Evolving design & Control Strategies for Production Systems

This thesis focuses on two issues: the design of operator walk cycles for Flexible Manpower Lines and the performance of the recently developed Control Point Policy [Gershwin, 1999] with a view to its application in multi part type flexible manpower lines and Flexible Machining Lines. A flexible manpower line forms an individual cell of a Linked-Cell Manufacturing System (L-CMS), developed within the Toyota Production System [Ohno, 1988]. The system, and consequently each cell, is designed to achieve Just-In-Time objectives. The production rate of a cell is dictated by (is, in fact, the inverse of) the Takt time. The Takt time is typically small, i.e.. comparable to the processing times of machines. Cells are operated with very little space allocated for the storage of in-process inventory and are required to exhibit the ability for quick and efficient changes in the rate of production. Extensive analysis, via the use of discrete event simulation, highlights the advantages of the CPP over Critical Ratio, a popular control policy for such systems. Particular attention is awarded to cases with small buffer sizes and Takt times; characteristics typical of flexible manpower lines. The results indicate that the CPP lends itself well to these situations and, as such, provides an ideal candidate for the control of multi part type flexible manpower lines and flexible machining lines. There are no readily available methods for the selection of parameter values for the CPP, i.e., the buffer sizes and hedging times, due to the policy’s recently being designed. An integrated GA-simulation search procedure has been established as part this research and is used for the identification of suitable values for these parameters. The technique and the results obtained provide further understanding of the benefits of the policy and insight into how and when the policy should be applied. Extensive analysis, via the use of discrete event simulation, highlights the advantages of the CPP over Critical Ratio, a popular control policy for such systems. Particular attention is awarded to cases with small buffer sizes and Takt times; characteristics typical of flexible manpower lines. The results indicate that the CPP lends itself well to these situations and, as such, provides an ideal candidate for the control of multi part type flexible manpower lines and flexible machining lines. There are no readily available methods for the selection of parameter values for the CPP, i.e., the buffer sizes and hedging times, due to the policy’s recently being designed. An integrated GA-simulation search procedure has been established as part this research and is used for the identification of suitable values for these parameters. The technique and the results obtained provide further understanding of the benefits of the policy and insight into how and when the policy should be applied. Extensive analysis, via the use of discrete event simulation, highlights the advantages of the CPP over Critical Ratio, a popular control policy for such systems. Particular attention is awarded to cases with small buffer sizes and Takt times; characteristics typical of flexible manpower lines. The results indicate that the CPP lends itself well to these situations and, as such, provides an ideal candidate for the control of multi part type flexible manpower lines and flexible machining lines. There are no readily available methods for the selection of parameter values for the CPP, i.e., the buffer sizes and hedging times, due to the policy’s recently being designed. An integrated GA-simulation search procedure has been established as part this research and is used for the identification of suitable values for these parameters. The technique and the results obtained provide further understanding of the benefits of the policy and insight into how and when the policy should be applied. Extensive analysis, via the use of discrete event simulation, highlights the advantages of the CPP over Critical Ratio, a popular control policy for such systems. Particular attention is awarded to cases with small buffer sizes and Takt times; characteristics typical of flexible manpower lines. The results indicate that the CPP lends itself well to these situations and, as such, provides an ideal candidate for the control of multi part type flexible manpower lines and flexible machining lines. There are no readily available methods for the selection of parameter values for the CPP, i.e., the buffer sizes and hedging times, due to the policy’s recently being designed. An integrated GA-simulation search procedure has been established as part this research and is used for the identification of suitable values for these parameters. The technique and the results obtained provide further understanding of the benefits of the policy and insight into how and when the policy should be applied. Variation in the throughput rate of a cell is achieved by altering the manning levels. Frequent changes in the number of operators within a cell require frequent redesign of operator walk cycles. Currently, walk cycles are constructed using a time-consuming, manual method. One aim of this thesis is to provide a flexible and efficient technique for the design of walk cycles, based on Genetic Algorithms (GAs), where either the Takt time or manning levels may be specified as the design constraint. A direct consequence of the efficiency of the technique developed is the ability to obtain greater knowledge of the relationships between system productivity, flexibility and manpower utilisation. The Control Point Policy (CPP) is a strategy for the real-time scheduling and control of production systems. The policy uses hedging points or hedging times to determine when machines should process parts. This thesis includes a continuation of recent investigations into the behaviour of the CPP but in a make-to-order., rather than make-to-stock, environment.