Dynamic relative positioning of AGVs in a loop layout to minimize mean system response time

One of the control decisions in the operation of an automated guided vehicle (AGV) system is to determine the home locations of idle vehicles. In this paper, the problems of selecting home location of a vehicle when idle in a single loop AGV network is presented. As the number of unit loads to be picked up at each workstation dynamically changes over time, the optimum home location of vehicles may also change. Based on the objective of minimizing the expected response time of a vehicle, models are constructed. Example problems are given to illustrate the use of the solution algorithms. The results of the system response times obtained using the dynamic dwell point models are compared with those of other dwell point rules.     

Selection of circulatory loops for patrol vehicles operating in a network

Many applications exist in which a vehicle or set of vehicles have the responsibility of patrolling and providing coverage over a wide area or network for the purpose of responding quickly to service requests generated randomly at demand points in the network. One of the decisions in the operation of such patrol vehicles is that of determining the home location or circulatory loop for free or idle vehicles. A vehicle is said to be idle if it is not responding or attending to a service request. Such home location can be a point, a set of points in a network, a circulatory loop, or a set of circulatory loops in the network. The actual number of home locations is system, fleet size, and analyst dependent. Whether a point or loop strategy is implemented, the specification of the home locations or loops is usually done with the objective of improving the response time of idle vehicles to service requests originating from demand stations. In cases where circulatory loop strategy is adopted, the problem is one of determining the best overall circulatory loop or loops that would improve the overall system response time. In a typical conventional or generalized network, several loops may exist. Unless a systematic procedure is developed, non-optimal loops could be selected. In this paper, methodologies for determining the best circulatory loops for patrol vehicles for a given problem scenario in both unidirectional and bidirectional networks are presented. The problem is modelled as a mathematical programming problem. Example problems are given to illustrate the application of the approach. The criterion for optimization is the minimization of the maximum system response time.     

Optimally Locating Multiple Dwell Points in a Single Loop Guide Path System

Dwell points for idle vehicles affect the performance of automated guided vehicle systems. An exact polynomial-time algorithm to solve idle vehicle positioning problems in both unidirectional and bidirectional single loop systems is developed to minimize the maximum response time. The proposed algorithm considers several potential locations for the first dwell point and applies a dynamic programming procedure to obtain the remaining dwell points for each initial location. The algorithm is streamlined by applying certain optimality properties. Computational results show that the algorithm can solve large-scale problems in reasonable time.     

Flow path design for automated guided vehicle systems

An important factor in the design of automated guided vehicle systems (AGVS) is the flow path design. This paper presents an approach to determining the optimal flow path. The objective is to find the flow path which will minimize total travel of loaded vehicles. The problem is formulated as a zero-one integer program. Examples are presented to demonstrate the approach.     

Coloured timed Petri net model for real-time control of automated guided vehicle systems

Automated guided vehicle systems (AGVS) are material-handling devices representing an efficient and flexible option for products management in automated manufacturing systems. In AGVS, vehicles follow a guide-path while controlled by a computer that assigns the route, tasks, velocity, etc. Moreover, the design of AGVS has to take into account some management problems such as collisions and deadlocks. The paper presents a novel control strategy to avoid deadlock and collisions in zone-controlled AGVS. In particular, the control scheme manages the assignments of new paths to vehicles and their acquisition of the next zone. Moreover, the use of coloured Petri nets is proposed to model the dynamics of AGVS and implement the control strategy stemming from the knowledge of the system state. Additionally, extending the coloured Petri net model with a time concept allows investigation of system performance. Several simulations of an AGVS with varying fleet size while measuring appropriate performance indices show the effectiveness of the proposed control strategy compared with an alternative policy previously presented.     

Optimal dwell point location of automated guided vehicles to minimize mean response time in a loop layout

An important control issue in operating an automated guided vehicle system is where to locate idle vehicles. Dwell points for idle vehicles affect the performance of manufacturing systems. A polynomial time algorithm has been developed to determine dwell points for idle automated guided vehicles that minimize the mean response time. Both uni- and bidirectional loop layouts are considered. The proposed algorithm, based on a dynamic programming model, partitions the set of pick-up stations into subsets so that a single vehicle serves all stations in a subset. The dynamic programming algorithm has been streamlined by applying certain optimality properties. Computational results show that the proposed algorithm can solve large-scale problems in a reasonable time. A simulation experiment has also been conducted to compare several idle vehicle positioning rules, and the results show that best performance is obtained by positioning idle vehicles in dwell points that minimize the mean response time.     

Closed form models for determining the optimal dwell point location in automated storage and retrieval systems

This paper examines the problem of where a storage/retrieval machine should reside, or dwell, when an automated storage and retrieval system (AS/RS) becomes idle to minimize the expected value of the next transaction time. After a review of the relevant literature on AS/RS dwell point strategies, this paper proposes several analytical models of these expected response times of the AS/RS based on the relative locations of the input and output ports of the AS/RS. It uses a continuous rack approximation to provide analytical models of the dwell point location problem. These models provide closed form solutions for the dwell point location in an AS/RS. Extensions are made to consider AS/RS with a variety of configurations including multiple input and output ports. These models not only provide solutions to the dwell point location problem, but they provide considerable insight into the nature of this problem, which is particularly valuable when the requirements facing the AS/RS are uncertain.     

Potentials for bi-directional guide-path for automated guided vehicle based systems

Most current applications of automated guided vehicle systems (AGVS) in manufacturing shop environments employ uni-directional guide-paths for vehicle routing despite the fact that bi-directional vehicles exist. In this paper comparisons and issues regarding unidirectional and bi-directional flows are presented. Also presented is a model of a bi-directional traffic flow guide-path. The effect of the traffic flow pattern on the shop throughput is demonstrated and compared to that of a uni-directional flow system of an equivalent facility. The model is implemented using computer simulation.     

Modelling empty vehicle traffic in AGVS design

Empty vehicle traffic plays a critical role in the operating performance of automated guided vehicle systems (AGVSs). While significant research has been conducted on intelligent AGVS dispatching-which, in essence, is the management of empty vehicle traffic-research in AGVS design has sometimes ignored or simplified the role of empty vehicle traffic. In this paper, we show how empty vehicle traffic strongly influences several important AGVS design problems, such as flowpath design and vehicle requirements. We present analytical models to predict empty vehicle travel under two popular vehicle dispatching rules for systems facing stochastic trip demand. We show that using our models' estimates of empty vehicle traffic in the AGVS design process can dramatically improve the resulting system performance. We also analyse shop-floor transfer patters and find that empty traffic has a larger impact on systems with discernible trends in job movement.     

Guide path design and location of load pick-up/drop-off points for an automated guided vehicle system

The guide path layout for an automated guided vehicle system (AGVS) is a critical component in the overall design of a flexible manufacturing system (FMS) that utilizes AGVs for materials handling. Not only does it affect the total distance travelled by the vehicles but it also affects vehicle requirements and space utilization. In this study, the problem of selecting the guide path as well as the location of pick-up and drop-off points for outward and inward bound parts to departments is addressed. The problem is modelled and solved as a linear integer program with the objective of minimizing the total distance travelled.     

Impact of empty vehicle flow on performance of single-loop AGV systems

With the increasing trend in the use of automated guided vehicles systems (AGVS) in manufacturing, the need for design tools becomes greater. The problem is that optimized static design models do not ensure expected performance due to the difficulty in capturing the dynamic operational characteristics of the vehicle system. This study shows that by using the single-loop guide path configuration some of the dynamic features of the system, like the impact the empty vehicle flow has on the system's performance, is reduced. Thus, a more reliable prediction of system performance is achieved     

Location of vehicle identification sensors to monitor travel-time performance

Traffic agencies are increasingly using automatic toll collection technologies to collect fees on toll roads. Associated with such technologies are toll tags on vehicles that identify the vehicle and its account for charging the appropriate toll, and roadside readers (or overhead readers) that read the toll tags. A by-product of such technologies is the data collected on traffic measures such as travel times from the point when the vehicle is first tagged to the point it leaves the system. That is, the toll reader becomes a traffic sensor that measures travel times of some tagged vehicles and volumes of such vehicles that go from intermediate origins to intermediate destinations. Two problems have been studied on the optimal locations of such automatic vehicle identification (AVI) readers on traffic network using criteria: (i) to maximise the total vehicle-miles monitored, and (ii) to minimise the variance of predicted travel times. Optimisation models are formulated through which it is shown that these problems are NP-hard. Subsequently, greedy heuristics are proposed. Exact optimal solutions, using CPLEX, and approximate solutions, using the heuristics are compared for several small hypothetical networks for a case study dealing with a real application as well as in Harris County, Texas, USA.     

Developing exact and Tabu search algorithms for simultaneously determining AGV loop and P/D stations in single loop systems

There are some issues which have to be addressed when designing an automated guided vehicles system (AGVS) such as flow-path layout, traffic management, the number and the location of pick up and delivery points, vehicle routing and so on. One of the AGVS guide path configurations discussed in the previous researches includes a single-loop which is the subject of this paper. In unidirectional single loop systems, determining the loop for the motion of an AGV, and the location of pick up and delivery (P/D) stations in the cells, are prominent points which, when considered simultaneously, lead to better results than determining each one independently. However, in the literature it is proved that the problem of separately determining the shortest feasible loop is a NP-complete problem. In this paper, by considering a from-to chart and a block layout as the input of problem, we try to determine: (1) a single loop, with at least one shared edge with each cell, (2) the direction of the flow and (3) the location of P/D stations on the loop, all at the same time, in a way that the total travel distance on the loop be minimised. In this regard, first a new exact algorithm is presented and then three heuristic algorithms are developed utilising a Tabu search (TS) method. Solving randomly generated test problems shows that our exact algorithm is capable of solving small size problems; also all three TS algorithms work efficiently in solving problems that could not be solved by exact algorithms.     

An optimization algorithm for a capacitated vehicle routing problem with time windows

In this paper, vehicle routing problem (VRP) with time windows and real world constraints are considered as a real-world application on google maps. Also, tabu search is used and Hopfield neural networks is utilized. Basic constraints consist of customer demands, time windows, vehicle speed, vehicle capacity and working hours. Recently, cost and on-time delivery are the most important factors in logistics. Thus, the logistic applications attract attention of companies. In logistic management, determining the locations of delivery points and deciding the path are the vital components that should be considered. Deciding the paths of vehicles provides companies to use their vehicles efficiently. And with utilizing optimized paths, big amounts of cost and time savings will be gained. The main aim of the work is providing the best path according to the needs of the customers, minimizing the costs with utilizing the VRP and presenting an application for companies that need logistic management. To compare the results, simulated annealing is used on special scenarios. And t-test is performed in the study for the visited path in km with p-value of 0.05.     

Relative pre-positioning of storage/retrieval machines in automated storage/retrieval system to minimize expected system response time

This paper is focused on the control problem of determining the dwell point of an idle storage/retrieval (S/R) machine in an automated S/R system to improve the overall system service level as measured by the turnaround time of requests to the system. The emphasis of the paper is on the minimization of the expected response time over all retrieval and storage requests executed by the system. The model presented is developed under the operating condition that each machine serves several dedicated aisles of the system. The aisles are assumed to exist in the same region of the automated storage/retrieval system (AS/RS) and thus form a valid order S/R zone. A solution algorithm for determining the optimal dwell point location is developed. The paper assumes a dynamic environment where orders arrive over time and are to be serviced by the S/R machine. A dwell point or strategic point to pre-position the machine is to be determined whenever the machine becomes idle in anticipation of an incoming service request. A numerical example is provided to demonstrate how the technique can be used in an actual production setting to improve the overall order turnaround time.     

A model for the design of zone control automated guided vehicle systems

An analytical modelling strategy is proposed as a screening device for use prior to the simulation phase in the design of zone control automated guided vehicle systems (AGVS). The proposed model predicts the effects of the major AGVS design variables on system-performance measures including maximum throughput capacity and risk factors associated with shop locking. This is done through a process which specifically evaluates the system operating dynamics associated with vehicle fleet size, guidepath layout, workstation storage capacity, routeing, and, to a less precise extent, vehicle dispatching. Operating dynamics are defined to include vehicle blocking resulting from guidepath contention and variation of in-process storage levels. The objective of the analytical model is to reduce the extent of simulation modelling necessary to design a zone-control AGVS for a given materials-handling workload. The model is illustrated for a sample AGVS with variation in the fleet size, workstation storage capacity and dispatching rule demonstrated.     

Relative pre-positioning of storage/retrieval machines in automated storage/retrieval systems to minimize maximum system response time

In this paper, the problem of pre-positioning a storage/retrieval (S/R) machine in an automated storage/retrieval system (AS/RS) when the machine is idle is addressed. The objective of the pre-positioning strategy is to minimize the maximum response time to service incoming requests into the AS/RS. A model is developed under the operating condition that each machine can serve one or multiple dedicated aisle(s) of the system. The aisles are assumed to exist in the same region of the AS/RS and thus form a valid order storage/retrieval zone. A solution algorithm for determining the optimal dwell point or location is developed. The paper assumes a dynamic environment where orders arrive over time and are to be serviced by the S/R machine. A dwell point or strategic point to pre-position the machine is to be determined whenever the S/R machine becomes idle in anticipation of an incoming service request. A numerical example is provided to demonstrate how the technique can be used in an actual production setting to improve the overall order turnaround time.     

A capacitated vehicle routing problem for just-in-time delivery

This paper focuses on the formulation and solution of the problem of planning vehicle routes for material delivery within the premises of a plant working under a just-in-time production system. The unique characteristic of this problem is that the quantity to be delivered at each of the demand nodes is a function of the route taken by the vehicle assigned to serve that node. The problem is modeled by adding a non-linear capacity constraint to the standard vehicle routing model, such that vehicle idle times and inventories at the customer locations are minimized. A heuristic solution procedure is outlined, and the formulation of a lower-bound relaxation is suggested. The performance of the heuristic solution procedure is evaluated in comparison to the lower-bound relaxation, and the heuristic procedure is shown to provide generally good results.     

The bi-objective two-dimensional loading vehicle routing problem with partial conflicts

The two-dimensional loading vehicle routing problem with partial conflicts combines two NP-hard problems: the classical vehicle routing problem and the two-dimensional bin-packing problem with partial conflicts. This new variant of transportation problems is inspired from hazardous materials classification and compatibilities, where some materials can be partially conflicting. In this case, they can be loaded together but an additional constraint on the distance separating them must be respected. We propose in this paper an NSGA-II algorithm to perform a bi-objective study in which the minimization of the total cost of transportation as well as the load balancing between different routes in terms of used area of vehicles are the considered objectives. The first results for this new problem are presented, using benchmarks available in the literature that have been adapted to deal with the problem. Moreover, the adding value of path relinking is proved with different statistical measurements.     

Radiative-convective heat transfer of a spherically shaped space vehicle in carbon dioxide

A system of equations of motion of viscous, heat-conducting, chemically nonequilibrium, and selectively radiating and absorbing gas is used for solving the problem of radiative-convective heat transfer of a space vehicle of the simplest (spherical) shape in carbon dioxide used for simulating the Mars atmosphere. Comparison is made of the convective and radiative heating of the entire surface of the space vehicle (from forward to rear stagnation point) for the typical conditions of entry of space vehicles into the Mars atmosphere, in particular, for individual points of the Pathfinder trajectory. It is demonstrated that, in some regions of space vehicle entry into the atmosphere, the radiative thermal heating of some elements of space vehicle surface may exceed the convective thermal heating. There is also demonstrated the need for analysis of the level of radiation heat fluxes on the entire surface of the Mars-bound space vehicle rather than on its frontal surface alone.