ME 6703- Computer Integrated Manufacturing Mechanical Engineering 2017-2018 UNIT IV Flexible Manufacturing System (FMS) Automated Guided vehicle system (AGVS) PART-A 1. Define FMS. A Flexible Manufacturing System is an integrated approach to automating a production system with a view to accommodate fast response to product changes. It is designed to be flexible so that it can manufacture a variety of products at relatively low volumes, with minimum lead-time between product changes. 2. Define FMC (Nov/ Dec 2015) The simplest, hence most flexible type of FMS is a flexible manufacturing cell. It consists of one or more CNC machine tools, general purpose or of special design interfaced with automated material handling and tool changers. FMC s are capable of automatically machining a wide range of different work piece. They are usually employed in one off and small batch production as independent machining centers, but are frequently the starting point for FMS. 3. List the benefits of FMS. (Nov/ Dec 2015) (Nov/Dec 2016) Reduced cycle times; Lower work-in-process (WIP) inventory; Low direct labour costs; Ability to change over to different parts quickly; Improved quality of product (due to consistency); Higher utilization of equipment and resources (Utilization better than standalone CNC machines); Quicker response to market changes; Reduced space requirements; Ability to optimize loading and throughput of machines. 4. List down the major elements of FMS Production systems, Support systems; materials handling systems; automated storage and retrieval systems, buffer storing of parts; chip removal and washing systems; computer control systems. 5. List the function of FMS computer control system. Workstation control, Material Handling system control, Work piece Monitoring, Tool control, Failure Diagnosis & Safety Monitoring, Production control, Reporting etc., 6. List the types of data associated with the FMC A typical FMC system handles four different types of data: Master data, control data, status data and general management data. These data are generated from CAD, CAP and CAM functions. 7. Write any two applications of FMS. Gantry (Robot) for Loading and Unloading (Automatic Part Changing); Automatic Tool Change Systems; automated storage and retrieval systems; Tool Condition Monitoring System 8. List the typical FMS layout subsystems. a)automatic storage and retrieval system; b) CNC machines; c) Work piece carriers (AGV s); d) Palletizing station; e) Washing station; f) Tool presetting station; g) Computer control system. 9. What are the types of FMS layout? In-line layout; Loop layout; Ladder layout; Open field layout; Robot centered layout.
10. List some MHS system. Materials handling includes parts transfer from conveyors and palletization. Robots perform well in these applications as well as handling fragile components and heavy ones and parts that are very hot or very cold. The tasks can be simple or complex involving tracking in one or two axes or a moving conveyor. Some robots equipped with multiple tooling can handle more than one part at a time. A variety of end effectors have been used including mechanical grippers, magnets and vacuum cups, conveyor, cranes, industrial trucks, monorails automated guided vehicle, industrial robots. 11. State the purpose of primary and secondary material handling system. Primary handling system - establishes the basic layout of the FMS and is responsible for moving work parts between stations in the system. Secondary handling system - consists of transfer devices, automatic pallet changers, and similar mechanisms located at the workstations in the FMS. 12. Differentiate between FMS and FMC. a)a FMC has two or three machine, while an FMS has four or more machine. b) FMS generally include non-processing workstations. However, FMC does not any non-processing workstations. c) The computer control systems of an FMS are generally larger and more sophisticated than that of a FMC. 13. What are the different types of flexibility? a)machine flexibility b) Production flexibility c) Mix flexibility d) Volume flexibility e) Expansion flexibility f) Routing flexibility g) Product flexibility. 14. Write the different components of FMS? (1) Workstations, (2) material handling and storage system, and (3) computer control system. In addition, even though an FMS is highly automated, (4) people are required to manage and operate the system.
PART B 1. Explain the component of FMS and FMS layout configuration. (Nov/Dec 2016) Flexible Manufacturing System Layout Configurations
2.Write notes on various materials handling equipment that are commonly used in a FMS A FMS typically needs several materials handling systems to service the machines. A transport system to move workpieces into and out of the FMS (e.g. overhead conveyors, gantry systems, AGV s, RGV s) A buffer storage system for queues of workpieces at the machines (e.g., pallets) A transfer system to load and unload the machines (e.g. robots, transfer fixtures) For these systems to work effectively, they must be synchronized with the machine operations. The location and movement of workpieces must be tracked automatically. This is done by using sensors on the materials handling system and workstations. They may be either contact devices (e.g. switches) or non-contact devices (e.g. optical, tags or proximity devices). Automatic Guided Vehicles (AGV) AGV is one of the widely used types of material handling device in an FMS. These are battery-powered vehicles that can move and transfer materials by following prescribed paths around the shop floor. They are neither physically tied to the production line nor 90 driven by an operator like forklift. Such vehicles have on-board controllers that can be programmed for complicated and varying routes as well as load and unload operations. The computer for the materials handling system or the central computer provides overall control functions, such as dispatching, routing and traffic control and collision avoidance. AGV s usually complementing an automated production line consisting of conveyor or transfer systems by providing the flexibility of complex and programmable movement around the manufacturing shop. Advantages of using AGV systems in FMS (i) Flexibility: The route of the AGV s can be easily altered, expanded and modified, simply by changing the guide path of the vehicles. This is more cost effective than modifying fixed conveyor lines or
rail guided vehicles. It provides direct access materials handling system for loading and unloading FMS cells and accessing the automated storage and retrieval system. (ii) Real time monitoring and control: Because of computer control, AGV s can be monitored in real time. If the FMS control system decides to change the schedule, the vehicles can be re-routed and urgent requests can be served. AGV s are usually controlled through wires implanted on the factory floor. The control is effected using a variable frequency approach. Radio control, an alternative to in-floor mounted communication lines, permits two way communications between the on-board computer and a remote computer, independent of where the vehicle is i.e. whether it is in the parking place or whether it is in motion. To issue a command to a vehicle, the central computer sends a bit stream via its transmitter using frequency shift keying methods to address a specific vehicle. The signal transmitted from the base station is, therefore, read by the appropriate vehicle only. The vehicle is also capable of sending signals back to the remote controller, to report the status of the vehicle, vehicle malfunction, battery status, and so on. (iii) Safety: AGV s can travel at a slow speed but typically operate in the range 10 to 70 m/min. They have on-board microprocessor control to communicate with local zone controllers which direct the traffic and prevent collisions between vehicles as well as the vehicle and other objects. A bumper is attached to some designs of AGV s to prevent collision. AGV s may also incorporate warning lights, fire safety interlocks and controls 91 for safety in shops. During design, the use of simulation can help detect whether there are enough vehicles to perform the necessary load, unload and transportation tasks and thus optimize the utilization of the AGV system. Because these vehicles have to work in a tandem with highly organized FMS cells as well as with automated warehouses under computer control, their level of performance will affect the entire efficiency of the FMS. Automated Storage and Retrieval Systems A key part of any materials handling system is storage. Major advances have been made in recent years to automate the storage and retrieval of product and materials by employing sophisticated materials handling machines, high-density storage techniques and computer control. Such systems come in a variety of forms and sizes depending on the materials handling and storage job that has to be done. They often take the form of automated warehouses which use automatic storage and retrieval systems, conveyors and computers to control the materials handling machines and to track and control the inventory. The characteristics of such warehouses include: (i) (ii) (iii) High density storage (in some cases, large, high-rise rack structures) Automated handling systems (such as elevators, storage and retrieval carousels and conveyors). ( Materials tracking systems (using optical or magnetic sensors) In such a storage system, the computer can keep track of a large number of different parts, products and materials and can assign bin locations to optimize the use of storage space. When such a system is tied into the production control system, parts and materials can be replenished as they are consumed on the factory floor, keeping the work in process (WIP) to a minimum.
3.Explain about FMS workstation. List the applications of FMS. (Nov/ Dec 2015) FMS workstation types Workstation Load/Unload Machining Other Assembly Description Physical interface between the FMS and the rest of the factory, it is where raw parts enter the system, and completely-processed parts exit the system. Loading and unloading can be performed manually by personnel, or it can be automated as part of the material handling system. Should be designed to permit the safe movement of parts, and may be supported by various mechanical devices (e.g. cranes, forklifts). The station includes a data entry unit and monitor for communication between the operator and computer system, regarding parts to enter the system, and parts to exit the system. In some FMSs, various pallet fixtures to accommodate different pallet sizes may have to be put in place at load/unload stations. The most common FMS application occurs at machining stations. These are usually CNC machine tools with appropriate automatic tool changing and tool storage features to facilitate quick physical changeover, as necessary. Machining centres can be ordered with automatic pallet changers that can be readily interfaced with the FMS part handling system. Machining centres used for non-rotational parts; for rotational parts turning centres are used. Milling centres may also be used where there are requirements for multi-tooth rotational cutters. Other possible stations may be found in specific industries, such as for example sheet metal fabrication, which has stations for press-working operations, such as punching, shearing, and certain bending and forming processes. Forging is another labour intensive operation which may be broken into specific station categories, such as heating furnace, forging press, and trimming station. Associated with assembly FMSs, the assembly operation usually consists of a number of workstations with industrial robots that sequentially assemble components to the base part to create the overall assembly.
Supporting They can be programmed to perform tasks with variations in sequence and motion pattern to accommodate the different product styles assembled in the system. Supporting stations may include inspection stations where various inspection tasks may be carried out. Co-ordinate measuring machines, special inspection probes, and machine vision may all be used here. Other supporting stations may include pallet and part washing stations for particularly dirty or oily FMSs, and temporary storage stations for both parts and pallets. Applications of FMS The applications of FMS are realized in the following areas: 1.Machining, 2. Assembly, 3. Sheet-metal press working 4. Forging, 5. Plastic injection molding, 6. Welding, 7. Textile machinery manufacture, and 8.Semiconductor component manufacture
4.Explain the functions of FMS Computer control system. Computer Control System The third major component of FMS is the computer control system.in flexible manufacturing systems, computers are required to control theautomated and semi-automated equipment and to participate in the overallcoordination and management of the manufacturing system. A typical FMS computer control system consists of a central computer and microcomputers controlling the individual machines and other components. The centralcomputer coordinates the activities of the components to achieve smooth overalloperation of the system. Functions of a FMS Computer Control System The functions that are performed by the FMS computer control 1. Workstation/processing station control. Computer control system controls the operations of the individual processing or assembly stations in the factory. For controlling the machining centres, CNC is used. 2. Distribution of control instructions to workstations. A direct numerical control (DNC) is used in a machining FMS to download the part programs to the machines. The DNC computer control system also stores the programs, allows entering and editing of programs, and performs the other DNC functions. 3. Production control. Computer control system, based on data entered into the computer, helps to take decisions on part mix and rate of input of the various parts onto the system. As a part of the production control, computer control system communicates instructions to the operators for performing different tasks on different work units. Also certain production scheduling functions are accomplished atihe manufacturing site by the computer control system. Structure of FMS Application Software Systems Fig.4.14 illustrates the modular structure of the FMS application software for system control, presented by Human Resources The fourth and final component in the FMS is human labour. Like in any other manufacturing approaches, the operations of the FMS are also managed by human labours. In FMS, human labors are needed to perform the following functions: 1. To load raw work parts into the system. 2. To unload finished work parts from the system. 3. For tool changing and tool setting. 4. For equipment maintenance and repair. 5. To furnish NC part programming in a machining system. 6. To program and operate the computer system. 7. To accomplish overall-management of the system. 5. write short notes on the following. 1) Quantitative Analysis of Flexible Manufacturing Systems
2) Benefits of FMS Flexible manufacturing system 3) Advantages of Flexible Manufacturing Systems 4)Disadvantages of Flexible Manufacturing Systems Quantitative Analysis of Flexible Manufacturing Systems FMSs can be analysed under four different approaches; these are: Deterministic models that is, models that are used to gain starting estimations of system performance, but not for complex phenomena, such as the build-up of queues and other dynamics that can impair system performance. Queuing models these attend to issues of queuing not examined in deterministic models, and they examine various simple system configurations. Discrete event simulation often used in the latter parts of system design to determine the most accurate methods for modelling specific attributes of the FMS. Characteristics handled here include layout configuration, number of pallets in the system, and production scheduling rules. Other techniques this is a hold-all title that includes various approaches, from mathematical programming, heuristic approaches, and a number of operational research techniques. Benefits of FMS Flexible manufacturing system benefits are listed as follows: Reduction of inventories throughout the complete chain of manufacturing including work-in-progress Reduction of lead time by 40% Improved machine utilization by 30% Reduction of labour times by 30% Reduction of direct and indirect labour costs Increased management control over the entire manufacturing process Substantially reduced scrap levels. The ability to adapt quickly to new work pieces. Advantages n Faster, lower-cost changes from one part to another which will improve capital utilization n Lower direct labor cost, due to the reduction in number of workers FMS Introduction and Description 11 n Reduced inventory, due to the planning and programming precision n Consistent and better quality, due to the automated control n Lower cost/unit of output, due to the greater productivity using the same number of workers n Savings from the indirect labor, from reduced errors, rework, repairs and rejects Disadvantages n Limited ability to adapt to changes in product or product mix (ex. machines are of limited capacity and the tooling necessary for products, even of the same family, is not always feasible in a given FMS) n Substantial pre-planning activity n Expensive, costing millions of dollars n Technological problems of exact component positioning and precise timing necessary to process a component n Sophisticated manufacturing systems
6. With neat schematic layout explain an example offmsimplemented manufacturing system.
7.What are the bottleneck model and the extended bottleneck model? Major issues of planning for the creation of FMSs include: part family considerations; processing requirements; physical characteristics of the workparts; and production volume. Major issues of design for the creation of FMSs include: workstation type; consideration of the variations in process routings and FMS layout; choice of material handling system; work-in-process and storage capacity decisions; tooling choices; and pallet fixture considerations. FMSs are generally analysed under deterministic and queuing methodologies in the early stage of system; while, in later stages, discrete event simulation is used to handling general system characteristics. Other techniques, including mathematical programming, heuristics, and operational research techniques, are used to capture various realties within the FMS, as necessary. In deterministic modelling, the bottleneck model is a model that examines the production system s upper limit, given that the product mix flowing through the FMS is fixed. The bottleneck model allows us to determine initial values for the average workload of individual stations in the FMS, the average number of transports in the system, and the average workload of the material handling system. The extended bottleneck model assumes a closed queuing network in which there are always a certain number of workparts in the FMS. There are two cases considered in the extended bottleneck model, where the number of parts in the system is the critical determinant. 8. What is AGV? Explain its Components? An AGVS is a system consisting of an unmanned, battery-powered vehicle, a guidance system, and other associated components. The vehicle can be programmed to pick up a load at one location and take it to another location automatically. Unlike a conveyor, AGVs are actual vehicles that can take materials/products from one location to another using a variety of different paths based on traffic in the area and programming that is done that will direct the AGV down the path that will get it to its location the quickest. Since AGVs are unmanned, they can run 24 hours a day, 7 days a week, and 365 days a year. AGVSs can use a PLC to set its course and to stop the AGV when it has an object in close proximity that it might hit. Several types of AGVSs can be used to do the necessary work and various components on the AGVS help it perform its functions and keep it and the objects/people around it safe.
AGVS Components AGVSs use many different components to assist it in getting a load from point A to point B. These components range from sensors on the vehicle used to detect objects or people in the near vicinity to the control system that is used to control the actions of the vehicle as well as interface it with the server that receives the work requests. See Appendix A for pictures of a magnet scanner and a laser scanner. Controller The controller typically consists of a microprocessor that is used to gather data and make decisions based on a program. The controller tells the vehicle how fast to go, where to go, where to turn, and when to stop. The controller gathers information from its guidance system about its location and about routes that are available. Information is also collected from the sensors to stop the vehicle, sound warnings, and maneuver if obstructions are in the vehicles path. New programs can be loaded into the controller to tell the vehicle of new routes that must be taken due to new obstructions such as construction areas. See Figure 1 for an example of a controller configuration. A controller will maintain communication with another PC/workstation that supervises all of the AGV controllers. The supervisor controls traffic, tells the AGV where the load is to be picked up and where to take it. When the routing of the vehicles must change, a user can install the program on the supervisor, and the supervisor can download it to the individual AGV controllers. Wireless Steering Motor Steering Motor Driver Modem Command and Diagnosis Termination Running Motor Running Motor Driver Barrier Detector Off-track and Guided Signal Detector AGV Controller Cruising Antenna Manual Control Device Signpost Identifying Sensor SENSORS Control Configuration (Zhan 426) Sensors come in all shapes and sizes and have a variety of different uses from temperature sensors to proximity sensors. A temperature sensor on an AGV can monitor battery temperature to prevent the battery from being damaged. Proximity sensors can be used to determine if there are objects in the path of the AGV such as inanimate obstructions or personnel. Although guidance systems will be discussed later, it is important to note that guidance systems use sensors to determine position by sensing targets on the wall, chemicals on the floor, magnets in the floor, etc.
RF Modem Since AGVs are not wired directly to a supervisor, and the AGV must know where loads are and where to drop them off, an RF modem is typically used for communication between the controller on the AGV and the supervisor computer. Without this modem, the AGV would have to be controlled manually, which defeats a large portion of the purpose (reduction in manning) behind using AGVS. Another possible scenario that doesn t involve an RF modem requires information be transmitted through a wire in the floor. This requires a wire-guided guidance system, which, as will be discussed later, limits the flexibility of the system. AGVS Vehicles Many different vehicles can be used to move loads. These vehicles differ in functionality and capacity. When choosing which vehicle is best for the application, consult with the system designers and analyze the process and vehicles available to determine which will work best. These vehicles include but are not limited to forklifts; tow trucks, conveyor decks, and scissor lift transporters. See Appendix A for pictures of some of the vehicles. 9.How the issue of Routing of AGVSis managed? Guidance systems are used to get an AGV from point A to point B. However, once the guidance system is chosen, how does one determine the path the guidance system uses to get the AGV from point A to point B? The routing of an AGVS defines the path the guidance system will take based on a number of design factors from the number of vehicles to be used to the amount of space available to run the AGV. Number of Vehicles The number of vehicles that are used in an AGVS depend on several factors. To determine the number of vehicles that are required in a system, the following analysis by Singh can be used. Dd = total average loaded travel distance De = total average empty travel distance Ndr = number of deliveries required per hour Th = loading and unloading time Tt = traffic factor that accounts for blocking of vehicles and waiting of vehicles in line and at intersections. If there is no congestion (that means there is only one vehicle in the system), the traffic factor is one. However, when more vehicles are involved, the traffic factor value will certainly be less than one. Normally, Tf lies between 0.85 and 1. v = vehicle speed Tdv = total time per delivery vehicle Nd = number of deliveries per vehicle per hour
T dv D v d De Th v N 60T d T dv f Number of Automated guided vehicles = Ndr/Nd (Singh 273-274) Floor Space The amount of floor space that is available restricts the number of vehicles that can pass through an area at the same time. There might be areas that only one vehicle can pass through, or there may be areas where all the vehicles could pass each other at the same time if required. Floor space availability will affect the guide path due to some areas having more obstructions or foot traffic than others will. Although there is no equation that determines the amount of floor space required for a certain number of vehicles, common sense must be exercised when determining if there is enough floor space available for the system that is being designed. Guide Path Determination Two types of guide paths can be set up for an AGV to use. An AGV can use either a static path, which remains constant or a dynamic path, which will be different to some degree nearly every time. Vehicle use and application will play a large part in determining if the path should be static or dynamic. Static Path Static guide paths are used when every possible pick-up or drop-off point is known. If these points are known, there will be a finite number of routes that can be taken to get from one to the other. Static guide paths typically use guidance systems similar to the ones discussed above. However, if a static guide path is used, the designer must also decide whether to use a unidirectional system in which the vehicles travel only one direction or a bi-directional system in which the vehicles can travel either forwards or backwards. Unidirectional Systems In this type of system, the vehicles are only allowed to travel in one direction. The vehicles are given a single lane, and they travel in somewhat of a loop to and from the destinations. For example, on a oneway street, a car is only permitted to travel in one direction. If the driver decides they would like to go back to their original location, they must find another series of one-way streets that will lead them back to their original location. Even though AGVs are only allowed to travel a certain direction in a given lane, if a particular aisle can support more than one lane, it can still be unidirectional as long as each lane only has traffic proceeding in one direction. An exception to this occurs when short bi-directional segments are used at load/unload stations (Peters 5). Bi-directional Systems A bi-directional system allows vehicles to use the same lane to travel in either direction. functionality can be accomplished by providing turnaround points for vehicles or by using bi-directional vehicles Egbelu and Tanchoco (1986) have shown a marked improvement in productivity and a This
reduction in the number of vehicles required in bi-directional systems (Peters 5). Bi-directional systems do increase the complexity of dispatching due to multiple vehicles going two directions on the same path, but as previously noted, there are some benefits to it. Dynamic Path A dynamic path system is not typically used in the industrial environment. In this type of system, the vehicle must have a map of the area. Once it receives its next location, it plots its own course, and uses ultrasonics or other sensors to avoid collisions. For the industrial application, this system is not feasible due to the cost of the components and the overall system, and it provides more capability than is needed in a manufacturing type application. As the components decrease in price and the knowledge of dynamic path systems increases, this system may become more prevalent. This is an extremely flexible system because there are no beacons to set, not target to hit, and no wires to follow. If there are changes in machine locations or obstacles on the floor, the AGVS control system is just updated with the new machines and the new locations relative to its coordinate system (Peters 5). Source and Delivery Points Source and delivery points are critical in route planning. One must know the number of source and delivery points there will be as well as the approximate number of times the AGV will visit these points in a period of time. There are three basic source delivery options. The first is one source point and many delivery points. The AGV may go to a single area to pick up parts for ten different machines. In this case, the AGV would make one pickup and ten deliveries. The second possibility is one source point and one delivery point. In this situation, the AGV might pick several parts up in one area for deliver to a single machine/area. The third possibility is many source points and one delivery point. This is common in factories that have several machines making different parts for assembly at another location with one machine/person assembling the parts. Routing Efficiencies Throughput requirements for the AGVS must also be examined closely when determining the route the vehicles must travel. If a company can minimize the time spent moving between two locations and the time it takes to load vehicles, the number of vehicles will decrease, and the number of paths for the vehicles will decrease in quantity. A carefully planned route can lead to an efficient, cost effective implementation of an AGVS. 10. Discuss in detail the three technologies used in vehicle guidance. A guidance system is what directs the vehicle on the path it will take to get from point A to point B. The decision about the type of guidance system that will be used is one of the most crucial that will be made. Some guidance systems have a high reliability and very rarely fail, but these same guidance systems restrict the flexibility of the factory. Other systems are slightly less reliable, but, if the factory
decides to expand or change the layout of an AGV area, components can easily be moved and added to accommodate the changes. Wire guidance, optical guidance, laser guidance, inertial guidance, dead reckoning guidance, and beacon guidance are the most common guidance systems. WIRE GUIDANCE Wire guidance is the most common type of guidance system. It is known to be reliable, but not very flexible. The wire that carries an electric current is embedded in concrete along the pre-determined path. The groove that carries the guidance wire can also be used to carry communication cables so messages can be transmitted to the AGV without the use of an RF modem. Technical Details Wire guidance is based on the fact that an electrical conductor through which an AC current is flowing will create an electromagnetic field around itself (Wire Guidance 1). See Figure 2. Since the field is stronger closer to the wire, the AGV is able to steer itself using an antenna that detects the strength of the field. More than one AGV can be located on a single wire, and different guide wires can carry electricity at different frequencies, which enables a single system to control AGVs on different paths. The guiding antenna consists of two coils that straddle the guide wire. The difference in electric voltage between the two coils will create the steering signal (Wire Guidance 1). The steering signal indicates to the steering motor which direction to turn. As long as the antenna coils are centered over the guide wire, there is no difference in voltage, and the steering signal will be zero. If the vehicle begins to wander off center, the voltage will rise in one coil on the antenna and drop in the other coil which will change the steering signal and cause the vehicle to steer in one direction or the other (Wire Guidance 1). FIELD AROUND WIRE (WIRE GUIDANCE 1)
Even though the vehicle may be on path, it must also know where it is with respect to its target, pick up point, or drop off point. Guide wires are used for indicating position relative to these points as well. The vehicle position is updated using a cross antenna that detects guide wires that are perpendicular to the guide path. From these perpendicular guide wires, the controller or supervisor can deduce where the AGV is. Advantages/Disadvantages The most obvious disadvantage to using the guide wire is its inflexibility. Systems that are frequently changed should not use the wire guidance system. Changing an AGV to a route that has not been used before requires the company to cut new grooves in the concrete and lay new guide wire. This can be costly and disruptive to the manufacturing process. Another disadvantage to the wire guided system is the fact that, over time, the wires will become brittle and break. When this happens, the wire will have to be dug out and new wire will have to be lain. Finally, in order to use this guidance system, one must have an environment that will allow the wire to be embedded in concrete. If the vehicles have to range across a dirt floor, the wire system would be difficult to use. Even though the disadvantages may be difficult to overlook, there are several strong advantages. Wire guidance is one of oldest AGV guidance technologies and has been proven to work repeatedly with many different companies. Some companies offer safety features that stop all vehicles when a control wire or guide wire is disconnected (Wire Guidance 1). Another advantage is that the wire-guided system can handle a dirty environment with heavy traffic since the wire is set in the concrete floor. In addition, the system does not have difficulty with interference from outside signals since it is not relying on lasers or lights for operation. OPTICAL GUIDANCE There are two basic types of optical guidance systems. The first is spot guidance system that uses spots of reflective material on the ground to guide the vehicle. The second system is the painted line guidance system that uses a paint or chemical path for guidance. Spot Guidance A spot guidance system uses lights on the bottom of the vehicle such as halogen lamps. This light is reflected off reflective spots on the floor such as a small glass beads to a camera or other sensor on the bottom of the vehicle. Different shapes of glass bead can have different meanings to the vehicle such as: Stop, turn left, turn right, etc. Paint/Chemical Guidance This type of system uses a line of fluorescent particles or dye to indicate the path for the vehicle to follow. Once again, sensors on the vehicle are used to detect reflected light to indicate to the vehicle its position in relation to the specified path. Information codes can also be set on the path and the sensor will read the codes and perform the action instructed by the code such as: Stop, turn left, slow down, etc.
Advantages/Disadvantages The optical systems have advantages over the wire systems since they are so easily moved. A new path can be laid out or painted with very little expense. It is also easy to repair a path if it is damaged. The disadvantages to the optical systems include the necessity of a clean environment. These optical systems must have a clean environment, and the path must remain clear of all obstructions including a piece of paper. The optical systems are also easily damaged. Paint/chemicals/glass beads will wear rapidly in a high traffic area. Finally, the networks must be kept fairly simple, since junctions are not as easy to manage as in the wire-guided case (Henlich 1). 11. Write in detail the three types of AGVS vehicles. AGVS Vehicles Many different vehicles can be used to move loads. These vehicles differ in functionality and capacity. When choosing which vehicle is best for the application, consult with the system designers and analyze the process and vehicles available to determine which will work best. These vehicles include but are not limited to forklifts; tow trucks, conveyor decks, and scissor lift transporters. See Appendix A for pictures of some of the vehicles. Forklift Forklifts can be used to pick up loads at varying heights. The forks of the vehicle are raised or lowered to the height of the load, and they are, equipped with sensors at the fork end, so it can handle high-level stacking on its own (Singh 265). A forklift can be used to pick a pallet off the floor and transport it from one place to another. It can be used to place loads on racks, load automatic storage and retrieval systems, or unload truck trailers. Fork trucks come with varying capacities, and the FMC SGV2000 has capacities up to 4000 lbs. Tow Truck Tow trucks are used to pull carriers around a facility. Products are loaded onto a trailer and the tow truck hooks up to the trailer and pulls it to its destination. Tow trucks can tow trailers, ranging from 8,000 to 50,000 lbs (Singh 261). Conveyor Deck A conveyor deck has a series of rollers on its load surface to assist in the loading and unloading of the vehicle. Pallets or bins can be loaded onto the conveyor surface without the vehicle changing deck height, which simplifies the vehicle and decreases the cost. Scissor Lift Transporter A scissor lift is used to transport unit loads such as pallets and bins similar to the conveyor deck. However, a scissor lift s deck changes height to easy the loading and unloading, but it is not equipped with a conveyor deck. This means that a lift of some sort or a person must place the load on the AGV instead of pushing it onto the deck over the rollers.
12. Write the factors influencing FMS planning issues. Planning issues for FMSs Issue Part family considerations Description A choice has to be made regarding group technology, and the part family to be produced on the FMS. The FMS cannot be completely flexible, and so cannot handle any part whatsoever; there must be some consideration on the creation of a composite part, with all possible physical attributes of the parts that may be processed in the FMS. Processing requirements Once the entire range of possible parts to be processed are known, we must use this information to help choose associated processing requirements for each part, and thus the type of equipment that should be used to process the parts. Physical characteristics of the workparts Size and weight of workparts determine size of the machines required to process the parts. It also determines the size of the material handling system needed. Production volume Production quantities must be determined, as these tell us how many machines of each type will be required. 13. Explain the Vehicle guidance technology of AGVs (Nov /Dec 2016) A guidance system is what directs the vehicle on the path it will take to get from point A to point B. The decision about the type of guidance system that will be used is one of the most crucial that will be made. Some guidance systems have a high reliability and very rarely fail, but these same guidance systems restrict the flexibility of the factory. Other systems are slightly less reliable, but, if the factory decides to expand or change the layout of an AGV area, components can easily be moved and added to
accommodate the changes. Wire guidance, optical guidance, laser guidance, inertial guidance, dead reckoning guidance, and beacon guidance are the most common guidance systems. WIRE GUIDANCE Wire guidance is the most common type of guidance system. It is known to be reliable, but not very flexible. The wire that carries an electric current is embedded in concrete along the pre-determined path. The groove that carries the guidance wire can also be used to carry communication cables so messages can be transmitted to the AGV without the use of an RF modem. Technical Details Wire guidance is based on the fact that an electrical conductor through which an AC current is flowing will create an electromagnetic field around itself (Wire Guidance 1). See Figure 2. Since the field is stronger closer to the wire, the AGV is able to steer itself using an antenna that detects the strength of the field. More than one AGV can be located on a single wire, and different guide wires can carry electricity at different frequencies, which enables a single system to control AGVs on different paths. The guiding antenna consists of two coils that straddle the guide wire. The difference in electric voltage between the two coils will create the steering signal (Wire Guidance 1). The steering signal indicates to the steering motor which direction to turn. As long as the antenna coils are centered over the guide wire, there is no difference in voltage, and the steering signal will be zero. If the vehicle begins to wander off center, the voltage will rise in one coil on the antenna and drop in the other coil which will change the steering signal and cause the vehicle to steer in one direction or the other (Wire Guidance 1). FIELD AROUND WIRE (WIRE GUIDANCE 1) Even though the vehicle may be on path, it must also know where it is with respect to its target, pick up point, or drop off point. Guide wires are used for indicating position relative to these points as well. The vehicle position is updated using a cross antenna that detects guide wires that are perpendicular to the
guide path. From these perpendicular guide wires, the controller or supervisor can deduce where the AGV is. Advantages/Disadvantages The most obvious disadvantage to using the guide wire is its inflexibility. Systems that are frequently changed should not use the wire guidance system. Changing an AGV to a route that has not been used before requires the company to cut new grooves in the concrete and lay new guide wire. This can be costly and disruptive to the manufacturing process. Another disadvantage to the wire guided system is the fact that, over time, the wires will become brittle and break. When this happens, the wire will have to be dug out and new wire will have to be lain. Finally, in order to use this guidance system, one must have an environment that will allow the wire to be embedded in concrete. If the vehicles have to range across a dirt floor, the wire system would be difficult to use. Even though the disadvantages may be difficult to overlook, there are several strong advantages. Wire guidance is one of oldest AGV guidance technologies and has been proven to work repeatedly with many different companies. Some companies offer safety features that stop all vehicles when a control wire or guide wire is disconnected (Wire Guidance 1). Another advantage is that the wire-guided system can handle a dirty environment with heavy traffic since the wire is set in the concrete floor. In addition, the system does not have difficulty with interference from outside signals since it is not relying on lasers or lights for operation. OPTICAL GUIDANCE There are two basic types of optical guidance systems. The first is spot guidance system that uses spots of reflective material on the ground to guide the vehicle. The second system is the painted line guidance system that uses a paint or chemical path for guidance. Spot Guidance A spot guidance system uses lights on the bottom of the vehicle such as halogen lamps. This light is reflected off reflective spots on the floor such as a small glass beads to a camera or other sensor on the bottom of the vehicle. Different shapes of glass bead can have different meanings to the vehicle such as: Stop, turn left, turn right, etc. Paint/Chemical Guidance This type of system uses a line of fluorescent particles or dye to indicate the path for the vehicle to follow. Once again, sensors on the vehicle are used to detect reflected light to indicate to the vehicle its position in relation to the specified path. Information codes can also be set on the path and the sensor will read the codes and perform the action instructed by the code such as: Stop, turn left, slow down, etc. Advantages/Disadvantages The optical systems have advantages over the wire systems since they are so easily moved. A new path can be laid out or painted with very little expense. It is also easy to repair a path if it is damaged.
The disadvantages to the optical systems include the necessity of a clean environment. These optical systems must have a clean environment, and the path must remain clear of all obstructions including a piece of paper. The optical systems are also easily damaged. Paint/chemicals/glass beads will wear rapidly in a high traffic area. Finally, the networks must be kept fairly simple, since junctions are not as easy to manage as in the wire-guided case (Henlich 1). LASER GUIDANCE Laser guidance, as with many of the systems that will be described later allow a vehicle to be guided off line. Using reflective targets and a laser scanner, the exact location of the AGV can be calculated. For an understanding of laser guidance system, see Figure 3. Unlike the wire guided and optical systems, a laser guidance system does not rely on floor-based reference points (Forger 2). The targets are mounted on the wall and are scanned by the scanner on the AGV. Two or three targets are used to pinpoint the location of the AGV, and the on board controller or the supervisor can make adjustments based on expected location and traffic. LASER GUIDANCE SYSTEM CONFIGURATION LASER GUIDANCE SYSTEM CONFIGURATION (NDC 1)
Scanner Details In this type of system, the information gathered from the laser scanner is used to guide the vehicle, but how does the scanner work? The laser scanner measures distance and angle to each target and gives out X and Y coordinates (Laser Guidance 1). Since the locations of these targets are preprogrammed into the controller of the AGV, the AGV can pinpoint its exact location based on its distance and angle from the reflective targets. Important Facts The lasers can be hazardous to the eyes depending on the class of the laser and the duration of exposure. For this reason, care must be taken when choosing the laser scanner and the location of the reflective targets. AGVs come equipped with a mast that can have a laser scanner mounted on it, which will move the location of the reflective targets to a height that will be above typical foot or vehicle traffic. The targets for the vehicles can be up to 30 meters away (Laser Guidance 1). There should also be a minimum of three targets in sight of the vehicle, but five targets are preferred to get a good bearing on location. Advantages/Disadvantages Although the system is easy to install, it does require additional setup time over the wire guided system and optical system. More programming is required and the AGV must be taught where the reflective targets are in relation to pick up and drop off points as well as obstacles that may be permanent fixtures on the factory/warehouse floor. Another disadvantage is the cost of the laser system compared with that of the wire system. The price difference comes from the, higher cost for the onboard navigation. It is (a) 15 to 30 percent increase in vehicle price (Laser Guidance 1). Furthermore, the accuracy of the laserguided vehicles is slightly less than the wire guided or optical systems. The laser guidance system does offer a great deal of flexibility as compared to a wire guided system or optical system. Reflective targets can easily be moved with minimal disruption to the process, or new targets can be added to the system for expansion without much difficulty. The laser system can also be used in areas that are not conducive to wires being run in the ground or lines being painted on the floor; however, there must be places to mount the targets so that they have good visibility of the scanner. INERTIAL GUIDANCE The inertial guidance system is different from any of the other guidance systems previously discussed. The inertial guidance system uses a gyroscope to determine the direction the AGV is moving. The vehicle can be programmed to take a variety of courses without additional costs due to moving wire or setting up new targets. The Gyroscope
A gyroscope operates on the principle of acceleration. Accelerations cause forces in different directions that are detected by the gyroscope. The axis of the gyroscope must be setup so that it is parallel to the motion of the AGV. When the AGV deviates from the path, acceleration forces will be perpendicular to the axis of the gyroscope. Integrating this acceleration twice gives the position deviation from the path, which can then be corrected (Henlich 3). See Figure 4. FIGURE 4. GYROSCOPE SYSTEM (FORGER 1) The information from the gyroscope is sent to the controller so that location and direction can be calculated. If the AGV begins to turn prematurely, the controller can turn on the servos that operate the steering in order to steer the AGV back on course. Proprietary Changes Some companies have made positive changes to the inertial guidance system. Companies use different technologies, but the concepts are similar. Companies have begun to install small devices, such as magnets, in the floor along the guide path. These devices contain unique position codes to communicate to the AGV its location in comparison to its expected location. Some might suggest that if things must be placed in the floor a wire guidance system should be used, but AGV Electronics uses magnets in its system and suggests that a pair of magnets should be placed in the floor every five to ten meters. Since the magnets are so spaced out and are easy to install, they can be used as a feasible alternative to wire. Advantages/Disadvantages Flexibility is one key advantage to this system even if magnets are installed in the floor. Magnets can be installed at a relatively low cost compared to the installation of wire (Magnet-Gyro Guidance 1). However, it is not as flexible as a laser guidance system that could be moved ten feet off its original path without any trouble. As with any mechanical system, the parts will wear over time. The gyroscope is a high precision device, and a small amount of wear can cause an AGV to be steered off course. Furthermore, path deviation at constant velocity cannot be corrected (Henlich 3). For these reasons, it would be essential to have the additional guidance devices placed in the floor if the system is to be expected to have a fair amount of accuracy over time.
DEAD-RECKONING GUIDANCE SYSTEM Dead-reckoning refers to another non-wire system. This system uses an optical encoder on the drive wheels to measure rotation and steer angle. A controller is used to calculate the position of the AGV based on the starting position, the internal map, and the information that is provided by the optical encoder. Wheel slippage is the biggest cause for AGVs using dead-reckoning to get off track. Since the optical encoder acts more or less as an odometer, when the wheels are turning, the controller thinks the AGV is moving. Heavy loads can also cause problems due to a deformed wheel. For these reasons, it is very important that the surface the vehicle is being used on is smooth. The dead-reckoning guidance system adds flexibility over the wire guided system or optical systems, but they require smooth surfaces to run on. Debris and foreign material in the path or on the wheels of the AGV will cause serious errors. This system is less expensive than the laser system since its controller is not as high priced; however, the system is not as reliable since it does not have absolute reference points that don t change. BEACON GUIDANCE The beacon guidance system operates on a concept similar to that of the laser guidance system. In the beacon system, beacons are placed in strategic locations around the plant. The AGV is programmed with the exact location of these beacons. As the AGV traverses through the plant, it receives signals from the various beacons and is able to use that information to determine where it is and how it needs to get to its destination. As with all of the other non-wire guidance systems, the beacon system offers higher flexibility as compared to the wire guidance system or optical systems. Typically, depending on the scope of the change, the route of an AGV with a beacon system can be changed with software. There are drawbacks to the beacon system. The first is the required mounting of at least three transmitting beacons so that the AGV will be able to pinpoint its location. There is also additional programming that must be made during installation so that the AGV knows the exact location of all of the beacons. Finally, there is also an increased cost involved with this system over the wire-guided system due to the more complex controller, the beacon transmitters, and the vehicles tracing device (Henlich 2). THE FUTURE Where are guidance systems going and how are they advancing? As with all technological advances, one can only speculate as to what the future is of guidance systems. There are systems that will be in use for a long time such as wire guidance systems. There are also systems such, as laser guidance that are currently not in great demand but the industry is beginning to realize the benefit of them. Speculation might lead one to believe that the systems will eventually be equipped with on board navigational systems with a Global Positioning System, GPS, for positioning. If this were the case, coordinates of obstacles could be loaded into the system. The source and delivery points could be sent to the system by the supervisor as well as the location of any other AGVs in the area. With this information, the AGV could direct itself to the source/delivery point using the information gained from the supervisor and it could pinpoint its current position using GPS. This may be many years down the road, but this is one possibility of where guidance systems may be going.
14. Explain the LASERVehicle guidance technology of AGVs. LASER GUIDANCE Laser guidance, as with many of the systems that will be described later allow a vehicle to be guided off line. Using reflective targets and a laser scanner, the exact location of the AGV can be calculated. For an understanding of laser guidance system, see Figure 3. Unlike the wire guided and optical systems, a laser guidance system does not rely on floor-based reference points (Forger 2). The targets are mounted on the wall and are scanned by the scanner on the AGV. Two or three targets are used to pinpoint the location of the AGV, and the on board controller or the supervisor can make adjustments based on expected location and traffic. LASER GUIDANCE SYSTEM CONFIGURATION Scanner Details LASER GUIDANCE SYSTEM CONFIGURATION (NDC 1) In this type of system, the information gathered from the laser scanner is used to guide the vehicle, but how does the scanner work? The laser scanner measures distance and angle to each target and gives out X and Y coordinates (Laser Guidance 1). Since the locations of these targets are preprogrammed