Introduction 1302
Airtrans System 1302
Vehicles, Guideways and Stations 1302
Automatic Vehicle Control 1303
Automatic Vehicle Protection and Operation 1304
Automatic Vehicle Supervision 1305



Personal Rapid Transit (PRT) provides public transportation over a limited area, as in the central business district of a city or between the scattered facilities at an airport. It is designed to handle a high volume of short trips at a service level superior to that possible on surface roadways. The goal is to attract patronage by offering service approximating direct response to individual passenger demand, i.e., ‘personal” service.
High speeds are not necessary, but delays waiting for service or en route must be minimal. Such service requires a large number of small vehicles operating with close headway on an isolated right of way. To keep costs at an acceptable level vehicles typically function without attendants, thus fully automated operation is essential.
To achieve these objectives, it is necessary to use vehicles and guideways of novel design which differ considerably from those of conventional rail transit. Although vehicles and right of way are different, safety requirements are essentially the same as for rail transit. Sufficient separation must be maintained between vehicles to provide adequate braking distance, and routes must be locked against hazardous changes. Additionally, speed must be limited as required by grades, curves and civil restrictions, and vehicle doors must open only when it is safe to do so.
GRS has developed and installed control systems for PRT which achieve objectives of safety, efficiency, and passenger satisfaction. The system supplied for the Airtrans PRT installation at the Dallas/Fort Worth Airport affords a good example and is described briefly in this section to illustrate how signal technology is adapted to new forms of transportation. It must be remembered, however, that PRT control systems are individually designed to meet specific needs. Thus, while the safety and efficiency objectives of the Airtrans control system are representative, the specific technology described would be modified to fit the requirements of PRT systems elsewhere.
Vehicles, Guideways, and Stations
Airtrans vehicles are electrically propelled, and run on pneumatic tires over a concrete guideway, Figure 1301. Two types of vehicles are used. Type A vehicles are equipped for the control system and

may operate as single units; Type B vehicles have propulsion systems but are not fitted for independent control. Type B vehicles are thus used only as trailing units coupled to a Type A vehicle. When a Type B vehicle is used, the coupling to the associated Type A vehicle is verified by on-board circuits. If the vehicles part both stop automatically. In addition to passenger vehicles, Airtrans includes utility vehicles for baggage transfer, delivery of supplies, and trash handling at designated stations.
The guideway is a concrete trough somewhat wider than the vehicle. It consists of a flat running surface bordered by two low walls. Horizontal guide wheels, projecting laterally from the vehicle, bear against the side walls. Through a steering linkage, the guide wheels direct the vehicle along the guideway.
Where routes diverge, GRS Model 55 switch machines, installed in vaults beneath the running surface, are controlled to position guide “blades” on the guideway walls. The blades intercept additional lateral guide wheels which are normally idle. Trapped by the blade, these guide wheels steer the vehicle as required by the route. Merging routes are handled automatically by an arrangement which spring-biases the guideway guidance blades for the “through” movement but which permits vehicles merging from the “side track” to deflect the blade against spring pressure.
The guideway supports a multiple-conductor bus structure, Figure 1302, used for propulsion power and vehicle control. Vehicle-carried brushes engage the bus bars, also called “rails,” as shown in Figure 1303. The three central conductors provide three-phase propulsion power. The top and

Figure 1301. Personal rapid transit vehicle in guideway.



bottom rails are used for control with the top rail, called the signal rail, carrying command signals and the bottom rail at ground. The bottom rail also serves as a heavy earth ground return for propulsion current faults and imbalances.
Stations are relatively small, since platforms need accommodate only two coupled vehicles, and service frequency minimizes space needed for waiting passengers. Stations for general passenger service are equipped with platform doors which remain closed except when a vehicle is in loading position. Stations used by airport personnel omit this feature.
The GRS Automatic Vehicle Control (AVC) system for Airtrans is similar in general functions to Automatic Train Control (ATC) for rail rapid transit. Thus there are three basic subsystems.
Automatic Vehicle Protection (AVP)
Automatic Vehicle Operation (AVO)
Automatic Vehicle Supervision (AVS)
Automatic vehicle protection performs the vital safety functions required for vehicle separation, prevention of excessive speed, assurance of route

Figure 1302. Power and signal busses are mounted on guideway wall.
integrity at switching points, and door control at stations. Automatic vehicle operation relates to speed maintenance en route between stations, stops at stations, and departures from stations. Automatic vehicle supervision serves non-vital functions of communications, route selection, supervision, and performance monitoring.

F. -


Figure 1303. Vehicle brushes contact wayside busses. Middle three busses deliver 3-phase propulsion power. Top and bottom busses serve for occupancy detection and vehicle control.




Automatic Vehicle Protection and Operation
Protection against inter-vehicle collision is provided by a block system which assures positive separation between vehicles. Blocks are defined by isolated sections of the wayside signal rail. At one end of the block, relatively high-voltage d.c. (48 volts) is fed via a 270-ohm limiting resistor to the signal rail. At the other end of the block, d.c. is fG from the signal rail to a GRS Type B vital relay. A 2000-ohm resistor in series with the relay limits coil current to an appropriate value. The common ground rail provides the return path for the circuits. The arrangement is thus analogous to a single-rail track circuit.
Each Type A vehicle establishes low-resistance shunts across the signal and ground rails via (1) front-end brushes and the AVP command receiver, (2) rear-end brushes and the relatively low-resistance wiliding of an on-board relay which is picked up by the shunt current flowing from the wayside. Either shunt is sufficient to produce track relay release and thus vehicle detection. Type B vehicles have front and rear relay-energizing shunts similar to the rear-end shunt on Type A vehicles.
Integrity of vehicle shunts is verified by the system. Loss of the front brush shunt causes the vehicle to stop because command signals, received via the same shunt connection, are also lost. Loss of a relay-energizing shunt releases the on-board relay, which prevents the vehicle from leaving the next station it enters. Such malfunctions are indicated to central control over a communication link.
Possible loss of a rear shunt, which could result in loss of detection of vehicle presence in a block before the rear end has completely cleared, is recognized in the block length and stopping distance safety factors. Verification that a vehicle actually shunts is established by check-in checkout wayside circuits. Once a track relay is released it cannot pick up again until the next downstream track relay is down. With occupancy detected, general concepts of rapid transit automation can be applied.
Safe vehicle speed is determined by downstream block occupancy, guideway switch conditions, grade or curve restrictions, and civil limits that may apply. Safety limits are stablished by commands from the wayside, received via the

signal rail. Conformance with speed limits is monitored by an FR governor. Violation of a speed limit initiates an irrevocable vehicle stop.
Speed limits are transmitted to vehicles by means of two carrier frequencies, identified as fi and f2, superimposed on the signal rail. Reception of fi establishes the low-speed limit (6ft./sec.), reception of f2 establishes the medium-speed limit (16 ft./sec.). Reception of fi and f2 together sets the high-speed limit (28.5 ft./sec.). Absence of both carriers mandates a vehicle stop.
Protection against spurious signals is obtained by continuous frequency-shift of the carriers above and below their nominal center values. The vehicle recognizes fi or f2 inputs only if the requisite modulation is present.
In addition to frequencies fi and f2, two supplementary frequencies, f3 and f4, are utilized. These are used in conjunction with fi and f2 for transmitting commands involving speed reduction profiles for normal stops, and for modification of running speeds. Since the command information conveyed by f3 and f4 cannot result in violation of the maximum speed limits established by fi and f2, f3 and f4 are not frequency-shifted. Figure 1304 tabulates the carrier frequencies transmitted to establish maximum speed limits and operating commands.
Route control is established by each vehicle on the basis of instructions received from supervisory control and stored on board. Vehicle identification and desired route are transmitted from the vehicle to the wayside. The wayside remembers vehicle positions in the queue and positions switches as required for each vehicle as it approaches.
Station stops are triggered by a command from the wayside to the vehicle. The vehicle measures the distance traversed from the reception of the command and tapers its speed to zero at the platform. Platform position is checked by circuits which are closed only when the vehicle’s brushes are in contact with designated sections of the signal rail. With the vehicle properly positioned, no motion detected, speed commands off, power off and brakes on, vehicle and platform doors are permitted to open.
Vehicles may depart from stations only when all doors, including platform, are closed. Vehicle doors (including rear doors not used in normal passenger service) must remain locked between


Figure 1304. Speed limits, operating commands, and associated carrier frequencies.

stations. Violation of this requirement stops the vehicle.
Automatic Vehicle Supervision
Automatic supervision of the Airtrans system provides for optimizing system performance by adjustment of vehicle speeds, selection of routes, and insertion or withdrawal of vehicles. Automatic monitoring and record keeping provide informa tio

useful for subsequent modification and improvement of supervisory practices.
Wayside-vehicle and vehicle-wayside communication via the signal rail establishes two-way digital data links. Vehicles can be addressed and commanded individually from central control. Similarly, central control can identify the specific vehicles from which incoming messages originate.











Zero Speed

Stop, Short Profile

Stop, Long Profile

Proceed, Low Speed

Proceed, Med. Speed

Proceed, High Speed