自动化立体仓库(英文)

关于自动化立体仓库使用双货叉问题的探讨-An Analysis Of Dual Shuttle Automated Storage/Retrieval SystemsAn Analysis Of Dual ShuttleAutomated Storage/Retrieval SystemsAdhinarayan KeserlaBrett A. PetersAugust 1, 1994This working paper is not to be copied, quoted, or cited without the permission of the authors. Address correspondence to Brett A. Peters, Dept. of Industrial Engineering, Texas A&M University, College Station, TX 77843-3131 or email to bpeters@AbstractThis paper addresses the throughput improvement possible with the use of a dual shuttle automated storage and retrieval system. With the use of such a system, travel between time in a dual command cycle is virtually eliminated resulting in a large throughput improvement. The dual shuttle system is then extended to perform an equivalent of two dual commands in one cycle in a quadruple command mode (QC). A heuristic that sequences retrievals to minimize travel time in QC mode is developed. Monte Carlo simulation results are provided for evaluating the heuristic's performance and show that it performs well, achieving large throughput improvements compared with that of the dual command cycle operating under the nearest neighbor retrieval sequencing heuristic.Keywords:Automated Storage/Retrieval Systems Design; Automated Storage/Retrieval Systems Operation; Material Handling Systems; Performance Modeling and AnalysisIntroductionAutomated storage/retrieval systems (AS/RS) are widely used in warehousing and manufacturing applications. A typical unit load AS/RS consists of storage racks, S/R machines, link conveyors, and input/output (I/O) stations. An important system performance measure is the throughput capacity of the system. The throughput capacity for a single aisle is the inverse of the mean transaction time, which is the expected amount of time required for the S/R machine to store and/or retrieve a unit load. The service time for a transaction includes both S/R machine travel time and pickup/deposit time. This time typically depends on the configuration of the storage rack and the S/R machine specifications.Han et al. [2] improved the throughput capacity of the AS/RS through sequencing retrievals. Intelligently sequencing the retrievals can reduce unproductive travel between time when the S/R machine is traveling empty and thereby increase the throughput. They develop an expression for the maximum possible improvement in throughput if travel between is eliminated for an AS/RS that is throughput bound and operates in dual command mode. In essence, this means that if the S/R machine travels in a single command path but performs both a storage and a retrieval operation, the above throughput improvement could be obtained.In this paper, we analyze an alternative design of the S/R machine that has two shuttles instead ofone as in a regular AS/RS. The new design eliminates the travel between the storage and retrieval points and performs both a storage and a retrieval at the point of retrieval, thereby achieving the maximum throughput increase calculated by Han et al. [3].The dual shuttle AS/RS is a new design aimed at improving S/R machine performance. most studies on AS/RS systems have been based on a single shuttle design. In our analysis of the dual shuttle AS/RS performance, we build upon these previous research results.Alternative S/R Machine DesignA typical unit-load AS/RS has an S/R machine operating in each aisle of the system. The S/R machine has a mast which is supported at the floor and the ceiling and travels horizontally within the aisle. Connected to this mast is a shuttle mechanism that carries the unit load and moves vertically up and down the mast. The shuttle mechanism also transfers loads in and out of storage locations in the rack. Figure 1 provides an illustration of the single shuttle S/R machine.Figure 1. Single Shuttle S/R Machine DesignA typical single shuttle AS/RS can perform a single command cycle or a dual command cycle. A single command cycle consists of either a storage or a retrieval. For a storage, the time consists of the time to pickup the load at the I/O point, travel to the storage point, deposit the load at that point, and return to the I/O point. The time for a retrieval is developed similarly.A dual command cycle involves both a storage and a retrieval in the same cycle. The cycle time involves the time to pickup the load at the I/O point, travel to the storage location, place the load in the rack, travel empty to the retrieval location, retrieve a load, return to the I/O point, and deposit the load at the I/O point.If we critically analyze the dual command cycle of the S/R machine (shown by the solid line in Figure 2), a potential open location for a future storage is created when a retrieval is performed. Furthermore, if both a retrieval and a storage are performed at the same point, the travel between time (TB) is eliminated, and the travel time will be equal to the single command travel time. With the existing AS/RS design, this mode of operation is not possible; therefore, an alternative to the S/R machine, a dual shuttle R/S machine, is proposed.Figure 2. Dual Command Travel Paths of S/R and R/S MachinesR/S Machine OperationConsider an S/R machine with two shuttle mechanisms instead of one. This new S/R machine could now carry two loads simultaneously. Each shuttle mechanism could operate independently of the other, so that individual loads can still be stored and retrieved. An illustration of the dual shuttle S/R machine is shown in Figure 3. This new S/R machine would operate as described below.Figure 3. Dual Shuttle S/R Machine DesignThe S/R machine picks up the item to be stored from the I/O point, loads it into the first shuttle, and moves to the retrieval location. After reaching the retrieval location, the second shuttle is positioned to pickup the item to be retrieved. After retrieval, the S/R machine positions the first shuttle and deposits the load. The S/R machine then returns to the I/O point. The operation can easily be seen as a single command operation plus a small travel time for repositioning the S/R machine between the retrieval and storage (as well as the additional pickup and deposit time associated with the second load). Therefore, the S/R machine now operates as an R/S machine performing a retrieval first then a storage in a dual command cycle.Since the R/S machine has two shuttles, the position of the shuttles has a role in the operation ofthe system. With two shuttles, the R/S machine is able to perform a dual command cycle at one location in the rack. This operation is accomplished by first retrieving the load onto the empty shuttle, transferring the second shuttle into position, and storing the load into the empty location in the rack. However, the choice of shuttle configuration does not impact the analysis in this paper. To perform these operations, the R/S machine must move the second shuttle into position after the first shuttle has completed the retrieval. Due to the small distance involved, the R/S machine will use a slower creep speed for positioning, but this travel time is generally small. Furthermore, an amount of creep time is usually included in the pickup and deposit time to account for this required positioning. A second design characteristic is that additional clearance beyond the first and last row and column of the rack must be provided for overtravel of the R/S machine to accommodate both shuttle mechanisms.Throughput ImprovementTo estimate the throughput improvement by the dual shuttle system over existing designs, we use the expressions for single command and dual command cycle times developed by Bozer and White [1] and the tabulated values for the nearest neighbor heuristic from Han et al. [4]. In developing the expressions, the authors in [1] and [4] made several assumptions. The same assumptions hold for the new design and include the following.1. The rack is considered to be a continuous rectangular pick face where the I/O point is located at the lower left-hand corner of the rack.2. The rack length and height, as well as the S/R machine velocity in the horizontal and vertical directions, are known.3. The S/R machine travels simultaneously in the horizontal and vertical directions. In calculating the travel time, constant velocities are used for horizontal and vertical travel. Acceleration and deceleration effects are implicitly accounted for in either a reduced top speed or an increased pickup and deposit time. A creep speed is used for repositioning the dual shuttle.4. Pickup and deposit times associated with load handling are assumed constant and, therefore, these could be easily added into the cycle time expressions.5.The S/R machine operates either on a single or dual command basis, i.e., multiple stops in the aisle are not allowed. (This assumption is later relaxed for the new R/S machine to perform a quadruple command cycle.)6. For the nearest neighbor heuristic, a block of n retrievals is available for sequencing and there are m initial open locations in the rack face.Dual Shuttle S/R SystemsThe new design of the S/R machine has two shuttles and therefore could be operated as a dual shuttle system: carrying two loads and depositing them, retrieving two loads, and returning to the I/O point to deliver them as shown in Figure 4. The above operation can be performed by storing and retrieving the loads at four different locations. Therefore, the travel time would consist of the time for a single command travel plus three travel between times. To more efficiently perform the 4 operations, a retrieval and storage performed at one location is interspersed with a dual command operation. This mode of operation, termed the quadruple command (QC) cycle, eliminates one travel between and is more efficient than the previous mode mentioned above (see Figure 5). The QC cycle can be performed with storages at randomized locations and retrievals processed in a first-come-first-served (FCFS) manner. However, by intelligently sequencing the retrieval list, the travel time in performing the four operations can be significantly reduced. Thistype of analysis was used by Han et al. [4] to improve the throughput of a single load AS/RS. In our paper, we build on the results of their analysis. The notation and the assumptions mentioned in section 2.2. still hold, except that multiple stops of the S/R machine are now allowed.Figure 4. S/R Machine Path Performing Four Operations At Four Locations.Figure 5. S/R Machine Path Performing Four Operations At Three Locations.ConclusionsThis paper performs an analysis of dual shuttle automated storage and retrieval systems. Several contributions have been made including the following.1.Throughput improvements in the range of 40-45% can be obtained using the quadruple command cycle relative to dual command cycles with a single shuttle system.2.With the dual shuttle design, travel between is virtually eliminated for a dual command cycle. The dual shuttle system shows promise for situations requiring high throughput. The main disadvantage with the new design is the extra cost of the S/R machine. An economic evaluation is needed to determine if it is appropriate for a particular situation. However, based on throughput performance, the dual shuttle design appears promising.The concept of dual shuttle systems can also be extended to other material handling systems. Furthermore, research is needed to consider other storage strategies, such as class based storage policies, to examine their impact on throughput in conjunction with the dual shuttle design. This paper provides a framework for analyzing dual shuttle AS/RS, and it provides a foundation for other material handling research related to this concept.References[1] Bozer, Y.A. and J.A. White, "Travel-Time Models for Automated Storage/Retrieval Systems," IIE Transactions, V ol. 16 , No. 4, 1984, 329-338.[2] Elsayed, E.A. and O.I. Unal, "Order Batching Algorithms and Travel Time Estimation for Automated Storage/Retrieval Systems," International Journal of Production Research, V ol. 27, No. 7, 1989, 1097-1114.[3] Han, M.H., L.F. McGinnis, J.S. Shieh, and J.A. White, "On Sequencing Retrievals In An Automated Storage/Retrieval System," IIE Transactions, March 1987, 56-66.。