Simulation Approach for Logistical Planning in a Warehouse : A Review

Simulation approach has been widely utilized for design and planning of a warehouse logistic system. As technology advances, better simulation tools have been developed. This enables decision makers to create more realistic models to represent the actual scenario. As a result, better evaluation could be made especially regarding dynamic factors within system. Thus, complex logistic problems could be solved. This paper reviews the fundamentals of warehouse logistic and the simulation tools available to analyze logistic problems. Oriental Journal of Computer Science and Technology Journal Website: www.computerscijournal.org ISSN: 0974-6471, Vol. 11, No. (4) 2018, Pg. 201-208


Introduction
One of the most important aspects of Logistics Warehouse (LWs) is queuing systems. 1 The systems are used for managing traffic flow and goods distribution within a warehouse.During all stage of a logistic process, raw materials and finished goods are stored in a warehouse facility.It is critical for warehouse management to operate to achieve the important goals of logistic systems that are to ensure customers' satisfactory and to operate at minimum cost.As the world is becoming a more interconnected marketplace, customers have also become more demanding.Therefore, increasing productivity, flexibility and reliability of supply chains are the most important aspects to improve logistic processes in a warehouse.
Capacity planning, technical requirement and material handling are among the factors when selecting strategies for warehouse operations.In measuring the effectiveness, time and cost could be considered as the most commonly used indicators for individual storages operations for warehouse space or labour resources.Figure 1 shows the operation of a logistics warehouse.Material and information flows of the related operations are shown in the figure .Besides, Sulírová et al., 2 explained the important part of logistic management is to provide a satisfactory level of customer service.However, as technology advances, customer's expectation also increases.] Basically there are two main aspects that need to be focused which are the fundamental of warehouses and the simulation tools that can be used to optimize the processes.
The remainder of the article is organized as follows: Section 2 provides the fundamentals of process flow in a warehouse.Meanwhile, Section 3 describes the automated material handling system through Automated Guided Vehicle (AGV) implementation.Section 4 presents the simulation methodology and illustrates its effectiveness in supporting decision-making.Finally, Section 5 provides the concluding remarks and potential directions for future extensions.

Warehouse Process Flows
The design of warehouse layout can be classified as the systematic way to organize the relative location of function, department and equipment of warehouse facility.There are six main material flow activities for a warehouse which include receiving material, transfer and shipping out, accumulation or sortation, order picking or selection, shipping and cross-docking.The importance of designing appropriate warehouse layout is to minimize the output costs of processing in the warehouse.In this study, warehouse layout can easily reduce the time for the process need order or a bunch of order. 4The total travel distance required for the process to take an average order are the important thing that can affect the cost. 5Figure 2 shows an examples of typical warehouse function and flows which is in a U-shape of warehouse layout.The U-shape layout are commonly use in the industry especially in a shopfloor that requires cross-docking operation.The various configuration of warehouse design layouts provide the efficiency for warehouse operations and the capacity to add storage volume for future expansion.Among the important objectives that need to be considered when designing a layout for warehouse include: [2][3][4][5] 1.
To minimize transportation requirement 2.
To minimize the labour requirement 3.
To minimize the space utilization 4.
To minimize use of equipment and tools 5.
To increase work safety The same case applies for warehouse that is located within a manufacturing environment where the material handling for intra-warehouse and the production floor-warehouse need to be considered. 7n many cases, intelligent techniques are applied in the various aspects of intelligent vehicles. 3,5Safety, efficiently and effective of material transportation are the main objectives.There are several of material which are usually carried by material handling which are: 7 1.

Raw material 2.
Part purchase from supplies 3.

Disposed material 7. Tooling
In an automated system such as warehouse, Automated Guided Vehicle (AGV) system is used to pick-up and drop-off load because it provide more accurate and precise operation.The main characteristic for AGV system is to carry pallet loading and unloading over the floor.
A warehouse is a place to store of goods and product.
It also known as a distribution center.In a warehouse management, the distribution of goods include the process of allocating of incoming good, storage and material tracking. 7Nowadays, warehouse management system is able to be intergrated with an automated system.These includes radio frequency identification (RFID) and voice recognition.The effective control of the movement of material in the warehouse critical for any warehouse management system.

AGV as a Material Transportation
Culler and Long 8 described AGVs as the commonly used in manufacturing and material handling.Predetermined lanes are typically used to enable the AGV to travel within the facility.Meanwhile, Shneier and Bostelman 9 stated that aside from factories and warehouse, there are other workplaces where AGVs were utilized including in cargo port and hospital. The main goal of optimizing the AGV system is to maximize the efficiency of materials flow.Additionally, transportation system in a warehouse need to be flexible in order to adapt to the system dynamic.Leite et al., 11 agreed that in order to achieve the objectives of flexible manufacturing system (FMS), the material handling and delivery times need to be shorten.AGV have to provide the required materials in the right quantity and at the right time.There are several advantages of AGV in flexible manufacturing system which is: 1.
Reduce of the broken parts due to manual handling In order to automate the internal material flow an automated guided vehicles (AGV) are commonly used in warehouse, distribution centers and manufacturing plant.Typically, the design of AGV system to transport big and heavy transport units such as Euro pallets or mesh pallets.For a small transportation units such as just-in-time (JIT) inventory management and lean production need to allow one-piece-flow.An AGV system are used to solve the problem.In this study, they localization and tracking of small AGV system for the solution.
RFID transponders to detect a localization.

Simulation Methods
There are various features of AGV system which is design of system.The difficult task in AGV system are to design a control system.Number of the vehicle are the important thing to start the design of system.Simulation will help the designers to build a model.Leite et al., 11 describes the flow of material in the production system that can be optimized by having an efficient AGV system.In the paper, they state simulation software Promodel 7.0 are used to optimize the use of AGV system.
Although there are some criticism on the accuracy obtained by simulation software, Negahban and Smith 12 found that the Discrete Event Simulation (DES) approach for a manufacturing system is capable to provide a good insight on how the proposed solutions could affect the system.AGV are the important element in the material handling system.They state that productivity will increase by used an AGV system.The advantages of conceptual simulation of the AGV are can built a complicated design and operation of AGV system.
There are several control strategies focusing on AGV steering system which is neutral network, fuzzy control and an intelligent coordinate control. 13ethod of least square is utilized to minimize distance error and orientation angle error.In order to design the AGV controller, the intelligent coordinate control strategy are used to improve the dynamic performance of the system.
The simulation model is capable to show the result for the logistic system that can fulfil the requirement of the future company to optimize material supply, increase the operation areas and able to create layout. 14Using simulation, it is feasible to do the same process such as logistic, production areas and assembles in the real manufacturing industry.In this study, number of vehicles is used in static calculation as a method to build a simulation model.For the basic model to initial validate only used a calculation base for dynamic simulation as depicted in Figure 3.
Based on the result of simulation experiment, static calculation shown different result.Therefore, for the dynamic simulation it need more input parameters.
6][17] In that study, they evaluated nine objective functions which are makespan, maximum flow time, quantity of tardy jobs, total setup time, mean setup time, mean flow time, maximum tardiness, mean tardiness and other performance measures.A discrete event simulation was used in that experiment using nine dispatching rules.NP-hard scheduling problem is an important in assembly job scheduling problem (AJSSP).
Relationship between jobs and product structure are complex so it very difficult to solve the problem.In this paper, they used systematic multi-attribute and preference selection index method in dispatching rules to ranking priority for scheduling in assembly job shop.For the experiment a simulation model is used.To determine a multi-objective environment are the best effect priority dispatching rule.

Fig. 3: The Method of Simulation Model Development for the Factory 14
In assembly line, during supplying part the just-intime (JIT) conept has been widely used in production [18].An automated transportation of the AGV system are commonly used to transport parts and to guide the part to be fed to assembly units.It is because AGV is the efficient transportation method.However, there are some potential resource misallocation in AGV system such as inappropriate AGV dispatching rules which can lead to queuing part and traffic vehicles.
] The problem for the AGV system is to determine the vehicle to transfer and load to be transfer.The time to transport of loading and loading pallet need to fast.In the study, the objectives are to reduce the costs by reduce the number of vehicles and travel time to fulfil the demand.8] For the task, comparison between the shortest processing time dispatching rule and meta-heuristic Tabu Search technique was conducted.Simulation result show the operation cost for material handling can be reduce and the number of vehicles can be estimate.The technological advancement of AGV has been accelerated throughout the years. 19This is partially due to the fact that computing technology has advances and demand for automated transport soars rapidly as previously reviewed. 20

Simulation Software
There are several criteria that are typically considered in selecting software for simulation.Among the common criteria are price, functionality, model development, case problem requirement, decision support functionality, open-ended programming functionality, data analysis, compatibility and exchangeability, animation and technical support.Nevertheless, more often than not, the options will be narrowed based on budget and the application requirements.This research select some of most widely used software for consideration.The software are: • Simul8 -SIMUL8 21 could be utilized to simulate logistic systems that involve processing the activities of discrete entities at discrete times.
Apart from warehouse logistics, other suitable applications include optimization of production and other processes in services [22][23][24] .The main strength of SIMUL8 is that it allows user to add advanced logic to the simulation model created. 21This allows user to take into account real life constraints, capacities and other dynamic factors affecting the system performance.

Conclusion
Research on simulation approaches for warehouse logistics is entering a new phase with the availability of commercial software capable of modeling dynamic and realistic environment.Furthermore, as it is becoming ever critical to optimize warehouse operations, simulation is a viable option to forecast the effects when changes are made.Furthermore, some of the softwares are even equipped with artificial intelligence and advanced analytics.These approaches should be fully utilized in order to enable Industrial Revolution 4.0 to be adapted more easily.

Fig. 1 :
Fig. 1: The Operation of a Logistics Warehouse 1

Fig. 2 :
Fig. 2: Conventional Warehouse Function and Flows 6 23 [39][40][33]a user-friendly simulation software containing objects with pre-built logic and task execution to imitate the resources utilization activities in realworld operations.The software support 3D animation thus very graphical and very suitable for presentation.[31][32][33]• Ar-Arena is very suitable to simulate and analyze existing and proposed systems as well as operational analysis.[34][35]Ithasbeen widely used to model manufacturing and warehouse operations 36-37 • Witness -Witness 38 is a desktop software for professional modelling and application development.The software is user-friendly and is capable to predict simulation model quickly.[39][40]