# Simulation-Based Performance Analysis for a Novel AVS/RS Technology with Movable Lifts
## 4 Analyses of the Novel AVS/RS/ML
### Simulation Model of the Proposed AVS/RS/ML  
In the simulation model, we assume that there are two MLs per aisle. For the easiest way of collision prevention of MLs, in one design, a warehouse aisle is divided into two separate zones and each of the MLs operates in a dedicated zone. Furthermore, in another design, we assume that there are more than a single AGV in the system. Remember that there are two types of transaction requests arriving at the system: Storage and retrieval. The processes completed for those requests are summarized below. Storage requests:  
An available AGV, waiting at the I/O point, is selected randomly by the arriving storage transactions. While transaction is charged at the selected AGV, an appropriate ML is also activated and both travel to the target column location simultaneously. As a note, if the storage compartment is at the ground level, ML is not utilized for that transaction. * AGV with the tote is clamped on the lifting mechanism and it is lifted to the target tier of the storage rack. Note that once the AGV reaches the target tier, it is at the position of the required storage compartment at the same time. * AGV drops off the tote at the storage compartment. * AGV is moved downwards at the ground level and the ML is released. * AGV travels to the output point to take another waiting transaction. If there is no waiting tote, the AGV waits idle in the system. Retrieval requests:  
* An available AGV waiting at the I/O point is selected randomly by an arriving retrieval transaction. * While the selected AGV travels to the requested column of the target storage compartment, it also requests the ML; if it is seized, then both travel to the regarding column simultaneously. * AGV is clamped on the lifting mechanism of the ML, and it is lifted up to the target tier of the storage rack. Note that once the AGV reaches the target tier, it is at the position of the required storage compartment at the same time. * AGV picks up the tote at the storage compartment. * AGV is lifted down at the ground level and the ML is released. AGV travels to the output point and drops off the tote. The details of the operating rules considered in the simulation are shown in a flow chart in Figure 6.  
By simulating the system, we aim to compare two alternative operating designs for the described system. In the first design (Figure 7a), we assume a single I/O point that is located at periphery of enter point of each aisle. In the second design (Figure 7b), we assume that there are two separated I/O points, located at both enter points of a divided zone. It is clear that in both designs, there is an extra space at the middle of each aisle, needed for prevention of collision of two MLs (e.g., note that the w  
Figure 6: Flow chart of storage and retrieval transaction requests. upper levels; see Figure 2). Note that, in the Figure 7a design, we also allow AGVs to leave the aisle at the middle point to travel to the I/O point. In this design, we also consider one-way travel pattern for AGVs so that, while they enter the warehouse aisle from the I/O point side, they leave it either form the middle or from the other end side according to their last completion location. The design with single I/O location is experimented with two and three AGVs. For simple prevention of collision of the AGVs, it is assumed that an AGV cannot pass through another AGV that is in front of it. It stops and waits until that AGV completes its process even if that AGV is in the lifting mechanism. The design with two I/O locations is modelled by considering two AGVs each of which is dedicated to a divided zone. Hence, there is no possibility of collision of two AGVs. In this design, after AGVs complete their process they do not move to the I/O position to wait for further deployment. Instead, they stay at the ground location where they completed their last task. For each of different physical configurations based on I/O locations, we also considered different rack configurations with different number of tiers and columns scenarios.