1,721,118 research outputs found
Discrete-time model for two-machine one-buffer transfer lines with buffer bypass and two capacity levels
No full textThis article deals with the analytical modeling of transfer lines consisting of two machines decoupled by one finite buffer. The innovative contribution of this work consists in representing a particular behavior that can be found in a number of industrial applications, such as in the ceramics and electronics industries. Specifically, the buffer significantly affects the line’s performance as, when it is accumulating or releasing material (i.e., when one machine is operational and the other machine is under repair), it forces the operational machine to slow down. Conversely, when both machines are operational they can work at a higher capacity since the buffer is bypassed. Thus, two levels for the machine capacity can be identified, based on the conditions of the machines and, consequently, the state of the buffer. The system is modeled as a discrete-time, discrete-state Markov process. The resulting two-Machine one-Buffer Model with Buffer Bypass is here called 2M-1B-BB model. The analytical solution of the model is obtained and mathematical expressions of the most important performance measures are provided. Finally, some numerical results are discussed
A MULTIPLE SINGLE-PASS HEURISTIC ALGORITHM FOR THE STOCHASTIC ASSEMBLY LINE RE-BALANCING PROBLEM
No full textAssembly line re-balancing is a problem frequently tackled by companies, as continuous changes in product features and volume demand caused by the volatility of today’s markets produce assembly tasks re-definition and line cycle time fluctuations. Hence, managers have to adapt the balancing of their lines to accomplish with the new conditions, while trying to keep to a bare minimum increases in completion costs and in costs related to changes in tasks assignment. In particular, modifications in line balancing impact on operators training, equipment switching and moving, along with quality assurance. The stochastic assembly line re-balancing problem basically consists in a multi-objective problem where two objectives, total expected completion cost of the new line and similarity between the new and the existing line, have to be jointly optimized. In this paper, a multiple single-pass heuristic algorithm is consequently developed with the aim to find the most complete set of dominant solutions representing the Pareto front of the problem. Multiple single-pass procedures iterate the execution of single-pass algorithms, in order to generate a set of solutions, rather than to create a unique purpose. Given such a set, the best-performing solutions, in accordance with the multi-objective nature of the problem, are presented to the line designer, who selects the final assembly line balancing considering also additive factors that can be hardly inserted in a mathematical approach (i.e. simplicity of learning re-assigned tasks, time requested for the re-allocation of tools necessary for executing re-assigned tasks, experience requested for maintenance of tools necessary for executing re-assigned tasks). By means of a wide experimentation including comparisons with a multi-objective genetic algorithm, the behaviour of the proposed methodology is set and optimized
The two-machine one-buffer continuous time model with restart policy
No full textThis paper deals with the performance evaluation of production lines in which well defined machine start/stop control policies are applied.
A modeling approach has been developed in order to reduce the complexity of a two-machine one-buffer line where a specific control policy, called “restart policy”, is adopted. The restart policy exercises control over the start/stop condition of the first machine: when the buffer gets full and, as a consequence, the first machine is forced to stop production (i.e., it is blocked), the control policy keeps the first machine in an idle state until the buffer becomes empty again. The rationale behind this policy is to reduce the blocking frequency of the first machine, i.e. the probability that a blockage occurs on the first machine due to the buffer filling up. Such a control policy is adopted in practice when outage costs (e.g., waste production) are related to each restart of the machine.
The two-machine one-buffer line with restart policy (RP line) is here modeled as a continuous time Markov process so as to consider machines having different capacities and working in an asynchronous manner. The mathematical RP model is described along with its analytical solution. Then, the most critical line performance measures are derived and, finally, some numerical examples are reported to show the effects of such a policy on the blocking frequency of the first machine
Fleet sizing of laser guided vehicles and pallet shuttles in automated warehouses
No full textThe paper deals with emerging automation technologies for pallet handling in warehouses characterized by multiple-depth rack storage systems. In particular, this studyaddresses the adoption of Pallet Shuttles (PSs) for material handling within the rack structure, i.e., for put-away and retrieval operations from the rack face to the specific pallet location and viceversa. PSs are used in combination with Laser Guided Vehicles (LGVs) so as to obtain a fully automated warehousing system. The study develops an analytical model for the joint LGV–PS fleet sizing problem, also taking into consideration stochastic phenomena and queueing implications. This enables the analyst to compute the utilization of LGVs and PSs as a function of sizes of the two fleets. Then, a queueing model can be applied to estimate the cycle time of the orders in the warehousing system. An example to clarify the application of the method is also provided
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