Planning and scheduling approaches to operate a particular FMS

This paper contains a detailed description of a thirteen machine COMAU FMS for a company in Torino, Italy. The monthly and daily problems that need to be addressed and the approaches that are suggested to operate this system efficiently are detailed. The trickier problems and constraints are those of tool management, especially tool loading. Detailed tooling data and their analysis are also presented. The complete spectrum of operational problems addressed range from aggregate planning to detailed scheduling, including fixture and inventory management. Breakdown situations are also addressed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29836/1/0000183.pd

A hierarchical approach to solving machine grouping and loading problems of flexible manufacturing systems

A flexible manufacturing system (FMS) is an integrated system of computer numerically controlled (CNC) machine tools, each having an automatic tool interchange capability, and all connected by an automated material handling system. One or more computers control most real-time functions.Flexible manufacturing is realized to be an efficient alternative to conventional manufacturing that allows simultaneous machining of small to medium batches of a variety of part types. Parts can flow through the system in unit batch sizes. These systems typically machine five to forty different part types.In managing these systems, technological requirements indicate that several decisions must be made prior to system start-up. With these requirements in mind, previous research has defined a set of production planning problems, providing a conceptual framework to aid an FMS manager in setting up his/her system to enable efficient production. Several approaches have been taken to solve several of these problems and we describe those here. The main focus in this paper is on only two of these planning problems, the machine grouping and loading problems. In brief, the FMS machine grouping problem is to partition the mi machine tools of type i into gi groups to maximize expected production, subject to FMS technological and capacity constraints. Machines in a group are identically tooled and hence can perform the same operations during production. The FMS loading problem is to allocate operations and associated tooling of a selected set of part types among the machine groups, according to some appropriate (system dependent) loading objective, also subject to technological and capacity constraints.This paper ties some previous results together by suggesting a hierarchical approach to solve actual grouping and loading problems. Both problems are first defined at an aggregated level of detail and in the context of a queyeing network model. At this level, much information is suppressed. However, the robustness of the model allows the application of the obtained theoretical results to a lower level in the hierarchy that considers all details of these problems. In addition, results obtained using the aggregate model can be used as input to the detailed models. Here, the grouping and loading problems are formulated in all detail as nonlinear integer programs, using all available and required information. The use of these models to solve realistic machine grouping and loading problems is then described. Finally, future research needs are suggested.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26252/1/0000333.pd

On the nonconcavity of throughput in certain closed queueing networks

Analytic queueing network models are being used to analyze various optimization problems such as server allocation, design and capacity issues, optimal routing, and workload allocation. The mathematical properties of the relevant performance measures, such as throughput, are important for optimization purposes and for insight into system performance.We show that for closed queueing networks of m arbitrarily connected single server queues with n customers, throughput, as a function of a scaled, constrained workload, is not concave. In fact, the function appears to be strictly quasiconcave. There is a constraint on the total workload that must be allocated among the servers in the network. However, for closed networks of two single server queues, we prove that our scaled throughput is concave when there are two customers in the network and strictly quasi-concave when there are more than two customers. The mathematical properties of both the scaled throughput and reciprocal throughput are demonstrated graphically for closed networks of two and three single server queues.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25952/1/0000018.pd

Algorithms for efficient planning and operation of a particular FMS

This article is a detailed case study of a particular FMS that will be operational in 1989. It describes the daily planning and operating problems that will need to be addressed. The algorithms that will operate this system are presented. Given the daily changing production requirements, the algorithms begin with an aggregate planning feasibility check. Then planning, scheduling, inventory management, and breakdowns are addressed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45447/1/10696_2004_Article_BF00183873.pd

Performance evaluation for systems of pooled machines of unequal sizes: Unbalancing versus balancing

This research explores the appropriateness of unbalancing the workload per machine in certain types of flexible manufacturing systems (FMSs) configured as pooled machines of unequal sizes. Studies are conducted to examine the applicability of following an objective to unbalance workloads when solving the FMS planning problems of selecting part types to be machined together and determining their production ratios. Simulation is used to compare unbalancing and balancing on realistic, detailed models of flexible flow system (FFSs). The experiments are constructed to evaluate the impact of operational factors such as blocking, transportation, buffer utilizations, fixture requirements of various types, and different workload distributions among the machine types. The research results indicate that the aggregate and theoretically optimal unbalanced workloads provided by Stecke and Solberg (1981, 1985) using a closed queueing network model can be appropriate in a realistic FMS. Production rate and system and machine utilizations can all be higher when unbalancing workloads in systems of pooled machines of unequal sizes. It is also observed that: (1) system performance in terms of system utilization or production rate is sensitive to the appropriate number of pallets in the system, when either unbalancing or balancing; and (2) unbalanced part mix ratios conversely can lead to balanced machine utilizations among unequally sized pooled machine types. Overall system utilization seems to be more sensitive to the number of pallets in the system when unbalancing than when balancing. Further research needs are also noted.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27760/1/0000153.pd

The optimality of balancing workloads in certain types of flexible manufacturing systems

Symmetric mathematical programming is used to analyze the optimality of balancing workloads to maximize the expected production in a single-server closed queuing network model of a flexible manufacturing system (FMS). In particular, using generalized concavity we prove that, even though the production function is not concave, balancing workloads maximizes the expected production in certain types of m-machine FMS's with n parts in the system. Our results are compared and contrasted with previous models of production systems.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25713/1/0000270.pd

Profit-based FMS dynamic part type selection over time for mid-term production planning

The dynamic part type selection problem for mid-term scheduling to maximize profit over time in a general flexible manufacturing system is considered. Some mathematical programming models are developed to address this problem and their method of solution is based on a column generation technique. In the solutions to these models, the production plan is represented as a sequence of steady-state, periodical, cyclic schedules. Each cyclic schedule allows a subset of parts of different types to be released periodically in appropriate production ratios. Parts can be ratios and cycles until the production requirements for some part type are types can enter production and new ratios and cycles can easily be found. A two-level procedure is developed. At the upper level, a large-scale linear programming Master Problem is solved, the columns of which are generated by solving a nonlinear, mixed-integer, profit-based that selects part types for simultaneous production over a period. Numerical experiments demostrate that this model is computationally tractable to solve problems of practical size and over several periods of time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29721/1/0000055.pd

Reducing work-in-process inventory in certain classes of flexible manufacturing systems

In this paper, we establish that maintaining a balanced workload on each machine over time stochastically minimizes the work-in-process inventory in certain types of flexible manufacturing systems (FMSs) with finite or infinite common input buffer storage and an ample buffer et each machine. The results obtained here complement those obtained by [Stecke and Morin, 1985], in which it is established that balancing workloads maximizes expected production, again for the. same, particular types of FMSs. [Stecke and Morin, 1985] treats a static FMS loading problem, while this paper addresses a dynamic problem which considers three strategies to control the release of parts into the system.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26086/1/0000162.pd

A study of FMS part type selection approaches for short-term production planning

This research compares seven approaches from the literature to the selection of part types for simultaneous production over the next time horizon. A flexible approach to the selection of part types and the simultaneous determination of their mix ratios so as to balance aggregate machine workloads is presented. Constraints on tool magazine capacity are considered. Simulation studies are conducted on realistic, detailed models of flexible flow systems (FFSs) configured as pooled machines of equal sizes. The simulated settings are constructed to evaluate the impact of such factors as blocking, transportation, buffer utilizations, and fixture requirements and limitations of various types.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45438/1/10696_2004_Article_BF00713157.pd

Dynamic analysis of repetitive decision-free discreteevent processes: The algebra of timed marked graphs and algorithmic issues

A model to analyze certain classes of discrete event dynamic systems is presented. Previous research on timed marked graphs is reviewed and extended. This model is useful to analyze asynchronous and repetitive production processes. In particular, applications to certain classes of flexible manufacturing systems are provided in a companion paper. Here, an algebraic representation of timed marked graphs in terms of reccurrence equations is provided. These equations are linear in a nonconventional algebra, that is described. Also, an algorithm to properly characterize the periodic behavior of repetitive production processes is descrbed. This model extends the concepts from PERT/CPM analysis to repetitive production processes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44155/1/10479_2005_Article_BF02248590.pd