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.
Principles of operation
AWS is a system based on trains detecting magnetic fields. These magnetic fields are created by permanent magnets and electromagnets installed on the track. The polarity and sequence of magnetic fields detected by a train determine the type of indication given to the train driver.
A magnet, known as an AWS magnet is installed on the track center line. The magnetic field of the magnet is set based on the next signal aspect. The train detects the polarity of the magnetic field via an AWS receiver, permanently mounted under the train.
An AWS magnet is made up of a permanent magnet, and an optional electromagnet. The permanent magnet is generally uncontrollable, and always produces a constant magnetic field of unchanging polarity. A train running over the permanent magnet will deliver an AWS warning indication to the train driver.
The optional electromagnet can be used to provide the train driver with an AWS clear indication. If the train AWS detects a second magnetic field of a certain polarity after the first permanent magnet, then the AWS displays a clear indication instead of a warning indication. The train detects the electromagnet polarity after the permanent magnet polarity. This is because the optional electromagnet is always installed after the permanent magnet (in the direction of travel). The electromagnet is connected to the green signal aspect, so the driver will only receive an AWS clear indication if the signal is clear (green).
The permanent magnet always produces a south pole. If the electromagnet is energized to produce a north pole, the AWS will give the driver an AWS clear indication.
Multiple unit trains have an AWS receiver at each end. Vehicles that can operate singly (single car DMUs and locomotives) only have one; this could be either at the front or rear depending on the direction the vehicle is traveling in.
The equipment on a train consists of; An AWS receiver (a magnetic field detector located under the driving cab of the train) An AWS visual indicator (known as the 'sunflower') 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.
Limitations This list of limitations is not exhaustive:
The AWS only has two output states; a clear indication and a warning indication. For warning indications, AWS does not provide any extra information about the reason for the warning. It is therefore reliant on the driver to observe the surroundings and determine the reason for the warning indication. AWS warning indications require the AWS acknowledge button to be pressed by the driver. It is possible for a driver on a crowded suburban railway to run all day on caution aspects, acknowledging hundreds of AWS warning indications. Continually acknowledging AWS warning indications can lead to a state where the driver fails to take appropriate action. This has led to a number of fatal accidents. It is possible for AWS to develop a wrong-side failure and the driver is given either no indication or a clear indication instead of a warning indication. The handbook notes that "AWS does not relieve the driver of the responsibility of observing and obeying lineside signals and indicators." There is also no special treatment of stop signals. A driver can still pass a stop signal aspect (SPAD) if they acknowledge the AWS warning indication. Other protection systems such as the Train Protection & Warning System (TPWS), can overcome this limitation.
History
Early devices
Early devices used a mechanical connection between the signal and the locomotive. In 1840, the locomotive engineer Edward Bury experimented with a system whereby a lever at track level, connected to the signal, sounded the locomotive's whistle and turned a cab-mounted red lamp. Ten years later, Colonel William Yolland of the Railway Inspectorate was calling for a system that not only alerted the driver but also automatically applied the brakes when signals were passed at danger but no satisfactory method of bringing this about was found.Vanns (1997) p.129
In 1873, United Kingdom Patent No. 3286 was granted to Charles Davidson and Charles Duffy Williams for a system in which, if a signal were passed at danger, a trackside lever operated the locomotive's whistle, applied the brake, shut off steam and alerted the guard. Numerous similar patents followed but they all bore the same disadvantage – that they could not be used at higher speeds for risk of damage to the mechanism – and they came to nothing. In Germany, the Kofler system used arms projecting from signal posts to engage with a pair of levers, one representing caution and the other stop, mounted on the locomotive cab roof. To address the problem of operation at speed, the sprung mounting for the levers was connected directly to the locomotive's axle box to ensure correct alignment. When Berlin's S-Bahn was electrified in 1929, a development of this system, with the contact levers moved from the roofs to the sides of the trains, was installed at the same time.
The first useful device was invented by Vincent Raven of the North Eastern Railway in 1895, patent number 23384. Although this provided audible warning only, it did indicate to the driver when points ahead were set for a diverging route. By 1909, the company had installed it on about 100 miles of track. In 1907 Frank Wyatt Prentice patented a radio signalling system using a continuous cable laid between the rails energized by a spark generator to relay "Hertzian Waves" to the locomotive. When the electrical waves were active they caused metal filings in a coherer on the locomotive to clump together and allow a current from a battery to pass. The signal was turned off if the block were not "clear"; no current passed through the coherer and a relay turned a white or green light in the cab to red and applied the brakes. The London & South Western Railway installed the system on its Hampton Court branch line in 1911, but shortly after removed it when the line was electrified.
GWR automatic train control The first system to be put into wide use was developed in 1905 by the Great Western Railway (GWR) and covered by UK patents 12661 & 25955. Its benefits over previous systems were that it could be used at high speed and that it sounded a confirmation in the cab when a signal was passed at clear.