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Feedback controls are widely used in modern automated system. Figure 2.7 is a typical diagram of feedback control systems. It consists of five basic components as shown below:

(a) Input

(b) Process being controlled (c) Output

(d) Sensing element

(e) Controller and actuating devices.

- The five components of a feed back control system is represent

Figure 2.11: Feedback Control System.

- The input to the system is the reference value, or set point, for the system output. This represent the desired operating value output

- The output of the variable of the process that is being measured and compared to the input.

- The sensing element is the measuring devices used in the feedback loop to monitor the value of the output variable.

PROCESS OUTPUT

SENSING ELEMENT CONTROLLER INPUT

CONTROLLER AND ACTUATING DEVICES

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- The purpose of the controller and actuating devices in the feedback system is to compare the measured output value with the reference-input value and to reduce the difference between them. In general the controller and actuator of the system are the mechanism by which changes in the process are accomplished to influence the output variable.

2.7 Concept of SMS-Based Remote Monitoring and Control

The introduction of the Global System for Mobile Communication (GSM) and particularly the use of hand-held mobile phones brought the innovation of distance communication at remote location.

The case of remote control capability and the possibility of achieving it at a reasonably low cost have motivated the need to use the GSM. GSM systems are becoming increasingly popular and have found application in almost all wireless applications because one can use it at any point and time. The capability of controlling industrial processes and many other numerous processes in a wireless and remote fashion has provided a great convenience to many people in life. The GSM can be used to monitor and control plants/field devices via short message service (SMS). SMS stands for Short Message Service. It is a mobile technology that allows for sending and receiving text or even binary messages to and from a Mobile phone (Hachenburg 1997). With an SMS based computer control system, monitoring and control can be achieved at all times. This is as a result of the ease of accessibility that comes with the use of a mobile phone. Therefore in this project the GSM has many advantages over other wireless systems. It is easy to install and covers a wide range. It can also monitor the signal strength and is more adaptable.

This system will be a powerful and flexible tool that will offer this service at any time, and from anywhere with the constraints of the technologies being applied. The proposed approach for designing this system is to implement a microcontroller-based control module that can receive instructions and commands from a cellular phone over the GSM network. The microcontroller then will carry out the issued commands and then communicate the status of a given parameter or device back to the Cellular phone.

2.8 Control System Network

There have been two major revolutions in process control and control system in the last 20 years.

The first was the replacement of analogue controller with digital controller using embedded computer to perform the control function. The second revolution is the replacement of the standard 4 to 20mA analogue signals for communication throughout a process plant by serial communication

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via a network. In this recent past, a plant would typically house nearly all controllers in building room centrally located in the plant facility.

All measurement data taken in the plant (field) was communicated to the control over pair of wires carrying a 4 to 2mA signals with one pair for each sensor. They were connected to the controllers. The feedback control signals to the final control element were sent to the field in order pairs of wires carrying 4 to 20mA signals.

The 4 to 20mA range was an open standard that all manufacturers of controllers, sensors and final control element agreed upon.

A petrol chemical plant extends over several areas so that wire runs could be very long. In the early development of computers as controllers, data was still carried back and forth between the control room and field over the bus of 4 to 20mA wires pairs. With the present advancement in computer and computer technology, industrial process monitoring and control has moved to a very advanced level.

2.9 Direct Digital Control (DDC)

As computers have become more reliable and miniaturized, they have taken over the controller function. The operations of the controller have been replaced by Software in the computer.

The ADC and DAC provide interface with the process measurement and control action.

2.10 Industrial Control Systems

Industrial control system (ICS) is a general term that encompasses several types of control systems, including supervisory control and data acquisition (SCADA) systems, distributed control system configurations such as skid-mounted programmable logic controllers (PLC) often found in the industrial sectors and critical infrastructures.

ICSs are typically used in industries such as electrical, water, oil, gas and data. Based on information received from remote stations, automated or operator-driven supervisory commands can be pushed to remote station control local operations such as opening and closing valves and breakers, collecting data from sensor systems, and monitoring the local environment for alarm conditions.

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Programmable Logic Controller (PLC) evolved, out of a need to replace racks of relays in ladder form. The latter were not particularly reliable, were difficult to rewrite, and were difficult to diagnose. PLC control tends to be used in very regular, high-speed binary controls, such as controlling a high-speed printing press. Originally, PLC equipment did not have remote 1/0 racks, and many couldn‘t even perform more than rudimentary analog controls.

SCADA’s history is rooted in distribution applications, such as power, natural gas, and water pipelines, where there is a need to gather remote data through potentially unreliable or intermittent low-bandwidth/high-latency links SCADA systems use open-loop control with sites that are widely separated geographically. A SCADA system uses RTUs (remote terminal units, also referred to as remote telemetry units) to send supervisory data back to a control center. Most RTU system always did have some limited capacity to handle local controls while the master station is not available. However, over the years RTU systems have grown more and more capable of handing local controls.

The boundaries between these system definitions are blurring as time goes on. The technical limits that drove the designs of these various systems are no longer as much of an issue. Many Plc platforms can now perform quite well as a small DCS, using remote 1/0 and analog control loops, and are able to communicate supervisory data. It is not uncommon to have telecommunications infrastructure that is so responsive and reliable that some SCADA systems actually manage closed loop control over long distances. With the increasing speed of today‘s processors, many DCS products have a full line of PLC-like subsystems that weren‘t offered when they were initially developed.

This led to the concept of a PAC (programmable automation controller or process automation controller), that is an amalgamation of these three concepts. Time and the market will determine whether this can simplify some of the terminology and confusion that surrounds these concepts today.

2.11 Distributed Control System

DCSs are used to control industrial processes such as electric power generation, oil and gas refineries, water and wastewater treatment, and chemical, food, and automotive production. DCSs are integrated as a control architecture containing a supervisory level of control, overseeing

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multiple integrated sub-systems that are responsible for controlling the details of a localized process.

Product and process control are usually achieved by deploying feed back or feed forward control loops whereby key product and / or process conditions are automatically maintained around a desired set point. To accomplish the desired product and/or process tolerance round a specified set point, only specific programmable controllers are used.

2.12 A GENERALIZED VIEW OF THE PROCESS MODEL

The concept of representing process behavior with purely mathematical expressions is called modeling and the resulting form is called a model.

The most important model form in process Dynamic and control studies still remains the transfer function form. Ogunaike (1994) and Stephanopolous (1984) discussed some fundamental principles of how transfer functions are used for dynamic analysis.

Types of models 1. State-space model

The input, u (t), is a time-domain function, and the output, the time-domain function y(t) is generated by the process of solving differential or difference equations that model the process.(Ogunaike, 1994)