Plc report

AMITY UNIVERSITYRajasthan

PLC AND SCADA

SUBMITTED TOMs. Pushpa Gotwal

SUBMITTED BYPriya Hada

B.tech(ECE)7th sem

October 13, 2014

CERTIFICATE

This is to certify that PRIYA HADA, student of B.Tech. in Electronics andCommunication Engineering has carried out the work presented in Training en-titled ”PLC and SCADA” as a part of fourth Year programme of B.Tech. inElectronics and Communication Engineering from Amity School of Engineer-ing and Technology, Amity University Rajasthan, under my supervision.

FACULTYMr. Sudhir Kumar MishraASET(AUR)

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ACKNOWLEDGMENTS

It has come out to be a sort of great pleasure and experience for me to workon the Plc and SCADA, from Prime Vision Automation.I wish to express myindebtedness to those who helped us i.e. the faculty of our Institute Ms. PushpaGotwal, ASET during the preparation of the manual script of this text. Thiswould not have been made successful without his help and precious sugges-tions.

Priya Hada

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Contents

CERTIFICATE i

ACKNOWLEDGMENTS ii

1 INTRODUCTION 1

1.1 AUTOMATION . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 PLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 SCADA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Features of PLCs . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.5 History of PLCs . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.6 Hard Wired Relay Comparison . . . . . . . . . . . . . . . . . . 6

2 PROGRAMMABLE LOGIC CONTROLLER 9

2.1 INPUT MODULE . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2 OUTPUT MODULE . . . . . . . . . . . . . . . . . . . . . . . 12

2.3 Wiring In a PLC . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3.1 Generation of Input Signal . . . . . . . . . . . . . . . . 15

2.4 PLC compared with other control systems . . . . . . . . . . . . 16

3 Ladder Logic 17

3.1 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.2 Comparison to Relay Logic . . . . . . . . . . . . . . . . . . . . 17

3.3 Ladder Logic Programming . . . . . . . . . . . . . . . . . . . . 19

3.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 19

3.4 EXAMPLES OF LADDER PROGRAMMING . . . . . . . . . 24

4 SCADA 27

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.2 Types of SCADA . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.3 Features of SCADA . . . . . . . . . . . . . . . . . . . . . . . . 27

4.4 Manufacture of SCADA . . . . . . . . . . . . . . . . . . . . . 28

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4.5 What does SCADA MEAN? . . . . . . . . . . . . . . . . . . . 28

4.6 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.6.1 Hardware Architecture . . . . . . . . . . . . . . . . . . 29

4.6.2 Generic Software Architecture . . . . . . . . . . . . . . 29

4.7 Communications . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.7.1 Internal Communication . . . . . . . . . . . . . . . . . 29

4.7.2 Access to Devices . . . . . . . . . . . . . . . . . . . . 29

4.8 Interfacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.8.1 Application Interfaces / Openness . . . . . . . . . . . . 30

4.8.2 Database . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.8.3 Scalability . . . . . . . . . . . . . . . . . . . . . . . . 30

4.8.4 Redundancy . . . . . . . . . . . . . . . . . . . . . . . . 31

4.8.5 Functionality . . . . . . . . . . . . . . . . . . . . . . . 31

4.8.6 Trending . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.8.7 Alarm Handling . . . . . . . . . . . . . . . . . . . . . 32

4.8.8 Logging/Archiving . . . . . . . . . . . . . . . . . . . . 32

4.8.9 Report Generation . . . . . . . . . . . . . . . . . . . . 33

4.8.10 Automation . . . . . . . . . . . . . . . . . . . . . . . 33

5 Application Development 34

5.1 Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.2 Development Tools . . . . . . . . . . . . . . . . . . . . . . . . 34

5.3 Object Handling . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.4 Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.5 Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.6 Potential benefits of SCADA . . . . . . . . . . . . . . . . . . . 36

5.7 Where SCADA is used ? . . . . . . . . . . . . . . . . . . . . . 37

6 CONCLUSION 38

REFERENCES 39

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List of Figures

1 Complete Automation System Components . . . . . . . . . . . 1

2 Block diagram of PLC . . . . . . . . . . . . . . . . . . . . . . 4

3 Typical Small Scale Control Panel . . . . . . . . . . . . . . . . 7

4 Typical PLC Control Panel . . . . . . . . . . . . . . . . . . . . 7

5 Diagram of Counter . . . . . . . . . . . . . . . . . . . . . . . . 11

6 Diagram of Timer . . . . . . . . . . . . . . . . . . . . . . . . . 12

7 Input module of PLC . . . . . . . . . . . . . . . . . . . . . . . 12

8 Output module to PLC . . . . . . . . . . . . . . . . . . . . . . 13

9 Output module from PLC . . . . . . . . . . . . . . . . . . . . . 13

10 PLC scan cycle . . . . . . . . . . . . . . . . . . . . . . . . . . 14

11 Wiring in PLC . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

12 Basic Ladder Logic Program . . . . . . . . . . . . . . . . . . . 17

13 Electromechanical Relay . . . . . . . . . . . . . . . . . . . . . 18

14 Basic Relay logic Circuit . . . . . . . . . . . . . . . . . . . . . 18

15 Relay Logic Circuit with Jog function and Status Indicators . . . 19

16 Complex Ladder Diagram . . . . . . . . . . . . . . . . . . . . 20

17 Basic Ladder Logic Program . . . . . . . . . . . . . . . . . . . 20

18 Basic Program to show input and output . . . . . . . . . . . . . 22

19 Normally Open Contact . . . . . . . . . . . . . . . . . . . . . . 22

20 Normally Open Coil . . . . . . . . . . . . . . . . . . . . . . . 22

21 Normally Closed Contact . . . . . . . . . . . . . . . . . . . . . 23

22 Normally Closed coil . . . . . . . . . . . . . . . . . . . . . . . 23

23 Basic And Gate using Ladder Logic . . . . . . . . . . . . . . . 23

24 Basic And Gate using Ladder Logic . . . . . . . . . . . . . . . 23

25 Start/Stop of Motor by PLC . . . . . . . . . . . . . . . . . . . . 25

26 Starting of Motor . . . . . . . . . . . . . . . . . . . . . . . . . 25

27 Continous Running of motor when Start Button is Released . . . 26

28 To Stop the Motor . . . . . . . . . . . . . . . . . . . . . . . . . 26

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1 INTRODUCTION

1.1 AUTOMATION

Automation or industrial automation is the use of control systems such as com-puters, controllers to control industrial machinery and processes, to optimizeproductivity in the production of goods and delivery of services. Automationis a step beyond mechanization. Whereas mechanization provides human op-erators with machinery to assist them with the muscular requirements of work,automation greatly decreases the need for human sensory and mental require-ments.

Automation Impacts:

1. It increases productivity and reduce cost.

2. It gives emphasis on flexibility and convertibility of manufacturing pro-cess. Hence gives manufacturers the ability to easily switch from manu-facturing Product A to manufacturing product B without completely re-built the existing system/product lines.

3. Automation is now often applied primarily to increase quality in the man-ufacturing process, where automation can increase quality substantially.

4. Increase the consistency of output.

5. Replacing humans in tasks done in dangerous environments.

Figure 1: Complete Automation System Components

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1.2 PLC

A Programmable Logic Controller, PLC, or Programmable Controller is a dig-ital computer used for automation of industrial processes, such as control ofmachinery on factory assembly lines. Unlike general-purpose computers, thePLC is designed for multiple inputs and output arrangements, extended tem-perature ranges, immunity to electrical noise, and resistance to vibration andimpact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time systemsince output results must be produced in response to input conditions within abounded time, otherwise unintended operation will result. PLC are registeredtrademarks of the Allen-Bradley Company.

PLCs have become the most predominant control elements for the discreteevent control of a mechatronics system. Simplification of engineering and pre-cise control of manufacturing process can result in significant cost savings. Themost cost-effective way which can pay big dividends in the long run is flexibleautomation; a planned approach towards integrated control systems. It requiresa conscious effort on the part of plant managers and engineers to identify areaswhere automation can result in better deployment and/or utilization of humanresources and savings in man-hours or down time. Controls automation neednot be high ended and extremely sophisticated; it is the phased, step-by-stepeffort to automate, employing control systems tailored to ones specific require-ments that achieves the most attractive results. This is where programmablelogic controls have been a breakthrough in the field of automation and controltechniques.

A constant demand for better and more efficient manufacturing and pro-cess machinery has led to the requirement for higher quality and reliabilityin control techniques. With the availability of intelligent, compact solid stateelectronic devices, it has been possible to provide control systems that can re-duce maintenance, down time and improve productivity to a great extend. Byinstalling an efficient and user friendly electronics systems for manufacturingmachinery or processors, one can obtain a precise and reliable means for pro-ducing quality products. One of the latest techniques in solid state controls thatoffers flexible and efficient operation to the user is programmable controllers.The basic idea behind these programmable controllers was to provide meansto eliminate high cost associated with inflexible, conventional relay controlledsystems. Programmable controllers offer a system with computer flexibility thatis suited to withstand the harsh industrial environment, has simplicity of oper-ation/readability, can reduce machine down time and provide expandability forfuture and is able to be maintained by plant technicians.

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1.3 SCADA

SCADA is widely used in industry for Supervisory Control and Data Acqui-sition of industrial processes, SCADA systems are now also penetrating theexperimental physics laboratories for the controls of ancillary systems such ascooling, ventilation, power distribution, etc. More recently they were also ap-plied for the controls of smaller size particle detectors such as the L3 moondetector and the NA48 experiment, to name just two examples at CERN.

SCADA systems have made substantial progress over the recent years interms of functionality, scalability, performance and openness such that they arean alternative to in house development even for very demanding and complexcontrol systems as those of physics experiments.

1.4 Features of PLCs

1. PLC is an industrial computer control system that continuously monitorsthe state of input devices and makes decisions based upon a custom pro-gram to control the state of output devices.

2. It is designed for multiple inputs and output arrangements, extended tem-perature ranges, immunity to electrical noise, and resistance to vibrationand impact.

3. Almost any production process can greatly enhanced using this type ofcontrol system, the biggest benefit in using a PLC is the ability to changeand replicate the operation or process while collecting and communicat-ing vital information.

4. Another advantage of a PLC is that it is modular i.e. you can mix andmatch the types of input and output devices to best suit your application.

With each module having sixteen ”points” of either input or output, this PLC hasthe ability to monitor and control dozens of devices. Fit into a control cabinet, aPLC takes up little room, especially considering the equivalent space that wouldbe needed by electromechanical relays to perform the same functions.

The main difference from other computers is that PLC are armored for se-vere condition (dust, moisture, heat, cold, etc) and has the facility for extensiveinput/output (I/O) arrangements. These connect the PLC to sensors and actua-tors. PLCs read limit switches, analog process variables (such as temperatureand pressure), and the positions of complex positioning systems. Some evenuse machine vision. On the actuator side, PLCs operate electric motors, pneu-matic or hydraulic cylinders, magnetic relays or solenoids, or analog outputs.

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The input/output arrangements may be built into a simple PLC, or the PLC mayhave external I/O modules attached to a computer network that plugs into thePLC.

Many of the earliest PLCs expressed all decision making logic in simpleladder logic which appeared similar to electrical schematic diagrams. This pro-gram notation was chosen to reduce training demands for the existing techni-cians. Early PLCs used a form of instruction list programming, based on astack-based logic solver. The functionality of the PLC has evolved over theyears to include sequential relay control, motion control, process control, dis-tributed control systems and networking. The data handling, storage, process-ing power and communication capabilities of some modern PLCs are approxi-mately equivalent to desktop computer.

Figure 2: Block diagram of PLC

1.5 History of PLCs

PLCs were first introduced in the 1960s. The primary reason for designing sucha device was eliminating the large cost involved in replacing the complicatedrelay based machine control systems. Bedford Associates (Bedford, MA) pro-posed something called a Modular Digital Controller (MODICON) to a majorUS car manufacturer. The MODICON 084 brought the world’s first PLC intocommercial production.

When production requirements changed so did the control system. This

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becomes very expensive when the change is frequent. Since relays are mechan-ical devices they also have a limited lifetime because of the multitude of mov-ing parts. This also required strict adhesion to maintenance schedules. Trou-bleshooting was also quite tedious when so many relays are involved. Nowpicture a machine control panel that included many, possibly hundreds or thou-sands, of individual relays. The size could be mind boggling not to mention thecomplicated initial wiring of so many individual devices. These relays would beindividually wired together in a manner that would yield the desired outcome.The problems for maintenance and installation were horrendous.

These new controllers also had to be easily programmed by maintenanceand plant engineers. The lifetime had to be long and programming changeseasily performed. They also had to survive the harsh industrial environment.The answers were to use a programming technique most people were alreadyfamiliar with and replace mechanical parts with solid-state ones which have nomoving parts.

Communications abilities began to appear in approximately 1973. The firstsuch system was Modicon’s Modbus. The PLC could now talk to other PLCsand they could be far away from the actual machine they were controlling. Theycould also now be used to send and receive varying voltages to allow them touse analog signals, meaning that they were now applicable to many more controlsystems in the world. Unfortunately, the lack of standardization coupled withcontinually changing technology has made PLC communications a nightmareof incompatible protocols and physical networks.

The 1980s saw an attempt to standardize communications with General Mo-tor’s manufacturing automation protocol (MAP). It was also a time for reducingthe size of the PLC and making them software programmable through sym-bolic programming on personal computers instead of dedicated programmingterminals or handheld programmers.

The 1990s saw a gradual reduction in the introduction of new protocols, andthe modernization of the physical layers of some of the more popular protocolsthat survived the 1980’s. PLCs can now be programmable in function blockdiagrams, instruction lists, C and structured text all at the same time. PC’s arealso being used to replace PLCs in some applications. The original companywho commissioned the MODICON 084 has now switched to a PC based controlsystem.

1. The first PLCs were designed and developed by Modicon as a relay re-placer for GM and Landis.

2. The primary reason for designing such a device was eliminating the largecost involved in replacing the complicated relay based machine control

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systems for major U.S. car manufacturers.

3. These controllers eliminated the need of rewiring and adding additionalhardware for every new configuration of logic.

4. The first PLC, model 084, was invented by Dick Morley in 1969.

5. The first commercial successful PLC, the 184, was introduced in 1973and was designed by Michel Greenberg.

Communications abilities began to appear in approximately 1973. The firstsuch system was Modicon’s Modbus. The PLC could now talk to other PLCsand they could be far away from the actual machine they were controlling.

1.6 Hard Wired Relay Comparison

At the outset of industrial revolution, especially during sixties and seventies,relays were used to operate automated machines, and these were interconnectedusing wires inside the control panel. In some cases a control panel covered anentire wall. To discover an error in the system much time was needed, especiallywith more complex process control systems. On top of everything, a lifetime ofrelay contacts was limited, so some relays had to be replaced. If replacementwas required, machine had to be stopped and production as well. Also, it couldhappen that there was not enough room for necessary changes. A control panelwas used only for one particular process, and it wasnt easy to adapt to the re-quirements of a new system. As far as maintenance, electricians had to be veryskillful in finding errors. In short, conventional control panels proved to be veryinflexible. Typical example of conventional control panel is given in the follow-ing picture. In Figure 3 you can see a large number of electrical wires, relays,timers and other elements of automation typical for that period. The picturedcontrol panel is not one of the more complicated ones, so you can imagine whatcomplex ones looked like.

The most frequently mentioned disadvantages of a classic control panel are:

1. Large amount of work required connecting wires.

2. Difficulty with changes or replacements.

3. Difficulty in finding errors; requiring skillful/experienced work force.

4. When a problem occurs, hold-up time is indefinite, usually long.

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Figure 3: Typical Small Scale Control Panel

Figure 4: Typical PLC Control Panel

With invention of programmable controllers, much has changed in how a pro-cess control system is designed. Many advantages appeared. Typical exampleof control panel with a PLC controller is given in the following picture.

Advantages of control panel that is based on a PLC controller can be pre-sented in few basic points:

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1. Compared to a conventional process control system, number of wiresneeded for connections is reduced by approximately 80

2. Diagnostic functions of a PLC controller allow for fast and easy errordetection.

3. Change in operating sequence or application of a PLC controller to a dif-ferent operating process can easily be accomplished by replacing a pro-gram through a console or using PC software (not requiring changes inwiring, unless addition of some input or output device is required).

4. Needs fewer spare parts.

5. It is much cheaper compared to a conventional system, especially in caseswhere a large number of Input/Output instruments are needed and whenoperational functions are complex.

6. Reliability of a PLC is greater than that of an electro-mechanical relay ora timer, because of less moving parts.

7. They are compact and occupy less space.

8. Use of PLC results in appreciable savings in hardware and wiring cost.

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2 PROGRAMMABLE LOGIC CONTROLLER

Definition:

A Programmable controller is a solid state user programmable control sys-tem with functions to control logic, sequencing, timing, arithmetic data manip-ulation and counting capabilities. It can be viewed as an industrial computerthat has a central processor unit, memory, input output interface and a pro-gramming device. The central processing unit provides the intelligence of thecontroller. It accepts data, status information from various sensing devices likelimit switches, proximity switches, executes the user control program stored inthe memory and gives appropriate output commands to devices such as solenoidvalves, switches etc.

Input output interface is the communication link between field devices andthe controllers. Through these interfaces the processor can sense and measurephysical quantities regarding a machine or process, such as, proximity, position,motion, level, temperature, pressure, etc. Based on status sensed, the CPUissues command to output devices such as valves, motors, alarms, etc. Theprogrammer unit provides the man machine interface. It is used to enter theapplication program, which often uses a simple user-friendly logic.

What is inside a PLC?

The PLC, being a microprocessor based device, has a similar internal struc-ture to many embedded controllers and compute rs. They consist of the CPU,Memory and I/O devices. These components are integral to the PLC controller.Additionally the PLC h as a connection for the Programming and MonitoringUnit or to connect to other PLCs in the field.

Components:

The PLC mainly consists of a CPU, memory areas, and appropriate circuitsto receive input/output data. We can actually consider the PLC to be a box fullof hundreds or thousands of separate relays, counters, timers and data storagelocations. They don’t physically exist but rather they are simulated and canbe considered software counters, timers, etc. Each component of a PLC has aspecific function:

1. The CPU is the brain of a PLC system. It consists of the microprocessor,memory integrated circuits and circuits necessary to store and retrieveinformation from memory. It also includes also includes communicationports to the peripherals, other PLCs or programming terminals. The jobof the processor is to monitor status or state of input devices, scan andsolve the logic of a user program, and control on or off state of outputdevices.

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2. RAM or Random Access Memory is a volatile memory that would loseits information if power were removed , hence some processor units areprovided with battery backup. Normally CMOS (Complementary MetalOxide Semiconductor) type RAM is used.

3. ROM is a non volatile type of memory. This means it stores it is dataeven if no power is available. This type of memory information can onlybe read, it is placed there for the internal use and operation of processorunits.

4. EEPROME or Electrically Erasable Programmable Read Only Memoryis usually an add on memory module that is used to backup the mainprogram in CMOS RAM of the processor. In many cases, the processorcan be programmed to load the EEPROMS program to RAM, if RAM islost or corrupted.

5. Input Relays (contacts) - These are connected to the outside world. Theyphysically exist and receive signals from switches, sensors, etc. Typicallythey are not relays but rather they are transistors.

6. Internal Utility Relays - These do not receive signals from the outsideworld nor do they physically exist. They are simulated relays and arewhat enables a PLC to eliminate external relays. There are also somespecial relays that are dedicated to performing only one task. Some arealways on while some are always off. Some are on only once duringpower-on and are typically used for initializing data that was stored.

7. Counters - These are simulated counters and they can be programmed tocount pulses. Typically these counters can count up, down or both up anddown. Since they are simulated they are limited in their counting speed.Some manufacturers also include high-speed counters that are hardwarebased. We can think of these as physically existing.

8. Timers - These come in many varieties and increments. The most com-mon type is an on-delay type. Others include off-delay and both retentiveand non-retentive types. Increments vary from 1 millisecond through 1second.

9. Output Relays (coils) - These are connected to the outside world. Theyphysically exist and send on/off signals to solenoids, lights, etc. They canbe transistors, relays, or triacs depending upon the model chosen.

10. 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

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the PLC. Upon power-up they will still have the same contents as beforepower was removed

A counter is a simple device intended to do one simple thing - count. Usingthem can sometimes be a challenge however because every manufacturer seemsto use them a different way. There are several different types of counters. Thereare up-counters called CTU CNT, or CTR that only count up, such as 1, 2, and3. There are also down counters called CTD that only count down, such as 9,8, 7, etc. In addition to these two, there are up-down counters, typically calledUDC (up-down counter). These count up and/or down (1,2,3,4,3,2,3,4,5,...)

Figure 5: Diagram of Counter

A timer is an instruction that waits a set amount of time before doing some-thing. As usual in industry, different types of timers are available with differentmanufacturers. The most common type of timer is an On-Delay Timer. Thistype of timer simply delays turning on its respective output. In other words,after our sensor (input) turns on we wait x number of seconds before activatinga solenoid valve (output). This is the most common timer. It is often calledTON (timer on-delay), TIM (timer) or TMR (timer). Another type of timer isan Off-Delay Timer. This type of timer is the opposite of the on-delay timerlisted above. This timer delays turning off its respective output. After a sensor(input) sees a target we turn on a solenoid (output). When the sensor no longersees the target we hold the solenoid on for x number of seconds before turningit off. It is called a TOF (timer off-delay) and is less common than the on-delaytype listed above. Very few manufacturers include this type of timer, althoughit can be quite useful. The last type of timer is a Retentive or Accumulatingtimer. This type of timer needs 2 inputs. One input starts the timing event (i.e.the clock starts ticking) and the other resets it. The on/off delay timers abovewould be reset if the input sensor wasn’t on/off for the complete timer duration.This timer however holds or retains the current elapsed time when the sensorturns off in mid-stream. For example, we want to know how long a sensor ison for during a 1 hour period. If we use one of the above timers they will keepresetting when the sensor turns off/on. This timer however, will give us a to-

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tal or accumulated time. It is often called an RTO (retentive timer) or TMRA(accumulating timer).

Figure 6: Diagram of Timer

2.1 INPUT MODULE

Input Module: Input modules interface directly to devices such as switches andtemperature sensors. Input modules convert many different types of electricalsignals such as 120VAC, 24VDC, or 4-20mA, to signals which the controllercan understand since all electrical systems are inherently noisy, electrical isola-tion is provided between input and processor. The component most often usedfor this purpose is optocoupler .Input signal from the field devices are usually 4to 20 ma or 0-10 V.

Figure 7: Input module of PLC

2.2 OUTPUT MODULE

Output module : It interface directly to devices such as motor starters and lightsOutput modules take digital signals from the PLC and convert them to electricalsignals such as 24VDC and 4 mA that field devices can understand. D to Aconversion is carried out in there modules. Usually Silicon Controlled Rectifier

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Figure 8: Output module to PLC

(SCR), triac, or dry contact relays are used for this purpose. Normally the outputsignal is 0-10 V or 4-20 ma.

Figure 9: Output module from PLC

PLC Operation: PLC operates by continually scanning the program andacting upon the instructions , one at a time, to switch on or off the variousoutputs. In order to do this PLC first scans all the inputs and stores their statesin memory. Then it carries out program scan and decides which outputs shouldbe high according to the program logic.

Then finally it updates these values to the output table, making the requiredoutputs go high. At his point PLC checks its own operating system and if ev-erything is ok, it goes back to scanning inputs all over again.

PLC SCAN CYCLE: A PLC works by continually scanning a program.Thefirst step is to check the input status. This step is therefore generally referredto as the Check Input Status stage. First the PLC takes a look at each input todetermine if it is on or off. In other words, is the sensor connected to the firstinput on? How about the second input? How about the third? This goes on and

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on through the entire program. It records this data into its memory to be usedduring the next step.

Next the PLC executes your program one instruction at a time, called theExecute Program stage. For example, if your program said that if the first inputwas on then it should turn on the first output. Since it already knows whichinputs are on/off from the previous step it will be able to decide whether thefirst output should be turned on based on the state of the first input. It will storethe execution results for use later during the next step.

Finally the PLC updates the status of the outputs. It updates the outputsbased on which inputs were on during the first step and the results of executingyour program during the second step. Based on the example in step 2 it wouldnow turn on the first output because the first input was on and your programsaid to turn on the first output when this condition is true.

A new style of scanning has been implemented in the more recent years,called rung scanning. This type basically scans each ladder rung individually inthe entire ladder logic program, updating the outputs on that rung after scanningthrough the inputs. This changes the type of programming that will be used aswell. If an output is in a rung above the inputs it depends on, you will not getthe output updated until the next scan, as the program will keep scanning downuntil the last rung, then start over. This style is very advantageous in certainsituations. If you want your outputs updated at the soonest possible moment,this is the style of scanning that you want to use.

Figure 10: PLC scan cycle

SCAN TIME

Time taken by plc to execute these three steps (Checking Input status, Exe-cuting Program, Updating Output Status) is denoted by its scan time.

COMMUNICATION

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There are several methods to communicate between a PLC and a Program-mer or even between two PLCs. PLCs have built in communications ports, usu-ally 9 pin RS 232, RS-485, TTY but optionally EIA-485 or Ethernet Modbus,BACnector DF1 os usually included as one of the communications protocols.Other options include various fieldbuses such as Device Net or Profibus. Mostmodern PLCs can communicate over a network to some other system , suchas a computer running a SCADA ( Supervisory Control And Data Acquistion )system or web browser.

2.3 Wiring In a PLC

2.3.1 Generation of Input Signal

Inside the PLC housing, connected between each input terminal and the Com-mon terminal, is an opto-isolator device (Light-Emitting Diode) that providesan electrically isolated ”high” Logic signal to the computer’s circuitry (a photo-transistor interprets the LED’s light) when there is 120 VAC power applied be-tween the respective input terminal and the Common terminal. An indicatingLED on the front panel of the PLC gives visual indication of an ”energized”input.

Figure 11: Wiring in PLC

Output signals are generated by the PLC’s computer circuitry activating aswitching device (transistor, or even an electromechanical relay), connectingthe ”Source” terminal to any of the ”Y-” labeled output terminals. The ”Source”terminal, correspondingly, is usually connected to the L1 side of the 120 VACpower source. As with each input, an indicating LED on the front panel of thePLC gives visual indication of an ”energized” output

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In this way, the PLC is able to interface with real-world devices such asswitches and solenoids. The actual logic of the control system is establishedinside the PLC by means of a computer program. Although the program itselfappears to be a ladder logic diagram, with switch and relay symbols, there areno actual switch contacts or relay coils operating inside the PLC to create thelogical relationships between input and output. These are imaginary contactsand coils, if you will. The program is entered and viewed via a personal com-puter connected to the PLC’s programming port.

2.4 PLC compared with other control systems

PLCs are well-adapted to a certain range of automation tasks. These are typ-ically industrial processes in manufacturing where the cost of developing andmaintaining the automation system is high relative to the total cost of the au-tomation, and where changes to the system would be expected during its oper-ational life. PLCs contain input and output devices compatible with industrialpilot devices and controls; little electrical design is required, and the designproblem centers on expressing the desired sequence of operations in ladderlogic (or function chart) notation. PLC applications are typically highly cus-tomized systems so the cost of a packaged PLC is low compared to the cost ofa specific custom-built controller design. For high volume or very simple fixedautomation tasks, different techniques are used.

A microcontroller-based design would be appropriate where hundreds orthousands of units will be produced and so the development cost (design ofpower supplies and input/output hardware) can be spread over many sales, andwhere the end-user would not need to alter the control. Automotive applicationsare an example; millions of units are built each year, and very few end-users al-ter the programming of these controllers. However, some specialty vehiclessuch as transit buses economically use PLC’s instead of custom-designed con-trols, because the volumes are low and the development cost would be uneco-nomic.

PLCs may include logic for single-variable feedback analog control loop,a ”proportional, integral, derivative” or ”PID controller.” A PID loop could beused to control the temperature of a manufacturing process, for example. His-torically PLCs were usually configured with only a few analog control loops;where processes required hundreds or thousands of loops, a distributed controlsystem (DCS) would instead be used. However, as PLCs have become morepowerful, the boundary between DCS and PLC applications has become lessclear.

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3 Ladder Logic

3.1 Definition

Ladder logic is one form of drawing electrical logic schematics, and is a graphi-cal language very popular for programming PLC’s. Ladder logic was originallyinvented to describe logic made from relays. The name is based on the observa-tion that programs in this language resemble ladders, with two vertical ”rails”and a series of horizontal ”rungs” between them. Figure 12 below is a verybasic example of ladder logic used in a programmable logic controls program.

Figure 12: Basic Ladder Logic Program

3.2 Comparison to Relay Logic

The program used in a controls schematic, called a ladder diagram, is similar toa schematic for a set of relay circuits. An argument that aided the initial adop-tion of ladder logic was that a wide variety of engineers and technicians wouldbe able to understand and use it without much additional training, because ofthe resemblance to familiar hardware systems. This argument has become lessrelevant lately given that most ladder logic programmers have a software back-ground in more conventional programming languages, and in practice imple-mentations of ladder logic have characteristics such as sequential execution thatmake the analogy to hardware somewhat imperfect. Electricians and data ca-bling or control technicians still argue that this is the best graphical interface as

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they generally do not have any computer science or digital systems background,and are therefore taught with this interface in sequence with relay logic. Relay

Figure 13: Electromechanical Relay

logic is the precursor to ladder logic, and is a method of controlling industrialelectronic circuits by using relays and contacts. Figure 13 above shows an aver-age mechanical relay used in older relay logic systems. The schematic diagramsfor relay logic circuits are often called line diagrams, because the inputs and out-puts are essentially drawn in a series of lines, with the lines representing actualwires run in the circuit. A relay logic circuit is an electrical network consistingof lines, in which each input/output group must have electrical continuity withall components in that group of devices to enable the output device. The Relaylogic diagrams represent the physical interconnection of devices, while the re-lay logic circuit forms an electrical schematic diagram for the control of inputand output devices. This is why electricians and control technicians can easilyunderstand and interpret relay logic and ladder logic diagrams. Figure 14 belowshows a basic relay logic circuit.

Figure 14: Basic Relay logic Circuit

Figure 14 is a small, basic relay logic circuit. You can see how in relay logiccircuits the push buttons are represented with graphical drawings of a normallyclosed push button for the stop button, and a normally open push button for thestart button. The coil that is marked M is a motor coil, and is a physical piece

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of equipment in the same location as the motor, which is represented by a circlewith the letter M in the middle. The over current or overload device is repre-sented by a normally closed coil symbol with O.L. over it. There would only beseven wires to connect in this circuit, so this would not be very difficult to wire,but when more inputs and outputs are added, the difficulty grows exponentially.

Figure 15 below adds four more components to the system. Two of themare just coils from the motor apparatus that are used as inputs and the other twoare a red and green light to be utilized as output/motor status indicators for theuser.

Figure 15: Relay Logic Circuit with Jog function and Status Indicators

Ladder logic is the most widely used program for PLC where sequentialcontrol of a process or manufacturing operation is required. Ladder logic isuseful for simple but critical control systems, or for reworking old hardwiredrelay circuits. As programmable logic controllers became more sophisticatedit has also been used in very complex automation systems. Figure 15 aboveshows a much more complicated ladder logic diagram than the one shown inFigure 5.In addition there are holding/latching contacts included, but they arenot a piece of hardware. In fact, they are just the address of the respective outputbeing referenced, which will be discussed in greater detail later. This is still nota very large program. Ladder logic programs can easily grow to more than 500rungs to finish some functions.

3.3 Ladder Logic Programming

3.3.1 Introduction

Ladder logic or ladder diagrams are the most common programming languageused to program a PLC. Ladder logic was one of the first programming ap-proaches used in PLCs because it borrowed heavily from the relay diagrams

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Figure 16: Complex Ladder Diagram

that plant electricians already knew. The symbols used in relay ladder logicconsist of a power rail to the left, a second power rail to the right, and individualcircuits that connect the left power rail to the right. The logic of each circuit (orrung) is solved from left to right. A common mistake made by most people istrying to think of the diagram as having to have current across the rung for theoutput to function. This has given many people trouble because of the fact thatsome inputs are not inputs, which will be true when there isnt current throughthis sensor. These concepts will be discussed more later. The symbols of thesediagrams look like a ladder - with two side rails and circuits that resemble rungson a ladder.

Figure 17: Basic Ladder Logic Program

While Ladder Logic is the most commonly used PLC programming lan-guage, but it is not the only one. Following table lists some of the Languages

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that are used to program a PLC.

1. Ladder Diagram()LD.

2. Functional block Diagram (FBD)

3. Structured Text (ST)

4. Instruction List (IL)

5. Sequential Functional Chart (SFC)

Ladder logic is a method of drawing electrical logic schematics. It is now agraphical language very popular for programming Programmable Logic Con-trollers (PLCs). It was originally invented to describe logic made from relays.The name is based on the observation that programs in this language resembleladders, with two vertical ”rails” and a series of horizontal ”rungs” betweenthem. It is a graphical programming language, initially programmed with sim-ple contacts that simulates the opening and closing of relays. Ladder Logic pro-gramming has been expanded to include functions such as Counters, Timers,shift Registers and math operations.

A program in ladder logic, also called a ladder diagram, is similar to aschematic for a set of relay circuits. An argument that aided the initial adoptionof ladder logic was that a wide variety of engineers and technicians would beable to understand and use it without much additional training, because of theresemblance to familiar hardware systems.

Ladder logic is widely used to program PLCs, where sequential control of aprocess or manufacturing operation is required. Ladder logic is useful for sim-ple but critical control systems, or for reworking old hardwired relay circuits.As programmable logic controllers became more sophisticated it has also beenused in very complex automation systems.

Ladder logic can be thought of as a rule-based language, rather than a pro-cedural language. A ”rung” in the ladder represents a rule. When implementedwith relays and other electromechanical devices, the various rules ”execute” si-multaneously and immediately. When implemented in a programmable logiccontroller, the rules are typically executed sequentially by software, in a loop.By executing the loop fast enough, typically many times per second, the effectof simultaneous and immediate execution is obtained.Figure 18 shows a simpli-fied ladder logic circuit with one input and one output. The logic of the rungabove is such:

1. If Input1 is ON (or true) - power (logic) completes the circuit from theleft rail to the right rail - and Output1 turns ON (or true).

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Figure 18: Basic Program to show input and output

2. If Input1 is OFF (or false) - then the circuit is not completed and logicdoes not flow to the right - and Output 1 is OFF (or false).

There are many logic symbols available in Ladder Logic - including timers,counters, math, and data moves such that any logical condition or control loopcan be represented in ladder logic. With just a handful of basic symbols such asa normally open contact, normally closed contact, normally open coil, normallyclosed coil, timer and counter most logical conditions can be represented.

Normally Open Contact

Figure 19: Normally Open Contact

This can be used to represent any input to the control logic such as a switchor sensor, a contact from an output, or an internal output. When solved thereferenced input is examined for a true (logical 1) condition. If it is true, thecontact will close and allow logic to flow from left to right. If the status isFALSE (logical 0), the contact is open and logic will NOT flow from left toright.

Normally Open Coil

This can be used to represent any discrete output from the control logic.When ”solved” if the logic to the left of the coil is TRUE, the referenced outputis TRUE (logical 1).

Figure 20: Normally Open Coil

Normally Closed Contact

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When solved the referenced input is examined for an OFF condition. If thestatus is OFF (logical 0) power (logic) will flow from left to right. If the statusis ON, power will not flow.

Figure 21: Normally Closed Contact

Normally Closed Coil

When ”solved” if the coil is a logical 0, power will be turned on to thedevice. If the device is logical 1, power will be OFF. Normally Open Contact

Figure 22: Normally Closed coil

Basic AND and OR Gates The AND is a basic fundamental logic conditionthat is easy to directly represent in Ladder Logic. Figure 8 shows a simplifiedAND gate on a ladder rung.

Figure 23: Basic And Gate using Ladder Logic

Figure 24: Basic And Gate using Ladder Logic

In order for Light1 to turn TRUE, Switch1 must be TRUE, AND Switch2must be TRUE. If Switch1 is FALSE, logic (not power) flows from the left rail,

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but stops at Switch1. Light1 will be TRUE regardless of the state of Switch2. IfSwitch1 is TRUE, logic makes it to Switch2. If Switch2 is TRUE, power cannotflow any further to the right, and Light1 is FALSE. If Switch1 is TRUE, ANDSwitch2 is TRUE - logic flows to Light1 solving its state to TRUE. The OR isa logical condition that is easy to represent in Ladder Logic. Figure 13 showsa simple OR gate. Notice the differences in logic between the OR and ANDgates.

If Switch1 is TRUE, logic flows to Light1 turning it to TRUE. If Switch2is TRUE, logic flows through the Switch2 contact, and up the rail to Light1turning it to TRUE. If Switch1 AND Switch 2 are TRUE Light1 is TRUE. Theonly way Light1 is FALSE is if Switch1 AND Switch2 are FALSE. In otherwords, Light1 is TRUE if Switch1 OR Switch2 is TRUE.

Basic Timers and Counters

Many times programs will call for action to be taken in a control programbased on more than the states of discrete inputs and outputs. Sometimes, pro-cesses will need to turn on after a delay, or count the number of times a switchis hit. To do these simple tasks, Timers and Counters are utilized.

A timer is simply a control block that takes an input and changes an outputbased on time. There are two basic types of timers. There are other advancedtimers, but they wont be discussed in this report. An On-Delay Timer takesan input, waits a specific amount of time, allows logic to flow after the delay.An Off-Delay Timer allows logic to flow to an output and keeps that outputtrue until the set amount of time has passed, then turns it false, hence off-delay.Figure 14 above shows an On-Delay Timer with a 10 second delay before itpasses the logic through it.

A counter simply counts the number of events that occur on an input. Thereare two basic types of counters called up counters and down counters. As itsname implies, whenever a triggering event occurs, an up counter increments thecounter, while a down counter decrements the counter whenever a triggeringevent occurs.

3.4 EXAMPLES OF LADDER PROGRAMMING

Programming For Start/Stop of Motor by PLC Often we have a little green”start” button to turn on a motor, and we want to turn it off with a big red”Stop” button. The pushbutton switch connected to input X1 serves as the”Start” switch, while the switch connected to input X2 serves as the ”Stop.”Another contact in the program, named Y1, uses the output coil status as a seal-in contact, directly, so that the motor contactor will continue to be energizedafter the ”Start” pushbutton switch is released. You can see the normally-closed

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contact X2 appear in a colored block, showing that it is in a closed (”electricallyconducting”) state.

Figure 25: Start/Stop of Motor by PLC

Starting of Motor If we were to press the ”Start” button, input X1 wouldenergize, thus ”closing” the X1 contact in the program, sending ”power” to theY1 ”coil,” energizing the Y1 output and applying 120 volt AC power to the realmotor contactor coil. The parallel Y1 contact will also ”close,” thus latching the”circuit” in an energized state.

Figure 26: Starting of Motor

Logic for Continous Running of motor When Start Button is ReleasedNow, if we release the ”Start” pushbutton, the normally-open X1 ”contact” will

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return to its ”open” state, but the motor will continue to run because the Y1 seal-in ”contact” continues to provide ”continuity” to ”power” coil Y1, thus keepingthe Y1 output energized.

Figure 27: Continous Running of motor when Start Button is Released

To Stop the Motor To stop the motor, we must momentarily press the”Stop” pushbutton, which will energize the X2 input and ”open” the normally-closed ”contact,” breaking continuity to the Y1 ”coil:” When the ”Stop” push-button is released, input X2 will de-energize, returning ”contact” X2 to its nor-mal, ”closed” state. The motor, however, will not start again until the ”Start”pushbutton is actuated, because the ”seal-in” of Y1 has been lost.

Figure 28: To Stop the Motor

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4 SCADA

An industrial SCADA system will be used for the development of the controls ofthe four LHC experiments. This paper describes the SCADA systems in termsof their architecture, their interface to the process hardware, the functionalityand the application development facilities they provide.

4.1 Introduction

Widely used in industry for Supervisory Control and Data Acquisition of in-dustrial processes, SCADA systems are now also penetrating the experimentalphysics laboratories for the controls of ancillary systems such as cooling, ven-tilation, power distribution, etc.

SCADA systems have made substantial progress over the recent years interms of functionality, scalability, performance and openness such that they arean alternative to in house development even for very demanding and complexcontrol systems as those of physics experiments.

4.2 Types of SCADA

1. D+R+N ( Development +Run + Networking)

2. R+N ( Run +Networking )

3. Factory focus

4.3 Features of SCADA

1. Dynamic process Graphic

2. Alarm summery

3. Alarm history

4. Real time trend

5. Historical time trend

6. Security (Application Security)

7. Data base connectivity

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8. Device connectivity

9. Scripts

10. Recipe management

4.4 Manufacture of SCADA

Modicon (Telemecanique) Visual look

1. Allen Bradly : RS View

2. Siemens: win cc

3. KPIT : ASTRA

4. Intelution : Aspic

5. Wonderware : Intouch

4.5 What does SCADA MEAN?

SCADA stands for Supervisory Control And Data Acquisition. As the nameindicates, it is not a full control system, but rather focuses on the supervisorylevel. As such, it is a purely software package that is positioned on top of hard-ware to which it is interfaced, in general via Programmable Logic Controllers(PLCs), or other commercial hardware modules.

SCADA systems are used not only in industrial processes: e.g. steel mak-ing, power generation (conventional and nuclear) and distribution, chemistry,but also in some experimental facilities such as nuclear fusion. The size of suchplants range from a few 1000 to several 10 thousands input/output (I/O) chan-nels. However, SCADA systems evolve rapidly and are now penetrating themarket of plants with a number of I/O channels of several 100 K: we know oftwo cases of near to 1 M I/O channels currently under development.

SCADA systems used to run on DOS, VMS and UNIX; in recent years allSCADA vendors have moved to NT and some also to Linux.

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4.6 Architecture

4.6.1 Hardware Architecture

One distinguishes two basic layers in a SCADA system: the ”client layer” whichcaters for the man machine interaction and the ”data server layer” which handlesmost of the process data control activities. The data servers communicate withdevices in the field through process controllers. Process controllers, e.g. PLCs,are connected to the data servers either directly or via networks or fieldbusesthat are proprietary (e.g. Siemens H1), or non-proprietary (e.g. Profibus). Dataservers are connected to each other and to client stations via an Ethernet LAN.The data servers and client stations are NT platforms but for many productsthe client stations may also be W95 machines. Fig.1. shows typical hardwarearchitecture.

4.6.2 Generic Software Architecture

However, it is possible to have dedicated servers for particular tasks, e.g. his-torian, datalogger, alarm handler. Fig. 2 shows a SCADA architecture that isgeneric for the products that were evaluated.

4.7 Communications

4.7.1 Internal Communication

Server-client and server-server communication is in general on a publish-subscribeand event-driven basis and uses a TCP/IP protocol, i.e., a client application sub-scribes to a parameter which is owned by a particular server application andonly changes to that parameter are then communicated to the client application.

4.7.2 Access to Devices

The data servers poll the controllers at a user defined polling rate. The pollingrate may be different for different parameters. The controllers pass the requestedparameters to the data servers. Time stamping of the process parameters is typ-ically performed in the controllers and this time-stamp is taken over by the dataserver. If the controller and communication protocol used support unsoliciteddata transfer then the products will support this too.

The products provide communication drivers for most of the common PLCsand widely used field-buses, e.g., Modbus. Of the three fieldbuses that are rec-

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ommended at CERN, both Profibus and Worldfip are supported but CANbusoften not.

4.8 Interfacing

4.8.1 Application Interfaces / Openness

The provision of OPC client functionality for SCADA to access devices in anopen and standard manner is developing. There still seems to be a lack of de-vices/controllers, which provide OPC server software, but this improves rapidlyas most of the producers of controllers are actively involved in the developmentof this standard The products also provide:

1. an Open Data Base Connectivity (ODBC) interface to the data in thearchive/logs, but not to the configuration database,

2. an ASCII import/export facility for configuration data,

3. a library of APIs supporting C, C++, and Visual Basic (VB) to access datain the RTDB, logs and archive. The API often does not provide access tothe product’s internal features such as alarm handling, reporting, trending,etc.

The PC products provide support for the Microsoft standards such as Dy-namic Data Exchange (DDE) which allows e.g. to visualise data dynamically inan EXCEL spreadsheet, Dynamic Link Library (DLL) and Object Linking andEmbedding (OLE).

4.8.2 Database

The configuration data are stored in a database that is logically centralised butphysically distributed and that is generally of a proprietary format. System(RDBMS) at a slower rate either directly or via an ODBC interface.

4.8.3 Scalability

Scalability is understood as the possibility to extend the SCADA based controlsystem by adding more process variables, more specialised servers (e.g. foralarm handling) or more clients. The products achieve scalability by havingmultiple data servers connected to multiple controllers. Each data server has itsown configuration database and RTDB and is responsible for the handling of asub-set of the process variables (acquisition, alarm handling, archiving).

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4.8.4 Redundancy

The products often have built in software redundancy at a server level, whichis normally transparent to the user. Many of the products also provide morecomplete redundancy solutions if required.

4.8.5 Functionality

Access Control Users are allocated to groups, which have defined read/writeaccess privileges to the process parameters in the system and often also to spe-cific product functionality.

MMI The products support multiple screens, which can contain combi-nations of synoptic diagrams and text. They also support the concept of a”generic” graphical object with links to process variables. These objects canbe ”dragged and dropped” from a library and included into a synoptic diagram.

Most of the SCADA products that were evaluated decompose the processin ”atomic” parameters (e.g. a power supply current, its maximum value, itson/off status, etc.) to which a Tag-name is associated. The Tag-names used tolink graphical objects to devices can be edited as required. The products includea library of standard graphical symbols, many of which would however not beapplicable to the type of applications encountered in the experimental physicscommunity.

Standard windows editing facilities are provided: zooming, re-sizing, scrolling...On-line configuration and customisation of the MMI is possible for users withthe appropriate privileges. Links can be created between display pages to navi-gate from one view to another.

4.8.6 Trending

The products all provide trending facilities and one can summarise the commoncapabilities as follows:

1. the parameters to be trended in a specific chart can be predefined or de-fined on-line

2. a chart may contain more than 8 trended parameters or pens and an unlim-ited number of charts can be displayed (restricted only by the readability)

3. real-time and historical trending are possible, although generally not inthe same chart

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4. historical trending is possible for any archived parameter

5. zooming and scrolling functions are provided

6. parameter values at the cursor position can be displayed

The trending feature is either provided as a separate module or as a graphicalobject (ActiveX), which can then be embedded into a synoptic display. XY andother statistical analysis plots are generally not provided.

4.8.7 Alarm Handling

Alarm handling is based on limit and status checking and performed in thedata servers. More complicated expressions (using arithmetic or logical expres-sions) can be developed by creating derived parameters on which status or limitchecking is then performed. The alarms are logically handled centrally, i.e., theinformation only exists in one place and all users see the same status (e.g., theacknowledgement), and multiple alarm priority levels (in general many morethan 3 such levels) are supported.

It is generally possible to group alarms and to handle these as an entity(typically filtering on group or acknowledgement of all alarms in a group). Fur-thermore, it is possible to suppress alarms either individually or as a completegroup. The filtering of alarms seen on the alarm page or when viewing the alarmlog is also possible at least on priority, time and group. However, relationshipsbetween alarms cannot generally be defined in a straightforward manner. E-mails can be generated or predefined actions automatically executed in responseto alarm conditions.

4.8.8 Logging/Archiving

The terms logging and archiving are often used to describe the same facility.However, logging can be thought of as medium-term storage of data on disk,whereas archiving is long-term storage of data either on disk or on another per-manent storage medium.

Logging is typically performed on a cyclic basis, i.e., once a certain file size,time period or number of points is reached the data is overwritten. Logging ofdata can be performed at a set frequency, or only initiated if the value changesor when a specific predefined event occurs. Logged data can be transferred to anarchive once the log is full. The logged data is time-stamped and can be filteredwhen viewed by a user. The logging of user actions is in general performedtogether with either a user ID or station ID. There is often also a VCR facilityto play back archived data.

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4.8.9 Report Generation

One can produce reports using SQL type queries to the archive, RTDB or logs.Although it is sometimes possible to embed EXCEL charts in the report, a ”cutand paste” capability is in general not provided. Facilities exist to be able toautomatically generate, print and archive reports.

4.8.10 Automation

The majority of the products allow actions to be automatically triggered byevents. A scripting language provided by the SCADA products allows theseactions to be defined. In general, one can load a particular display, send anEmail, run a user defined application or script and write to the RTDB.

The concept of recipes is supported, whereby a particular system configura-tion can be saved to a file and then re-loaded at a later date.

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5 Application Development

5.1 Configuration

The development of the applications is typically done in two stages. First theprocess parameters and associated information (e.g. relating to alarm condi-tions) are defined through some sort of parameter definition template and thenthe graphics, including trending and alarm displays are developed, and linkedwhere appropriate to the process parameters. The products also provide anASCII Export/Import facility for the configuration data (parameter definitions),which enables large numbers of parameters to be configured in a more efficientmanner using an external editor such as Excel and then importing the data intothe configuration database.

However, many of the PC tools now have a Windows Explorer type devel-opment studio. The developer then works with a number of folders, which eachcontains a different aspect of the configuration, including the graphics.

The facilities provided by the products for configuring very large numbersof parameters are not very strong. However, this has not really been an issueso far for most of the products to-date, as large applications are typically about50K I/O points and database population from within an ASCII editor such asExcel is still a workable option.

On-line modifications to the configuration database and the graphics is gen-erally possible with the appropriate level of privileges.

5.2 Development Tools

The following development tools are provided as standard:

1. a graphics editor, with standard drawing facilities including freehand,lines, squares circles, etc. It is possible to import pictures in many formatsas well as using predefined symbols including e.g. trending charts, etc. Alibrary of generic symbols is provided that can be linked dynamically tovariables and animated as they change. It is also possible to create linksbetween views so as to ease navigation at run-time.

2. a data base configuration tool (usually through parameter templates). It isin general possible to export data in ASCII files so as to be edited throughan ASCII editor or Excel.

3. a scripting language

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4. an Application Program Interface (API) supporting C, C++, VB

5. a Driver Development Toolkit to develop drivers for hardware that is notsupported by the SCADA product.

5.3 Object Handling

The products in general have the concept of graphical object classes, which sup-port inheritance. In addition, some of the products have the concept of an objectwithin the configuration database. In general the products do not handle objects,but rather handle individual parameters, e.g., alarms are defined for parameters,logging is performed on parameters, and control actions are performed on pa-rameters. The support of objects is therefore fairly superficial.

5.4 Evolution

SCADA vendors release one major version and one to two additional minorversions once per year. These products evolve thus very rapidly so as to takeadvantage of new market opportunities, to meet new requirements of their cus-tomers and to take advantage of new technologies.

As was already mentioned, most of the SCADA products that were evalu-ated decompose the process in ”atomic” parameters to which a Tag-name is as-sociated. This is impractical in the case of very large processes when very largesets of Tags need to be configured. As the industrial applications are increas-ing in size, new SCADA versions are now being designed to handle devicesand even entire systems as full entities (classes) that encapsulate all their spe-cific attributes and functionality. In addition, they will also support multi-teamdevelopment.

As far as new technologies are concerned, the SCADA products are nowadopting:

1. Web technology, ActiveX, Java, etc.

2. OPC as a means for communicating internally between the client andserver modules. It should thus be possible to connect OPC compliantthird party modules to that SCADA product.

5.5 Engineering

Whilst one should rightly anticipate significant development and maintenancesavings by adopting a SCADA product for the implementation of a control sys-

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tem, it does not mean a ”no effort” operation. The need for proper engineeringcan not be sufficiently emphasised to reduce development effort and to reacha system that complies with the requirements, that is economical in develop-ment and maintenance and that is reliable and robust. Examples of engineeringactivities specific to the use of a SCADA system are the definition of:

1. a library of objects (PLC, device, subsystem) complete with standard ob-ject behaviour (script, sequences, ...), graphical interface and associatedscripts for animation,

2. templates for different types of ”panels”, e.g. alarms,

3. instructions on how to control e.g. a device ...,

4. a mechanism to prevent conflicting controls (if not provided with theSCADA),

5. alarm levels, behaviour to be adopted in case of specific alarms, ...

5.6 Potential benefits of SCADA

The benefits one can expect from adopting a SCADA system for the control ofexperimental physics facilities can be summarised as follows:

1. a rich functionality and extensive development facilities. The amount ofeffort invested in SCADA product amounts to 50 to 100 p-years!

2. the amount of specific development that needs to be performed by theend-user is limited, especially with suitable engineering.

3. reliability and robustness. These systems are used for mission criticalindustrial processes where reliability and performance are paramount.In addition, specific development is performed within a well-establishedframework that enhances reliability and robustness.

4. technical support and maintenance by the vendor.

For large collaborations, as for the CERN LHC experiments, using a SCADAsystem for their controls ensures a common framework not only for the devel-opment of the specific applications but also for operating the detectors. Opera-tors experience the same ”look and feel” whatever part of the experiment theycontrol. However, this aspect also depends to a significant extent on properengineering.

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5.7 Where SCADA is used ?

Electric power generation, transmission and distribution: Electric utilities useSCADA systems to detect current flow and line voltage, to monitor the opera-tion of circuit breakers, and to take sections of the power grid online or offline.

Water and sewage: State and municipal water utilities use SCADA to mon-itor and regulate water flow, reservoir levels, pipe pressure and other factors.

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6 CONCLUSION

With the speed of changing technology today it is easy to lose sight or knowl-edge of the basic theory or operation of programmable logic. Most people sim-ply use the hardware to produce the results they desire. Hopefully, this reporthas given the reader a deeper insight into the inner workings of programmablelogic and its role in mechanical operations. The idea of programmable logic isvery simple to understand, but it is the complex programs that run in the lad-der diagrams that make them difficult for the common user to fully understand.Hopefully this has alleviated some of that confusion.

SCADA is used for the constructive working not for the destructive workusing a SCADA system for their controls ensures a common framework notonly for the development of the specific applications but also for operating thedetectors. Operators experience the same ”look and feel” whatever part of theexperiment they control. However, this aspect also depends to a significantextent on proper engineering

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References

[1] A Daneels, W Salter. ”Technology Survey Summary of Study Report” ,IT-CO/98-08-09, CERN, Geneva 26th Aug 1998.

[2] A Daneels, W Salter, ”Selection and Evaluation of Commercial SCADASystems for the Controls of the CERN LHC Experiments” Proceedings ofthe 1999 International Conference on Accelerator and Large ExperimentalPhysics Control Systems, Trieste, 1999, p.353.

[3] G Baribaud et al., ”Recommendations for the Use of Fieldbuses at CERNin the LHC Era” Proceedings of the 1997 International Conference on Ac-celerator and Large Experimental Physics Control Systems, Beijing, 1997,p.285.

[4] www.wikipedia.com

[5] literature.rockwellautomation.com/idc/groups

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