Automated Guided Vehicle

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The Automated Guided Vehicle (AGV) is a mobile robot used highly in industrial applications to move materials from point to point.

Contents 1 Introduction 1.1 Flexible manufacturing system 2 Navigation 2.1 Wired 2.2 Target Navigation 2.3 Gyroscopic Navigation 2.4 Steering control 3 Path Decision 3.1 Frequency select mode 3.2 Path select mode 3.3 Magnetic Tape mode 4 Traffic Control 4.1 Zone control 4.2 Forward sensing control 4.3 Combination control 5 System Management 6 Vehicle Types 7 Battery Charging 7.1 Battery Swap 7.2 Automatic / Opportunity Charging 7.3 Automatic Battery Swap 8 References

[edit] Introduction

Automated guided vehicles (AGVs) help to reduce costs of manufacturing and increase efficiency in a manufacturing system.^[1] 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.

[edit] 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.^[2] 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.^[2] 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.^[2] 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 time one or more AGV’s are used in FMS to connect work cells together.

[edit] 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.^[1]

[edit] Wired

The wired sensor is placed on the bottom of the robot and is placed facing the ground. A wire is placed approximately 1 inch below the ground and a slot is cut in the ground to expose the wire. The sensor senses the radio frequency being transmitted from the wire and follows it. If the AGV is put off course the AGV will correct itself and continue to follow the wire.

[edit] Target Navigation

The wireless navigation is done by mounting reflective targets on poles or machines. The AGV carry a laser transmitter and receiver on a rotating turret. The laser is sent off then received again the angle and distance are automatically calculated and stored into the AGV’s memory. The AGV has a grid stored in its memory and can find its location by reflecting the laser off the targets.^[3] It can then navigate to a destination target through the grid and the constantly updating sensor.

[edit] Gyroscopic Navigation

Another form of a wireless AGV is gyroscope navigation. This method involves using a gyroscope to detect the slightest change in the direction of the robot. Magnets are installed in the ground of the work place and the AGV detects these magnets with a sensor.^[1] The magnets give the robot an insurance factor that it is on course. The gyro detects the slightest turn and corrects it in order to keep the AGV on its path. The margin of error for the magnet gyro method is ±1 inch.^[1]

[edit] 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.^[1] Steered wheel control is used for towing and can also at times have an operator control it.

[edit] 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 (CREFORM in Germany).

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] Zone control

Zone control is the favorite of most environments because it is simple to install and easy to expand.^[1] 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.

[edit] 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.^[4] 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.

[edit] 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.

[edit] 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.^[1]

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

[edit] 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.

[edit] 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.^[5]

[edit] Battery Swap

"Battery swap technology"^[5] 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.

[edit] Automatic / Opportunity Charging

"Automatic and opportunity battery charging"^[5] 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.

[edit] Automatic Battery Swap

"Automatic battery swap"^[5] 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.

[edit] References

1. ^ ^{a b c d e f g} “The Basics of Automated Guided Vehicles”. AGV Systems. Siemens. 5 March 2006
2. ^ ^{a b c} “Flexible Manufacturing Systems”. University of Kentucky. 5 March 2006
3. ^ “Nav 200 Absolute Navigation System”. Mobile Platforms. 5 March 2006
4. ^ “Sonar sensor and mounting”. University of Birmington. 5 March 2006
5. ^ ^{a b c d} “Battery Charging Systems for Automated Guided Vehicles”. AGV Battery Charging Systems. Egemin Automation Inc. 26 October 2006

• "AGVs Provide Fully Automated Transport of Your Products". Egemin Automation Inc. 22 October 2006
• “Automatic Guided Vehicle-Description”. Swisslog. 5 March 2006
• “The Salmoiraghi Approach.”. SALMOIRAGHI Automatic Handling. 5 March 2006