# Simulation-Based Performance Analysis for a Novel AVS/RS Technology with Movable Lifts
## 1 Introduction  
Modern high bay warehouses processing small and light tones usually adopt a shuttle-based storage and retrieval system (SBS/RS), which is an automated warehousing technology with automatic vehicles (i.e., shuttle carriers). Figure 1 shows an SBS/RS. In that system, shuttle vehicles move totes horizontally within a tier aisle. Shuttles are tier-captive vehicles whose main duty is to transport tones from their storage compartments to elevator's lifting tables and vice versa. Lifts (elevator's lifting tables) are located at the periphery of each aisle to transport tones between tiers and input/output (I/O) points.  
By a dedicated shuttle vehicle in each tier, SBS/RS provides a high throughput rate. However, on the other hand, works on SBS/RS design show that lifts usually become bottleneck in those systems and shuttle vehicle utilizations are very low [1, 2, 3]. To improve that unbalanced condition, a double capacitated lifting system is adopted as shown in Figure 1. However, lifting systems are still mostly the bottleneck resources in those systems.  
Different from the described SBS/RS with tier-captive shuttle vehicles, an autonomous vehicle solution with free-moving automated guided vehicle (AGV) that can enter the aisle at the ground floor that can lift itself vertically along each rack column has also been introduced in material handling market [4]. The main disadvantage of that design is the large amount of electricity consumption from their batteries during the lifting operation.  
Alternatively, in this paper, we propose a novel autonomous vehicle-based storage and retrieval system (AVS/RS) with movable lifts (MLs) where autonomous vehicles (i.e., AGVs) can travel inside and outside of storage aisles. This system is described in moredetail in Section 3. It is referred to as AVS/RS with MLs (AVS/RS/ML) throughout the paper.  
Note that the purpose of this paper is to study a novel AVS/RS/ML by utilizing a discrete event simulation modelling in order to verify the feasibility of the system and to evaluate the throughput performance for two separate sample designs. Under those designs, we also experiment cases with different number of AGVs as well as different storage rack designs.  
Here, the main research questions are:  
RQ1--Is there an alternative automated storage and retrieval system design to existing SBS/RS that would be cost efficient and have balanced utilization levels of lifts and autonomous vehicles?  
RQ2--By the proposed novel AVS/RS/ML system, by the decreased number of AGVs, since the throughput rate per unit time may decrease, would there be good designs under that proposed novel system helping to increase the throughput performance of the system?  
Note that for RQ1, we propose the studied AVS/RS/ML. For RQ2 we study two alternative warehouse designs for that AVS/RS/ML, which are zoning and non-zoning scenarios. Besides, we also experiment different number of AGVs for the non-zoning scenario. To the best of our knowledge, this paper first studies a novel system design developed as an alternative to tier captive SBS/RS. This paper is organised as follows. In Section 2, we provide a literature review about the related works. In Section 3, we describe the proposed novel AVS/RS/ML in detail by its operation rules, main characteristics, and design of MLs and AGVs. In Section 4, we describe the research methodology and the methods used, including the system simulation model assumptions. In Section 5, we provide the results of the experimental work and the discussion. Last, in Section 6, we conclude the work.