Introduction to
Instrumentation and
Control Engineering
Objectives:
1. To gain understanding about the basic concepts and principles in
Instrumentation and Control Engineering.
2. To explain the difference of the classifications of instruments.
3. To know the basic elements of a control loop.
Measurement
Measurement (also called metrology) is the science of determining
values of physical variables. It is a method to obtain information
regarding the physical values of the variable. Measurement of a given
quantity is essentially an act or result of comparison between the
quantity (whose magnitude is unknown) and predetermined or
predefined standards. Two quantities are compared the result is
expressed in numerical values.
Why do we measure?
For a few minutes think of an answer for this question and be ready
to share it to the class.
The Needs for Measurement
The measure for the weight of the precious stones, such as diamond,
carat is used. Carat was the weight of four carob (Keçiboynuzu) beans.
Today carat is standardized as 0.2 gr.
Metric System
SI Units: Systemes Internationales d’Unites
Two different units are defined:
- Fundamental Units
- Derived Units
Standards
• International Organization for Standardization (ISO)
• International Electrotechnical Commission (IEC)
• American National Standards Institute (ANSI)
• Standards Council of Canada ( SCC)
• British Standards (BS)
• Institute of Turkish Standards (TSE)
Standard Bodies
1. International standards:
Defined by international agreements.
2. Primary standards:
Maintained at institutions around the world.
Main function is checking accuracy of secondary standards.
Instrumentation
Instrumentation is used in almost every industrial process and
generating system, where consistent and reliable operations are
required. Instrumentation provides the means of monitoring, recording
and controlling a process to maintain it at a desired state. A typical
industrial plant such as an electric generating station yields many
process variables that have to be measured and manipulated.
Process Variables
Variables such as boiler level, temperature, pressure turbine speed,
generator output and many others have to be controlled prudently to
ensure a safe and efficient operation. With instrumentation, automatic
control of such presses can be achieved. Specific instrumentation can
be selected to measure, and to indicate process conditions so that a
corrective action could be initiated if required.
Instrumentation based on industrial
application:
“It is a collection of instruments, devices, hardware
or functions or their application for the purpose of
measuring, monitoring or controlling an industrial
process or machine, or any combination of these.”
What is an instrument?
“It is device used for direct or indirect measurement,
monitoring, and/or control of a variable including
indicators, controllers, and other devices such as
annunciators, switches and pushbuttons.”
Measurement Instrument
A measurement instrument is a
device capable of detecting change,
physical or otherwise, in a particular
process.
It
then
converts
these
physical changes into some form
of information understandable by the
user.
Instrument Examples:
Classification of Instruments
Critical Instruments - an instrument which, if not conforming to
specification, could potentially compromise product or process
quality and safety.
Non-critical Instrument - an
instrument whose function is
not critical to product or
process
quality,
function
is
but
more
operational significance.
whose
of
an
Reference Only Instrument - an instrument whose function is
not critical to product quality, not significant to equipment
operation, and not used for making quality decisions.
Control Systems and Process Control
Control in process industries it refers to the regulation,
command or direction of all aspects of the process.
2 Types of Control
• Manual Control
• Automatic Control
System is an arrangement, set or collection of physical
components connected or related in such a manner as to form
and/or act as an entire unit. Therefore control system is an
arrangement of physical components connected or related in such
a manner as to command, direct or regulate itself or another
system. A process simply refers to the methods of changing or
refining raw materials to create end products and
Process Control
Process Control play an important role in how a plant process
upset can be controlled and subsequent emergency actions executed.
Without adequate and reliable process controls, an unexpected process
occurrence cannot be monitored, controlled, and eliminated. Process
controls can range from simple manual actions to computer logic
controllers, remote from the required action point, with supplemental
instrumentation feedback systems.
Control Systems
A system, whose output can be managed, controlled or
regulated by varying its input is called Control System. A control
system can also be a combination of smaller control systems and
are normally used to get desired/required output. If we look
around, we will find many control systems in our surroundings i.e.
Refrigerator, Air Conditions, Washing Machines etc.
Block Diagram of Control System
The above figure represents a simple control system and we can
think of this control system as a mathematical equation i.e.
X+5=Y
where, X is input, Y is output and Constant 5 is acting as a Control
System. So, by changing the value of input parameter ( X ), we can
change our output value (Y). Similarly, if we want a particular
output value, we can achieve it by fixing input value.
Explain
the
shown
example
of
a
simple
Control System.
Control Systems are classified into two main categories, which are:
1. Open Control Loop – exist when the process variable is
not compared, and action is taken not in response to on the
condition of the process variable.
2. Closed Control Loop – exists when a process variable is
measured, compared to a setpoint and action is taken to
correct any deviation from setpoint.
In Open Loop Control Systems, we have three main components i.e.
Input, Controller & Output. Input signal is directly fed to the controller, which
utilizes it and generates the required output. In Open Loop systems, generated
output has no affect on the Input signal i.e. no feedback provided.
Example of an Open Loop Control Systems:
Clothes Dryer is a very simple example of an open loop system.
When damp clothes are put in the dryer machine, the operator / user
sets the time for drying the clothes. This time acts as the input signal
for the dryer. Correspondingly at the end of that time, the machine
stops and clothes can be taken out. Now the thing to be noted here is
that no matter if the clothes are dry enough or not, the machine will
stop because of the time (input signal) fed to it.
So the output of the system does not affect the input in this
case. For a better understanding the block diagram of a cloth dryer
control system is shown below:
Traffic Light system is another easy to understand example of
an open loop system. Certain input signals are fed to the controller,
which then displays one of the three lights at the output turn by
turn. The direct input signals can be altered to change the output
light but the output has no affect on the input. As we are not
passing any feedback i.e. which light turned ON or OFF.
Closed Loop Control System (Feedback control system) is
an advanced automated system, which generates the desired output
by using inputs, Controllers and feedback elements. These systems
use feedback element to fed the Output back to the controller. By
doing that, we can compare the current output with input to get
errors.
Here's the block diagram of a Closed Loop Control System:
The block diagram above is an excellent representation of a
closed loop control system. As seen, the system output is being fed
back to the controller through an error detector. The function of
the error detector is to find the difference in the input and output
signal, and feed this difference to the controller so that the output
can be adjusted. In this way the system output is being
automatically adjusted all the time with the help of the feedback
signal and the operator does not have to worry about it.
Example of an Closed Loop Control Systems:
Air conditioner is a very typical example of a closed loop control system.
The input signal in the form of required room temperature is fed into the
controller of the air conditioner. The compressor along with its various electrical
and mechanical components help in achieving the required temperature. Now
whenever the room temperature changes, the temperature sensor at the output
senses the change in the room temperature and the signal from the sensor is
calculated by the error detector and fed back to the controller through the
feedback loop to maintain the required room temperature.
In this way the required output is always maintained automatically without
any manual interference. The block diagram illustration of this process is shown
below:
Four Basic Elements of a Control Loop
1. Primary Element/Sensor
2. Secondary Element/Signal-generating Element
3. Controlling Element/Controller
4. Final Control Element
Primary Elements
It measures process parameters and variables. Measurement of
the variables or properties are based on certain unique phenomena,
such as physical, chemical, thermo-electrical factors.
Note: Process Variables sensed by the primary element cannot be
transmitted unless converted to an electrical (or pneumatic) signal
by a secondary element.
Primary Element Examples
1. Sensors
Integral part of loop that first senses the value of a process variable that
assumes a corresponding predetermined state and generates an output signal
indictive of or proportional to the process variable.
2. Detectors
It is a device that is used to detect the presence of something, such as
flammable or toxic gases or discrete parts.
Secondary Elements/Signal Generating Element
1. Transducer
Transducers are often employed at the boundaries of
automation, measurement, and control systems, where electrical
signals are converted to and from other physical quantities (energy,
force, torque, light, motion, position, etc.). The process of converting
one form of energy to another is known as transduction.
2. Converter
Converters are used to convert AC power to DC power. Virtually
all the electronic devices require converters. They are also used to
detect amplitude modulated radio signals. A power electronic
converter uses power electronic components such as SCRs, TRIACs,
IGBTs, etc. to control and convert the electric power. The main aim
of the converter is to produce conditioning power with respect to a
certain application.
3. Transmitter
As its name implies, the general purpose of a transmitter is to transmit
signals. These signals contain information, which can be audio, video, or data.
It converts a reading from one sensor or transducer into a standard and
transmits that signal to a monitor or controller. Transducers and transmitters
are virtually the same thing, the main difference being the kind of electrical
signal each sends. A transducer sends a signal in volts (V) or millivolt (mV)
and a transmitter sends a signal in milliamps (mA).
Types of Signal
1. Analog Signal - a signal that has no discrete positions or states and
changes value.
• Pneumatic : 3-15 psi
• Electrical : 4-20 mA (Current)
: 1-5 VDC (Voltage)
2. Digital Signal - a signal that generates or uses binary digit signals to
represent continuous values or discrete states.
Controlling Element
Known as the controller and is the brain of the control system.
It performs appropriate functions for maintaining the desired level
(set point) of parameters to restore quality and rate of production.
A controller is a device that receives data from a measurement
instrument , compares that data to a programmed setpoint, and, if
necessary, signals a control element to take corrective action.
Common examples of controller:
Programmable Logic Controller (PLC) – usually computers
connected to a set of input/output (I/O) devices. The computers are
programmed to respond to inputs by sending outputs to maintain all
processes at setpoint.
Distributed Control System (DCS) – are controllers that, in addition
to performing control functions , provide readings of the status of the
process, maintain databases amd advance man-machine-interface.
Final Control Element
The part of the control system that acts to physically change the
manipulated variable. Typically used to increase or decrease fluid
flow.
Common Final Control Elements
1. Actuator - the part of a final control device that causes a
physical change in the final control device when signaled to do so.
2. Control Valves - manipulate the flow rate of gas or liquid;
whereas, the control switches manipulate the electrical energy
entering a system.
Instrument applications:
❖Factory automation instruments
❖Plant safety or safeguarding instruments
❖Product Quality monitoring/control instruments
❖Environmental condition monitoring /control
instruments.
❖Process variable measurement and control instruments.
“You are worth more than just your grades.”
-end-