This Simple-cost effective-easy to maintain track circuit can be used to identify an incoming train so that it can automatically trigger an alarm system as well as the railway gate. This system is assured to be fail-safe.

1. 1
EN 1140 ENGINEERING DESIGN
ELECTRONIC AND TELECOMMUNICATION DEPARTMENT
UNIVERSITY OF MORATUWA
BELL AND ALARM RAILWAY LEVEL CROSSING SYSTEM-PROGRESS
REPORT
120063M CHANDIKA K.S.G.N.
120167K GANEWATTHE A.V.
120171R GAYAN V.G.T.
120187V GUNATHILAKE S.A.
120318C KUMARA H.P.D.P.
120613C SHANIKA L.U.K.T.
120680C WANNIGE V.M.
120708X WICKRAMASURIYA P.I.L.
120729L WIJETHILAKA M.D.S.R. TEAM 05
120740L ZOYSA H.K.G. TRACK CIRCUIT

2. 2
CONTENTS
1. Abstract----------------------------------------------------------------------------------- 05
2. Introduction------------------------------------------------------------------------------ 06
2.1. Overview in Railway level Crossing Systems in Sri Lanka--------------------07
2.2. Present Track-Relay System and track circuits in Sri Lanka-----------------08
2.2.1. Track circuit-----------------------------------------------------------------------09
2.2.2.Track Relay System---------------------------------------------------------------11
3. Literature research-----------------------------------------------------------------------12
3.1. Track circuit--------------------------------------------------------------------------13
3.1.1. Joint less track circuits-------------------------------------------------------14
3.1.2. History of track circuit-------------------------------------------------------15
3.2. Audio frequency track circuit---------------------------------------------------16
3.2.1. Dual code high frequency track circuits---------------------------------16
3.2.2. Train detection----------------------------------------------------------------17
3.2.3. Automatic speed commands----------------------------------------------17
4. Field visit
4.1. Field visit no 01--------------------------------------------------------------------20
4.2. Field visit no 02--------------------------------------------------------------------20
5. Suggested system designs-------------------------------------------------------------21
5.1. Concept no1: Usage of electromagnetic theory--------------------------22
5.1.1. Overview---------------------------------------------------------------------22
5.1.2. Circuit design----------------------------------------------------------------22

3. 3
5.1.3. Method-------------------------------------------------------------------------22
5.1.4. Components used in the circuit ------------------------------------------23
5.2. Concept no 02---------------------------------------------------------------------24
5.2.1. Circuit diagram----------------------------------------------------------------24
5.2.2. Principles and operations--------------------------------------------------25
6. Implemented concept for the project------------------------------------------------26
6.1. Overview ----------------------------------------------------------------------------26
6.2. How it works------------------------------------------------------------------------26
6.3. schematic design-------------------------------------------------------------------27
6.4. PCB design---------------------------------------------------------------------------28
6.5. Important parts in the circuit---------------------------------------------------29
6.5.1. Performed loops------------------------------------------------------------29
6.5.2. Loop extension cable------------------------------------------------------29
6.5.3. Loop train detector---------------------------------------------------------29
6.6. System implementation---------------------------------------------------------30
6.6.1. Location of the loop--------------------------------------------------------30
6.6.2. Loop installation------------------------------------------------------------31
6.6.3. Detector installation-------------------------------------------------------31
6.7. Sensitivity of the implemented system ------------------------------------32
6.7.1. Sensitivity to temperature -----------------------------------------------32
6.7.2. Trouble shooting------------------------------------------------------------32
7. Fail safe system available with the implemented system-----------------------33

4. 4
8. Identical track circuits currently existing in the world railway industry------34
10. Summary-----------------------------------------------------------------------------------35

5. 5
ABSTRACT
The objective of this project is to identify and investigate on problems related with
unprotected railway level crossings existing in Sri Lanka and provide some solution by
implementing an automated bell and alarm railway level crossing system. Having taken into
consideration the increasing number of accidents at unprotected level crossings, it essential
to have new inventions to help reduce the number of accidents at unprotected level
crossings.At the end of this project it is intended to develop a cost effective, reliable, easily
maintainable automated system for unprotected railway systems. As team 05 we are
implementing the track circuit part of that automated system. This report provides the
detailed progression of the particular section track circuit.

6. 6
1. INTRODUCTION
The place where track and highway/road intersects each other at the same level is known as
“level crossing”. There are mainly two types of level crossing they are manned level crossing
and unmanned level crossing.
Railways being the cheapest mode of transportation are preferred over all the other means
.When we go through the daily newspapers we come across many railway accidents
occurring at unmanned railway crossings. This is mainly due to the carelessness in manual
operations or lack of workers. We come up with a solution for the same. Using simple
electronic components we have tried to automate the control of railway gates.
Here, in this paper we are working on the track circuit. A track circuit is a simple electrical
device used to detect the absence of a train on rail tracks, used to inform signallers and
control relevant signals.
The Purpose of Track Circuits in a railway level crossing
The track circuit is a device designed to continuously prove the absence of a train from a
given section of track; it cannot absolutely prove the presence of a train, since its designed
failure mode is to give the same indication as if a train is present. By proving the absence of
a train, a clear track circuit can be used to confirm that it is safe to set a route and permit a
train to proceed.
Track circuit provides locations on the location of the trains, and this information is used to
command train speed so that trains operate safely.

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1.1 Overview in Railway level Crossing Systems in Sri Lanka
The island's first railroad line, from Colombo to Kandy, was opened in 1867, and in the
1980s Sri Lanka Railways had 1,944 kilometres of railroad track. In early 1988, service in
Northern and Eastern provinces had been irregular for several years. The network's
passenger-kilometres amounted to 1.9 billion in 1986, about 38 percent less than its total in
1982 and now the railway network of Sri Lanka consist of 1,422 kilometres.
According to the records (Nov 2013) of Sri Lankan Railway Department there are nearly 612
unprotected level crossings throughout the island.
The highest number of 225 unprotected level crossings is along the coastal rail track from
Colombo Fort to Galle.
There are 121 unprotected crossings on the Ragama-Puttalamline, Polgahawela-Vavuniya
line 92, Maho-Batticoloa line 81, Peradeniya-Matale line 9, Colombo-Badulla main line 59,
Kelani Valley line 29 and Gal-Oya to Trincomalee16.
Sri Lanka cannot afford to spend Rs. 3 million a month for the safety of thousands of
vehicular passengers who cross the railway lines daily at about 600 points throughout the
country.This question is so pertinent in the light of the increasing number of fatal accidents
at unprotected railway crossings since lately. The last fortnight might go down in the railway
history as the fortnight with highest number of accidents or at least one of such
fortnights.Accidents at railway crossings are not unavoidable phenomena as natural hazards
such as tsunamies or volcanoes. It is simple logic that the only remedy for these accidents
and loss of lives is to prevent the motorists from crossing the railway lines at times when
trains pass through these vulnerable points or to inform them of the approaching trains.
However, there is no doubt that the authorities would argue that public coffers cannot
afford to protect such a large number of railway crossings by putting up gates.
" At the present rate the Railway Department would have to pay such a gateman at least
Rs. 5000, a month in addition to an initial expenditure for the installation of crude
wooden or iron barricades, and there is no point in requesting the authorities to install a
sophisticated protective system. "
Hence there’s essential need in inventing a cost effective reliable way of protecting these
unprotected railway systems.

8. 8
1.2 Present Track-Relay System and track circuits in Sri Lanka
The heart of the signaling system is the interlocking plant. This can be claimed as the
decision making part of the system. The signal outputs are based on the track occupancy,
motor point status, output of the remote end signal and the input from traffic controller.
This plant ensures that before a signal goes in to 'clear' (green) state, it is absolutely safe for
a train to enter into the track segment. The traffic controller’s commands are not executed
if it is not safe to do so.
The interlockingplant is built out of electromechanical relays.
1.2.1. Track Circuit
Track circuit is one of the primary inputs for a signal interlocking plant. An 'interlocking
plant' is the control logic behind the signaling system. The signal cannot be 'green' while
there is another train on track segment ahead. The system should able to detect the
condition of the track segment: occupied or not.

9. 9
The tracks are segmented into 'blocks'. Each block is track circuited separately. The figure
below illustrates a track circuit.
Figure: Track Circuit
The track circuit consists of a power supply on one end and a directional (polarized) relay on
the other end. The power supply has a 6V battery kept charged by a 6V/6A rectifier. In case
of power failure the battery will supply power to the circuit.
The track relay (TR), which has a resistance of 30 ohms and a pickup voltage of 1.4 volt, is
normally held in picked-up state the circuit being completed via the rails. When a train
enters the segment the axels of the train short circuit the supply to the relay and the relay
drops. The contacts of the track relays appear in most of the safety circuits of the
interlocking plant. The interlocking logic is arranged such that only one train can be
permitted to enter a section. If you carefully observe, the track circuit is fail safe; if the
circuit fails it will indicate occupancy.
The variable resister is introduced into the circuit such that it can be tuned to make the
system works under all weather conditions.
The rails are insulated to separate the adjacent track circuits. The polarity of the adjacent
track circuit is always reversed, so that the power supply of one circuit cannot operate the
relay of the other circuit should the insulate between the circuits breakdown. Within one
track circuit the rails are electrically connected by two wires (for safety).
The minimum length of track circuit is depends on the degree of control necessary and the
maximum length is limited by the weather conditions. On the Northern line from Loco
Junction (Maradana) to Veyangoda, the segments have a maximum length of 2000 feet. On
the Southern line from Loco Junction to Wadduwa, due to the saline atmosphere along the
coast line, the track circuits are limited to 1500 feet max.

10. 10
1.2.2. Track Relay System
Relays are electro-mechanical devices used for switching. Relays are used to make the
signaling logic circuits in the interlocking plants. They consists of one or two magnetic coils
(electro magnets) and a set of contacts.
The magnetic system of the relay illustrated below (JRK 10 type) consists of a cylindrical iron
core with coil (pale blue near the bottom), two pole pieces and an armature. Larger relays
(JRK 11) have two iron cores united at the rear with a yoke and the front end being provided
with pole pieces. The armature extends across both pole pieces.
Iron core, pole pieces and armature are made out of iron with excellent magnetic
properties. The armatures are so balanced that the vibration on the unit will not affect the
relay operation
Figure: Magnetic System

11. 11
The relay contacts can be classified into four types. A relay unit will contain a
combination of these types,
Front contact – NO
Back contact - NC
Front/ Back contact
Make before break contact
Figure : Contact System
The contact springs are made out of nickel and the contacts tips are silver. The front
contacts are of twin contacts and the back contacts are single contact type. The rear
end of the contact springs are fixed between two blocks of transfer molded carbonate
plastic reinforced with glass fibre. The stationary contact springs are supported at their
free ends by a strip with notches, which limits the spring movement. The lower end of
this strip is attached to the magnet support.
The movable contact springs are guided by an actuating strip which at the lower end
attached by bearings to the armature and at the upper end to the upper most movable
contact spring. The front edge of the actuating strip provided with slots, which lock the
spring and guide the movement of the contacts.
The rear end of every contact spring has eight forked terminals. This provides a very
dependable connections to the plug board terminals, when the relays are plugged in.

12. 12
3. Literature research
Date:05 03 2014
Link:http://en.wikipedia.org
Source: web page
3.1 TRACKCIRCUIT
A track circuit is a simple electrical device used to detect the absence of a train on rail
tracks. It was used to inform signalers and control relevant signals. The basic principle
behind the track circuit lies in the connection of the two rails by the wheels and axle of
locomotives and rolling stock to short out an electrical circuit. This circuit is monitored by
electrical equipment to detect the absence of the trains. Since this is a safety appliance, fail-
safe operation is crucial; therefore the circuit is designed to indicate the presence of a train
when failures occur. On the other hand, false occupancy readings are disruptive to railway
operations and are to be minimized.
Track circuits allow railway signaling systems to operate semi-automatically, by displaying
signals for trains to slow down or stop in the presence of occupied track ahead of them.
They help prevent dispatchers and operators from causing accidents, both by informing
them of track occupancy and by preventing signals from displaying unsafe indications.
A track circuit typically has power applied to each rail and a relay coil wired across them.
When no train is present, the relay is energized by the current flowing from the power
source through the rails. When a train is present, its axles short (shunt) the rails together;
the current to the track relay coil drops, and it is de-energized. Circuits through the relay
contacts therefore report whether or not the track is occupied.
Figure 1: Schematic drawing of occupied track circuit

13. 13
Each circuit detects a defined section of track, such as a block. These sections are separated
by insulated joints, usually in both rails. To prevent one circuit from falsely powering
another in the event of insulation failure, the electrical polarity is usually reversed from
section to section. Circuits are powered at low voltages (1.5 to 12 V DC) to protect against
line power failures. The relays and the power supply are attached to opposite ends of the
section to prevent broken rails from electrically isolating part of the track from the circuit. A
series resistor limits the current when the track circuit is short-circuited.
In some railway electrification schemes, one or both of the running rails are used to carry
the return current. This prevents use of the basic DC track circuit because the substantial
traction currents overwhelm the very small track circuit currents.
Where DC traction is used on the running line or on tracks in close proximity then DC track
circuits cannot be used, similarly if 50 Hz AC electrification is used then 50 Hz AC track
circuits cannot be used.
To accommodate this, AC track circuits use alternating current signals instead of direct
current (DC) but typically, the AC frequency is in the range of audio frequencies, from 91 Hz
up to 10 kHz. The relays are arranged to detect the selected frequency and to ignore DC and
AC traction frequency signals. Again, failsafe principles dictate that the relay interprets the
presence of the signal as unoccupied track, whereas a lack of a signal indicates the presence
of a train. The AC signal can be coded and locomotives equipped with inductive pickups to
create a cab signaling system.
There are two common approaches to provide a continuous path for traction current that
spans multiple track circuit blocks. The simplest method installs insulated track circuit joints
on only one of the two rails with the second being a path for the return current and a
ground for the track circuit rail. This has the disadvantage of only being able to detect
breaks in one rail so the more popular two rail system uses impedance bonds to permit
traction current to pass between isolated track circuit blocks while blocking current at track
circuit frequencies.
AC circuits are sometimes used in areas where conditions introduce stray currents, which
interfere with DC track circuits.
In some countries, AC-immune DC track circuits are used on AC electrified lines. This is the
predominant method of track circuiting on overhead electrified parts of the UK rail network.
One method provides 5 V DC to the rails, one of the rails being the traction return and the
other being the signal rail. When a relay is energized and attached to the track, normal
voltage is 5 V DC. When there is a break in the circuit and there is no train, the voltage rises
to 9 V DC which provides a very good means for fault finding. This system filters out the
voltage induced in the rails from the overhead lines. These track circuits are limited in length
to about 300m.

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3.1.1. Jointless track circuits
Modern track is often continuously welded, the joints being welded during installation. This
offers many benefits to all but the signaling system, which no longer has natural breaks in
the rail to form the block sections. The only method to form discrete blocks in this scenario
is to use different audio frequencies (AF) in each block section. To prevent the audio signal
from one section passing into an adjacent section, pairs of simple tuned circuit are
connected across the rails at the section boundary. The tuned circuit often incorporates the
circuit to either apply the transmitted signal to the track or recover the received signal from
the other end of the section.
Consider a railway with two block sections as in the diagram. Section 1 has frequency A
injected at the left-hand end and received at the right-hand end. Section 2 continues from
the right hand end of section 1 where frequency B is injected and then received at the right-
hand end of section 2.
There is often a gap between where frequency A is received and frequency B is injected.
This is referred to as a 'tuned zone' and is a section of track where the amplitude of
frequency A reduces in the direction of section 2 and the amplitude of frequency B reduces
in the direction of section 1. The tuned zone can be of the order of 20 m long.
Advantages of jointless track circuits:
 Eliminates insulated block joints, a component liable to mechanical failure (both of
insulation and by introducing stress to adjoining rails) and maintenance.
 In electrified areas, jointless track circuits require fewer impedance bonds than any
other double rail traction return track circuits.
Disadvantages of jointless track circuits:
 Restrictions on placing impedance bonds, hence any connection for electrification
purposes, in or near tuned zones as this may upset the filter properties of the tuned
zone.
 Electronic circuits are more vulnerable to lightning strikes.
3.1.2. History of track circuits
The first use of track circuiting was by William Robert Sykes on a short stretch of track of the
London Chatham and Dover Railway at Brixton in 1864. The failsafe track circuit was
invented in 1872 by William Robinson, an American electrical and mechanical engineer. His
introduction of a trustworthy method of block occupancy detection was key to the
development of the automatic signaling systems now in nearly universal use.

15. 15
The first railway signals were manually operated by signal tenders or station agents. When
to change the signal aspect was often left to the judgment of the operator. Human error or
inattentiveness occasionally resulted in improper signaling and train collisions.
The introduction of the telegraph during the mid-nineteenth century showed that
information could be electrically conveyed over considerable distance, spurring the
investigation into methods of electrically controlling railway signals. Although several
systems were developed prior to Robinson's, none could automatically respond to train
movements.
Robinson first demonstrated a fully automatic railway signaling system in model form in
1870. A full-sized version was subsequently installed on the Philadelphia and Erie Railroad at
Ludlow, Pennsylvania (aka Kinzua, PA), where it proved to be practical. His design consisted
of electrically operated discs located atop small trackside signal huts, and was based on an
open track circuit. When no train was within the block no power was applied to the signal,
indicating a clear track.
An inherent weakness of this arrangement was that it could fail in an unsafe state. For
example, a broken wire in the track circuit would falsely indicate that no train was in the
block, even if one was. Recognizing this, Robinson devised the closed loop track circuit
described above, and in 1872, installed it in place of the previous circuit. The result was a
fully automatic, failsafe signaling system that was the prototype for subsequent
development.
Although a pioneer in the use of signals controlling trains, the UK was slow to adopt
Robinson's design. At the time, many carriages on UK railways had wooden axles and/or
wheels with wooden hubs, making them incompatible with track circuits.

16. 16
Date: 07.04.2014
Link: http://www.alstomsignalingsolutions.com/Data/Documents/Track_Circuit_09.pdf
Source: PDF document
3.2. AUDIO FREQUENCY TRACK CIRCUITS
Dual code audio frequency (AF) track circuits to mitigate electromagnetic interference
(EMI) with the consequence of preventing the unsafe condition of falsely energizing track
relays during track circuit occupancy. The use of dual code AF track circuits enables AF
track circuits to operate under tight constraints with immunity to EMI and adjacent track
circuit interference. Dual code AF track circuits were initially implemented systemwide on
MARTA in 1980 with great success.
3.2.1 Dual Code High-Frequency Track Circuit reliably operates in the presence of electrical
noise generated by trains. The track circuit is ideally suited for use on continuous welded
rail, where insulated joints are neither desired nor required. Combining solid-state devices
with proven vital circuit relays, this track circuit eliminates the need for insulated joints,
protects against interference from foreign current picked up in the rails, increases track
circuit versatility, and provides a simple and proven method of transmitting function
commands to the train for cab signals and speed control or for complete automatic
operation. Track circuit lengths of up to 1,800 feet can be obtained, with suitable ballast
conditions. A unique feature of the dual code high-frequency track circuit is the flexibility
afforded by the "building block" concept. This allows the basic track circuit to be upgraded
with more features added to accommodate stages of expanding facilities.
The Dual Code High-Frequency Track Circuit equipment consists of WEE-Z® bonds, a solid
state transmitter and receiver, and a track relay. With the exception of the WEE-Z bond, this
equipment can be housed in a central equipment room or in a wayside case. The WEE-Z
bond couples information between the rails and the electronic wayside equipment (via a
single twisted-pair line circuit), defines block limits, and confines pertinent frequencies to
the applicable track circuits. The transmitter and receiver consist of printed circuit boards
which plug into a rack-mounted cabinet located near the associated track relays. The
transmitter supplies, and the receiver responds to, the high-frequency signals in the rails,
which provide track occupancy detection and automatic train protection commands. Up to
eight frequencies are available for track occupancy detection. One or two additional
frequencies can be used to transmit cab signal and/or speed control information, depending
on the amount of information required. The WEE-Z bond traction current return can be
connected to a traction return feeder, to a substation, or to a bond on an adjacent track.

17. 17
System Operation
3.2.2. Train Detection
The track is divided into blocks, with each block checked for occupancy by high-frequency
track circuits. Except at interlockings, there are no insulated joints for block separation. The
bonds also inject into the track speed commands that are picked up by the train. WEE-Z
bonds are located at the ends of each track circuit, with one circuit usually in each block but
two or more circuits in longer blocks. Except at interlockings, a particular bond serves as a
track circuit boundary, the transmitter coupling for one circuit and the receiver coupling for
the next downstream track circuit. An ATP transmitter at the leaving end of the track circuit
feeds high-frequency energy to the track, using the WEE-Z bond as a coupling transformer.
Acting as a receiver, a bond at the entering end of the circuit energizes a track relay if the
signal from the transmitter is not shunted by the axles of a train. The transmitter and
receiver associated with one particular track circuit are tuned to the same frequency.
3.2.3. Automatic Speed Commands
The high-frequency track circuit equipment is the communication channel between the
wayside and train for the ATP speed limit commands. In addition, the WEE-Z bonds and rails
are the transmission mediums for the train to wayside communications (TWC) system. The
ATP speed command channel has a frequency separate from the train detection and TWC
frequencies. When a train is detected in a circuit, a speed command generated by the
wayside track transmitter at the leaving end of the circuit is transmitted through the rails to
the train to control its speed.

18. 18
WeeZ bonds
1. Filters the train detection carrier frequency unique to that track circuit by the use of a
bandpass filter.
2. Amplifies the coded carrier frequency.
3. Removes the carrier frequency element of the signal and leaves only the code rate. This is
called the demodulator.

19. 19
4. The code rate signal has spikes and is not a clean square wave form. The spikes are
removed and a clean square wave is formed by the use of a level detector.
5. Sends square wave representing the code rate to the decoder driver board. The interface
for the correct voltage and current to decoder, and eventually the track relay, is
accomplished by the decoder driver board.
6. Filters the signal from the decoder driver for the correct code rate and then sends it to
the track relay. The decoder contains a series tuned circuit on the primary winding of the
tuned decoder transformer. The secondary winding of the transformer contains a full wave
bridge rectifier and filtering capacitor to convert the code signal to DC to drive the track
relay. It is the decoder that makes the dual code track immune to EMI and induced
interference.
7. The track relay is a GRS vital B1 relay designed for the dual code application. This relay is a
direct replacement for the original track relay.

20. 20
4. Field visit
4.1. Field visit no 1
Our group member have been visited to some rail way tracks regarding the project to get an overall
idea on the designing requirements of the track circuit we are going to implement.
Date: 08.03.2014
Place: Galle-Akurassa road, Katugoda railway crossing.
Habaraduwa railway crossing
General description of the visited area:
Katugoda railway crossing is situated in the southern coastal railway line towards Matara. Katugoda
railway station also situated near to this railway crossing and this station has double railway lines
since it is used as an exchange centre.
Description of the environment:
Climate:
Katugoda is an area within the coastal line of southern region, very close to the ocean. The region's
rainfall is unimodal and obtains high rainfall from May to October. The annual rainfall is about 860
mm. The duration of the rainy season decreases from south to north. The major part of the region,
about 75%, is lowlands with an altitude below 30m. It is hot for most of the year with average
temperature ranging from >27.50c.The main areas affected by the project generally have a relatively
high moisture deficit. The area around the site is normally hot.
4.2. Field visit no2
As our second field visit we have visited Ceylon Government Railway (CGR).
Date: 24 04 2014
Place: chief mechanical engineers office, Rathmalana.
Description: we contact the chief mechanical engineer there and he adviced us to conjtact
signalling department of the Sri Lanka railway which situated at dematagoda.

21. 21
5. Suggested track system designs
After our two field visits and investigating in to existing railway track systems our group
came up with 2 system designing concepts. Information related to these two system
designing are noted below.
5.1. Concept No 1; usage of electromagnetic theory
5.1.1. Overview
This concept consists of a method which uses electromagnetic theories. When a
steady current flows in one coil a magnetic field is produced in the other coil. But if that
magnetic field is not changing ,Faraday's law tells us that there will be no induced voltage in
the secondary coil. But if a switch is opened to stop the current there will be a change in
magnetic field in the coil and a voltage will be induced in the other coil. A coil is a
reactionary device, not liking any change! The induced voltage will cause a current to flow in
the secondary coil which tries to maintain the magnetic field which was there. The fact that
the induced field always opposes the change is an example of Lenz' Law. Once the current is
interrupted and the switch is closed to cause the current to flow again an induced current in
the opposite direction will oppose that buildup of magnetic field. This persistent generation
of voltages which oppose the change in magnetic field is the operating principle of a
transformer. The fact that a change in the current of one coil affects the current and voltage
in the second coil is quantified in the property called Mutual inductance.
In our concept we use not only the mutual inductance between two coils,but also
the variation of the induced voltage.

22. 22
We use an AC power supply to give an AC current to the coil so that in other coil, a voltage is
induced. This induced voltage is then going through a bridge rectifier so that the voltage
signal will be DC. This DC voltage signal is then amplified and goes through a comparator.
This way we give the signal to the controller.
5.1.2. Circuit Design.
5.1.3. Method
The two coils illustrated above are placed on both side of a single railway track (A single
railway track is also shown above.) When a train goes through these coils, the medium
between the cores are changed. Therefore the induced voltage will also change and this
change is now amplified using the amplifier and then compared using the comparator. This
can give us the idea whether there is a train on the particular track or not. Using the flip-flop
we can give a steady signal once the train passes by the track circuit. After passing the level
cross, Track circuit in the direction gives the reset signal to the flip-flop.
5.1.4. Components used in circuit
• LM324
used as the amplifier and the comparator.

23. 23
The LM324 series are low cost, quad operational amplifiers with true differential inputs.
They have several distinct advantages over standard operational amplifier types in single
supply applications. The quad amplifier can operate at supply voltages as low as 3.0 V or as
high as 32 V with quiescent currents about one fifth of those associated with the MC1741
(on a per amplifier basis). The common mode input range includes the negative supply,
thereby eliminating the necessity for external biasing components in many applications. The
output voltage range also includes the negative power supply voltage
• 74LS74 flip-flop
The SN54/74LS74A dual edge-triggered flip-flop utilizes Schottky TTL circuitry to produce high speed
D-type flip-flops. Each flip-flop has individual clear and set inputs, and also complementary Q and Q
outputs. Information at input D is transferred to the Q output on the positive-going edge of the clock
pulse. Clock triggering occurs at a voltage level of the clock pulse and is not directly related to the
transition time of the positive-going pulse. When the clock input is at either the HIGH or the LOW
level, the D input signal has no effect.
• Bridge Rectifier
• AC Power supply

24. 24
5.2. Concept no2; vibration sensing
5.2.1. Circuit diagram
5.2.2. Principles and operation
The basic principle of above track circuit is induction of current using vibration of
railway track by piezoelectric sensor which happens on train is coming. This circuit is
monitored by electrical equipment to detect the absence of the trains. Since this is a safety
appliance, fail-safe operation is crucial; therefore the circuit is designed to indicate the
presence of a train when failures occur. On the other hand, false occupancy readings are
disruptive to railway operations and are to be minimized.
Basic circuit:
A track circuit typically has power applied. Piezoelectric sensor fixed on railway track.
When no train is present, no voltage generate in sensor. When a train is present and it’s
coming, huge vibration generate and sensor generate voltage and detects train is coming or
passing sensor position. Circuits through the relay contacts therefore report whether or not
the track is occupied.
Piezoelectric sensors require some precautions when connecting to sensitive
electronic components. First and foremost, the voltage levels created by hard shock can be
very high, even around 100-V spikes.
More than likely, an op amp will be used to interface these sensors to an A/D converter,
either discrete or on a microcontroller. One tip is to choose a high-input-impedance op amp
to minimize current. One possible candidate is the Linear Technology JFET input dual op
amp. It has 10¹² Ω input resistance and a 1 MHz gain bandwidth product, good enough to
easily handle the vibration ranges of piezoelectric sensors.

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Another suitable part is the TLV2771from Texas Instruments. This rail-to-rail low-power op-
amp also has a 10¹² Ω differential input resistance and a 5 MHz unity-gain bandwidth. Signal
conditioning in a single stage can prepare the input from the shock sensor directly into an
A/D converter. Op-amp circuits can be designed to operate in voltage mode or charge
mode. Charge mode is used when the amplifier is remote to the sensor. Voltage mode is
used when the amplifier is very close to the sensor.
Another tip is to attenuate the input signal and use the op amp’s gain to bring into the
desired range. Be aware that you may need snubbing protection on the inputs of the op
amp, especially if the design could be subjected to harsh hits.
Also note that you may think that a pressure sensor would generate only a positive voltage,
but, in reality, the signal from the sensor can ring and introduce negative voltage spikes.
This means that you may need to squelch negative voltage levels on the op-amp inputs,
especially if using only a single rail power supply on the op amp.

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6. Implemented concept for the project:
Basics of vehicle loop detectionwith electromagnetic induction
theories
6.1. Overview
There are a number of ways to detect vehicles, for traffic control or drive-thru, inductive
loop technology is the most reliable.
An inductive loop vehicle detector system consists of three components: a loop (preformed
orsaw-cut), loop extension cable and a detector. When installing or repairing an inductive
loop system the smallest detail can mean the difference between reliable detection and an
intermittent detection of vehicles. Therefore, attention to detail when installing or
troubleshooting an inductive loop vehicle detection system is absolutely critical.
6.2. How it Works:

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The preformed or saw-cut loop is buried in the traffic lane. The loop is a continuous run of
wire that enters and exits from the same point. The two ends of the loop wire are
connected tothe loop extension cable, which in turn connects to the vehicle detector. The
detector powers the loop causing a magnetic field in the loop area. The loop resonates at a
constant frequency that the detector monitors. A base frequency is established when there
is no vehicle over the loop. When a large metal object, such as a vehicle, moves over the
loop, the resonate frequency increases. This increase in frequency is sensed and, depending
on the design of the detector, forces a normally open relay to close. The relay will remain
closed until the vehicle leaves the loop and the frequency returns to the base level. The
relay can trigger any number of devices such as an audio intercom system, a gate, a traffic
light, etc.
There is a misconception that inductive loop vehicle detection is based on metal mass. This
is simply not true. Detection is based on metal surface area, otherwise known as skin effect.
The greater the surface area of metal in the same plane as the loop, the greater the increase
in frequency. For example, a one square foot piece of sheet metal positioned in the same
plane of the loop has the same effect as a hunk of metal one foot square and one foot thick.
Another way to illustrate the point is to take the same one square foot piece of sheet metal,
which is easily detected when held in the same plane as the loop, and turn it perpendicular
to the loop and it becomes impossible to detect.
By looking in to overall matching of this vehicle loop detector with required track circuit
features we decided to implement the project further with this concept.
Newly implemented track circuit with induction loop concept is as follows.
6.3. Schematic design

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6.4. PCB design

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6.5. Important parts in the design
6.5.1. Preformed and Loops
A preformed loop is typically 3 to 5 turns of loop wire encased in PVC pipe for use in new
construction before the track is installed. The loop wire is 14 or 16 awg stranded machine
tool wire with an insulation of XLPE (cross-linked polyethylene) encased in PVC pipe to hold
the loop’s shape and to protect the loop wire from damage while the pavement is installed.
A saw-cut loop is used when the pavement is already in place. The installation involves
cutting the loop shape in the pavement with a concrete saw, laying the loop wire in the slot,
pressing in a polyfoambacker rod to keep the wire compacted and finishing with saw-cut
loop sealant or street bond to fill the slot and protect the wire. It is best to use the
recommended 14 or 16-awg machine tool wire for loop installation. The insulationhas a high
resistance to water, heat, abrasions, oils and gasoline. Purchase the wire from the same
source you bought the saw-cut loop sealant to be sure to get the correct wire.
6.5.2. Loop Extension Cable
Loop extension cable is used to extend the distance from the preformed or saw-cut loop to
the vehicle detector, which is usually located indoors or in a weatherproof enclosure. The
characteristics of the extension cable are just as important as the characteristics of the loop
wire. Use only 14, 16, or 18 awg stranded 2 conductor twisted, shielded cable with a
polyethylene insulation jacket. The extension cable connections to the loop wire and the
vehicle detector wires must be soldered. Do not use any other method for connection. The
distance between the loop and the detector can safely be extended to 300 feet with proper
extension cable, however check with the vehicle detector manufacturer for confirmation.
6.5.3. Loop train Detector
The proper installation and material is critical! In general, loop vehicle detectors from all
manufacturers work under the same principle and will all work reliably if the installation is
done properly and the correct materials are used. Train detector features differ between
manufacturers, and most are straight forward. The following features need special
consideration.

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Number of Outputs.
Ourdetector provide a switch closure via a relay, which is typicallyconfigured as normally
open. It is the number of outputs provided that may be importantand how they can be
configured. More and more devices, particularly in the drive-thru environments, need to be
triggered by vehicle detection, such as audio communication, car timing, message greeting,
electronic menu boards, gates, etc. Determine the number of devices that will be used now
and in the future with the train detector and can match or exceed that number with the
number of available relay outputs.
Signal Type. Detector provide a constant presence style of signal output. In otherwords, the
relay output is closed the entire time that a vehicle is present over the loop, and does not
open again until the vehicle drives away. Most devices require this style of output
signal,however some devices require a pulse style, which will only momentarily close the
relay at thetime when the train is detected. Check the requirements of the devices that you
are connectingto the detector. If you are connecting more than one device to the detector,
make sure that the detector can provide the required signal types at the same time. Some
detectors can only provide one or the other style of signal output at a time.
Diagnostics.
This train detector provides PC diagnostics via a communication port on thedetector.
Diagnostic software gives you a visual picture of what is happening at the loop, and will help
you troubleshoot any problems you may experience during installation or in the future.
Detectors with this feature are usually in the same cost range as other detectors and can
help to save time solving a detection problem. The PC software and cable is usually
additional. Diagnostics software can also help determine the depth and position of the loop.
6.6. System implementation
6.6.1. Location of Loop
The position of the loop relative to the train you are trying to detect is extremely important.
Vehicles entering a fast food restaurant drive-thru lane will stop at the menu board with the
driver’s window positioned in line with the speaker post. The front axle is the only metal
surface whose relative position to the driver is consistent from vehicle to vehicle. Because of
this fact, the vehicle detector is designed to pick up the front axle, not the vehicle’s engine.
Therefore, the loop should be positioned 1 ½ to 2 feet ahead of the speaker post, with the
long axis of the loop running perpendicular to the traffic lane. This positions the axle of the
vehicle directlyover the loop in the same direction as the loop.
The proper installation and location of the loop are the most important aspects of reliable
vehicle detection. In recent years, there has been an increase in the number of missed and
false detectionsdue to the popularity of SUVs. The missed detections can be attributed to
two factors. First, and most obvious, is that the metal surface area of the taller vehicles is
farther away from the loop which makes the vehicle more difficult to detect. Second, and

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less obvious, is that larger vehicles have a greater turn- front axle. The driver finds it difficult
in some drive-thru lanes to round the corner prior to theloop and as a result, the vehicle
becomes positioned further away from the curb and not properly positioned over the loop.
Compound the poor position of the vehicle with the height of the vehicle and you have a
difficult vehicle to detect.
6.6.2. Loop Installation
There are a couple of important points to make with regard to saw-cut loop and preformed
loop installation. It is important that when the installation is complete the loop be no more
than 2” below the surface of the asphalt or concrete. The deeper the loop the less sensitive
the loop detection system becomes. It is also important that the lead-in wires from the
detector to the beginning of the loop be twisted a minimum of five times per foot.
6.6.3. Detector installation
The proper installation and material is critical! In general, loop vehicle detectors work under
the same principle and will all work reliably if the installation is done properly and the
correct materials are used.

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6.7. Sensitivity of the implemented system
Our train detector have adjustable settings for sensitivity. If the detector is missing trains
then the sensitivity is set too low. If the detector is jumpy or is creating false detections, it
may be set too sensitive. However, all inductive loop detectors are dealing with the same
physical characteristics of a magnetic field in a loop. The maximum height of detection is
roughly 2/3 the length of the short side of the loop. For example, if you have a loop that is
18”x 60”, the maximum height of detection is 12” from the loop.
The most effective way to increase sensitivity is to lengthen the short side of the loop. Most
Drive-thru loops are 18 to 24 inches wide. If you take an 18” x 60” loop and increase the
short
Side to 24”, you have increased the height of detection by 4”. However, making the loop too
Wide can cause a different problem. In the drive-thru scenario where train move slowly, and
bumper to bumper, a system that is too sensitive may not be able to identify the gap
between trains causing a missed detection Another misconception about loop sensitivity is
that increasing the number of turns in the loop will increase sensitivity. Increasing or
decreasing the number of turns does not affect sensitivity. Increasing the number of turns
increases stability. Three to five turns is ideal for maintaining the proper stability and
sensitivity combination. The frequency of the loop will change as the environment changes,
as a result most detectors are designed to constantly adjust to this slow change in frequency
over time.
6.7.1. Sensitivity to temperature
The detector’s purpose is to detect rapid changes in frequency. However, inductive loops
and detector is sensitive to temperature. When the temperature of the inductive loop
increases, the frequency will decrease, and the opposite is true of the detector. When the
temperature of the detector increases the frequency will increase. If the temperature of
either the loop or the detector increases or decreases too fast, false detections will occur.
The loop, buried in the track is not likely to change temperature rapidly, however mounting
the detector in the wrong place can cause such a problem. For example, mounting the
detector directly in line with a window where it can get a cold blast of air whenever the
window is open, can result in problems.
6.7.2. Troubleshooting
Most detectors provide LEDs that will indicate a problem with the loop, such as a short or an
open. It is possible for a problem to occur that will cause the error indicating LED to stay on
and yet the installation is ok, but simply needs a reset. Lightning can cause such a problem.
Electrical storms can cause havoc with equipment, especially vehicle detectors because the
loop is outside. If problems persist, check the connections to the extension cable and to the
loop lead-in wires. Bad connections are a very common problem with inductive loops. If the
installed detector has a communications port for diagnostics, beg, borrow or steal, a copy of
the software and cabling needed to utilize this feature. Diagnostics software is an amazingly
powerful tool for diagnostics, is less expensive than the test equipment needed to do the

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same job and will provide more information. Some diagnostics software will even capture
the data to disk. This is especially useful if you have an intermittent problem. You can leave
the computer running for days in order to capture the problem. In addition to using
diagnostics software to capture or see a problem as it occurs it can often be used to help
locate the loop in the pavement in order to determine if it’s been properly positioned and
buried at the right depth. If a communications port is not available, the next best thing is a
mega ohm meter. After disconnecting the loop from the detector, place one lead of the
meter to one of the lead wires of the loop and the other to earth ground. The resistance
should be greater than 100 mega ohms. If the resistance is between 50 and 100 mega ohms
then it is possible that the loop wire is nicked or the extension cable has been damaged. If
the resistance is less than 50 mega ohms, the loop is shorted to ground. In either case the
loop or the extension cable must be replaced.

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7. Fail safe system available with the implemented system
Although our theories were well evaluated and tested, there would be some occasions
where external disturbances will occur at our system. Since our circuit is placed near the
railway tracks, there would definitely be heavy vibration and exposure to critical weather
conditions. We consider these disturbances are the major threats that our system will go
through. So for each threat, we came up with a solution.
7.1For Critical weather conditions and Vibration
We planned to design the enclosure with two materials. Outer Layer would be Steel or
Aluminum as they are design to withstand to vibration and critical weather. Inner layer will
be design using plastic since the inner circuit will need protection from heat. Circuit is
mounted on several springs/dampers to withstand the heavy vibration. A sample design of
our enclosure and placement is shown below.
7.2. Circuit fail safe system
In our circuit design, there are three major functions. A Sine wave generator, tuning circuit
including loop and comparators. In our fail-safe method, if there is any component failure in
the sine wave generator or in loop circuit, our circuit will inform the central controller as an
occupancy of a train in that particular track. So the barrier gates will be closed.
As you see in the diagram there are three comparators which compares the same signal
from the loop. So if there is any trouble in one comparator, other two will work accordingly.
In such situation, technicians must manually over ride this system in order to avoid road
traffic.

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8. Identical track circuits currently existing in the world railway
industry
we found that there are many companies use track circuits identical to our implemented
track circuit. The world-wide company names as follows.

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8. Summary
Inductive loop detection is relatively simple as a system, but it is important to arm yourself
with the knowledge of how it works and how the pieces interrelate. There is no question
that a problematic installation can be extremely frustrating, but if you break it down to
basics it can be solved more efficiently.
 The loop should be buried no more than 2” below the asphalt or concrete surface.
 Replace any loop wire that has nicks or splices in the insulation.
 Loop wire should be 14 or 16 awg machine tool wire with XLPE insulation.
 Loops should be no less than three turns and no greater than five.
 The number of turns increases stability of the signal over long runs between the loop
and detector.
 The number of turns does not affect sensitivity.
 Extension cable should be 14, 16, or 18 awg twisted/shielded 2-conductor cable with
polyethylene jacket.
 The wires that lead into the loop must be twisted a minimum of five turns per foot.
 The maximum height of detection is roughly 2/3 the length of the short side of the
loop.
 Connections to the detector, the loop and the extension cable should be soldered.
 The frequency decreases as the temperature of the loop increases
 The frequency increases as the temperature of the detector increases

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References
http://en.wikipedia.org
http://www.alstomsignalingsolutions.com/Data/Documents/Track_Circuit_09.pdf
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