Loading and scheduling for flexible manufacturing systems with controllable processing times

This study addresses the problem of determining the allocation of operations and their tools to machines, the operation processing times and the allocation/sequence of the parts to be processed on each machine for flexible manufacturing systems with controllable processing times. Tool lives, tool copies and tool sharing are also considered. An integer programming model is developed for the objective of minimizing the sum of operation processing and tardiness costs. Then, iterative algorithms are proposed that solve the two subproblems iteratively, where the loading subproblem is solved by a modified bin packing algorithm under initial processing times and the resulting scheduling subproblem is solved by a priority scheduling method while modifying the loading plans and operation processing times iteratively. Computational experiments were carried out, and the results are reported.     

Autonomous agents architectures and algorithms in flexible manufacturing systems

This paper investigates possible implementations of the autonomous agents concept in flexible manufacturing control. The implementation issues and the effectiveness of different control architectures and algorithms are analyzed by means of a simulation model of a flexible job shop. Extensive experimental results are reported, allowing the evaluation of the trade-off between the degree of autonomy and system performance.     

Loading a flexible manufacturing system

This paper examines the problem of master scheduling for an FMS in Scotland containing six CNC horizontal boring machines. A multiple criteria approach is used to choose the compatible subset of candidate orders for processing by this FMS, subject to resource constraints and potentially conflicting performance objectives. A structured framework for conflict resolution is described and compromise solutions are obtained using standard mathematical programming software.     

Loading and control policies for a flexible manufacturing system

An experimental investigation of operating strategies for a computer-controlled flexible manufacturing system is reported. The system is a real one, consisting of nine machines, an inspection station and a centralized queueing area—all interconnected by an automatic material-handling mechanism. The operating strategies considered involve policies for loading (allocating operations and tooling to machines) and real-time flow control. A detailed simulation was employed to test alternatives. The results are different from those of classical job shop scheduling studies, showing the dependence of system performance on the loading and control strategies chosen to operate this flexible manufacturing system. Loading and control methods are defined that significantly improve the system's production rate when compared to methods which were previously applied to the system. Finally, some conclusions are presented concerning the control of these automated systems.     

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.     

Performance evaluation of flexible manufacturing systems with blocking

Analytical approximations for the performance of flexible manufacturing systems (FMS) with blocking of machines due to limited local buffers are presented. The approximations are based on a detailed analysis of FMS configurations used in industry. The method proposed uses informations generated by applying the classical closed queueing network (CQN) model to the FMS. The approximations developed are tested against simulation models for a wide variety of FMS configurations. The results presented show that the approximations are very good.     

Locating cells with bottleneck machines in cellular manufacturing systems

Because of bottleneck machines, the assignment of machine-cells to locations is interrelated with the machines' relative locations, and it makes the problem complicated to solve optimally. This paper prescribes an assignment of machine-cells to linear locations in order to minimize the inter-cell material handling cost incurred due to bottleneck machines in a cellular manufacturing system. This problem is formulated as a quadratic assignment problem (QAP). The optimal results can be obtained for a limited size of QAP. A 3-pair comparison heuristic is devised to partially overcome the dimensional problem for solving a large example. Later, an improvement heuristic called the 'bubble search' technique is developed to obtain a better solution, followed by the development of a lower bound on the QAP problem. Numerical examples are presented to illustrate the two heuristic procedures. A comparison between the optimal and heuristic solutions is also provided to evaluate the performance of the heuristics. Empirical tests conducted on two sets of data yield impressive test results.     

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.     

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 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.     

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.     

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.     

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.     

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.     

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.     

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.     

Tool management in flexible manufacturing systems with network part program

Recent technological developments have allowed a new concept of a part program to execute the machining operations that remove the software constraints introduced by the numerical control of machining centres. This change allows machines executing a part program to be structured as a net of operations and not linearly as normally happens in shop floors. The main advantage of using the network part program in numerically controlled machines is that it gives the system more flexibility by increasing the number of alternatives that machines can follow during work. In such a way, machines select their path in the network part program taking into account the dynamic state of resources (e.g. spindles, tools, carriers, pallets, etc.). The paper contains a study on the flexibility related to network part program exploitation in numerically controlled machines. Easy-to-calculate indicators are also defined. They estimate the potential flexibility of a network part program related to a particular product. The study has allowed the definition of a new tool management rule to be used in a flexible manufacturing system where the tools are shared dynamically among machines. The numerical results carried out on two different real cases demonstrate the validity of the new concept by quantifying the advantages in terms of throughput increment and machine idle-time reduction.     

A rule-based,object-oriented framework for operating flexible manufacturing systems

A methodology for operating a flexible manufacturing system (FMS) is presented which consists of a decomposition of the problem, where heuristics, rules, and simulation are integrated to make the control decisions needed for random FMS's. Expert systems are used to make operating decisions, to choose heuristics, and make operating policies. Multiple passes of discrete-event simulation are employed to evaluate decision alternatives. Object-oriented programming provides a suitable environment to implement the integrative framework     

New modelling formalism for control programs of flexible manufacturing systems

Proposed in this paper is a modelling formalism, called the FMS control model, which is able to provide a detailed guideline for the implementation of the FMS control program (PLC program). Although, there are numerous simulation models for FMSs describing the system dynamics at the level of discrete event simulation, they cannot be fully utilised for the generation of a real control program involving sensors and actuators. Due to this, PLC programs for FMSs are usually done by the repetitive method of code writing, testing, and debugging until the control objectives are achieved. This is an error-prone and time-consuming task. The proposed FMS control model is described at the level of sensors and actuators. It can support the PLC program generation from the model. Since the FMS control model has been developed by expanding the DEVS formalism, it has a well-defined formalism. The proposed methodology has been implemented and test runs were made.