History of AGVS
1953 First AGV
Automatic Guided Vehicles have played a role in moving material and product for more than 40 years. The first AGV system was built and introduced in 1953. It was a modified towing tractor that was used to pull a trailer and follow an overhead wire in a grocery warehouse. By the late 50’s and early 60’s towing AGVs were in operation in many types of factories and warehouses.
History of AGVS
1973 Volvo Assembly Plant
In 1973, Volvo in Kalmar, Sweden set out to develop non-synchronous assembly equipment as an alternative to the conventional conveyor assembly line. The result was 280 computer-controlled assembly AGVs.
History of AGVS
1970s First Unit Load
The first big development for the AGV industry was the introduction of a unit load vehicle in the mid 1970s. These unit load AGVs gained widespread acceptance in the material handling marketplace because of their ability to serve several functions; a work platform, a transportation device and a link in the control and information system for the factory. Today there are several hundred systems using unit load vehicles in operation which were produced by a number of manufacturers. These systems transport material in warehouses, factories, mills, hospitals, and other industrial and commercial settings
History of AGVS
Smart Floors and Dumb Vehicles
In the 1970’s the principal guidance technology was to induce an electronic frequency through a wire that was buried in the floor. A device called a ‘floor controller’ turned the frequency on the wires on and off and directed the AGV through its intended route. The AGV was considered ‘dumb’ since the vehicle was just following signals in the floor. The intelligence for the routing of the vehicles was in the floor controllers. So, the systems of this day were considered “smart floors” and “dumb vehicles”.
An antenna on the AGV would seek out the frequency and guide the vehicle based on the strength of the signal. This technology required embedding multiple wires in the floor in order to handle intersections or other decision points. The system would energize the wire that would correspond to the intended direction of travel. For example, at an intersection three separate wires might be required.
These first generation navigation schemes were expensive to install. All floor cuts needed to follow the exact path of the AGV. The cut for a turn had to follow the radius curve that the vehicle would make when turning. Many systems had to embed four wires - three for guidance and one for communications. Often, rebar or electronic signals would interfere with the guidance signals imposed on the wires.
History of AGVS
Dead Reckoning Capability
As electronics and microprocessors advanced, so did AGV applications. As the vehicles became more intelligent, the path became less sophisticated. One of the first major breakthroughs was the development of dead reckoning capabilities. Dead reckoning is a term that describes the ability of a vehicle to traverse steel expansion joints on the factory floor or to cross a steel grate. The biggest advantage was that dead reckoning eliminated the need to make the cut radius turns at intersections. The vehicles could leave the wire, turn at a programmed radius, and then pick-up the wire to continue its course of travel. The path still required multiple wires in the floor, but the installation was greatly simplified.
History of AGVS
1980s Non-Wire Guidance
During the late 1980s, non-wire guidance for AGV systems was introduced. Laser and inertia guidance are two examples of non-wire guidance which allow for increased system flexibility and accuracy. When changes to the original guidepath are needed, there is no need for floor alterations or production interruption. These and other methods of navigation are explained more fully in the section on navigation.
History of AGVS
Computer Functions
As with all high-technology products based on electronic and computer software, without a doubt, AGVs have been greatly influenced and have benefited from the advances and reductions in cost of microcomputers and microelectronics. The computers used in AGV systems can store instructions, make decisions and execute procedures. In practice, AGVs are capable of performing almost all of the decisions and control functions currently controlled by humans in managing the material handling process. They can schedule factory time, maintain inventories, manage system details and control many types of mechanical systems in the overall operation.
History of AGVS
Applications and Controls
The use of AGVs has evolved drastically from traditional distribution-oriented applications at one end of the spectrum to complex computer-controlled automobile assembly systems with robotic interfaces at the other end. They can be stand-alone systems, an integral part of another system, or aid in pulling together islands of automation. Originally designed for horizontal transportation of palletized material, the design and application of vehicles and controls are now as varied as those of industrial robots.
History of AGVS
AGV Manufacturers
The market demand for AGVs can be measured by the increased number of AGV manufacturers. During the late 1970s there were fewer than 6 AGV vendors in the United States and only 3 different types of vehicles. By 1990 there were more than 40 worldwide vendors and more than 15 vehicle types, with an increasing emphasis on standard design. The future will be marked by still more change which will be driven partly by even more technological advances. But it will also be driven by increases in the use of AGVS, which in turn will foster more investment in research and development
Vehicle Navigation
The principles which make it possible for an AGV to navigate its way between any two locations are really quite simple. All navigation methods use a path. The vehicle is instructed to FOLLOW a Fixed Path or TAKE an Open Path.
Fixed Path Navigation
Following a Path
Vehicles that FOLLOW a path use the earliest methods of AGVS navigation. The general features of these methods are:
· The paths are well marked on the floor
· The paths are continuous
· The paths are fixed, but can be changed
Fixed Path Navigation:
Creating a Path
The principle techniques for creating paths are to:
- Apply a narrow magnetic tape on the surface of the floor
- Apply a narrow photo sensitive chemical strip on the surface of the floor
- Apply a narrow photo reflective tape on the surface of the floor
- Bury a wire just below the surface of the floor
The first three methods require a sensor on the underside of the vehicle which can detect the presence of the surface mounted path. The sensor’s mission is to keep the vehicle directly over the guide path. If the path makes a turn the sensor detects the turn, provides feedback to the onboard vehicle controller, which in turn causes the vehicle to steer in the direction of the path. The sensor’s role in connection with the onboard controller and steering mechanism is to cause the vehicle to FOLLOW the path.
Buried Wire Path
- Apply a narrow magnetic tape on the surface of the floor
- Apply a narrow photo sensitive chemical strip on the surface of the floor
- Apply a narrow photo reflective tape on the surface of the floor
- Bury a wire just below the surface of the floor
When the fourth method of “path following” is employed using a current-carrying buried wire, the under vehicle sensor takes the form of a small antennae consisting of magnetic coils. With current flowing, a magnetic field surrounds the buried wire. The closer the buried wire is to the AGV antennae, the stronger the field.
The magnetic field is completely symmetrical around the conductor or the buried wire. At a given distance from the wire the field has the same strength on either side of the cable. The field strength is detected by the antennae’s magnetic coil and induces voltage in the coil.
Fixed Path Navigation:
Steering Correction Coils
Like the three other path following methods, the vehicle steers itself to FOLLOW the magnetic field surrounding the buried wire. To get a steering correction signal the vehicle’s sensing antennae consists of two coils. When the vehicle is centered directly above the buried wires equal voltages are induced in the two coils. If the vehicle moves a bit to one side of the wires, the induced voltages would be of different strength. The difference in signal strength in the coils is proportional to side displacement of the coil. This difference is amplified and fed back to control an onboard servo motor, which turns the guide wheel or wheels until both coils receive equal signals, and the course is corrected.
Fixed Path Navigation:
Path Selection
In this illustration, a vehicle at “A” has two choices on how to get to “B”. A computer either on board the vehicle or at some central location selects a path based on established criteria. That criteria may be the shortest distance or the path with the least traffic at the present time. Once selected the vehicle is dispatched and navigation begins. All of the “PATH FOLLOWING” navigation methods permit routing options that include guide path switching and merging.
Open Path Navigation:
Taking a Path
Navigation methods that direct a vehicle to “TAKE A PATH” employ developments that came into their own in the early to mid 90’s. They utilize a different set of technologies. Unlike “path following navigation,” where the guide paths are fixed, and more or less permanent, vehicles operating in the “Take a Path” category are actually offered more variation if not an infinite number of ways to navigate the open space between two points.
Open Path Navigation:
Navigation Requirements
From a practical point of view, the choices are limited by known permanent or temporary obstructions and the path selection criteria, such as take the shortest path.
To navigate in an open, unrestricted space without the benefit of a fixed path, a vehicle must have a way of knowing where it is and be capable of taking a heading to where it wants to go. All open space navigation methods require:
· A map of the area in which the vehicle can operate that is contained in the vehicle’s computer memory.
· Multiple, fixed reference points located within the operating area that can be detected or “seen” by the vehicle.
Open Path Navigation:
Navigation Methods
The three most common open space navigation methods are Laser Guidance, Inertial Guidance, and Cartesian Guidance. The choice of navigation method for a particular application is often a simple matter of preference. Each method offers different benefits and costs associated with system set up and operation which is application dependent. Unless there is a definite preference, users should work closely with suppliers to evaluate the options in relation to the intended application. Each method is backed by a rich history of its successful use in practice.
· Laser Guidance
· Inertial Guidance
· Cartesian Guidance
AGVS Dispatching
AGVS dispatching is essential to every AGVS, whether simple or complex. Dispatching AGVS is much the same as dispatching taxi cabs. The dispatch function makes sure that all customers get timely services from the vehicle best able to service a request. Without a dispatching function, nothing would move. With inefficient dispatching, a system will not obtain maximum benefits.
The dispatch function maximizes the benefits of any AGVS and ensures that all customers get serviced in a timely manner. Remote and local dispatch are most commonly described as offboard and onboard dispatchers respectively.
Remote Dispatching
Methods of Communication
Remote dispatch occurs when the AGVs receive dispatching information from a central controller. This type of dispatch requires methods of communications to send the command from the Dispatcher to the AGVs. These methods of communications include:
· An RF Network (Broadband or Spread Spectrum) including a base station and antennas transmitting a signal throughout the airwaves with a receiver mounted aboard each vehicle.
· Inductive RF through a wire in the floor with antennas mounted aboard each vehicle. In many cases, the guidance wire will be used as the RF communication antenna and in other cases, the inductive signal is communicated through a separate buried wire.
· Infrared communications with multiple IR transmitters mounted throughout a facility with an IR receiver mounted aboard the AGVS.
Remote Dispatching
The Dispatcher
The remote dispatch function generally resides in a computer (PC), Programmable Controller (PLC), or other microprocessor, known as the Dispatcher. The Dispatcher accepts input from the various system components (generally transport requests) and directs the AGVS to fulfill the command in the most efficient manner. This function directly compares to the taxi Dispatcher back at the terminal, receiving calls from many customers and then dispatching each driver via radio to the pickup station. Remote dispatch can occur with vehicles at single or various dispatch points.
Remote Dispatching
Single Point Dispatch
A single dispatch point requires the AGVs to return to the same location every time to receive commands and transport requests. Single point dispatch generally has predetermined shortest path routings and works on strict FIFO requests. As vehicles are available and reach the dispatch point, they are given a transport command.
Remote Dispatching
Vehicle Manager
Using various dispatch locations, vehicles are given transport commands as soon as they complete their previous transport. The Dispatcher attempts to choose the closest available vehicle to fulfill the command. Should a vehicle be given a command and a closer vehicle becomes available, the more complex Dispatchers will be able to reassign the transport request to the closer vehicle. The Dispatcher, also known as the Vehicle Manager, generally includes an algorithm for priorities governing system movement.
Remote Dispatching
Dispatcher Commands
The Dispatcher commands can also take multiple forms where an AGV can be sent a full command, a half command, or a point-to-point command.
For example:
Full command (pick up at “A” and deliver to “B”)
Half command (pick up at “A”)
Point-to-point (move from point 1 to 2, from point 2 to 3, etc.)
With full and half commands, the AGV has a system map resident in its onboard vehicle controller. With point-to-point routing, the system map is resident with the Dispatcher.