Automated guided vehicle

The automated guided vehicle or automatic guided vehicle (AGV) is a mobile robot used in industrial applications to move materials around a manufacturing facility or a warehouse.

Introduction

Automated guided vehicles (AGVs) help to reduce costs of manufacturing and increase efficiency in a manufacturing system. [http://www.agvsystems.com/basics/vehicle.htm “The Basics of Automated Guided Vehicles”] . AGV Systems. Siemens. 5 March 2006] AGVs can tow objects behind them in small trailers which they can autonomously hook up to. These trailers can be used to move raw materials into line to get them ready to be manufactured. The AGV can also store objects on a bed. The objects can be placed on a set of motorized treads and then pushed off by reversing them. Some AGVs use fork lifts to lift objects for storage. Transporting materials such as medicine in a hospital situation is also done.

An AGV can also be called a laser guided vehicle (LGV) or self-guided vehicle (SGV). In Germany the technology is also called "Fahrerlose Transportsysteme" (FTS) and in Sweden "förarlösa truckar".

The first AGV was brought to market in the 1950s, by Barrett Electronics of Northbrook, Illinois, (now Savant Automation of Walker, Michigan) and at the time it was simply a tow truck that followed a wire in the floor instead of a rail. 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 LGV plays an important role in the design of new factories and warehouses, safely moving goods to their rightful destinations.

Flexible manufacturing system

To begin to understand AGV it is necessary to understand the fundamentals of flexible manufacturing systems (FMS). FMS is a means by which to manufacture a product. FMS is more of a philosophy rather than a tangible item. [http://www.uky.edu/~dsianita/611/fms.html “Flexible Manufacturing Systems”] . University of Kentucky. 5 March 2006] FMS is the idea that faster is better and uses machines to produce their products. Rather than using humans to perform repetitive tasks a machine is used to perform that task 24 hours a day. FMS uses computer numerical controlled machines (CNC) to form a work cell. Each cell performs a specific task to assist in the manufacturing of a product. Although FMS is fast and efficient it is not cheap as it requires a lot of expensive machines in order to work. Typically, it costs millions of dollars to introduce an FMS into a factory. Rather than using a complete FMS, most companies use part of an FMS called a flexible manufacturing cell. This is used to produce part of a product by machine and maybe part by other methods. Often one or more AGV’s are used in FMS to connect work cells together.

Navigation

AGVs in FMS are used to transport an object from point A to point B. AGVs navigate manufacturing areas with sensors. There are two main sensors AGVs use for navigation, a wired and a wireless sensor.

Wired

The wired sensor is placed on the bottom of the robot and is placed facing the ground. A slot is cut in the ground and a wire is placed approximately 1 inch below the ground. The sensor detects the radio frequency being transmitted from the wire and follows it.

Guide Tape

Many light duty AGVs (some known as automated guided carts or AGCs) use tape for the guide path. The tapes can be one of two styles: magnetic or colored. The AGV is fitted with the appropriate guide sensor to follow the path of the tape.One major advantage of tape over wired guidance is that it can be easily removed and relocated if the course needs to change. It also does not involve the expense of cutting the factory or warehouse floor for the entire travel route. Additionally, it is considered a "passive" system since it does not require the guide medium to be energized as wire does. Colored tape is initially less expensive, but lacks the advantage of being embedded in high traffic areas where the tape may become damaged or dirty.

Laser Target Navigation

The wireless navigation is done by mounting retroreflective tape on walls, poles or machines. The AGV carrys a laser transmitter and receiver on a rotating turret. The laser is sent off then received again the angle and (sometimes) distance are automatically calculated and stored into the AGV’s memory. The AGV has reflector map stored in memory and can correct its position based on errors between the expected and received measurements. [http://www.robosoft.fr/SHEET/02Local/1005NAV/NAV200.html “Nav 200 Absolute Navigation System”] . Mobile Platforms. 5 March 2006 ] It can then navigate to a destination target using the constantly updating position.

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 inertial is method is ±1 inch.

Inertial can operate in nearly any environment including tight aisles or extreme temperatures and has a longer lifespan than other guidance options. [ [http://www.jervisbwebb.com/Products/guidance_options.aspx?pid=308&qs=1_6_ "Guidance options for AGVs"] Jervis B. Webb Company, 2007.]

Natural Features Navigation

Navigation without retrofitting of the workspace is called Natural Features Navigation. One method uses one or more range-finding sensors, such as a laser range-finder, as well as gyroscopes and/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 and can handle failure without bringing down the entire manufacturing operation, since AGVs can plan paths around the failed device. [ [http://www.mobilerobots.com/MRinsideSpecs.html Specifications for Platforms Deploying MOBILEROBOTSinside Natural Features Navigation] ]

Steering control

To help and AGV navigate it can use two different steer control systems. The differential speed control is the most common. In this method there are two sets of wheels being driven. Each set is connected to a common drive train. These drive trains are driven at different speeds in order to turn or the same speed to allow the AGV to go forwards and/or backwards. The AGV turns in a similar fashion to a tank. This method of steering is good in the sense that it is easy to maneuver in small spaces. 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 other type of steering used is steered wheel control AGV. This type of steering is similar to a cars steering. It is more precise in following the wire program than the differential speed controlled method. This type of AGV has smoother turning but cannot make sharp turns in tight spots. 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.

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 guide cutting meaning more money.

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 sort strips of the tape in different combos of the strip tell the AGV to change lane and also speed up slow down and stop with north and south magnetic combos, this is used by TOYOTA USA and TOYOTA JAPAN.

Traffic Control

Flexible manufacturing systems containing more than one AGV may require it to have traffic control so the AGV’s will not run into one another. Methods include zone control, forward sensing control, and combination control each method has its advantages and disadvantages.

Zone control

Zone control is the favorite of most environments because it is simple to install and easy to expand. Zone control uses a wireless transmitter to transmit a signal in a fixed area. Each AGV contains a sensing device to receive this signal and transmit back to the transmitter. If the area is clear the signal is set at “clear” allowing any AGV to enter and pass through the area. When an AGV is in the area the “stop” signal is sent and all AGV attempting to enter the area stop and wait for their turn. Once the AGV in the zone has moved out beyond the zone the “clear” signal is sent to one of the waiting AGVs. Another way to set up zone control traffic management is to equip each individual robot with its own small transmitter/receiver. The individual AGV then sends its own “do not enter” message to all the AGVs getting to close to its zone in the area. A problem with this method is if one zone goes down all the AGV’s are at risk to collide with any other AGV. Zone control is a cost efficient way to control the AGV in an area.

Forward sensing control

Forward sensing control uses collision avoidance sensors to avoid collisions with other AGV in the area. These sensors include: sonic, which work like radar; optical, which uses an infrared sensor; and bumper, physical contact sensor. Most AGVs are equipped with a bumper sensor of some sort as a fail safe. Sonic sensors send a “chirp” or high frequency signal out and then wait for a reply from the outline of the reply the AGV can determine if an object is ahead of it and take the necessary actions to avoid collision. [http://www.cs.bham.ac.uk/resources/courses/robotics/halloffame/1999/team2/Sonar1.html “Sonar sensor and mounting”] . University of Birmingham. 5 March 2006] The optical uses an infrared transmitter/receiver and sends an infrared signal which then gets reflected back; working on a similar concept as the sonic sensor. The problems with these are they can only protect the AGV from so many sides. They are relatively hard to install and work with as well.

Combination control

Combination control sensing is using collision avoidance sensors as well as the zone control sensors. The combination of the two helps to prevent collisions in any situation. For normal operation the zone control is used with the collision avoidance as a fail safe. For example, if the zone control system is down, the collision avoidance system would prevent the AGV from colliding..

System Management

Industries with AGVs need to have some sort of control over the AGVs. There are three main ways to control the AGV: locator panel, CRT color graphics display, and central logging and report.

A locator panel is a simple panel used to see which area the AGV is in. If the AGV is in one area for too long, it could mean it is stuck or broken down. CRT color graphics display shows real time where each vehicle is. It also gives a status of the AGV, its battery voltage, unique identifier, and can show blocked spots. Central logging used to keep track of the history of all the AGVs in the system. Central logging stores all the data and history from these vehicles which can be printed out for technical support or logged to check for up time.

AGV is a system often used in FMS to keep up, transport, and connect smaller subsystems into one large production unit. AGVs employ a lot of technology to ensure they do not hit one another and make sure they get to their destination. Loading and transportation of materials from one area to another is the main task of the AGV. AGV require a lot of money to get started with, but they do their jobs with high efficiency. In places such as Japan automation has increased and is now considered to be twice as efficient as factories in America. For a huge initial cost the total cost over time decreases

Vehicle Types

*"AGVS Towing Vehicles" were the first type introduced and are still a very popular type today. Towing vehicles can pull a multitude of trailer types and have capacities ranging from 8,000 pounds to 60,000 pounds.

*"AGVS Unit Load Vehicles" are equipped with decks, which permit unit load transportation and often automatic load transfer. The decks can either be lift and lower type, powered or non-powered roller, chain or belt decks or custom decks with multiple compartments.

*"AGVS Pallet Trucks" are designed to transport palletized loads to and form floor level; eliminating the need for fixed load stands.

*"AGVS Fork Truck" has the ability to service loads both at floor level and on stands. In some cases these vehicles can also stack loads in rack.

*"Light Load AGVS" are vehicles which have capacities in the neighborhood of 500 pounds or less and are used to transport small parts, baskets, or other light loads though a light manufacturing environment. They are designed to operate in areas with limited space.

*"AGVS Assembly Line Vehicles" are an adaptation of the light load AGVS for applications involving serial assembly processes.

Battery Charging

AGVs utilize a number of battery charging options. Each option is dependent on the users preference. The most commonly used battery charging technologies are "Battery Swap", "Automatic/Opportunity Charging", and "Automatic Battery Swap". [http://www.egeminusa.com/pages/agvs/agvs_battery_charging.html “Battery Charging Systems for Automated Guided Vehicles”] . AGV Battery Charging Systems. Egemin Automation Inc. 26 October 2006]

Battery Swap

"Battery swap technology" requires an operator to manually remove the discharged battery from the AGV and place a fully charged battery in its place approximately 8 - 12 hours (about one shift) of AGVs operation. 5 - 10 minutes is required to perform this with each AGV in the fleet.

Automatic / Opportunity Charging

"Automatic and opportunity battery charging" allows for continuous operation. On average an AGV charges for 12 minutes every hour for automatic charging and no manual intervention is required. If opportunity is being utilized the AGV will receive a charge whenever the opportunity arises.When a battery pack gets to a predetermined level the AGV will finish the current job that it has been assigned before it goes to the charging station.

Automatic Battery Swap

"Automatic battery swap" is an alternative to manual battery swap. It requires an additional piece of automation machinery, an automatic battery changer, to the overall AGV system. AGVs will pull up to the battery swap station and have their batteries automatically replaced with fully charged batteries. The automatic battery changer then places the removed batteries into a charging slot for automatic recharging. The automatic battery changer keeps track of the batteries in the system and pulls them only when they are fully charged.

References

:* [http://www.hksystems.com/resources/educational/ES_AGV_2006.pdf "The Appropriate Application of Automated Guided Vehicles "] . HK Systems, Inc. 01 October 2007:* [http://www.swisslog.com/hcs-index/hcs-systems/hcs-agv/hcs-agvcomponents.htm “Automatic Guided Vehicle-Description”] . Swisslog. 5 March 2006:* [http://www.salmoiraghi-spa-monza.com/textureplants.cfm?menu3=2 “The Salmoiraghi Approach.”] . SALMOIRAGHI Automatic Handling. 5 March 2006:* [http://www.elettric80.com/products-applications/AGuidetoRoboticLogistics.aspx “A Guide to Robotic Logistics – LGVs (AGVs)”] . Elettric 80 S.p.A. 26 April 2008