58455204-PLC-SCADA

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rx automation controls and equipment services Inc.,

145 A. Bonifacio St., Mandaluyong City

Philippines, 1550

Telefax: +63 2 358-4819

Email: [email protected] / [email protected]

What is SCADA?

Introduction

Widely used in industry for Supervisory Control and Data Acquisition of industrial processes,

SCADA systems are now also penetrating the experimental physicslaboratories for the

controls of ancillary systems such as cooling, ventilation, power distribution, etc.

SCADA systems have made substantial progress over the recent years in terms of

functionality, scalability, performance and openness such that they arean alternative to in

house development even for very demanding and complex control systems as those of

physics experiments.

Features of SCADA- Dynamic process Graphic

- Alarm summary

- Alarm History

- Real Time Trend

- Security system

- Database connectivity

- Recipe management

- Device connectivity

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. As such, it is a purely

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.

SCADA systems are used not only in industrial processes: e.g. steel making, power

generation (conventional and nuclear) and distribution, chemistry, but also in some

experimental facilities such as nuclear fusion. The size of such plants range from a few 1000

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 100 K: we know of two cases of near to 1 M I/O channels currently under

development.

ARCHITECTURE

Hardware Architecture

One distinguishes two basic layers in a SCADA system: the "client layer" which caters for the

man machine interaction and the "data server layer" which handles

8/2/2019 58455204-PLC-SCADA




Philippines, 1550

Telefax: +63 2 358-4819


most of the process data control activities. The data servers communicate with devices in

the field through process controllers. Process controllers, e.g. PLCs, are connected to thedata 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. The data servers and client stations are NT platforms but

for many products the client stations may also be W95 machines. Fig.1. shows typical

hardware architecture.

Software Architecture

The 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 (e.g.polling

controllers, alarm checking, calculations, logging and archiving) on a set of parameters,

typically those they are connected to.

8/2/2019 58455204-PLC-SCADA




Philippines, 1550

Telefax: +63 2 358-4819


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 supportunsolicited data transfer then the products will

support this too.

What is PLC?

Introduction

The first Programmable Logic Controller (PLC) was developed by a group of engineers at

General Motors in 1968, when the company was looking for an alternative toreplace

complex relay control systems.

The new control system had to meet the following requirements:

_ Simple programming

_ Program changes without system intervention (no internal rewiring)

_ Smaller, cheaper and more reliable than corresponding relaycontrol systems

_ Simple, low cost maintenance

8/2/2019 58455204-PLC-SCADA




Philippines, 1550

Telefax: +63 2 358-4819


Subsequent development resulted in a system, which enabled the simple connection of

binary signals. The requirements as to how these signals were to be connected were

specified in the control program. With the new systems it became possible for the first time

to plot signals on a screen and to file these in electronic memories.

Since then, three decades have passed, during which the enormous progress made in the

development of microelectronics did not stopshort of programmable logic controllers. For

instance, even if program optimization and thus a reduction of required memory capacity

initially still represented an important key task for the programmer, nowadays this is hardly

of any significance.

Moreover, the range of functions has grown considerably. 15 years ago, process

visualization, analogue processing or even the use of a PLC as a controller, were considered

as Utopian. Nowadays, the support of these functions forms an integral part of many PLCs.

Areas Application of a PLC

Every system or machine has a controller. Depending on the type of technology used,

controllers can be divided into pneumatic, hydraulic, electrical and electronic controllers.

Frequently, a combination of different technologies is used. Furthermore, differentiation is

made between hard-wired programmable (e.g. wiring of electro-mechanical or electronic

components) and programmable logic controllers. The first is used primarily in cases, where

any reprogramming by the user is out of the question and the job size warrants the

development of a special controller.

Typical applications for such controllers can be found in automatic washing machines, video

cameras, and cars. However, if the job size does not warrant the development of a special

controller or if the user is to have the facility of making simple or independent program

changes, or of setting timers and counters, then the use of a universal controller, where the

program is written to an electronic memory, is the preferred option. The PLC represents

such a universal controller. It can be used for different applications and, via the program

installed in its memory, provides the user with a simple means of changing, extending and

optimizing control processes.

The PLCs currently on offer in the market place have been adapted to customer

requirements to such an extent that it has become possible to purchase an eminentlysuitable PLC for virtually any application. As such, miniatures PLCs are now available with a

minimum number of inputs/ outputs starting from just a few hundred Pounds. Also

available are larger PLCs with 28 or 256 inputs/outputs.

8/2/2019 58455204-PLC-SCADA




Philippines, 1550

Telefax: +63 2 358-4819


Basic Design of PLC

The term programmable logic controller is defined as follows by EN 61131-1 (IEC 61131-1):

A digitally operating electronic system, designed for use in an industrial environment,

which uses a programmable memory for the internal storage of user-oriented instructions

for implementing specific functions such as logic, sequencing, timing, counting and

arithmetic, to control, through digital or analogue inputs and outputs, various types of

machines or processes. Both the PC and its associated peripherals are designed so that they

can be easily integrated into an industrial control system and easily used in all their

intended functions."

A programmable logic controller is therefore nothing more than a computer, tailored

specifically for certain control tasks.

The function of an input module is to convert incoming signals into signals, which can be

processed by the PLC, and to pass these to the central control unit. The reverse task is

performed by an output module. This converts the PLC signal into signals suitable for the

actuators. The actual processing of the signals is effected in the central control unit in

accordance with the program stored in the memory. The program of a PLC can be created invarious ways: via assembler type commands in statement list, in higher-level, problem-

oriented languages such as structured text or in the form of a flow chart such as

represented by a sequential function chart. In Europe, the use of function block diagrams

based on function charts with graphic symbols for logic gates is widely used. In America, the

ladder diagram is the preferred language by users. Depending on how the central control

unit is connected to the input and output modules, differentiation can be made between

compact PLCs (input module, central)

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