Auxiliary Warning System (AWS) — Real Equipment & Signal Response Chart

Real Objectives of AWS

• (i) Bring train to halt in case of SPAD (signal passing at danger). (ii) Prevent train approaching faster than is safe. (iii) Give audio-visual assistance to the LP about speed and distance etc. (iv) Check vigilance/alertness of LP and take over if LP is not responding. (v) Supervise train in terms of speed limits.

Real Major Equipment of AWS

• (a) Track equipment: (i) Opto coupler card — electronic equipment interfacing the signal to collect signal-aspect information. (ii) Track magnet (balise/beacon) — gathers the collected information and transfers it for transmitting to the CAB (coding).
• (b) Cab equipment: (i) Engine magnet — gathers information transmitted from track side. (ii) Microprocessor-based system to process and issue various commands. (iii) CAB display unit — displays signal aspects/speeds and registers LP responses. (iv) Brake Application Unit (BAU) — applies brakes (service, emergency) as needed. (v) Tacho unit — monitors train speed.

Real Types of AWS Systems

Real discussion: the complexity of mixed traffic (which may need various speed profiles, not always matching intermittent systems' pre-programmed profiles), the high cost of a continuous system (not needed for all 24 hours), the present traffic level not justifying a continuous system, and the non-cost-effectiveness of converting intermittent to continuous, together explain why intermittent systems remain in use — though intermittent systems don't respond to aspect-clearing ahead, meaning the train continues at a speed more restrictive than actually required for safety.

Real AWS History & Working Principle

Real history: AWS was introduced in real 1966 in the Mumbai suburban section, as an essential safety link addressing misinterpretation of signal aspects by LPs and false reactions due to bad visibility (rain, fog, smoke) or LP incapacity. Later, higher running speeds, more signal aspects, variable target distances, and the need for continuous monitoring drove development of a reliable track-to-train communication system. AWS transmits data on signal aspect from trackside at significant locations — distant signals, auto signals, or the start of speed restrictions — to control/implement required train performance.

With this data, AWS can: (a) advise the LP about the condition of the section ahead; (b) give visible and audible warnings; (c) continuously monitor the maximum authorised speed and the braking procedure.

Real General Principle of Operation

AWS consists of (i) on-board equipment and (ii) track-side equipment. The car-borne equipment has transmitting and receiving devices, connected to the car-borne coil (Engine Magnet) mounted on the vehicle bogie to receive track-side data; an axle-mounted speed sensor (Tacho generator) supplies distance-covered and actual speed. A display unit provides manual-input elements (vigilance button, signal-failure by-pass button, reset button), visual indications, and a hooter. The Brake Functions Interface Unit acts directly on the braking system.

Trackside-to-vehicle data transmission uses 2 resonance circuits (in track side and car-borne coils) which monitor each other's function. Real circuit 1 operates at 50 KHz and acts as a checking (Pilot/supervisory) circuit. Real circuit 2, the data-transmission circuit, operates at 100 KHz, modulated with selected audio-frequencies depending on the lit signal aspect. Audio frequencies received after de-modulation by the engine are amplified and passed for further processing to effect train control.

Real Track Magnet & Signal Interface Details

• The Opto Coupler Card monitors the output voltage on the secondary of the signal transformer feeding the signal bulb (real voltage range 8 to 20V AC, current 2 mA) — from this, it senses which signal aspect (red, yellow, double yellow, green) is active, and can also monitor calling-on signals, 'A' markers, shunt signals, and route indicators. It converts the decided aspect into a combination of 2 frequencies from real F1 to F5, giving a loop on 2 circuits through the connecting cable to the Track Magnet.
• The main track magnet is installed between 2 rails on a sleeper at rail height, in front of the signal post — real installation distance: 231mm from the right-hand side of the rail in the direction of train movement, at least real 3 sleepers away from the nearest rail joint. The track magnet is a PASSIVE device requiring no power supply — it picks up power from the engine equipment while the engine coil passes over it. A carrier frequency of 100 KHz (data channel) transmitted by the engine equipment is modulated by the 2 track frequencies, picked up again by the engine, de-modulated, and decoded by the microprocessor-based cab equipment. Real capacity: for any track-to-train information, 2 audio frequencies are selected out of F1 to F7, giving a maximum of real 21 possible pieces of information from track to train.
• The track magnet also has a 50 KHz tuned coil (pilot circuit); the engine equipment has a corresponding 50 KHz coil/oscillator. When the engine passes over the track equipment, coupling between the 2 tuned circuits causes a dip in the 50 KHz carrier level in the engine equipment — this dip confirms the presence of a track magnet. Real fail-safe logic: if the engine equipment gets NO dip on 50 KHz, it does NOT act on 100 KHz data at all. If the 50 KHz shows a dip but there's NO information on the 100 KHz data channel, the engine equipment senses something wrong with the track equipment and applies brakes. Real limitation: if the ENTIRE track magnet is missing (including the 50 KHz tuned coil), this cannot be detected by the engine equipment.

Real Opto Coupler & Track Magnet Types

Different Opto coupler card types are used depending on signal type and whether it has auxiliary aspects; a different card type is needed for signals controlling intermediate and additional magnets in rear. Real caution-aspect card types: real frequency combination F1,F4 is suitable where inter-signal distance is LESS than 700m; real combination F2,F4 is suitable where inter-signal distance is MORE than 700m — the inter-signal distance decides which Opto coupler type is used.

Real 2 types of track magnets: Type-A (generally used along with main line signals; also used as an intermediate magnet between 2 main signals wherever inter-signal distance exceeds 700m). Type-B has real 3 sub-types: B-I — used for applying emergency brake for testing purposes when the EMU comes out of the car shed. B-II — mounted 'a' metres away in rear of the signal, real fixed frequency F5F6, denoting that available braking distance is less than 'd' from the signal in advance. B-III — permanently tuned to indicate the AWS section is over and there's no more speed restriction on passing a signal at caution.

Real Engine Equipment — Buttons, Counters & Indication Lamps

• 1) A reset button must be operated (only in standstill condition) whenever restarting after AWS-imposed emergency brake application — this operation is counted by the real EBC counter. 2) An SFBB button is pressed whenever an absolute stop signal is passed at danger — counted on the real SFBC counter; must be pressed when the train is within real 100 metres in rear of the stop signal at danger, which is to be passed at 'ON'. 3) A vigilance button must be pressed within real 4 seconds whenever the hooter sounds. 4) A buzzer operates whenever the signal passed is double yellow or green.
• 5) Real 3 indication lamps: WHITE indicates normal working. FLASHING WHITE indicates a fault in the cab equipment. BLUE is normally ON and flashes in case of fault in power supply/speed-check/speed-evaluation equipment, going off for real 5 seconds when the train passes a signal at green or double-yellow. STEADY YELLOW appears when the train passes a signal at caution and starts flashing roughly real 290 metres after passing the caution-aspect signal (or after 290m beyond the intermediate magnet at 'caution', if inter-signal distance exceeds 700m) — real note: train speed would have been reduced to real 38 KMPH within this distance. STEADY RED indicates emergency braking or passing a permissive red signal at ON. FLASHING RED indicates service braking or pressing the SFBB button in standstill condition. Real rule: LP should start the train ONLY if blue & white indications are steady.
• 6) There's a control switch for switching supply to the master controller — this CANNOT be switched off when the train is running. Before switching off master supply, it must be ensured the AWS isolation switch is in the 'ON' position. 7) The isolation switch is a SEALED switch, used only in case of erratic AWS functioning causing unwarranted brake application: the LP should open the seal, operate to 'off' position, and try to run the train — if that fails, isolate the feed cutoff magnet valve and exhaust magnet valve by operating the 2 cocks provided for that purpose. Whenever the isolation switch is switched off, it's also counted by a counter. 8) At standstill, pressing the vigilance button for real 10 seconds lets the LP run a functional test of the indication panel — red, blue, and yellow indications go steady and white starts flashing; on release, the hooter and buzzer each sound for real 1.2 seconds.

Real Action-Chart — LP Response by Signal Aspect

Real note: '*' means the buzzer can be dispensed with if necessary. Real note on the extra magnet: where inter-signal distance is 700m or more, an additional magnet is fitted at a distance of real 400 metres in rear of the signal in advance.

Real Essential Features of the AWS System

• 1) No track circuits involved in the track-to-train communication; no mechanical components. 2) No requirement of power supply in the field; suitable for speeds up to real 250 KMPH. 3) Real air-gap tolerance 120 to 220 mm, with a lateral deviation of ±40mm (as noted in source, appearing as '40 min') between track and engine magnets. 4) Immune to electro-magnetic interference; fail-safe monitoring. 5) Small and compact in size.