20-04-2010, 03:57 PM
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Automated Guided Vehicles
Topics
History
What is AGVs /AGVSs
Components of AGVS
Types of AGVS
Important issues for the AGVS
System design of AGVS
Automated Guided Vehicle System
Transport material from loading to unloading stations
Highly flexible, intelligent and versatile material-handling systems.
A very flexible solution for the problem of integrating a new automated transportation line into an existing transportation environment by using automatic
guided vehicle.
HISTORY
First AGV developed in 1954 by A.M.Barrett,Jr.
Using a overhead wire to guide a modified towing truck pulling a trailer in a grocery warehouse.
Subsequently, commercial AGV were introduced by Barrett.
1973, Volvo developed automated guided vehicles to serve assembly platforms for moving car bodies through its final assembly plants.
Later, Volvo marketed their unit load AGVs to other car companies.
What is AGV?
Driverless Vehicle
Electric motors, battery powered
Programming capabilities
Destination
Path selection
Positioning
Collision avoidance
System Discipline
Components of AGVS
The Vehicle “ No operator
The guide path “ The path for the AGV
The control Unit “ Monitors and Directs system operations including feedback on moves, inventory, and vehicle status.
The computer interface “ Interfaces with other mainframe host computer, the automated storage and retrieval system (AS/RS), and the flexible manufacturing
system.
Types of AGVSs
AGVS towing vehicle
AGVS unit load carriers
AGVS pallet trucks
AGVS forklift trucks
AGVS light-load transporters
AGVS assembly-line vehicles
AGVS Towing Vehicles
First type of AGV introduced.
Towing vehicle is called an automated guided tractor
Flatbed trailers, pallet trucks, custom trailers can be used.
Generally, used for large volumes (>1000 lb) and long moving distances (>1000 feet).
AGVS Unit load Carriers
To transport individual unit load onboard the vehicle.
Equipped with powered or non-powered roller, chain or belt deck, or custom deck.
Loads can be moved by Pallet truck, forklift truck, automatic loading/unloading equipment, etc.
AGVS Pallet Trucks
No special device is needed for loading except the loads should be on pallet
Limited to floor level loading and unloading with palletized load
Widely used in distribution functions
Capacity 1000-2000 lb
Speed > 200ft/min
Pallet truck can be loaded either manually or automatically
AGVS Forklift Trucks
Ability to pickup and drop palletized load both at floor level and on stands.
Pickup and drop off heights can be different
Vehicle can position its fork according to load stands with different heights
Very expensive
Selected where complete automation is necessary/required.
AGVS Light Load Transporters
Capacity < 500lb
To handle light and small loads/parts over moderate distances
Distribute between storage and number of workstations
Speed 100ft/min, turning radius 2ft
For areas with restricted space
AGVS Assembly-Line Vehicles
Variation of an light load transporter
For serial assembly processes
As the vehicle moves from one station to another, succeeding assembly operations are performed
This kind provides flexibility for the manufacturing processes
Lower expenses and ease of installation
Complex computer control and extensive planning is required to integrate the system.
Important issues for AGVâ„¢S
Guidance system
Routing
AGVS control systems
Load transfer
Interfacing with other subsystems
AGVS Guidance system
The goal of an AGVS guidance system keep the AGV on track/predefined path
One of the major advantage of AGV is ease in modification given by the guidance system for changing the guide path at low cost compare to conveyors, chains,
etc.
Another benefit is: guide path is flexible which means intersection of path is possible.
Generally, guide path does not obstruct another systems.
The guidance systems can be selected based on the type of AGV selected, its application, requirement and environmental limitation.
Wire-guided
Optical
Inertial
Infrared
Laser
Teaching type
Wire-guided:
An energized wire is rooted along the guide path.
The antenna of the AGV follows the rooted wire.
Optical:
Colorless florescent particles are painted on the concrete/tiled floor.
Photosensors are used to track these particles.
Inertial:
The guide path is programmed on a microprocessor which is fixed on the AGV
Sonar system is incorporated for finding obstacles.
Infrared:
Infrared light transmitters are used to detect the position of the vehicle.
Reflectors are affixed on the top of vehicle to reflect the light.
Laser:
Laser beam is used to scan wall-mounted
bar-coded reflectors.
Accurate positioning can be obtained.
Teaching type:
AGV learns the guide path by moving the required route.
Sends the information to the host computer.
AGVS Routing
A routing system is used to select the vehicle which is positioned with the optimum path.
A network controller gives the destination, while the on-board controller navigates the vehicle.
Commonly used methods:
Frequency select method
Path-switch select method
Frequency select Method
At the bifurcation of path (decision point), the vehicle reads a code in the floor in the form of metal plate, or coded device.
The vehicle selects one of the frequencies as per the direction required.
A continuous wire is used to loop the frequencies.
AGVS Control Systems
Computer controlled system
The path controller controls the guide path of AGVS.
Sends information to AGVS process controller.
Process controller directs movement of vehicles
Interchanges information with the host computer
Most Expensive and complex type of control.
AGVS Load Transfer
Load Transfer of the vehicle means loading/unloading. The two types of load transfer operations are:
Manual Load Transfer
Manually loading and unloading the Vehicle.
Use of forklift trucks, pallet trucks, roller, etc.
Manually coupling/uncoupling towed vehicles
Automatic Loading and unloading
Efficient system
Use of Powered roller, belt, and chain
Powered lift/lower devices, push/pull devices
Automatic couple/uncouple
Interfacing with other subsystems
AGVS system can be interfaced with other subsystems through the distributed processing network or using Host computer. Such subsystems are:
Automated storage and retrieval system
Flexible Manufacturing systems
Computer Numerical control (CNC)
Process Control Equipment
Shop Floor control system
AGVS System Design
Many issues must be considered before designing system for an AGVS:
Selection of guidepath and vehicle
Guidepath layout / Flow path design
Number of vehicles
There are several other issues regarding timing of AGVs, dispatching rules, routes, etc. Also, there must be interaction between design and operational
issues for system design.
Attributes for selection of guidance & AGVS
Vehicle Related attributes:
Cost of the vehicle system
Cost of guidance system
Vehicle dimensions
Load capacity
Maximum weight
Maximum Volume (depending on AGV inner dimensions)
Maximum speed at loaded/unloaded condition
Maintenance facilities: Modular components for maintenance, self diagnosis, etc.
Charging related attributes such as charging time, on-line charging.
Turning Radius
Position sensors
Loading system i.e. unit load, pallet, or other attachments required.
Flow path design
Type of flowpath within the layout i.e. unidirectional, bidirectional or combination
Type of guidepath layout
Position of load transfer or loading /unloading stations
Number of stoppage stations
Storage space of the stations.
For developing a flow path design simulation software can be used. These software takes into consideration the layout, locations of P/D stations, timings
of AGV, material flow intensities between stations, etc.
Number of vehicles
General notations:
Dd = total average loaded travel distance
De = total average empty travel distance
Ndr = Number of deliveries required per hour
Nd = Number of deliveries per vehicle per hour
Th = loading and unloading time
Tdv = total time per delivery per vehicle
Tf = traffic factor that accounts for blocking and waiting of vehicles and at intersections.
if only 1 vehicle than Tf =1
if Number of vehicles > 1 than Tf <1
v = vehicle speed
Tdv = (Dd / v) + Th + (De / v)
= loaded travel time + loading/unloading time + empty travel time
Nd = ( 60*Tf ) / Tdv
Number of automated guided vehicles = Ndr / Nd
Advantages of AGVâ„¢s
Unobstructed movement
Flexibility
Locations, path, P/D points can be reprogrammed
Easy to change guide path system
Number of vehicles can be altered depending on requirement
Greater reliability
Less environmental problems
AGV can be replaced by another, in case of failure.
Lower investment
Higher operating savings on long run
Minimal labor cost
Easy maintenance
Easy to interface with other systems
Best choice for AS/RS, FMS
