized stochastic Petri nets (gspn) have emerged as the principal performance modelling tools for flexible manufacturing systems (fms). In this paper, we present integratedpfqn-gspn models, which combine the computational efficiency ofpfqn and representational power ofgspn by employing the principle of flow-equivalence. We show thatfms that include nonproduct form characteristics such as dynamic routing and synchronization can be evaluated efficiently and accurately using the integrated models.     

A heuristic procedure for loading problems in flexible manufacturing systems

The loading problem in a flexible manufacturing system (FMS) is viewed as selecting a subset of jobs from a job pool and allocating the jobs among machines. In this paper a heuristic solution to the loading problem has been suggested by developing the concept of essentiality ratio for the objective of minimizing the system unbalance and thereby maximizing the throughput. The proposed heuristic is tested on ten problems and the results show that the algorithm developed is very reliable and efficient.     

A systemic framework for the recovery of flexible production systems

In this paper, the real-time failure-recovery of flexible production systems is considered. Initially, the problem is structured and formulated within a systemic framework, and then, a generic local-in-time, schedule-repair approach is proposed. The coloured timed Petri net formalism is used to represent, in a modular and compact way, the production system's discrete-event dynamics. In connection with this, an intelligent opportunistic recovery technique has been developed for obtaining, in real time, the recovery trajectory which conforms best to a chosen production strategy. The application of the approach is demonstrated by considering recovery cases in a batch-processing chemicals plant.     

A framework for characterising energy consumption of machining manufacturing systems

Energy consumption in machining manufacturing systems is increasingly of interest due to concern for global climate change and manufacturing sustainability. To utilise energy more effectively, it is paramount to understand and characterise the energy consumption of machining manufacturing systems. To this end, a framework to analyse energy consumption characteristics in machining manufacturing systems from a holistic point of view is proposed in this paper. Taking into account the complexity of energy consumption in machining manufacturing systems, energy flow is described in terms of three layers of machining manufacturing systems including machine tool layer, task layer and auxiliary production layer. Furthermore, the energy consumption of machining manufacturing systems is modelled in the spatial and temporal dimensions, respectively, in order to quantitatively characterise the energy flow. The application of the proposed modelling framework is demonstrated by employing a comprehensive analysis of energy consumption for a real-world machining workshop. The characteristics of energy consumption for machine tool layer, task layer and auxiliary production layer are, respectively, obtained using quantitative models in the spatial and temporal dimensions, which provides a valuable insight into energy consumption to support the exploration of energy-saving potentials for the machining manufacturing systems.     

A method for scheduling in parallel manufacturing systems with flexible resources

We address the Parallel-Machine Flexible-Resource Scheduling (PMFRS) problem of simultaneously allocating flexible resources, and sequencing jobs, in cellular manufacturing systems where the cells are configured in parallel. We present a new solution methodology for the PMFRS problem called the Nested Partitions (NP) method. This method combines global sampling of the feasible region and local search heuristics. To efficiently apply the NP method we reformulate the PMFRS problem, develop a new sampling algorithm that can be used to obtain good feasible schedules, and suggest a new improvement heuristic. Numerical examples are also presented to illustrate the new method.     

A neural network model for on-line control of flexible manufacturing systems

This research investigates the potential of using a neural network approach in real-time control of flexible manufacturing systems. A hierarchical manufacturing controller, consisting of two neural network structures, is proposed. The first neural system participates in the feasibility analysis, and the other, at the lower level, in the process of dispatching and control. At the first level, a Sigma-Pi type of connection is used to translate work-in-process (WIP) move requests into directed arcs. Through a filter scheme, infeasible arcs are identified and eliminated from further consideration. At the second level, a modified Hopfield-Tank model is developed to determine the correct moves. Its goal is to deliver the right WIP to the right workstation, and process it at the right time. An example is used throughout the paper to illustrate the architecture developed. This two-phase control procedure provides adaptability, speed, and good solution quality which are important for real-time control of flexible manufacturing systems.     

A queueing network model for flexible manufacturing systems with tool management

The analytical model in this paper allows the evaluation of the performance of a flexible manufacturing system (FMS) with a tool management system. Design parameters such as the transportation time for tools to machines, as well as the number of transportation vehicles for tools, are explicitly considered. The part and the tool transportation system are modeled as two interacting closed queueing networks. The classical convolution algorithm is used to evaluate the part transportation system and mean value analysis approximation is applied to evaluate the tool transportation system. The resulting set of nonlinear equations allows then to estimate important system parameters such as the throughput of parts, the utilization of the tool transportation vehicles and the service interruptions caused by a tool supply order.     

Performance-based dynamic scheduling model for flexible manufacturing systems

A performance-based dynamic scheduling model for random flexible manufacturing systems (FMSs) is presented. The model is built on the mathematical background of supervisory control theory of discrete event systems. The dynamic FMS scheduling is based on the optimization of desired performance measures. A control theory-based system representation is coupled with a goal programming-based multi-criteria dynamic scheduling algorithm. An effectiveness function, representing a performance index, is formulated to enumerate the possible outputs of future schedules. Short-term job scheduling and dispatching decisions are made based on the values obtained by optimizing the effectiveness function. Preventive actions are taken to reduce the difference between actual and desired target values. To analyse the real-time performance of the proposed model, a software environment that included various Visual Basic Application® modules, simulation package Arena®, and Microsoft Access® database was developed. The experimentation was conducted (a) to determine the optimum look-ahead horizons for the proposed model and (b) to compare the model with conventional scheduling decision rules. The results showed that the proposed model outperformed well-known priority rules for most of the common performance measures.     

Production reliability in flexible manufacturing systems

A typical flexible manufacturing system, Westland Helicopters' sheet metal detail manufacturing complex, has been analysed for reliability. The techniques of fault tree analysis and event tree analysis are presented and their applicability to this study investigated. Event tree analysis has been found to be a more effective method for analysing manufacturing systems. The failure states of the system have been identified from the construction of an event tree which considers random hardware faults that influence production. Failure rate data have been used to quantify the critical production failure states in terms of machine failures. Estimates are made of the system's MTTF and percentage availability using typical MTTR figures. The probability that a selected production route fails to complete the manufacture of a set of parts is also evaluated. A dependency of systems reliability on the production demand has been discovered, and a possible method for modelling and assessing the reliability of systems capable of producing several products is proposed.     

Guidelines for implementing robust supervisors in flexible manufacturing systems

Over the past several years, researchers have developed numerous control policies that assure deadlock-free operation for flexible manufacturing systems. Using this research base as a foundation, we have developed several supervisory policies that assure robust operation in the face of resource failure. Along with deadlock-free operation, these policies guarantee that failure of unreliable resources does not block production of part types not requiring failed resources. In our previous work, we developed two types of robust policies, those that ‘absorb’ all parts requiring failed resources into the buffer space of failure-dependent resources (resources that support only parts requiring failed resources), and those that ‘distribute’ parts requiring failed resources among the buffer space of shared resources (resources shared by parts requiring and parts not requiring failed resources). These two types of robust controllers assure different levels of robust system operation and impose very different operating dynamics on the system, thus affecting system performance in different ways. In this research, we use extensive simulation and experimentation on a highly complex and configurable system to develop guidelines for choosing the best robust supervisor based on manufacturing system characteristics and performance objectives. We validate these guidelines using seven randomly generated complex systems and find a better than 88% agreement.     

Optimization-based parts-machines matching in flexible manufacturing systems

A new dynamic scheduling strategy, Parts-Machines Matching (PMM), is developed and tested in simulated flexible manufacturing systems. This strategy is aimed to achieve globally optimal matching between parts and machines by a semi-qualitative optimization algorithm, originally developed for the Stable Marriage Problem. Global and Partial implementations of PMM are presented and compared with other conventional part-flow rules. They are found to achieve better shop performance than conventional rules, in terms of system throughput, robustness against travel time uncertainties, and recovery from machine breakdowns. The prospect of bringing about system-wide optimization-based performance improvements into bidding schemes makes the proposed framework very significant.     

The optimal design of flexible manufacturing systems

The optimal design and control of flexible manufacturing systems is essential to minimize operating costs and enhance productivity. We present a hybrid mathematical model of a flexible manufacturing system as a closed network of queues for which the optimal cost effective system configuration is determined by a. partial implicit enumeration algorithm. The optimal configuration of both reliable and unreliable flexible manufacturing systems are considered.     

A response surface approach to developing composite dispatching rules for flexible manufacturing systems

We present a new approach to scheduling Flexible Manufacturing Systems based on Response Surface Modelling methodology. This experimental technique for model building is used to generate scheduling rules which are then used by the machines in the process of job selection. A systematic procedure to identify factors of interest and utilize them to develop a scheduling mechanism is presented. Issues of automation revision of the rules to adapt to changes in the shop are also discussed. The approach was tested as part of a larger framework of distributed decision making for scheduling and was found to perform significantly better than static heuristic procedures.     

A simulated annealing procedure for single row layout problems in flexible manufacturing systems

In this paper we address a layout problem in flexible manufacturing systems (FMSs). A layout type that has been extensively implemented in FMSs is the single row machine layout. In such a configuration machines are arranged along a straight track with a material handling device moving jobs from one machine to another. The single row layout problem (SRLP) deals with the optimal arrangement of the machines for the above configuration. We propose a simulated annealing (SA) heuristic for the solution of SRLP. Extensive computational results, and ways to improve the performance of the SA algorithm through parameter fine-tuning procedures, are reported.