PROGRAMMABLE LOGIC CONTROLER

Definition

A programmable logic controller (PLC) or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or lighting fixtures. PLCs are used in many industries and machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a bounded time unintended operation will result.

Historical Background

The Hydramatic Division of the General Motors Corporation specified the design criteria for the first programmable controller in 1968 .

Their primary goal

To eliminate the high costs associated with inflexible, relay-controlled systems.

The controller had to be designed in modular form, so that sub-assemblies could be removed easily for replacement or repair.

The control system needed the capability to pass data collection to a central system.

The system had to be reusable.

The method used to program the controller had to be simple, so that it could be easily understood by plant personnel.

Programmable Controller Development Early PLCs were designed to replace relay logic systems. These PLCs were programmed in "ladder logic", which strongly resembles a schematic diagram of relay logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a stack-based logic solver.

1968 - Programmable concept developed

1969 - Hardware CPU controller, with logic

instructions, 1 K of memory and 128 I/O

points

1974 - Use of several (multi) processors within a

PLC - timers and counters; arithmetic

operations; 12 K of memory

and 1024 I/O points

1976 - Remote input/output systems introduced

1977 - Microprocessors - based PLC introduced

1980 - Intelligent I/O modules developed

Enhanced communications facilities

Enhanced software features

(E.g. documentation)

Use of personal microcomputers as

Programming aids

1983 - Low cost small PLC’s introduced

1985 on - Networking of all levels of PLC, computer and machine using SCADA software.

Programmable Logic Controllers( Definition according to NEMA standard ICS3-1978)

A digitally operating electronic apparatus which uses a programming memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic to control through digital or analog modules, various types of machines or process.

Leading Brands Of PLC

AMERICAN

1. Allen Bradley2. Gould Modicon 3. Texas Instruments 4. General Electric5. Westinghouse6. Cutter Hammer7. Square D

EUROPEAN 1. Siemens

2. Klockner & Mouller 3. Festo 4. Telemechanique

JAPANESE 1. Toshiba

2. Omron3. Fanuc4. Mitsubishi

Major Components of a Common PLC

POWER SUPPLY

PROCESSOR

POWERSUPPLY

I MN O P D U UT L E

O M U OT DP UU LT E

PROGRAMMING DEVICE

From SENSORSPushbuttons

,contacts,

limit switches,

etc.

ToOUTPUTSolenoids, contactors,

alarmsetc.

Provides the voltage needed to run the primary PLC componentsI/O MODULES

Provides signal conversion and isolation between the internal logic- level signals inside the PLC and the field’s high level signal.

PROCESSOR Provides intelligence to command and govern the activities

of the entire PLC systems.PROGRAMMING DEVICE

used to enter the desired program that will determine the sequence of operation and control of process equipment or driven machine.

I/O Module

• The I/O interface section of a PLC connects it to external field devices.

• The main purpose of the I/O interface is to condition the various signals received from or sent to the external input and output devices.

• Input modules converts signals from discrete or analog input devices to logic levels acceptable to PLC’s processor.

• Output modules converts signal from the processor to levels capable of driving the connected discrete or analog output devices.

I/O ModuleDC INPUT MODULE

OPTO-ISOLAT

OR

IS NEEDED TO: Prevent voltage transients from damaging the processor.Helps reduce the effects of electrical noise

CurrentLimitingResistor

FROM INPUTDEVICE

USE TO DROP THE VOLTAGE TO LOGIC LEVEL

Buffer, Filter, hysteresis Circuits

TOPROCESSOR

I/O ModuleAC INPUT MODULE

OPTO-ISOLAT

OR

IS NEEDED TO: Prevent voltage transients from damaging the processor.Helps reduce the effects of electrical noise

Rectifier,ResistorNetwork

FROM INPUTDEVICE

CONVERTS THE AC INPUT TO DC AND DROPS THE VOLTAGE TO LOGIC LEVEL

Buffer, Filter, Hysteresis Circuits

TOPROCESSOR

I/O ModuleDC / AC OUTPUT MODULE

OPTO-ISOLAT

OR

IS NEEDED TO: Prevent voltage transients from damaging the processor.Helps reduce the effects of electrical noise

FROM PROCESSOR

TTLCircuits

AmplifierRELAYTRIACX’SISTOR

TOOUTPUTDEVICE

Processor

The processor module contains the PLC’s microprocessor, its supporting circuitry, and its memory system.The main function of the microprocessor is to analyze data coming from field sensors through input modules, make decisions based on the user’s defined control program and return signal back through output modules to the field devices. Field sensors: switches, flow, level, pressure, temp. transmitters, etc. Field output devices: motors, valves, solenoids, lamps, or audible devices.

The memory system in the processor module has two parts: a system memory and an application memory.

Memory Designs

Memory Map Organization

SYSTEM

System memory includes an area called the EXECUTIVE, composed of permanently-stored programs that direct all system activities, such as execution of the users control program, communication with peripheral devices, and other system activities.The system memory also contains the routines that implement the PLC’s instruction set, which is composed of specific control functions such as logic, sequencing, timing, counting, and arithmetic.System memory is generally built from read-only memory devices.

APPLICATIONThe application memory is divided into the data table area and user program area.The data table stores any data associated with the user’s control program, such as system input and output status data, and any stored constants, variables, or preset values. The data table is where data is monitored, manipulated, and changed for control purposes. The user program area is where the programmed instructions entered by the user are stored as an application control program.

Data TableUser Program

VOLATILE . A volatile memory is one that loses its stored information

when power is removed. Even momentary losses of power will erase any information stored or programmed on a volatile memory chip.Common Type of Volatile Memory

RAM. Random Access Memory(Read/Write) Read/write indicates that the information stored in the memory can be retrieved or read, while write indicates that the user can program or write information into the memory.

The words random access refer to the ability of any location (address) in the memory to be accessed or used. Ram memory is used for both the user memory (ladder diagrams) and storage memory in many PLC’s.RAM memory must have battery backup to retain or protect the stored program.

Several Types of RAM Memory:

1. MOS 2. HMOS 3. CMOSThe CMOS-RAM (Complementary Metal Oxide Semiconductor) is probably one of the most popular. CMOS-RAM is popular because it has a very low current drain when not being accessed (15microamps.), and the information stored in memory can be retained by as little as 2Vdc.

NON-VOLATILEHas the ability to retain stored information when power is

removed, accidentally or intentionally. These memories do not require battery back-up. Common Type of Non-Volatile Memory ROM, Read Only Memory

Read only indicates that the information stored in memory can be read only and cannot be changed. Information in ROM is placed there by the manufacturer for the internal use and operation of the PLC.

Other Types of Non-Volatile Memory

PROM, Programmable Read Only Memory

Allows initial and/or additional information to be written into the chip. PROM may be written into only once after being received from the PLC manufacturer; programming is accomplished by pulses of current. The current melts the fusible links in the device, preventing it from being reprogrammed. This type of memory is used to prevent unauthorized program changes.

EPROM, Erasable Programmable Read Only Memory

Ideally suited when program storage is to be semi-permanent or additional security is needed to prevent unauthorized program changes. The EPROM chip has a quartz window over a silicon material that contains the electronic integrated circuits. This window normally is covered by an opaque material, but when the opaque material is removed and the circuitry exposed to ultra violet light, the memory content can be erased. The EPROM chip is also referred to as UVPROM.

EEPROM, Electrically Erasable Programmable Read Only MemoryAlso referred to as E2PROM, is a chip that can be

programmed using a standard programming device and can be erased by the proper signal being applied to the erase pin. EEPROM is used primarily as a non-volatile backup for the normal RAM memory. If the program in RAM is lost or erased, a copy of the program stored on an EEPROM chip can be down loaded into the RAM.

PLC Operation

Basic Function of a Typical PLC Read all field input devices via the input interfaces, execute

the user program stored in application memory, then, based on whatever control scheme has been programmed by the user, turn the field output devices on or off, or perform whatever control is necessary for the process application. This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning.

PHASE 1 – Input Status scan

While the PLC is running, the scanning process includes the following four phases, which are repeated continuously as individual cycles of operation:

PHASE 2Progra

mExecuti

on

PHASE 3Diagnosti

cs/ Comm

PHASE 4Output

Scan

PHASE 1Read InputsScan

A PLC scan cycle begins with the CPU reading the status of its inputs

PHASE 2– Logic Solve/Program Execution

The application program is executed using the status of the inputs

PHASE 3– Logic Solve/Program Execution

Once the program is executed, the CPU performs diagnostics and communication tasks

PHASE 4 - Output Status Scan

An output status scan is then performed, whereby the stored output values are sent to actuators and other field output devices. The cycle ends by updating the outputs.

As soon as Phase 4 are completed, the entire cycle begins again with Phase 1 input scan.The time it takes to implement a scan cycle is called SCAN TIME. The scan time composed of the program scan time, which is the time required for solving the control program, and the I/O update time, or time required to read inputs and update outputs. The program scan time generally depends on the amount of memory taken by the control program and type of instructions used in the program. The time to make a single scan can vary from 1 ms to 100 ms.

PLC CommunicationsSerial Communications

PLC communications facilities normally provides serial transmission of information.

Common Standard

RS 232 Used in short-distance computer communications, with the

majority of computer hardware and peripherals. Has a maximum effective distance of approx. 30 m at 9600

baud.

Local Area Network (LAN)

Local Area Network provides a physical link between all devices plus providing overall data exchange management or protocol, ensuring that each device can “talk” to other machines and understand data received from them.

LANs provide the common, high-speed data communications bus which interconnects any or all devices within the local area.LANs are commonly used in business applications to allow several users to share costly software packages and peripheral equipment such as printers and hard disk storage.

Examples of PLC Programming Software:

1. Allen-Bradley – Rockwell Software RSLogix5002. Modicon - Modsoft

3. Omron - Syswin 4. GE-Fanuc Series 6 – LogicMaster65. Square D- Power Logic 6. Texas Instruments – Simatic 7. Telemecanique – Modicon TSX Micro8. Siemens-Simatic Manager

PROGRAMMINGPLC programs are typically written in a special application

on a personal computer, then downloaded by a direct-connection cable or over a network to the PLC. The program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory. Often, a single PLC can be programmed to replace thousands of relays.

Under the IEC 61131-3 standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. Initially most PLCs utilized Ladder Logic Diagram Programming, a model which emulated electromechanical control panel devices (such as the contact and coils of relays) which PLCs replaced. This model remains common today.

IEC 61131-3 currently defines five programming languages for programmable control systems: FBD (Function block diagram), LD (Ladder diagram), ST (Structured text, similar to the Pascal programming language), IL (Instruction list, similar to assembly language) and SFC (Sequential function chart). These techniques emphasize logical organization of operations.

While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible.

PLC programming using LADDER LANGUAGE

Power flows through these contacts when they are closed. The normally open (NO) is true when the input or output status bit controlling the contact is 1. The normally closed (NC) is true when the input or output status bit controlling the contact is 0.

Normally Open(NO)

Normally Closed(NC)

Coils

Coils represent relays that are energized when power flows tothem. When a coil is energized it cAauses a correspondingoutput to turn on by changing the state of the status bit controlling the output to 1. That same output status bit maybe used to controlnormally open or normally closed contact anywhere in the program.

Boxes

Boxes represent various instructions or functions that areExecuted when power flows to the box. Some of these Functions are timers, counters and math operations.

AND OPERATION

Each rung or network on a ladder program representsa logic operation. In the rung above, both inputs A and Bmust be true (1) in order for the output C to be true (1).

RungA B C

OR OPERATION

In the rung above, it can be seen that either input A or Bis be true (1), or both are true, then the output C is true (1).

RungA

B

C

NOT OPERATION

In the rung above, it can be seen that if input A is be true (1),then the output C is true (0) or when A is (0), output C is 1.

RungA

C

Advantages of PLCs

• Less wiring.• Wiring between devices and relay contacts are done in

the PLC program.• Easier and faster to make changes.• Trouble shooting aids make programming easier and

reduce downtime.

Reliable components make these likely to operate for years before failure

Features

Control panel with PLC (grey elements in the center). The

unit consists of separate elements, from left to right; power supply, controller, relay units for in- and output

The main difference from other computers is that PLCs are armored for severe conditions (such as dust, moisture, heat,

cold) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays, solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.

Need of PLCThe basic application of PLC is to control the entire process

of the Traffic Control System. The parameter like timer,counter,comparator,memory bit & the operations like latching ,interlocking are used in the control logic . If we would have connected above parameters and operation physically then it would have been very complex and also the troubleshooting very critical and more critical .By using the plc, this type of problem are being solved easily. Through this we can use any no. of parameters to smoothly operation of the process.

It can easily replace the complex logic in a simple ladder programme and also easily diagnostic the error. We can simulate the system error and modify it immediately thorough the software. So this type of process is getting a great advantage by the plc.

SCADA

Definition Definition of SCADA or Supervisory Control and Data

Acquisition software is:

Computer based  Alarms  Data acquisition Operator interface Non real-time control Database and Log Files Reports and Information Sharing

 

 

Computer Based

We feel that SCADA software must have all possible types of connectivity and integration.  This means serial ports, Ethernet, PCI slots, and the ability to run a wide variety of applications.  PLCs and simple operator interfaces (i.e. not based on the regular Windows operating system) are too limited in their functionality and capabilities.  Click for industrial computers or Visual Basic and C#. 

 

Alarm and Event Monitoring

A SCADA system must be able to detect, display, and log alarms and events.  When there are problems the SCADA system must notify operators to take corrective action.  Alarms and events must be recorded so engineers or programmers can review the alarms to determine what caused the alarm and prevent them happening again. 

Data Acquisition

SCADA must be able to read data from PLCs and other hardware and then analyze and graphically present that data to the user.  SCADA systems must be able to read and write multiple sources of data.  

Operator Interface

A SCADA system collects all of the information about a process.  The SCADA system then needs to display this data to the operator so that they can comprehend what is going on with the process. 

 

 

 

 

Non Real-Time Control

For simple control requirements, the SCADA system should be able to perform control instead of a PLC.  However, for anything other than simplistic control we prefer a PLC or soft PLC to do the real-time control with SCADA doing the non-real-time control.  The SCADA system is the medium between the operator and the real-time controller.  It allows the operator to control the system, such as start a new batch, load a new recipe, etc.   

Databases and Data Logging

Most applications require recipes, data logging, and other means of reading and writing databases.  The great thing about SCADA systems is that they can log incredible amounts of data to disk for later review.  This is helpful for solving problems as well as providing information to improve the process.  Many different methods should be available including, plain text, binary fixed column, Comma Separated Variable (CSV), XML, Excel, Access, SQL, SQL Server, ODBC, web services. 

Reports and Information Sharing

What good is a SCADA system and all this information if you can't share it with others?  Some of the reports overlap with the previous description of databases and data logging.  For example, your users might prefer that you put the results into an Excel spreadsheet or database so that they can use their own tools for creating the report.  Or the users might want you to create reports in Microsoft Word format. 

Explanation of SCADA.

SCADA is the acronym for Supervisory Control And Data Acquisition. The term refers to a large-scale, distributed measurement (and control) system. SCADA systems are used to monitor or to control chemical or transport processes, in municipal water supply systems, to control electric power generation, transmission and distribution, gas and oil pipelines, and other distributed processes.

Systems concepts A SCADA system includes input/output signal hardware,

controllers, HMI, networks, communication, database and software. It mainly comes in the branch of Instrumentation Engineering

The term SCADA usually refers to a central system that monitors and controls a complete site or a system spread out over a long distance (kilometres/miles). The bulk of the site control is actually performed automatically by a Remote Terminal Unit (RTU) or by a Programmable Logic Controller (PLC). Host control functions are almost always restricted to basic site over-ride or supervisory level capability. the SCADA system may allow an operator to change the control set point , and will allow any alarm conditions are to be recorded and displayed. The feedback control loop is closed through the RTU or PLC; the SCADA system monitors the overall performance of that loop.

Data acquisition begins at the RTU or PLC level and includes meter readings and equipment statuses that are communicated to SCADA as required. Data is then compiled and formatted in such a way that a control room operator using the HMI can make appropriate supervisory decisions that may be required to adjust or over-ride normal RTU (PLC) controls. Data may also be collected in to a Historian, often built on a commodity Database Management System, to allow trending and other analytical work.

SCADA systems typically implement a distributed database, commonly referred to as a tag database, which contains data

elements called tags or points. A point represents a single input or output value monitored or controlled by the system. Points can be either "hard" or "soft". A hard point is representative of an actual input or output connected to the system, while a soft point represents the result of logic and math operations applied to other hard and soft points. Most implementations conceptually remove this distinction by making every property a "soft" point (expression) that can equal a single "hard" point in the simplest case. Point values are normally stored as value-timestamp combinations; the value and the timestamp when the value was recorded or calculated. A series of value-timestamp combinations is the history of that point. It's also common to store additional metadata with tags such as: path to field device and PLC register, design time comments, and even alarming information.

It is possible to purchase a SCADA system, or Distributed Control System (DCS) from a single supplier. It is more common to assemble a SCADA system from hardware and software components like Allen-Bradley or GE PLCs, HMI packages from Wonder ware, Rockwell Automation, Inductive Automation, Citect, or GE. Communication typically happens over ethernet.

Introduction:What is SCADA?

SCADA stands for Supervisory Control And Data Acquisition. As the name indicates, it is not a full control system, but rather focuses on the supervisory level. It is a software package that is positioned on top of hardware to which it is interfaced, in general via Programmable Logic Controllers (PLCs), or other commercial hardware modules. Systems similar to SCADA systems are routinely seen in factories, treatment plants etc. These are often referred to as Distributed Control Systems (DCS). They have similar functions to SCADA systems, but the field data gathering or control units are usually located within a more confined area. Communications may be via a local area network (LAN), and will normally be reliable and high

speed. Basically, SCADA is a computer system for gathering and analyzing real time data.

What is data acquisition?

Data acquisition is the process of retrieving control information from the equipment which is out of order or may lead to some problem or when decisions are need to be taken according to the situation in the equipment. So this acquisition is done by continuous monitoring of the equipment to which it is employed. The data accessed are then forwarded onto a telemetry system ready for transfer to the different sites. They can be analog and digital information gathered by sensors, such as flow meter, ammeter, etc. It can also be data to control equipment such as actuators, relays, valves, motors, etc.

So why or where would you use SCADA?

SCADA can be used to monitor and control plant or equipment. The control may be automatic, or initiated by operator commands. The data acquisition is accomplished firstly by the RTU's (remote Terminal Units) scanning the field inputs connected to the RTU ( RTU's may also be called a PLC - programmable logic controller). This is usually at a fast rate. The central host will scan the RTU's (usually at a slower rate.) The data is processed to detect alarm conditions, and if an alarm is present, it will be displayed on special alarm lists. Data can be of three main types. Analogue data (i.e. real numbers) will be trended (i.e. placed in graphs). Digital data (on/off) may have alarms attached to one state or the other. Pulse data (e.g. counting revolutions of a meter) is normally accumulated or counted.

These systems are used not only in industrial processes. For example, Manufacturing, steel making, power generation both in conventional, nuclear and its distribution, chemistry, but also in some experimental facilities such as laboratories research, testing and evaluation centers, nuclear fusion. The size of such plants can range from as few as 10 to several 10 thousands input/output (I/O) channels. However, SCADA systems evolve rapidly and are now penetrating the market of plants with a number of I/O channels of several 100K.

The primary interface to the operator is a graphical display (mimic) usually via a PC Screen which shows a representation of the plant or equipment in graphical form. Live data is shown as graphical shapes (foreground) over a static background. As the data changes in the field, the foreground is updated. E.g. a valve may be shown as open or closed. Analog data can be shown either as a number, or graphically. The system may have many such displays, and the operator can select from the relevant ones at any time.

SCADA systems were first used in the 1960s.SCADA systems have made substantial progress over the recent years in terms of functionality, scalability, performance and openness such that they are an alternative to in house development even for very demanding and complex control systems as those of physics experiments. SCADA systems used to run on DOS, VMS

and UNIX; in recent years all SCADA vendors have moved to NT and some also to Linux.

Architecture:

In this section we are going to details which describe the common architecture required for the SCADA products.

Hardware Architecture

The basic hardware of the SCADA system is distinguished into two basic layers: the "client layer" which caters for the man machine interaction and the "data server layer" which handles most of the process data control activities. The data servers communicate with devices in the field through process controllers. Process controllers, e.g. PLC’s, are connected to the data servers either directly or via networks or fieldbuses that are proprietary (e.g. Siemens H1), or non-proprietary (e.g. Profibus). Data servers are connected to each other and to client stations via an Ethernet LAN. Fig.1. shows typical hardware architecture.

Figure 1: Typical Hardware Architecture

Software Architecture

The SCADA products are multi-tasking and are based upon a real-time database (RTDB) located in one or more servers. Servers are responsible for data acquisition and handling like polling controllers, alarm checking, calculations, logging and archiving) on a set of parameters, typically to which those are connected.

However, it is possible to have dedicated servers for particular tasks, e.g. historian, datalogger, alarm handler. Fig. 2 shows a SCADA architecture that is generic for the product.

Figure 2: Generic Software Architecture

Communication:

Internal Communication:

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

Access to Devices:

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

The products provide communication drivers for most of the common PLCs and widely used field-buses, e.g., Modbus. Of the three fieldbuses that are recommended are, both Profibus and Worldfip are supported but CANbus often not. Some of the drivers are based on third party products (e.g., Applicom cards) and therefore have additional cost associated with them. VME on the other hand is generally not supported.

A single data server can support multiple communications protocols; it can generally support as many such protocols as it has slots for interface cards. The effort required to develop new drivers is typically in the range of 2-6 weeks depending on the complexity and similarity with existing drivers, and a driver development toolkit is provided for this.

Interfacing

Application Interfaces / Openness

The provision of OPC client functionality for SCADA to access devices in an open and standard manner is developing. There still seems to be a lack of devices/controllers, which provide OPC server software, but this improves rapidly as most of the producers of controllers are actively involved in the development of this standard.

The products also provide

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

an ASCII import/export facility for configuration data, a library of APIs supporting C, C++, and Visual Basic (VB)

to access data in the RTDB, logs and archive. The API often does not provide access to the product's internal features such as alarm handling, reporting, trending, etc.

The PC products provide support for the Microsoft standards such as Dynamic Data Exchange (DDE) which allows e.g. to visualize data dynamically in an EXCEL spreadsheet, Dynamic Link Library (DLL) and Object Linking and Embedding (OLE).

Database

The configuration data are stored in a database that is logically centralized but physically distributed and that is generally of a proprietary format. For performance reasons, the RTDB resides in the memory of the servers and is also of proprietary format. The archive and logging format is usually also proprietary for performance reasons, but some products do support logging to a Relational Data Base Management System (RDBMS) at a slower rate either directly or via an ODBC interface.

Scalability

Scalability is understood as the possibility to extend the SCADA based control system by adding more process variables, more specialized servers (e.g. for alarm handling) or more clients. The products achieve scalability by having multiple data servers connected to multiple controllers. Each data server has its own configuration database and RTDB and is responsible for the handling of a sub-set of the process variables (acquisition, alarm handling, archiving).

Functionality:

Access Control

Users are allocated to groups, which have defined read/write access privileges to the process parameters in the system and often also to specific product functionality.

MMI

The products support multiple screens, which can contain combinations of synoptic diagrams and text. They also support the concept of a "generic" graphical object with links to process variables. These objects can be "dragged and dropped" from a library and included into a synoptic diagram. Most of the SCADA products that were evaluated decompose the process in "atomic" parameters (e.g. a power supply current, its maximum value, its on/off status, etc.) to which a Tag-name is associated. The Tag-names used to link graphical objects to devices can be edited as required. The products include a library of standard graphical symbols, many of which would however not be applicable to the type of applications encountered in the experimental physics community. Standard windows editing facilities are provided: zooming, re-sizing, scrolling... On-line configuration and customization of the MMI is possible for users with the appropriate privileges. Links can be created between display pages to navigate from one view to another.

Trending

The products all provide trending facilities and one can summarize the common capabilities as follows:

the parameters to be trended in a specific chart can be predefined or defined on-line

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

real-time and historical trending are possible, although generally not in the same chart

historical trending is possible for any archived parameter zooming and scrolling functions are provided parameter values at the cursor position can be displayed

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

Alarm Handling

Alarm handling is based on limit and status checking and performed in the data servers. More complicated expressions (using arithmetic or logical expressions) can be developed by creating derived parameters on which status or limit checking is then performed. The alarms are logically handled centrally, i.e., the information only exists in one place and all users see the same status (e.g., the acknowledgement), and multiple alarm priority levels (in general many more than 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). Furthermore, it is possible to suppress alarms either individually or as a complete group. The filtering of alarms seen on the alarm page or when viewing the alarm log is also possible at least on priority, time and group. However, relationships between alarms cannot generally be defined in a straightforward manner. E-mails can be generated or predefined actions automatically executed in response to alarm conditions.

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 permanent 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 of data can be performed at a set frequency, or only initiated if the value changes or when a specific predefined event occurs. Logged data can be transferred to an archive once the log is full. The logged data is time-stamped and can be filtered when viewed by a user. The logging of user actions is in general performed together with either a user ID or station ID. There is often also a VCR facility to play back archived data.

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 "cut and paste" capability is in general not provided. Facilities exist to be able to automatically generate, print and archive reports.

Automation

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

The concept of recipes is supported, whereby a particular system configuration can be saved to a file and then re-loaded at a later date. Sequencing is also supported whereby, as the name indicates, it is possible to execute a more complex sequence of actions on one or more devices. Sequences may also react to external events. Some of the products do support an expert

system but none has the concept of a Finite State Machine (FSM).

Evolution:

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

As was already mentioned, most of the SCADA products that were evaluated decompose the process in "atomic" parameters to which a Tag-name is associated. This is impractical in the case of very large processes when very large sets of Tags need to be configured. As the industrial applications are increasing in size, new SCADA versions are now being designed to handle devices and even entire systems as full entities (classes) that encapsulate all their specific attributes and functionality. In addition, they will also support multi-team development.

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

Web technology, ActiveX, Java, etc. OPC as a means for communicating internally between the

client and server modules. It should thus be possible to connect OPC compliant third party modules to that SCADA product.

Potential benefits of SCADA:

The benefits one can expect from adopting a SCADA system for the control of experimental physics facilities can be summarized as follows:

A rich functionality and extensive development facilities. The amount of effort invested in SCADA product amounts to 50 to 100 p-years!

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

Reliability and robustness. These systems are used for mission critical industrial processes where reliability and performance are paramount. In addition, specific development is performed within a well-established framework that enhances reliability and robustness.

Technical support and maintenance by the vendor.

For large collaborations, using a SCADA system for their controls ensures a common framework not only for the development of the specific applications but also for operating the detectors. Operators experience the same "look and feel" whatever part of the experiment they control. However, this aspect also depends to a significant extent on proper engineering.

Scada and DCS. Systems similar to SCADA systems are routinely seen in

factories, treatment plants etc. These are often referred to as Distributed Control Systems

(DCS). They have similar functions to SCADA systems, but the field

data gathering or control units are usually located within a more confined area.

In DCS Communications may be via a local area network (LAN), and will normally be reliable and high speed.

A DCS system usually employs significant amounts of closed loop control.

SCADA systems on the other hand generally cover larger geographic areas, and rely on a variety of communications systems that are normally less reliable than a LAN.

Closed loop control in this situation is less desirable.

Development Tools

Project editor Graphics editor Configuration through parameter templates Scripting language Driver Development Tool Kit And more

Data Access Mechanism's

Alarm Server typically poll data from the data server( -> impact on network bandwidth)More advanced techniques like publish / subscribe are available in some cases

Data from field buses are mainly polled.

Conclusion

SCADA is a control system with

• More front end functionality

• More interfaces and efficient storage • More record or device oriented configuration• but System wide configuration tools are needed• are less expensive than DCS, but offer different

functionality than DCS• And finally various applications

SCADA Software Linkshttp://www.rockwellautomation.com/rockwellsoftware/performance/view32/

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