An automated guided vehicle (AGV) is a portable robot that, unlike an autonomous mobile robot (AMR), follows along marked long lines or wires on the floor, or uses radio waves, vision cameras, magnets, or lasers for navigation. They are most often used in industrial applications to transport heavy materials around a large industrial building, such as a factory or warehouse. The application of AGVs broadened during the late 20th century.
Introduction
Automated guided vehicles can tow objects behind them in trailers to which they can autonomously attach. The trailers can be used to move raw materials or finished products. The AGV can also store objects on a bed. The objects can be placed on a set of motorized rollers (conveyor) and then pushed off by reversing them. AGVs are employed in nearly every industry, including pulp, paper, metals, newspaper, and general manufacturing. Transporting materials such as food, linen or medicine in hospitals is also done.
An AGV can also be called a laser guided vehicle (LGV). In Germany the technology is also called Fahrerloses Transportsystem (FTS) and in Sweden förarlösa truckar. Lower cost versions of AGVs are often called Automated Guided Carts (AGCs) and are usually guided by magnetic tape. The term autonomous mobile robot is sometimes used to differentiate the mobile robots that do not rely in their navigation on extra infrastructure in the environment (like magnetic strips or visual markers) from those that do; the latter are then called AGVs.
AGVs are available in a variety of models and can be used to move products on an assembly line, transport goods throughout a plant or warehouse, and deliver loads. The first AGV was brought to market in the 1950s, by Barrett Electronics of Northbrook, Illinois, and at the time it was simply a tow truck that followed a wire in the floor instead of a rail. Out of this technology came a new type of AGV, which follows invisible UV markers on the floor instead of being towed by a chain. The first such system was deployed at the Willis Tower (formerly Sears Tower) in Chicago, Illinois to deliver mail throughout its offices. Over the years the technology has become more sophisticated and today automated vehicles are mainly Laser navigated e.g. LGV (Laser Guided Vehicle). In an automated process, LGVs are programmed to communicate with other robots to ensure product is moved smoothly through the warehouse, whether it is being stored for future use or sent directly to shipping areas. Today, the AGV plays an important role in the design of new factories and warehouses, safely moving goods to their rightful destination.
Navigation
Wired A slot is cut into the floor, and a wire is placed below the surface along the path the AGV is to follow. This wire generates a magnetic field, which is detected by a sensor on the bottom of the AGV, close to the ground. The sensor detects the relative position of the magnetic field generated by the current in the wire. This information is used to regulate the AGV's steering system, ensuring it follows the wire path accurately.AUTOMATED GUIDED VEHICLE SURVEY by ALEXANDER J. TREVOR and HENRIK I. CHRISTENSEN
Optical guidepath
A stripe of ultraviolet reactive material is applied to the floor. This can be paint or UV reactive tape. The AGVs are equipped with ultraviolet light emitters and detectors. The control system for the vehicle identifies the stripe on the floor, and makes the necessary adjustments to the vehicle heading to remain on the guidepath. This navigation method requires less augmentation of the environment than guide wire localization.
Laser target navigation Navigation for AGVs can be achieved using retro-reflective laser guidance. In this system, reflective markers are placed on walls, poles, or fixed machines within the AGV's operational area. The AGV is equipped with a laser scanner that emits laser beams to detect these reflectors. The position of the AGV is determined by measuring the angle and distance to the reflectors within its line of sight. This information is compared to a stored map of the reflector layout in the AGV's memory, allowing the navigation system to triangulate its current position. The AGV then adjusts its steering based on its position relative to the programmed path defined by the reflector layout.
This method also doesn't necessarily restrict the AGVs to fixed paths; they are capable of localization and navigation in any area where two to three reflective markers are visible. Modulated Lasers: Emits a continuous beam of light, providing uninterrupted reflections and high accuracy. A two-dimensional laser scanner is mounted atop each AGV, typically centered along the width axis of the vehicle and towards the front control panel. The scanner emits a modulated laser light, rotating and covering a 360° angle ten or more times per second up to several hundred feet in distance. Pulsed Lasers: This method involves emitting rapid pulses of laser light. This technique is crucial for creating detailed 3D maps of the environment.
Inertial (Gyroscopic) navigation Another form of an AGV guidance is inertial navigation. With inertial guidance, a computer control system directs and assigns tasks to the vehicles. Transponders are embedded in the floor of the work place. The AGV uses these transponders to verify that the vehicle is on course. A gyroscope is able to detect the slightest change in the direction of the vehicle and corrects it in order to keep the AGV on its path. The margin of error for the inertial method is ±1 inch."The Basics of Automated Guided Vehicles" . AGV Systems. Savant. 5 March 2006
Inertial can operate in nearly any environment including tight aisles or extreme temperatures."Comparing Magnetic Navigation to Other Robot Guidance Techniques" Naviq Company, 2024 Inertial navigation can include use of magnets embedded in the floor of the facility that the vehicle can read and follow."Inertial (Magnet)Navigation" Egemin Automation Inc., 2014.
Natural feature (Natural Targeting) navigation Navigation without retrofitting of the workspace is called Natural Features or Natural Targeting Navigation. One method uses one or more range-finding sensors, such as a laser range-finder, as well as gyroscopes or inertial measurement units with Monte-Carlo/Markov localization techniques to understand where it is as it dynamically plans the shortest permitted path to its goal. The advantage of such systems is that they are highly flexible for on-demand delivery to any location. They can handle failure without bringing down the entire manufacturing operation, since AGVs can plan paths around the failed device. They also are quick to install, with less down-time for the factory.
Vision guidance Vision-guided AGVs can be installed with no modifications to the environment or infrastructure. They operate by using cameras to record features along the route, allowing the AGV to replay the route by using the recorded features to navigate. Vision-guided AGVs use Evidence Grid technology, an application of probabilistic volumetric sensing, and was invented and initially developed by Dr. Hans Moravec at Carnegie Mellon University. The Evidence Grid technology uses probabilities of occupancy for each point in space to compensate for the uncertainty in the performance of sensors and in the environment. The primary navigation sensors are specially designed stereo cameras. The vision-guided AGV uses 360-degree images to build a 3D map, which allows the vision-guided AGVs to follow a trained route without human assistance or the addition of special features, landmarks or positioning systems.
Geoguidance A geoguided AGV recognizes its environment to establish its location. Without any infrastructure, the forklift equipped with geoguidance technology detects and identifies columns, racks and walls within the warehouse. Using these fixed references, it can position itself, in real time and determine its route. There are no limitations on distances to cover number of pick-up or drop-off locations. Routes are infinitely modifiable.
Steering control To help an AGV navigate it can use three different steer control systems.AGV Drive and Steering Options Transbotics Corp., 2009 The differential speed control is the most common. In this method there are two independent drive wheels. Each drive is driven at different speeds in order to turn or the same speed to allow the AGV to go forwards or backwards. The AGV turns in a similar fashion to a tank. This method of steering is the simplest as it does not require additional steering motors and mechanism. More often than not, this is seen on an AGV that is used to transport and turn in tight spaces or when the AGV is working near machines. This setup for the wheels is not used in towing applications because the AGV would cause the trailer to jackknife when it turned.
The second type of steering used is steered wheel control AGV. This type of steering can be similar to a car's steering. But this is not very manoeuvrable. It is more common to use a three-wheeled vehicle similar to a conventional three wheeled forklift. The drive wheel is the turning wheel. It is more precise in following the programmed path than the differential speed controlled method. This type of AGV has smoother turning. Steered wheel control AGV can be used in all applications; unlike the differential controlled. Steered wheel control is used for towing and can also at times have an operator control it.
The third type is a combination of differential and steered. Two independent steer/drive motors are placed on diagonal corners of the AGV and swivelling castors are placed on the other corners. It can turn like a car (rotating in an arc) in any direction. It can crab in any direction and it can drive in differential mode in any direction.
Path decision AGVs have to make decisions on path selection. This is done through different methods: frequency select mode (wired navigation only), and path select mode (wireless navigation only) or via a magnetic tape on the floor not only to guide the AGV but also to issue steering commands and speed commands.
Frequency select mode Frequency select mode bases its decision on the frequencies being emitted from the floor. When an AGV approaches a point on the wire which splits the AGV detects the two frequencies and through a table stored in its memory decides on the best path. The different frequencies are required only at the decision point for the AGV. The frequencies can change back to one set signal after this point. This method is not easily expandable and requires extra cutting, which makes the process more expensive.
Path select mode An AGV using the path select mode chooses a path based on preprogrammed paths. It uses the measurements taken from the sensors and compares them to values given to them by programmers. When an AGV approaches a decision point it only has to decide whether to follow path 1, 2, 3, etc. This decision is rather simple since it already knows its path from its programming. This method can increase the cost of an AGV because it is required to have a team of programmers to program the AGV with the correct paths and change the paths when necessary. This method is easy to change and set up.
Magnetic tape mode The magnetic tape is laid on the surface of the floor or buried in a 10mm channel; not only does it provide the path for the AGV to follow but also strips of the tape in different combinations of polarity, sequence, and distance laid alongside the track tell the AGV to change lane, speed up, slow down, and stop.
Traffic control