Implementation of distributed control system

CHAPTER 1 Introduction

“Implementation of distributed control system” is a project of industrial application. Before starting project, here is a small introduction of the project

1.1 What is Distributed control system(DCS)

“A Distributed control system (DCS) refers to a control system usually of manufacturing system, process or any kind of dynamic system, in which the controller elements are not centrally involved but are distributed throughout the system with each component sub system controlled by one or more controllers. The entire system of controllers is connected by networks for communication and monitoring.”[1]

DCS is a very broad term used in variety of industries , to monitor and control distributed equipment.

Examples[1]include,

Environmental control system
Traffic signals
Radio signals
Oil refining plants
Chemical plants
Pharmaceutical manufacturing
Sensor networks
Metallurgical process plants
Dry cargo and bulk oil carrier ships

1.2 Why DCS?

With the advent of the Internet, companies have realized that a whole new market nowadays, industrial network applications are increasingly based on the Internet, TCP/IP (Transmission Control Protocol / Internet Protocol), and World Wide Web technologies. However, web-based distributed control systems have not yet been used due to difficulties in coping with the large amount of data, the lack of standardization of communicate with on-line devices, and the unreliability of data transmission on the Internet. But we use Ethernet & internet and make sure these following options

We choose DCS due to the following reasons[2],

Reliability
Compatibility and operatibility
Graphical interface
Control algorithms
Processing speed and cycle time
Ease of configuration
Ease of use
Support and upgradeability
Cost

1.3 Advantages & Disadvantages[2]

1.3.1 Reasons for using DCS

Robust
Flexible
Secure
Emergent behavior
Less expensive and installation and better reliability
Less expensive to expand
It could do analog and discrete control

1.3.2 Reasons for not using DCS

Component cost
A distributed system can be somewhat difficult to maintain
Programming them is not easy

Chapter 2 Literature Review

2.1 Introduction of the project

The computer-based control systems, which can monitor, control and simulate industrial automation process must provide enough means to deal with time, concurrency, and decentralization. In addition, control systems should still function and be available during maintenance and also available during fail-safe behavior. In the past control systems are centralized nw as the trend changes Control systems are now moved from centralized to distributed control, because now real-world problems are distributed, as in telecommunication, traffic control or building automation, and because the implementation should be flexible and reliable. There is currently growing interest in new technologies and architectures for exploring the next generation technologies for distributed control systems. Our project was to develop a DCS system in which one PLC acts as a master and others would act as a slave. Master is responsible to monitor all the processes those are connected to all other slaves which return their status to master. MODBUS protocol is used for communication between two PLC’s. We have used LCD display to view the monitored data. Our major objective was to develop a DCS system using a network of PLC’s that would be economical for industry.

2.2 selected controls

After the discussion with our advisor we decide that whatg we have to control like our DCS system will control an AC motor, Temperature sensor in which PT100 sensor is involved and last part is that we control level sensor used in industry in boilers in which we will use magnet switches and contacts

2.3Reasons for selecting this project

We have selected this project because it has vast industrial applications such as in manufacturing system, quality control system (QCS) etc.
This project is very helpful for learning purpose in the field of automation.
Broader applications in Pakistan.
Easy to implement.
Handling of DCS system through PLC’s.

2.4Hardware Description

Hardware used in our project is given below,

PLC
Power supplies
Connection cables
LCD display
PUMA mini-series AC drive
Motor

2.4.1 PLC

2.4.2 History of PLC

In the old days humans were the main method for controlling a system. Lately electricity has been used for control, early electrical control was based on relays. The development of low cost computer has brought the most recent revolution, the programmable logical controller(PLC).The invention of plc began in 1970s, and has been used mostly for manufacturing controls.[3]

Before solid state logic circuits came into existence, logical control systems were designed and built exclusively around electromechanical relays. A relay control panel consists of a single to thousands of relays. Relays are far no longer used in modern design but have been replaced in many of their former roles as logic level control devices, dismissed to an inferior position most often to those applications requiring high current and high voltage switching.[4]

Systems and processes demanding switching control is plentiful in modern industry, but such control system are seldom built from either electromechanical relays or discrete logic gates. Instead digital computers fill the need which may be programmed to perform logical function. [4]

2.4.3 Definition of PLC

A programmable logical controller is a computer which is mostly used in industrial applications. PLCs are different from office computers because they use different hardware and software to perform their tasks. All the PLCs monitor inputs and make their decision depending on the stored program and control the output to automate any process. The components of PLC are, [5]

CPU Module:

CPU module consists of central processing unit and programmable read only memory(PROM). PROM contains PLC operating system, driver programs and RAM to store user working data and programs. RAM is protected by long life battery backup so that user programs would remain there in the memory after power is removed from PLC. Removable memory i.e EEPROM is also plugged into the cpu module so that user can write programs to memory modules at higher voltage levels. Flash memory is also built in CPU module. It is just like EEPROM, when a program is running on PLC and power is lost then there would be no loss to working data when you restore power.[4]

Power Supply Module:

Power supply module converts the DC supply to the supply required by the CPU and I/O module. Some of the power supply modules converts the supply to 24V for the digital outputs. If the PLC based control system requires power supplies other than 5V or 24V to sensors or actuators then user should have to provide additional power supplies.[4]

I/O Module:

Input output modules connect the PLC to the sensors or actuators. I/O modules manufactures in the way that there would be no problem of compatibility with the CPU module. Manufactures offer different modules including, [4]

1. Digital I/O Module:

This module is used to connect sensors and actuators to switch them on or off. Every module can be connected to different digital sensors and digital actuators with similar electrical characteristics. [4]

2. PWM Outputs:

Pulse width modulation (PWM) is a highly efficient way of controlling the output voltage to devices. PWM works by first turning on output to it’s full swing and then shut it off for another while and so on. Digital output #7 and #8 are of PWM type.[6]

3. Analog Module:

Analog modules are used because some control systems require the PLC to monitor an analog voltage or produce analog voltage. Analog modules are used in applications like temperature control because they can provide continuous signal.[4]

2.5 Connection Cables

1) Desktop to PLC (RS-232 with RS-485 connector to PLC)

2) Twisted pair cable RS-485 with RS-485 connector to PLC

2.6 Sensors

Sensors are devices that convert a physical condition into an Electrical signal for use by a controller such as a PLC .Sensors are connected to the input of a PLC. A pushbutton is one example of a sensor that is often connected to a PLC input. An electrical signal indicating the condition (open or closed) of the pushbutton contacts is sent from the pushbutton to the PLC.

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Chapter 3 Requirements Specification

3.1 Non-functional Requirements

Product requirements

Priority

Details

NR-01-001

Platform: Window Operating System

NR-01-002

Language: Ladder Logic

NR-01-003

Compiler: FXGP-WIN

NR-01-004

Machine: Personal Computer

Organisational requirements

Priority

Details

NR-02-001

Delivery: The system development process and deliverable documents shall conform to the process and deliverables defined in the document “project proposal report” and “project progress report”

NR-02-002

Standard: The standard of the final product shall be of undergraduate level.

External requirements

Priority

Details

NR-03-001

Security: This is a degree project having no strict security requirements.

NR-03-002

Ethical: The application will not use any type of un-ethical electronic material while project development and execution.

NR-03-003

Legislative: The application shall not use any private or confidential data, or network information that may infringe copyrights and/or confidentiality of any personnel not directly involved in this product.

3.2 Functional Requirements

Category 1(PLC)

PLC requires following tools for its operation.

Priority

Details

FR-01-001

Personal Computer

FR-01-002

Software to program PLC

FR-01-003

Programming Cable

FR-01-004

Power Supply

Category 2(HMI)

To run HMI it requires following.

Priority

Details

FR-02-001

Personal computer

FR-02-002

Software to program HMI

FR-02-003

USB Cable

FR-02-004

Power supply from PLC

FR-02-005

Communication cable with PLC

Category 3 (Sensors)

Priority

Details

FR-03-001

Bimetallic switch

FR-03-002

Floating magnet

FR-03-003

Switches

Category 4(Outputs)

Priority

Details

FR-04-001

Frequency inverter

FR-04-002

3-phase motor

FR-04-003

Wires

Chapter 4 Project Design

4.1 Methodology

In today’s competition and fast production environment, automation industries demand a totally integrated control & solutions which increase production and minimize cost. In this project we use master slave combination and if any of the slaves fall down during the process of the system than master would treat all processes as a slave and handle the status failed slave plc. The PC is an operating station that is used to configure all functioning blocks and monitor the processes. Communication between the PC and the controllers is implemented in a master-slave type broadcasting on the communication network and in this type of communication the slave nodes are not allowed to transmit data without a request from the master, and do not directly communicate with each other if they needs to send a message to another, they have to send the message to master and then the master forwards the message to the receiver.

4.2 Architecture Overview

4.2.1 MODBUS protocol

In this diagram we show that we will handle our all processes with the communication of port RS-485 .first we make a daisy chain fashion of the twisted pair wire which is used between the master ha all slaves PLC’S .than this MODBUS protocol need commands from master to run a slave program ..we gave commands to slave through and these command give 5 reference volts to the ac drive/frequency inverter through which we can run our 3 phase induction motor .

4.2.2 Remote Access

In this remote access process we use internet/intranet .in this we use any internet modem device .for this remote access make sure that both systems should be attach to Internet/Intranet .TLserver provide URL Addresses for the other systems..which we should provide other system to access that particular PLC

4.3 SENSORS

Sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer. For example temperature sensors are PT100, Ac drive etc. level sensors such as bimetallic switch. The sensors used in this project are

Digital Level Sensor

It is a magnetic reed-switch component and composed of a magnetic reed-switch sensor connected with PVC cable. This sensor is placed in a two-pieces (sensor and cover) anti-magnetic box, filled with an epoxy material approved for electrical and electronic devices. It is be characterized as a class 3 electronic device as it is a passive component, which is working as a contact.

The DLS senses the pass of the magnet, which is transported together to the condition we want to read and changes the status of an internal magnetic-electric contact. This magnet, for example, may be a floating one dived in the liquid or a common magnet or anything else that generates a magnetic field.

The DLS is appropriate for industrial use. It is made to be custom installed. Its flexibility in the wiring and its good insulation and temperature specifications gives the ability to use it everywhere there is a need to read the movement of a magnet. The DLS is an independent magnetic sensor. It is suitable for industrial applications such as height readings or liquid level sensor.

Installation

Most of the tanks have an indicator tube besides. In this tube dives a floating magnet. The magnet has only the one pole as active magnetic field. Outside the tube DLS is fixed at the level we want to read. The sensor distance from the tube depends upon the strength of the magnet. More than one level sensor can be placed but there must be some distance of 125mm between them.

The DLS has enough liquid and temperature resistance. Some strong chemicals can cause damage to the body of the box. The extremely high temperature (150 C) may damage the sensor or its insulation or the cable if it has direct contact with the hot surface.

PT100

It is Platinum resistance which ranges from 0-200 degree centigrade. A main advantage of the platinum sensors is naturally the very high linearity of their resistance vs. temperature. The variation of current from 4-20 milli ampere varies the ranges of temperature from 0-200 Celsius.

4.4 Phases Motor

4.4.1Mini series Inverter

Mini series frequency conversion is developed on the basis of Mini series frequency converter. It has kept the main functions of Mini Series, increased by some functions according to the user requirement. Its specification is

INPUT:3 phase 400V 50/60Hz

OUTPUT:3 Phase 400V 4.0A 1.5KW

Freq.Range:0.1 – 400Hz

To operate an inverter, following diagram shows its operation.

4.4.2 Main Circuit Terminal Arrangement

4.5.3 Function Description of Main Circuit Terminals

Symbolic function description

R.S.T

Input terminals of AC line

U.V.W

Output terminals to motor

P/Pr

External breaking resister terminal

E

Ground terminal

Table 4.4: Symbolic function [10]

4.4.4 Function Description of Control Circuit Terminals

Symbol

Function

Factory Setting

For

Multi-Function input1

Forward Run

REV

Multi-Function input2

Reverse Run

RST

Multi-Function input3

Reset

SPH

Multi-Function input4

High speed

SPM

Multi-Function input5

Medium speed

SPL

Multi-Function input6

Low speed

GND

Ground common for input terminals

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+ 10

Power supply for analog freq.

+ 10

VI

Analog frequency reference input(voltage)

0 ~ +10V corresponding to high operating frequency

AI

Analog frequency reference input(current)

4 ~ 20mA corresponding to high operating frequency

DRV

Multi-function output terminal 1

Optical couple output DC 24V/100mA

AM

Digital frequency output terminal

0 – 10V

FA

FB

FC

Multi-Function output terminal 3

Fault relay output 3A/250VAC, 3A/30VDC

Table 4.5: Control circuit terminals [10]

Interfacing of Inverter with Motor

Chapter 5 Implementation

5.1 Development stages

Following are the different stages which we face during the implementation of our project

Understanding the process

First we understand

Trainer Board designing

At the start of our project we design a trainer board for the PLC available in our electric machine in which we install LCD display,RS-485 port and RS-232 port through which we connect our PC and connect I/O port to the trainer board

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