A continuous cab display of the maximum permitted APT speeds on sections of track is given by the control system based on transponders mounted on the track at specific intervals.
The men who drive the APT will also drive conventional trains over the same routes, yet APT is allowed to exceed existing speed limits by a substantial margin. To prevent confusion, the higher limits are displayed automatically in the APT cab. This advisory system (C-APT) leaves the driver firmly in control of the train, but gives him a digital advance warning on his desk of the higher APT speed limits.
On the track, beacons, called transponders, store permanent speed limit information in coded form. Sealed in glassfibre re-inforced cases the transponders, containing electronics and a loop aerial, are waterproof and need no external power supply.
The transponders are powered by a radio beam transmitted by a loop aerial under the front of the train. A coded message (the message is 80 bits long) is re-transmitted by the transponder and fed to the train-bourne processor unit. Micro-processor circuits check the validity of the code and display the approaching speed limit to the driver.
When the train approaches a speed restriction the display changes to the new limit at the appropriate braking distance. An audible warning sounds which the driver must acknowledge, otherwise the brakes are applied automatically.
The driver selects a suitable braking rate to bring the train down to the new limit displayed. At the start of the speed restriction an indicator light on his desk is briefly illuminated, while at the end he receives a short warning sound to alert him to the higher speed.
C-APT has to fail safe so transponders are bolted to the sleepers at intervals of 1km or less, if the equipment fails to detect a transponder after 1km, the display goes blank and an audible warning is initiated which must be acknowledged by the driver. With a blank display the driver reverts to conventional line speeds.
To eliminate the risk of wrong speed limits being displayed, all the train-borne equipment, except for the display, is duplicated, while the electronic system has an inbuilt self-checking routine.
A secondary use of C-APT is to close air intakes when approaching tunnels to prevent ear discomfort to passengers.
|The C-APT processor units each contained two Intel
4004 Single Chip 4-Bit microprocessors.