PROCESS CONTROL & AUTOMATION
BY
SOBUKOLA, O.P. (PhD)/KAJIHAUSA, O.E. (MRS)
Department of Food Science & Technology,
University of Agriculture, PMB 2240, Abeokuta, Nigeria.
sobukolaop@unaab.edu.ng
Grading
• Continuous Assessment Test – CAT - 20%
• Examination
- 70%
• Attendance
- 10%
• Total
- 100%
Course Outline
• Lecture 1- Introduction
• Lecture 2- Control Systems
• Lecture 3- Measuring and Detecting Elements
• Lecture 4- Control Actions
• Lecture 5- Frequency response Analyses
• Lecture 6- Computer-based Systems
LECTURE ONE
INTRODUCTION
Automatic control has played a vital role in the
advance of engineering and science. In addition to
its extreme importance in space-vehicle systems,
robotic systems and the like, automatic control has
become an important and integral part of modern
manufacturing and industrial processes.
- A process – is the transformation of a set of
inputs, which may be material, actions,
methods and operations into desired outputs
in the form of a product.
- Control - means measurement of the
performance of a process and the feedback
required and corrective actions where
necessary.
- Automation – Automation means reductions
in the use of direct labour during food
processing.
Advantages of Automation include:
• Consistency and accuracy in the positioning of
moving parts of an equipment.
• A more consistent product.
• The more economic use of existing plant by
saving of fuel/and or electrical energy.
• The release of skilled personnel for other
productive work .
• Reduction of physical effort with consequent
reduction of fatigue and boredom
• Improved working conditions.
Limitations of automation:
– Initial cost is high
– power fluctuations,
– Lack of skilled personnel etc.
Basic steps in process control are:
• Measurement of the process variable;
• Evaluation and comparison with desired level;
and
• Control of the required level of the parameter
involved
LECTURE TWO
Definition of some terms in process control
• Controller – A device that measures a variable
condition (Temperature, pressure, humidity, moisture
content) like thermostats, humidistat or pressure
controllers.
• Control system – consist of controller, controlled device
and source of energy or input.
• Controlled device – it reacts to the signal received from
a controller and varied the flow of the controlled
agents. Valve, damper, electric relay or a motor driving
a pump, fan etc.
• Control agents – the medium being manipulated by the
action of controlled device e.g. air or gas.
• Controlled variables – are system parameters
which are under control e.g. Temperature,
pressure, humidity etc.
• Manipulated variable – is the quantity or condition
that is varied by the controller so as to affect the
value of the controlled variable.
• Plant – This may be a piece of equipment or a set
of machine parts functioning together, the purpose
of which is to perform a particular operation.
• Disturbances – A disturbance is a signal that tends
to adversely affect the value of the output of a
system.
CONTROL SYSTEMS
Self controlled systems
Pneumatic system
Hydraulic system
Electrical system
Electronic System
LECTURE THREE
MEASURING AND DETECTING ELEMENTS
(SENSORS)
They are the main interfere between the
control system and process. In food
processing, they are required for incoming
material selection; material waste control;
process quality control; packaging inspection;
equipment maintenance/failure prediction,
environmental control.
Variable
Detecting element
Strain or Load
Resistance strain gauge
Displacement/
Potentiometer, differential
Position
Transformer, capacitance
Transducer
Speed
Weights moved by centrifugal force,
Tachometer generator
Rate of flow
Orifice plate or venturi tube
Temperature
Thermometer, Thermocouple, Thermopile
Pressure
Bourdon tube, diaphragm, and bellows
Liquid level
Electrodes at various depths,
Dip-tube (pneumatic), capacity probe
pH
Glass and calomel electrodes
Block diagram
• A control system consist of a number of
components that perform certain factors
which are represented by block diagram in
control engineering.
• It is a pictorial representation of functions
performed by each component and of the
flow of signals.
• It depicts the interrelationships that exist
among the various components.
Open loop control system – Those systems in
which the output has no effect on the control
action. In other words, in an open loop control
system the output is neither measured nor fed
back for comparison with the input.
Feedback/Closed loop system- The term closedloop control always implies the use of
feedback control action in order to reduce
system error.
LECTURE FOUR
CONTROL ACTIONS
 The relationship between the deviation and the signal sent
from the controller to the correcting unit determines the
control action.
 Most control actions derive their names on the basis of
mathematical or functional relationship between the
output and the error.
 An automatic controller compares the actual value of the
plant output with the reference input (desired value),
determines the deviation, and produces a control signal
that will reduce the deviation to zero or to a small value.
 The manner in which the automatic controller produces
the control signal is called the control action.
Industrial controllers are usually classified
according to their control actions:
• Two step Action or on-off controllers
• Proportional Action
• Integral Control Action
• Proportional plus integral control Action
• Proportional plus derivative control Action
• Proportional plus Integral plus derivative
control actions
Frequency response of controller
A controller may be regarded as an amplifier.
For sinusoidal input signals, a controller with
proportional action only gives an output
proportional to the input, but in phase
opposition to it, whatever the frequency may
be.
LECTURE FIVE
FREQUENCY RESPONSE ANALYSIS
Frequency response – Is the relationship
between output signal and input signal when
the sinusoidal input is a component or system
is varied over a wide range of frequencies.
When the plant and the controller are
connected together to form a closed loop, we
have a system similar to a voltage amplifier
with feedback mechanism.
• The input to the controller is a signal representing the
output condition of the process and the output of the
controller is fed into the process.
• The process may be broken down into a series of
individual stages, called transfer stages and time lags
due to the finite time taken for signals to travel from
one point to another.
• As the frequency of the input signal is increased the
angle of lag increases, the largest possible lag for a
signal stage being 90o.
• The attenuation also increases as frequency is
increased. Attenuation – is the production of an
output signal smaller than the corresponding input.
Frequency response of controller
A controller may be regarded as an amplifier. For
sinusoidal input signals, a controller with
proportional action only gives an output
proportional to the input, but in phase opposition
to it, whatever the frequency may be.
Transient response and stability
It is time variation of the output signal when a
specified step input signal or disturbance is
applied. The transient response of a speed
control system is illustrated by the shape of the
speed/time graph immediately following the
sudden application of load or a sudden change in
the desired speed setting.
LECTURE SIX
Computer-based systems
• The increasingly widespread use of microprocessor-based
process controllers over the last twenty years is due to their
flexibility in operation, their ability to record (or ‘log’) data
for subsequent calculations and the substantial reduction in
their cost.
• Computers can not only be programmed to read data from
sensors and send signals to process control devices, but
they can also store and analyse data and be connected to
printers, communications devices, other computers and
controllers throughout a plant.
• They can also be easily reprogrammed by operators to
accommodate new products or process changes.
• Examples of the different types of computer controlled
systems are described below.
Programmable logic controllers (PLCs)
• A significant development in process control
during the 1980s was the introduction of PLCs.
• They are based on microcomputers, and have
the same functions as relays, but with vastly
greater flexibility.
• Historically, they were first used to replace
relays in simple repetitive applications, but
the greater power was quickly used to develop
other functions, including recipe storage, data
transfer and communications with higher level
computers.
Types of control systems
The different combinations in which PLCs and
larger computers can be linked together in an
integrated control system can be described in
three categories:
1. dedicated systems
2. centralised systems
3. distributed systems.
Neural networks
• Where complex relationships exist between a
measured variable and the process or product,
it has not yet been possible to automate the
process.
• Recent developments of ‘expert systems’ or
‘neural networks’ may have the potential to
solve such problems.
• These are able to automatically deduce
complex relationships and also to quickly
‘learn’ from experience.