The dual-retrieval （DR） operation sequencing problem in the flow-rack automated storage and retrieval system （AS/RS） is modeled as an assignment problem since it is equivalent to pairing outgoing unit-loads for each DR operation. A recursion symmetry Hungarian method （RSHM）, modified from the Hungarian method, is proposed for generating a DR operation sequence with minimal total travel time, in which symmetry marking is introduced to ensure a feasible solution and recursion is adopted to break the endless loop caused by the symmetry marking. Simulation experiments are conducted to evaluate the cost effectiveness and the performance of the proposed method. Experimental results illustrate that compared to the single-shuttle machine, the dual-shuttle machine can reduce more than 40% of the total travel time of retrieval operations, and the RSHM saves about 5% to 10% of the total travel time of retrieval operations compared to the greedy-based heuristic.
针对截止期限约束下有向无环图DAG(directed acyclic graph)表示的工作流费用优化问题,提出两个新的费用优化算法:时间约束的前向串归约算法FSRD(forward serial reduction within deadline)和时间约束的后向串归约算法BSRD(backward serial reduction within deadline).算法利用DAG图中串行活动特征给出串归约概念;基于分层算法对串归约组的时间窗口重定义,并提出动态规划的求解策略实现组内费用的最优化.两种归约算法综合考虑DAG图中活动的串并特征,改变分层算法中仅对单一活动的费用优化策略,实现了串归约组的时间收集和最优利用.模拟实验结果表明:BSRD和FSRD能够显著改进相应分层算法的平均性能,且BSRD优于FSRD.
