Automatic Warning System

showing a warning indication has been acknowledged. Automatic Warning System (AWS) is a railway safety system invented in the United Kingdom. It provides a train driver with an audible indication of whether the next signal they are approaching is clear or at caution. Depending on the upcoming signal state, the AWS will either produce a 'horn' sound (as a warning indication), or a 'bell' sound (as a clear indication). If the train driver fails to acknowledge a warning indication, an emergency brake application is initiated by the AWS; if the driver correctly acknowledges the warning indication, by pressing an acknowledgement button, a visual 'sunflower' is displayed to the driver, as a reminder of the warning.

showing a warning indication has been acknowledged.]] Automatic Warning System (AWS) is a railway safety system invented in the United Kingdom. It provides a train driver with an audible indication of whether the next signal they are approaching is clear or at caution. Depending on the upcoming signal state, the AWS will either produce a 'horn' sound (as a warning indication), or a 'bell' sound (as a clear indication). If the train driver fails to acknowledge a warning indication, an emergency brake application is initiated by the AWS; if the driver correctly acknowledges the warning indication, by pressing an acknowledgement button, a visual 'sunflower' is displayed to the driver, as a reminder of the warning. An AWS audible indicator (capable of producing two different sounds, clear indication sound = bell, warning indication sound = 'horn'. On modern rolling stock the electromechanical bell and horn is replaced with an electronically generated "chime" or "buzz". An AWS Acknowledge button (used to acknowledge the audible AWS warning indication. If not pressed within 2-3 seconds of an AWS warning indication, emergency brakes are applied) An AWS isolation/fault indicator (a visual indicator advising the driver when the AWS has been isolated or has a fault) ===AWS practical example=== The polarities in this example are relevant to the UK. The permanent magnet produces a south pole in the UK. Other countries may use permanent magnet that produces a north pole. The key operational principle is that the electromagnet produces the opposite pole of the permanent magnet. ====AWS clear indication example==== A train is driving towards a signal that shows clear (green). The train runs over the AWS magnet (which is two magnets, first a permanent magnet and then an electromagnet). The electromagnet is energized. The AWS receiver detects a magnetic field in the sequence: South, North. The south pole comes from the permanent magnet, and the north pole comes from the electromagnet. This south then north sequence gives an AWS clear indication to the driver. ====AWS warning indication example==== A train is driving towards a signal that shows caution (yellow). The train runs over the AWS magnet (which is two magnets, first a permanent magnet and then an electromagnet). The electromagnet is de-energized (i.e. it is not powered). The AWS receiver detects only one magnetic field in the sequence: South. The reason only one magnetic field was detected is because the electromagnet was not energized. This makes the electromagnet invisible to the AWS receiver. This south pole by itself results in an AWS warning indication to the driver. ===AWS at signals=== As the train approaches a signal, it will pass over an AWS magnet. The AWS visual indicator ('sunflower') in the driver's cab will change to all black. If the signal being approached is displaying a 'clear' aspect, then AWS will sound a bell tone (modern trains have an electronic sounder that makes a distinctive 'ping') and leave the 'sunflower' black. This AWS clear indication lets the driver know that the next signal is showing 'clear' and that the AWS system is working. If the next signal is displaying a restrictive aspect (e.g. caution or danger) the AWS audible indicator will sound a continuous alarm. The driver then has approximately 2 seconds to press and release the AWS acknowledgement button (if the driver keeps the button held down, the AWS will not be cancelled). After pressing the AWS acknowledgement button, the AWS audible indicator is silenced and the AWS visual indicator changes to a pattern of black and yellow spokes. This yellow spoke pattern persists until the train reaches the next AWS magnet and serves as a reminder to the driver of the restrictive signal aspect they passed. As a fail-safe mechanism, if the driver fails to press the AWS acknowledgement button for a warning indication in sufficient time, the emergency brakes will automatically apply, bringing the train to a stop. After stopping, the driver can now press the AWS acknowledgement button, and the brakes will automatically release after a safety time out period has elapsed. ===AWS for speed warnings=== AWS works in the same way as for signals, except that a fixed magnet is located at the service braking distance before the speed reduction and no electromagnet is provided (or needed). A single fixed magnet will always cause a warning indication to the driver, which the driver must acknowledge to prevent the emergency brake applying. A trackside warning board will also advise the driver of the speed requirement ahead. ===Literature=== ==Further reading==

Text derived from https://en.wikipedia.org/w/index.php?oldid=1378451147 under CC BY-SA 4.0; modifications have been made.