Design of assembly/disassembly systems with unreliable machines

Abstract

We develop efficient methods for performance analysis and design of tree- structured assembly/disassembly (AD) systems, which consist of stations to process assembly and/or disassembly operations and buffers to connect the stations. Each buffer has finite capacity, and each station consists of parallel identical machines of which times between failures, times to repair, and processing times are exponentially distributed with different parameter values. Computational algorithms are developed for evaluating the performance (throughput rate, average work-in process inventories, etc.)of the AD systems. Using the proposed computational algorithms as a performance evaluation tool, algorithms for designing the AD systems are developed. Based on an extension of decomposition methods originally applied to performance evaluation of transfer lines, an efficient method is developed for performance evaluation of AD systems with single-machine stations. This method decomposes a K-machine tree-structured AD system to a system of K-1 two-machine lines and then finds processing rates, failure rates, and repair rates of the decomposed system which make performance of the two systems be close to each other. To determine those rates of the decomposed system, we derive 6(K-1) equations based on interruption of flow, resumption of flow, and flow rate-idle time relations and suggest two computational algorithms, which use the equations in different ways. For calculating throughput rates of the AD systems with parallel-machine stations, we develop a method that transforms such an AD system to an approximately equivalent AD system with single-machine stations, of which the throughput rate is computed with the decomposition methods. Results of computational experiments indicate that the suggested methods give good estimates for the throughput rates of the AD systems in a short time. Using the proposed methods for performance evaluation, we develop efficient methods for buffer allocati...