AGV Kennis Instituut
The 'AGV Knowledge Centre' has been set up to provide general information regarding AGV-systems. The AGV Education Centre will feature general information regarding AGV-technology and AGV-applications.
1. History of Automated Guided Vehicles
2. General Technology Description of AGV-systems
1. History of Automated Guided Vehicles
The patented FROG-technology has been developed over a time span of more than 20 years. Frog AGV Systems was founded in 1984 and at that time one of the pioneers with Automated Guided Vehicles (AGVs) with free ranging capabilities.
Automated Guided Vehicles (AGVs) are driverless industrial trucks, usually powered by electric motors and batteries. Applications of AGV-systems, with loads ranging from (cardboard) boxes to pallets and (steel) coils, can typically be found in and between production- and storage environments.
Automated Guided Vehicles (or Automatic Guided Vehicles) have been moving material and product over 50 years. The first AGV system, a modified towing tractor with trailer following an overhead wire, was built and introduced in 1953 in a grocery warehouse. By the late 50's and early 60's towing AGVs were in operation in many types of factories and warehouses. This type of AGV, a tugger, is still applied today. In 2003 Frog AGV Systems supplied four automated 7tons Tuggers for a chassis marriage process within an automotive factory.
In 1973, Volvo in Kalmar, Sweden set out to develop non-synchronous assembly equipment as an alternative to the conventional conveyor assembly line. The result was 280 computer-controlled assembly AGVs.
In the mid 1970s the unit load AGV was introduced - a big development for the AGV industry. The unit load Automated Guided Vehicle gained widespread acceptance in the material handling marketplace because of its' ability to serve multiple functions: assembly platform, transportation device and linking the control and information systems within a production facility. Nowadays AGVs are applied in all types of markets for the transportation of material: warehouse environments, factories, hospitals and other industrial and commercial settings. Frog also supplies AGVs for outdoor applications in harbors and for the transportation of people - in a public transportation setting, but also for rides in the entertainment industry.
Smart Floors and Dumb Vehicles
Wire guidance was the principal AGV guidance technology in the 1970's. An electronic frequency is induced in a wire that is buried in the floor. A device called a 'floor controller' turned the frequency on the wires on and off and directed the AGV through its intended route. As the intelligence of the system was in the floor controllers, these systems were typified as 'smart floors, dumb vehicles'.
The AGVs were equipped with an antenna that would seek out the frequency and guide the vehicle based on the strength of the signal. For decision points and intersections, multiple (costly) wires need to be installed. The system would energize the wire that would correspond to the intended direction of travel.
These first generation navigation schemes were expensive to install. The exact path of the AGV needed to be 'cut' in the floor to bury the wire in. The cut for a turn had to follow the radius curve that the vehicle would make when turning. Many systems had to embed four wires - three for guidance and one for communications. Often, rebar or electronic signals would interfere with the guidance signals imposed on the wires.
Today the wire-guided technology is outdated. New AGV guidance systems, such as the FROG-technology, offer many advantages (installation costs, flexibility, maintainability) over the wire-guided AGV technology. For very basic applications, this technology is sometimes still applied.
Dead Reckoning Capability
As electronics and microprocessors advanced, so did AGV applications. More intelligent Automatic Guided Vehicles were created and the need for a sophisticated path was reduced. The first major development was 'dead reckoning'. Dead reckoning is the ability to traverse space without having to rely on the physical presence of a guidance wire. The biggest advantage was that dead reckoning eliminated the need to make the cut radius turns at intersections. The AGVs could leave the wire, turn at a programmed radius, and then pick-up the wire to continue its course of travel. The path still required multiple wires in the floor, but the installation was greatly simplified.
During the 1980s, non-wire guided AGV systems were introduced. Laser and inertia guidance are two AGV guidance technologies allowing for increased system flexibility and accuracy. Changes to the path can be made without costly, time-consuming floor alterations or production interruptions.
Modern AGVs are computer-controlled vehicles with onboard microprocessors (such as the FrogBox and FrogBox Light). Most AGV-systems also have a supervisory control system (e.g. SuperFROG) to optimise the AGV utilization, generate and/or distribute transport orders, tracking and tracing modules and acting as 'traffic cop' based on priorities.
Frog AGV Systems was founded in 1984. Frog was commission to undertake a study of the feasibility of an automated transport system in an outdoor environment. It soon became apparent that the existing wire guided AGV technology would not suffice. As a solution the Free Ranging On Grid (FROG) concept was developed.
The development of the FROG-technology is a continues effort, ensuring that it is still state-of-the-art AGV-technology today. Because of the evolution of the technology, the applications have also evolved.
Applications and Controls
Within industrial environments, the use of AGVs has evolved drastically: from traditional distribution-oriented applications at one end of the spectrum to complex computer-controlled automobile assembly systems with robotic interfaces at the other end. They can be stand-alone systems, an integral part of another system, or aid in pulling together islands of automation.Â Originally designed for horizontal transportation of palletized material, the design and application of AGVs and controls are now as varied as those of industrial robots.
Outside industrial environments, Automated Guided Vehicles are now also being applied for transhipment at ports, as people movers and in the entertainment industry. Frog AGV Systems is a pioneer in this field and has been the first AGV manufacturer to enter these new markets.
2. General Technology Description AGV-systems
Automated Guided Vehicles (AGVs) are used for material handling. Material handling is the movement, storage, control and protection of material and goods throughout the process of their manufacture, distribution, consumption and disposal.
An AGV-system consists of vehicles, a supervisory control system and the required auxiliary equipment. The following sections detail the different aspects related to an AGV system. These sections are intended to inform and educate regarding the technologies used and applied for AGV-systems. They do not specifically describe technologies as they are applied by Frog AGV Systems.
Supervisory Control System
The information for these sections is derived from different sources, among others the educational lessons on AGVs of the Materials Handling Industry of America. Please also refer to their website for more information.
There are two basic principles for the navigation of an Automated Guided Vehicle (AGV):
Fixed path AGV navigation is the oldest method of navigation. To date it is sometimes still used for basic applications. Most of the systems that are being installed are now free ranging AGV systems, allowing for greater flexibility and better maintainability.
Fixed path AGV navigation
Fixed path AGV navigation could also be described as 'dumb vehicles, smart floors'. The general characteristics of fixed path AGV navigation are:
The AGV paths are well marked on the floor;
The AGV paths are continuous;
The AGV paths are fixed, but can be changed.
Different types of fixed AGV paths can be distinguished:
Apply a narrow magnetic tape on the surface of the floor
Apply a narrow photo sensitive chemical strip on the surface of the floor
Apply a narrow photo reflective tape on the surface of the floor
Bury a wire just below the surface of the floor
The first three methods require a sensor on the underside of the Automated Guided Vehicle which can detect the presence of the surface mounted path. The sensor's mission is to keep the AGV directly over the guide path. If the path makes a turn the sensor detects the turn, provides feedback to the onboard vehicle controller, which in turn causes the Automated Guided Vehicle to steer in the direction of the path. The sensor's role in connection with the onboard controller and steering mechanism is to cause the AGV to follow the path (smart floor, dumb vehicles).
In case of a current-carrying wire, the sensor underneath the AGV is usually a small antennae consisting of magnetic coils. With current flowing, a magnetic field surrounds the buried wire. The closer the buried wire is to the AGV antennae, the stronger the field. The magnetic field is completely symmetrical around the conductor or the buried wire. At a given distance from the wire the field has the same strength on either side of the cable. The field strength is detected by the antennae's magnetic coil and induces voltage in the coil.
The Automated Guided Vehicle steers itself to follow the magnetic field surrounding the buried wire. To get a steering correction signal the AGV's sensing antennae consists of two coils. When the vehicle is centered directly above the buried wires equal voltages are induced in the two coils. If the Automated Guided Vehicle moves a bit to one side of the wires, the induced voltages would be of different strength. The difference in signal strength in the coils is proportional to side displacement of the coil. This difference is amplified and fed back to control an onboard servo motor, which turns the guide wheel or wheels until both coils receive equal signals, and the course is corrected.
Free Ranging AGV Technology
During the late 1980s, non-wire guidance for AGV systems was introduced. Laser and inertia guidance are two examples of non-wire AGV guidance, which allow for increased system flexibility and accuracy. When changes to the original guide path are needed, there is no need for floor alterations or production interruption. Non-wire guided AGV technologies are actually offering more variation - if not an infinite number of ways to navigate the open space between two points.
To navigate in an open, unrestricted space without the benefit of a fixed path, an Automated Guided Vehicle must have a way of knowing where it is and be capable of taking a heading to where it wants to go. All open space navigation methods require:
A map of the area in which the AGV can operate that is contained in the vehicle's computer memory.
Multiple, fixed reference points located within the operating area that can be detected or "seen" by the AGV.
Typically a free-ranging AGV technology will be very similar to the FROG-technology. Automated Guided Vehicles will have a route/map on-board of the vehicle (route planning). During travel the AGV will measure distance and direction by counting the number of wheel revolutions and measuring the steering angle (odometry). Encoders attached to the wheels provide the data that determine the covered distance as well as the change of direction. This technology ensures that the vehicle can operate automatically, but because of uneven surfaces or wheel spin, small inaccuracies occur.
Inaccuracies are corrected by comparing the calculated position of the vehicle with the actual position of the vehicle. The actual position of the vehicle is determined by means of external reference points. There are three different types of calibration for free ranging AGVs:
The GPS option is not valid indoors. The accuracy of even dGPS navigation is comparable yet (!) to the accuracy that can achieved with laser or grid navigation. Grid and laser are calibration alternatives suited for indoor environments, while the grid can also be applied in outdoor AGV applications.
Laser is a commonly used technique in industrial environments. With laser guidance, the reference points are strategically located targets (Super-high reflective reflectors) mounted to a vertical surface such as a wall or column. A beacon on top of the vehicle emits a rotating laser beam, which is reflected back to the vehicle when it strikes (sees) the reflector. The targets are known (X,Y) locations. An Automated Guided Vehicle needs only two (but ideally three or four) targets in order to calculate a relative coordinate location and heading using simple geometry.
Calibration by means of grid is less common as it is patented by Frog AGV Systems. Only Frog and a select group of licensees are able to offer this AGV calibration technology. The grid nowadays consists of small magnets embedded in the road surface. When a grid of permanent small magnets embedded in the floor is used, the AGV re-calibrates its position every time the vehicle travels over one of the magnets. A magnet ruler (sensor) mounted underneath the AGV is used to detect the magnets.
Laser and grid calibration are just as accurate. Frog AGV Systems will always select the calibration principle most suited to the AGV application.
AGV Supervisory Control System
Running a multiple number of Automated Guided Vehicles, active infrastructure supports (like pick-up & delivery stations) and/or control and coordination with other installations, requires a supervisory control system. A supervisory control system tasks range from AGV traffic control, to AGV dispatching, tracking and tracing to communication and monitoring.
The supervisory control system of Frog AGV System is called SuperFROG. SuperFROG and consists of two software programs: a User Interface and an Internal Program, which is responsible for communication with external systems, implementation of the logistic concept, Frog AGV management and control of infrastructure equipment. The Graphical User Interface handles all user interactions with the control system. SuperFROG may support multiple User Interface modules at different locations in the layout at the expense of additional computer hardware. The user interface contains different overviews of the AGV system: layout definition, AGV dispatch monitoring, AGV monitoring, graphical layout monitoring (real time visual display) and statistics monitoring.
The different aspects of the supervisory control system are:
Planning and scheduling
The main tasks of a supervisory control system are traffic control, lay-out management, communication and job generation and assignment.
Automatic stopping, starting and routing of Automated Guided Vehicles is essential to all AGV systems. To ensure against one AGV entering an already occupied zone or intersection of a guide path and to provide for orderly and efficient routing in general, the location of each AGV is monitored and decisions are made based on this knowledge. In the context of traffic control, all forms of automatic stopping and starting are known as blocking.
Two types of blocking exist:
- Zone blocking;
- Accumulative blocking.
Zone blocking is controlled from the supervisory control system. A specific section (zone) of the track can only be granted to one Automated Guided Vehicle at the time. If the zone is granted to a specific AGV, it can not be granted to another AGV wanting to use the same section. This Automated Guided Vehicle will have to wait until the zone is clear again and permission to access it can be granted. These zones will typically be located over intersections, station and turns.
A typical scenario of events:
- AGV 1 approaches zone A and requests permission to access zone A.
- The supervisory control system checks is zone A is clear.
- If the zone is clear access is granted to AGV 1.
- As AGV 2 now ask permission to access zone A, the supervisory system checks is zone A is clear.
- As zone A is granted to AGV 1, AGV 2 will have to wait before accessing zone A.
- Once AGV 1 signals zone A is clear, the supervisory control system will grant access to AGV 2.
- If multiple AGVs are waiting to access a particular zone, access could be granted to either the first AGV to arrive, the AGV with the highest priority, or multiple AGVs coming from a single direction.
Accumulative blocking is not controlled by the supervisory control system. It is performed by the AGVs and the obstacle detection sensors mounted on them.
Accumulative blocking is utilized on long, straight sections of guidepath and is performed independent of AGV Supervisory control system logic. An Automated Guided Vehicle will sense a stopped or slower AGV on the guidepath ahead and stop behind it. When the stopped or slower moving AGV (ahead) resumes travel and is clear of the trailing AGVs object detection, the trailing AGV will resume travel. This means that Automated Guided Vehicles can accumulate immediately behind each other and move up to the next controlled intersection area instead of waiting up to several hundred feet for a zone to be cleared. The result is greater throughput and faster vehicle response.
AGVs can be equipped with different sensors looking in the direction of travel to detect obstructions in the AGVs path. After detecting an object an AGV will slow down first and finally come to a controlled stop. For more information on obstacle detection system, please review the section on safety.
Layout Management is a functionality only available in the more sophisticated supervisory control systems. Usually the AGV layout manager or editor will be a CAD-like program, consisting of a background with structures depicted in it. The layout editor will then facilitate the creation of routes and stations for the Automated Guided Vehicles, taking into account the structures of the facility. In some cases, special conditions can be added to the routes, such as reduced maximum speed, speed limitation, maximum weight on a route section, or for instance a temporary route blockage.
Communications include message commands such as where to go, when to start, when to slow down and when to stop. It may also include fault condition reporting and in some cases even 'AGV-heart-beat' monitoring. Computer-controlled systems overseeing remote objects need a means of communicating commands, and in many cases confirming replies, between a supervisory computer and the objects being controlled. Depending on the application, there are four types of basic communication media being used within AGV Systems:
Guide Wire Data communication;
Inductive Loops communication.
Radio provides maximum flexibility in system control. AGVs can be programmed "on the fly", new routings or maps can be downloaded quickly, and system speed of response to changing load movement demands is improved. It provides almost constant communication between the vehicles and the system and makes the AGV system a very responsive tool that can react to the changing dynamics of the work environment.
Radio waves can be used to communicate information and data, from a fixed base station to the modems on each vehicle. Radio waves simply perform the function of delivering energy to the remote receiver. The actual information is superimposed on the radio wave so that it can be accurately extracted from the wave at the receiving end. This provides a continuous two-way data link with the AGVs. There are two basic systems in use today, narrow-band and spread spectrum. A plant survey is generally done to determine what other frequencies are operating in the environment, if there are any dead zones in the system that would inhibit radio transmission and to determine the number, type and location of antennas.
A narrow-band radio system transmits and receives user information on a specific radio frequency. Narrow-band radio keeps the radio signal frequency as narrow as possible to pass data. Undesirable cross talk between communications channels is avoided by carefully coordinating different users on different channel frequencies. The radio receiver filters out all radio signals except the ones on its designated channel frequency.
There are Federal Communications Commission (FCC) licensed frequency channels assigned to qualified users who request them. To allow unlicensed radio usage the FCC established spread spectrum radio communication. Spread spectrum radios do not rely on licensed frequency channels to obtain privacy. Instead, the signals are purposely spread over a large band of frequencies and rely on the fact that others in that band are doing the same. Each receiver must know what spreading pattern or code the transmitter is using in order to follow the selected pattern. This action is known as frequency hopping.
The FCC allows frequency hopped systems to define their own channel width up to a maximum of 500 kHz in the 900 MHz band or 1 MHz in the 2.4 GHz band. These systems may not spend more than 0.4 seconds on any one channel and they must hop through at least fifty channels in the 900 MHz band and seventy-five channels in the 2.4 GHz band. This is to reduce the chance of repeated collisions between users.
Communication can be initiated by the supervisory computer or by an Automated Guided Vehicle. A typical communication involves vehicle ID, location, load status, and traffic condition requests. These exchanges are brief but can be extended by an AGV if more than the requested data is required, such as alarm conditions, low battery, emergency bumper stop, or a lost condition. On-board microprocessors manage this data, receive and decode messages from the supervisory controller, and arrange and encode return messages to the controller.
Optical infrared communication is highly reliable but has the disadvantage of not being continuous; it is point to point. AGVs may be stopped during this data exchange which usually occurs at load stations where the fixed and mobile units are aligned and in close proximity (e.g. hokuyo sensors). Or, the AGV communicates at fixed points along its guide path as the AGV travels through a given zone. Usually infrared communication is not used any longer to communicate between Automated Guided Vehicles and the supervisory control system. Infrared is now being used for local communication (e.g. lining up with a conveyor belt).
Guide wire data communication
Data transmitted on the guide wire by the guide line driver provides almost the same flexibility as radio, with the exception of vehicle movement off the wire. Since the distance between the guide wire and the on board responders is constant, there are no transmission dead spots, as there may be with radio. The techniques to accomplish this type of data link are not widespread.
Inductive Loops Communication
Inductive loops are another means of point-to-point communication. In-floor wire loops are located adjacent to the guide wire and connected to the central controller for data transmission. They are usually 3 to 10 feet long and must be located at every point where communication with vehicles is desired. Electronic messages or simple commands in the form of prescribed frequencies are sent out via the wire loops. Antennas on the under side of the vehicles sense the frequencies which are then decoded and acted on by the AGV. A vehicle can likewise send messages back to a central controller. This is an inexpensive but limited method of data transfer. Most systems using this method do not require AGVs to stop while receiving data from inductive loops.
Job Generation and Assignment
Job generation and assignment (dispatching) is essential to every AGV system - whether simple or complex. The operation can be compared to that of dispatching taxi cabs. All customers are to receive timely services from the vehicle best able to service a request. If the job generation and assignment is done inefficiently, a system will not obtain maximum benefits. Job generation and assignment is usually performed by the supervisory control system (remote dispatching), but can also be performed locally by means of on-board dispatchers.
For job generation and assignment by a supervisory control system, an RF network (broadband or spread spectrum) including base station(s) and antennas with a receiver aboard each AGV is required. The generation of transport requests is application specific. The supervisory control system can be connected to plant control systems for the generation of transport request or are connected to the various systems directly. The supervisory control system will combine the transportation requests if possible and consequently assign them to the most appropriate AGV. These AGVs can be located in single or multiple dispatching positions, or can still be working on a current transport request.
A single dispatch point requires the AGVs to return to the same location every time to receive commands and transport requests. Single point dispatch generally has predetermined shortest path routings and works on strict FIFO requests. As AGVs are available and reach the dispatch point, they are given a transport command.
Using various dispatch locations, AGVs are given transport commands as soon as they complete their previous transport. The supervisory control system attempts to choose the closest available AGV to fulfill the command. Should a vehicle be given a command and a closer AGV becomes available, the more sophisticated control systems will be able to reassign the transport request to the closer vehicle. These sophisticated control systems also have the ability to assign jobs to vehicles that are currently still servicing a job - creating a job load for each AGV.
Local job generation is less used but can be better depending on the application. Local dispatch occurs when an AGV is dispatched by means of a signal or occurrence originating at the location of the AGV. Local dispatch is generally associated with simpler AGV systems and where repetitive tasks are predominant, but this does not have to be true in every case. Because of safety, some operations require local job generation by a person to enhance safety within the working environment.
AGV Safety Systems
The purpose of an AGV obstacle detection system is to detect any obstacle on the path, in time, so that the AGV can slow down and stop, if necessary - until the path is clear. As the path clears, the AGVs will automatically continue their trip. Of course, immobile structures (such as a storage rack or a wall) have to be detected as well, so that a collision can be prevented.
Each AGV is equipped with bumpers and/or contact sensitive strips. Both work upon impact: the vehicle stops immediately when the bumper or contact strips 'hit' an obstacle. It is the most basic form of obstacle detection, but it may be perfectly adequate for certain AGV applications. The most common situation for AGVs however, is having a bumper and/or contact sensitive strips and additionally a non-contact obstacle detection sensor system.
Sensors assisting truck drivers when backing up are a familiar example of obstacle detection sensors based on non-contact detection: a warning is given before a collision occurs. This is especially important in environments involving people. Sensors used in AGV-applications function in the same manner: slowing down or stopping the AGV instead of generating a warning signal.
The sensor system is capable of transmitting two digital signals: upon early detection of obstacles, the "caution" signal is activated, causing the AGV to gradually reduce speed; as the AGV approaches the obstacle and reaches a pre-set distance, the "stop" signal is activated, bringing the AGV to a full stop. The range of "caution" and "stop" zones is adjustable to the speed of the AGV.
A continuous or near-continuous field of view in front of the AGV will be covered by the object detection system at some minimal distance. Various techniques exist (laser, infrared, ultrasonic, vision) for generating the field of view. It is generally possible to detect obstructions as small as 1 inch by 1 inch. Even smaller obstructions may be detected using specialized sensory devices.
When an obstacle is detected, an AGV will start to slow down. If an object is not removed, the AGV will come to a controlled stop. Once the object moves or is removed, the AGV will delay for a prescribed amount of time, and then start in creep speed. After achieving creep speed and finding no obstructions, the AGV will resume its previous travel speed.
AGV Energy Management
Power on virtually all AGVs is provided by batteries. Different battery technologies are available for AGVs, but the selection of the most appropriate battery type for each AGV-application is determined by factors such as ambient temperature, throughput, number of shifts in operation, the type of AGV used to handle the loads, the load weight, and specific customer requirements.
Flooded Lead Acid Batteries
Flooded lead acid batteries are the most common type of battery for AGV-systems. Flooded lead acid battery chargers are close to an AGV standard, and customers usually understand the maintenance required. Lead acid batteries have the highest power-to-size ratio of all the standard batteries used in AGVs. Up to 80% of the ampere-hour capacity can be used before swap out or charging is required. This adds up to more hours of operation between charges.
Battery manufactures recommend eight hours of charge time for eight hours of run time with an additional eight hours of cooling. Nowadays new batteries and new charging processes aim to reduce the number of hours required for charging and cooling.
For an AGV-system requiring long operational hours, various options exist for the energy management. First, sizing the lead acid battery to allow it to work longer than 8 hours is possible when ample battery compartment space on the AGV is available, and when duty cycle allows. Secondly, it is also possible to switch from capacity charging to opportunity charging (charger on or off board). AGVs can run for long periods of time (up to 15 hours) with little or no charging). At the end of this period, the AGV batteries must be recharged. The large ampere-hour size of the battery and the 80% allowable depth of discharge is a great advantage over nickel cadmium batteries; however, there must be ample time to charge the lead acid battery before it is needed again. The third option is a battery swap out, where the batteries are exchanged for a new set.
The frequency of maintenance for the flooded lead acid battery depends on how hard the AGV (and thus the battery) is being used - maintenance ranges from weekly to monthly depending on usage. This maintenance is greatly reduced when an automatic battery swap out system is used. The battery receives frequent watering and conditioning equalization, or charges automatically. Automatic watering or equalization charge systems can further reduce manual maintenance requirements.
Valve-Regulated Lead Acid Batteries
Valve regulated lead acid batteries are ideal in locations where cleanliness is important and gassing is not permitted. There is little maintenance required for these batteries. Because of the valve regulated technology, no watering or special equalized charging is needed for the life of the battery under normal conditions.
There are different technologies for valve regulated batteries which have different charging / discharging characteristics. By mid 1996 the gelled electrolyte type had changed the discharging limit from 50% to 80% depth of discharge. The absorbed glass mat also allows an 80% depth of discharge before swap out or charging is required. The type of technology that is used depends on the AGV application's location and conductors such as ambient temperature, throughput, and customer's requirement of the battery manufacturer.
These batteries may be used the same way as flooded lead acid batteries including automatic opportunity charging (charging on or off board) and manual/automatic swap out. Regardless of which technology is being used, a battery charger that is designed for valve regulated batteries may be used. Because of the valve-regulated technology, the charge current is reduced compared to flooded lead acid chargers. The time to charge a fully discharged battery takes from 8 to 16 hours. However, because of the low current rate, the battery does not require much (if any) cooling time. The valve-regulated batteries provide a large throughput surge (at least 8 hours) with little or no charging, common in non-cyclical applications. At the end of that shift, the AGV must be allowed to recharge.
Nickel Cadmium Batteries
Nickel Cadmium (NiCad) batteries are ideal for an AGV-system when an automatic charging system is required. One of the benefits is that the battery has a low maintenance requirement when a 10% depth of discharge or less is used: monthly or quarterly. This battery can work 24 hours a day by using the automatic charging system.
The Nickel Cadmium battery has a faster recharge time compared to the lead acid battery. Thus, opportunity charging has the potential of minimizing the time an AGV is out of circulation. Also, locating the system chargers strategically will minimize the trip to the charger, which reduces the AGV charging time. Because of the low depth of discharge recommended, the run time before charging is greatly reduced compared to a lead acid battery. It may vary from 30 minutes to 2 hours depending on the current consumption of the AGV, throughput, and battery ampere-hour size. Nickel cadmium batteries provide the maximum continuous duty AGV cycle time for all opportunity charging applications.
The charge time for an AGV ranges from 2 to 10 minutes. This is possible because the charge current can be up to double of the ampere-hour rating of the battery. The utilization of the AGV (run time compared to the charge time) may vary from 60% to 90% depending on the size of battery, charge rate, application, and manufacturer. The AGV utilization must be taken into account when verifying throughput. The nickel cadmium batteries and chargers are more expensive than the traditional lead acid equipment, but only 1 battery per AGV and fewer chargers are required in many applications. Also, the cost may be less for battery maintenance.
Battery charging methods range from manually swapping out batteries to fully automating the swap out process, to charging a battery while the AGV is still in operation but idle.
Manual Battery Swap Out
Automatic Battery Swap Out
On-Board Automatic Opportunity Charging
Off-Board Automatic Opportunity Charging
Manual Battery Swap Out
When a battery reaches a pre-set level, or at the end of an 8 hour work shift, for example an 80% depth of discharge, the AGV will travel to the maintenance / battery area and wait for a worker to remove the depleted battery and install a fresh one. The AGV must then be entered back into the system.
The depleted battery is then charged and cooled. This method is ideal when a customer or user desires to keep good records on the battery maintenance.
Automatic Battery Swap Out
This method of swap out basically automates the manual process of changing, charging, watering, and equalizing the AGVs battery. This system is optimal when the conventional or sealed lead acid batteries are designed with enough ampere-hour capacity to last at least 12 hours of run time. This allows two batteries per AGV in the system, thereby allowing only one set of batteries to be charging/cooling while the other set is in operation. This type of operation allows the AGVs to operate for about 12 hours and 5 to 10 minutes of down time for the battery replacement procedure. The normal battery maintenance is greatly reduced compared to the manual operation. The system's cost must be weighed against the AGV fleet size and the number of AGVs that can be reduced because of the high AGV utilization.
On-Board Automatic Opportunity Charging
On-Board Opportunity charging requires a charger to be installed in the AGV. It can only be used with lead acid batteries because the nickel cadmium system chargers are too large to fit on the AGV. The AGV, when not working on an order, is sent to a charging queue area (also known as home queue). This area contains overhead rails that supply AC voltage to the onboard charger. The attractive feature about this system is several AGVs can be charging at the same time while moving throughout the queue area. This minimizes the charging area(s) and allows the lead acid batteries to charge optimally.
Off-Board Automatic Opportunity Charging
This style of charging can be applied to any battery technology. The AGV automatically goes to the battery charging area anytime the vehicle's battery is below a certain level. This level is configurable to allow it to continue to work until there are no more pending orders, or when the battery level goes critical. When charge shoes are used in the system (typically with nickel cadmium and valve regulated lead acid batteries), they are usually mounted in the floor to allow the AGV's battery to connect to the charger. These charge shoes are usually mounted in close proximity to the system chargers. Other connections are possible, such as over head or side mounted rails.
Capacity charging entails charging at fixed intervals or after one continues shift has finished. Charging for the next day after a continuous shift has finished is common when the duration of the shift is limited to maximum 12 hours per day. The fixed interval charging is applicable when the AGV is used in a production environment of which the path and timing of the AGV is predictable. This scenario is common when using valve regulated lead acid or Nickel Cadmium batteries; however it is ideal when using nickel cadmium. In NiCad battery applications, utilizing a lower depth of discharge can extend battery life.
AGV Load Transfer and Handling
The early AGVs were tuggers towing trailers or were shaped as platform vehicles. They had only one single function: to transport. Today, AGVs can be equipped with robotic arms and perform any type of robotic handling functions. AGVs no longer have a single function, but are also used as storage machines - equipped with forks and handling loads in storage racks up to 10 meters in height or more.
Automatic load transfer and handling is used in most AGV-systems, however manual load transfer is also possible. AGVs can pick up and drop off their loads usings fork attachement, conveyers, lift tops and an array of other devices depending on the type and size of the load units to handle.
The basic AGV load transfer and handling mechanisms that can be distinguished are:
Manual load transfer and handling
Automatic (un)couple transfer and handling
Powered roller/belt/chain conveyor transfer and handling
Powered lift transfer and handling
Powered Push/Pull transfer and handling
Some AGV applications will feature a specific transfer and load handling system, but other AGV-systems may combine multiple systems in one single vehicle or multiple transfer mechanisms in the same application.
Manual Load Transfer and Handling
There is a large variety of manual load transfer and handling mechanisms for AGVs. These include manually (un)coupling trailers to tugger, (un)loading by manually driven vehicles (e.g. a fork lift) and manually (un)loading AGVs - such as with trolley's.
When trailers are (manually) uncoupled from the AGV, it becomes possible to move the trailer to a given work station. It can also involve transferring roller beds from the AGVs to fixed roller station by manually pushing the load off the vehicles. The most common manual load transfer however, is applying fork trucks to (un)load either the AGVs themselves or trailers of tuggers.
Automatic (un)couple transfer and handling
Automatic uncouple load transfer methods are relatively easy to implement. The AGV is directed to a separate section of the track where it automatically uncouples its trailer. The AGV consequently pulls forward and is available to service other transportation requests. This includes picking up a trailer at another location. Coupling of the new trailer to the AGV can be done either manually or automatically.
In some cases manual coupling is selected, as automatic coupling is more challenging. The trailer has to be positioned such that the AGV hitch and the trailer engage properly. The accuracy of positioning of both trailer and AGV are thus critical. The supervisory control system is also important in this perspective, to carefully plan the process, avoid collisions with other AGVs during the operation and for job generation and assignment once the trailer is ready for pick-up by an AGV.
Powered roller/belt/chain conveyor transfer and handling
A standard technique for automatic transfer and handling of loads is by means of a powered roller belt or chain conveyor. AGVs can be equipped with a single or multiple power decks, or have a trailer with single or multiple power decks, to automatically transfer loads to and from fixed stations. The fixed stations can either be active or passive stations. Active stations are also powered, while passive stations are not. Passive stations usually require the AGVs to have a shuttle system to be able to transfer loads.
For every automatic transfer the AGVs have to align very precisely with the transfer station before the load transfer can be achieved. A positioning accuracy of + or -1/2" is normally achieved, which (for most applications) is well within the tolerances required at conveyor transfer stations. For the automatic load transfer to take place, a 'handshake' is required. A 'handshake' ensures that the load transfer mechanism of the AGV and the fixed station are simultaneously activated to achieve the load transfer. The signal also turns off the drives when the transfer is complete. The handshake method usually involve sensors onboard the AGV, which interface with sensors on the stand - exchanging signals when a load transfer is about to occur. If the proper signals are not exchanged, then the AGV will not attempt the transfer. As a consequence it will alert its surroundings by means of sound and send a signal to the supervisory control system.
Powered lift transfer and handling
A powered lift transfer and handling mechanism can be installed on different types of AGVs, enabling them to pick up their loads either from the ground surface or from a fixed station.
A common application with a powered lift transfer and handling mechanism is applying forklift AGVs (or side loader AGVs). A forklift AGV is capable of picking up and dropping of loads directly to ground surface. However it is also possible that a lifting device is installed in other types of applications. The lifting device is very convenient in applications with (existing) fixed stations, which have different handling heights. In this case the lifting device can be used to accommodate the changes in handling heights. The 'liftback' AGV is very low and positions itself underneath its load to lift the load on its back. Other applications using powered lifts for load transfer can be found in automotive factories and the 'chassis-marriage' process more in particular.
Powered Push/Pull transfer and handling
A powered push- or pull transfer and handling mechanism is not frequently applied for AGV systems. In these applications the vehicles remain relatively simple and the load transfer mechanism is included in the stations that are set up. The AGV with a non-powered deck or trailer, positions itself in front of a fixed station. The station is equipped with an automatic push- or pull mechanism. This mechanism will reach out to the load and push or pull it off the AGV or trailer. The method is quite useful if the automatic load transfer is centralized as the cost of multiple load transfer points can be prohibitive.