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INSTRUMENTATION ENGINEERING
COURSE OUTLINE
Review of Instrumentation
Instrumentation standards
Instrumentation characteristics
Elements of measurements
Sensors and transducers
Signal conditioning
Application of Instrumentation
Introduction
There is always an increasing need for precise, efficient instruments and measurement
schemes in industrial environments.
Industrial control and automation are meaninglesswithout a proper instrumentation
scheme. Therefore, technological advancement requires advancement in both
instrumentation and control.
Measurement is the result of comparison b/n unknown quantity and predefined standard
Instrumentation is the design and use of an electrical system to collect and process
physically meaningful data
Industrial processes to be handled by measurement and instrumentation can vary
widely from simple to complex processes
Elements of measurement
Measuring device (to get information about the system and to control that system); To get
information the following processes are must,
Transduction (using sensors and transducers)
Signal processing (amplification, rectification, filtration..) using Opamps
Displaying (indicating, recording, and controlling)
Accuracy and precision, basics of pmmc(permanent magnet moving coil), the 3 different
torques (deflecting,controlling,damping)
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Q. Discuss on each of the above torques
Instrumentation standards
It is a physical representation of unit of measurement
The term standard is a tool piece of equipment having a known measure of physical
quantity precisely
There are Two methods of measurement
Direct method -voltage measurement
Indirect method- power measurement
Classification of instruments
They are basically classified in to three categories
Absolute and secondary
Deflection and null type
Analog and digital
Q. Try to discuss on each of them
Characteristics of instruments
This can be divided in to two categories
Static: this is considered when the instrument is used to measure a
condition not varying with time
Calibration(a process of comparing to the standard and
readjusting)
Accuracy(It is the closeness of instruments reading to the true
value )
Precision(It is the closeness of individual measurements by thatinstrument)
Example: assume true value=101v
M1=100 m1=98 m1=98.0
M2=99 m2=99 m2=98.2
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M3=101 m3=97 m3=98.2
A N P
Dynamic : This is considered when the instrument is used to measure
conditions varying with time
Sensitivity
Drift (zero,span,zonal drifts)
Resolution
Fidelity
Speed of response
Lag
Linearity
Backlash
Q . Discuss on each of them
Sensors
The devices that inform the control system about what is actually occurring are calledsensors (also known as transducers).
As an example, the human body has an amazing sensor system that presents our brain
with a reasonably complete picture of the environment whether we need it all or not. For a
control system, the designer must ascertain exactly what parameters need to be monitored.
Example:- position, temperature, pressure, level, flow and other parameters
The choice of sensors would be dictated by
system requirements, cost, and reliability.
Most sensors work by converting some physical parameter such as temperature or
position into an electrical signal. This is why sensors are also called transducers, which
are devices that convert energy from one form to another.
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POSITION SENSORS
Position sensors report the physical position of an object with respect to a reference point. The
information can be an angle, as in how many degrees a radar dish has turned, or linear, as in how
many inches a robot arm has extended.
i. Potentiometers
A potentiometer (pot) can be used to convert rotary or linear displacement to a voltage.
Actually, the pot itself gives resistance, but as we will see, this resistance value can easily be
converted to a voltage. A pot used to measure angular position has linear taper, which means the
resistance changes linearly with shaft rotation.
Block diagram
Example
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A pot is supplied with 10 V and is set at 82 the range of this single turn pot is 350. Calculate
the output voltage.
Loading error
A loading error occurs when the pot wiper is connected to a circuit with an input resistance that
is not considerably higher than the pots resistance
Example
A 10-k pot is used as a position sensor (Figure above). Assume that the wiper is in the
middle of its range. Find the loading error when
a. The interface circuit presents an infinite resistance
b. The interface circuit presents a resistance of 100 k
Example
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Optical Rotary Encoders
An optical rotary encoder produces angular position data directly in digital form,
eliminating any need for the ADC converter.
A light source and photocell arrangement are mounted so that the slots pass the light
beam as the disk rotates. The angle of the shaft is deduced from the output of the
photocell. There are two types of optical rotary encoders:
1. Absolute encoder
2. Incremental encoder
Incremental Optical Encoder
It has only one track of equally spaced slots. Position is determined by counting the
number of slots that pass by a photo sensor, where each slot represents a known angle.
This system requires an initial reference point, which may come from a second sensor on
an inner track or simply from a mechanical stop or limit switch
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Examples
1. A position sensor system uses a 250-slot disk. The current value of the counter is 00100110.
What is the angle of the shaft being measured?
SOLUTION
For a 250-slot disk, each slot represents 360/250 = 1.44, and a count of 00100110 = 38decimal, so the position is 38 1.44 = 54.72.
Linear Variable Differential Transformers
The linear variable differential transformer (LVDT) is a high-resolution position
sensor that outputs an AC voltage with a magnitude proportional to linear position. It has
a relatively short range of about 2 in., but it has the advantage of no sliding contacts.
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ANGULAR VELOCITY SENSORS
Angular velocity sensors, or tachometers, are devices that give an output proportionalto
angular velocity. These sensors find wide application in motor-speed control
systems.They are also used in position systems to improve their performance.
Tachometers
Optical Tachometers
The optical tachometer, a simple device, can determine a shaft speed in terms of
revolutions per minute (rpm).
Position sensors including potentiometers, optical rotary encoders, and linear variable
differential transformers
Velocity sensors including optical and direct current tachometers.
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Proximity sensors including limit switches, optical proximity switches, and Hall-effect
switches.
Load sensors including bonded-wire strain gauges, semiconductor force strain gauges,
and low-force sensors.
Pressure sensors including Bourdon tubes, bellows, and semiconductor pressure sensors
Temperature sensors including bimetallic temperature sensors, thermocouples, resistance
temperature detectors, thermistors, and IC temperature sensors.
Flow sensors including orifice plates, venturis, pitot tubes, turbines, and magnetic flow
meters.
Liquid-level sensors including discrete and continuous types
Toothed-Rotor Tachometers
A toothed-rotor tachometer consists of a stationary sensor and a rotating, toothed,
iron-based wheel
There are two kinds of toothed-rotor sensors in use.
1. variable reluctance sensor
2. Hall-effect sensor
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Direct current tachometer
It is is essentially a DC generator that produces a DC output voltage proportional to shaft
velocity. The output polarity is determined by the direction of rotation. Typically, these units
have stationary permanent magnets
PROXIMITY SENSORS
Limit Switches
A proximity sensor simply tells the controller whether a moving part is at a certain place.
A limit switch is an example of a proximity sensor. A limit switch is a mechanical push-
button switch that is mounted in such a way that it is actuated when a mechanical part or
lever arm gets to the end of its intended travel.
For example, in an automatic garage-door opener, all the controller needs to know is if the door
is all the way open or all the way closed.
Limit switches can detect these two conditions. Switches are fine for many applications,
but they have at least two drawbacks: (1) Being a mechanical device, they eventually
wear out, and (2) they require a certain amount of physical force to actuate. (Chapter 4
has more on limit switches.) Two other types of proximity sensors, which use either
optics or magnetics to determine if an object is near, do not have these problems. The
price we pay for these improved characteristics is that they require some support
electronics.
Optical Proximity Sensors
Optical proximity sensors, sometimes called interrupters, use a light source and a photo
sensor that are mounted in such a way that the object to be detected cuts the light path.
Figure below illustrates two applications of using photo detectors.
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Four types of photo detectors are in general use: photo resistors, photodiodes, photo
transistors, and photovoltaic cells.
LOAD SENSORS
Load sensors measure mechanical force. The forces can be large or small
Example
weighing heavy objects or detecting low-force tactile pressures. In most cases, it is the
slight deformation caused by the force that the sensor measures, not the force directly
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PRESSURE SENSORS
Pressure is defined as the force per unit area that one material exerts on another
Pressure sensors usually consist of two parts: The first converts pressure to a force or
displacement, and the second converts the force or displacement to an electrical signal.
Pressure measurements are made only for gases and liquids.
The simplest pressure measurement yields a gauge pressure, which is the difference
between the measured pressure and ambient pressure.
Bourdon Tubes
A Bourdon tube is a short bent tube, closed at one end. When the tube is pressurized,it
tends to straighten out. This motion is proportional to the applied pressure.
Bellows
This sensor uses a small metal bellows to convert pressure into linear motion
Bourdon-tube sensors
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Bellows pressure sensors
TEMPERATURE SENSORS
Temperature sensors give an output proportional to temperature. Most temperature sensors have
a positive temperature coefficient (desirable), which means that the sensor outputgoes up as the
temperature goes up, but some sensors have a negative temperature coefficient, which means that
the output goes down as the temperature goes up. Many control systems require temperature
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sensors, if only to know how much to compensate other sensors that are temperature-dependent.
Some common types are discussed next.
Bimetallic Temperature Sensors
The bimetallic temperature sensor consists of a bimetallic strip wound into a spiral
The bimetallic strip is a laminate of two metals with different coefficients of thermal
expansion. As the temperature rises, the metal on the inside expands more than the metal
on the outside, and the spiral tends to straighten out
These sensors are typically used for on-off control as in a household thermostat where a
mercury switch is rocked from on to off
Thermocouples
The thermocouple was developed over 100 years ago and still enjoys wide use,
particularly in high-temperature situations.
The thermocouple is based on the See beck effect, a phenomenon whereby a voltage that
is proportional to temperature can be produced from a circuit consisting of two dissimilar
metal wires.
Resistance Temperature Detectors
The resistance temperature detector (RTD) is a temperature sensor based on the fact that
metals increase in resistance as temperature rises
Thermistors: A thermistor is a two-terminal device that changes resistance with
temperature.
FLOW SENSORS
Flow sensors measure the quantity of fluid material passing by a point in a certain
time.Usually, the material is a gas or a liquid and is flowing in a pipe or open channel.
Summary
Sensors, also called transducers, are devices that sense physical parameters such as
position, temperature, or pressure. In most cases, the sensor outputs an analog voltage (or
digital value) that is proportional to the parameter being measured.
Position sensors measure the physical position of an object. Potentiometers (variable
resistors) measure angular position and give an analog output. The optical encoder,
another type of position sensor, uses a slotted disk and a photo sensor. The output of the
optical encoder is in digital form. Linear motion can be detected with a linear motion
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potentiometer or a linear variable differential transformer (LVDT). The LVDT uses a
movable slug inside a special transformer. The phase and magnitude of the AC output
can be processed to provide position information.
Position sensors can determine velocity by processing the data from two sequential
position samples.The more direct method to measure velocity is to use a tachometer.
A DC tachometer is actually a small generator that gives a DC voltage proportional to
velocity.
Optical tachometers give a pulse for each revolution of a shaft.
Proximity sensors sense whether an object has arrived at a certain place. The simplest
way to do this is with a mechanical limit switch. Other methods would include using a
photo sensor or a Hall-effect switch. The Hall-effect switch gives an output if a magnet is
brought near a specially configured semiconductor.
Load sensors can determine force by measuring the small deformation that the force
causes.The traditional method for measuring large forces is with a bonded-wire strain
guage.
This device incorporates a pattern of thin wires. When stretched, the resistance of the
wires change. Another method for measuring force uses the piezo-resistive effect of
semiconductorsthat is, the resistance changes when the material is compressed.
Pressure sensors measure the pressure of liquids and gases. One class of pressure sensors,
such as a bellows, uses the pressure to cause mechanical motion. Semiconductor pressure
sensors convert pressure directly into electrical resistance.
A wide variety of temperature sensors are in use. Simple bimetallic strips will bend when
heated and can then activate switch contacts.
The thermocouple is a traditional high temperature- sensing device that makes use of the
fact that the junction of two dissimilar metals will create a small voltage when heated.
The resistance temperature detector (RTD) uses the fact that a wire will increase in
resistance when heated.Numerous semiconductor devices are available that convert
temperature directly into resistance or voltage.Flow sensors measure the flow of a fluid ina pipe or open channel. Many flow sensors work by placing a restriction in the pipe and
then measuring the pressure before and after the restriction. The pressure difference
between the two places is proportional to fluid velocity. Turbine flow sensors use the
moving fluid to spin a propeller. The rpm of the propeller is proportional to velocity.
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Liquid-level sensors determine the level of liquid in a tank. The discrete type can sense
only if the level is at or above a certain point. Examples of the discrete level detectors are
a float activating a limit switch or a photo sensor.
The continuous-level detector gives an analog output proportional to fluid level. There
are many different ways this can be donefor example, connecting a float to apotentiometer, monitoring the pressure at the bottom of the tank, or monitoring the
weight of the tank with load cells.
Vision sensors are being used more and more for such things as inspecting parts and
guiding robots in assembly operations. A vision system consists of a TV camera
connected to a computer, typically a PC. Special vision software analyzes the video
image and makes specific recommendations to the work area.
SIGNAL CONDITIONING TECHNIQUES
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What is signal conditioning?
Most of signals found from sensors are too weak which needs conditioning. It is a process
of modifying the coming signal as per the requirement.There are different conditioning
techniques some of them are the following
1. Amplification: the process of increasing the amplitude or strength of the sensor output
signal without varying it in any other way.
2. Modulation and Demodulation : the process of imposing or removing a signal (the
information) upon another signal (the carrier) that is used to convey the original
information. Modulation puts the information on the carrier, and demodulation recovers
the original information from the carrier.
3. Frequency Selection ( Filtration) : the process whereby a signal containing a group
of different frequencies is filtered, allowing only certain desired frequencies to pass,
while blocking all other frequencies. (All pass, low pass, high pass, band pass, notch
filter)
4. Transmission: the process of taking a signal from one point in space and conveying it,
undistorted, to another point.
5. Wave Shaping: the process of purposely distorting a signal to give it certain desired
characteristics.
6. Isolation: the process of maintaining a signal so that it cannot be easily modified by
interfering signals or random noise.
7. Logic: the processes whereby certain signals interact with one another according to
preset rules that allow elementary decisions to be made.
8. Conversion: the process of transferring a signal from analog to digital format or
viceversa.
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OPAMPS
An inverting amplifier reverses the polarity of the input signal while amplifying it. But
A non-invertingamplifier is an opamp circuit designed to provide a positive voltage
gain.
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Design an op amp circuit with inputs v1 and v2 such that vo = 5v1+3v2.
One of the most useful and versatile opamp circuits for precision measurement and
process control is the instrumentation amplifier (IA
).Typical applications of IAs includeisolation amplifiers, thermocouple amplifiers, and data acquisition systems.
DIPLAYING DEVICES
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Display devices
A display device presents instantaneous data so that it can be read from the instrument by
a human, but it does not remember any of the data. Thus, a display must be continuously
watched if the data is to be carefully observed.
There are several types of display devices that are useful in any field of instrumentation.
These are listed as follows:
1. Analog scale. A common electrical meter is an example of an analog scale. Here, some sort
of a pointer indicates a location on a graduated scale calibrated in the proper units. This
represents the value of the measured variable.
2. Digital readout. This device indicates the numeric value of a variable by havingthe actual
numerals displayed. In some cases, letters and other symbols can also be displayed.
3. Loud speaker or other sound source. This device can indicate by means ofgenerating soundwaves. Some instruments even use an artificial voice generated by a computer to announce the
results of a measurement.
4. Cathode ray tube or flat panel solid state display. This familiar picture tube orLCD readout
of a television set or a computer monitor can display complete photographic-quality images,
graphical data, and other computer generated illustrations and text material.
5. Indicator lamp orLight Emitting Diode. These are binary or "go-no-go" displays in that they
can only indicate one of two states: light on or light off. Some light emitting diodes can also
change color based on the input signal.
Storage devices
Storage devices differ from display devices in that they keep a permanent record of all
data. This record may appear as a chart, a printed page, or invisible electrical, optical or
magnetic signal. Examples of storage devices are listed below.
1. Chart recorders. Graphic data is permanently plotted by these devices. There are several
types of chart recorders:
a.) Strip chart recorder. A variable is plotted on a long, narrow strip of graph paper that is
moved past the plotting mechanism such as a pen, at a uniform rate thus giving a graph of the
variable as a function of time.
b) Trend recorder. This is very similar to the strip chart recorder, only thatthe time scale is very
long, and that several hours of data can be placed on a single 8 " x 11" sheet of paper, thus
making it possible to easily observe long term trends in the data.
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A digital spectrum analyzer computes the discrete Fourier transform (DFT), a
mathematical process that transforms a waveform into the components of its frequency
spectrum.
Some spectrum analyzers (such as "real-time spectrum analyzers") use a hybrid technique
where the incoming signal is first down-converted to a lower frequency using super-heterodyne techniques and then analyzed using fast Fourier transformation (FFT)
techniques.
Parts of Spectrum Analyzer
Certain functions and basic controls :
FREQUENCY: allows to fix the window of frequencies to visualize in the screen.
MARKER/PEAK SEARCH: controls the position and function of markers and indicates
the value of power.
BW/AVG: is a filter of resolution. The spectrum analyzer captures the measure on having
displaced a filter of small bandwidth along the window of frequencies.
AMPLITUDE: is the maximum value of a signal in a point.
VIEW/TRACE: manages parameters of measurement. It stores the maximum values in
each frequency and a solved measurement to compare it.
INTERFACING CIRCUIT
The signals are often processed before interfacing between the instrument and the
computer. Some of the processing operations are as follows :
1. Matching or termination for a standard signal type
2. Protection or barrier
3. Filtering
4. Conditioning
5. Manipulation such as linearization
Signal Matching
In a process control system the signals of various forms are transformed to a standard
form of voltage or current. For example, optically transmitted signals are converted to
electrical voltage signals by opto-electrical converters.
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In many industries pneumatic transducers transmit pneumatic signals. Such signals are
converted to electrical signals by pneumatic to voltage or current converters.
AC signals produced by some sensors are rectified to DC.
In many cases, signals generated by high-impedance sensors are matched by using op-amp impedance matching modules.
Protection
The digital computers and the interfacing circuits operate at low voltage levels and these
are always prone to accidental damage due to probable high current flowing from the
plant side. Proper protection against this is incorporated by using a series of current-
limiting resistors, circuit-breaking fuses, polarity protectors, opto-isolators, isolation
transformers, and so on.
Apart from protection, fault detectors are also used to detect faults like short circuits,open circuits in transducers, and so on.
Filtering
Low-level signals are corrupted with noise generated in the transducer itself or noise due
to other electrical or electromagnetic interferences.
Power line interference causes undulation to the transducer signals at power line
frequency (5060 Hz), which must be removed by RC or active high-pass filters.
Similarly high-frequency noises generated by logic switching, microwave,radio waves, and so oncan be filtered by low-pass filters.
Interface Standards
When a set of transducers, sensors, and actuators are required to be interfaced between plant and
computer several parameters must be matched: the interrupt structure, data timing and control,
physical connections, signal levels, and programming.
Biomedical applications
What are biomedical instruments ?
The term medical instrumentstood for simple hand-held instruments used by physicians
for observing patients, examining organs, making simple measurements, or administering
medication. These small instruments, such as stethoscopes, thermometers, tongue
depressors, and a few surgical tools, typically fit into a physicians hand bag.
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Todays medical instruments are considerably more complicated and diverse, primarily
because they incorporate electronic systems for sensing, transducing, manipulating,
storing, and displaying data or information.
Furthermore, medical specialists today request detailed and accurate measurements of a
vast number of physiologic parameters for diagnosing illnesses and prescribe complicatedprocedures for treating these
Surge Protection
The isolation amplifiers described in the preceding paragraph are primarily used for the
protection of the patient from electric shock
Interferences
The most critical point in the measurement of biopotentials is the contact between
electrodes and biological tissue. Both the electrode offset potential and theelectrode/tissue impedance are subject to changes due to relative movements of electrode
and tissue
Electromyography (EMG)
EMG measures muscle activity by detecting and amplifying the tiny electrical impulses
that are generated by muscle fibers when they contract.
EMG sensors are required to be placed on the muscle belly and its positive and negative
electrodes are parallel to the muscle fibers.
ELECTROCARDIOGRAPHY (EKG or ECG)
EKG is similar to EMG: The sensor detects and amplifies the small electrical voltage that
is generated by the heart muscle when it contracts.
The actual EKG signal is measured in micro-volts (V).Most often, though, the
clinically useful measures are those that are computed from the raw EKG
Electroencephalogram (EEG)
Brain cells (neurons) generate small electrical voltages when they fire.
Electroencephalography is utilized to measure the electrical activity of the brain via
electrodes and the resulting traces are known as an electroencephalogram (EEG)
What is a CAT/CT Scan(computed axial tomography)?
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Moisture Content Measurement
When food items are processed, water is either mixed in or driven off by the process of
drying
Most food items absorb moisture from the air during processing or preservation theunwanted or free water in food helps the growth of microorganisms, which can cause
food poisoning on consumption
Humidity measurement
food items like potato chips, dry breakfast cereals, and crackers absorb water particles
when exposed to relatively humid environments. As a result, the quality deteriorates due
to physical, chemical, or biological changes that take place in the food. Physically, food
can become soggy, rubbery, and unappetizing
Color ofFood
Measurements of color is important quality control method in many food processing
industries.
In many food items the chemical and the food values are reflected in their color. Apart
from the processed food, the color of the raw materials also plays an important role in
quality monitoring in certain food processing industries
Food and Process Temperature Measurement
The primary considerations required for food temperature measurement are type of thefood item (solid, liquid, viscous, etc.), accuracy needed, dynamic behaviors (stationary or
moving), atmospheric conditions, thermal coupling, and so on. The simplest situation of
temperature measurement is in a stationary
phase of a food (i.e., while storing)
Food Flow Metering
Flow measurement of liquid food materials is primarily conducted to determine the
quantity or proportion of input material introduced into a processing stage.
The important aspect of flow metering in the food industry is the quantity control; hence,
special attention should be paid to designing a flow meter for a special applications.
pH Values ofFood
pH is a parameter that determines the quality of a food or any other quality dependent on
pH, particularly in packaging, preservation, or canning.
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pH Scale pH is defined as the negative logarithm of the hydrogen ion (i.e., proton)
concentration in a solution, which can be expressed as pH = log(H+)
Food Enzymes
Because enzyme activities are one of the most important facets of food processing,researchers are currently trying to detect and quantify enzymes using various solid state
sensors developed so far.
Flavor (taste) Measurement
Food items are mostly natural vegetative products that have a certain characteristic
flavor. When food items are processed, the flavor pattern might change and new flavor
components might be produced.
Food Texture and Particle Size
Texture plays an important role in many food products to make it more appealing.
Analysis of the texture-related qualities of food products is an area for development of
new products or improvement of existing ones
Milk processing
Milking machine
Milk churner
Milk processor
Packing machine
Reserving
Transporting
