Optimal flow path design of unidirectional AGV systems

This paper describes an alternative formulation of the AGV flow path layout (FPL) problem which was first formulated by Gaskins and Tanchoco (1987) as a zero-one integer programming problem. A computationally efficient procedure is proposed which is based on the branch-and-bound technique. An algorithm for satisfying the reachability condition for nodes in the AGV flow path network is also presented. A simple illustrative example is discussed to demonstrate the procedure, and a more complex problem is also given.     

Depth resolution in stereoscopic systems

In television stereoscopic systems the magnification and the effective eye separation can be varied at will. This study shows that, under carefully chosen experimental conditions, depth resolution can be closely predicted by the use of geometrical optics and can be enhanced by increases in magnification and effective eye separation. The limited spatial resolution of conventional television or other photoelectronic systems may, however, prevent high levels of stereoacuity, comparable with naked-eye viewing, from being achieved in a wide-field system. In more complex environments the inevitable conflict of perceptual cues might be expected to reduce depth resolution further.     

Terminal location and guide path design in terminal based AGV systems

In this paper, an operational scheme based on a terminal option is adopted for movements of an Automated Guided Vehicle (AGV). An integer programming model is proposed to obtain optimal discrete locations of teminals along with guide path directions. Sensitivity analysis with respect to key parameters is provided. Various penalty parameters are recommended to gain insight into different practical difficulties.     

Solution methods for the mathematical models of single-loop AGV systems

Guide path simplification can potentially reduce the complexity inherent in conventional, multi-loop automated guided vehicle systems (AGVs). A single-loop configuration is one alternative. A procedure for designing single-loop AGV systems, the OSL method was presented in a previous paper. In this paper, we suggest faster and more efficient methods for solving the two mathematical models in the OSL procedure. The first model called the valid single-loop problem (VSLP) is used to determine an initial single loop for the procedure. The method suggested is a heuristic procedure that starts from a loop around one of the departments and keeps adding departments to the loop until a valid single loop is constructed. The second model called the single-loop station location problem (SLSLP) is used to determine the location of the pick-up and delivery stations along a given loop. The method suggested converts the mixed integer formulation into a linear formulation.     

Determination of unit load sizes of AGV in multi-product multi-line assembly production systems

This study deals with the problem of determining unit load sizes of automated guided vehicles (AGVs) in multi-product and multi-line assembly production systems. It is assumed that an AGV delivers assembly parts to each workstation of assembly lines from a miniload automated storage/retrieval system (AS/RS), and conveyor lines transport subassemblies between workstations. In this system, determination of the unit load sizes of parts carried by the AGV becomes an important decision factor due to its effects on the work-in-process inventory level and the operating cost of the system. Under a proposed AGV dispatching policy, a nonlinear mathematical model which allows unequal unit load sizes among assembly lines is formulated. Based on the characteristics of the objective function and feasible regions, a heuristic solution algorithm is developed. To illustrate the model, problems are chosen and solved.     

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     

Conflict detection and resolution for goal formation in the fractal manufacturing system

The fractal manufacturing system (FrMS) is based on the concept of autonomously cooperating agents referred to as fractals. A fractal is a set of self-similar agents whose goal can be achieved through cooperation, coordination, and negotiation among the agents for themselves. A fractal has fractal-specific characteristics (e.g. self-similarity, self-organization, self-optimization, goal-orientation, and dynamics), and it also has the characteristics of an agent (e.g. autonomy, mobility, intelligence, cooperation, and adaptability) at the same time. In the FrMS, a goal can be regarded as the status which the system aspires to be in. The goal-formation process (GFP) in the FrMS is a process of generating goals and modifying them by coordination between agents. In the GFP, conflicts may occur between goals, which can drive a system to become inefficient. In this paper, a conflict resolution mechanism via agent-based negotiation is proposed for facilitating the GFP. The scheme deals with non-fixed goals. The mobile agent-based negotiation process (MANPro), in which a mobile agent is used for information-exchanging and problem-solving, is used for negotiations in this scheme. The proposed mechanism is illustrated with a goal formation scenario in an exemplary FrMS.     

A max-algebra approach to the robust distributed control of repetitive AGV systems

This paper presents a new approach for distributed control of automatic guided vehicle systems (AGVS) that uses max-algebra formalism to model system operation. The proposed method differs from previous works on performance analysis and control of such systems in that it constructs a feasible schedule by exploiting the repetitive character of the flow of automated guided vehicles. The AGVS is treated as a distributed system composed of autonomous and repetitive processes of AGV flows. Its periodic behaviour follows from the type of operation characterizing the job shops considered. The proposed approach employs a concept of asynchronous traffic semaphores that provide the local mechanism for vehicle flow synchronization. Setting the timing of particular semaphores results in distributed control of vehicle flows possessing self-synchronization capabilities.     

Performance evaluation of tandem and conventional AGV systems using generalized stochastic Petri nets

This paper investigates the different policies and concepts followed in the traffic management of automated guided vehicle (AGV) systems, and develops the controls for automatically eliminating potential vehicle conflicts in an AGV system. The planning of the AGV system is performed in such a way that there are no conflicts or deadlocks for the vehicles using stochastic Petri nets (SPNs). The major effort is devoted to determining the benefits of the tandem AGV control in comparison with the conventional AGV control method. SPNs have been used to model the different designs of flexible manufacturing systems (FMSs) and with different policies for the movement of material, vehicle path control, inventory planning and tool control. The SPN model is solved and the performance of the system can be evaluated. In this study, the effort is directed to model an FMS with two different types of AGV traffic management methods, namely the conventional and tandem AGV control. A SPN program is used which takes the FMS Petri net model as the input and evaluates the different properties of the Petri net. Finally the performance measures are obtained, which helps in evaluating and comparing the two different AGV traffic management methods.     

AGV controlled FMS

One of the key factors that prevent the implementation of Flexible Manufacturing Systems (FMS) is the elaborate cost associated with the control software. In a FMS there is often a wide range of equipment, such as personal computers, Programmable Logical Controllers (PLCs), CNC Machines or robots, each of these having their own 'intelligence' and library of data. Providing a means of communication between these individual controllers has traditionally been achieved using a Local Area Network (LAN). This proves more expensive when traditional manual operated machines have to be integrated into the system. The model described in this paper provides an alternative to the conventional use of a LAN in a FMS environment. In the design solution, an Automatically Guided Vehicle (AGV) is used as both the materials handling unit and the communications line linking each station to the host controller. Communications between the AGV and peripheral equipment is achieved using a standard infrared data link, eliminating hard-wiring and network protocols. A simulation model has been developed to demonstrate the feasibility of such a system, using industrial data. The software package Witness is used to develop the simulation model. The objective from developing this simulation model is to test whether an AGV is capable of meeting the demands of such a scenario. The research undertaken aims to test this by modelling an existing factory layout. Using this layout and captured machining times and part routes, from the factory database, the feasibility of such AGV controlled production system is established. The model shows that such a system is plausible in a scenario where machine times are high and the distance between machines is large.     

A machine-to-loop assignment and layout design methodology for tandem AGV systems with single-load vehicles

In this paper, we propose a method for design of tandem automated guided vehicle (AGV) systems with single-load vehicles. We consider the concurrent design of machines layout and AGV guide paths for a tandem system. Our goal is to devise a method that can achieve the following objectives: (1) maximise the workload balance between loops; (2) minimise the inter-loop flow; and (3) minimise the total flow distance. Our method solves the problem in four stages, considering the machines layout and the tandem paths at the same time. It assigns machines to loops, determines the layout of each loop, arranges loops on the floor, and finally designs a transportation centre to link the loops. We compare the performance of our method with a sequential design method that first determines the layout and then assigns the machines to loops. We solve a number of randomly generated problems for both methods. Results indicate that the proposed algorithm performs faster and achieves lower values of inter-loop flows and inter-loop flow distances.     

Heuristic,storage space minimization methods for facility layouts served by looped AGV systems

An analytical method for predicting work-in-process storage requirements resulting from a fixed number of looping, automated guided vehicles (AGVs) serving a line layout is described. The method is based on a model of storage queue dynamics that predicts material flow rates and vehicle response times resulting from vehicle dispatching within a single loop system. The method is applied in the development of two heuristic, random number driven procedures designed to perform sequential search for WIP storage minimizing line layouts over alternative AGV fleet sizes. The first algorithm is a greedy, ‘CRAFT like’ procedure based on local improvement in storage requirements using random, pairwise exchanges of workcentre locations. The second algorithm is a modification to the greedy procedure using simulated annealing methods designed to avoid trapping at local minima. Limited computational studies using the methods are reported.     

AGV dispatching based on workload balancing

We consider a manufacturing job shop with automated guided vehicles. Such a system is characterized by several complex features such as the random pattern of material flows between machines, the limited capacity of machine buffers, shop deadlock phenomena, floating bottlenecks, etc. We will describe a heuristic procedure for the control of materials flows and automated guided vehicles in such a system. The procedure is based on the idea of workload balancing; it tries to balance the workload among machines as well as the workload between the machines and the transporters. A deadlock prevention algorithm is embedded in the procedure. We then present the results of simulation experiments to evaluate the performance of the proposed procedure.     

Cystic resolution: a performance metric for ultrasound imaging systems

This paper describes a metric that can be used to characterize the resolution of arbitrary broadband coherent imaging systems. The metric is particularly suited to medical ultrasound because it characterizes scanner performance using the contrast obtained by imaging anechoic cysts of various sizes that are embedded in a speckle-generating background, accounting for the effect of electronic noise. We present the theoretical derivation of the metric and provide simulation examples that demonstrate its utility. We use the metric to compare a low-cost, handheld, C-scan system under development in our laboratory to conventional ultrasound scanners. We also present the results of simulations that were designed to evaluate and optimize various parameters in our system, including the f/# and apodization windows. We investigate the impact of electronic noise on our system and quantify the tradeoffs associated with quantization in the analog to digital converter. Results indicate that an f/1 receive aperture combined with 10-bit precision and a signal-to-noise ratio (SNR) of 0 dB per channel would result in adequate image quality.     

Spatial-spectral volume holographic systems: resolution dependence on effective thickness

The resolution dependence of spatial-spectral volume holographic imaging systems on angular and spectral bandwidth of nonuniform gratings is investigated. Modeling techniques include a combination of the approximate coupled-wave analysis and the transfer-matrix method for holograms recorded in absorptive media. The effective thickness of the holograms is used as an estimator of the resolution of the imaging systems. The methodology, which assists in the design and optimization of volume holographic simulation results based on our approach, are confirmed with experiments and show proof of consistency and usefulness of the proposed models.     

Decentralized autonomous AGV system for material handling

Decentralized control of an autonomous automated guided vehicle system (AGVS) used for material handling is expected to lead to high system flexibility and robustness. A complete control methodology for decentralized autonomous AGVS control was developed and implemented in a computer-integrated manufacturing environment. The methodology addresses all aspects of AGVS functionality: system management, navigation and load transfer. Hierarchical fuzzy behaviour-based control, a reactive navigation scheme, was expanded to multirobot control in semistructured environments by incorporating a priori path optimization and right-of-the-way determination.     

A new extrapolation technique for resolution enhancement of pulse-echo imaging systems

A new, simple extrapolation technique to enhance the lateral resolution of pulse-echo imaging systems is presented. The method attempts to build an image that could be obtained with a transducer aperture larger than that physically available, extrapolating the information contained in the image to be enhanced. The extrapolation process requires small hardware modifications of the standard echographic systems. The computational cost is very low compared with Fourier-based deconvolution approaches. The obtained computer simulations give very interesting and promising results     

Foundations for a theory of computer assisted superhigh resolution measurement systems

We present the elements of a theory for computer aided measurement systems with superhigh resolution. We present the concept of an ideal measurement instrument that does not distort the parameters of an object of study. Translated from Izmeritel'naya Tekhnika, No. 2, pp. 3–10, February, 1998.     

Intersection graph method for AGV flow path design

In this paper, we introduce the Intersection Graph Method for solving the AGV Flow Path Optimization Model developed by Kaspi and Tanchoco (1990). A branch-and-bound procedure is described wherein only a reduced subset of all nodes in the flow path network is considered. Only intersection nodes are used to obtain optimal solutions. Two examples are given to illustrate the proposed method.     

A structured approach to deadlock detection,avoidance and resolution in flexible manufacturing systems

Production scheduling models that determine part mix ratios and detailed schedules do not usually account for deadlocks that can be caused by part flow. Deadlocks must be prevented for operational control (especially in automated systems). The major thrust of this paper is in developing a structured model for deadlock detection, avoidance and resolution caused by part flow in manufacturing systems. A system status graph can be constructed for the parts currently in the system. Deadlock detection amounts to determining deadlocks in the system status graph. On the other hand, deadlock avoidance amounts to restricting parts movement so that deadlocks are completely avoided in the future. While deadlock detection is a one-step look ahead procedure, deadlock avoidance is a complete look ahead procedure. Deadlock resolution or recovery amounts to judiciously using a limited queue to recover from deadlocks. Deadlock detection and avoidance are absolutely crucial to uninterrupted operation of automated manufacturing systems. A model based in graph theory has been formulated to detect and avoid deadlocks in automated manufacturing systems.