Mathematical model for estimating direct tool to tool transport ratio

In designing a rail layout for Overhead Hoist Transport system (OHT) and determining appropriate number of Overhead Hoist Vehicles (OHVs), detail transport request volume data, so-called a from-to matrix, is very important. But it strongly depends on estimates of direct tool to tool delivery ratio (henceforth, it is abbreviated as `direct ratio´). So how to estimate direct ratios is critical in designing an OHT system for a particular fab. This paper proposes a reasonable method to estimate direct ratios.