The Basics of Automatic Guided Vehicle Systems

Routing

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 I 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 it ultimate distination.

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 aver 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.