CHAPTER I
INTRODUCTION
Petri nets as graphical and mathematical tools provide a uniform environment for modelling, formal analysis, and design of discrete event systems. One of the major advantages of using Petri nets is that the same model is used for the analysis of behavioural properties and performance. Petri nets as graphical tools provide a powerful communication medium between the user, typically requirements engineer, and customer. Complex requirements specification instead of using ambiguous textual description or mathematical notations difficult to understand by the customer can be represented graphically using Petri nets. This combination with existence of computer tools allowing for interactive graphical simulation of Petri nets puts in hands of the development process of complex system. The ability of Petri nets to verify the model formally is especially important for real time safety-critical systems such as air-traffic control system, rail-traffic control systems etc. Petri nets were used to model real-time fault tolerant and safety-critical -system. Fault detection and in process monitoring were modelled and analyzed.
Discrete Event Systems (DES) that have features like non-determinism, asynchronism, event-driven and simultaneity in their structures can be modelled by Petri Nets. However ordinary PNs do not deal with actuators or sensors, therefore in addition to all these methods for modelling and analyzing DES a new method is proposed as an extension of PNs called as Automation Petri Nets (APNs) that can embrace both actuators and sensors. One of the main advantages of using APNs is that; implementing the model of the system into programmable logic controller (PLC) can be achieved easily by using Token Passing Logic (TPL). PNs are m re comprehensible then other modelling tools like Finite State Machines (FSM) and Grafcet. In addition to this, because of its easiness in graphical illustration and easiness in practice on implementing on a PLC, PNs have wide field of application areas in industry.
Beside industrial applications PNs are also used in railway applications like scheduling rail operations, supervisory control approach for railway networks, modelling train control systems for level crossing, constructing PN model automatically for subways, simulation of railways and development of interlocking and signalling systems with safety verifications. In this paper interlocking and signalization of a sample railway yard is modelled by APNs and resulting model is implemented on a PLC by using TPL to verify its accuracy. This paper is structured as follows.
CHAPTER II
BLOCK DIAGRAM
TRAFIC LIGHT POST1
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TRAFIC LIGHT POST1
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RELAY
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SENSOR 1
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SENSOR 2
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TRAFIC LIGHT POST1
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TRAFIC LIGHT POST1
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PLC MODULE
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SENSOR 4
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SENSOR 3
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SENSOR
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RELAY
|
FIGURE 1
Block diagram for Signalization and Interlocking
2.1 BLOCK DIAGRAM DISCRIPTION
Relay based and digital based systems are required if there is a special requirement for speed or resistance to electric noise. Here plc module used is GeFanuc, plc software is Versa Pro. Sensors are those devices which are used for detecting any change in the system. Here the sensor used is a mechanical contact type sensor which works same as that of a press button. While the train reaches the corresponding positions the sensor transfers an electrical signal such that the respective changes in the system get processed. 24V, 1 amp 2 normally opened 2 normally closed relay is used . We can use this relay for rail interlocking, by the action of the relay the inter lock part of the rail get moved to corresponding position since the train used to move to the corresponding destination.
CHAPTER III
COMPONENTS OF RAILWAY INTERLOCKING AND SIGNALIZATION SYSTEM
3.1 COMPONENTS
Interlocking and signalization system consists of the following parts:
3.1.1 Switches
Railway vehicles do not have a steering mechanism like the other road vehicles. For this reason, shifting from one rail to another or changing a route can be actualized by switches. An example crossover switch is shown in Figure 2.
FIGURE 2 Cross over switch
3.1.2 Track Circuits (TC) and Signals
A track circuit (see Figure 3) is simple electrical equipment used to detect the presence or absence of a train on railway, and are used to inform signallers and other control relevant equipment. Signal lights are found on every railway yard in front of TCs to inform trains if the rail in front is occupied or not. Likewise on road traffic lights, green means that railway is unoccupied (i.e. it is free to go and red means that railway is occupied (stop).
FIGURE 3 A Track circuit
3.1.3 Interlocking and Traffic Command Center
Railway traffic is controlled from Traffic Command Center (TCC) by the help of track circuits and the other railway equipments given in this section. TCC reserves the desired route for an incoming train. Interlocking enables reservations by changing the signals and positions of switches on that route when it is possible. Therefore the other routes that intersect with this reserved route are prohibited to assure safety.
After the train completes the route reserved for it, prohibited railway tracks become enabled for new route reservations. If a train enters to a previously reserved route when it is prohibited, the train can be stopped by collision prevention systems such as Automatic Train Stop (ATS) and Automatic Train Protection (ATP).
CHAPTER IV
AUTOMATION PETRI NETS
Discrete Event Systems (DES) that has features like non-determinism, a synchronism, event-driven and. Simultaneity in their structures can be modelled by Petri Nets. However ordinary PNs do not deal with actuators or sensors, therefore in addition to all these methods for modelling and analyzing DES a new method is proposed as an extension of PNs called as Automation Petri Nets (APNs) that can embrace both actuators and sensors. One of the main advantages of using APNs is that; implementing the model of the system into programmable logic controller (PLC) can be achieved easily by using Token Passing Logic (TPL) .In this paper interlocking and signalization of a sample. Railway yard is modelled by APNs and resulting model is implemented on a PLC by using TPL to verify its accuracy.
An extension of the Petri Net definition in can be done easily by adding four terms to ordinary PNs and known as Automation Petri Nets (APN). An APN is defined as follows and can be seen on,
APN = (P, T, Pre, Post, In, En, χ, Q, M0)
· P : {p1, p2, …, pn}, finite set of places.
· T : {t1, t2, …, tn}, finite set of transitions.
· Pre : (PxT) → N, directed ordinary arcs from places to transitions (N is a set of nonnegative integers).
· Post : (TxP) → N, directed ordinary arcs from transitions to places.
· M0 : P → N, initial marking.
· In : (PxT) → N, inhibitor arcs from places to transitions.
· En : (PxT) → N, enabling arcs from places to transitions.
· χ : {χ1, χ2, …, χm}, firing conditions associated with the transitions.
· Q : {q1, q2, …, qn}, finite set of actions that might be assigned to the places.
FIGURE 4 Petri Net logic
It is shown in figure 5 that if a place is connected to a transition with an enabling arc that transition can be fired, if, and only if, the token number on that place is equal to or greater than the number of enabling arc. Likewise if a place is connected to a transition with an inhibitor arc that transition can be fired, if, and only if, the token number on that place is less than the number of inhibitor arc.
FIGURE 5 Petri Net logic
(a) Number of tokens on P1 is less than the number of enabling arcs from P1 to t1 therefore t1 is not fired.
(b) Number of tokens on P3 is less than the number of inhibitor arcs from P3 to t2 therefore t2 is fired.
More than one action can be assigned to places, and by putting enough number of tokens on a place, the assigned actions are activated. However, a transition can be fired under two conditions: firstly, number of tokens on a place must be greater than or equal to the number of ordinary directed arcs, and secondly transition firing condition χ (information coming from sensors that is associated with transition t) must be satisfied.
When these two conditions are satisfied, tokens can be passed from one place to the other through transition t and number of tokens on both places decrease and increase, suitably as it is shown, which is shown on figure 8.
FIGURE 6 Token passing
In Figure 6(a) the number of tokens on P1 is equal to number of ordinary arcs between P1 - t1 therefore t1 is enabled. If firing condition χ1 is satisfied than transition t1 is fired and number of tokens on P1 are decreased (by the number of ordinary arcs from P1 to t1) and the number of tokens on places connected to t1 are increased (by the number of ordinary arcs from t1 to P2 and t1 to P3) as can be seen on Figure 6(b).
4.1 INTERLOCKING & SIGNALIZATION DESIGN AND TOKEN PASSING LOGIC
Railway yard shown in Figure 7 is used for application. The entrance of any train on this railway yard can be sensed via track circuits which are established on regions A, B, C and D. After entering the section the train follows the route which is reserved by TCC.
FIGURE 7 Railway yard
There is a crossover switch in the section where Xo-Xc shows the position of the switch. When switch X is on Xo position, crossing from upper track to lower track is enabled, otherwise crossing is disabled.
For the chosen sample railway yard in Figure 7, the APN model can be seen in Figure 8. For every track circuit M (bit) memories (because of only one train can exist on a railway track) are used on S7-200 PLC. Incoming signals (ascending (↑) and descending (↓) edges) from track circuits (inputs of the PLC) and reserved route information provided by TCC are compared with each other by PLC, then lights on the reserved routes are turned to green and all the others are kept on red.
While a train is moving on its route all lights which are passed by turn back to red immediately for new reservations. Note that all lights are red and switch is on Xc position when there is no train on the railway yard.
If an unwanted situation occurs like reservation of track B and X or reservation of track D and track X at the same time, APN model disables all the crossings on switches and keeps all the related lights on red to prevent derailing or any collision by using enabling and disabling arcs as can be seen from APN model in Figure 9.
Memory addresses and their definitions are given on Tablen1 and input-output configuration of PLC is given on Table 2. After modelling sample railway yard by APNs it can easily be turned into a ladder diagram for PLC using TPL [5-11]. An example is given in Figure 5.
FIGURE 8 Example of APN’s and its ladder logic
As it can see from figure 8, in order to fire transition t1 five conditions must be satisfied. Inhibitor and enabling arcs give flexibility to ordinary PNs and very useful in modelling DES and it is easy to convert the model to a ladder code for PLC.
APN model of the railway yard consists of 24 transitions with 9 places. Firing conditions associated with the transitions are ascending and descending edges of the input signals incoming from track circuits. If all conditions are satisfied then related transitions is fired and token (train) passes from that place (track circuit) to another place (track circuit).
For example, in order to fire transition t16, four conditions have to be satisfied which are as follows: There have to be a train on TC-C, TC-D must be reserved (place PC and PDm have one tokens) and there mustn’t be any train between TCD and TC-C, TC-X mustn’t be reserved (place PDC and PXm must have no tokens) and if the descending edge signal of TC-C is received afterwards t16 will be fired and train (token) can pass from one place to another.
While the train is moving along its route related lights and switch (if necessary) change their position. At the beginning, it is assumed that all lights are on red and switch X is on Xc position. When a track circuit is reserved by TCC then the related light turns green while the others remain red. This is provided by enabling arcs, for example in order to turn light A into green TC-A must be reserved that is provided by putting a token into place PAm. As can be seen from figure 9.
FIGURE 9 Automation Petri Net’s model
CHAPTER V
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 signaling appliances and to ensure their safe operation. Beginning around the 1930s, electrical relay interlocking were used. Since the late 1980s, new interlocking systems have tended to be of the electronic variety.
First of all, a switch or signal could be operated in error, since it was not evident which lever or stirrup went with which switch or signal. Secondly, the signalman was not able to observe the points of switches he operated, to see that they fit properly and were set for the proper route, since they were now remote from him. Thirdly, if more than one man was required to operate the switches and signals, they might work at cross purposes. Fourthly, if a driver did correctly identify the signal applying to him, he could still be in doubt of the proper place to stop if the signal commanded it. Of all the possible ways to operate the levers, most were useless, and many positively dangerous. The purpose of interlocking is to connect the switches and signals so that a dangerous condition cannot arise, and in doing this, to make operation clearer and more logical.
The levers operating switches and signals are placed side by side, from 3-1/2" to 6" apart (the pitch) in an interlocking frame, and are numbered from left to right. Each lever assumes two positions, normal when back in the frame, and reversed when pulled forward. The lever is latched in each position by a spring-loaded dog fitting into a notch in a quadrant. The dog is removed from its notch by depressing the catch handle, after which the lever can be moved. The pivot for a 6-7 ft lever is 3-4 ft below the floor, and the rod or wire operating the switch or signal is attached to the tail of the lever. A back tail on the other side could be provided for counterweights or electric locks. The apparatus for connecting the levers to the operating rods and wires on the ground is called the lead-out. The levers are colour-coded depending on their function (some British and American colours are: black--points; blue--FPL's; red--stop signals; yellow--distant signals; white--spares). Signals are generally located on the ends, switches in the middle.
A switch can be operated with 1" ID gas pipe (the usual operating rod) at distances up to about 400 yards. The limit in Britain was set at 350 yards in 1925. A signal can be wire-operated at distances up to perhaps 1500 yards, but 900 yards is a more practical limit. Wire operation is applicable only to two-position signals. Switches and signals can be operated at up to about 800 yards with a two-wire transmission (described in another article on this website). This was never used to any extent in Britain, America or France, but was very popular in Germany. Two-wire transmission is necessary when three-aspect semaphore signals are used, as in Belgium. In America, three-aspect signals were always pipe connected, if they were not electrically worked, which was already available at the time of their introduction, after 1900. Light signals, which appeared after 1920, require only electrical connections. Levers that operated only electrical contractors were specially identified (say, by making them shorter) so that excessive force would not be used on them.
Thermal expansion must be taken into account in the connections. A length of 100 yd of steel expands by 1.7" with a temperature change of 72°F. Rods are divided into equal sections of compression and tension by reversing levers, called compensating levers. The most widely used compensator, the lazy jack, uses two right-angle cranks and a connecting link arranged so that the rod is not offset. Expansion then has no effect, since compression and tension segments expand by the same amounts. Wire is a much greater problem, if only because the runs are longer. The signalman can adjust the tension in the wire manually as temperature changes. Automatic wire compensators have been used, but no design has been completely satisfactory. The general idea is to use a weight to establish a standard tension, and then to grab the wire when it is pulled. It is easier to design a good two-wire compensator than a single-wire one. The solution by mechanisms of the problems of interlocking is inherently interesting and historically important. The general principles are applicable to any type of interlocking, however. It is significant that all early power interlocking, whether electro pneumatic or all-electric, used mechanical locking beds. Electrical methods at first were supplementary to mechanical methods, in devices like electrical locks, track circuits and motor-operated signals. Then, using relays and later digital logic, they eventually replaced the mechanical devices.
CHAPTER VI
PROGRAMMABLE LOGIC CONTROLER (PLC)
6.1 INTRODUCTION TO PLC
Programmable controller equipment can be based on any of the following four principles:-
a) Relay based systems
b) Digital logic based systems
c) Computer based systems
d) Programmable logic controller (PLC) based systems
Relay based and digital based systems are required if there is a special requirement for speed or resistance to electric noise. Computer based control systems are preferred superior as compared to other controllers for handling complex functions. But, the overall best choice is a PLC, which today, is built with increasingly complex features.
6.2 PROGRAMMABLE LOGIC CONTROLERS (PLC)
Sequencing has traditionally been realized with relay techniques. Until the beginning of the 1970s, electromechanical relays and pneumatic couplings dominated the industrial applications. During the 1970s, PLCs became more and more common, and today sequencing is normally implemented in software instead of relays.
PLCs are industrially hardened micro computers that perform discrete or continuous functions in a variety of processing plant and factory environment. Originally intended as relay replacement equipment for the automotive industry, the PLCs are now used in virtually every type industry imaginable. A PLC produces on/off voltage output and can actuate elements such as electric motors, solenoids, fans, heaters and light switches. They are vital parts of industrial automation equipment found in all kinds of industries.
In the process industry and in industrial automation there is a wealth of application of switching circuit. PLCs were originally developed for use in discrete manufacturing applications. The batch process control requires sequencing and interlocking schemes similar to discrete manufacturing, and a plc can easily replace relays and tenor drum control common to many batch/filtration systems. A PLC can also be a substitute for process operators in certain areas. Traditionally many PLCs have been used in plant control were they provide interlock logic and alarm monitoring of purely digital signals in such applications, input would typically come from digital sensors such as micro switches, and outputs would derive digital devices such as relays and indicators.
Relay circuits either normally open (no) or normally closed were ‘normally’ refers to the state in which the coil is not energized. Relays remain a necessary interface between the control electronics and powered devices.
FIGURE 10 A Programmable Logic Controller
6.2.1 EVOLUTION OF PLC:
The PLC was originally and developed by a group of engineers of general Motor Corporation in 1968 to eliminate costly scrapping of assembly-line relays during model changeover of cars. These PLCs had to be easily programmed and reprogrammed, preferably in-plant, easily maintained and repaired, smaller than its relay equivalent and cost competitive with the solid-state and relay panels then in use. This provoked great interest from engineers of all disciplines using the PLC for industrial control. A microprocessor-based PLC was introduced in 1977 by Allen Bradley-Corporation in the USA, using an Intel 8080 microprocessor with circuitry to handle bit logic instructions at a high speed.
The early PLCs where designed only for logic based sequencing operators (ON/OFF signals). Today there are hundreds of different PLC models in the market. They differ in their memory size (from 256 bytes to several kilobytes and megabytes) and I/O capacity (from few lines to thousands). The difference also lies in the features they offer. The smallest PLCs serve just as replaces with added timer and counter capabilities. This is an extremely basic controller that is amazingly inexpensive. This small dedicated controller is enclosed in a single-mounted hardened case. It provides reliable control to a stand-alone section of a process.
The modern medium-sized PLCs perform all the relay replacement functions expected of it but also adds many other functions such as counting; timing and complex mathematical applications to it repertoire. Most medium-sized PLCs perform PID, feed forward and other control functions as well. In addition, medium sized and large scale PLCs now has data highway capabilities and they function well in distributed control systems (DCS) environment.
6.2.2 PLC/RELAY COMPARISON
To see how far we have progressed sine the time of the relay, consider the chart below. It summarizes the value of the PLC over the relay.
RELAYS
· Large complicated systems
that take up a lot of space.
· Hardwired devices used to configure relay ladder.
· Difficult to modify or
update program.
· Limited mechanical life.
· Require separate hardwired
timers and counters.
PLCs
· One PLC can control a large system. Takes up less floor space than a relay-based system.
· Only the input and output devices are hardwired. The inner workings of the PLC are solid-state.
· With the programming software it is simple to write a new program (or modify an existing one) and then downloads it into the PLC.
· The PLC, itself, is a solid-state device. It has a very long life and requires little maintenance.
· Counters and timers are internal, solid-state, devices.
6.3 PLC ARCHITECTURE
A PLC manufactured by any company has several common functional units. PLC architecture consists of the following main units.
· Power supply.
· Input/output (I/O) systems.
· Real-time central processing unit.
· Memory unit.
· Programmer unit
· Peripheral devices.
6.3.1 BLOCK DIAGRAM OF PLC:
PROGRAMMING DEVICE
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CENTRAL PROCESSING UNIT
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MEMORY
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POWER SUPPLY
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I/O SYSTEM MODULES
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OUTPUT DEVICES (SOV, MOTORS)
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INPUT DEVICES (SWITCHES, ETC.)
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FIGURE 11 Block diagram of PLC
6.3.2 POWER SUPPLY
The power supply unit provides the isolation necessary to protect solid state components from most high voltage line spikes. The power supply unit converts power line voltages to those required by the solid state components. All PLC manufacturers provide the option to specify line voltage conditions. In additions, the power supply is rated for heat dissipation requirements for plant flow operation. This dissipation capability allows PLCs to have high-ambient-temperature specifications and represents an important difference between PLCs and personal computers central processing unit, the memory unit and some peripheral devices.
6.3.3 INPUT/OUTPUT (I/O) SYSTEMS
Inputs are defined as real world signals giving the controller real time status of process variables. These signals can be analog or digital, low or high frequency, maintained or momentary. Typically they are represented to the programmable as a varying voltage, current, or resistance value. Signals from thermocouple and resistance temperature detectors (RTD) are common examples of analog signals. Some flow meters and strain gauges provide variable frequency signals while push buttons, limit switches or even electro mechanical relay contacts are examples of digital, contact closure type signals. Register input is another type of input signal that reflects the computer matcher of the programmable controller. The register input is particularly useful when the process condition is represented by a collection of digital signal delivered to the PLC at the same time. A binary code decimal (BCD) thumb wheel is an example of an input device that is compatible with a register input port.
There are three common categories of outputs: discrete, register and analog. Discrete outputs can be pilot like, solenoid valves, or annuciator windows. Register outputs can derive panel meters or displays. Analog outputs can drive signals to variable speed drives or to I/P (current to converters) and thus to control valves.
Today, all I/O systems are modular in nature, ie, systems are arranged in modules that contain multiples of I/O points. These modules can be clubbed into the existing bus structure. The bus structure is a high speed multiplexer that carries information back and forth between the I/O modules and the central processing unit. One of the most important functions of I/O is its ability to isolate real world signals from the low signal levels in the I/O bus. This is accomplished by use of optical isolators, which trigger a process switch to transfer data in or out to a solenoid valve without violating bus integrity.
6.3.4 REAL TIME CENTRL PROCESSING UNITS
The central processing unit (CPU) also called central control unit (CCU) ,performs the tasks necessary to fulfil the PLC function such as scanning , I/O bus traffic control program execution, peripheral and external device communications, special function or data handling execution, and self-diagnostics.
6.3.5 MEMORY UNIT
The memory unit of the PLC serves several functions. It’s the library, where the application program is stored. It’s also where the PLCs executive program is stored. An executive program functions as the operating system of the PLC. It is the program that interprets, manages and executes the user’s application program. Finally, the memory unit is the part of the programmable controller, where the process data from the input modules and control data for the output modules are temporarily stored as data tables. Typically, an image of these data tables is used by the CPU and when appropriate, sends to the output modules.
Memory can be volatile or non-volatile. The content of the volatile memory is erased if power is removed. Obviously, this is undesirable and the units with volatile memory provide battery backup to ensure that there will be no loss of program in the event of power failure. Non-volatile memory does not change state on loss of power and is used in cases in which extended power failures or long transportation times to job sites are anticipated.
6.3.6 PROGRAMMER UNITS:
The programmer unit provides an interface between the PLC and the user during program development, start-up and trouble shooting. The instructions to be performed during each scan are coded and inserted into memory with the programmer unit. The programmer unit varies from small hand-held (size of a large calculator) to desktop stand-alone intelligent CRT-based units. PLC manufactures are now providing controller models that use personal computer which allows the computer to interface with a serial input module installed in the programmable controller.
Programming units are the liaison between what the PLC understands and what the engineers desires to occur during the control sequence. Some programmer units store programs on other media such as cassettes, tapes and floppy disks. It provides automatic documentation of the existing program using a printer attached to it. With offline programming, the user can write a control program on the programming unit, then take the unit to the PLC in the field and load the memory with the new program, all without removing the PLC. Online programming allows cautious modification of the program while the PLC is controlling the process or the machine.
6.3.7 PERIPHERAL DEVICES
Peripheral devices are grouped into several categories such as programming aids, operational aids, I/O enhancement and computer interface devices. Programming aids provide documentation and program recording capabilities. The definite trend in programming aids is PC compatible that allows the PLC to be emulated by the personal computer. The software is sold by the PLC manufacturer or a licensee and is often model specific.
Operational aides include a variety of resources that range from color graphics CRTs to equipment or support programs that can give the operator specific access to processor parameters. In this situation the operator is usually allowed to read and modify timer, counter and loop parameters but not have access to the program itself. Some aids facilitate the interaction between the PLC and dumb terminals, such as printers, to deliver process information in a desired format. Some devices have the ability to setup an entire panel and plug into a PLC through RS232Cports, there by having enormous panel and wiring costs.
The I/O enhancement group is a large category of peripheral equipment. It includes all types of modules, from dry contact modules to intelligent I/O to remote I/O capabilities. Some I/O simulators used to develop and debug programs that can be categorized in the I/O enhancement group. These are hardware modules which can be plugged into the PLC.
The interface device group is a rapidly expanding section of PLC peripheral devices. These, peripheral devices allow peer-to-peer communications (i.e. one PLC is connected directly to another), as well as network interaction with various computer systems.
6.4 PLC PROGRAMMING
The use and understanding of PLC programming depends on the following factors,
· Knowledge of the process to be controlled.
· Understanding of electrical schematics.
· An appreciation for logic operations and for various types of logic and relay devices.
One popular programming technique involves defining the sequential logic in electrical schematic formats, using tag numbers and then translating this diagram into the appropriate programming language. The figure shows the translational of some examples of typical circuits of ladder diagram, Boolean algebra and mnemonics. Because this translation is relatively simple, maintenance and engineering personnel have accepted programmable controllers.
Clearly the diagram of figure resembles a ladder and thus, these diagrams are commonly called ladder diagrams. This ladder diagram approach of PLC programming was straightforward to program for plant operators and maintenance personnel. This programming language is called ladder language.
A PLC is usually programmed via an external unit, called programming unit (or terminal). The programming units range from small handheld portable units, to personal computers. The personal computer as programming unit has become very popular with graphical display. The display typically shows several ladder diagram lines at a time and also indicates the power flow within each line during the operation to make debugging and testing simpler. Other units are programmed with logical gates instead of ladder diagram. The program is entered by moving a cursor along the screen. When the cursor reaches the location, where the next element is to be added, confirmation is given via additional keys.
6.4.1 LADDER DIAGRAM:
Ladder diagrams are traditional methods of describing relay logic of the control circuits. Many switches are produced today from solid state gates, but electromechanical relays are still used in many applications. Relays remain a necessary interface between the control electronics and the powered devices. They are popular for describing combinatorial circuits.
FIGURE 12 Traditional Relay Logic FIGURE 13 PLC Logic
They are also a basis for writing programs for programmable logic controllers. A ladder diagram reflects a conventional wiring diagram of the physical arrangement of the various components (switches, relays, motors, valves, etc) and their interconnections. It is used by electrician (field maintenance engineer) to do the actual wiring of a control panel.
A ladder diagram consists of individual rungs just like on a real ladder. Each rung must contain one or more inputs and one or more outputs. The first instruction on a rung must always be an input instruction and the last instruction on a rung should always be an output (or its equivalent).
6.4.2 LADDER DIAGRAM SSYMBOLS:
CREATING A LADDER DIAAGRAM
First step:-We have to translate all of the items we are using into symbols the PLC understand. The PLC doesn’t understand terms such as switch, relay, and bell. It prefers input, output, coil, contact, etc. It doesn’t care what the input or an output device is. It only cares that it’s an input or an output.
Second step:-We must tell the PLC where everything is located. In other words, we have to give all the devices an address. Where is the push button going to be physically connected to the PLC? How about the light? We start with a blank road map in the PLC’s town and give each item an address. Could you know they live in the same town but which house? The PLC town has a lot of houses (input and output) but we have to figure out who lives where (what device is connected where). For now, let’s say that our input will be called “0000” and our output will be called “500”. (Please note that each PLC manufacturer uses different addressing method).
Final step:-We have to convert the schematic into a logical sequence of event. This is much easier than it sounds. The program we’re going to write tells the PLC what to do when certain events take place. In our example, we have to tell the PLC to make the light illuminate when the operator presses the button. The picture below is the final converted diagram.
Completed Ladder Diagram
HOW A PLC WORKS: EXAMPLE
Let’s say that we have the following program in our PLC, where M is a motor starter that controls a conveyor motor.
FIGURE 14 Example Program
Action: The operator pushes the start button to start the conveyor.
Step One: The PLC will see that the Start button, an input, has been activated. (The diagram below illustrates the status of the system after this action.)
Step Two: The PLC will run the logic and see that if the Start button has been pushed, there is a complete path to the motor starter.
Step Three: Since there is now a complete path or circuit to the motor starter, the PLC turns the motor starter (an output) on. (Because the Start pushbutton is traditionally a momentary pushbutton, a latching contactor maintains a closed circuit path.)
Status of the System When Start Pushbutton Is Released
|
When the Stop button is pushed, the PLC will see that the path to it is broken and turn the motor starter off.
This simply means that the CPU always initiates and controls all communication to remote racks or other devices on the network. The communication details of such networks are beyond the scope of this PLC overview.
6.5 PLC SELECTION
The selection of a PLC can be determined and an analysis of the following system characteristics
· I/O quantity and type
· I/O remoting requirement
· Memory, sized and type
· Programming requirements
· Programmer units
· Peripheral requirements
Although sizing of a PLC is straight forward, the selection of the right PLC requires a considerable judgment regarding trade-offs between future requirements and present cost.
I/O QUANTITY AND TYPE
In all modern PLCs plug-in modules are used to convert the I/O signal level to one that is compatible with the best architecture. These modules can be composed 1, 4, 8, or 16 points depending on the manufacturer’s standard design. It is easy to define a group I/O requirement for small process control application. However, a systematic approach is required for medium applications in order to avoid confusion of I/O application. A careful planning is required for the organization of I/O for large systems.
I/O REMOTING REQUIREMENTS
A unique feature of the PLC is the multiplexed nature of the I/O bus. This is used to the great advantage to reduce overall wiring cost. If I/O racks are centralized in logical cluster, plant wiring requirements can be greatly reduced. Wiring between racks and CPU is reduced to a few twisted pairs of wires or signal cables. This tremendous cost saving is realized without compromising on control accuracy or capability.
Remote I/O is divided into two distinct types such as the integral type and the transmitter/receiver type. The integral type remote I/O allows a limited transmission distance. The transmitter/receiver type remote I/O allows virtually unlimited transmission capability. Technology is greatly advancing bin this field as systems change from fibre optics to micro wave and radio transmission.
The major weakness of remote I/O system is that if the bus is cut or interrupted the effects of I/O failure will be relatively unpredictable. For this reason smaller CPUs in distributed control mode at each remote location is often considered preferable to a large central CPU.
MEMORY SIZE AND TYPE
The type and size of PLC memory used, depends on the controller’s size and its manufacturing company. Most small PLCs come with fixed quantity of RAM. Midsize and large PLCs provide users an option for almost any type of memory desired. This includes various types of non-volatile memory.
PROGRAMMER UNITS
Following three basic programming tools are provided by the manufacturers of midsize PLCs
· Hand-held programmers
· CRT programmers
· CRT programmer simulators that can run on personal computers
The hand held programmer enables the operator to enter a program one contact at a time. These units are widely used because they are rugged, portable and easy to operate. They are cost, devices. The CRT programmer provides the engineer with a visual picture of the program in the PLC. Ladder diagrams are drawn on the screen just as they would be drawn on paper. Design and troubleshooting time is reduced with the use of CRT. With memory driven software, programmer training is decreased.
6.6 ADVANTAGES OF PLC
· Flexibility: The PLC is a general purpose machine that is usually programmed from scratch for process control. It’s not loaded with dedicated programs of loops and screens, then configured to an application. Thus a PLC based process control system allows a more customized final product
· Maintainability: PLCs are typically maintained by electricians while computers are maintained by engineers and programmers with PLCs, plant operating personnel seldom require engineering assistance after start up
· Connectivity: PLC based control systems offer superior communications with external devices such as drivers, temperature controls, islands of automation and so o. A PLC has numerous ports that can be dedicated to 3rd party devices.
· Ruggedness and Reliability: PLCs don’t need to be housed in air-conditioned environments. They have battery backed memory or data storage and offer hot standby configurations of redundant CPUs and I/O cabling.
· Small physical sized, moderate to low initial investment cost, modular design.
CHAPTER VII
RELAY
A relay is a simple electromechanical switch made up of an electromagnet and a set of contacts. Relays are found hidden in all sorts of device. In fact, some of the first computers ever build used relays to implement Boolean gates. It is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in1835. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered to be, in a broad sense, a form of an electrical amplifier.
It consists of a coil of wire surrounding a soft iron, an iron yoke, which provides a low reluctance path for magnetic flux, a movable iron armature, and a set, or sets, of contacts; two in the relay pictured. The armature is hinged to yoke and mechanically linked to a moving contact or contacts. It is held in place by a spring so that when the relay is de-energised there is an air gap in the magnetic circuit.
When an electric current is passed through the coil, the resulting magnetic field attracts the armature, and the consequent movement of the movable contacts either make or breaks a connection with the fixed contact. If the set of the contacts was closed when the relay was de-energized, then the movement opens the contact and breaks the connections, and vice versa if contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position.
Since relays are switches, the terminology applied to switches is also applied to relays. A relay will switch one or more poles, each of whose contacts can be thrown by energized the coil in one of the two ways:
· Normally-open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called Form A contact or “make” contact.
· Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called a Form B contact or “break” contact.
RELAYS ARE USED TO AND FOR:
· Control a high voltage circuit with a low voltage signal as in some types of modem or Audio amplifier.
· Control a high current circuit with low current signal, as in the starter solenoid of an automobile.
· Detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays).
· Isolates the controlling circuit from the controlled circuit when the two are at different potential.
· Logic functions.
· Time delay functions. Relay can be modified to delay opening or delay closing a set of contacts.
CHAPTER VIII
SCOPE OF STUDY
We will extent the mini project “RAILWAY SIGNALIZATION AND INTERLOCKING DESIGN VIA AUTOMATION PETRINETS USING PLC” to main project. In the mini project only we used PLC Programming, but in the main project the whole signalization and interlocking will be controlled by SCADA. And we are updating this project by including Railway Gate control.
RESULT
We designed a simple railway signalization and interlocking system using the Petri Net logic and it was implemented on the Programmable Logic Controller. The system designed was checked on a GeFanuc PLC and it was interfaced with the hardware, following observations are made. When the path selected the corresponding traffic signal glows green which is initially at red, when the train crosses the position, signal turns back to red. The path is selected according to the controller’s decision, the track interlocking and signalling occurs. When one path is selected other one interfering to the path will remains red. If two paths selected at a time the first path only will get selected, if its wrong all the process can be reseted by pressing the reset switch.
Consider an example train is initially at track C,path selected is track X and track A.In track A and C light glown should be green while all others are red .Track X and track A are selected from command center(controller) instuctions are given for interlocking.when train passes track C,the light turns red and track moves to intial position
CHAPTER IX
CONCLUSION
In this mini project, interlocking and signalization design of a sample railway yard that has a single crossover switch is examined. Besides gathering more visuality and simple improvability to Discrete Event Systems, APN models can easily be implemented on a programmable logic controller; therefore error tracing and improving of the APN model for different scenarios can be realized rather easily. Signalization and interlocking systems for light rail and metro systems can also be modelled with APNs in order to provide more efficient control and safety without spending too much effort. Code generated by the developed APN
Model is tested for the given railway yard and its safety is verified for different scenarios. The results show that modelling with APNs is feasible and safe. For further work, APN model and PLC code for real systems which have more safety specifications considering time-delays including those between the commands and actualizing the commands will be developed.
CHAPTER X
BIBLIOGRAPHY
· Railway Signalization and Interlocking Design via Automation Petri Nets.
By M. S. Durmuş, M. T. Söylemez, Student Member, Member, IEEE
· 1 C. G. Cassandras and S. Lafortune, “Introduction to Discrete Event Systems”, Kluwer Academic Publishers, 1999.
· T. Murata, “Petri Nets: Properties, Analysis and Applications”, Proc. of IEEE, vol. 77, no. 4, pp. 541-580, 1989
· 3M. Uzam and A. H. Jones, “Discrete Event Control System Design Using Automation Petri Nets and Their Ladder Diagram Implementation,” The International Journal of Advanced Manufacturing Technology, vol. 14, no. 10, pp. 716-728, 1998.
· M. Uzam and A. H. Jones, “Design of a Discrete Event Control System for a Manufacturing System Using Token Passing Ladder Logic,” Proc. of the Symposium on Discrete Events and Manufacturing Systems, CESA’96 IMACS Multiconference, pp. 513- 518, 1996.
· H. Jones, M. Uzam, A. H. Khan, D. Karımzadgan and S. B. Kenway, “A General Methodology for Converting Petri Nets into Ladder Logic: The TPLL Methodology,” Proc. of the 5th Int. Conference on Computer Integrated Manufacturing and Automation Technology–CIMAT’96, pp. 357-362, 1996. M.
· Uzam and A. H. Jones, “Conversion of Petri Net Controllers for Manufacturing Systems into Ladder Logic Diagrams,” Proc. of the IEEE Conf. on Emerging Technologies and Factory Automation, ETFA’96, vol. 2, pp. 649-655, 1996.
· B. Barnard, “Railway System Modelling – Not Just for Fun,” IEEE Seminar on Railway System Modelling, pp. 2-6, ISBN: 0 86341 457 5, London, UK, 30 Sept. 2004.