Applications: Pallet Trucks
Applications: Fork Trucks
Applications: Unit Load
Applications: Light Load
Applications: Assembly Line
AGVS towing applications were the earliest and are still the most numerous AGV type. Towing applications can involve the bulk movement of product into and out of warehouse areas or direct service to a manufacturing/assembly operation. Usually side path spurs are placed in receiving or shipping areas so that trains can be loaded or unloaded off the main line and thereby not hinder the movement of other trains on the main path.
Chain movement of product with AGVS trains is also popular. In this case, the AGVS trains are loaded with product destined for specific destinations along the guide path route. The train will make several stops in order for the product to be unloaded at the correct locations.
Trains systems are generally used where movement of product is over long distances, sometimes between buildings, outdoors or in very large distributed systems where the runs are long. Since each train can move as much as 16 pallet loads at a given time, this becomes a very efficient method and can usually be justified easily based on the elimination of fork trucks or manual trains and operators.
Applications: Pallet Trucks
AGVS pallet trucks are used generally in distribution functions. Vehicles can be loaded in two ways, either they are capable of automatically reversing into pallets on the floor or operators will manually board the vehicles and back them into pallets.
For the product delivery the vehicles normally proceed down the path to specific destinations in storage areas, pull off onto a spur, lower their pallet forks to the floor and pull out from under the pallets, then automatically return empty to the loading areas. Many applications have been done whereby the vehicles are manually boarded in the loading areas and driven off the path to load staging areas where they are manually loaded. The vehicles are backed up under the loads, driven back to the path, given a destination by an operator and automatically proceed to the drop off spurs in the warehouse areas. Automatically reversing a guided pallet truck adds considerable expense to the system and the necessity for accurate positioning loads on the floor for pickup. They can only be justified in limited applications at this time. Manually loading the vehicle gives operators flexibility to position loads anywhere off the path and still be able to retrieve them with the vehicles which then automatically proceed without operators into the warehouse drop locations.
Applications: Fork Trucks
AGVS fork truck applications are relatively new. Guided fork trucks are used when the system requires automatic pickup and drop off of loads from floor or stand level and where the heights of load transfer vary at stop locations. The guided fork truck has the ability to automatically pickup a load or discharge the load without any human interface.
The vehicle can position its forks to any height so that conveyors or load stands of varying height in a given system can all be serviced.
Since these vehicles are some of the most expensive AGVS types, they can only be justified where total automation is required. AGVS for trucks require more intricate path layout and a method of accurately positioning loads on the floor or on stands for vehicles to service. This normally requires greater system discipline than with other systems, but the benefits include greater flexibility in integrating other subsystems together through the AGVS system.
Applications: Unit Load
AGVS unit load applications – usually involve specific mission assignments for individual load movement. Unit load carriers are quite popular in applications integrating conveyors with manufacturing/assembly operations or storage retrieval systems. Here they are a very efficient means for horizontal transportation between hardware intensive material handling subsystems. The unit load carrier, over moderate distances, can move high volumes of material linking other automated subsystems in a totally integrated facility. Usually the unit load systems involve an automatic pickup and delivery of product with remote management of the vehicles in the system.
Unit load carriers are normally used in warehousing and distribution systems where the guide path lengths are relatively short, but the volumes are high. Here the unit load carriers have the ability to maneuver in tight areas where AGVS trains would be too awkward to use. Load transfer to conveyors or load stands is easily accomplished with unit load carriers either using roller decks or lift/lower decks. The unit load carriers allow good system versatility for product movement because they usually operate independent of one another and can pass each other to get to specific destinations.
Applications: Light Load
Light load AGVS applications are used in light manufacturing processes. The product can be distributed from a small parts storage area to individual workstations where operators do light assembly. The AGVS system can be driven by product demand at the various assembly stations. The light load AGVS is smaller and has only a several hundred pound capacity. It is ideal for moving small parts in trays or baskets and for maneuvering in very small, tight areas. Electronic fabrication, small assembly manufacturing and parts kitting applications are proper uses for light loads AGVS.
Applications: Assembly Line
Assembly line AGVS applications are only now being introduced in the U.S. This is an adaptation of the small light load AGVS for an assembly line process. Here the guided vehicles carry major subassemblies such as motors or transmissions to which parts are added in a serial assembly process. Prior to each assembly area is a parts staging area where small parts are placed in a tray onboard the vehicle beneath the major subassembly. The vehicle the proceeds into an assembly area where it stops at assembly work station. The assembler takes the parts from the tray onboard the vehicle and then assembles them onto the major subassembly. When that process is completed, he then releases the vehicle, which proceeds to the next parts assembly area the process repeats several more times. When the assembly process is complete, the finished assembly such as an engine block or chassis is unloaded from the vehicle, which is then sent to the start area for the assembly process. There it is again loaded with a raw subassembly.
AGVS assembly systems give good flexibility to a manufacturing process by allowing parallel operations. They also allow for individual tracking of items and measured work rates. Normally these systems are integrated into an overall production system, which requires computer control and extensive planning.
A common misconception is that guided vehicles can be made to do anything and with so many new vendors on the market it is very difficult to differentiate what is practical from what is not. It is very useful to have a basic understanding of guided vehicle controls in order to appreciate what you can and cannot do with them.
Navigation & Guidance allows the vehicle to follow a predetermined route which is optimized for the material flow pattern of a given application
Routing is the vehicle’s ability to make decisions along the guidance path in order to select optimum routes to specific destinations
Traffic Management is a system or vehicle ability to avoid collisions with other vehicles while at the same time maximizing vehicle flow and therefore load movement throughout the system.
Load Transfer is the pickup and delivery method for an AGVS system, which may be simple or integrated with other subsystems.
System Management is the method of system control used to dictate system operation.
The proper method selection for each function and its ability to work with the other functions determines in great measure the degree of success of a given system.
Navigation & Guidance
AGVs guide and navigate using one of 3 principal methods. Although nearly all new systems employ some type of non-wire navigation, some systems still utilize wire guidance. Here the AGV uses a sensor under the vehicle to detect the RF signal emanating from a wire placed in a slot approximately 1 inch below the floor surface.
In the mid-1980s non-wire guidance and navigation was first introduce using laser target triangulation. Here, reflective targets are mounted above the floor on columns, walls, machines or posts approximately 25 feet apart. Each facility target is surveyed and given a unique ‘X,Y’ coordinate. These coordinates are loaded into each AGV’s memory. Onboard each AGV is a rotating laser light beam source and receiver. When the laser light reflects off a facility target its distance and angle are automatically measured. Be processing several laser reflections at a time, and comparing them to the stored target coordinates, the AGV can calculate its position. The AGV then compares its calculated position to a coordinate map of the preplanned path stored in its memory and determines it steering instructions as it proceeds throughout the facility.
A new form of non-wire guidance and navigation was introduced in the mid-1990’s. This technology is called inertial or gyro navigation. Each AGV is equipped with a solid-state gyroscope. This device senses very small deviations in the AGV direction of travel. Like laser navigation, the AGV path is a virtual set of coordinates stored in each AGV’s memory. A small marker (magnet) is installed in the floor approximately every 25 feet along the AGV virtual path. The markers are flush with the floor surface and surveyed for their unique X/Y coordinates in the facility. This information is also stored in the AGV’s memory. As the AGV negotiates the virtual path the onboard gyro detects slight change in travel direction and this is compared with the actual stored travel path. The AGV corrects course as needed to stay ‘on’ the prescribed path in its memory. The in floor markers are used as reference points to correct any slight error accumulated over the distance between markers. Typically, the AGVs track the actual path within 1″.
AGVS steering control allows AGVS vehicles to physically maneuver in different ways. There are two basic types of AGVS steering control, “differential speed steer control” and “steered wheel steer control”.
- Differential speed control uses two fixed wheel drives and varies the speeds between the two drives on either side of the guide path to permit the vehicles to negotiate a turn; much in the way a tank or a tracked vehicle turns.
- Steered wheel control uses automotive type steering control in which a front steered wheel turns to follow the guide path.
In either case, the guide paths look the same for most applications. Steered wheel control is used in all type of AGVS vehicles; however, differential steer control is not used in towing applications or on vehicles which have man onboard controls.
In very space limited applications not requiring high throughput volume, differential steer control vehicles are used in pivot steer modes. The vehicle stops on the main line, rotates 90 degrees and proceeds into or out of a station. If sufficient room is present, a normal radius curve is often better from the standpoint of flow and controls simplicity.
AGVS routing is another fundamental control function of an AGVS system. How does the vehicle negotiate the path to take the shortest route from point A to point B? Generally, there are two approaches to the routing function. A simple layout will demonstrate those two concepts. This layout has two locations where the guide path splits into two separate directions. These are called decision points or path branches. It also has two locations where two guide paths merge together into a single guide path. These are known as convergences.
AGVS routing techniques center on two methods depending on whether the system is wire or non-wire guidance. The wire guided method is known as the “frequency select method” and the non-wire guidance method is known as the “path select method.”
In the frequency select method the wire guided AGVS vehicle approaches the decision point, reads a marker in the floor, which tells the vehicle what location it is at. This marker is typically a passive code device, which is usually in the form of buried magnets, metal plates, or other code devices. The vehicle uses frequency selection to follow the proper path. When the vehicle is approaching the decision point it is following a single frequency. At the decision point two frequencies are present in the same slot. The vehicle depending on which direction it wishes to go selects which frequency to follow and the routing is automatically accomplished.
In the path select method the non-wire AGVS vehicle approaches a decision point and chooses whether to follow path 1, 2, 3, etc. The choice of path is easily accomplished as it is a virtual path choice made within the vehicle’s onboard controller. Since the path map is stored in the AGV’s memory, it simply chooses witch of the memorized paths to follow based on its ultimate destination.
In our simple system layout, the wire based frequency select method requires that at least two different frequencies be present at a decision point for a two-path decision. If there were three paths to select from at a decision point, then three different frequencies would be required. The multiple frequencies are only required at the decision point or at a convergence point where multiple paths come together. In between divergences and convergences, single frequencies are used. In the frequency select method the frequencies are used over and over again whenever they are required. Most systems only require two or three frequencies. These frequencies loop through the system in a continuous wire and are always active.
The AGVS vendors using this technique produce frequency layout drawings whereby they design the system so that the necessary frequencies are available at intersections and convergences as required. When the frequency is no longer required, it is routed out of the main guide path slot into an interconnect slot and run to the next needed area in the interconnect slot. This results in some extra slot cutting other than for the main guide path.
In the non-wire path switch select method, the virtual guide path is divided into segments which are stored in the AGVs/ memory. Since there is no physical wire path installed in the floor, the AGVs simply determine their location on their internally stored maps and determine when they arrive at a path branch which path to take.
When an AGV system consists of more than one AGV, some form of traffic management is required to prevent AGVs from running into each other.
There are three types of traffic management in general use.
Zone control is easily the most popular and widely used type of traffic management. We can apply zone control to our simplified AGVS layout it by segmenting the path into separate zones. The rules of zone control are that only one vehicle is permitted in a given zone at a time.
When a vehicle occupies a zone the closest a trailing vehicle can get is into the next completely unoccupied zone behind the lead vehicle. The lead vehicle must then proceed into the next zone before the trailing vehicle can move ahead into its next zone. A zone may have multiple stop stations in it. If a vehicle is allowed to occupy a zone it can proceed to any stop in that zone.
Zone control is accomplished by two general methods.
“Central zone control” accomplishes the zone sequence action through the use of a central controller, which controls each block zone by use of a zone communication point for each zone. The central zone controller regulates the entire network of zone communication points itself. When a vehicle approaches a zone entrance, it communicates from the zone communication point to the central controller. If the central zone controller determines that the vehicle can enter the zone, it communicates to the vehicle at the communication point that it can proceed; otherwise, the vehicle waits for permission to go.
“Vehicle zone control” accomplishes the zone sequence action through the use of the vehicles themselves. Each vehicle communicates to one another without the need for a central zone controller. The vehicles know where they are and communicate that information by radio frequency communication to other vehicles in the area. When these vehicles receive the information, they then decide for themselves, based on their location, whether or not they can enter a given zone. If a vehicle is communicating that it is in that zone, any vehicle wishing to enter that zone will automatically stop and wait at the beginning of the occupied zone. When the lead vehicle passes into the next free zone it changes the information it is transmitted to indicate it new zone position. This new information is transmitted to other vehicles in the system and the vehicles take the appropriate blocking actions.
Briefly summarizing these two principal methods of zone control, they each have distinct advantages and disadvantages.
In a central zone control system, if the central zone controller should go down, there is no ability for the AGVS system to operate. The AGVs cannot operate if the system does not perform traffic control. The central zone control method does allow for flexibility and blocking sophistication allowing a high degree of vehicle movement for high throughput rates.
Vehicle zone control is based on an intelligent control system onboard the vehicle. Since there is no central, the failure case can be limited only to specific vehicles that fail and not to a central zone controller failure which would affect all the vehicles.
In any form of zone blocking, the more zones in the system the greater the degree of vehicle movement permitted. Fifty foot long zones permit much more vehicle movement than do 200 foot long zones because in a given length of path you can fit four times as many fifty foot zones as you can 200 foot zones. This would permit vehicles to move with a greater degree of freedom from one another and yields higher throughput.
Forward Sensing Control
Traffic management by forward sensing control uses a sensing system onboard the guided vehicles that detect the presence of a vehicle in front of it. Three types of sensors are used: sonic, which works on radar principal; optical, which uses infrared light sources; and bumper, which uses a physical contact of vehicles to cause traffic control. These methods are not effective in path intersection or convergence areas since the vehicle’s forward sensing controls look straight ahead and not to the side for intersecting or converging traffic. Therefore, in those areas, these methods have to be augmented by some form of zone traffic control.
In general, forward sensing control is useful where a lot of straight path is in the system and where that straight path is not interrupted by intersection curves. In these methods, wherever the sensing system sees a vehicle in front of it the vehicle goes into a hold and when the lead vehicle moves out of the range of the sensing device the trailing vehicle automatically restarts.
One advantage of forward sensing for traffic management is that vehicles can come quite close to one another and there are no fixed holding points as in zone control methods. This permits a greater density of vehicles in a given area. A drawback is that it cannot be used where paths are very dense and complex.
Systems can be done are done entirely with one method only, or a system layout can benefit from both types of traffic control known as a combination control traffic management system. There may be long runs in which forward sensing control can be used reducing the expense of the traffic management system significantly. In the remaining areas of the path where there are divergence path and convergence path areas, a form of zone control would be used. The vehicles would respond to both types of traffic control depending upon the area in which they are operating.
AGVS load transfer can be accomplished in many different ways:
Manual Load Transfer
Manual methods of load transfer include manually coupling and uncoupling towed trailers, loading and unloading by fork lift and manually loading and unloading AGVS vehicles.
Manual methods can include uncoupling trailers and moving the trailer off the AGVS system to given work stations or it can involve simple roller bed transfer from the AGVS to fixed roller stations by manually pushing the load off the vehicles. The most popular conventional AGVS approach uses fork trucks to manually load and unload the trailers towed by an AGVS vehicle. Fork trucks are also used to directly load and unload AGVS unit load carriers.
Automatic Couple and Uncouple
Automatic uncouple load transfer methods are relatively easy to implement. The vehicle usually stops on a side spur and automatically uncouples trailers, which it is pulling. The AGV then pulls forward and waits or proceeds to another location to receive a new string of trailers to tow. Typically, an operator will manually couple the new trailers to the AGV hugger.
Automatic couple capability is a more challenging capability than automatic uncouple. First, the carts/trailers to which the AGV is coupling must be in a precise position so that the AGV hitch and the trailer hitch engage properly. Second, controls must be provided to insure trailers left on the path are identified so that other AGVs do not collide with them. Third, sophisticated AGV management software and controls need to be provided to support an automatic coupling operation. For example, an operator must ‘call’ from a terminal when they have some carts ready for an AGV to pick up. This requires a software management program to decide where the closest AGV is that is 1) not currently towing carts and 2) is available for a new task. If operators move the waiting carts or override the automatic dispatching of AGVs to pick up carts, then the system will not function well. So, increased operator and supervisory discipline needs to be provided.
Power Roller, Belt, Chain
Powered roller belt or chain transfer for guided vehicles has been a standard technique for many years. Unit load carriers can be equipped with these power decks so that they may automatically transfer loads to and from fixed stations. In certain cases, under light load conditions, parasitic drives can be used which allow the AGV unit load carrier vehicle to interface with a non-powered roller stand. Most applications involve the AGV vehicle dropping off to a powered or gravity type conveyor. A roller deck transfer method can be used on AGVS towing systems where the towed trailers are equipped with powered transfer deck mechanisms. In all cases, the vehicles must precisely align with the given transfer station before the load transfer can be achieved. Normal stopping tolerances are + or -1/2″ which is well within the tolerances required at conveyor transfer stations.
Whenever a vehicle is automatically transferring a load, a method or “handshake” logic must be employed. Handshake logic is the method by which the vehicle checks to make sure a load stand is empty before it automatically transfers a load onto the stand. Or when a vehicle comes to pickup a load it must make sure that there is a load on the stand for pick up before the load transfer occurs. The handshake signals between the vehicle and the load stand allow transfer drives to be activated simultaneously to achieve the load transfer. The signals also turn off the drives when the transfer is complete. The handshake method usually involve sensors onboard the vehicle which interface with sensors on the stand that exchange signals when a load transfer is about to occur. If the proper signals are not exchanged, then the vehicle will not attempt the transfer. It can blow its horn or abort the mission and proceed to another destination.
There are three general power lift/lower methods of load transfer:
- Pallet fork lift/lower
- Unit load lift/lower
- Fork truck lift/lower
In the pallet truck lift/lower method the AGVS pallet truck can pick up loads or drop them off directly to the floor. The vehicle must pull straight ahead after depositing a load on the floor so that the pallet forks clear the load before it reenters the main guide path.
The lift/lower unit load carrier can interface with special ‘horseshoe’ conveyor load stands or with fixed load pickup deposit stations. The vehicle approaches a load transfer area and makes a right angle turn into a horseshoe shaped load station. It decelerates and pulls into the load station where it accurately stops and lowers or lifts its load deck. The load is then transferred to/from the load stand and the vehicle reverses its direction, and proceeds to its next assignment.
The lift/lower fork truck is a concept that allows the guided vehicle to pick loads up off the floor and deposit them on the load stands or in racks. This gives the AGVS system more versatility in larger systems where not all of the conveyors or load stands are at the same height. The vehicle can service an infinite number of different heights within a given lifting range.
A less frequently used method of automatic load transfer is the push or pull type. Here a vehicle with a non- powered deck or trailers positions itself in front of of fixed stations which is equipped with an automatic push or pull mechanism. This mechanism will reach out to the load and push or pull it off the vehicle or trailers. This permits the vehicles to be extremely simple. This method is quite useful if the automatic load transfer is centralized because the cost of multiple load transfer points can be prohibitive. A good example of this approach would be a central storage area supporting manufacturing. Parts baskets can be loaded onto an AGVS train by a powered shuttle automatically. The train proceeds into the manufacturing area. The parts baskets are unloaded manually or by fork truck and are moved to machining centers. Parts baskets are returned to the central storage area on the AGVS train and unloaded automatically by the powered shuttle. Another possible permutation would be to have a powered device load or unload the AGV at certain locations and employ a manual approach to transferring loads at other locations in the system.
System Management: System Monitoring
AGVS system monitoring is an important consideration in many systems. Simple systems do not require extensive monitoring or control. Sophisticated systems benefit greatly from monitoring. When a sophisticated system is installed, usually there is a high degree of automation and throughput required. A breakdown or slowdown in the system could cause serious problems if not detected immediately. There are three approaches to monitoring:
- Locator Panel
- CRT Color Graphics Display
- Central Logging and Report
A simple monitoring system for AGVS systems can be a locator panel which merely indicates if a vehicle is in a given area of the guide path. It does not identify the vehicle specifically or its condition. A light next to the area a vehicle occupies illuminates to indicate that there is a vehicle in that location. Sometimes a timer is used for each zone to indicate if that zone has been occupied for too long which might indicate that a vehicle problem exists in that location. A CRT color graphics display specifically shows where each vehicle is including its status.
CRT color graphics is usually a real time monitoring type, which can instantly detect a problem, identify specific vehicles, and show the location of the failure on the graphics display. Other useful information includes whether the vehicle is moving or is blocked by the other traffic, whether the vehicle is loaded or empty, if the battery is O.K. and where the vehicle is going. Operators can spot blockages and slowdowns quickly and take corrective action as required.The CRT color monitor can show system condition either in graphic form or in a table form.
The table form lists each vehicle ID, its location, destination, and condition, mode of control, load status, and alarm condition in a column format listing which is continually updated. A central logging and report capability for monitoring an AGVS system is helpful when attempting to develop historical data on the systems performance. Periodically, performance reports can be printed out indicating such things as how long the vehicles were moving, how many loads a given vehicle transported to a given stop station, and where were they taken or even when did a vehicle have a low battery or when did a specific load get picked up and where did it go. Performance data can be quite useful for keeping system efficiency at the highest possible levels.
System Management: Vehicle Dispatch
Onboard dispatch selector involves a control panel on each vehicle, which is used by an operator to dispatch the vehicle to a single, or series of stop stations. He may also select the function he wishes the vehicle to perform at the stop station. This is the most common form of system management and is usually the most flexible type.
An off-board call system can also be used for vehicle management in AGVS systems. These call systems vary in complexity from simple types which involve only a push button at a call location to stop the passing guided vehicle all the way to call terminals which not only can call a specific vehicle, but can also remotely dispatch that vehicle to other destinations without operator interface. Operators can input new destinations into the call box panel at their station, which will communicate those destinations to the guided vehicle after it leaves their stop location. This is useful in systems where load transfer is automatic.
A remote dispatch terminal can be used in systems to give a degree of centralized dispatching to the control of the guided vehicles in the system. The keyboard remote terminal approach allows an operator in a central location to control the individual guided vehicles from a central location. In order to do this the operator must have some visual status of the vehicles’ location and condition so that they might effectively dispatch vehicles to specific areas in the system where they are needed. This is usually a CRT graphic display or locator panel. Many times this approach is used in systems, which can’t justify totally computer-controlled solutions yet, where a high degree of selective movement with automatic transfer is required. For example, if loads are received at a receiving area and taken to selective locations in a storage area with automatic load transfer used by the AGVS vehicles, then a centralized control many be an effective solution. A central operator would use the keyboard to send the vehicles to the receiving area to automatically pickup loads and then dispatch those vehicles to selected storage locations where the loads would be automatically dropped off.
Central computer control for system management is the highest level of control possible. The AGVS vehicles would respond to a central computer’s commands for where they should go and what they should do. This would be automating the previous case and eliminating the central dispatcher completely. Most integrated material handling systems incorporate central computer control for AGVS system management. In smaller dedicated systems such as picking up at production machines (injection molding, stretch wrapping, palletizing, machining, etc.), the AGVs are called to the production machine automatically when a load is ready for pickup. The computer control system can select the closest, available AGV for the task and then dispatch it automatically. Once a load is picked up, the central control system will communicate a destination to the AGV. This level of control permits automatic tracking of every load in the system. Where the volume is high and automatic load transfer occurs, the computer form of system management is very efficient.
It is also possible to mix these various forms of system management together at the same time in the same system. This depends on the type of control offered by particular vendors, but in many cases systems can be operated in several different modes at the same time. This is particularly useful if you have a sophisticated level of operation in your system and wish to have a backup to it in case the main level of operation control fails. For example, if your system is computer controlled and the central computer fails, then resorting to remote terminal control or onboard vehicle control would allow the AGVS system to continue to operate in the automated mode. Not all central computer control systems permit this type of operation but it is very important in certain situations.