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How automatic guided vehicles link shop operations: employed for many years in European manufacturing plants, automatic guided vehicles are now showing up in US plants - not only in flexible manufacturing systems, but also in assembly and plant-integration systems.

How automatic guided vehicles link shop operations

Part I--The technology

Two General Motors plants in Lansing, MI, are the sites of new, automated material-handling installations that may well prove to be landmarks in manufacturing in the US.

At an Oldsmobile plant, two computer-controlled, automatic guided vehicle systems (AGVS)--being provided by Eaton-Kenway, Salt Lake City, UT-- will involve a total of about 130 wire-guided vehicles.

The first system will serve an engine-assembly department. The second will be employed in assembly, in a line where engines are mounted into chassis.

A second General Motors Lansing plant, this one operated by Fisher Body, will employ an AGVS to carry passenger cars through various stages of assembly. This system is being provided by the Automated Systems Div, Volvo of America, Sterling Heights, MI.

These installations are significant because they are the first of their kind in the US automobile industry. Never before have American automakers used wire-guided vehicles as moving assembly platforms.

The systems going into GM's Lansing plants also throw a spotlight onto a major type of workpiece and tooling transport that has been relatively little known and underutilized in this country. Neglect of the AGVS appears to be coming to an end, though; US system suppliers forecast annual dollar growth rates topping 30 percent over the next several years.

Wire guidance of vehicles isn't new, of course. Conceived in the early 1950s, AGV systems were first applied in warehouses and distribution centers. These early systems employed electromagnetic guidance on driverless tractors that towed trailers loaded with palletized goods.

During the 1960s and 1970s, application of transistor and then integrated circuit controls greatly expanded the potential for AGVS. More recently, onboard microprocessors and centralized computer controls have made the AGVS a powerful tool for factory automation and integration.

Two types of guidance

AGV systems employ unmanned vehicles powered by onboard batteries. The vehicles are quipped to automatically follow a guidepath on the building floor or outdoor pavement. With the aid of onboard controls, the vehicles can be routed and positioned from a remote location, either by direct human control or by computer program.

Two types of guidepath are in use today: one, the electromagnetic or buried-wire path, and two, the reflective or optical path. Used mainly for light-load installations, the latter employs reflective tape or fluorescing paint applied as a stripe on the floor.

A light source on the vehicle shines a beam onto the floor; light reflected from the guidepath stripe is picked up and fed to a servosystem that steers the vehicle and keeps it on track. AGV systems utilizing this type of guidance are supplied by Automated Systems Div of Bell & Howell, Zeeland, MI.

Most AGV systems operating today employ the electromagnetic or buried-wire type of guidepath. For this kind of guidance, the installer cuts a slot in the floor or pavement surface, and lays one or more wires in the slot (Figure 2).

Usually the slot is 1/8 to 3/8 wide and 7/8 to 1 1/4 deep; the wires are common 16 to 18 gage stranded with PVC coating. After emplacing the wires, the installer covers them with grout or epoxy filler to the level of the surface. Sometimes a layer of flexible rubber or neoprene is installed between the wires and the filler.

The low-frequency, low-amplitude alternating current that flows through the buried wire (or one of the multiple wires) generates a magnetic field (Figure 3). A two-coil sensor assembly on the lower front of the vehicle picks up signals for steering, speed, and start-stop control. These signals go to the vehicle's control unit and steering servos (Figure 4).

AGV system suppliers make use of two wire-guidance techniques. In one, the vehicle control unit meassures the amplitude of the signal in each coil. If the amplitudes differ, servo correction is automatically applied to the steering wheel or wheels, and the vehicle comes back on track.

In the other technique, the vehicle control unit detects and measures any vector phase picked up by one of the sensor coils. When the vehicle is on course over the wire, there is no vector and hence a "zero signal.' If the vehicle strays left of the wire, a positive vector-phase signal is generated; if right of the wire, a negative vector-phase signal results.

Also, if the vehicle has strayed either right or left of the guidepath, the amplitude of the signal is weaker than when the vehicle is on course. Both the amplitude and the vector-phase signals are processed by the vehicle control unit to apply corrective steering by the drive servos and motors.

Power onboard

Automatic guided vehicles draw their power from conventional lead-acid batteries carried within the vehicle (Figure 4). Ratings of the batteries range from 24 V, 150 A/hr to 48 V, 600 A/hr.

Usually the battery pack is sized for one shift or 8 hr of operation. Where travel is intermittent and the loads are light, the vehicles may be able to run 16 hr before recharged batteries are needed.

Most vehicles are designed so that human operators can quickly slide out the spent batteries and replace them with charged units. One supplier, Renault Industries, offers a battery-changing station that is totally automatic.

For assembly applications in which the vehicles stop at various stations, the vehicles may be equipped for automatic battery charging. In this configuration, when the vehicle pulls into the station and stops, contact is made automatically with a 24-V bus, and the central control computer turns on the charging current. As soon as the vehicle starts moving again, the computer shuts off the current.

Vehicle designs

AGV systems are being employed in manufacturing worldwide, not only for assembly but also for factory integration and in flexible manufacturing systems (FMS). Loads being transported include automobile, truck, and tractor bodies and chassis; engines (Figure 6), transmissions, and gearboxes; racks of oriented tools for automatic toolchangers (Figure 5); and tubs, both sheet metal and wire (Figure 7), for unoriented parts.

Also being transported are boxes and pallet loads of parts, oriented and unoriented (Figure 8), for loading and unloading by human operators, and pallet loads and trays of oriented parts for handling by robots.

Tractor trains can operate not only within a plant but also outdoors between plants. These trains are especially useful where loads are too bulky or heavy for forklift trucks, or where volume is too low to justify conveyors. Trains are also a good choice for facilities in which vehicles must negotiate gentle inclines or declines.

To accommodate the numerous kinds of loads, AGVS suppliers have designed several basic vehicle configurations plus a wide variety of fixturing, load/unload mechanisms, and pickup and drop-off station hardware. Most vehicles going into AGVS for manufacturing are customized for the loads in the particular installation.

Generally speaking, the vehicles are classified into four major categories: towing vehicles, unit-load transporters, pallet trucks, and light-load transporters. (There is also another type, the wire-guided stock selector. The activities of this vehicle are usually confined to storage-rack areas, however, so it is not considered here as a manufacturing transport vehicle.)

Towing vehicles--Also called driverless tractors, these vehicles pull trailers on which the loads are carried. A tractor can pull two to five trailers at speeds to 260 fpm. Total load capacity of a tractor train may range up to-50,000 lb. Rolling resistance of the towed trailers is about 2 percent of the total load.

Unit-load transporters--These vehicles carry their loads on their decks. Load capacities range from 1000 lb to 20,000 lb, with most loads falling in the 4000-lb to 8000-lb range. The vehicles can travel at speeds to 260 fpm.

Unit-load transporters are being provided with many different kinds of automatic load/unload mechanisms. One common type is a laterally moving, bidirectional, powered roller conveyor that can unload to or receive from a fixed, floor-mounted conveyor. Another type is an unpowered roller conveyor that is loaded or unloaded by pneumatic or hydraulic rams or push-pull mechanisms.

Still another common mechanism is a vertically mobile or elevator deck (Figure 1). This enables the vehicle--with deck and load raised--to drive into a load/unload stand, lower the deck, and drive off unloaded. Ideal for palletized loads, the elevator deck is being applied in flexible manufacturing systems and in plant-integration systems.

Unit-load transporters are also being equipped with special fixturing for assembly of vehicles, chassis, engines, and other products (Figure 9). In some cases, the fixturing is powered to provide vertical or lateral movement of the entire workpiece or one of its components, such as the hood or trunk lid of an auto body.

These vehicles may be unidirectional or bidirectional. In the later case, they can travel in either direction on a guidepath, or pull into or out of dead-end spurs.

Pallet trucks--Like unit-load transporters, pallet trucks carry unit loads directly on the vehicle. In this design, however, the load is carried at floor level on movable forks or on a platform at the front of the vehicle.

Capacities of pallet trucks usually range between 4000 lb and 6000 lb. Speeds up to 260 fpm are attainable.

Until recently, most pallet trucks were of the "stop-and-drop' variety. This type of vehicle pulls onto a loop or dead-end spur, drops the load on the floor (or picks up a load), then pulls back onto the main guidepath.

Within the past 12 months, AGVS suppliers have been introducing an interesting variant, a hybrid between the stop and drop and the automatic stock selector (Figure 10). This new type of vehicle, called the turret truck, can raise the load to several feet above the floor. The turret mechanism can also swing the load through 180 degrees without vehicle movement, and can extend or retract the forks under remote control.

This capability promises to make the turret truck highly useful in situations requiring side loading and unloading at different heights. The pickup and drop-off stations to be used will include load stands, machine-tool pallet changers and indexing tables, conveyor-loading mechanisms, and truck docks. Turret trucks will also be able to serve low storage racks in the shop area.

The special advantages of pallet trucks, both stop-and-drop and turret types, are their ability to move loads at floor level and to pick up or deposit loads without the need for stands of push-pull mechanisms.

Light-load transporters--These are designed to carry relatively small, light loads onboard the vehicle itself. Utilizing optical rather than buried-wire guidance, light-load transporters usually carry less than 1000 lb and travel at speeds to 100 fpm. Applications in manufacturing include electronic assembly and light mechanical assembly.

Control options

There are three major types of control for AGVS: manual, remote dispatch, and computer.

Manual control--With this type of control, the vehicles are equipped with an onboard control panel having input devices such as push buttons, toggle switches, or a keyboard. The operator loads or unloads the vehicle or trailers, then inputs a destination on the vehicle panel. The vehicle travels to the destination, stops, and waits for further input from another operator.

Manual control is the simplest and least expensive of the three control options. A potential drawback, though, is the total dependence on humans for proper loading, unloading, and command input.

Remote-dispatch control--In this option, the human operators do not interface directly with the vehicle. Instead, they make use of electronic devices called remote-dispatch terminals. The operators make inputs at the terminals, which relay traffic and load/unload signals through the guidewires to the vehicles.

Most systems utilizing remote-dispatch control also have provisions for automatic loading and unloading of the vehicles. This methods of control increases system utilization over manual control, but is still prone to operator error. Moreover, empty vehicles must continuously circulate on the guidepath, searching for their next assignments.

Computer control--With this type of control (Figure 11), all the vehicles as well as their load/unload mechanisms are under direct control of a central computer. This can be linked not only with the AGVS line driver but also with the plant host computer, monitoring devices, fixed or mobile call systems, and various data-entry terminals.

The central control computer can be programmed to interrogate the vehicles regularly, obtaining real-time data on vehicle identity, location, and load status. The central computer also watches for nonstandard conditions such as blocked path or interrupted signal, and can then command appropriate actions. In addition, the central computer serves to track loads, and assists the host in updating inventory files and generating management reports.

Computer-graphic aids are often used with this type of control. Video (CRT) screens or electronic display-boards enable human operators to easily monitor, in real time, the locations, movements, and status of all vehicles in the system.

Operators can use either fixed or mobile call stations to enter requests to the central computer for vehicles or materials. The mobile call stations employ FM radio links with the central computer.

Computer control is more expensive than either manual or remote-dispatch control. At the same time, though, this type of control offers the maximum in system utilization, flexibility, and accuracy. Once the exception, computer control for AGVS is fast becoming the norm.

Photo: 1. A wire-guided unit-load transporter, equipped with a deck that can raise and lower the load, brings a workpiece to a machining center. The vehicle will drive under the drop-off station at right, lower the deck, and drive on through the other end, leaving the load on the station. At far left in the background is another vehicle that is bringing a rack of tooling for the machining center. The vehicles in this demonstration FMS are provided by Eaton-Kenway.

Photo: 2. A workman lays guidance wires in a slot cut in a factory's concrete floor. Slots usually measure 1/8 to 3/8 wide 7/8 to 1 1/4 deep;. (Volvo Automated Systems)

Photo: 3. Floor-level representation of the magnetic field generated around a guidance wire. Sensor units on the vehicle use two coils; these measure amplitude or vector phase plus amplitude. Servosystems provide corrective steering. (NDC Technologies Inc)

Photo: 4. Diagram of major components in a vehicle employing single-wheel drive and steering. The sensor unit is at the front of the vehicle, lower left. (NDC Technologies Inc)

Photo: 5. A low-profile driverless tractor, left, tows a trailer loaded with a double-deck carousel of tooling for a machine-tool's automatic changer. The trailer docks at the toolchanger, which rotates the carousel and picks the proper tool. (JOBS SpA)

Photo: 6. A unit-load transporter automatically unloads a palletized diesel engine onto a test stand. Note the twin conveyors on the vehicle deck. (Portec)

Photo: 7. This wire-guided pallet truck transports raw material to a stamping shop, and carries the stamped parts, here being loaded, to the assembly department. (SPS Technologies)

Photo: 8. Here a unit-load transporter backs into a load/unload stand, delivering palletized parts to a machining station. The vehicle deck, now raised, will lower to deposit the load on the stand. The vehicle will then drive back onto the main guidepath. (Barrett Electronics)

Photo: 9. Four varieties of unit-load transporters, with different fixturing and load/unload mechanisms. The vehicle at center rear is designed for auto-body assembly; the powered scissor-lift holds the automobile truck lid or hood open during assembly. (Volvo Automated Systems)

Photo: 10. An example of a new type of pallet truck called the turret truck. The vehicle can raise and lower loads through a range of several feet, swing the load laterally through 180 degrees, and extend or retract the forks--all under remote, automatic control. (Portec)

Photo: 11. Schematic of the general arrangement for a centralized computer-control scheme for AGVS. Note that the central computer can be linked not only with the AGV system's line driver, but also with the plant's host computer, fixed and mobile call stations, and various monitoring and data-entry terminals. (The Material Handling Institute)
COPYRIGHT 1984 Nelson Publishing
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Copyright 1984 Gale, Cengage Learning. All rights reserved.

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Title Annotation:The technology, part 1 of 2; General Motors
Author:Quinlan, Joseph C.
Publication:Tooling & Production
Date:Jan 1, 1984
Previous Article:Making hydroforming tools with a copy mill.
Next Article:Computer-aided NC programming - coming slow but sure.

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