Equipment 101: Automatic guided vehicles basics
In addition to choosing the right vehicle for the right job, there are also choices to be made when it comes to AGV guidance, also referred to as navigation systems. Navigation systems can be closed path or open path.
Closed path systems use a predetermined set of rules that define the path plan, explains Bill Torrens, director of sales and marketing at RMT Robotics. But in an open path, the AGV finds the best path. Autonomously, robots adapt to the conditions they see in real time and are able to create a new path if necessary. And, according to Torrens, technology is enabling a higher level of autonomy. The units are capable of making their own decisions because the intelligence is moving from the control platform to the vehicle platform.
Wire-guidance is the simplest form of navigation, designed for a set, predictable path. Torrens likens wire-guidance to a hound dog following a scent trail that has verifications along the way. An RF signal is transmitted from the wire that’s buried in a slot below the floor to a sensor under the vehicle. The sensor detects the signal and adjusts the position of the vehicle to keep it on the path. Because the slot must be cut into the floor, wire-guided systems are most commonly used in applications that require a high degree of accuracy on the path, like an AGV traveling back and forth between two workstations in a congested area.
Magnetic tape and magnetic paint are used to guide vehicles in applications that are relatively simple and where flexibility is paramount. “Magnetic tape allows customers to easily change their guide paths by simply pulling up and reapplying the tape,” says Daifuku Webb’s Carlson.
They are also less expensive than other guidance systems.
Laser-guidance is a non-wire-guided navigation system that uses targets in predictable locations. With more than half of the AGVs using laser-guidance, it’s the most popular navigation system in North America and Western Europe. An on-board laser reflects off targets mounted above the floor on columns, walls, machines or posts that are positioned about 25 feet apart. The system automatically measures the distance and angle of the reflected light to calculate and adjust the AGV’s position on the preplanned guide paths. While the combination of accuracy and flexibility make laser-guidance systems very popular, they can pose challenges for environments with obstacles that can obstruct the laser signal.
Inertial, or gyro, navigation systems are often used in facilities with a significant amount of random floor storage that might interrupt a laser signal. An on-board gyroscope senses very small deviations in the AGV direction or travel, while small magnets or passive RF tags are installed in the floor approximately every 25 feet to serve as position points.
Camera-based imaging is the newest guidance technology, and according to JBT’s Longacre, is becoming increasingly popular. The main reason for its growing popularity isn’t so much for obstacle detection, as for load recognition. For example, a pallet stored by a manual lift truck might not be perfectly aligned in its position, but the AGV’s camera can see the pallet fork pockets and adjust accordingly.
Camera-based imaging is used in several mobile robotic applications. While camera-based imaging requires a lot of processing power for its dynamic route planning, there are no rules. “A mobile robot sees what it sees and can evaluate its environment in real time,” explains Torrens. “It looks for the ability to go around obstacles and can change its mind in a moment’s notice.”