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AIRTRANS - Automated Transit System |
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Controls The automatic control system is divided into three subsystems: Automatic Vehicle Protection (AVP), Automatic Vehicle Operation (AVO), and Central Control. Their functions are summarized in the following:
Automatic Vehicle Operation
Central Control
Components of the AVP and AVO subsystems are located both on the vehicle and in the wayside control rooms. All vehicles may be driven manually with a plug-in unit, which overrides the AVO system; the AVP system cannot be overridden. Vehicle route information is stored in an onboard control logic assembly. This device responds to an interrogation from the wayside every 0.2-sec. and sends back route information as well as malfunction information. The wayside controls decode the route information and set the switches to the proper position. The proper speed command for each vehicle, depending on its location and other traffic, is transmitted to the vehicle from the wayside control units by the block control system. The system was designed for a nominal operating speed of 25 ft./s (17 mph). A higher speed would not significantly reduce the trip time because of the many station stops. Also, because of the many curves, both laterally and vertically, a higher speed would have reduced the ride quality. Vehicle operating safety is obtained through a five-block control system. The guideway was divided into 708 blocks by insulators spaced at intervals along the signal rail. A nominal block is 90-ft. long. The central control, from which the system is supervised, is located in the central heating and air conditioning building. The console in the Central Control illustration shows the status of the system and permits the operator to override the automatic controls of the system, if it is safe to do so. The route map shows the location by block of each vehicle and its status. TV screens permit viewing all passenger terminals, and two-way voice communication is possible with any and all vehicles. A CRT display also displays vehicle status and malfunction information. A smaller route map on the console displays power distribution information.
AIRTRANS was also designed and constructed to carry United States Postal Service mail to and from terminals to a Post Office in the maintenance area shown in the Guideway Configuration illustration as "AMF" (Air Mail Facility). This service was demonstrated as satisfying contract requirements but was terminated by the USPS as being too demanding in that it required their employees to interface with an automatic system. An incinerator was built by the DFW Airport in the transportation maintenance area, and AIRTRANS cargo vehicles were designated to carry trash there from the terminals. Airtrans: Program Background |
The vehicle control units are located under the baggage rack in the center of the vehicle opposite the entrance door. The wayside controls are located in each terminal and at the maintenance building in secure rooms. These units are made up of standard fail-safe relays that have been in use for many years on the nation’s railroads. One room is shown in the Wayside Control Room Illustration. The use of this equipment in an innovative way played a role in the long life, low cost and high reliability of AIRTRANS.
The vehicle maximum stopping distance under emergency conditions is 165 ft. In a five-block system, two blocks must be allowed for emergency stopping. With provisions for some margin, the block length was thus established at 90 ft. During any operation, at least one full block must separate the vehicles. For the various speed commands sent to the vehicles, see Vehicle Speed Controls illusttration. A green signal (tan in the photo) to proceed at full speed is sent to the vehicle from the wayside whenever a vehicle is cruising at high speed and separated from other vehicles by five full blocks or more. When the separation becomes less than four blocks, a signal is sent to the vehicle to slow to medium speed (14 ft./s). Within two blocks, the command is to stop. This allows one clear block between queuing vehicles. In a high-speed dynamic case, the vehicles have a minimum separation of 450 ft. At 25 ft./s the minimum headway is 18 seconds (that is, a vehicle passes a given point every 18 seconds ).
The supervisor does not operate the system but he may add or subtract cars, change the routes, dispatch service crews or interface with the passenger. A printed copy of all operations and malfunctions of vehicles and stations is available from the line printer and computer storage. The supervisory system consists of a hierarchy of computers located in terminals and maintenance areas, all reporting to the central computer as shown in the Supervisory Computers illustration
AIRTRANS was designed to carry all baggage between terminals in 89 containers. These had rollup doors and three were carried on a cargo vehicle. (NOTE: The baggage system was constructed and demonstrated but was never implemented because interline transit times from airline to airline were changed after the system was installed.) Powered conveyors on the vehicle matched an array of powered conveyors in the terminals to move the containers from the vehicle to positions for unloading by airline baggage handlers. The Terminal Cargo Handling Conveyors illustration shows the array of conveyors in a terminal. These have subsequently been removed.
This facility was never able to operate as planned and the service was never implemented. An additional system capability was to carry supplies to the terminals from a large supply depot adjoining the transportation center. This system was an “on-demand” system and used the same cargo vehicles and container types as the other cargo services It was a highly successful operation and was used until 1991 at which time it was reluctantly terminated to increase passenger capability on AIRTRANS. All cargo systems, although of great complexity and far ahead of their time for automated systems, performed to the contract specification and would have remained in use except for changed requirements.