Browsing by Subject "Flexible manufacturing systems."

Now showing 1 - 7 of 7

Open Access
Analysis of scheduling problems in dynamic and stochastic FMS environment : comparison of rescheduling polices
(2001) Kızılışık, Ö Batuhan
In this thesis, we study the reactive scheduling problems in a dynamic and stochastic flexible manufacturing environment. Specifically, we test different scheduling policies (how-to-schedule and when-to-schedule policies) under process time variations and machine breakdowns in a flexible manufacturing system. These policies are then compared with on-line scheduling schemes. The performance of the system is measured for the mean flowtime criterion. In this study, a beam search based algorithm is used. The algorithm allows us to generate partial or full schedules. The results indicate that on-line scheduling schemes are more robust than the off-line algorithm in dynamic and stochastic environments.
Open Access
Analytical loading models and control strategies in flexible manufacturing systems: a comparative study
(1990) Kırkavak, Nureddin
There are three problem areas in designing and implementing a manufacturing line : the part family selection and grouping, system configuration and toolings and the operational control of manufacturing. The manufacturing process has to be stream-lined by considering resources and products to achieve flow lines operating around product families with acceptable levels of utilization. The stream-lined processes have to be assigned to tandem machines in the manufacturing lines. Then, interactions between production and inventory levels should be controlled at the operational level. Based on this framework, first a system configuration and tooling problem is modeled. The model turns out to be a large mLxed integer linear program, so that some alternative optimal seeking or heuristic techniques are used to solye the model for constructing a flow line structured Flexible Manufacturing System. Push systems of the Material Requirements Planning type or pull systems like the base-stock or Kanban schemes are often seen as alternatives for controlling manufacturing systems. The differentiating features of push, pull and a hybrid strategy are studied by discrete event simulation under different system and environmental characteristics for Flexible Manufacturing Systems. The impact of assignment of operations to machines on the performance of the system is also discussed.
Open Access
A Comparison of the aggregation and disaggregation task allocation approaches in a random flexible manufacturing system
(1997) Lahmar, Maher
The increased use of flexible manufacturing systems to efficiently provide customers with diversified products has created a significant set of operational challenges for managers. This technology is not only becoming more complex to control, but also presents a number of decision problems. Many issues concerning long-run utilizations and operational policies are still unresolved. The primary objective of this study is to examine the effects of aggregation and disaggregation task allocation approaches of random flexible routings on the performance of flexible manufacturing systems (FMS) and to analyze the effect of routing and sequencing flexibility on FMS and their interactions with other factors. The other experimental factors considered are machine load, alternative processing time ratio, local buffer capacities, set-up time, machine breakdown, processing time variation and scheduling rules. For this purpose, a simulation study is conducted and its results are analyzed by statistical methods.
Open Access
Joint lot sizing and tool management in a single CNC environment
(1996) Önen, Siraceddin
111 most of the studies on tool management, lot sizes are taken as pi'edetermined input while deciding on tool allocations and machining |)arameters. In this study, we considered the integration of lot sizing and tool rnaricigement problems for single and multi i:>eriod cases. For the single period case, we proposed a new algorithm. By this algorithm we not only improved the overall solution by exploiting interactions, but also prevented any infeasibility that might occur lor the tool management problem due to the decisions made at the lot sizing level. The computational experiments showed that in a set of randomly generated problems 22.5% of solutions found by a. traditional approach were infeasible and the proposed joint approach improved the overall solution by 6.8%. For the multi period case, we proposed live new algorithms. Among these algorithms, the most promising one was tlie Look Atiea.d-LUC algorithm, which improved the overall solution on the average by 6.5% compared to the best known algorithm, Wa.gner-Whitin, used in traditional approcich, over a set of randomly generated problems.
Open Access
Manufacturing flexibility in the justification of advanced automation investments
(1991) Çetinkaya, Sıla
A substantial amount of literature pertaining to flexibility has accumulated over the last decade. Nevertheless, there are several strategically important issues underlying this concept which are not understood properly, Understanding flexibility is made difficult by its multidimensional nature. Based on a detailed review of the literature we classify the conceptual frameworks on formalizing flexibility as: • type based understanding • change based understanding. We suggest the change based approach can provide a greater understanding of flexibility to managers whose knowledge about technological details is limited. Thus we expand Suresh’s (1990.b) capability-ease definition to provide a basis for the understanding. There have been a prevailing discussion between researchers on how flexibility relates to system performance. In fact type based and change based approaches are two different ways of determining relevant performance measures associated with flexibility. We suggest a framework capability-ease approach for the analysis of relevant performance measures. If it is followed by a task force capability-ease approach can contribute to a greater understanding of flexibility which leads to the selection of more appropriate performance measures. It is well recognized that for the future of manufacturing, flexibility is a crucial concept. However flexibility investments have been difficult to justify because of their high initial costs and strategic implications. Over the last few years incremental implementation of flexible technology has been suggested as a remedy for the investment justification problems, because it leads to lower annual capital outlays. We develop a m ixed-zeroone, nonlinear program m ing, m ultim achine, m ultiperiod, replacem ent model for incremental implementation of flexible automation. Capability and ease notions are adapted for modeling flexibility and a reclassification of costs is considered. Thus some specific aspects of designing flexibility are modeled in contrast to machine-level equipment replacement problem.
Open Access
Multiple part-type scheduling in flexible robotic cells
(2009) Batur, Gül Didem
This thesis considers the scheduling problem arising in two-machine manufacturing cells which repeatedly produce a set of multiple part-types, and where transportation of the parts between the machines is performed by a robot. The cycle time of the cell depends on the robot move sequence as well as the processing times of the parts on the machines. For highly flexible CNC machines, the processing times can be adjusted. As a result, this study tries to find the robot move sequence as well as the processing times of the parts on each machine that minimize the cycle time. The problem of determining the best cycle in a 2- machine cell is first modeled as a travelling salesman problem. Then, an efficient 2-stage heuristic algorithm is constructed and compared with the most common heuristic approach of Longest Processing Time.
Open Access
Two-machine flowshop scheduling with flexible operations and controllable processing times
(2011) Uruk, Zeynep
In this study, we consider a two-machine flowshop scheduling problem with identical jobs. Each of these jobs has three operations, where the first operation must be performed on the first machine, the second operation must be performed on the second machine, and the third operation (named as flexible operation) can be performed on either machine but cannot be preempted. Highly flexible CNC machines are capable of performing different operations as long as the required cutting tools are loaded on these machines. The processing times on these machines can be changed easily in albeit of higher manufacturing cost by adjusting the machining parameters like the speed of the machine, feed rate, and/or the depth of cut. The overall problem is to determine the assignment of the flexible operations to the machines and processing times for each job simultaneously, with the bicriteria objective of minimizing the manufacturing cost and minimizing makespan. For such a bicriteria problem, there is no unique optimum but a set of nondominated solutions. Using ǫ constraint approach, the problem could be transformed to be minimizing total manufacturing cost objective for a given upper limit on the makespan objective. The resulting single criteria problem is a nonlinear mixed integer formulation. For the cases where the exact algorithm may not be efficient in terms of computation time, we propose an efficient approximation algorithm.