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In The Name of Allah
The Most Beneficent The Most Merciful
ECE4545:
Control Systems
Lecture:
Elements of
Control Systems
Design
Engr. Ijlal Haider
UoL, Lahore
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A modern industrial plant: A section of
the OMV Oil Refinery in Austria
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Outline
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Control Talk
Open Loop to Closed Loop
Instruments
Desirable Attributes of Instruments
System Integration
Process Facility Consideration
Cost Benefit Analysis
Various Sensor Types
Actuators and Control
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Control Talk
 Manual
vs. Automatic Control in a Heat
Exchanger
(an example of process control)
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Control Talk
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In a process-control facility the controller is not
necessarily limited to one variable, but can
measure and control many variables. A good
example of the measurement and control of
multivariable that we encounter on a daily basis is
given by the processor in the automobile engine.
List of some functions performed by the engine
processor. Most of the controlled variables are six
or eight devices depending on the number of
cylinders in the engine. The engine processor has
to perform all these functions in approximately 5
ms.
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Control Talk
 Automobile
Engine Control Processor:
Multivariable
Control Talk
Signals & Systems Terminology
Control Talk
Typical Control Hierarchy
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Open Loop to Closed Loop Approach
Control System Design, Goodwin, Graebe, Salgado
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Instruments
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Instrument is the name of any of the various
device types for measuring, indicating or
computing physical quantities or conditions,
performance, position, direction, and the like.
Transducers are devices that can change one
form of energy to another, e.g.
a resistance thermometer or a thermocouple
 gives output that is proportional to temperature
Converters are devices that are used to change the
format of a signal without changing the energy form,
i.e. a change from a voltage to a current signal.
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Instruments
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Sensors are devices that can detect physical variables
and have the ability to give a measurable output that
varies in relation to the amplitude of the physical
variable.
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The human body has sensors in the fingers that can detect
surface roughness, temperature, and force.
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A thermometer is a good example of a line-of-sight sensor
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Diaphragm pressure sensor,
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a strain transducer may be required to convert the
deformation of the diaphragm into an electrical or
pneumatic signal before it can be measured.
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Instruments
 Actuators
are devices that are used to
control an input variable in response to a
signal from a controller.
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typical actuator will be a flow-control valve
that can control the rate of flow of a fluid in
proportion to the amplitude of an electrical
signal from the controller.
magnetic relays that turn electrical power
on and off.
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Instruments
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Controllers are devices that monitor signals
from transducers and take the necessary
action to keep the process within specified
limits according to a predefined program
(algorithm) by activating and controlling the
necessary actuators.
PLC, Microcontrollers, PC etc.
PLC are industrial computers, Graphical
programming (Ladder Logic)
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Transmissions
 Transmitters
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Current 4-20 mA, 10-50mA
Voltage 0-5 V, 0-10 V, 0-12 V
Pneumatic
3-15 psi, 6-30 psi
 Digital
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Transmission
Fast and requires less power
Samplers
and
Reconstructors
Conversions
 Smart
Sensors
for
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Transmissions
 Industrial
Networks
 Foundation Fieldbus (US) and Profibus (EU)
 Telemetry (Wireless transmission)
Desirable Attributes of Instruments
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Reliability. It should operate within the necessary range.
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Accuracy. For a variable with a constant value, they
should settle to the correct value.
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Responsiveness.
If
the
variable
changes,
the
measurement should be able to follow the changes.
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Slow responding measurements can, not only affect the
quality of control but can actually make the feedback loop
unstable. Loop instability may arise even though the loop
has been designed to be stable assuming an exact
measurement of the process variable.
Desirable Attributes of Instruments
 Noise
immunity. The instruments, including the
transmission path, should not be significantly
affected by exogenous signals such as
measurement noise.
 Linearity.
If the instrument system is not linear, then
at least the nonlinearity should be known so that it
can be compensated.
 Non
intrusive. The instrument should not significantly
affect the behaviour of the plant.
System Integration
Success in control engineering depends on taking a
holistic viewpoint. Some of the issues are:
 plant, i.e. the process to be controlled
 objectives
 sensors
 actuators
 communications
 computing
 architectures and interfacing
 algorithms
 accounting for disturbances and uncertainty
 homogeneity
Plant
The physical layout of a plant is an intrinsic
part of control problems. Thus a control
engineer needs to be familiar with the
"physics" of the process under study. This
includes a rudimentary knowledge of the
basic energy balance, mass balance and
material flows in the system.
Objectives
Before designing sensors, actuators or control
architectures, it is important to know the goal, that is,
to formulate the control objectives. This includes
what does one want
reduction, yield increase,...)
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achieve
(energy
what variables need to be controlled to achieve
these objectives
what level of
(accuracy, speed,...)
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to
performance
is
necessary
Sensors
Sensors are the eyes of control enabling one
to see what is going on. Indeed, one
statement that is sometimes made about
control is:
If you can measure it, you can control it.
Actuators
Once sensors are in place to report on the
state of a process, then the next issue is the
ability to affect, or actuate, the system in
order to move the process from the current
state to a desired state
A Modern Rolling Mill
A typical industrial control problem will usually
involve many different actuators - see below:
Typical flatness control set-up for rolling mill
A Modern Rolling Mill
Communications
Interconnecting sensors to controllers and
actuators,
involves
the
use
of
communication systems. A typical plant can
have many thousands of separate signals to
be sent over long distances. Thus the design
of communication systems and their
associated protocols is an increasingly
important aspect of modern control
engineering.
Computing
In modern control systems, the connection
between sensors and actuators is invariably
made via a computer of some sort. Thus,
computer issues are necessarily part of the
overall design. Current control systems use a
variety of computational devices including
DCS's (Distributed Control Systems), PLC's
(Programmable Logic Controllers), PC's
(Personal Computers), etc.
Architectures and Interfacing
The issue of what to connect to what is a nontrivial one in control system design. One may feel
that the best solution would always be to bring all
signals to a central point so that each control
action would be based on complete information
(leading to so called, centralized control).
However, this is rarely (if ever) the best solution in
practice. Indeed, there are very good reasons
why one may not wish to bring all signals to a
common point. Obvious objections to this include
complexity, cost, time constraints in computation,
maintainability, reliability, etc.
Algorithms (Controller)
Finally, we come to the real heart of control
engineering i.e. the algorithms that connect the
sensors to the actuators. It is all to easy to
underestimate this final aspect of the problem.
As a simple example from our everyday
experience, consider the problem of playing tennis
at top international level. One can readily accept
that one needs good eye sight (sensors) and strong
muscles (actuators) to play tennis at this level, but
these attributes are not sufficient. Indeed eye-hand
coordination (i.e. control) is also crucial to success.
In summary:
Sensors provide the eyes
and
actuators the muscle
but
control science provides the finesse
 Better Sensors
Provide better Vision
 Better Actuators
 Better Control
Provide more Muscle
Provides more finesse by combining sensors and
actuators in more intelligent ways
Disturbances and Uncertainty
One of the things that makes control
science interesting is that all real life systems
are acted on by noise and external
disturbances. These factors can have a
significant impact on the performance of
the system. As a simple example, aircraft are
subject to disturbances in the form of windgusts, and cruise controllers in cars have to
cope with different road gradients and
different car loadings.
Homogeneity
A final point is that all interconnected systems,
including control systems, are only as good as
their weakest element. The implications of this in
control system design are that one should aim to
have all components (plant, sensors, actuators,
communications,
computing,
interfaces,
algorithms, etc) of roughly comparable
accuracy and performance.
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Process Facility Consideration
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The process facility has a number of basic
requirements including
Safety Precautions for Human, Machine and
Facility
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Environmental and Social Impact
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Waste treatment, Disposal, Emissions, Noise
Well-regulated, reliable supply of
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Sensors, Alarms, Switches, Grounding, EMI
(isolating transformers)
Electricity, Water and Air
Installation and Maintenance
Cost Benefit Analysis
In order to make progress in control
engineering (as in any field) it is important to
be
able
to
justify
the
associated
expenditure. This usually takes the form of a
cost benefit analysis.
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Various Sensors
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Temperature and Heat
Flow
Pressure
Level
Light and Sound
Humidity, Density, Viscosity and pH
Motion and Position
Force, Torque and Load
Smoke, Fire and Chemical
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Actuators and Control
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Pressure Control Actuators
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Flow Control Actuators
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Globe Valve
Other Valves
Power Control Actuators
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Regulators
Valves
Electronic Control Devices
Magnetic Control Devices
Motors
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Servo
Stepper
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Thank You!