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.