Kenyatta University
BSc Electrical and Electronic Engineering
EEE414 INSTRUMENTATION
LECTURE NOTES by A. M. Muhia
March 21, 2016
EEE414 INSTRUMENTATION
Prerequisites:
Electrical Measurements, Analogue Electronics III
Objectives:
• To provide an in-depth understanding of the different types of sensors and transducers
• To develop a basic understanding of the electronic circuits used to convert sensor outputs to useful
signals.
• To develop an appreciation for the non-ideal characteristics of actual sensors and sensor circuits
Expected Outcomes:
At the end of this course, the students should be able to:
1. describe different instrument systems and their characteristics
2. identify and select appropriate transducers for given applications
3. understand various types of signal processing circuits used for processing transducer outputs.
4. be well acquainted with the measurement techniques of variables like stress, strain, displacement, acceleration, temperature, and humidity
5. describe the techniques of microprocessor-based data acquisition and storage.
6. Engineering Workbench) and the IEEE-488 General Purpose Interface Bus (GPIB) for data acquisition
and storage.
Description:
Instrument systems: classification, factors affecting system selection; linearity, accuracy, precision, resolution,
sensitivity, hysteresis.
Transducers: Types of transducers. Transducer selection. Transducer characteristics. Resistive, inductive,
capacitive, and Hall-effect types. Applications of transducers in measuring devices. Measurement systems:
measurement of stress and strain; displacement; acceleration; temperature; humidity. Opto-electronic measurements. Measurement of Low Level Quantities:
Signal Processing circuits: bridge circuits; instrumentation amplifiers, choppers and chopper-stabilized amplifiers, charge amplifiers, Voltage-to- frequency and frequency-to-voltage conversion. Analogue-Digital and
Digital- to-Analogue conversion.
Noise sources. Effects of noise on small signals. Noise and interference reduction. Introduction to
Digital Instrumentation: Principles and examples of digital instruments. Microprocessor applications in
instrumentation, RS-232 and IEEE-488 General Purpose Interface Bus (GPIB), digital data acquisition and
storage.
Remote sensing techniques.
i
Teaching Organization
Lectures: 2 hours per week; Tutorials: 2 hours per week; Laboratory Exercises: At least four experiments
per semester, with each lab/practical session 3 hours long.
Assessment
Regular Examination at end of Semester: 70
Practical work/Laboratory Exercises
The practical work/laboratory exercises are to cover the following topics:
• Displacement measurement by Linear Variable Differential Transducer (LVDT)
• Measurement of position and speed using Hall effect sensors
• Temperature measurement using Resistance Temperature Detector (RTD), Thermocouple, Thermistor
• Thermocouple calibration.
• Angular position measurement using an optical encoder.
• Measurement of bending strength or force using strain gauges.
• Force measurement using a load cell
• Water level measurement using a capacitive transducer
• Velocity measurement using air ow transducer
• RPM measurement using electromagnetic transducers
• Rotational speed using a tachometer
• Measurement of distance using ultrasonic method
• Analogue-to-Digital (A/D) and Digital-to-Analogue (D/A) conversion
• Data acquisition and storage using LabVIEW and the IEEE-488 General Purpose Interface Bus (GPIB).
Textbooks and References
1. Process Control Instrumentation Technology, by Curtis Johnson, 8th Edition, Prentice Hall, 2005
2. Measurement and Instrumentation Principles, Third Edition, by Alan S Morris, Butterworth-Heinemann
Publishers, 2001
ii
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Instrument Systems
Instrumentation is defined as "the art and science of measurement and control". Instrumentation can be used
to refer to the field in which Instrument technicians and engineers work in, or it can refer to the available
methods and use of instruments.
In abstract terms, an instrument is a device that transforms a physical variable of interest (the measurand)
into a form that is suitable for recording(the measurement).
1.1
Basic Measuring Instruments
The Structure of Measurement Systems:- Figure 1 shows the general structure of measurement systems, since
Figure 1: Structure of Basic Measurement System
a measurement system can consist of as little as one of the above steps.
• The Transducer consists of a Sensing Element Signal Conditioning (Variable Conversion Element).
The sensor detects a parameter in a physical form and the Variable Conversion Element converts it
into an electrical form. (N.B: Sometimes, sensors can do the sensing and converting, then a Variable
Conversion Element isn’t needed)
• The Signal Processing Element changes the small electrical quantities into quantities that are compatible
with the rest of the systems.
• Data Presentation is used to record the reading for the measured quantity and then it presents it in
the final display step.
A measurement system may also contain Auxiliary Functional Elements; those which may be incorporated
in a particular system depending on the type of requirement, the nature of measurement technique, e.t.c.
They are:
• Calibration Element to provide a built-in calibration facility.
• External Power Element to facilitate the working of one or more of the elements like the transducer
element, the signal conditioning element, the data processing element or the feedback element.
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• Feedback Element to control the variation of the physical quantity that is being measured. In addition,
feedback element is provided in the null seeking potentiometric or Wheatstone bridge devices to make
them automatic or self-balancing.
• Microprocessor Element to facilitate the manipulation of data for the purpose of simplifying or accelerating the data interpretation. It is always used in conjunction with analog-to-digital converter which
is incorporated in the signal conditioning element.
1.2
Classification of Measuring Instruments
Instruments can be subdivided into separate classes according to several criteria as follows:
1. Passive and active instruments
The passive instrument does not contain any electrical power source. In self-generating (or passive)
instruments, the energy requirements of the instruments are met entirely from the input signal.
On the other hand, power-operated (or active) instruments are those that require some source of
auxiliary power such as compressed air, electricity, hydraulic supply, e.t.c. for their operation. The
active instrument contains a power source. The energy in the output signal comes from the external
power source
Active instruments have better resolution but the passive types are cheaper in cost.
2. Contacting and Non-Contacting Types
A contacting type of instrument is one that is kept in the measuring medium itself. A clinical thermometer is an example of such instruments.
On the other hand, there are instruments that are of non-contacting or proximity type. These instruments measure the desired input even though they are not in close contact with the measuring medium.
For example, an optical pyrometer monitors the temperature of, say, a blast furnace, but is kept out of
contact with the blast furnace
3. Null Type and Deflection Type Instruments
A null type instrument is the one that is provided with either a manually operated or automatic balancing device that generates an equivalent opposing effect to nullify the physical effect caused by the
quantity to be measured. The equivalent null-causing effect in turn provides the measure of the quantity. Measurement is made in terms of the value of the weights needed to reach a null position.
A deflection type instrument is that in which the physical effect generated by the measuring quantity
produces an equivalent opposing effect in some part of the instrument which in turn is closely related
to some variable like mechanical displacement or deflection in the instrument. For example, the
4. Analog and digital instruments
An analog instrument gives an output that varies continuously as the quantity being measured changes
e.g. analog voltmeter
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A digital instrument has an output that varies in discrete steps and so can only have a finite number
of values e.g. digital voltmeter
The advantages of digital instruments over analogue instruments are
• Easy readability
• Better resolution
• Greater accuracy
• Automatic polarity and zeroing
5. Indicating instruments and instruments with a signal output
Indicating Instruments
Gives an audio or visual indication of the magnitude of the physical quantity measured e.g. liquid in
glass thermometer
Instruments with a signal output
Give an output in the form of a measurement signal whose magnitude is proportional to the measured
quantity e.g. strain gauge
6. Smart and non smart instruments
Smart or intelligent instruments have or incorporate a microprocessor
Non-Smart or dumb instruments don’t have or don’t incorporate a microprocessor
1.3
1.3.1
Characteristics of Instrument and Measurement Systems
Static Characteristics
1. Accuracy
Accuracy is the closeness with which the reading measured value approaches an accepted standard
value or the true value of the measured quantity.
2. Precision
Precision is the closeness with which individual measurements are distributed about the mean value.
It refers to the degree of agreement of a set of measurements among themselves.
3. Sensitivity
This is the ability of an instrument to respond to small changes in the quantity that it measures.
4. Reproducibility
It is the degree of closeness with which a given value may be repeatedly measured.
5. Drift
This is the variation of the measured value with time.
6. Resolution
This is the smallest increament in the input or quantity being measured which can be detected with
certainty by an instrument. Hence resolution defines the smallest measurable input change which.
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7. Threshold
This is the smallest measurable value of the quantity being measured.
8. Static error
This is the difference between the measured value and the true value of the quantity being measured.
1.3.2
Dynamic Characteristics
1. Speed of response
This is defined as the rapidity with which a measurement system responds to changes in the measured
quantity.
2. Measuring lag
It is the retardation or delay in the response of a measurement system to changes in the measured
quantity.
3. Fidelity
This is the degree to which a measurement system indicates changes in the measured quantity without
any dynamic error.
4. Dynamic error
It is the difference between the true value of the quantity being measured changing with time and the
value indicated by the measurement system.
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2
Transducers
2.1
Introduction
A generalized measurement system consists of 3 major components
1. An input device
2. A signal conditioning or processing device
3. An output device
The input device receives the measurand or the quantity under measurement and delivers a proportional or
analogous electrical signal to the signal conditioning device. Here the signal amplified, attenuated, filtered,
modulated or otherwise modified in a format acceptable to the output device.
The input quantity for most instrumentation systems is a non electrical quantity. In order to use electrical
methods and techniques for measurement, manipulation or control, the non electrical quantity is generally
converted into an electrical form by a device called a transducer. A transducer can be broadly defined as a
device which converts a non electrical quantity into an electrical quantity and vice versa.
For example these devices which convert mechanical force into an electrical signal which form a very large
and important group of transducers commonly used in industrial instrumentation.
Many other physical parameters such as heat, intensity of light, flow rate, liquid level, humidity and pH value
may also be converted into electrical form by means of transducers.
These transducers provide an output signal when stimulated by a mechanical or non mechanical input.
A photoconductor converts light intensity into a change of resistance.
A thermocouple converts heat energy into electrical voltage, a force produces a change of resistance in a
strain gauge, an accelerator produces a voltage in a piezoelectric crystal etc
In all cases however, the electrical output is measured by standard methods giving the magnitude of the
input in terms of an analogous output
2.2
Functional Parts of a Transducer
The transducers may be thought of consisting of 2 important and closely related parts. These 2 parts are
1. sensing element
2. transduction element
In addition there may be many other auxiliary parts such as amplifiers and other signal processing equipment,
power supplies, calibrating and reference sources and mechanical mounting features.
2.2.1
Sensing element / detector element
A detector or a sensing element is that part of a transducer which responds to a physical phenomena. The
response of the sensing element must be closely related to the physical phenomena.
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2.2.2
Transduction elements
A transduction element transforms the output of a sensing element to an electrical output. The transducer
element in a way acts as a secondary transducer.
Characteristics of the transducer
i. The transducer should recognize and sense the desired input signal and should be insensitive to other
signals present simultaneously in the measurand. For example, a velocity transducer should sense the
instantaneous velocity and should be insensitive to the local pressure or temperature.
ii. It should not alter the event to be measured.
iii. The output should preferably be electrical to obtain the advantages of modern computing and display
devices.
iv. It should have good accuracy.
v. It should have good reproducibility
vi. It should have amplitude linearity.
vii. It should have adequate frequency response (i.e., good dynamic response).
viii. It should not induce phase distortions (i.e. should not induce time lag between the input and output
transducer signals).
ix. It should be able to withstand hostile environments without damage and should maintain the accuracy
within acceptable limits.
x. It should have high signal level and low impedance. .
xi. It should be easily available, reasonably priced and compact in shape and size (preferably portable).
xii. It should have good reliability and ruggedness. In other words, if a transducer gets dropped by chance,
it should still be operative.
xiii. Leads of the transducer should be sturdy and not be easily pulled off.
xiv. The rating of the transducer should be sufficient and it should not break down.
2.3
2.3.1
Sensor Technologies
Capacitive and Resistive Sensors
Capacitive sensors consist of two parallel metal plates in which the dielectric between the plates is either air
or some other medium. The capacitance C is given by
r A
C= 0
d
where 0 is the absolute permittivity, r is the relative permittivity of the dielectric medium between the
plates, A is the area of the plates and d is the distance between them. Capacitive devices are often used as
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displacement sensors, in which motion of a moveable capacitive plate relative to a fixed one changes the capacitance. Often, the measured displacement is part of instruments measuring pressure, sound or acceleration.
Alternatively, fixed plate capacitors can also be used as sensors, in which the capacitance value is changed by
causing the measured variable to change the dielectric constant of the material between the plates in some
way. This principle is used in devices to measure moisture content, humidity values and liquid level.
Resistive sensors rely on the variation of the resistance of a material when the measured variable is applied
to it. This principle is most commonly applied in temperature measurement using resistance thermometers
or thermistors, and in displacement measurement using strain gauges or piezoresistive sensors. In addition,
some moisture meters work on the resistance-variation principle.
2.3.2
Magnetic Sensors
Magnetic sensors utilize the magnetic phenomena of inductance, reluctance and eddy currents to indicate
the value of the measured quantity, which is usually some form of displacement.
Inductive sensors translate movement into a change in the mutual inductance between magnetically coupled
parts. One example of this is the inductive displacement transducer shown in Figure 2
Figure 2: Inductive sensor
In this, the single winding on the central limb of an ’E’-shaped ferromagnetic body is excited with an alternating voltage. The displacement to be measured is applied to a ferromagnetic plate in close proximity to the
’E’ piece. Movements of the plate alter the flux paths and hence cause a change in the current flowing in the
V
winding. By Ohm’s law, the current flowing in the winding is I = . For fixed values of ω and V , this equa1
tion becomes I =
, where K is a constant. The relationship between L and the displacement, d, applied
KL
to the plate is a non-linear one, and hence the output-current/displacement characteristic has to be calibrated.
In variable reluctance sensors, a coil is wound on a permanent magnet rather than on an iron core as in
variable inductance sensors. Such devices are commonly used to measure rotational velocities. Figure 3
shows a typical instrument in which a ferromagnetic gearwheel is placed next to the sensor.
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Figure 3: Reluctance sensor
As the tip of each tooth on the gearwheel moves towards and away from the pick-up unit, the changing
magnetic flux in the pick-up coil causes a voltage to be induced in the coil whose magnitude is proportional
to the rate of change of flux. Thus, the output is a sequence of positive and negative pulses whose frequency
is proportional to the rotational velocity of the gearwheel.
Eddy current sensors consist of a probe containing a coil, as shown in Figure 4, that is excited at a high
frequency, which is typically 1 MHz.
Figure 4: Eddy Current sensor
This is used to measure the displacement of the probe relative to a moving metal target. Because of the high
frequency of excitation, eddy currents are induced only in the surface of the target, and negative pulses whose
frequency is proportional to the rotational velocity of the gearwheel and the current magnitude reduces to
almost zero a short distance inside the target.
This allows the sensor to work with very thin targets, such as the steel diaphragm of a pressure sensor. The
eddy currents alter the inductance of the probe coil, and this change can be translated into a d.c. voltage
output that is proportional to the distance between the probe and the target.
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2.3.3
Hall-effect Sensors
Basically, a Hall-effect sensor is a device that is used to measure the magnitude of a magnetic field. It consists
of a conductor carrying a current that is aligned orthogonally with the magnetic field, as shown in Figure 5.
This produces a transverse voltage difference across the device that is directly proportional to the magnetic
field strength. For an excitation current I and magnetic field strength B, the output voltage is given by
V = KIB, where K is known as the Hall constant.
Figure 5: Hall-effect sensor
The conductor in Hall-effect sensors is usually made from a semiconductor material as opposed to a metal,
because a larger voltage output is produced for a magnetic field of a given size. In one common use of the
device as a proximity sensor, the magnetic field is provided by a permanent magnet that is built into the
device. The magnitude of this field changes when the device becomes close to any ferrous metal object or
boundary. The Hall effect is also commonly used in keyboard push buttons, in which a magnet is attached
underneath the button. When the button is depressed, the magnet moves past a Hall-effect sensor. The
induced voltage is then converted by a trigger circuit into a digital output. Such push button switches can
operate at high frequencies without contact bounce.
2.3.4
Piezoelectric Sensors
Piezoelectric transducers produce an output voltage when a force is applied to them. They are frequently
used as ultrasonic receivers and also as displacement transducers, particularly as part of devices measuring
acceleration, force and pressure.
Piezoelectric transducers are made from piezoelectric materials. These have an asymmetrical lattice of
molecules that distorts when a mechanical force is applied to it. This distortion causes a reorientation of
electric charges within the material, resulting in a relative displacement of positive and negative charges. The
charge displacement induces surface charges on the material of opposite polarity between the two sides. By
9
implanting electrodes into the surface of the material, these surface charges can be measured as an output
voltage.
For a rectangular block of material, the induced voltage is given by:
V =
kF d
A
where F is the applied force in g, A is the area of the material in mm, d is the thickness of the material
and k is the piezoelectric constant. The polarity of the induced voltage depends on whether the material is
compressed or stretched.
2.3.5
Optical Sensors
Optical sensors are based on the modulation of light travelling between a light source and a light detector
as shown in Figure 6. The transmitted light can travel along either an air path or a fibre-optic cable. Either
form of transmission gives immunity to electromagnetically induced noise, and also provides greater safety
than electrical sensors when used in hazardous environments. Light sources suitable for transmission across
an air path include tungsten-filament lamps, laser diodes and light-emitting diodes (LEDs).
Figure 6: Optical sensor
As an alternative to using air as the transmission medium, optical sensors can use fibre optic cable instead
to transmit light between a source and a detector. The basis of operation of fibre-optic sensors is the
translation of the physical quantity measured into a change in one or more parameters of a light beam.
The light parameters that can be modulated are one or more of the following: intensity, wavelength, phase,
transmission time e.t.c.
2.3.6
Piezoresistive Sensors
A piezoresistive sensor is made from semiconductor material in which a p-type region has been diffused into
an n-type base. The resistance of this varies greatly when the sensor is compressed or stretched. This is
frequently used as a strain gauge, where it produces a significantly higher gauge factor than that given by
metal wire or foil gauges. It is also used in semiconductor-diaphragm pressure sensors and in semiconductor
accelerometers.
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2.3.7
Strain gauges
Strain gauges are devices that experience a change in resistance when they are stretched or strained. They
are able to detect very small displacements, usually in the range 0-50 , and are typically used as part of other
transducers, for example diaphragm pressure sensors that convert pressure changes into small displacements
of the diaphragm.
2.3.8
Ultrasonic sensors
Ultrasound is a band of frequencies in the range above 20 kHz, that is, above the sonic range that humans can
usually hear. Measurement devices that use ultrasound consist of one device that transmits an ultrasound
wave and another device that receives the wave. Changes in the measured variable are determined either by
measuring the change in time taken for the ultrasound wave to travel between the transmitter and receiver,
or, alternatively, by measuring the change in phase or frequency of the transmitted wave.
2.4
2.4.1
Typical Transducers
Light Transducers
Light transducers are devices which transform the light radiation into an electrical quantity (resistance, current) and can be used in industry as light transducers and also as indirect transducers of other physical
quantities such as position, angular speed and so on.
When incident on a material, the light radiation produces different effects, among which, there is the Photoelectric Effect which consists in the liberation of electrons by electromagnetic radiation incident on a metal
surface and in case of semiconductors, in the generation of electron-hole pairs.
Photoelectric effects on semiconductors can be divided into two kinds and precisely:
• Photoconductive Effect
The conductivity of a semiconductor bar depends on the intensity of the light radiation which strikes
it.Devices belonging to this category are called photoresistors
• Photovoltaic Effect
The current across a reversely biased P-N junction depends on the intensity of the light radiation. If
the junction is not biased, an electromotive force is generated across it (Photovoltaic effect).
Devices belonging to this category are called photodiodes, photoelectric cells and phototransistors.
a) Photoresistors
The photoresistor is a passive semiconductor component without junction. When crossed by light radiation, it varies its resistance as a result of the photoconductive effect: the resistance drops when the light
increases.
In dark conditions, the photoresistor practically acts as an insulating piece, as it has resistance values
measured in M Ω (dark resistance); if strongly illuminated it has very low resistance values measured up
to some tens of Ω
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Figure 7: Photoresistor
b) Photodiode
The photodiode is a device which structure is similar to a common semiconductor diode, with a P-N
junction, and, for this kind of use, it is reversely biased.
In dark conditions, the photodiode operates as a common semiconductor diode, while, when the junction
is crossed by a light radiation, the reverse current increases.
Figure 8: Photodiode
c) Phototransistor
The phototransistor is a device with a structure similar to the one of a standard transistor, but with a
photosensible base. It is generally NPN kind, it is powered with a positive voltage between Collector and
Emitter and the Base can be left open or connected to the emitter with a resistor.
In this second case, the sensitivity of the phototransistor can be adjusted by varying the value of the resistor
used. On dark conditions, the current of the collector Ic is minimum and increases with illumination.
Photo Conductive Cell
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Figure 9: Phototransistor
This works on the principle that the resistance of the photo cell varies with incident light. Typical control
circuit showing the application of a photo cell is shown in Figure 10 below
Figure 10: Photo Conductive Cell
The potentiometer is used to make adjustment to compensate for manufacturing tolerances in photocell
sensitivity and relay operating sensitivity. When light is incident on the photocell, its resistance becomes low
and the current through the relay is consequently high to operate the relay.
When the light is intercepted, the resistance increases reducing the current through the relay. This drop in
current may de-energize the relay. These transducers are used
1. for counting packages moving in a conveyor belt
2. in burglar alarm circuit where the interception of the light activates an alarm circuit. This application
mostly uses infra red light which a burglar cannot see.
2.4.2
Temperature Transducers
Instruments to measure temperature can be divided into separate classes according to the physical principle
on which they operate. The main principles used are:
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• The thermoelectric effect
• Resistance change
• Sensitivity of semiconductor device
• Radiative heat emission
• Thermography
• Thermal expansion
• Resonant frequency change
• Sensitivity of fibre optic devices
• Acoustic thermometry
• Colour change
• Change of state of material.
a) Thermoelectric effect (Thermocouple)
This works on the principle that an emf is generated across the junction of two dissimilar metals or
semiconductors when that junction is heated. Thermocouples are used to measure temperature, heat
flow and radiation. The emf produced in a thermocouple junction is given by
E = a (∆θ) + b (∆θ)2
(1)
where
∆θ = temperature difference between the hot thermocouple and the reference junction of the thermocouple in °C
a, b are constants
a is usually large as compared to b and therefore the emf is approximated as
E ' a (∆θ)
∆θ '
E
a
The thermocouple temperature measuring circuit , the emf setup is measured by sending a current
through a moving coil instrument whereby the deflection is directly proportional to the emf.
Since the emf is a function of the temperature difference ∆θ , the instrument can be calibrated to read
the temperature
Thermocouples are used for measurement of temperatures upto 1400°C. Note that thermocouples are
active transducers unlike resistance transducers and thermistors which are passive transducers
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Figure 11: Thermocouple
Construction of thermocouples
In industrial application the choice of material used to make a thermocouple depends upon the temperature range to be measured, the kind of atmosphere to which the material will be exposed, the output
emf and its stability, mechanical strength and its accuracy required in measurement.
Thermocouple materials are divided into 2 categories
(a) Rare metal types - using platinum, rhodium etc
(b) Base metal type
Several combinations of dissimilar metals make good thermocouples for industrial use. These combinations apart from having linear response and high sensitivity should be physically strong to withstand
high temperature, rapid temperature changes and the effect of corrosive and reducing atmosphere.
The pair of two dissimilar metals in physical contact may be twisted, screwed, clamped or welded
together. The most commonly used method for fabricating is to weld the metals together.
Industrial thermocouples employ protective sheathing surrounding the junction and a portion of the
extension leads. The leads and the junction are internally insulated from the sheath using various
potting compounds, ceramics beads or oxides.
The type of insulation used depends upon the process being monitored. The most common pairs of
metals used for thermocouples are
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+
-
1.Copper
Constantine
temp range
−250°C − 400°C
2. Iron
Constantine
temp range
−200°C − 850°C
3. Chrome
Constantine
temp range
−200°C − 850°C
4. Chrome 90%
Alumel
Nickel 10%
Al, Si and Mn 2%
temp range
−200°C − 1100°C
5. Platinum 90%
Platinum
Rhodium 10%
temp range
0°C − 1400°C
6. Tungsten 95%
Tungsten 75%
Rhenium 5%
rhenium 28%
temp range
0°C − 2600°C
The five standard base-metal thermocouples are chromel-constantan (type E), iron-constantan (type J),
chromel-alumel (type K), nicrosil-nisil (type N) and copper-constantan (type T). These are all relatively
cheap to manufacture but they become inaccurate with age and have a short life. In many applications,
performance is also affected through contamination by the working environment. To overcome this,
The sensitivity of thermocouples can be increased by reducing the mass of the measuring junction.
One method of reducing the mass is to butt-weld the two thermocouple wires. In application where
the mechanical strength of the butt-weld is not sufficient, the two wires are twisted together and the
ends are welded
Measurement of thermocouple output
The output emf of a thermocouple as a result of the difference between temperatures of the reference
junction and the measuring junction can be measured by the following methods
(a) Measuring the output voltage directly with a micrometer since the output is in the order of mV
(b) Measuring the output voltage with the help of a dc potentiometer.
(c) measuring the output voltage after amplifying it
Lead Compensation
In many applications it is desirable to place the reference junction at a point far moved from the
measurement junction. The connecting wires from thermocouple head to the meter are thus very long
and are usually not at the same temperature throughout their length. These causes errors which can
be avoided by using connecting wires made of the same materials as the thermocouple wires.
The implementation of this arrangement may not be possible in many cases due to cost and other
considerations. Under these circumstances, materials are chosen such that the relationship between
emf and temperature is the same or almost the same as that of the thermocouple wires.
These wires are known as compensating leads.
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Advantages of Thermocouples
(a) Are cheaper than resistance thermocouples
(b) They follow temperature changes with a small time lag and as such are suitable for recording
comparatively rapid changes in temperature.
Disadvantages
(a) They have a lower accuracy and hence cannot be used for precision wire
(b) They need to be protected in an open or closed end metal protecting tube to prevent contamination
of the thermocouple from precision metals e.g platinum or its alloys are being used, the protective
tube has to be made chemically inert or vacuum type.
(c) The compensating leads may be very long for a thermocouple placed in a remote place.
EXAMPLE
A thermocouple circuit uses a Chrome - Alumel which gives an emf of 33.3mV when measuring a
temperature of 800o °C with reference temperature of 0o °C. The resistance of the metal coil is Rm =
50Ω and current of 0.1mA gives a full scale deflection. The resistance of the junction and leads Re =
12Ω. Calculate
(a) Resistance of the series resistance if a temperature of 800o °C is to give full scale deflection
(b) The approximate error due to rise of 1Ω in Re
(c) The approximate error due to rise of 10o °C in a copper coil of the meter. The resistance temperature
coefficient of the coil is 0.00426/o °C
SOLUTION
(a)
emf = i (Rm + Re + Rs )
33.3 × 10−3 = 1 × 10−4 (50 + 12 + Rs )
Rs =
33.3 × 10−3
− 62
1 × 10−4
= 271Ω
(b) current in the circuit with increased resistance Re = 13Ω
=
33.3 × 10−3
= 0.0997mA
(50 + 271 + 13)
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approximate error in temperature
=
0.0997 − 0.1
× 800 = −3.4o °C
0.1
(c) change in resistance with a temperature increase of 10o °C is
= 50 × 0.00426 × 10
= 2.13Ω
current in the circuit with increase in resistance of the coil
=
33.3 × 10−3
= 0.09936mA
50 + 2.13 + 271 + 12
therefore approximate error in temperature
=
0.09936 − 0.1
× 800
0.1
= −5.12o °C
b) Resistance Varying Devices
i. Resistance Temperature Devices
Resistance thermometers, which are alternatively known as resistance temperature devices (or
RTDs), rely on the principle that the resistance of a metal varies with temperature according to the
relationship:
R = R0 (1 + a1 T )
This equation is approximately true over a limited temperature range for some metals, notably
platinum, copper and nickel, whose characteristics are summarized in Figure 12. Platinum has the
most linear resistance-temperature characteristic, and it also has good chemical inertness, making
it the preferred type of resistance thermometer in most applications. Its resistance-temperature
relationship is linear within ±0.4% over the temperature range between −200o C and +40o C
Besides having a less linear characteristic, both nickel and copper are inferior to platinum in terms
of their greater susceptibility to oxidation and corrosion. This seriously limits their accuracy and
longevity. However, because platinum is very expensive compared with nickel and copper, the latter
are used in resistance thermometers when cost is important. Another metal, tungsten, is also used
in resistance thermometers in some circumstances, particularly for high temperature measurements.
The working range of each of these four types of resistance thermometer is as shown below:
Platinum-−270o C to +1000o C
18
Copper: −200o C to +260o C
Nickel: −200o C to +430o C
Tungsten: −270o C to +1100o C
The characteristics of these resistance thermometers are as shown in Figure 12
Figure 12: Typical resistance-temperature characteristics of metals
ii. Thermistors Thermistors are manufactured from beads of semiconductor material prepared from
oxides of the iron group of metals such as chromium, cobalt, iron, manganese and nickel. Normally,
thermistors have a negative temperature coefficient, i.e. the resistance decreases as the temperature
increases, according to:
β
R = R0 e
1 1
−
T T0
This relationship is illustrated in Figure 13
However, alternative forms of heavily doped thermistors are now available (at greater cost) that
have a positive temperature coefficient. The form of equation (14.8) is such that it is not possible
to make a linear approximation to the curve over even a small temperature range, and hence the
thermistor is very definitely a non-linear sensor. However, the major advantages of thermistors are
their relatively low cost and their small size. This size advantage means that the time constant
of thermistors operated in sheaths is small, although the size reduction also decreases its heat
dissipation capability and so makes the self heating effect greater. In consequence, thermistors have
to be operated at generally
2.4.3
Force and Weight Transducers
a) Electronic load cell
19
Figure 13: Typical resistance-temperature characteristics of thermistor materials
In an electronic load cell, the gravitational force on the body being measured is applied to an elastic
element. The electronic load cell uses the physical principle that a force applied to an elastic element
produces a measurable deflection. The elastic elements used are specially shaped and designed, some
examples of which are shown in Figure 14
The design aims are to obtain a linear output relationship between the applied force and the measured
deflection and to make the instrument insensitive forces that are not applied directly along the sensing
axis. Load cells exist in both compression and tension forms. In the compression type, the measured mass is placed on top of a platform resting on the load cell, which therefore compresses the cell.
In the alternative tension type, the mass is hung from the load cell, thereby putting the cell into tension.
Electronic load cells have significant advantages over most other forms of mass-measuring instrument
in terms of their relatively low cost, wide measurement range, tolerance of dusty and corrosive environments, remote measurement capability, tolerance of shock loading and ease of installation.
One problem that can affect the performance of load cells is the phenomenon of creep. Creep describes
the permanent deformation that an elastic element undergoes after it has been under load for a period
of time. This can lead to significant measurement errors in the form of a bias on all readings if the
instrument is not recalibrated from time to time. However, careful design and choice of materials can
largely eliminate the problem.
b) Resistance Strain Gauge
This works on the principle that a resistance of a wire of a semiconductor is changed by elongation
or compression due to externally applied stress. It is commonly used in the measurement of force ,
torque and displacement. Consider the block of Figure 15 below (load cell) which is a short column
with resistance wire strain gauge bonded to it.
20
Figure 14: Elastic elements used in load cell
Figure 15: Resistance Strain Gauge
In this case the measurand is a force and is applied in a column therefore producing strain. The force
is first detected by the first column and is converted into strain which is a mechanical displacement.
This strain changes the resistance of the strain gauge. Hence we have an output which is a change in
the value of resistance.The measurement of force is a 2-stage process i.e first conversion of force into
strain and second conversion of strain into a change in electrical resistance.
NB: It is common phenomena that when a metal conductor is stretched or compressed, its resistance
changes on account of the fact that both length and diameter of the conductor change. Also there
21
is a change in the value of resistivity of the conductor when it is strained and this property is called
piezo-resistive effect.
Hence resistance strain gauges are also known as piezo resistive gauges. Apart from load cells, other
meters or gauges which employ strain gauges are torque meters, diaphragm type pressure gauges,
temperature sensors, accelerometer and flow-meters may employ strain gauges as secondary transducers.
Consider a strain gauge made up of a circular wire. The dimensions are length L ,area A and diameter
D by being strained. Let wire resistivity be ρ and the resistance of the strain wire be R. Therefore
R=
ρL
A
(1)
Let a tensile stress S be applied to the wire as shown in Figure 16 below
Figure 16: Tensile stress
Let ∆L , ∆D and ∆R be the changes in length , area , and resistance respectively. Given equation 3
that
R=
ρL
dR
ρ δL
ρL δA L δρ
⇒
=
−
+
A
dS
A δS
A δS
A2 δS
(2)
Dividing equation 4 by equation 3 we have
1 δL
1 δA 1 δρ
1 dR
=
−
+
R dS
L δS
A δS
ρ δS
From equation 5 the per unit change in resistance is due to
(a) per unit change in length ∆L
L
∆ρ
(b) per unit change in resistivity ρ
(c) per unit change in area ∆A
A
A=
It follows that
22
πD2
4
(3)
2πD δD
δA
=
δS
4 δS
Hence
2π D
1 dA
δD
2 δD
4
= =
π D2 δS
A dS
D δS
4
(4)
Hence equation 5 can be rewritten as
1 dR
1 δL
2 δD 1 δρ
=
−
+
R dS
L δS
D δS
ρ δS
(5)
Introducing Poisson ratio
v=
−δD
Lateral strain
= D
δL
Longitudinal strain
L
(6)
Equation 7 for small variation can be written as
∆R
∆L
∆L ∆ρ
=
+ 2v
+
R
L
L
ρ
(7)
The gauge factor is defined as the ratio of per unit change in resistance to per unit change in length.
Gauge factor (Gf )
∆R
Gf = R
∆L
L
OR
(8)
∆R
∆L
= Gf
= Gf R
L
Where
= strain =
∆L
L
The Gf can be written as
∆ρ
ρ
Gf = 1 + 2v +
∆ρ
ρ
= 1 + 2v +
∆L
L
(9)
Note that if the change in the value of resistivity of a material due to strain is neglected the gauge
factor is
Gf = 1 + 2v
The Poisson ratio for the metal is between 0 to 0.5. This gives a gauge factor of approximately 2. The
most common value of Poisson ratio for wires is 0.3. This gives a value of 1.6 for wire wound strain
gauges.
23
EXAMPLE
A strain gauge is bonded to a beam 0.1m long and has a cross section area 4cm2 . Young modulus
for steel is 207GN/m2 . The strain gauge has unstrained resistance of 240Ω and a gauge factor of 2.2.
When a load is applied the resistance of the gauge changes by 0.013Ω. Calculate the change in length
of the steel beam and the amount of force applied to the beam.
SOLUTION
∆R
∆R
Gf = R ⇒ ∆L = R L
∆L
Gf
L
0.013
= 240 × 0.1
2.2
= 2.46 × 10−6 m
stress S = E =
207 × 109 × 2.46 × 10−6
0.1
= 5.092 × 106 N/m2
F = SA = 5.092 × 106 × 4.0 × 10−4
= 2.037 × 103 N
The resistance change in strain gauges is small and requires the use of a bridge circuit for measurement,
as shown in Figure 17. The strain gauge elements are mounted in two arms of the bridge, and two
resistors, R1 and R2 , form the other two arms. The output signal from the bridge is amplified and
impedance matched.
c) Piezo Electric Transducers
These transducers use the principle that an emf is generated when an external force is applied to certain
crystalline material such as quartz.
A piezo - electric material is one in which an electric potential appears across certain surfaces of a
crystal if the dimensions of the crystal are changed by application of a mechanical force.
This potential is produced by the displacement of charge (electric charge). This effect is reversible i.e
if a varying potential is applied to the proper axis of the crystal, it will change the dimensions of the
crystal thereby deforming it.
This effect is known as piezo-electric effect
Common piezo electric materials include;
24
Figure 17: Strain gauge bridge circuit
Rochelle salt, Ammonium dihydrogen phosphate, Lithium sulphate, Quartz, Ceramic dipotassium tartarate and potassium dihydrogen phosphate. Except quartz and ceramics the rest are man made
crystals.
The ceramic materials are polycrystalline in nature and basically are made of barium titanate.
Materials exhibit significant and useful piezo electric effect are divided into two categories
(a) Natural group e.g quartz and Rochelle salt
(b) Synthetic group e.g Lithium phosphate and ethylene diamine tartarate
Figure 18: Piezoelectric crystal measuring acceleration
A piezo electric element used for converting mechanical motion to electrical signals may be thought
of as a charge generator and a capacitor. Mechanical deformation generates a charge and the charge
appears as a voltage across the electrodes. The voltage is given by
Eo =
ϕ
C
The piezo electric effect is direction sensitive.
A tensile force produces a voltage of one polarity while a compressive force produces a voltage of
opposite polarity.
25
The magnitude and polarity of the induced surface charges are proportional to the magnitude and
direction of the applied force F . The polarity of the induced charge depends upon the direction of the
applied force
ϕ = d ∗ F coulombs
(10)
where d=charge sensitivity of the crystal in coulombs/newton and is constant for a given crystal
F = applied force in Newtons
The force F causes a change in thickness of the crystal and
F =
AE
∆t N
t
(11)
Where
A = Area of crystal in m2
t = thickness of the crystal in m
E = young modulus of elasticity in N/m2
Therefore
E=
Stress
=
Strain
E=
F
A
×
1
∆t
t
Ft
N/m2
A∆t
(12)
but A = wl Where w is the width of the crystal in meters and l is the length of the crystal in meters
ϕ = dEA
∆t
t
(13)
The charge at the electrode give rise to an output voltage
Eo =
ϕ
Cp
(14)
where Cp is the capacitance between the electrodes in Farads
C p = 0 r
Eo =
A
t
(15)
ϕ
dF
dt F
=
=
A
Cp
0 r A
0 r t
(16)
d
tp = gtp
0 r
(17)
2
BUT F
A = p pressure or stress in N/m
Thus
Eo =
where
g=
26
d
0 r
(18)
g is the voltage sensitivity of the crystal. This is a constant for a given crystal circuit. Its units are
V − m/N
Now
Eo
Eo
= t
tp
p
(19)
Electric f ield
E
=
stress
p
(20)
d = 0 r g C/N
(21)
g=
But Eto = Electric field strength in V /m
g=
EXAMPLE 1
A quartz piezo electric crystal has a thickness of 2mm and a voltage sensitivity of 0.055V − m/N . It
is subjected to a pressure of 1.5M N/m2 . Calculate the voltage output.
If the permittivity of quartz is 40.6 × 10−12 F/m , calculate the charge sensitivity
SOLUTION
Eo = gtp
= 0.055 × 2 × 10−3 × 1.5 × 106
= 165V
charge sensitivity d
d == 0 r g
= 40.6 × 10−12 × 0.055
= 2.23 × 10−12 C/N
EXAMPLE 2
A Barium titanic pick up has the dimensions of 5 mm by 5 mm by 1.25 mm. The force acting on it
is 5 N , its charge sensitivity is 150 pC/N and its permittivity is 12.5 × 10−9 F/m . If the modulus of
elasticity of Barium titanic is 12 × 106 N/m2 , calculate the strain , the charge and the capacitance
SOLUTION
Area of plate A
A = 5 × 5 × 10−6 = 25 × 10−6
27
Pressure p
p=
5
= 0.2 × 106 N/m2
25 × 10−6
Voltage sensitivity g
g=
d
150 × 10−12
=
0 r
12.5 × 10−9
= 12 × 10−3 V m/N
Voltage generated Eo
Eo = gtp = 12 × 10−3 × 1.25 × 10−3 × 0.2 × 106
= 3V
Strain =
=
stress
∆t
=
t
young modulus
0.2 × 106
= 0.0167
12 × 106
charge ϕ
ϕ = dF = 150 × 10−12 × 5
= 750pC
Capacitance Cp
Cp =
ϕ
750 × 10−12
=
Eo
3
= 250pF
Charge produced q = kq xi coulombs
kq = Sensitivity c/m
xi = displacement m
RP = leakage resistance of transducer Ω
Cp = capacitance of transducer F
Cc = capacitance of cable F
CA = capacitance of amplifier F
RA = resistance of amplifier Ω
The charge generator in second figure is converted into a constant current generator as in the third
figure
28
Figure 19: Piezoelectric transducer set-up
Figure 20: Piezoelectric transducer equivalent circuit
Figure 21: Reduced equivalent circuit
The total capacitance across the current generator C is given by
C = Cp + Cc + CA
R=
RA Rp
RA + Rp
29
(22)
(23)
Since the leakage resistance of the transducer is very large of the order 1 × 1011 Ω ,
R ' RA
Converting this charge generator into a constant current generator we have
iCR =
= kq
dq
dt
dxi
dt
(24)
where iCR is the current of constant current generator
iCR = iC + iR
output voltage at the load eL
1
eL = eC =
C
OR
Z
1
iC dt =
C
Z
iCR − iR dt
deL
1
=
iCR − iR
dt
C
OR
C
deL
e
dx
= iCR − iR = kq i − L
dt
dt
R
RC
But RC = τ
τ
deL
dx
+ eL = kq R i
dt
dt
(25)
deL
dx
+ eL = kτ i
dt
dt
(26)
kq
where k = sensitivity constant = C V /m
Taking Laplace transform of equation 29 we have
(τ s + 1) EL (s) = kτ sXi (s)
(27)
EL (s)
kτ s
=
Xi (s)
τs + 1
(28)
Transfer function
For sinusoid function the transfer function is
EL (jω)
jωkτ
=
Xi (jω)
1 + jωτ
The amplitude ratio is
m=
EL
(jω) = q
Xi
30
ωτ
1 + (ωτ )2
1
=r
1+ 1 2
(ωτ )
The phase shift
φ=
π
− tan−1 ωτ rad
2
(29)
(30)
At very high frequencies ω 1 , m = 1 and φ = 0
Hence high frequency sensitivity
e
k= L
xi
(31)
From equation 33 it is evident that the steady state response of a piezoelectric transducer to a constant displacement xi is zero. Therefore piezoelectric transducers cannot be used to measure static
displacement.
EXAMPLE
A piezoelectric transducer has a capacitance of 1000pF and a charge sensitivity of 40 × 10−3 C/m. The
connecting cable has a capacitance of 300pF , while the ocr used for read out has a resistance of 1M Ω
with a parallel capacitance of 50pF .
(a) What is the sensitivity in V/m of the transducer alone?
(b) What is the high frequency sensitivity in V/m of the entire measuring system?
(c) What is the lowest frequency that can be measured with 5% amplitude error by the entire system?
(d) What is the value of an external shunt capacitance that can be connected in order to extend the
range of 5% error down to 10Hz
(e) With external capacitance calculated in (b) above connected in the circuit what is the system high
frequency sensitivity?
SOLUTION
(a)
kq = 40 × 10−3 C/m
Cp = 1000 × 10−12 F
k=
kq
40 × 10−3
=
Cp
1000 × 10−12
= 40 × 106 V /m
31
(b)
C = Cp + Cc + CA
= 1000 + 300 + 50 = 1350pF
k=
kq
40 × 10−3
=
C
1350 × 10−12
= 29.63 × 106 V /m
(c)
τ = RC = 1 × 106 × 1350 × 10−12 = 1.35ms
m = 0.95
1
0.95 = r
1+ 1 2
(ωτ )
ωτ = 3.04
ω=
3.04
= 2254rad/s
1.35 × 10−3
2πf = 2254
f=
2254
= 358.7Hz
2π
(d)
τ=
3.04
3.04
=
= 48.38ms
ω
2π × 10
Total capacitance
C=
τ
48.38 × 10−3
=
R
1 × 106
= 48.380nF
External capacitance
Ce = 48380pF − 1350pF
= 47020pF
32
(e) High frequency sensitivity with Ce
=
40 × 10−3
= 827kV /m
48380 × 10−12
Uses of Piezoelectric Transducers
(a) Used for stabilizing electronic oscillators
(b) Used for the measurement of surface roughness and in accelerometers and vibration pick ups
(c) Used in under water detection systems known as sonar and ultrasonic generation element
(d) Used in measurement of force in rolling miles
Advantages of Piezoelectric Transducers
(a) These transducers need no external power and is therefore self generating (active transducers)
(b) It has a very good high frequency response
Disadvantages of Piezoelectric Transducers
(a) Cannot measure static force or displacement
(b) The output of the transducer is affected by changes in temperature. Hence temperature compensating devices have to be used.
2.4.4
Displacement, Velocity and Acceleration Transducers
a) Potentiometer
This works on the principle that positioning of a slider by an external force varies the resistance in a
potentiometer.
It is used to measure electrical pressure or voltage or mechanical displacement. The schematic diagram
of a potentiometer (POT) is as shown in Figure 22 below.
Figure 22: Potentiometer
33
Let L be the total length of the potentiometer and Rt be its total resistance. The input displacement
is xi . The output voltage eo is given by
x
eo = i ei
L
(1)
eo
L
ei
(2)
and
xi =
b) Linear Variable Differential Transformer (LVDT)
This is the most widely used inductive transducer to translate the linear motion electrical signals.
LVDT works on the transformer principle that an emf is induced in a coil whenever the magnetic force
linking that coil changes. It is used to measure pressure , force and displacement (position)
Figure 23: Normal circuit of LVDT
Figure 24: Differential circuit of LVDT
A transformer consists of a single primary winding (p) and two secondary windings s1 and s2 wound
on a cylindrical former. The secondary windings have equal number of turns and are identically placed
34
at either side of the primary winding. The primary winding is connected to an ac source. A movable
soft iron core is placed inside the transformer. The displacement to be measured is applied to the arm
attached to the soft iron core. In practice the core is made up of high permeability nickel iron which is
annealed. This means low harmonics, low null voltage and a high sensitivity. The assembly is placed
in a stainless steel housing and the end lids provide electrostatic and electromagnetic shielding. The
frequency of ac provided to the primary winding maybe between 50Hz - 20kHz. Since the primary
winding is exited by an alternating current source it provides alternating magnetic field which in turn
induces alternating current in the two secondary windings. The output voltage of s1 is Es1 and that of
s2 is Es2 .In order to convert the outputs from s1 and s2 into a single voltage signal, the two secondary
are connected in series opposition.
Then the output of the transducer is the difference between the two voltages
The differential output voltage
Eo = Es1 − Es2
(3)
When the core is at its normal position (null position) the flux linking both the secondary winding is
equal and hence equal emfs are induced in them.Thus at Null position
Es1 = Es2
Now if the core is moved to the left of the null position, more flux links with the winding s1 and less
with the winding s2
Accordingly output voltage Es1 is more than Es2 . This magnitude of output voltage is thus
Eo = Es1 − Es2
and the output is in phase with say, the primary voltage.
Similarly if the core is moved to the right of the null position, the flux linking the winding s2 becomes
larger than that linking s1 . These results in Es2 becoming larger than Es1 . The output voltage in
these case is Eo = Es1 − Es2 and is 180° out of phase with the primary voltage.
The two secondary voltages are πrad out of phase with each other. This amount of voltage change in
either secondary winding is proportional to the amount of movement of the core. Hence we have the
indication of the amount of linear motion.
By noting which output voltage is increasing or decreasing, we can determine the direction of motion .
In other words any physical displacement in the position of the core causes the voltage of one secondary
winding to increase while simultaneously reducing the voltage in the other secondary winding.
The difference of the voltages appear across the output terminals of the transducer and gives a measure
of the physical position of the core and hence the displacement. The amount of output voltage may
be measured to determine the displacement. The output signal may also be applied to a recorder or a
controller that can restore the moving system to its normal position.
Advantages of LVDTs
(a) Have high range for measurement of displacement . This can be used to measure displacement of
the order 1.25mm - 250mm
35
(b) It is a frictionless device and hence no mechanical wear. LVDT has infinite mechanical life. This
feature is vital in high reliability mechanisms and systems found in aircraft , missile and space
vehicles and critical industrial equipment
(c) Have high sensitivity which is typically above 40V/mm
(d) Ruggedness i.e LVDT can tolerate high degree of shock and vibrations especially when the core is
spring induced without any adverse effects.
(e) It has low hysterisis and hence repeatability is excellent under all operating conditions
(f) It is light in weight , stable, easy to align and maintain
(g) It has low power consumption of less than 1W
Disadvantages of LVDTs
(a) Relatively large displacement is required for appreciable differential output
(b) LVDTs are sensitive to stray magnetic field but shielding is possible
(c) They are affected by strong mechanical vibrations
(d) They can only operate on ac signal no dc equivalent
(e) The dynamic response of these transducers is limited mechanically by the mass of the core and
electrically by the frequency of the applied voltage
(f) Temperature affects the performance of the transducer and hence temperature compensating schemes
need to be employed
c) Electrical Tachometer (Tachogenerator)
This uses the principle that motion of a coil in a magnetic field generates a voltage. This is accomplished
through the use of electromagnetic induction.There are two types of electromagnetic tachometers
(a) Dc tachometer
(b) Ac tachometer
Dc tachometer consists of a small armature which is coupled with the machine whose speed is to be
measured. This armature revolves in the field of permanent magnet.The emf generated is proportional
to the product of speed and flux. Since the flux of a permanent magnet is constant, the voltage
generated is proportional to speed.
The polarity of output voltage indicates the direction of resistance. The emf is measured with the help
of a moving coil voltmeter having a uniform scale and calibrated directly in terms of speed.
Ac tachometer have a rotating magnet which may either be permanent or electro magnet. The coil is
wound on the stator and therefore the problems associated with commutators as in dc tachometers are
absent. The rotation of the magnet causes an emf to be induced in the stator coil. The amplitude and
frequency of emf are both proportional to the speed of rotation.
Thus either amplitude or frequency of the induced voltage may be used to measure the rotational speed.
The output voltage of ac tachometer generator is rectified and is measured with a permanent magnet
moving coil instrument (PMMC)
36
2.4.5
Level Measurement
1. Dipstick
Dipsticks offer a simple means of measuring level approximately. The ordinary dipstick is the cheapest
device available. This consists of a metal bar on which a scale is etched. The bar is fixed at a known
position in the liquid-containing vessel. A level measurement is made by removing the instrument from
the vessel and reading of how far up the scale the liquid has wetted. As a human operator is required
to remove and read the dipstick, this method can only be used in relatively small and shallow vessels.
2. Float Systems
Float systems, whereby the position of a float on the surface of a liquid is measured by means of
a suitable transducer, have a typical measurement inaccuracy of ±1%. This method is also simple,
cheap and widely used. The system using a potentiometer is very common, and is well known for its
application to monitoring the level in motor vehicle fuel tanks.
3. Pressure measuring devices(hydrostatic system) The hydrostatic pressure due to a liquid is directly
proportional to its depth and hence to the level of its surface. Several instruments are available that use
this principle, and they are widely used in many industries, particularly in harsh chemical environments.
In the case of open-topped vessels (or covered ones that are vented to the atmosphere), the level can
be measured by inserting a pressure sensor at the bottom of the vessel, as shown in Figure 25
Figure 25: Pressure measuring device
P
,whereρ is the liquid
ρg
density and g is the acceleration due to gravity. One source of error in this method can be imprecise
The liquid level h is then related to the measured pressure P according to h =
knowledge of the liquid density. This can be a particular problem in the case of liquid solutions and
mixtures and in some cases only an estimate of density is available.
Even with single liquids, the density is subject to variation with temperature, and therefore temperature
measurement may be required if very accurate level measurements are needed.
4. Capacitive devices Capacitive devices are widely used for measuring the level of both liquids and solids
in powdered or granular form. They perform well in many applications, but become inaccurate if the
measured substance is prone to contamination by agents that change the dielectric constant. Ingress of
moisture into powders is one such example of this. They are also suitable for use in extreme conditions
37
measuring liquid metals (high temperatures), liquid gases (low temperatures), corrosive liquids (acids,
e.t.c.) and high pressure processes.
For non-conducting substances two bare-metal capacitor plates in the form of concentric cylinders are
immersed in the substance, as shown in Figure 26
Figure 26: Capacitive level sensor
The substance behaves as a dielectric between the plates according to the depth of the substance. For
concentric cylinder plates of radius a and b (b>a), and total height L, the depth of the substance h is
related to the measured capacitance C by:
b
− 2πεo
Cloge
a
h=
2πεo (ε − 1)
where ε is the relative permittivity of the measured substance and εo is the permittivity of free space.
5. Ultrasonic level gauge The principle of the ultrasonic level gauge is that energy from an ultrasonic
source above the liquid is reflected back from the liquid surface into an ultrasonic energy detector.
Measurement of the time of flight allows the liquid level to be inferred. In alternative versions, the
ultrasonic source is placed at the bottom of the vessel containing the liquid, and the time of flight
between emission, reflection off the liquid surface and detection back at the bottom of the vessel is
measured.
38
2.5
Classification of transducers
Transducers can be classified as follows
1. on the basis of transduction form used
2. As primary and secondary transducers
3. As active and passive transducers
4. As analogue and digital transducers
5. As transducers and inverse transducers
2.6
Factors influencing the choice of transducers
These factors are
1. Operating principle
2. Sensitivity
3. Operating range
4. Accuracy
5. Cross sensitivity
6. Errors
7. Transient and frequency response
8. Stability and Reliability
9. Insensitivity to unwanted signal
10. Usage and ruggedness
39
3
Signal Processing
Signal processing is concerned with improving the quality of the reading or signal at the output of a measurement system, and one particular aim is to attenuate any noise in the measurement signal that has not been
eliminated by careful design of the measurement system as discussed above. However, signal processing performs many other functions apart from dealing with noise, and the exact procedures that are applied depend
on the nature of the raw output signal from a measurement transducer. Procedures of signal filtering, signal
amplification, signal attenuation, signal linearization and bias removal are applied according to the form of
correction required in the raw signal.
Traditionally, signal processing has been carried out by analogue techniques in the past, using various types
of electronic circuit. However, the ready availability of digital computers in recent years has meant that
signal processing has increasingly been carried out digitally, using software modules to condition the input
measurement data.
Digital signal processing is inherently more accurate than analogue techniques, but this advantage is greatly
reduced in the case of measurements coming from analogue sensors and transducers, because an analogueto-digital conversion stage is necessary before the digital processing can be applied, thereby introducing
conversion errors. Also, analogue processing remains the faster of the two alternatives in spite of recent
advances in the speed of digital signal processing.
3.1
3.1.1
Analogue Signal Processing
Analogue Filtering
Signal filtering consists of processing a signal to remove a certain band of frequencies within it. The band of
frequencies removed can be either at the low-frequency end of the frequency spectrum, at the high-frequency
end, at both ends, or in the middle of the spectrum. Filters to perform each of these operations are known
respectively as low-pass filters, high-pass filters, band-pass filters and band-stop filters.
Analogue filters exist in two forms, passive and active form. The very simplest passive filters are circuits that
consist only of resistors and capacitors.
The term active filters comes from the active (amplifying) circuit elements that can generate signal energy.
Active filters are implemented using the operational amplifier.
The most simple low pass filter is the RC low pass network shown below.
40
1
V2
RC
H (s) =
=
Vin
s + RC
1
RC
H (jω) =
jω +
1
RC
1
RC
H (jω) = s
1 2
2
ω +
RC
ω=
1
,
RC
ω=∞,
ω=0,
1
H (jω) = √
2
H (jω) → 0
Low pass active filter
1
R2
sC
Zα =
=
1
sR2 C + 1
R2 +
sC
R2 ∗
41
H (jω) = 1
R2
V0
R1
=−
Vin
sR2 C + 1
R2
C
R1 R2
=−
1
s+
R2 C
1
C
R1
=−
1
s+
R2 C
Highpass passive filter
V0
R
sRC
=
=
1
Vin
sRC + 1
+R
sC
s
H (s) =
s+
H (jω) =
1
RC
jω
jω +
1
RC
H (jω) = s
ω2 +
ω
1 2
RC
when
ω=0,
ω=
H (jω) = 0
1
,
RC
ω→0,
1
H (jω) = √
2
H (jω) → 1
High pass active filter
First order inverting High pass filter
42
Vin
1
R1 +
sC
V
=− 0
R2
−sR2 C
V2
=
V1
sR1 C + 1
R
−s 2
R1
=
1
s+
R1 C
3.1.2
Amplification
Signal amplification is carried out when the typical signal output level of a measurement transducer is
considered to be too low. Amplification by analogue means is carried out by an operational amplifier.
R
Vo = − 2 Vi
R1
The amount of signal amplification is therefore defined by the relative values of R2 and and R1 . This ratio
between R2 and R1 is known as the amplifier gain. The sign of the processed signal is inverted. This can be
corrected for if necessary by feeding the signal through a further amplifier set up for unity gain (R1 = R2 ).
This inverts the signal again and returns it to its original sign.
43
Figure 27: Signal amplifier
Instrumentation amplifier
For some applications requiring the amplification of very low level signals, a special type of amplifier known
as an instrumentation amplifier is used. This consists of a circuit containing three standard operational
amplifiers,
Figure 28: Instrumentation amplifier
The advantage of the instrumentation amplifier compared with a standard operational amplifier is that its
differential input impedance is much higher. In consequence, its common mode rejection capability is much
better.
3.1.3
Signal attenuation
One method of attenuating signals by analogue means is to use a potentiometer connected in a voltagedividing circuit.
For the potentiometer slider positioned a distance of x along the resistance element of total length L, the
44
Figure 29: Signal attenuator
voltage level of the processed signal Vo is related to the voltage level of the raw signal Vi by the expression
Vo =
x
V
L i
The amplifier network of figure 27 can also be used with R2 being smaller than R1
3.1.4
Differential amplification
The amplifier configuration that is used to amplify the small difference that may exist between two voltage
signals VA and VB
The output voltage Vo is given by
Vo =
R3
VB − VA
R1
With all the resistors in the circuit equal, the output becomes
Vo = VB − VA
which is the difference between the two inputs
3.1.5
Signal addition
The most common mechanism for summing two or more input signals is the use of an operational amplifier
connected in signal-inversion mode, as shown in Figure 31. For input signal voltages V1 , V2 and V3 the
output voltage Vo is given by:
Vo = −(V1 + V2 + V3 )
45
Figure 30: Differencing amplifier
Figure 31: Summing amplifier
3.1.6
Signal integration
Connected in the configuration shown in Figure 32, an operational amplifier is able to integrate the input
signal Vi such that the output signal V0 is given by:
1
V0 = −
RC
3.1.7
Z
Vi dt
Signal differentiation
Signal differentiation is achieved by interchanging the resistor and capacitor in the integrator network of
Figure 32. The output becomes
Vo = −RC
46
d
V
dt i
Figure 32: Integrator
3.1.8
Signal linearisation
Several types of transducer used in measuring instruments have an output that is a non-linear function of the
measured quantity input. In many cases, this non-linear signal can be converted to a linear one by special
operational amplifier configurations that have an equal and opposite non-linear relationship between the
amplifier input and output terminals. For example, light intensity transducers typically have an exponential
relationship between the output signal and the input light intensity, i.e.:
V0 = Ke−αQ
where Q is the light intensity, V0 is the voltage level of the output signal, and K and α are constants. If a
diode is placed in the feedback path between the input and output terminals of the amplifier as shown in
Figure 33, the relationship between the amplifier output voltage V0 and input voltage V1 is given by:
V0 = C loge (V1 )
Figure 33: Operational amplifier connected for signal linearisation
If the output of the light transducer is conditioned by an amplifier of characteristic given by equation above,
47
the voltage level of the processed signal is given by
V0 = C loge (K) − αCQ
This shows that the output signal now varies linearly with light intensity Q but with an offset of C loge (K).
This offset would normally be removed by further signal conditioning, as described below.
3.1.9
Bias (zero drift) removal
Sometimes, either because of the nature of the measurement transducer itself, or as a result of other signal
conditioning operations, a bias (zero drift) exists in the output signal. This can be expressed mathematically
for a physical quantity x and measurement signal y as:
y = Kx + C
where C represents a bias in the output signal that needs to be removed by signal processing. The bias
removal circuit shown in Figure 34 is a differential amplifier in which a potentiometer is used to produce
a variable voltage Vp equal to the bias on the input voltage Vi . The differential amplification action thus
removes the bias.
Figure 34: Bias removal circuit
Referring to the circuit, for R1 = R2 and R3 = R4 the output V0 is given by:
V0 =
R3
Vp − Vi
R1
where Vi is the unprocessed signal y equal to (Kx − C) and Vp is the output voltage from a potentiometer
supplied by a known reference voltage Vref that is set such that Vp = C. Now, substituting these values for
Vi and Vp into the above equation and referring the quantities back into equation original equation gives:
0
y=K x
0
0
R
where K is related to K by K = − 3 K. It is clear that a straight line relationship now exists between
R1
48
the measurement signal y and the measured quantity x. Thus, the unwanted bias has been removed.
3.1.10
Voltage follower
The voltage follower, also known as a pre-amplifier, is a unity gain amplifier circuit with a short circuit in
the feedback path, as shown in Figure 35, such that:
V0 = Vi
Figure 35: Voltage follower circuit
It has a very high input impedance and its main application is to reduce the load on the measured system.
It also has a very low output impedance that is very useful in some impedance-matching applications.
3.2
Digital Signal Processing
Digital techniques achieve much greater levels of accuracy in signal processing than equivalent analogue
methods. Whilst digital signal processing elements in a measurement system can exist as separate units, it is
more usual to find them as an integral part of an intelligent instrument. However, the construction and mode
of operation of such processing elements are the same irrespective of whether they are part of an intelligent
instrument of not. The hardware aspect of a digital signal-processing element consists of a digital computer
and analogue interface boards.
3.2.1
Sampling
A sampler is basically a switch that closes every T seconds as shown in Figure 36
Figure 36: Sampler
When a continuous signal r(t) is sampled at regular intervals T, the resulting discrete signal is as shown in
Figure 37 (b) where q is the amount of time that the switch is closed.
The switch closure time q is much smaller than the sampling time T and can be neglected.
A sample and hold circuit is normally an essential element at the interface between an analogue sensor
or transducer and an analogue-to-digital converter. It holds the input signal at a constant level whilst
49
Figure 37: Sampled signal
the analogue-to-digital conversion process is taking place. This prevents the conversion errors that would
probably result if variations in the measured signal were allowed to pass through to the converter. The
operational amplifier circuit shown in Figure 38 provides this sample and hold function.
Figure 38: Operational amplifier as a sample and hold circuit
The input signal is applied to the circuit for a very short time duration with switch S1 closed and S2 open,
after which S1 is opened and the signal level is then held until, when the next sample is required, the circuit
is reset by closing S2
3.2.2
Analog to digital conversion
Important factors in the design of an analogue-to-digital converter are the speed of conversion and the number
of digital bits used to represent the analogue signal level. The minimum number of bits used in analogueto-digital converters is eight. The use of eight bits means that the analogue signal can be represented to
a resolution of 1 part in 256 if the input signal is carefully scaled to make full use of the converter range.
However, it is more common to use either 10 bit or 12 bit analogue-to-digital converters, which give resolutions
respectively of 1 part in 1024 and 1 part in 4096. Several types of analogue-to-digital converter exist. These
differ in the technique used to effect signal conversion, in operational speed, and in cost.
50
3.2.3
Digital to analog conversion
Digital-to-analogue conversion is much simpler to achieve than analogue-to-digital conversion and the cost
of building the necessary hardware circuit is considerably less. It is required wherever a digitally processed
signal has to be presented to an analogue control actuator or an analogue signal display device.
3.2.4
Digital filtering
Digital signal processing can perform all of the filtering functions mentioned earlier in respect of analogue
filters, i.e. low pass, high pass, band pass and band stop. However, the detailed design of digital filters
requires a level of theoretical knowledge, including the use of z-transform theory, which is outside the scope
of this course.
3.2.5
Autocorrelation
Autocorrelation is a special digital signal processing technique that has the ability to extract a measurement
signal when it is completely swamped by noise, i.e. when the noise amplitude is larger than the signal
amplitude. Unfortunately, phase information in the measurement signal is lost during the autocorrelation
process, but the amplitude and frequency can be extracted accurately.
3.3
Bridge circuits
Bridge circuits are used very commonly as a variable conversion element in measurement systems and produce
an output in the form of a voltage level that changes as the measured physical quantity changes. They
provide an accurate method of measuring resistance, inductance and capacitance values, and enable the
detection of very small changes in these quantities about a nominal value. They are of immense importance
in measurement system technology because so many transducers measuring physical quantities have an output
that is expressed as a change in resistance, inductance or capacitance. The displacement-measuring strain
gauge, which has a varying resistance
3.3.1
Null type (DC) bridges
A null-type bridge with d.c. excitation, commonly known as a Wheatstone bridge, has the form shown in
Figure 39. The four arms of the bridge consist of the unknown resistance Ru , two equal value resistors R2
and R3 and a variable resistor Rv (usually a decade resistance box). A d.c. voltage V is applied across the
points AC and the resistance Ri is varied until the voltage measured across points BD is zero. This null
point is usually measured with a high sensitivity galvanometer.
Ru =
R3
× Rv
R2
Thus, if R2 = R3 ,then Ru = Rv . As Rv is an accurately known value because it is derived from a variable
decade resistance box, this means that Ru is also accurately known.
51
Figure 39: Null bridge
3.3.2
Deflection type DC bridge
A deflection-type bridge with d.c. excitation is shown in Figure 40. This differs from the Wheatstone bridge
mainly in that the variable resistance Rv is replaced by a fixed resistance R1 of the same value as the nominal
value of the unknown resistance Ru . As the resistance Ru changes, so the output voltage V0 varies, and this
relationship between V0 and Ru must be calculated.
Figure 40: Deflection type bridge
52
V0 =
3.3.3
R1
Ru
−
Ru + R3
R1 + R2
Vi
AC bridges
AC bridges are used for not only measurement of resistances but also for measurement of capacitance and
inductance.
Maxwell’s Inductance Bridge
Figure 41: Inductance Comparison Bridge
By this bridge,(Figure 41), unknown inductance Lx and its internal resistance Rx can be determined as
follows
R R
Rx = 2 3
R1
Lx =
R2 L3
R1
Maxwell’s Inductance Capacitance Bridge
Using the bridge of Figure 42, we can measure inductance by comparing with a variable standard capacitor
Rx =
R2 R3
R1
Lx = R2 R3 C1
Anderson’s Bridge
It is a modified Maxwell’s bridge in which also the value of self inductance is obtained by comparing it with
a standard capacitor. It is basically used for precise measurement of inductance over a wide range of value.
The bridge is shown in Figure 43
To find balance equations, we transform the star formed by R2 , R4 and r into its equivalent delta as shown
in Figure 44
53
Figure 42: Maxwell’s Inductance Capacitance Bridge
Figure 43: Anderson’s Bridge
The elements in the delta equivalent are given by
R5 =
R2 r + R4 r + R2 R4
R4
R6 =
R2 r + R4 r + R2 R4
R2
R7 =
R2 r + R4 r + R2 R4
r
R7 shunts the source and therefore does not affect the balance equation. The values of R1 and Lx are
obtained as
Lx = CR3 R5
R
R1 = R3 5
R6
54
Figure 44: Transformed Anderson’s Bridge
Substituting for values of R5 and R6 yields
Lx =
CR3
[R2 r + R4 r + R2 R4 ]
R4
R1 =
R2 R3
R4
Hay Bridge
Hay bridge is similar to the Maxwell’s inductance capacitance bridge only that C1 is in series with R1 . The
bridge is shown in Figure 45
Figure 45: Hay Bridge
At balance condition, the values of Rx and Lx are obtained as
Rx =
ω 2 C12 R1 R2 R3
1 + ω 2 C12 R12
55
Lx =
R2 R3 C1
1 + ω 2 C12 R12
De Sauty Bridge
It is used measure unknown capacitance by comparing it with a known capacitance. The bridge is shown in
Figure 46
Figure 46: De Sauty Bridge
At balance condition, the values of Rx and Cx are obtained as
Rx =
R2 R3
R1
R
Cx = C3 1
R2
Schering Bridge
It is used extensively for measurement of capacitance. The bridge is shown in Figure 47
Figure 47: Schering Bridge
56
At balance condition, the values of Rx and Cx are obtained as
C
Rx = R2 1
C3
R
Cx = C3 1
R2
Wien Bridge
Used to measure the frequency of a voltage source using series RC network in one arm and parallel RC in
the adjoining arm. The bridge is shown in Figure 48
Figure 48: Wien Bridge
At balance condition, the frequency is obtained as
f=
2π
p
1
C1 C3 R1 R3
In most cases R1 = R3 and C1 = C3 . The frequency is given by
f=
1
2πRC
57
4
Noise in Instrumentation and Measurement Systems
Errors are often created in measurement systems when electrical signals from measurement sensors and transducers are corrupted by induced noise. This induced noise arises both within the measurement circuit itself
and also during the transmission of measurement signals to remote points. The aim when designing measurement systems is always to reduce such induced noise voltage levels as far as possible. However, it is usually not
possible to eliminate all such noise, and signal processing has to be applied to deal with any noise that remains.
Noise voltages can exist either in serial mode or common mode forms. Serial mode noise voltages act in series
with the output voltage from a measurement sensor or transducer, which can cause very significant errors
in the output measurement signal. The extent to which series mode noise corrupts measurement signals is
measured by a quantity known as the signal-to-noise ratio. This is defined as:
signal to noise ratio = 20log10
Vs
Vn
where Vs is the mean voltage level of the signal and Vn is the mean voltage level of the noise. In the case of
a.c. noise voltages, the root-mean squared value is used as the mean.
Common mode noise voltages are less serious, because they cause the potential of both sides of a signal circuit
to be raised by the same level, and thus the level of the output measurement signal is unchanged.
4.1
4.1.1
Sources of noise
Inductive coupling
The primary mechanism by which external devices such as mains cables and equipment, fluorescent lighting
and circuits operating at audio or radio frequencies generate noise is through inductive coupling. If signalcarrying cables are close to such external cables or equipment, a significant mutual inductance M can exist
between them, as shown in Figure 49, and this can generate a series mode noise voltage of several millivolts
˙ where I is the rate of change of current in the mains circuit.
given by Vn = M I,
Figure 49: Noise induced by inductive coupling
58
4.1.2
Capacitive coupling
Capacitive coupling, also known as electrostatic coupling, can also occur between the signal wires in a
measurement circuit and a nearby mains-carrying conductor. The magnitude of the capacitance between
each signal wire and the mains conductor is represented by the quantities C1 and C2 in Figure 50. In
addition to these capacitances, a capacitance can also exist between the signal wires and earth, represented
by C3 and C4 in the figure.
Figure 50: Noise induced by capacitive coupling
The series mode noise voltage Vn is zero if the coupling capacitances are perfectly balanced, i.e. if C1 = C2
and C3 = C4 . However, exact balance is unlikely in practice, since the signal wires are not perfectly straight,
causing the distances and thus the capacitances to the mains cable and to earth to vary. Thus, some series
mode noise voltage induced by capacitive coupling usually exists.
4.1.3
Noise due to multiple earths
As far as possible, measurement signal circuits are isolated from earth. However, leakage paths often exist
between measurement circuit signal wires and earth at both the source (sensor) end of the circuit and also
the load (measuring instrument) end. This does not cause a problem as long as the earth potential at both
ends is the same. However, it is common to find that other machinery and equipment carrying large currents
is connected to the same earth plane. This can cause the potential to vary between different points on the
earth plane. This situation, which is known as multiple earths, can cause a series mode noise voltage in the
measurement circuit.
4.1.4
Noise in form of voltage transients
When motors and other electrical equipment (both a.c. and d.c.) are switched on and off, large changes
of power consumption suddenly occur in the electricity supply system. This can cause voltage transients
(’spikes’) in measurement circuits connected to the same power supply. Such noise voltages are of large
magnitude but short time duration. Corona discharge can also cause voltage transients on the mains power
supply. This occurs when the air in the vicinity of high voltage d.c. circuits becomes ionized and discharges
to earth at random times.
59
4.1.5
Thermoelectric potentials
Whenever metals of two different types are connected together, a thermoelectric potential (sometimes called
a thermal e.m.f.) is generated according to the temperature of the joint. This is known as the thermoelectric
effect and is the physical principle on which temperature-measuring thermocouples operate. Such thermoelectric potentials are only a few millivolts in magnitude and so the effect is only significant when typical
voltage output signals of a measurement system are of a similar low magnitude.
4.1.6
Short noise
Shot noise occurs in transistors, integrated circuits and other semiconductor devices. It consists of random
fluctuations in the rate of transfer of carriers across junctions within such devices.
4.1.7
Electrochemical potentials
These are potentials that arise within measurement systems due to electrochemical action. Poorly soldered
joints are a common source.
4.2
Techniques for reducing measurement noise
Prevention is always better than cure, and much can be done to reduce the level of measurement noise by
taking appropriate steps when designing the measurement system.
4.2.1
Location and design of wires
Both the mutual inductance and capacitance between signal wires and other cables are inversely proportional
to the square of the distance between the wires and the cable. Thus, noise due to inductive and capacitive
coupling can be minimized by ensuring that signal wires are positioned as far away as possible from such
noise sources. A minimum separation of 0.3 m is essential, and a separation of at least 1 m is preferable.
Noise due to inductive coupling is also substantially reduced if each pair of signal wires is twisted together
along its length. This design is known as a twisted pair.
4.2.2
Earthing
Noise due to multiple earths can be avoided by good earthing practices. In partic- ular, this means keeping
earths for signal wires and earths for high-current equipment entirely separate. Recommended practice is to
install four completely isolated earth circuits as follows:
• Power earth: provides a path for fault currents due to power faults.
• Logic earth: provides a common line for all logic circuit potentials.
• Analogue earth (ground): provides a common reference for all analogue signals.
• Safety earth: connected to all metal parts of equipment to protect personnel should power lines come
into contact with metal enclosures.
60
4.2.3
Shielding
Shielding consists of enclosing the signal wires in an earthed, metal shield that is itself isolated electrically
from the signal wires. The shield should be earthed at only one point, preferably the signal source end. A
shield consisting of braided metal eliminates 85% of noise due to capacitive coupling whilst a lapped metal
foil shield eliminates noise almost entirely. The wires inside such a shield are normally formed as a twisted
pair so that protection is also provided against induced noise due to nearby electromagnetic fields. Metal
conduit is also sometimes used to provide shielding from capacitive-coupled noise, but the necessary supports
for the conduit provide multiple earth points and lead to the problem of earth loops.
61
5
Remote sensing
5.1
Data Transmission and Telemetry
The distance between the primary transducer to the display device in instrumentation systems may be too
large. This necessitates sure means of data transfer technique.
Data transmission and telemetry refers to the process by which information regarding a quantity under
measurement using a transducer and signal conditioning devices is transferred to a remote location, perhaps
to be processed , recorded, stored or displayed.
Telemetry is important because it enables to collect data from several measurement points at inconvenient
locations or inaccessible areas , transmit that data to a convenient location and present the several individual
measurement in a usable form.
5.1.1
Methods of Data Transmission
The transmission of a measured variable to a remote point is an important function of modern day instrumentation systems because of the size, need, and complexity of modern industrial plants. The methods used
for data transmission depends upon the variables and distance over which it has to be transmitted. The most
commonly used methods of data transfer/transmission are
1. Pneumatic transmission
Pneumatic transmission consists of transmitting analogue signals as a varying pneumatic pressure
level that is usually in the range of 3-15 p.s.i. Pneumatic transmission has the advantage of being
intrinsically safe, and provides similar levels of noise immunity to current loop transmission. However,
one disadvantage of using air as the transmission medium is that transmission speed is much less than
electrical or optical transmission. A further potential source of error would arise if there were a pressure
gradient along the transmission tube. This would introduce a measurement error because air pressure
changes with temperature.
2. Electrical transmission
The simplest method of electrical transmission is to transmit the measurement signal as a varying
analogue voltage. However, this can cause the measurement signal to become corrupted by noise. If
noise causes a problem, the signal can either be transmitted in the form of a varying current, or else it
can be superimposed on an a.c. carrier system.
3. Fibre-optic transmission
Light has a number of advantages over electricity as a medium for transmitting information. It is intrinsically safe, and noise corruption of signals by neighbouring electromagnetic fields is almost eliminated.
The most common form of optical transmission consists of transmitting light along a fibre-optic cable,
although wireless transmission also exists.
4. Optical wireless telemetry
Wireless telemetry allows signal transmission to take place without laying down a physical link in the
form of electrical or fibre-optic cable. This can be achieved using either radio or light waves to carry
the transmitted signal across a plain air path between a transmitter and a receiver.
62
Optical wireless transmission consists of a light source (usually infra-red) transmitting encoded data
information across an open, unprotected air path to a light detector. Three distinct modes of optical
telemetry are possible, known as point-to-point, directed and diffuse:
5. Radio telemetry
In radio telemetry, data are usually transmitted in a frequency modulated (FM) format. Radio telemetry is normally used over transmission distances up to 400 miles, though this can be extended by special
techniques to provide communication through space over millions of miles. However, radio telemetry is
also commonly used over quite short distances to transmit signals where physical electrical or fibre-optic
links are difficult to install or maintain. This occurs particularly when the source of the signals is mobile.
The great advantage that radio telemetry has over optical wireless transmission through an air medium
is that radio waves are attenuated much less by obstacles between the energy transmitter and receiver.
Hence, as noted above, radio telemetry usually performs better than optical wireless telemetry and is
therefore used much more commonly.
6. Digital transmission protocols
Digital transmission has very significant advantages compared with analogue transmission because the
possibility of signal corruption during transmission is greatly reduced.
Many different protocols exist for digital signal transmission. The protocol that is normally used for
the transmission of data from a measurement sensor or circuit is asynchronous serial transmission, with
other forms of transmission being reserved for use in instrumentation and computer networks.
Asynchronous transmission involves converting an analogue voltage signal into a binary equivalent,
using an analogue-to-digital converter. This is then transmitted as a sequence of voltage pulses of equal
width that represent binary ’1’ and ’0’ digits. Commonly, a voltage level of 6 V is used to represent
binary ’1’ and zero volts represents binary ’0’. Thus, the transmitted signal takes the form of a sequence
of 6 V pulses separated by zero volt pulses. This is often known by the name of pulse code modulation.
Such transmission in digital format provides very high immunity to noise because noise is typically
much smaller than the amplitude of a pulse representing binary 1.
At the receiving end of a transmitted signal, any pulse level between 0 and 3 volts can be interpreted
as a binary ’0’ and anything greater than 3 V can be interpreted as a binary ’1’.
5.1.2
General Telemetry System
The generalized telemetry system is as shown below
The primary detector and the end device have the same position and functional roles as in a generalized
measurement system. The intermediate stage device which are peculiar to a telemetry system are
1. Telemeter transmitter
2. Telemeter channel
3. Telemeter receiver
63
Figure 51: General Telemetering System
The telemeter transmitter converts the output of a primary sensing element into an electrical signal and
transmits it over the telemetry channel. This signal is in electrical form and is received by the receiver placed
at a remote location.
This signal is converted into a so usable form by the receiver and is indicated or recorded by the end device.
The end device is graduated in terms of the measurand. The device may also be a control element which
may be used to control the input quantity through a feedback loop to produce desired output.
64
6
Microprocessor application in instrumentation
Microprocessor based systems are suitable for dedicated and complicated measurement systems available in
modern day industries and hospitals. These microprocessors perform complicated signal processing operations
e.g signal conditioning. They can handle thousands of signal conditioning and manipulation operations within
a very short time. They are very small, compact, requires little power and are precise.
The microprocessor cannot take most of the signal input from transducers directly. Conversion of transducer
output signal from analogue to digital in most cases is a must. This is done by an analogue digital converter.
If more than one input quantity (measurand) is to be measured and processed, multiplexers are used. If
an ac voltage is used , sample and hold circuits are used to keep the desired instantaneous voltage constant
during conversion.
The digital output obtained is fed to the microprocessor through an interfacing device.
In a microprocessor based instrumentation system the designer has to select suitable input/output devices
to interface to the microprocessor. If a particular device is not compatible additional electronic circuits have
to be designed through which the device may be interfaced to the cpu. Example of a simple microprocessor
based system for an industry or a hospital is as shown below.
Figure 52: Microprocessor based Instrumentation system
For industrial setup, the first to be measured maybe: position, temperature, pressure, displacement, speed,
force, current, voltage , etc
For hospital setup, the first to measured maybe ; heart beat, blood pressure, room-temperature, body
temperature, blood pH value, breathing rate, etc
All these quantities will be converted to an analogue electrical signal by the transducer. After the conversion
, they will be converted to digital signals then signal conditioned before being fed to the computer through
the interfacing card.
The computer through a specific installed program will perform the required data analysis , storage and
output functions. NB the microprocessor or microcomputer may be a programmable logic controller (PLC)
65
7
Data Storage, Recording and Display Devices
In measurement systems , the last stage is often the data presentation stage. This consist of display devices
and recorders. The display and recording devices are also called the output devices. The significance of these
devices is that they make the result of measurement meaningful through display of instant observation or
stage for observation at a later stage.
The choice between the display devices and recorders is influenced by the expected use of the output and the
information content of the output.
7.1
Display Devices
In electrical measurements, indicating or display instruments are used extensively in measurement of current
, voltage , resistance and power. These instruments can be broadly classified as
1. Analog
2. Digital
The analog instruments display the quantity to be measured in terms of the deflection of a pointer i.e. an
analog displacement or an angle corresponding to the electrical quantity.
On the other hand , the digital instrument indicates the value of the measurand in form of a decimal number.
They work on the principle of quantization.
Advantages of digital instruments over Analog
1. The digital instruments indicate the readings directly in decimal numbers and therefore errors on
account of human factors like parallax and approximation are eliminated.
2. The digital readings maybe carried to any number of significant figure by merely positioning the decimal
point.
3. Since the output of digital instruments is in digital form, it may be directly fed into memory devices like
tape recorders , digital printers, floppy disks, hard disks and computers for storage and future analysis
4. The power requirement for digital display instruments are considerably smaller with the advancement
of microprocessor based digital instruments technology. Several factors have to be considered when
choosing either analog or digital display systems. These are
(a) accuracy
(b) environmental compatibility
(c) resolution
(d) power requirements
(e) cost and portability
(f) range
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Types of Digital display Units
There are several types of digital display units which are classified as
• planar - i.e the entire read out characters are in the same place
• non-planar - where the characters are displayed in different planes
The planar displays include
1. seven segment display which is used for numeric display. It consists of seven segments a,b,c,d,e,f,g
Figure 53: Seven segment display
A segmental display forms the digit to be displayed by illuminating proper segment from the group. By
illuminating the proper combination of these seven segments , number 0-9 can be displayed. Example to
display one only segment b and c are illuminated and to display four segments b,c,f and g are illuminated.
2. 14 segmental display
This is used for displaying alphanumeric characters. It works on the same
principle as the seven segmental display
Figure 54: 14 segment display
e.g to display c the segment 1, 6, 5 and 4 are illuminated.
3. Dot matrix utilizing 27 dots
Where the dots maybe square or round with 0.4mm size or diameter.
LEDs and LCDs are used for illuminating the dots thus displays only numeric characters
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4. 5*7 Dot Matrix Used for displaying alphanumeric characters
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The non planar displays include
5. Rear projection displays
6. Nixie tube displays
They are rarely used
7. Light Emitting Diodes (LEDs)
Which is a p-n junction device which emits light when a current
passes through it in the forward direction. Charge carriers recombination takes place at the p-n junction as
electrons cross from n-side and recombine with holes at the p-side.
When this happens , the charge carriers give up energy in form of heat and light. If the semiconducting
material is translucent the light is emitted and the junction is a source of light. Materials used for manufacture
of LEDs are Gallium Arsenide Phosphide (GaAsP) and Gallium Arsenide.
8. Liquid Crystal Displays (LCDs) Used in similar application as LEDs. These application are display
of numeric and alphanumeric characters in dot matrix and segmental displays. LCDs are of two types
1. Dynamic scattering type
2. Field effect type
The construction of a dynamic scattering LCD cell is as shown below
The liquid crystal material maybe one of the several organic compounds which exhibit optical properties of
the crystal though they remain in liquid form. Liquid crystal is layered between glass sheets with transparent
electrode deposited on the inside faces.
When an electric potential is applied across the cell charge carriers flowing through the liquid disrupt the
nuclear alignment and produces turbulence. This causes the light to be scattered in all directions , and the
cell appears to be bright. The phenomena is called dynamic scattering.
NB when the liquid is not disturbed, the crystal is transparent
7.2
Recorders
Are required to keep a permanent record on the state of a phenomena being investigated. A recorder therefore
records electrical and non electrical quantities as a function of time.
The record may be written or printed and later on can be examined and analyzed to obtain a better understanding and control of a process. Recording requirement is one of the most important considerations in an
instrument system. There are two types of recording methods
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1. Analog recording methods : which are analog recorders. Examples are
• Graphic recorders which are devices which display and store a pen and ink record of the history
of some physical event. Examples are strip chart recorders also known as x-time recorders and
x-y recorders
• oscillographic recorders ; they record information in oscilographs eg ultra violet recorders and
duddel oscillographic recorders
• magnetic tape recorders; they are used to record data which can be retrieved or reproduced
in electrical form. Can be used to record dc to several MHz frequency range signals. The
basic component of a magnet tape are; recording head, magnetic tape, reproducing head, tape
transparent mechanism and conditioning devices
2. Digital recording methods : Uses digital normal recorders. Examples include
• computer hard disk
• flash disk, ROM, RAM etc
They can store thousands of bits of information in a very small space
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