探究關鍵鏈排程之緩衝時間計算

On Buffer Sizing Techniques in Critical Chain Scheduling

在關鍵鏈排程中，為確保能達成對客戶之交期承諾，會在專案尾端加入專案緩衝；而為避免非關鍵鏈作業之延誤影響到關鍵鏈作業的既定排程，會在匯入關鍵鏈之路徑的尾端加入路徑緩衝。文獻中最常見的緩衝時間計算方法為剪貼法與根方誤差法，而相關的模擬實驗顯示剪貼法可能嚴重高估所需的緩衝保護。此外，實驗結果也顯示專案總工期顯著地受到資源緊迫度與先行關係密度影響。因此，有學者將這兩個因素列入考慮，提出以根方誤差法為基礎之新的緩衝時間計算方法。然而，這些調整係數會有顯著且不必要的膨脹，有必要做更深入的探討。為避免不必要的膨脹，在資源相當充裕且專案網路為鏈型時，本文所提出的調整係數取值為1。

In the theory of Critical Chain/Buffer Management (CC/BM), to protect the due-date promised to the customer from the variation in activity durations, the safeties associated with the critical activities are shifted to the end of the critical chain in the form of a project buffer. Moreover, feeding buffers are placed whenever a non-critical chain activity joins the critical chain. The most widely used buffer sizing methods are the cut and paste method (C&PM) and the root square error method (RSEM). Herroelen and Leus performed a full factorial experiment on a set of benchmark problems to test the CC/BM scheduling mechanism, and then reported that the C&PM may lead to a serious overestimation of the required buffer protection. From the results of this experiment, it can be also found that the project makespan highly depends on the resource tightness and the density of precedence. Therefore, Tukel et al. took these two factors into consideration, and then proposed two adjustment factors for the RSEM. However, their adjustment factors may lead to a significant and unnecessary expansion. To avoid the unnecessary expansion, in this paper, some revision to their adjustment factors are proposed. The new adjustment factors take the value 1 in cases that the resource is sufficient enough and the network is a chain.