Theory and MethodologyTwo-stage heuristic algorithms for part input sequencing in flexible manufacturing systems
Introduction
A flexible manufacturing system (FMS) can be defined as an automated manufacturing system consisting of multi-functional machines that are interconnected by a material handling system, all controlled by a computer system. Before an FMS begins to produce parts, a number of decision problems must be solved. For example, a set of part types to be produced next should be selected, machines should be grouped into machine groups, and operations and associated cutting tools required for the selected part types should be allocated to the machines. These are called the part type selection, machine grouping, and loading problems, respectively. After an FMS is configured according to such decisions, the FMS scheduling problem is defined and can be solved.
There are two types of decisions to be made in the FMS scheduling problem. One is to determine a sequence to release parts into the system, which is called the part input sequencing problem, and the other is to determine a schedule for operations of released parts by determining where (on which machine) and when (in what sequence) operations of parts are to be processed. The latter decision can be considered after parts are released into the system. This paper focuses on the part input sequencing problem.
There are several previous studies on the input sequencing problem in flexible flow systems (FFSs) or assembly systems, most of which employ the cyclic sequencing approach based on a minimal part set (MPS). The MPS is a set of the numbers of part types that are proportional to the production requirements of the part types, i.e., a set of the smallest integers representing the product-mix of the part types. McCormick et al. [8] suggest sequencing algorithms for an MPS in an assembly line with buffers between machines, and Miltenburg [10] considers an input sequencing problem in a mixed-model assembly line with the objective of keeping a constant rate of usage of all parts, and suggests several heuristic algorithms to generate good feasible schedules. O'Keefe and Rao [11] suggest two dynamic methods for the part input sequencing problem in an FFS, a method employing look-ahead simulation and a rule-based method that incorporates the current state of the FFS, and show that the former works better than the latter. On the other hand, Stecke [16] suggests integer programming formulations that balance or unbalance the workloads assigned to machines in an FFS. These formulations are used to determine an input part set that is cyclically released into an FFS. Later, Smith and Stecke [15] present a simulation study on the robustness of the part input sequences generated by solving the integer programs given by Stecke [16]. They also report that determination of the input part sets is more important than sequencing of parts included in the part sets. For other types of part input sequencing or control in various manufacturing systems, see Erschler et al. [1], Hitz [3], Melnyk and Ragatz [9], Philipoom and Fry [12], Ragatz and Mabert [13], and Roderick et al. [14].
Not much work has been done for the input sequencing problem in FMSs of a job shop type or general flexible manufacturing systems (GFMSs), in which flows of parts are not unidirectional. Escudero [2] considers the part input sequencing problem in an FMS with several groups of pooled machines and develops an algorithm to balance workloads among the machine groups. Lee and Kim [7] compare several part input sequencing methods in which sequences are determined based on MPS. They also compare several dispatching rules devised for FMSs with partially grouped machines.
In this study, we focus on the part input sequencing problem in GFMSs with the objective of minimizing makespan. Several two-stage heuristic algorithms are developed based on decomposition of the entire problem into the input part grouping and sequencing problems. Here, the input part grouping problem is the problem of determining input part groups that are sequentially or cyclically released into the system, and the sequencing problem is the problem of determining the sequence of parts for each input part group. A solution of the part input sequencing problem can be obtained by solving these two subproblems.
This paper is organized as follows. In Section 2, the part input sequencing problem considered in this paper is described in more detail. Section 3 presents input part grouping methods and sequencing algorithms that are used in the two-stage heuristic algorithms. Simulation experiments are done to test the performance of the suggested algorithms and results are given in Section 4. Finally, Section 5 concludes the paper with recommendations for further research.
Section snippets
Problem description
The part input sequencing problem considered in this paper can be stated as the problem of determining the input sequence of a given set of parts to be released into an FMS with the objective of minimizing makespan. Minimizing makespan is used as the objective of the problem since makespan is closely related to production rate, throughput, and production capacity, which are common performance measures for manufacturing systems.
The FMS considered in this paper consists of several identical
Solution algorithms
In the solution approach suggested in this paper, the part input sequencing problem is decomposed into two subproblems, the input part grouping problem to group parts into one or more groups which will be sequentially released into the system or to select parts and determine their production quantities to be included in each of input part groups to be formed and the sequencing problem to determine the input sequence of parts included in each input part group. Here, an input part group is
Simulation experiments
The combination of four input part grouping methods and sixteen dispatching rules results in 64 input sequencing algorithms. To compare these algorithms, a series of simulation experiments were done in this study. In addition to these 64 two-stage heuristics, we can devise simple (one-stage) algorithms for input sequencing, in which parts are released into the system using the sixteen dispatching rules without being partitioned into input part groups. Therefore, in this method, called the
Concluding remarks
This paper considered the part input sequencing problem in FMSs of job shop type with the objective of minimizing makespan. In this research, a two-stage approach is suggested in which the entire problem is decomposed into the input part grouping problem and the sequencing problem. For the two-stage approach, three input part grouping methods and sixteen sequencing rules were developed. Results of simulation experiments showed that the two-stage heuristic algorithms suggested in this paper gave
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