ULTra Personal Rapid Transit System

Control Systems

The control system forms the heart of the PRT concept. Once the passenger has confirmed their destination (see Passenger Interface), central control immediately begins the process for carrying out their chosen journey, which is the first of three layers of control:

Central Synchronous Control: Ensures all vehicles launched on non-conflicting paths.
Autonomous Vehicle Control: Maintains vehicle to prescribed path using laser sensors.
Automatic Vehicle Protection System (AVP): Fixed block signalling system to ensure vehicle separation.

Central Synchronous Control

The central control system responds to the passenger’s request by allocating a vehicle for the journey and instructing the vehicle on the required path and timing for that journey. Each path is unique ensuring there is no interaction between vehicles. This provides the first level of active safety in the system. The central control system also controls the empty vehicle management process which ensures that vehicles are sent to where they are needed.

The central control function has been the subject of extensive simulations by ULTra PRT since the start of the project, and the functionality has been well developed and tested. Simulation results show that average waiting times will typically be under 15 seconds and the 95% of passengers will be served within a minute.

The control system uses a "synchronous" method which allocates vehicles to slots in the same way as an aircraft control process. Some further comments on this are provided in the ULTra Summary document.

Autonomous Vehicle Control

The vehicles are controlled autonomously: Once the vehicle has received its instructions form central control it will continue to its destination without any need for further central control input.

In order to ensure the vehicles were designed using the most effective control mechanism, extensive tests of different vehicle control systems were carried out. ULTra PRT performed full scale system evaluation tests at a number of test sites to examine control methods based on wire guidance, optical and radar sensing, embedded guideway transponders and local sensors based on ultrasonics or lasers.

The last two of these approaches were found to be significantly more robust and reliable than the others; laser sensor systems and embedded guideway transponders considerably out-performed the alternatives. These results led ULTra PRT to use a combination of these two approaches in the final ULTra system design - the vehicles use laser guidance during journeys, with embedded transponders taking over when the vehicle is within a station control area.

Automatic Vehicle Protection (AVP) System

Finally, the ultimate protection for passengers on the ULTra system is provided by an independent Automatic Vehicle Protection (AVP) system. The system installed at London Heathrow is based on a fixed block signalling system parallel to that used on railways, where inductive loops set into the guideway interact with sensing circuits on the vehicle.

In order for a vehicle to be in motion, it must be receiving the "Proceed" signal which is continuously transmitted from the guideway. The inductive loops inhibit the transmission of this signal in the area directly behind every vehicle, and so if in an emergency situation a vehicle was forced to stop suddenly on the guideway, any vehicle following directly behind would automatically be halted once it approached within a certain distance. The diagrams below illustrate the basic ideas behind the AVP system.

Operation of Automatic Vehicle Protection system: The diagram on the left shows the basic zone protection requirement, with the fixed-block implementation shown on the right.

Vehicles far enough behind the stopped vehicle would either be pulled into a nearby station or be rerouted onto a different branch of the network before reaching the incident, in order that they could avoid the blocked link - for more details on how the system can continue to operate in a variety of challenging scenarios, see the page on operational robustness.