The Basics of Automatic Guided Vehicle Systems

Traffic Management: Zone Control

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 fool 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 may 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.