Process ControlHow organizations use ICT:

Introduction

Technological advancements in process monitoring, control and industrial automation in recent years have improved the productivity of

virtually all manufacturing industries throughout the world.

Almost all the controls installed in new plants or plant expansions are Digital Control Systems (DCS) connected by digital networks.

What is Process Control?

Process control is extensively used in industry.

It is the use of computers or microprocessors to control a process.

Process control enables automation.

This means that a small staff of operating personnel can operate a complex process from a central control room.

Applications

Enables mass production of continuous processes eg oil refining, paper manufacturing, chemical processing, temperature control etc

It is also used in the food and beverage industries

For example

Chilling the temperature in an industrial refrigeration plant is a process that has a specific, desired outcome to reach:

To maintain a defined temperature (e.g. 2°C)

And keep it constant over time.

Variables

A variable is a piece of data that can change.

The temperature is the controlled variable.

It is also the input variable since it is measured by a thermometer and used to decide whether to heat or not to heat.

Setpoint

The desired temperature (2°C) is the setpoint.

The state of the chiller (e.g. the activation of the actuator to switch the compressor on or off) is called the manipulated variable since it is subject to control actions.

IPO

Using the example on the previous slide, the plant temperature would be an input to the PLC.

The logical statements would compare the setpoint to the input temperature and determine whether more or less compression was necessary to keep the temperature constant.

A PLC output would then either activate the actuator to switch the compressor on, an incremental amount, depending on whether more or less chilling was needed.

PLCs

Most process control is overseen by PLCs rather than by computers.

A PLC is a type of microprocessor that is used for a single purpose.

PLC

A programmable logic controller, or a PLC, is used to:1. read a set of digital

and analog inputs2. apply a set of logic

statements3. generate a set of

analog and digital outputs.

How it works

A PLC is able to accept analogue and digital inputs

It makes extensive use of analogue to digital conversion (as well as digital-to-analogue conversion!)

A set of logic statements is used to compare the input with a pre-set value.

Depending on the results of that comparison, it activates the output devices.

PLCs are not really used in home central heating systems (where the pre-set value might change to suit seasonal conditions)

They are used in situations where the pre-set value is a constant, ie industrial refrigeration systems

How PLCs are programmed

Proportional-Integral-Derivative (PID) algorithm

These are used with closed-loop systems

A physical variable (ie temperature) is continuously monitored by a sensor connected to the PLC

The outputs from the controller affect the input (ie the temperature)

A Closed Loop System

Closed Loop Systems

These control continuous processes

The purpose of the PLC is to make the input value equal to the pre-set one and maintain it there.

PID is the best means of doing this

How PID algorithms work

The PID calculates the difference between the input value and the pre-set value.

It causes the PLC to make proportional changes to the output

so that the pre-set temperature is eventually reached.

An example

If the temperature in an industrial heating system is lower than the required temperature, the PID calculates the difference

Instead of switching the heater on until the pre-set value is reached, the PLC switches it on for a short time

Then checks the difference again

If there is still a difference, it switches the heater on again for another small burst

This is repeated until the required temperature is reached.

Types of Process Control

There are three types of process control:

Batch Process ControlContinuous Process

ControlDiscrete Process Control

Batch Process Control

Some applications require that specific quantities of raw materials be combined in specific ways for particular durations to produce an intermediate or end result.

One example is the production of adhesives and glues, which normally require the mixing of raw materials in a heated vessel for a period of time to form a quantity of end product.

Batch Process Control

Other important examples are the production of food, beverages and medicine.

Batch processes are generally used to produce a relatively low to intermediate quantity of product per year (a few pounds to millions of pounds).

Batch Process Control

The amount of each ingredient that is added is controlled by the computer

So is the length of time for each stage

So is the temperature

Discrete Process Control

Found in many manufacturing and packaging applications.

Robotic assembly, such as that found in car manufacture, can be characterized as discrete process control.

Most discrete manufacturing involves the production of discrete pieces of product, such as metal stamping.

Discrete Process Control

Specific items are produced. It is like an on/off or stop/start process. Fitting car wheels:

A robot fits a wheel to a car The car moves on to the next stage The robot stops The next car comes along The robot fits the wheel to the car… And so on. In between waiting for each car to arrive, the

robot stops

Continuous Process Control

Continuous process control refers to processes that appear unending.

A good example is the maintaining of temperature in confined surroundings – eg industrial refrigeration

Other examples include oil/petroleum refining, the production of plastics or paper production.

Continuous Process Control

Some important continuous processes are the production of fuels, chemicals and plastics.

Continuous processes in manufacturing are used to produce very large quantities of product per year (millions to billions of pounds).

Hybrid Applications

Applications having elements of discrete, batch and continuous process control are often called hybrid applications.