automated guided vehicle

CHAPTER 1

INTRODUCTION:

                   An automated guided vehicle system is a material handling system that uses independently operated, self-propelled vehicles that are guided along defined pathways on the floor. The vehicles are powered by means of on-board batteries that allow operation for several hours (8-16 hrs.) between recharging. Sensors on the vehicles that can follow the guide wires achieve guidance. Off-board computer or micro- processor controls the vehicle. The controller sends commands to the vehicle such as identification of load; its destination and other special instructions. An AGV system provides a material handling system i.e. both flexible and readily adaptable to either product or production changes.

                   AGV systems were originally developed for the distribution of material in warehouse environments. Although this is still an important use, two major growth areas have been evolved the moment of material to and from production areas in manufacturing facilities, replacing manual work lifts and the use of carriers of work in progress in assembly plant, replacing serial type asynchronous or fixed index assembly conveyor system and small packages, in hospitals to deliver meals in offices to deliver mails and in clean room environments for material handling.

                   AGV system was first introduced in 1950 in USA and later in EUROPE in early 1960, the technology caught on much faster in Europe.

  

  CHAPTER-2

                                    TYPES OF VEHICLES

2.1 Towing vehicles:

                      These vehicles consist of an AGV with no load carrying facility but with a hitch or tow bar that can pull trailers, carts, pallet-jacks and wheeled racks. They are used where there is a large volume of product to move or in retro fit application where product has historically been moved by trailers. These vehicles can move loads upto 50000 pounds.

2.2    Unit load transporters:

                         These vehicles are designed to carry individual loads. Unit load transporters can have an extremely versatile deck design which permits them to be equipped with rollers, belt conveyors, power lifts, special fixtures, or on-board robot arms. These AGV’s canned either bi-directional or unidirectional and are used in warehousing as well as on the factory floor. Unit load transporters can lift loads ranging in between 12000 to 60000 pounds.

2.3       Standard automatic guided pallet trucks:

                         These vehicles are designed to service palletized loads to and from floor level positions .The shadow fork version has limited fork travel and is designed to move pallets to and from floor position exclusively .The fork truck version has travel of upto 20 feet and can move pallets both at floor level and on stands or racks.

2.4       Assembly line vehicles:

                          These vehicles has a fixture on board that accepts the frame on initial part of the product that is to be assembled .The vehicle is routed through the various manufacturing station of the factory where parts and assemblies are added to the product. These AGV’S can provide total automatic transfer of materials. These are preferred as material handling system on automated assembly lines. They can skip assembly sections if equipment in a particular section breaks down.

2.5       Light load transporters:

                            These vehicles are designed to carry boxes, baskets, small parts. They may be outfitted with a flat top, shelves, conveyors, custom handlers etc With any other unitized container .It generally has a small footprint allowing its use in tight spaces and narrow aisles. These vehicles are used for a wide range of functions from mailrooms to clean rooms in every type of manufacturing and office environment.

2.6    Advantages and disadvantages:

2.6.1:  Advantages of AGV-

           Following are the advantages of the AGV

1. Reduction in labour force.
2. Improved productivity and quality.
3. Job enrichment and worker satisfaction.
4. Reduction in space requirements.
5. Reduction in product damage.
6. Improvement in house keeping.
7. Ease of removal and relocation.
8. Integration with other types of automation.
9. System adaptability and flexibility.

2.6.2: Disadvantages of AGV-

      1. Expensive.

      2. External use is limited.

      3. Requirement of specially designed floor space.

      4. Performance is affected if guide path bed is not stable.

      5. Sufficient support from management is required.

      6. Equal support from workers is required.

      7. Obstructions are created.

      8. Maintenance is required.

      9. Other considerations are

a.       The vehicle must not be used for inventory storage or act as buffers.

b.      It is not possible to operate vehicles made by different manufacturers on same guide path and control systems.

c.       Vehicles are not well suited for situations where ambient temperature exceeds 49⁰c.

d.      It is difficult to upgrade a simple system into complex one.

CHAPTER 3

AGV SYSTEM COMPONENTS

              Although all AGV systems are different, in general they consist of following similar components:

3.1 Vehicles

The component of an AGV system that is most readily identified is vehicle itself. The vehicle consists of a frame, batteries, on-board charging unit, electrical system drive unit, steering, precision stop unit, on-board controller, communication unit, and safety system and work platform.

Frame:

                   The frame is usually constructed of welded steel members with aluminum cover plate.

A.  Batteries and charging:

                   24 or 48V D.C. Industrial batteries typically power AGV systems. Battery charging is usually accomplished by one of the two techniques viz. slow charging, quick charging, and charging more than one battery at a time.

B.  Drive unit:

                   The main components are the main motor speed controller and drive mechanism. The drive motor speed controller is usually a pulse width modulated four-quadrant server drive unit. The carrier drive commands are generated either through the microprocessor or at the hand control unit.

C.  Steering:

                   Vehicles are designed to maneuver in three different ways forward only, forward and reverse, four directional. The major components of steering system are the steering antenna, the steering motors and their speed controllers, the steering mechanical linkage and steering limits switches.

D.   Precision stop controller:

                   A precision stop controller is used to stop the AGV with close location accuracy at workstation and charging station. At same point before an upcoming precision stop location the vehicle will receive a precision stop command from the off board controller or by code bar on the floor. As it approaches the stop point the vehicle’s metal detector is activated and the AGV slows to the end of the plate.

E.  On board controller:

                   The vehicle controller is used to monitor    vehicle performance through encoder data to determine position and velocity. Discrete digital input, monitor functions as controls, activation of safety devices, battery conditions, steering limit, break release, running light drive controller status

F.  Communication unit:

                   Instructions to the vehicle microprocessor are usually generated by the area controller and then relayed to the vehicle. The communication system may be either continuous or discrete

G. Safety:

                   Safety systems may be divided into three specific categories, vehicle to vehicle, vehicle to object, and vehicle to people. The first system uses photocells mounted on AGV’s leading age and reflective material on the trailing age to avoid the collision of vehicles. Vehicles to object system uses bumpers, toe born limit switches, proximity sensors to protect both vehicle and any object in AGV path vehicle to people system are designed both to warn and protect people. Generally vehicles have warning light buzzer or toner, which flashes or sounds to indicate the automatic mode.

3.2 GUIDE PATH AND GUIDENCE SYSTEMS

                        Generally most AGV’s need guide path to follow. The guide path techniques are known as passive or active tracking. Passive tracking depends upon either optical or metal detection principles whereas active tracking involves inductive principles.

A.  Passive tracking:

                   The optical method may simply involve a light sensitive photocell mounted on the vehicle, which follows the tape on the floor. It depends upon contrasting floor surface so that variations in the reflecting light that is sensed by the photo cell can detect when the vehicle begins to stray from the guide path. If guide tape becomes dirty, faded, or damaged or if the ambient light distorts the light level sensed the vehicle may stray from the guide path.

                        A variation of the optical method is Litton patented optical system. It is based on bonding fluorescent particles to the floor surface and stimulating these particles with ultraviolet light and causing them to omit a generated light .In the sensing head, an oscillating mirror scans the guide path and reflects the generated light into a photoreceptor, which in turn relays the signal to a microprocessor.

            The other passive tracking techniques involve vehicle with metal detecting sensors following a stainless steel ribbon. Transcar patented guidance system consists of two sensors packs each containing five sensors and located at each end of the AGV. The three central sensors allow the vehicle to center itself on the guide path. The two remaining sensors assist the vehicle in traversing curves. The sensor locates the presence of the guide tape and transmits this information to the onboard microprocessor.

B.   Active tracking:  

                   Active tracking involves use of a guide wire and most commonly used technique in industry. A low voltage (less than 40v), low current (less than 400 mA), low frequency (1 to 15 KHZ) and signal is conducted through a wire buried in a slot in a floor. A small electromagnetic field is radiated from the wire and two inductive type sensors are compared and as long as they are equal, the vehicle is centered on the guide path. If the vehicle begins to stray, signal magnitudes sensed are no longer equal and sensor difference is used to steer the vehicle back on the guidepath.

3.3 Floor and system Controls:

                      The controller is the brain of the whole system. Tying the vehicle to the guidepath and integrating the system. Not only does it controls the AGV system but also integrates with automatic assembly facility. The AGV system itself will usually contain three levels of control architecture, vehicle control system, floor control unit and vehicle on the board processor.

A.  Vehicle control system:

                   The top level of vehicle control system often communicates with and under the control the facility’s host computer. Most of the decision-making takes place at this level as it oversees the system operation. The vehicle control system stores in memory exact vehicle location at all the times and provides network access.

B.  Floor control unit:

                   This level is also referred to as the data concentrator and act as traffic manager and communicating directly with the vehicles and providing them with formatted detailed commands.

C.  Vehicle processor:

                   Generally the vehicle processor the vehicle processor knows the vehicle location, and it can interpret commands received from floor control unit and can monitor on board safety devices. The two types of vehicle control processors are intelligent type and non-intelligent type.    

      

 CHAPTER 4

                      AGV GUIDENCE & CONTROLS

Introduction

                    Some AGV systems use vehicles, which have sophisticated microprocessor on board and are known as smart or intelligent vehicles. Other systems have minimal vehicle computing ability and use a central computer to process all functions. In such a system the central computer determines vehicle location, its destination and the proper route and it directs the vehicle path and velocity. This is accomplished by turning on and off the path at decision points or by commanding vehicle to follow a particular frequency .All decision making is thus performed by central computer with smart vehicles. The central computer dispatches the vehicle to next location, through it’s on board microprocessor, the vehicle itself makes its own decision as to which path it takes.

4.1 Method of programming:

                   The simplest system uses manual programming to direct the vehicle to specific destination and to dispatch the vehicle. These system ranges from basic toggle switches, thumb wheel switches or push button numeric pads for programming the vehicle to go to specific station. The advantage of manual system is that it is the least expensive and simplest available system. The disadvantage manual system is that its efficiency is dependent upon operators. Finally these types of system lacks tracking capability and so system controller can’t determine vehicle location while it is in transit.

                    The second level of sophistication in control system is referred to as remote dispatch. In such system operator interacts with the local controller who in turn transmits information such as destination, route and automatic load / unload commands to the vehicle. This control system allows the vehicle to circulate on the guidepath looking for work. This system doesn’t offer tracking capabilities. The third level is more complex and expensive and is referred to as the central computer controlled system. In such system all vehicle transactions are monitored by the system central to computer and are connected to the facilities host computer. This permits interfacing the AGV system with CNC machines and process controlled equipment. Tracking is also possible with this level of sophistication including color graphics displaying guide path, locations and status of all the vehicles.

4.2 Guide Path Techniques

       a. Passive techniques. 

                          It involves the use of chemicals, paint & adhesive strips or tape where by the AGV focuses a beam of light on the reflective tape and tracks the path by measuring the amplitude of reflected beam. Another passive method involves vehicles with metal detecting sensors following a stainless steel tape. Communication of commands and positional information to the vehicles may be accomplished by placing guided path codes along the guide path.

b. Active Techniques

                   By the most commonly used methods in industry is the wire guide path this method involves cutting a slot in the floor (1/8 to ¾ inch wide) & (1/2 to 1.5 inch deep), into which one or more wires are placed and grouted and epoxies. There are two different wire guide path techniques, one using either one wire in the slot operating on one frequency or with multiple overlaid frequencies and other using several wires in the slots each operating at a diff frequency. With the multiple wire method a path is selected at decision points according to the assigned frequencies. The vehicle can be programmed by the system controller at decision points to follow the appropriate frequencies and thus the vehicle is directed on the desired path.

4.3 Communication Technique

                   Irrespective of guidance technique used, it is essential for the individual vehicles to be able to communicate with the system controller. The vehicle must be able to receive such commands as work assignments, destination, route frequency, speed, blocking instructions, when to start and stop and auxiliary equipment commands used. Similarly vehicles must be able to transmit its status to the system controller by sending such information as vehicle identification, location, direction of travel, speed of travel and battery status. There are two types of   AGV systems continuous and discrete type. Continuous indicates that the area controller always communicates with any vehicle where as discrete means that the area controller can only communicate with a particular vehicle at certain times. Radio frequency communication is widely used form in a continuous communication. Each AGV is equipped with transmit / receive antenna. Each AGV may be on a different frequency.

                   The majority of AGV systems prefer discrete communication method. The two types of discrete control are inductive and optical. Inductive method utilizes a set of wires buried beneath the floor along the guide path in squares or rectangles to form communication points. Each communication point is assigned a unique address through the area controller. To communicate, vehicles either come to a stop with their communication antenna immediately above a communication loop or while in motion over an elongated loop.

                   The optical method involves stopping the AGV at set stations along the bad path where information is passed to the vehicles using infrared light.

CHAPTER 5

APPLICATIONS OF AGV’S

Introduction:

           AGV system is used in a growing number and a variety of applications.

5.1 Driverless Train Operation:

                   These applications involve the movement of large quantities of material over large distances. E.g. the moves are within a large warehouse or factory building or buildings in a large storage department. For the movements of the train consisting of 5-10 trailers, this becomes an efficient handling method.

5.2 Storage or Distribution System:

                   Unit load carriers and pallet trucks are typically used in these applications. In these storage and distribution operations, the movement of the material is in unit loads form. The application often interfaces by AGV with some other automated handling or storage system. This type storage or distribution --which work-in progress is stored in a central storage area and distributed to individual workstations for assembling or processing.

5.3  Assembly line operation:

                   AGV systems are being used in a growing number of assembly line applications. In this application the production rate is relatively low and there is variety of different models made on production line. Unit load carriers and light load guided vehicles are the types of AGVS used in these applications.

5.4     Flexible Manufacturing Systems:

                   In this application this guided vehicles are used as the material handling systems. The vehicle delivers work from the staging area to individual workstation in the system. At a work station the work is transferred from the vehicle platform in to the work area of the station for the processing. At the completion of processing vehicle returns to pick up the work and transport it to the next area. AGV system provides a versatile material handling system to complement the flexibility of the automated workstation.

5.5     Miscellaneous Applications:

                    Other applications of AGV systems include non manufacturing and non warehouse application such as a mail delivery in office building and hospital material handling applications Hospital guided vehicles transport metal trays, lives, medicals and laboratory supplies and other material between different floors of the hospitals.

 CHAPTER 6

Future trends:

Although it is difficult to predict the future with absolute certainty,

It can be concluded from the trends that seem to indicate future status of AGV systems.

6.1 Guidance:

                   The research is being accomplished to expand the capability and even to eliminate the guidance using guide path or board controller. On board controller is becoming more sophisticated and at the same time they are becoming smaller and less expensive .The vehicle controller is exhibiting such features as expanded diagnostic. Although vehicles cannot repair themselves, they can at least indicate their problems to the maintenances and repair personnel. Controller sophistication will also allow the vehicle to operate more intelligently in complex handling situation and will increase system integrity in the event of host computer failure.

6.2 Vehicle communication:

                    The trend is towards continuous is opposed to discrete communication so that vehicle will be able to communication and receive updated instructions at any time.

6.3 System controller:

                    System will be designed to increased capability to track material and store this information. They will be able to follow and control material flow to support just in time concepts. The system controller will be able to be integrated with network allowing it to communicate with any other facility controller.

6.4 Vehicles:

                   Vehicles will become more standardized requiring less engineering to adapt the vehicle to a particular task, thus lowering the cost of vehicles to a great extent. This will make them easier to justify for many users.

6.5             Improved graphical display:

                   There will be probably the increased use of color graphical displays showing the entire guide path, every vehicle location, the vehicle identification, the vehicle status, and the vehicle load.

6.6 Safety:

                   New safety sensors for proximity detection will be developed and coupled with the increased computing power of the onboard controller to produce on even safer vehicle that readily negotiate pedestrian clogged aisle.