25-05-2012, 11:27 AM
AUTOMATIC CONTROL OF RAILWAY SIGNALLING
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ABSTRACT
The objective of this project is to provide an automatic railway signalling operated by the microcontroller. It deals with two things. Firstly, it deals with the reduction of time for which the train is being kept waiting for line clear and secondly, to provide safety to the trains by reducing the accidents.
This project highlights the concept of managing trains at one particular station by developing a simulation through microcontroller. Emphasis has also been given about the Historical Development of Railway Signalling.
This project finally concluded with a practical simulation of the automatic railway signalling.
INTRODUCTION
All over the world Railway transportation is increasingly used, as this mode of transport is more energy efficient and environmentally friendly than road transportation. Trains move on iron rail tracks and wheels of the railway vehicle are also flanged Steel wheels. Hence least friction occurs at the point of contact between the tracks & wheels.
Therefore trains carry more loads resulting in higher traffic capacity since trains move on specific tracks called rails, their path is to be fully guided and there is no arrangement of steering. Clear of obstruction as available with road transportation, so there is a need to provide control on the movement of trains in the form of Railway signals which indicate to the drivers to stop or move and also the speed at which they can pass a signal.
Since the load carried by the trains and the speed which the trains can attain are high, they need more braking distance before coming to the stop from full speed. Without signal to be available on the route to constantly guide the driver accidents will take place due to collisions.
There are basically two purposes achieved by railway signaling.
1. To safety receive and dispatch trains at a station.
2. To control the movements of trains from one station to another
After ensuring that the track on which this train will move to reach the next station is free from movement of another train either in the same or opposite direction. This Control is called block working. Preventing the movement from opposite direction is necessary in single line track as movements in both directions will be on the same track.
Apart from meeting the basic requirement of necessary safety in train operation, modern railway signalling plays an important role in determining the capacity of a section .The capacity decides the number of trains that can run on a single day. By proper signalling the capacity can be increased to a considerable extent without resorting to costlier alternatives.
The fixed signals provided by the side of the railway track with indication in the form of colour lights are the actual authority to a driver to get in to the portion of the track beyond the signal. At stations the trains may be received on any one of the platform lines. To take the train to any specific track, points are provided. The purpose of the point on the track is to divert the train from one track to adjacent one.
Hence the signal has to be connected to the points in an arrangement called interlocking. This Interlocking arrangement only ensures that a point is correctly set for the particular route or a track and the signal conveys this information to the driver.
SIGNALLING
2.1 RAILWAY SIGNALLING
A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train drivers/engineers. The driver interprets the signal's indication and acts accordingly. Typically, a signal might inform the driver of the speed at which the train may safely proceed, or it may instruct the driver to stop.
Railway signalling is a safety system used on railways to prevent trains from colliding. Trains are uniquely susceptible to collision because, running on fixed rails, they are not capable of avoiding a collision by steering away, as can a road vehicle; furthermore, trains cannot decelerate rapidly, and are frequently operating at speeds where by the time the driver/engineer can see an obstacle, the train cannot stop in time to avoid colliding with it. Most forms of train control involve messages being passed from those in charge of the rail network or portions of it to the train crew. The set of rules and the physical equipment used to accomplish this determine what is known as the method of working.
2.2 TYPES OF SIGNALLS
On most railways, physical signals are erected at the line side to indicate to drivers whether the line ahead is occupied and to ensure that sufficient space exists between trains to allow them to stop.
2.2.1 MECHANICAL SIGNALS
Older forms of signal displayed their different aspects by their physical position. The earliest types comprised a board that was either turned face-on and fully visible to the driver, or rotated so as to be practically invisible. While this type of signal is still in use in some countries (e.g. France and Germany), by far the most common form of mechanical signal worldwide is the semaphore signal. This comprises a pivoted arm or blade that can be inclined at different angles. A horizontal arm is the most restrictive indication (for 'danger' or 'caution', depending on the type of signal).
Typically this comprises a permanently-lit oil lamp with movable coloured spectacles in front that alter the colour of the light. The driver therefore had to learn one set of indications for day time viewing and another for night time viewing.
Whilst it is normal to associate the presentation of a green light with a safe condition, this was not historically the case. In the very early days of railway signalling, the first coloured lights (associated with the turned signals above) presented a white light for 'clear' and a red light for 'danger'. Green was originally used to indicate 'caution' but fell out of use when the time interval system was discontinued.
A green light subsequently replaced white for 'clear', to address concerns that a broken red lens could be taken by a driver as a false 'clear' indication. It was not until scientists at Corning Glassworks perfected a shade of yellow without any tinges of green or red that yellow became the accepted colour for 'caution'. Mechanical signals are usually remotely operated by wire from a lever in a signal box, but electrical or hydraulic operation is normally used for signals that are located too distant for manual operation.
2.2.2 COLOUR LIGHT SIGNALS
On most modern railways, colour light signals have largely replaced mechanical ones. Colour light signals have the advantage of displaying the same aspects by night as by day, and require less maintenance than mechanical signals.
Although signals vary widely between countries, and even between railways within a given country, a typical system of aspects would be:
Green: Proceed at line speed. Expect to find next signal displaying green or yellow.
Yellow: Prepare to find next signal displaying red.
Red: Stop.
2.2.3 CAB SIGNALLING
Cab signaling is a system that communicates track status information to the train cab (driving position), where the engineer or driver can see the information. The simplest systems display the trackside signal aspect, while more sophisticated systems also display allowable speed and dynamic information about the track ahead. In modern systems, a train protection system is usually overlaid on top of the cab signalling system to warn the driver of dangerous conditions, and to automatically apply the brakes and bring the train to a stop if the driver ignores the dangerous condition.[1] Cab signalling systems range from simple coded track circuits, to transponders that communicate with the cab and communication based train control systems.
2.3 INTERLOCKING
In the early days of the railways, signalmen were responsible for ensuring any points (US: switches) were set correctly before allowing a train to proceed. Mistakes were made which led to accidents, sometimes with fatalities. The concept of the interlocking of points, signals, and other appliances was introduced to improve safety. This prevents a signalman from operating appliances in an unsafe sequence, such as setting a signal to 'clear' while one or more sets of points in the route ahead of the signal are improperly set.
Early interlocking systems used mechanical devices both to operate the signalling appliances and to ensure their safe operation. From about the 1930s, electrical relay interlockings were used. Since the late 1980s, new interlocking systems have tended to be of the electronic variety.
2.4 CONTROL AND OPERATION OF SIGNALS
Signals were originally controlled by levers situated at the signals, and later by levers grouped together and connected to the signal by wire cables, or pipes supported on rollers (US). Often these levers were placed in a special building, known as a signal box (UK) or interlocking tower (US), and eventually they were mechanically interlocked to prevent the display of a signal contrary to the alignment of the switch points. Automatic traffic control systems added track circuits to detect the presence of trains and alter signal aspects to reflect their presence or absence.
2.5 APPLICATION AND POSITIONING OF SIGNALS
Originally, signals displayed simple stop/proceed indications. As traffic density increased, this proved to be too limiting, and refinements were added. One such refinement was the addition of distant signals on the approach to stop signals. The distant signal gave the driver/engineer warning that he was approaching a signal which might require a stop. This allowed for an increase in speed, since trains no longer needed to be able to stop within sighting distance of the stop signal.
Under timetable and train order operation, the signals did not directly convey orders to the train crew. Instead, they directed the crew to pick up orders, possibly stopping to do so if the order warranted it.
2.5.1 SIGNAL INDICATES
1. That the line ahead is clear (free of any obstruction) or blocked.
2. That the driver has permission to proceed.
3. Those points (also called switch or turnout in the US) are set correctly.
4. Which way points are set?
5. The speed the train may travel.
6. The state of the next signal.
7. That the train orders are to be picked up by the crew.
2.5.2 SIGNAL POSITION
1. At the start of a section of track.
2. On the approach to a movable item of infrastructure, such as points/switches or a swing bridge.
3. In advance of other signals.
4. On the approach to a level crossing.
5. At a switch or turnout.
6. Ahead of platforms or other places that trains are likely to be stopped.
7. At train order stations.
'Running lines' are usually continuously signalled. Each line of a double track railway is normally signaled in one direction only, with all signals facing the same direction on either line. Where 'bi-directional' signalling is installed, signals face in both directions on both tracks (sometimes known as 'reversible working' where lines are not normally used for bi-directional working). Signals are generally not provided for controlling movements within sidings or yard areas.