Non-wire guidance systems take control of AGVs
Automatic guided vehicles (AGVs) can still be routed by in-floor wires, but the vast majority of users now opt for the flexibility of proven alternative guidance systems.
By Gary Forger -- Modern Materials Handling, 8/1/1998
Moving loads of materials from point A to point B is what an automatic guided vehicle (AGV) is all about. As a result, the guidance system that directs the vehicle between A, B, and all other points plays a critical role in the ultimate efficiency of any AGV.Until the past couple of years, in-floor wire systems were generally the most preferred guidance technology. However, non-wire guidance systems have now come into their own and are specified for the vast majority of new AGV systems being installed by industry.
Two major trends are behind this shift.
To begin, several AGV suppliers now offer non-wire guidance technology. That was not always the case. And as more companies offered non-wire guidance, the technology itself improved and costs dropped. The box on this page details the leading suppliers.
Meanwhile, U.S. industry has placed a steadily increasing value on flexibility of operations to meet changing business conditions. This is especially true of automated materials handling systems such as AGVs.
With the ability to alter an AGV route almost on command, a warehouse manager, for instance, is able to turn what used to be hard, fixed automation into a flexible system. That puts a premium on non-wire systems that can change AGV routes in a matter of hours using software and without input from suppliers. Re-routing wire guidance systems, on the other hand, requires outside contractors to cut a new path in the concrete floor for the wire, causing extensive disruption of operations.
Even at the time of original installation, non-wire systems are generally regarded as having other advantages too. They are typically less costly and time consuming to install than wire systems, making start-up that much easier.
There are three common families of non-wire guidance systems for AGVs today-optical, laser, and inertial.
Optical systems were the original alternative to non-wire technology. Today, there are three different variations of optical systems.
Two are similar. Either a magnetic tape or chemical path is placed on the floor, creating a physical path that the AGV will follow. An optical sensor on the vehicle constantly seeks the path, ensuring that the track is followed. If, for some reason, the AGV veers from the path, a course correction is made with some re-steering. New paths are created simply by removing the existing path and relaying a new one for the vehicles to follow.
A third type of optical guidance uses a floor-mounted checkerboard pattern of light and dark squares to guide the AGV. Using a distance encoder in the wheels and the checkerboard squares as reference points, an on-board computer determines the actual position of the vehicle against the expected position, making course corrections as needed. Changing the path is primarily a software change, leaving the checkerboard pattern as it was originally installed on the floor.
Unlike optical systems, a laser guidance system does not rely on floor-based reference points. As the artwork shows, an on-board laser searches for targets positioned on the walls of the facility. Using the information from two or three targets, the on-board computer triangulates to determine the AGV's actual position and compares it to the expected position at that point in its route. Adjustments are made as needed. Changes in the AGV route are done with software.
An inertial guidance system combines an on-board navigational gyroscope and encoded electronic signals to compare actual and expected positions (see artwork). Navigation computers that make the computations can be either on-board or at a central control room that communicates with the AGV by radio frequency. Route changes are made by software and uploaded to the control system.
Non-wire guidance systems have clearly opened the door to increased flexibility for AGVs. And as the systems continue to advance, the level of flexibility seems sure to increase still further.
Leading advantages of non-wire guidance systems
1. Minimum installation time and costs for initial system startup
2. Maximum rerouting flexibility
3. Route changes can be made without going back to the supplier
4. No floor modifications required to reroute vehicles
5. No disruption of standard operations to accommodate a change in guidance path
6. Proven reliability...these systems are not bleeding edge
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