[SEMINAR 26] DEVELOPMENT OF AN AGV MATERIAL HANDLING SYSTEM IN A FLEXIBLE MANUFACTURING ENVIRONMENT

Abstract

            In recent years the worldwide development and use of automated system within the manufacturing industry has led to proliferation of system designs mainly involving the integration of a wide range of suitable computerized machine tools and material handling equipment. To cope with this and for getting flexibility and adoptability "FLEXIBLE MANUFACTURING (FMS)'' comes in to picture.

            Automated guided vehicles (AGV) are use to provide the flexibility in the handling of the material. The AGV'S can be of many different designs with different degree of flexibility. To exploit the benefits of an AGV material handling system. This paper presents the development of the fleet manager. The sophisticated software in the fleet manager monitor and controls the AGV'S remotely through a wireless communication network.

            The presented paper deals with introduction to FMS and the AGV system. An automated or automatic guided vehicle system is a material handling system that uses independently operated, self propelled vehicles that are guided along defined pathways in the floor.

            As we know automation has been key to success for many factoring process in industries. In it FMS is the new trend. In the decade of 90, so due to material handling by conventional methods takes more time so AGV is the solution over it. so in this paper FMS a AGV is elaborated in detail solution over it. 

INTRODUCTION

            F.M.S. is nothing but the flexible manufacturing system which consist of a group of processing stations (mainly m/c tools), interconnected by means of an automated material handling and storage system and controlled by an integrated computer systems.

            The definition according to UK department of trade and industry is " Flexible manufacturing system is a system which combines micro-electronics and mechanical engineering to bring economics of scale to batch work. A central on line computer controls the machine tools, work station and the transfer at components and tooling. This combination of flexibiling and overall control makes possible the production of a wide range of products in small number's

            Sometimes FMS used to denote the term flexible machining system. The machining process is largest application area for FMS technology. But FMS has a wire range of possible applications beyond machining. A movement away from mass production and towards batch manufacturing is taking place throughout the industrialized world. In today's competitive environment manufacturing industries must be able for higher producing lower cost improve products better quick consumer service.

            So the answer to all above aspects are flexible manufacturing system. FMS are developed to fill the production rate gap between CNC machines and dedicated transfer lines. Hence AGV system is introduced in this paper.

BACKGROUND TO FMS :-

FMS starts worldwide in 1980's but in India actually FMS starts from decade of 90.

FMS is the latest level of automation along an evolutionary road to achieve ever more productivity and flexibility from manufacturing equipments.

NEED OF FMS :-

            Flexible manufacturing system is a technology of computer controlled configuration of semi – independent workstations and a  material handling system designed to efficiently manufacture more than one part at low to medium volumes.

            As many parts requires machining on several machines and may spend weeks on the shop floor  waiting and moving from machine to machine. Based on various and practical it is proved that work piece  spend only 5% of it's time in the shop on a machine and out of 5% only 1.5 to 30% time is spend in actual machining so lot of time wastage in shop floor so. FMS is one of that system which improves all that parameters with CNC system.

Figure i

            Taking this into consideration following countries adopted FMS.

USA – In automotive , aerospace, m/c tool and power industries.

JAPAN – The no. of different parts machined in each system varies from 7-150 material handling in most cases is performed by conveyors automated wore-guided vehicles products manufactured include diesel engines transmission system and machine tools.

FRANCE – First FMS installations numbering 25 are concentrated in me automotive aero space and machine tool industries with Peugeot and graffentaden considered to be leading FMS builders capacities range from a few parts of medium volume production

 WEST GERMANY : First FMS began operating in 1971 more than 30 systems are in operation now.

            These are employed in medium sized industries and are capable of manufacturing 50-250 different Trumf, fritz, Werner, Burkhart and Weber and Scharmam are the major FMS manufactures.

U.S.S.R. :- 60 FMS are operating in the soviet union just slightly les than in the U.S. and Japan.

            Flexible manufacturing systems may slightly differ in features depending on the manufactures of the systems, they are all based an the same principle of the same principle of incorporating several individual automation technologies into a unified , highly responsive production system. fig. provides a simplified diagrammatic representation of me architecture of an FMS. It can be seen that FMS – the new method of automation – is actually based on three main elements : (i) machine tools (ii) an automatic material handling system , and (iii) a computer control system that co-ordinates the functions of the other tow elements.

1) Machine Tools :-

            An FMS always consists of a mixed range of CNC machine tools. The kind and characteristic features of those machine tools depend on the flexibility of the system. It becomes necessary to keep al the tool information in a magazine with the machine tool for easy reference.

Figure ii

2) Material Handling System :-

            The material handling system involves two main elements. These include robots. Which are used for automatic loading and unbending of machine tools, and a means of transpiration that links the various machine tools together such a means for moving parts around can be a conveyor or automatic guided vehicles (AVG) on fixed tracks.

3) Computer Control :-

            A flexible manufacturing system cannot be operated without a digital computer because of the complexity of the system and the need for real-time planning and control of the different subsystems. The function of the computer is not limited to the storage and distribution of the NC part programs, the operation of load-unload robots , and the control of AGV's traffic between stations. In face it goes beyond that to cover tool monitoring and production control.

INTROUDCITON   TO AGV

            Automation has been key to success for many manufacturing process industries. This automation process normally involves heavy investment  in expensive machine nixies. To compliment this investment so as to exact the most benefits of automation, material handling system has been incorporated conveyors system of different designs are effectively used for transpiration of high volume of products. But, Comparives change their products very frequently to stay compentitive. This has created a requirement for flexibility in the manufacturing process.

            Enumerators routes have to be configured in a sophisticated manufacturing system. This is because a variety of operations have to be carried out on different products using different machines. These routes have highly complicated structure. Maximum flexibility has to be achieved in the routing for the purpose of reducing whatever congestion which may happen during dynamic operation period of the system.

            AGV design aspects pauses for reducing to the maximum congestion in routes inside routing network. They are used to serve the is/ands of automation created in the FMS environment. This gives the process the flexibiling because ; AGV's can transport material and products to any of these is/ands. The routes can be preplanned and changed in real time hence minimizing the usage. of expensive machineries and the through put of the plan figure shows the LAYOUT of such an FMS currently being implemented.

CONTROL STURCUTRE OF AGV TRANSPORATION SYSTEM IN FMS :-

            In this transportation system the supervisory to control of the AGVs are directed from a central computer, which is a PC. The PC is linked to all the AGVs through a wireless network. This wireless network is implemented using radio-frequency wireless modems connected to the PC and the microprocessors on the AGV. After the instructions are entered, the fleet manager, which is made up of the control modules, the transportation planner (TP) and the AGV workstation controller carries out the supervisory role. The fleet manager, together with the control software on- board the AGV controller , therefore give the AGV transportation system the vitality and flexibility.

            The main function of the TP is to plan route taken by a fleet of , which is in our present case 3 AGVs. The shortest route to be taken by each vehicle to deliver material between the islands of automation is computed and the routing instructions for each AGV generated in the TP. These instructions are then sent to the AGV workstation controller.

            The AGV workstation controller is responsible for the automated transportation of materials between the various islands within the FMS. It acknowledges AGV routing instruction from the TP and transmits the instructions to the AGV via the radio communication link. Information such as position and location of all AGVs are updated periodically and made available to the TP for route optimization. Thus the workstation controller keeps tracks of all AGVs under it's command.

            In the event of breakdown of either the AGVs or the machineries in the island, the TP will re-compute new routes for the AGVs and assign the task to the remaining available AGVs and machineries so as to keep the production optimized at all times. The main functions of the onboard controller are to keep the vehicle on track. To carry out the instructions received from the workstation controller to move material from one location to the next and to communicate with the AGV workstation controller. In addition it also monitors all the sensors such as obstacle detection sensors, bumper sensors and limit structures. With appropriate sensors, it keeps the workstation controller of its load status and through this direct communication new tasks are assigned.

AGV design structure :-

            The mechanical structure of the AGV can be constructed in many different forms. Some of the most common structures come with a three wheeled configuration while others with a four wheeled configuration. In the former, only one directional tracking is feasible while the later allows bi directional tracking depending upon the wheel configuration. A square wheel configuration provides maximum stability but lacks the flexibility of bi directional tracking. For bi directional tracking, a diamond shaped wheel configuration is universally endorsed. In this configuration , the driving and steering / tracking can be achieved through either the same or different driving motor mechanisms. AGV in the figure is based on the later.

BODY CONFIGURATION :-

            The AGV mechanical structure  has a simple design. This provides for easy maintenance and manufacture. Lightweight material is used, which helps to improve on the availability of the vehicle for a given charge in the battery. The vehicle structure is constructed from a space – frame and a chassis fabricated from square mild steel hallow tubes.

WHEEL CONFIGURATION :-

            The diamond shape wheel provides the vehicle with the required stability and necessary flexibility for bi directional tracking. The two middle wheels are linked together through a mechanical differential gear box driven b a servomotor by which the front and back wheels provide the steering. In  this design, these steering wheels are steered together like the two front wheels of he motor vehicles, through the toothed belt driven by a servomotor. Servomotors are used to provide the dynamics required for negotiating tight corners at relatively high speed.

SUSPENSION SYSTEM :-

            To attend correct tracking for the diamond shaped wheel configuration the wheels must be in contact with the undulating floor. This is necessary so as to reduce premature wear incurred on the driving wheels and also to provide sufficient friction on the steering wheel to prevent skidding. These problems are overcome by the use of a specially designed suspension system.

TRACKING SYSTEM :-

             Tracking is one of the vital functions of the AVG. This is provided by the use of infrared light sensors tracking the reflective tape, which is glued, to the floor. Three infra-red photo sensors are used to detect the reflective tape. The diffused reflection type sensor .

Is used to provide the high fidelity signal. As required by the closed loop control system. The sensors are mounted at an offset position from the central axis where the steering wheels are being mounted as shown in the fig. This is to prevent the steering wheel from running over the tape constantly and wearing it off prematurely for the tracking of the position of the AGV, with respect to the shop floor, to additional sensors are mounted next to the dry wheels to detect markers placed along the track. These sensors also allow the onboard controller to steer the vehicle to follow the branches of the main track.

           

SAFETY DEVICES :-

            Safety devices must be incorporated prevent any accidents and injuries. In this respect, safety devices confirming to statutory requirements such as collision avoiding sensors and bumpers with shock sensors are installed. Warning system such as rotating beacon and an audible chime are also incorporated together with an emergency stop switch.

COMMUNICATION SYSTEM :-

            To facilitate the flexibility of AGV system in FMS environment, two levels of communication are provided. The first level allows operators in the machineries in the island to communicate with the AGV processor while the second level is ties to the AFV workstation controller through a wireless network.

            For the first level communication four handshaking sensors are mounted at the side of the AGV. These sensors are used to communicate directly with the operators of the machineries at the island. When the AGV arrives at the transfer system of the island a series of signals are exchanges through these sensors to initiate the loading and unloading process and subsequent departure of the AGV.

In the second level of communication two modes are provided namely the local and remote mode. In the local mode, commands are entered directly into the AGV controller through the onboard keyboards. This allows AGVs to be dispatched individually in the event of failure in the fleet manager. In the remote mode wireless modem is connected to the AGV controller through the RS– 232 serial part for bi-directional communication with the fleet manager PC. This enables the AGV to be controller remotely by the fleet manager for the dispatch, loading and uploading of material. Through the constant updating of position and load status of the AGVs routing programs can be updated and changed for subsequent downloading to the AGV controller.

AGV SYSTEM CONTROLLER :-

            The AGV on board controller, microprocessor based system is the brain of the vehicle. It provides the vehicle with all the intelligence for the tracking and all the other necessary job functions of the material handling system. The hardware and the software are described in the following system.

SYSTEM HARDWARE :-

            The onboard controller uses the Intel 8052 microcontroller. A key based together with a display unit are attached to the controller card I/O interfaces are used to connect it to the various sensors and the controlled cards of the steering and driving motors. From information obtained from these sensors, the micro controls the velocity of vehicle and also manages the material handling devices and all the safety protocols as described. Through the wireless modem connected to the RS232 port, it communicates with the fleet manager. It also monitors the status of the battery and the load status of the vehicle.

SYSTEM SOFTWARE :-

            The control software structure is not very dissimilar from that of the programmable logic controller (PLC). The main path of the software runs in a continuous loop after the initial setup routine. The routing instructions are obtained from the local keyboard or remotely from the fleet manager. Once the instructions have been received and initiate the appropriate AGV motion commands then monitors the tracking sensors and executes the tracking algorithm. It also monitors the location marks and updates the next command to be executed. The information on the location of the AGV is sent back to the fleet manager together with other information. A block diagram of the controller software is shown in the fig. below.  commands set to form a complete command set for a given destination or in the case of remote controller mode, this series of command is downloaded through the wireless modem. For multiple destinations, the routes are normally computed from a TP. The command series transmitted to the AGV controller. At each market location the command is retrieved and executed.

FLEET MANAGER :-

            The fleet manager basically manages the AGVs and to ensure the smooth running of the AGV material handling system. It also carries out system optimization by computing and updating the routes to be taken by each individual AGV. Residing within the fleet manager is a TP and the AGV workstation manager. They are implemented on the IBM PC/AT microcomputer. The software is written in 'C' Language.

AGV WORKSTATION CONTROLLER :-

            The software of the AGV workstation controller is made up of the runtime screen display and a communication interface protocol while the hardware includes the ESTEEM wireless modem for the communication link. Its main function is to acknowledge the AGV control instruction from the TP, dispatch it to the appropriate AGV and to monitor the position and the load status of each AGV for the display screen. The information with others such as unplanned activity such as break down of vehicles is then made available to the TP for updating of its route plans. In the runtime screen display the movement of all the AGVs are shown on the screen along with the AGV route layout. figure below gives the copy of the layout and the runtime screen. This allows the operator to monitor the precise location and status of each of the AGVs as they move around the shop floor.

In order to receive and to transmit the information between the fleet manager and the AGVs a special communication protocol program is required. The fleet manager PC acts as a master, which polls the AGVs sequentially at a certain periodic rate. The rate is fixed in design depending on the number of AGVs used as to maximize the precisions of the system. Communication between the PC and the targeted AGVs are established by initiating a connection sequence using the latter's modem address. Once the link is established , data transfer is confined to only these modems. The data transfers between the PC and the targeted AGV basically includes the series of commands, which is then followed by the status of the AGV.

TRANSPORATION PLANNER :

            The TP consist of two software modules, namely, the graphic display editor and the shortest route planner. As with the workstation controller they are both written 'C' language. The TP initially makes use of the data keyed in from the keyboard or from another higher up in the hierarchy to carry out the planning. As real time data becomes available once the system is running, the planes are then updated periodically.

GRAPHICS DISPLAY EDITOR :-

            The graphic display editor (GDE) allows the user to map out the route layout of the AGV system on to the monitor. It provides graphics addition for modification or creating a new route layout due to planned expansion. Creating a route layout can be easily carried out using the editors' menu that provides various predefined junctions and symbols.

            The graphic display requires and EGA monitor with 640 x 350 pixel resolution to operate. The screen is divided into two dimensional 30 x 25 grid cells to facilitate the proper insertion of functions and other graphic symbols into the cells. In addition, the grid system allows the planner to compute the shortest route directly from the graphic display. Numerical codes are used to identify the difference between the various symbols.

SHORTEST ROUTE PLANNER :-

            This shortest route planner forms a very important part of the FMS in the optimization of the AGV material handling system. Its main function is to compute the shortest possible route that an AGV should take given the source and destination. Once the shortest route is found, the planner then generated a set of instructions to be passed down to the AGV routing for subsequent transmission to the AGVs.

CONCLUSION :-

            In FMS, the material handling system forms a very important integral component of the system. Without a flexible material handling system, bottle necks will occur and under productivity of the FMS. The AGV system together with the fleet manager provides an elegant solution to this problem. The local hierarchical control strategy in the fleet manager software enables the control of the flow of material smoothly and efficiently. The shortest route algorithm allows the automatic generation of the AGV commands. This together with the wireless communication network which allows the constant updating of the position and status of the AGVs result in a material handling system, which optimizes and compliments the FMS making the system truly feasible.

BIBLOGRAPHY

1] NEC research Index : http://citeseer_nj.nec.com

2] AGV product, Inc. : http://www.agvp.com

3] Mikell P. Grover (2001) Prentice Hall, Inc Englewood Cliffs. N.J. U.S.A.

      4] Rolend Lin, Loo KC " Design and construction of an AGV" school of MPE, Singapore 1999.

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