The Basics of Automatic Guided Vehicle Systems

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 infloor 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.

Steered wheel type vehicles have excellent guidance tolerance along the guide path. Differential steer vehicles normally have less tracking tolerance and therefore require external guides or positioning devices at load stations when used in automatic load transfer applications.