PLC BASED TRAFFIC CONTROL SYSTEM
A final year project report submitted in partial fulfilment of the regulations for the award of BEng (Hons) in Electrical and Electronics Engineering. 2010/2011
AZEEZ OLAREWAJU LAWAL
COLLEGE OF ENGINEERING AND TECHNOLOGY UNIVERSITY OF SUNDERLAND, SUNDERLAND UNITED KINGDOM
PLC BASED TRAFFIC CONTROL SYSTEM
BY
AZEEZ OLAREWAJU LAWAL
(099015901)
Report submitted in partial fulfilment of the requirements for the degree of BEng (Hons) in Electronics and Electrical Engineering.
MAY, 2011.
CHAPTER 1 INTRODUCTION
A traffic light is a collection of two or more coloured lights found at some junctions and pedestrian crossings which indicates whether it is safe and/or legal to continue across the path of other road users. In the United Kingdom, traffic lights are widely used both on major roads and in built-up areas. Their numbers have increased exponentially since they were first invented in 1868.
The operation of standard traffic lights which are currently deployed in many junctions, are based on predetermined timing schemes, which are fixed during the installation and remain until further resetting. The timing is no more than a default setup to control what may be considered as normal traffic. Although every road junction by necessity requires different traffic light timing setup, many existing systems operate with an over -simplified sequence. This has instigated various ideas and scenarios to solve the traffic problem. To design an intelligent and efficient traffic control system, a number of parameters that represent the status of the road conditions must be identified and taken into consideration.
1.1 Background History
The first traffic lights actually had their roots in the railway signals used at the time,where two gas lamps, one red and one green, would be alternately hidden by a semaphore arm depending on whether the arm was in a horizontal position or at a 30° angle. The first lights were installed outside the Houses of Parliament in London on 10 December, 1868 to control the increasing number of vehicles there. However, according to some sources, they later exploded and injured the policeman operating them.
The first electric lights were developed in the USA in the early 20th Century. Various people lay claim to the invention of the modern traffic light. These include:
(i) Lester Wire, Wire, a Salt Lake City policeman who set up the first red-green electric traffic lights in 1912. (ii) James Hoge, Hoge, from Cleveland, who in 1914 designed some red-green electric lights with a buzzer which sounded when the lights changed. (iii)William (iii) William Potts from Detroit, who designed the first three-colour electric traffic lights in 1920. (iv)John (iv)John Harriss, Harriss, a Police Commissioner from New York who developed the first interconnected three-colour electric traffic lights in 1922. (v) Garrett Morgan, Morgan, from Cleveland, who in 1923 designed a cross-shaped signalling device which is often mistakenly referred to as the first traffic light.
Once the USA had finished reinventing the traffic light, it was adopted in the UK. The first automatic lights were installed in Princes Square in Wolverhampton. Nowadays, traffic lights are often operated by complex computer software designed to optimise traffic flow [1]. This optimization is done using the Prog rammable Logic Controller (PLC).
1.2 Problem Definition The aim of this project is to design a program for Programmable Logic Controller(PLC) that could minimize the waiting time of the cars at intersections, when the trafficvolume is significantly high. Besides that, it can prevent the emergency car stuck in thetraffic jam at the intersections as well.
1.3 Objectives of Project 1. To understand the structure and operation of PLC 2. To study the ladder logic design and their programming technique 3. To understand how to make the interfacing to the PLC 4. To design a program that works together with a model of four- junction traffic light and sensors. 5. To build the model of four-junctions of intelligent traffic light that can overcome some of major problem of current traffic light.
1.4 Project Scope 1. Construct a model of four way junction of a traffic light model. 2. Programme a ladder logic diagram to control the traffic light. 3. Combine the software part and the hardware part to simulate a traffic light system.
1.5 Thesis Outline Chapter 1 is the introduction to traffic light systems. This chapter also explains about project objectives and scopes and discuss about problem statement. Chapter 2 will describe all techniques, the theory and concepts behind Traffic Lights and PLC automation. All requirements and preliminary design details will be explained in this chapter. The practical design will be di scuss later in Chapter Three.
Chapter 3 focuses on hardware development and configuration. This chapter explain every detail about PLC FESTO FEC FC34and traffic light model. The wiring diagram forthis hardware also will be discussed in this chapter. Chapter 4 deals with the software development using FESTO Software Tools FST 4.10 Programmer. These chapters also discuss the flowchart and development programfor traffic light systems. Chapter 5 presents all the results obtained and the configuration of doing simulation in the real world. Chapter 6 discusses the conclusion of this project, the development of traffic lightcontrol system using Programmable Logic Controller. This chapter also explains theproblem and the
recommendation
orsystemmodification.
for
this
project
and
for
the
future
development
CHAPTER 2 LITERATURE REVIEW
2.1 Introduction Traffic signals are the most convenient method of controlling traffic in a busy junction.But, we can see that these signals fail to control the traffic effectively when a particular lane has got more traffic than the other lanes. This situation makes that particular lane more crowdy than the other lanes. If the traffic signals can allot different time slots to different lanes according to the traffic present in each lane, then, this problem can be solved easily. 2.2 The Basics of British Traffic Lights
The most basic traffic light consists of three bulbs with different coloured lenses, which from top to bottom are red, amber and green. In the UK, the lights commonly use a sequence of four phases:
1. Red— this indicates that traffic must stop behind the line. It is compulsory for
all road
users to do so. Some traffic lights even have cameras to catch drivers breaking this law. 2. Red and Amber— this combination of bulbs indicates that the lights are about to change to green, and gives drivers time to release their handbrake and prepare to drive off as soon as they are allowed to do so. This phase was first introduced in 1958. 3. Green— this indicates that traffic may pass through the junction, provided that it is safe to do so and the way is clear. Some junctions are marked with a hash of yellow lines forming a box, which indicates that drivers must not stop on the box unless they are turning right and their exit is clear. 4. Amber— this warns traffic that it should stop unless it is unsafe to do so. In the UK it is legal to pass through an amber light, as the phase exists to warn drivers not yet at the junction that they will have to stop.
Traffic lights at junctions will always follow this pattern, with conflicting flows of traffic being forced to take turns. Often the green bulb is replaced with two or more green arrows or filter lights, which indicate that traffic turning left or right may go, while a red light remains to instruct oncoming traffic to wait. It is now quite common for vehicles turning right to have to wait for a separate filter light, even if the way is clear. Despite being relatively simple, filter arrows are often 'mistaken' for an instruction to go by drivers who want to turn a different way to th at shown. Problems are also known to arise from motorists watching the other lights at junctions and anticipating their own movement, and so shades are used to hide the lights from both drivers and from the sun, which would reduce their visibility [1].
It is interesting to note that the UK is one of only a few countries not to have a 'left on red' rule, where cars are allowed to pass through a red light if it is safe to turn left; in the UK, red lights and filter lights must always be obeyed.A recent improvement in traffic light technology has come with the development of red, amber and green light-emitting diodes (LEDs). Arrays of these tiny bulbs can be used to replace the existing light bulbs in traffic lights and are clearer and more energy-efficient. It is estimated that replacing all the traffic light bulbs in the UK with LEDs would save enough energy to power the city of Norwich.
2.21 Pedestrian Crossings
Many junctions also have pedestrian crossings built into them, where red and green signals in the shape of a walking (green) or standing (red) figure indicate to pedestrians whether it is safe to cross. There is also a blank phase where both signals are unlit; indicating that it is still safe to continue crossing but there is not enough time for the average 90-year-old to make it in time if they start now. These crossings often have associated push-buttons for use by pedestrians, but their only apparent action is to display the word
WAIT in
large, friendly letters. Some of these boxes do, however, have
a small knob underneath which revolves when it is safe to cross, which can be useful for
the visually impaired. It is important to note that in the UK, although it is not illegal to jaywalk, doing so violates the Highway Code and those responsible are liable for any resulting accident. Those using pedestrian crossings on side roads have right of way over vehicles once they have begun to cross [1].
A different sequence to the one mentioned above is used at pelican crossings, where the crossing is not associated with a junction, but is designed purely to allow pedestrians to cross busy roads. The push buttons at these crossings actually stop the traffic after a short delay, and the green figure is often accompanied by a beeping sound. The red and amber phase is replaced by a flashing one, indicating that drivers may continue if there are no pedestrians on the crossing; at the same time the beeping stops and a flashing green figure indicates to pedestrians still waiting to step out onto the crossing that they should wait for the next green man signal to give them right of way. Pedestrians already on the crossing should simply continue to the other side as normal.
Similar crossings are provided for cyclists (toucan crossings) and for horse riders (pegasus crossings). These crossings sometimes feature red and green cycles or horses. Another development on the theme of the pelican crossing is the puffin crossing, where a sensor detects if there are pedestrians on the crossing, making the flashing phase used on pelican crossings obsolete. These crossings do, however, cause confusion, as the red and green men are sighted above the push button and not on the opposite side of the road. There are some crossings that do not involve any coloured light sequences. The zebra crossing features a pair of flashing amber Belisha Beacons, while badger crossings do not have any lights at all.
Vehicle
Detection
Systems
is
either
Inductive
loops
or
sensors
or
Video
detectionsystem.For the last two decades most traffic lights at busy intersections and pedestrian crossings have been controlled by ‗inductive loop‘ sensors. Normally seen as dark square outlines on the road surface, they detect a passing vehicle by using a magnetic field to detect the metal components in the passing vehicle. They then send
information on location and speed to the computer controlling the traffic signals. The inductive loop system however has a number of important drawbacks, firstly is that they are often easily damaged by road degradation, utility works or road maintenance and secondly the need to close a section of road to install the system and its associated wiring, both inevitably increasing costs and congestion. Although the main purpose is to control traffic at junctions and to allow pedestrians to cross safely, traffic lights are used in a variety o f situations, including:
Traffic control at road works, where pair of three-bulb traffic lights has replaced the manual STOP/GO signs.
Lights at level crossings and drawbridges, where a single steady amber light precedes a pair of flashing red lights indicating that traffic must stop. These are also used to allow emergency services vehicles out of depots on busy roads, and to allow animals to be herded across main roads.
Lane control on motorways, where white arrows instruct drivers to change lane or leave the motorway, while red crosses indicate closed l anes.
Lane control on busy roads where the middle lane is used by rush-hour traffic heading one way in the morning and the other in the afternoon. Here, green arrows indicate open lanes and red crosses indicate closed ones.
As a colour-based system of rating something completely unconnected with driving, where red usually means 'bad' or 'unavailable' and green means 'good' or 'in plentiful supply'. Applications can range from rating the severity of an emergency to use at 'traffic light parties', where the colours give an indication of one's availability to the proposition of a relationship.
At the cheesy discos of the 1970s, where actual traffic lights were used as disco lights, mostly ignoring the standard sequences.
In traffic-light jelly.
2.22 Traffic Light Sensors and Vehicle Transducer Traffic signals are used to control the flow of vehicles through an intersection, which can have devices that detect the presence of vehicles in a traffic lane. Detection increases the efficiency of traffic signal operations. As part of optimum operation of traffic light intersections, there are all sorts of technologies for detecting vehicles. Some of these technologies are microwave and millimetre-wave radar, active LED infrared radar, video image detection system (VIDS) and loop detector among others [2]. Because the traffic flow rates change from time to time, it is often desirable to adapt the detector to the actual offered traffic light controller. Detectors that indicate the presence or absence of vehicles are necessary for this type of control. With the information from these detectors, the duration of phases, and/or the order of the phases can be changed.
Loop Detectors Loop detectors are strands of wire embedded into the pavement in a rectangular or round loop shape of standardized dimensions. It consists of an insulated electrical wire placed on or below the road surface. When energized, the loop creates a magnetic field. When a vehicle passes over the loop, the frequency of the magnetic field changes. A device in the traffic signal controller cabinet detects this change in frequency and signals the traffic signal controller to provide that vehicle with a green indication during the traffic signal cycle. The loop is attached to a signal amplifier and a power source, creating an electromagnetic field in the area of the loop. The wire loop is excited at frequencies from 10 kHz to 200 kHz. In conjunction with pull box electronics, the loop becomes an inductor, whose inductance decreases whenever a vehicle or other larger metallic object passes over it or stops on it. The resulting inductance change generates a signal to a controller [2].
Figure 2.0: Loop detector installed beneath the asphalt of a road intersection.
Video Detection
Video detection uses cameras mounted on poles over the travel lanes. Machine vision technology analyzes the video images and sends an electronic signal to the traffic signal controller when a defined change in the imagery occurs.
Radar Detection Radar detection uses microwave radar sensors mounted over the travel lanes. Energy is sent from the radar unit to the traffic lane and the reflected energy is measured by a sensor. A defined change in the reflected energy is used to signal the controller to serve that vehicle. Active LED infrared radar Infrared (IR) detectors operate on the same principles as microwave radars, but transmit low power energy from light emitting diodes (LEDs) or from laser diodes. The detector senses a portion of the reflected energy in its field of view. The distance of an object from the detector is found by measuring the two-way travel time of the infrared pulse, from the detector to the target and back. The IR detector then focuses the rebound energy from vehicles and translates it into electrical pulses. IR detectors can be used for passage of moving objects, presence or absence of objects and detecting speed of objects. Active IR detectors can be mounted on bridge overpasses or on existing poles.
More than one IR unit can be mounted to a pole without signal interference degrading performance. Units are typically mounted at heights between 15 and 30 feet [2].
Figure 2.1- Typical active IR detector.
Figure 2.2- An installed active IR detector
2.23 Selection Considerations
Public agencies consider a range of factors in selecting the most appropriate vehicle detection technology for a given location, including initial cost, accuracy, reliability and ongoing maintenance requirements. A traffic signal is typically controlled by a controller mounted on a concrete pad. Traffic controllers use the concept of phases, which are
directions of movement lumped together. For instance, a simple intersection may have two phases: North/South, and East/West and these phases are either controlled by controllers fixed time mode or detector which is through the use of transducers. Although some electromechanical controllers are still in use, modern traffic controllers are of programmable logic controller (PLC) technology. The typical controller consists of miniature circuit breaker, power panel, programmable logic controller and the dimming transformer[3].
figure 2.3: Functional block diagram of a traffic lights intersection system [3].
2.24 Pedestrian Push Buttons
Figure 2.4: Pedestrians push button installation.
The pedestrian push button assembly has a rigid frame having a piezoelectric material of a solid state switch positioned across a central aperture, and an elastic sealing ring positioned in a groove surrounding the piezoelectric material. A button is secured to the rigid frame such that a seal contact portion of the button sealable rests against the elastic sealing ring. A very small space separates an abutment surface of the button and a stopper surface of the rigid frame, and an elastic pressure portion of the button contacts the piezoelectric material. When operated, the elastic sealing ring is sufficiently biased to urge the elastic pressure portion against the piezoelectric material to generate a pulse signal which travels through wires to the controller to announce the presence of a pedestrian at the junction. The pedestrians push button is installed about 1.2 m from the surface of the ground on a traffic light pole with the help of bolts and nuts [3].
2.24 Traffic light Controller The miniature circuit-breaker provides efficient and reliable protection for traffic light cables and the controller cabinet in traffic light installations. Three different tripping characteristics provide the ideal solution for all applications from cable protection up to the protection of controller cabinet [3]. The power supply module takes 240 V ac and distributes 5 V dc power to the PLC‘s Central Processing Unit, 24V dc to the transducers and 240 V ac to both the dimming transformer and output devices. The dimming transformer is a single phase 240/110 V transformer, which in conjunction with the PLC reduces the illumination of the signal heads in the evening. This usually affects the vision of drivers.
Figure 2.5: An installed traffic light controller
Figure 2.6: 40A miniature circuit breaker
2.3 Programmable Logic Controller (PLC) Overview A programmable logic controller (PLC) is an industrially hardened computer based unit that performs discrete or continuous control functions in a variety of processing plant and factory environments. It is an industrial computer used to control and automate complex systems. A relatively recent development in process control technology. It was designed for use in an industrial environment, which uses a programmable memory for the integral storage of user-oriented instructions for implementing specific functions such as logic, sequencing, timing, counting, and arithmetic to control through digital oranalog inputs and outputs, various types of machines or proc esses. In the late 1960's PLCs were first introduced. The primary reason for designing such a device was eliminating the large cost involved in replacing the complicated relay based machine control systems. It was invented to replace the necessary sequential relay circuits for machine control. The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs. The user enters a program, usually via software, that gives the desired results. Bedford Associates (Bedford, MA) proposed something called a Modular Digital Controller (MODICON) to a major US car manufacturer. Other companies at the time proposed computer based schemes, one of which was b ased upon the PDP-8. The MODICON 084 brought the world's first PLC into commercial production.
When production requirements changed so did the control system. This becomes very expensive when the change is frequent. Since relays are mechanical devices they also have a limited lifetime which required strict adhesion to maintenance schedules. Troubleshooting was also quite tedious when so many relays are involved. Now picture a machine control panel that included many, possibly hundreds or thousands, of individual relays. The size could be mind boggling. How about the complicated initial wiring of so many individual devices! These relays would be individually wired together in a manner that would yield the desired outcome. Were there problems? You bet!
These "new controllers" also had to be easily programmed by maintenance and plant engineers. The lifetime had to be long and programming changes easily performed. They also had to survive the harsh industrial environment. That's a lot to ask! The answers were to use a programming technique most people were already familiar with and replace mechanical parts with solid-state ones.
In the mid 70's the dominant PLC technologies were sequencer state-machines and the bit-slice based CPU. The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs. Conventional microprocessors lacked the power to quickly solve PLC logic in all but the smallest PLCs. As conventional microprocessors evolved, larger and larger PLCs were being based upon them. However, even today some are still based upon the 2903. (Ref A-B's PLC-3) Modicon has yet to build a faster PLC than their 984A/B/X which was based upon the 2901.
Communications abilities began to appear in approximately 1973. The first such system was Modicon's Modbus. The PLC could now talk to other PLCs and they could be far away from the actual machine they were controlling. They could also now be used to send and receive varying voltages to allow them to enter the analog world. Unfortunately, the lack of standardization coupled with continually changing technology has made PLC communications a nightmare of incompatible protocols and physical networks. Still, it was a great decade for the PLC!
The 80's saw an attempt to standardize communications with General Motor's manufacturing automation protocol (MAP). It was also a time for reducing the size of the PLC and making them software programmable through symbolic programming on personal
computers
instead
of
dedicated
programming
terminals
or
handheld
programmers. Today the world's smallest PLC is about the size of a single control relay!
The 90's have seen a gradual reduction in the introduction of new protocols, and the modernization of the physical layers of some of the more popular protocols that survived the 1980's. The latest standard (IEC 1131-3) has tried to merge plc programming languages under one international standard. We now have PLCs that are programmable in function block diagrams, instruction lists, C and structured text all at the same time! PC's are also being used to replace PLCs in some applications. The original company who commissioned the MODICON 084 has actually switched to a PC based control system. What will the 00's bring? Only time will tell [4]. Compared with electromechanical relay systems, PLCs offer the following additional advantages:
Ease of programming and reprogramming the plant
A programming language that is based on relay wiring
High reliability and minimal maintenance
Small physical size
Ability to communicate with computer systems in the plant
Moderate to low initial investment cost
Rugged construction
Modular design
PLCs are used in many ―real world‖ applications like machining, packaging, material handling and automated assembly industries. PLCs can be employed in almost all applications that require some type of electrical control. For example, let‘s assume that when a switch turns on, we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for. We can do this
with a simple external timer. But what if the process included 10 switches and solenoids? We would require 10 external timers. What if the process also needed to count how many times the switches individually turned on? We need a lot of external counters. As you can see, the bigger the process, the more of a need we have for a PLC. Programmable logic controllers are used throughout industry to control and monitor a wide range of machines and other movable components and systems. PLC is used to monitor input signals from a variety of input points (input sensors) which report events and conditions occurring in a controlled process. Programmable logic controllers are typically found in factory type settings. PLCs are used to control robots, assembly lines and various other applications that require a large amount of data monitoring and control.
2.31 Basic PLC schema The basic PLC schema include CPU, power suppl y, memory, Input block, output block, communication and expansion connections.
Figure 2.7: PLC system overview and computer connection
CPU modules - The Central Processing Unit (CPU) Module is the brain ofthe PLC. Primary role to read inputs, execute the control program, update outputs.The CPU consists of the
arithmetic logic unit (ALU), timing/control circuitry,accumulator, scratch pad memory, program counter, address stack and instructionregister. A PLC works by continually scanning a program.Memory - The memory includes pre-programmed ROM memory containingthe PLC‘s operating system, driver programs and application programs and theRAM memory. PLC manufacturer offer various types of retentive memory to saveuser programs and data while power is removed, so that the PLC can resumeexecution of the user-written control program as soon as power is restored. Sometypes of memory used in a PLC include: i. ROM (Read-Only Memory) ii. RAM (Random Access Memory) iii. PROM (Programmable Read-Only Memory) iv. EPROM (Erasable Programmable Read-Only Memory) v. EEPROM (Electronically Erasable Programmable Read-Only Memory) vi. FLASH Memory vii. Compact Flash – Can store complete program information, read & write textfiles. viii. I/O Modules - Input and output (I/O) modules connect the PLC to sensorsand actuators. Provide isolation for the low-voltage, low-current signals thatthe PLC uses internally from the higher-power electrical circuits required bymost sensors and actuators. Wide range of I/O modules available including:digital (logical) I/O modules and analogue (continuous) I/O modules.
2.32 PLC Configurations Many PLC configurations are available, even from a single vendor. But eachof thesehas common components and concepts. These essential components are: i. Power Supply – This can be built into the PLC or be an external unit.Common voltage levels required by the PLC are 24Vdc, 120Vac and 220Vac. ii. CPU (central Processing Unit) – This is a computer where ladder logic isstored and processed.
iii. I/O (Input/output) – A number of input/output terminals must be provided sothat the PLC can monitor the process and initiate actions. Inputs to, andoutputs from, a PLC is necessary to monitor and control a process. Bothinputs and outputs can be categorized into two basic types: logical orcontinuous. Consider the example of a light bulb. If it can only be turned onor off, it is logical control. If the light can be dimmed to different levels, it iscontinuous. iv. Indicator lights – These indicate the status of the PLC including power on,program is running, and a fault. These are essential when diagnosing problems. v. Rack Type : A rack can often be as large as 18‖ by 30‖ by 10‖ vi. Mini: These are similar in function to PLC racks, but about the half size.Dedicated Backplanes can be used to support the cards OR DIN railmountable with incorporated I/O bus in module. vii. Shoebox: A compact, all-in-one unit that has limited expansion capabilities.Lower cost and compactness make these ideal for small applications. DINrail mountable. viii. Micro: These units can be as small as a deck of cards. They tend to havefixed quantities of I/O and limited abilities, but costs will be lowest. DIN railmountable
PLC's are normally constructed in modular fashion to allow them to be easily reconfigured to meet the demands of the particular process being controlled. The processor and I/O circuitry are normally constructed as separate modules that maybe inserted in a chassis and connected together through a common backplane using permanent or releasable electrical connectors.
Figure 2.8: PLC Construction Types 2.33 PLC Components
The PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive input/output data. We can actually consider the PLC to be a box full of hundreds or thousands of separate relays, counters, timers and data storage locations. Do these counters, timers, etc. really exist? No, they don't "physically" exist but rather they are simulated and can be considered software counters, timers, etc. These internal relays are simulated through bit locations in registers.
Figure 2.9: PLC Components [4].
INPUT RELAYS-(contacts): These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors.
INTERNAL UTILITY RELAYS-(contacts): These do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC
to eliminate external relays. There are also some special relays that are dedicated to performing only one task. Some are always on while some are always off. Some are on only once during power-on and are typically used for initializing data that was stored.
COUNTERS: These again do not physically exist. They are simulated counters and they can be programmed to count pulses. Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed. Some manufacturers also include high-speed counters that are hardware based. We can think of these as physically existing. Most times these counters can count up, down or up and down.
TIMERS: These also do not physically exist. They come in many varieties and increments. The most common type is an on-delay type. Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s.
OUTPUT RELAYS-(coils): These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or triacsdepending upon the model chosen.
DATA STORAGE: Typically there are registers assigned to simply store data. They are usually used as temporary storage for math or data manipulation. They can also typically be used to store data when power is removed from the PLC. Upon power-up they will still have the same contents as before power was removed. Very convenient and necessary!
2.4
PLC Operations
A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps. There are typically more than 3 but we can focus on the important parts and not worry about the others. Typically the others are checking the system and updating the current internal counter and timer values.
Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third... It records this data into its memory to be used during the next step.
Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step.
Step 3-UPDATE OUTPUT STATUS-Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true.
After the third step the PLC goes back to step one and repeats the steps continuously. One scan time is defined as the time it takes to execute the 3 steps listed above.
2.41 PLC Programming There are different types of Programming language which support people with different backgrounds. There are five programming languages that are supported by various Programmable Logic Controllers. They are: 1.
Ladder diagram (LD)2.Function block diagram (FBD)3.Instruction list (IL)
4.
Structured text (ST)5.Sequential function chart (SFC)
Figure 2.10: Programming Languages Examples
2.5 PLC Compared To Other Control Systems
PLCs are well-adapted to a range of automation tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation, and where changes to the system would be expected during its operational life. PLCs contain input and output devices compatible with industrial pilot devices and controls; little electrical design is required, and the design problem centres on expressing the desired sequence of operations in ladder logic (or function chart) notation. PLC applications are typically highly customized systems so the cost of a packaged PLC is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of massproduced goods, customized control systems are economic due to the lower cost of the components, which can be optimally chosen instead of a generic solution, and where the non-recurring engineering charges are spread over thousands or millions of units.
For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an electromechanical cam timer costing only a few pounds in production quantities.
A microcontroller-based design would be appropriate where hundreds or thousands of units will be produced and so the development cost can be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example; millions of units are built each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit buses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomic.
Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very
high-speed or precision controls may also require customized solutions; for example, aircraft flight controls.
PLCs may include logic for single-variable feedback analog control loop, a "proportional, integral, derivative" or "PID controller." A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops; where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be used. However, as PLCs have become more powerful, the boundary between DCS and PLC applications has become less clear-cut [6].
PLCs have similar functionality as Remote Terminal Units (RTU). An RTU, however, usually does not support control algorithms or control loops. As hardware rapidly becomes more powerful and cheaper, RTUs, PLCs and DCSs are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLC -like features and vice versa. The industry has standardized on the IEC 61131-3 functional block language for creating programs to run on RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated development environments.
2.6 Infrared Sensor This sensor provides the system with ability to detect the presence of object position. The theory is the IR emitter emits infrared light. If an object presence the signal will be reflected back to the receiver. Then, the IR detector implemented will detect the reflected light. Then, the correspondence signal sends to the PLC for being analyze. Based on the measurement of the intensity of the reflected light from the target area such a bottle, it has a light source sending light to the moving target and a light sensor receiving the light. The output signal from the sensor decreases exponentially with the increase of the distance to the measured object. Infrared light-emitting diodes (LED's) and photosensitive diodes are used in this transducer. The sensor output is inversely proportional to the amount of occupation. A multilink array of light sensitive elements
and a light-beam scanning technique determines and qualifies the shape of the measured object by processing data from the elements [7].
Figure 2.11: Basic IR Detector/Emitter circuit
CHAPTER 3 SYSTEM HARDWARE 3.1 Introduction The hardware part of this project is Programmable logic controller (PLC), Power Pack,a traffic light model and pairs of Infra-Red Sensors. Festo FEC FC34 is the type of PLC used in this project as the processor to control the traffic light. The four ways traffic light model was constructed to display how this trafficlight control system is running. This traffic light model has a complete set of trafficlight signal which are red, yellow and green as well as pedestrian red and green lights, for traffic signal on each lane. Each lane also has one limit switches represent as a sensor on the road. The sensors are placed on each lane to detect the presence of a car at the junction. The right connection between PLC and traffic light model is veryimportant in order to avoid problem or conflict when the program is transferred to PLC.
3.2 Festo FEC FC34 PLC Configuration Figure 3.1 shows the Festo FEC FC34 PLC configuration. The main body ofthis PLC is power supply unit, Central processor unit and input/output slot. Thepower supply unit receive the required PLC voltage which is 24Vdc. For safety thevoltage to PLC must be connected to the earth. The CPU covered by Analog input/outputslot, RS232 port, and processor. The inputs/outputs slots used for the system are usingdigital input and output. There are limited slot for input and output portand can be used for multiple inputs/outputs cards.
Figure 3.1: Festo FEC FC34 PLC Configuration
3.3 Traffic light model The four ways junction is developed using Woods, Steel, Bolts, Screws, Light Emitting Diodes, Resistors and paints. In order to display the simulation of the traffic light control system, each traffic light lane has a set of traffic light signal ―Red, Yellow, and Green‖. This traffic light signal operates similar like common traffic light signal in the UK. It changes from red to red and amber to green and then yellow and after that back to red signal. Each lane also has one limit switches represent as a sensor on the road.The sensor used for the design of these traffic light system is an infra-red detector which as an infra-red diode and transistor as a pair. The sensors are placed on each lane to detect and count the number of cars through that lane. From this combination of sensor, we will know the expected time for green signal on when each lane change to the green signal.
Figure 3.2: Four (4)-way intersection diagram [7].
Figure 3.3: Project Hardware with FestoDidatic Power Supply Unit
3.4 Hardware Wiring Once hardware is designed ladder diagrams are constructed to document thewiring. For this project, existed PLC cabinet box are use and connect with the trafficlight model. The wiring of the PLC is as shown in figure 3.4. The PLC and I/O card would be supplied with DC Power Supply of 24V.The common for input card is 24Vdc and for output card is
0Vdc. The PLC is connected to earth in order to avoid risk, hazards and damage to the PLC in case of fault.
Figure 3.4: PLC cabinet box wiring
The PLC input wiring address start with number I0.0 to I0.7 for every input card. The other input card which is installed to the PLC socket will carry the address for this input card as I1.0 to I1.3. The PLC outputs wiring address start with number O0.0 to O0.7 for every output card.
Four infra-red sensors (detectors) are placed on 4 lanes coming to a junction, one per lane. The sensor is placed at a distance away from the junction so that it doesn‘t get disturbed by the vehicles stopping at the signal. These sensors are connected to the PLC, which counts the pulses coming from the sensors.
CHAPTER 4 SYSTEM SOFTWARE
4.1 Introduction to FST software FST-Programmer (Software) is a PLC programming tool for the creation, testing and maintenance of programs associated FESTO PLCs. The FST software package supports the configurations, programming and commissioning of the following devices: – CPX terminal with integrated Front End Controller – FEC Compact – FEC Standard – PS1 Professional The FST software package is set-up on a Personal Computer (PC) in line with specific requirements. You can: – install FST in the language of your choice, – install example files – de-install FST.
4.11 The FST operating interface
When FST is started, the FST program window appears. First, a logo appears in the foreground which is then automatically hidden after a few seconds. Click on the logo make it to close immediately. The ―Tip of the Day‖ window is then shown. In the bottom section of the window you will see the ―Show Tips after on StartUp‖ checkbox. Tick to stop the tips appearing.
Figure 4.0: Operating interface of the FST software [9] FST uses what is referred to as the multiple document interface (MDI). A separate window within the FST program window opens for each document. The document window can be activated and arranged using the commands in the ―Window‖ menu.The size and position of the windows is saved between the FST sessions. If the screen resolution is changed, Windows adjusts the size and position of the windows. The FST software package is an application for the Windows operating system. As such, the program interface and operation are consistent with the usual Windows standard. The buttons, menu bar, picture scroll bars etc. of the FST software therefore behave as they do in most other Windows-based programs.
Figure 4.1: PLC Programming Tools [10]. 4.2 Project Workspace The project workspace can display a ladder program, the symbol table of that program or the Statement List view. The details displayed depend upon the selection made in the project workspace. When a new project is created or a new PLC added to a project, an empty ladder is automatically displayed on the rig ht-hand side to the project workspace. The symbol table and Mnemonics view must be explicitly selected to be displayed. All views can be opened at the same time and can be selected via options associated with the window menu. PLC program instruction can be entered as a graphical representation in ladder form. Programs can be created, edited, and monitored in the ladder diagram view. The figure below shows the diagram workspace appearance:
Figure 4.2:Workspace Appearance [1] Title bar [2] Menu bar [3] Toolbar [4] Project window [5] Program editor window [6] Workspace [7] Reduce to symbol [8] Status Bar
4.3 Program Development. Prior to the construction of a ladder logic diagram, program flowchart is ideal for aprocess that has sequential steps. The steps will be executed in a simple order that may change as the result of some simple decisions. The block symbol is connected using arrow to indicate the sequence of the steps and different types of program actions. The other functions may be used but are not necessary for most PLC applications. The concept of controlling a traffic light control system is introduced, which is the systematic approach of control system design using a PLC. The operation procedure of the systemapproach is shown in the figure below:
Figure 4.3: Programmable Control Design Flow Chart [10].
4.4 Programming the PLC The Festo FEC FC34 PLC is programmed according to the different variants using the following programming software: FST FEC/IPC in Statement List and Ladder diagram, which is largely based on the FST software for the FPC 100 or Multiprogwt in accordance with IEC 1131-3. An RS232 cable is required in order to connect the PLC to the serial port at the PC. A new project is created when a program is about to be written to the controller task. Once created, it is saved to the current project directory. Before creating a new project, the required project directory is set-up. When a new project is created, any project already open automatically closes.
In practice, programming is mostly started by entering the inputs and outputs into the allocation list. The allocation list can be found in the Project Window below:
Figure 4.4: Project window The allocation list consists of operands that match the physical input and output addresses or configurations of the PLC to the names of the devices attached to it. Note each symbol and its associated comments is used to identify the I/O it represents.
Figure 4.5: Example of Allocation List.
4.41 The Ladder Diagram Program Ladder diagram – LDR for short – is a graphic-based programming language developed from the circuit diagram. The diagram of a LDR program is therefore similar to the diagram of a circuit diagram – in relation to the diagram of logical links. However, for the LDR diagram, new symbols have been introduced for contacts and coils that are better suited for displaying on a monitor. Due to the similarities with circuit diagrams, the LDR diagramprogram is frequently preferred by developers who are familiar with relay technology. If a circuit diagram already exists for a control task, it can usually be transferred to a LDR program. A LDR diagram is based on two vertical lines. In the transfer sense, the left line is linked to the voltage source and the right is earthed. Between them, the LDR diagram is compiled in the form of horizontally arranged rungs with contacts, coils and other LDR symbols.Rungs consist of a condition part and an executive part. The left side of a rung represents the condition part, which contains the logical and/or arithmetical links, e.g. in the form of contacts and parallel branches. The right side of a rung represents the executive part. This is where the action to be executedis programmed, e.g. in the form of coils. A rung in the LDR therefore usually reads from left to right.As an example, the diagram below shows a small section of a LDR program in the online display.When the FST software is in online mode, contacts
and coils and all lines that report 1 signal are highlighted in blue. Operands that report 1 signal are tagged with ―ON‖, Operands that report 0 signal with ―OFF‖.
[1] Condition part [2] Executive part [3] Operand [4] Coil symbols [5] Parallel branch in the condition part [6] Contact symbols [7] Rung 2 [8] Rung 1 Figure 4.6: LDR program (online display)
4.42 Running the Ladder Program on the PLC When the LDR program is ready, clicking on the Build Project iconwill compile the program. The FST software does not always agree with everything written down. A syntax check, which searches the program for formal errors, is performed during compilation. Any error which requires debugging will be displayed in the status bar. If there is no error, you may proceed to go online and communicate with your PLC. This is set automatically if the PLC can communicate effectively with the PC using the RS232 port. The next step would be to transfer the program to the PLC. Click on Download Project icon. The message ‗Download Complete‘ must be given in the message window.
Figure 4.7: Transfer of Program to PLC
The controller after download is set to RUN status or configured to run automatically. In any case, if the RUN LED is green, then the controller is already operating in the RUN mode. If the LED is orange, then the controller must be set to RUN using the RUN/STOP slide switch. If it is red, that indicates an error in the program. Then, the devices could be checked whether it is been controlled as programmed.
CHAPTER 5 RESULTS AND DISCUSSION
As mentioned in Chapter 3 and Chapter 4, all the system of the desired project was implemented and the results of the systems illustrated in this Chapter. During the operation, all activities that occur can be observed by the PC using FST software. The system needs to debug along the way and fine tune if necessary. The system is test run thoroughly until it is safe to be operated.
5.1 The Prototype The prototype was mainly built by combining the wood design, steel design and the electrical designs. The main power supply is in built into the FESTO didactic power supply unit to supply 24V direct current needed by the PLC and other Devices.
5.2 Project Operation The operation of the traffic lights starts when the program is downloaded into the PLC.The traffic signal operation will start by the traffic lights illuminating in red for the North/South (NS) lane and green for the East/ West (EW) lane for the period of 20 seconds for the first timer (TON1). Then this timer operates the next timer and so on, in a way that a sequential system is formed. The second timer (TON2) makes the NS to stay at red and EW to change to amber (yellow), and is on for 5 seconds. The third timer (TON3) keeps the NS at red and changes EW to red for a period of 2 seconds. The fourth timer (TON4) changes the NS to red and amber, keeps the EW at red for 5 seconds. The fifth timer (TON5) changes the NS to green and keeps the EW at red for 20 seconds. The sixth timer (TON6) changes the NS to amber and held the EW at red for 5 seconds. The seventh timer (TON7) changes the NS back to red and held the EW at red for 2 seconds. The eighth timer (TON8) keeps the NS at red and changes the EW to red and amber for a period of 5 seconds. When the eighth timer (TON8) is off, it restarts the sequence, by restarting the first timer (TON1) which held the NS at red and EW changes to green. The
whole process is that if one timer finishes, it starts the other, and this is a continuous process, only if there is no power failure in the PLC. The Pedestrian Lights for the EW only are illuminated during the sequential process of the traffic lights due to the shortage of outputs on the PLC. The Pedestrian red light is illuminated during the timing period of the first, second and third timer and changes to green for the rest of the timing periods. The timing period for the eight timers is supposed to make the pedestrian green light to flash but this was not possible due to the limitation in the PLC operations. When the pedestrian button is pressed, it resets the third timer and changes the operation of the PLC outputs. The inclusion of monitoring devices such as infra-red sensors will give a rough indication of the traffic conditions, i.e., whether there is a high volume of traffic waiting to cross at a particular junction. With this information, we will be able to fine tune the traffic control system to change the traffic light timing to adapt to the traffic conditions. When the vehicular traffic is low, the traffic light can change more frequently to minimize waiting time. When the vehicular traffic is high, the traffic light can remain green for a longer period of time. Hence, the following set of logic/rules can be used: 1. There is one counter for the North/South lane and one counter East/West lane; so that the counters can compare the count wi th 2 different preset values (5 for each). 2. If the count is less than 5, the time allotted to that lane for the green light is 20s. If the count gets to 5, the time allotted will become longer because the timer resets and the counter resets as well for that particular lane. The sensors on the NS and EW lane are programmed check the each lane condition. It will check whether the sensors are triggered or not. In this project, four infrared sensors were used to detect the presence of vehicles in all four directions. This functions as when a vehicle blocks the sensor at a certain distance, the sensor is triggered and this will inform the PLC that there is a vehicle in the specific lane and the counter counts 1. The current design of a traffic light system in terms of mechanical, electrical, logic and instrumentationaspects take full advantage of the application of sensors in the real life
situation of traffic flow by optimizing the time between light changes. If there are no vehicles on the road in all four directions, then the lights will change as pre-programmed from red, to red-amber, to green, to amber, and back to red in both directions. This is a typical UK traffic lights sequence.
5.3 Advantages The traffic light system that had been developed presents several advantages. Since the waiting time of the vehicles for the lights to change is optimal, the emission of carbon monoxide from the vehicles is reduced. This will give a positive effect to the greenhouse effect towards the environment. The traffic light system will also save the motorists‘ time and reduce their frustration while waiting for the lights to change since it helps in reducing congestion at the traffic intersections. Another advantage is that there is no interference between the sensor rays and there is no redundant signal triggering. By being able to interface with the FST software, the PLC based traffic light system will easily accept feedback. Therefore there will be easier communication between the software and the hardware.
Figure 5: The Whole Hardware and Software.
CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS 6.1 Conclusion A traffic light system had successfully been designed and developed with proper integration of both the hardware and the software. The pedestrian light for North/South was not included due to insufficient output cards on the PLC, reasons stated in the next section below. The infra-red sensors were interfaced with the Festo FEC FC34 PLC. This interface is synchronized with the whole process of the traffic system. It could be seen from the objectives of this project, that knowledge and skills were combined together in order to complete this task. For this project, the knowledge of sequential systems, electrical and electronics applications had been proven. The skill involved in this project is the programming skills which makes you to think more as a student. The system will encounter problems without proper integration of both the hardware and software related to this project. Besides, this project, gave a challenge of having to learn some other craft related work like painting, drilling, cutting of metals and woodwork. Automatically, this project could be programmed in any way to control the traffic light model and will be useful for planning road system network in the United Kingdom. 6.2 Problems and Difficulties Encountered The Programmable Logic Controller that was originally chosen for this project was FESTO FEC Edutrainer FC34 which has enough inputs and outputs cards to cater for the whole outputs adjudged to the traffic light model but unfortunately it could not communicate with the computer. Due to this and unavailability of enough PLCs, FEC FC34 FST with less output was used and also the project progress was really affected.The available space on the model could not cater for two sensors on each lane, in which one will be detecting and the other will be counting. Also, having to learn another ladder logic program for the FST software really took time and more challenging because what I have simulated could no longer be used, since that was based on Allan Bradley Software.
The FESTO FEC FC34 PLC also has less functionality, unlike other PLC such as FEC standard; FPC 405; FESTO CPX/IPC etc. It does not have a flashing mode, the CFM that contains 4 flashing bits does not work with it. As a result, the pedestrian flashing mode is not included in the project.
6.3 Recommendation The efficient operation can be achieved when there are enough Input/output cards for the entire component used in this system. A more sophisticated and flexible PLC with enough input/output cards should be used to provide enough functionality for the traffic light system. The traffic light system should be programmed and necessary circuitry added to operate in three modes namely: Day, Emergency and Night modes. A wider area board should be used in order to achieve this. This prototype can easily be implemented in real life situations. Increasing the n umber of sensors to detect the presence of vehicles can further enhance the design of the traffic light system. Another room for improvement is to have the infrared sensors replaced with an imaging system/camera system so that it has a wide range of detection capabilities, which can be enhanced and ventured into a perfect traffic system. Different sensors should be used on each lane, to test communication strength with the PLC.
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