Editing Automatic Warning System (section)

== Principles of operation ==
[[File:Class 43 driver's AWS equipment.jpg|thumb|Driver's AWS equipment in a [[British Rail Class 43 (HST)|Class 43]] driving cab]]

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 [[magnetism|magnetic field]] of the magnet is set based on the next signal aspect.<ref name="Awsrules"/> The train detects the polarity of magnetic field via an AWS receiver, permanently mounted under the train.<ref name="Awsrules"/> 

An AWS magnet is made up of 1 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 [[Railway_signal#Aspects_and_indications|signal aspect]], so the driver will only receive an AWS clear indication if the signal is clear (green).

The permanent magnet always produces a [[Magnet#Polarity|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) <ref name="Awsrules"/>
* 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 stop) 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).<ref name="Awsrules"/> 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{{fact|date=March 2023}}.

* 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."<ref name="Awsrules"/>

* There is also no special treatment of stop signals. A driver can still pass a stop signal aspect [[Signal_passed_at_danger|(SPAD)]] if they acknowledge the AWS warning indication. Other protection systems such as the [[Train Protection & Warning System|Train Protection & Warning System (TPWS)]], can overcome this limitation.