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8/10/2019 ET ZC341-L8.pdf
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BITSPilaniPilani Campus Swapna Kulkarni
Instrumentation And Control
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Pressure Sensors(p>1 atmosphere)
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FIGURE 5.30 A diaphragm is used in many pressure sensors.
Displacement varies with pressure difference.
Curtis JohnsonProcess Control Instrumentation Technology, 8e]
Copyright 2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
One common element used to convert pressure information
into a physical displacement is the diaphragm (thin , flexiblepiece of metal).
F=(p-p)A
where A= diaphragm area in m
p,p = pressure in N/ m
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Bellows
A bellows is another device much like thediaphragm that converts a pressure
differential into a physical displacement,
except that here the displacement is much
more a straight-line expansion. The accordion-
shaped sides of the bellows are made from
thin metal. When there is a pressuredifference, a net force will exist on the flat,
front surface of the bellows.
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FIGURE 5.31 A bellows is another common method of converting
pressure to displacement. Here an LVDT is used to convert the
displacement tovoltage amplitude.
Curtis JohnsonProcess Control Instrumentation Technology, 8e]
Copyright 2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
An LVDT can be connected to the bellows so that pressure
measurement is converted directly form displacement to avoltage.
In addition, the displacement and pressure are nearly linearlyrelated, and because the LVDT voltage is linear with
displacement, the voltage and pressure are also linearly related.
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FIGURE 5.32 The Bourdon tube is probably the most common
pressure-to-displacement element.
Curtis JohnsonProcess Control Instrumentation Technology, 8e]
Copyright 2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
A hard metal tube, usuallya type of bronze or brass, is
flattened, and one end is
closed off. The tube is then
bent into a curve or arc,
sometimes even a spiral.
The open end is attachedto a header by which a
pressure can be introduced
to the inside of the tube .
Tube straighten if Pin>PoutCurve if Pin
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Electronic Conversions
Many techniques are used to convert thedisplacements generated into electronic signals.
The simplest technique is to use a mechanicallinkage connected to a potentiometer. In thisfashion, pressure is related to a resistancechange.
Other methods of conversion employ straingauges directly on a diaphragm. LVDTs and other
inductive devices are used to convert bellows orBourdon tube motions into proportionalelectrical signals.
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FIGURE 5.33 A differential pressure (DP) cell measures
pressure difference with a diaphragm. A feedback system
minimizes actual diaphragmdeflection.
Curtis JohnsonProcess Control Instrumentation Technology, 8e]
Copyright 2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
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Pressure sensors(p
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Pirani Gauge
This gauge determines the filamenttemperature through a measure of filament
resistance.
Filament excitation and resistancemeasurement are both performed with a
bridge circuit.
The response of resistance versus pressure ishighly nonlinear.
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Thermocouple
A second pressure transducer or gauge measuresfilament temperature using a thermocoupledirectly attached to the heated filament.
In this case, ambient room temperature serves asa reference for the thermocouple, and thevoltage output, which is proportional to pressure,is highly nonlinear.
Calibration of both Pirani and thermocouplegauges also depends on the type of gas for whichthe pressure is being measured.
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Ionization Gauge
This device is useful for the measurement ofvery low pressures from about 10-3 atm to
10-13atm.
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FIGURE 5.36 The ionization gauge is used to measure very low
pressures, down to about 10-13atm.
Curtis JohnsonProcess Control Instrumentation Technology, 8e]
Copyright 2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
This gauge employs electrons,usually from a heated
filament, to ionize the gaswhose pressure is to be
measured, and then measures
the current flowing between
two electrodes in the ionizedenvironment.
The number of ions per unitvolume depends on the gas
pressure, and hence the
current also depends on gas
pressure. This current is then
monitored as an approximately
linear indication of pressure.
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Flow Sensors
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Flow sensors
The measurement and control of flow can besaid to be the very heart of process industries.Continuously operating manufacturing
processes involve the movement of rawmaterials, products, and waste throughout theprocess. All such functions can be consideredflow, whether automobiles through an
assembly line or methyl chloride through apipe. The methods of measurement of floware at least as varied as the industry.
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Flow on three broad fronts-Solid, liquid andgas.
As with pressure, we will find that flowinformation is often translated into an
intermediate form, that is then measured
using techniques developed for that form.
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Solid Flow measurement
A common solid-flow measurement occurswhen material in the form of small particles,such as crushed material or powder, is carried
by a conveyor belt system or by some otherhost material. For example, if solid material issuspended in a liquid host, the combination iscalled a slurry, which is then pumped through
pipes like a liquid. We will consider theconveyor system and leave slurry to be treatedas liquid flow.
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Conveyor Flow Concepts
For solid objects, the flow usually is describedby a specification of the mass or weight perunit time that is being transported by the
conveyor system. The units will be in manyforms-for example, kg/min or lb/min. To makea measurement of flow, it is only necessary toweigh the quantity of material on some fixed
length of conveyor system. Knowing the speedof the conveyor allows calculation of thematerial flow rate.
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FIGURE 5.37 Conveyor system for illustrating solid-flow measurement.
Curtis JohnsonProcess Control Instrumentation Technology, 8e]
Copyright 2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Q=(WR)/L
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A mechanical valve controls that rate at whichmaterial can flow from the hopper onto the
conveyor belt.
The belt is driven by a motor system. Flow rate is measured by weighing the amount of
material on a platform of length L at any instant.
The conveyor belt slides over the platform, whichdeflects slightly due to the weight of material.
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A load cell measures this deflection as anindication of weight. In this case, flow rate can becalculated from
Q=(WR)/LWhere Q=flow(kg/min or lb/min)
W= weight of material on section of length L(kg orlb)
R=conveyor speed(m/min or ft/min)
L=length of weighing platform(m or ft)
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Another popular device for weightmeasurement of moving systems like this is an
LVDT that measures the droop of the conveyor
at the point of measurement because of thematerial that it carries.
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Liquid Flow
The conditions under which the flow occursand the vastly different types of material that
flow result in a great many types of flow
measurement methods.
The basic ideas of liquid flow measurementwill be presented.
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Flow units
The units used to describe the flow measured canbe of several types , depending on how thespecific process needs the information. The mostcommon descriptions are the following:
Volume flow rate: Expressed as a volumedelivered per unit time. Typical units aregals/min, m3/h, or ft3/h (1 gas=231in)
Flow velocity: Expressed as the distance the liquidtravels in the carrier per unit time. Typical unitsare m/min. This is related to the volume flow rateby
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V=Q/A
where V=flow velocity
Q= volume flow rateA=cross-sectional area of flow carrier(pipe, and
so on)
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Mass or weight flow rate: Expressed as mass orweight flowing per unit time. Typical units are
kg/h. This is related to the volume flow rate by
F=Q
where
F= mass or weight flow rate
=mass density or weight density
Q=volume flow rate
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Pipe Flow Principles
The flow rate of liquids in pipes is determinedprimarily by the pressure that is forcing the
liquid through the pipe. The concept of
pressure head, or simply head, is often usedto describe this pressure, because it is easy to
relate the forcing pressure to that produced
by a depth of liquid in a tank from which thepipe exists.
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Restriction Flow Sensors
One of the most common methods of measuringthe flow of liquids in pipes is by introducing arestriction in the pipe and measuring thepressure drop that results across the restriction.
When such a restriction is placed in the pipe, thevelocity of the fluid through the restrictionincreases, and the pressure in the restrictiondecreases. We find that there is a relationship
between the pressure drop and the rate of flowsuch that, as the flow increase, the pressuredrops.
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In particular, one can find an equation of the formQ=K
where
Q=volume flow rate
K=a constant for the pipe and liquid type
= drop in pressure across the restriction
The constant, K, depends on many factors, includingthe type of liquid, size of pipe, velocity of flow ,
temperature, and so on. The type of restrictionemployed also will change the value of the constantused in this equation.
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Pitot Tube
The pitot tube is a common way to measureflow rate at a particular point in a flowing
fluid(liquid or gas).
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The principle is that the fluid willbe brought to rest in the tube,and therefore its pressure will be
the sum of the static fluidpressure plus the effectivepressure of the flow.
The pressure in the pitot tube ismeasured in differential to the
static pressure of the flowing fluidin the same vicinity as the tube.
This differential pressure will beproportional to the square root ofthe flow rate. The flow rate in a
pipe varies across the pipe, so thepitot tube determines the flowrate only at the point of insertion.
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Obstruction Flow Sensor
Another type of flow sensor operates by theeffect of flow on an obstruction placed in the
flow stream.
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In a rotameter , the obstruction isa float that rises in a verticaltapered column.
The lifting force, and thus thedistance to which the float rises in
the column, is proportional to theflow rate.
The lifting force is produced bythe differential pressure that exists
across the float, because it is arestriction in the flow.
This type of sensor is used forboth liquids and gases.
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A Moving Vane
A moving vane flow meter hasa vane target immersed in theflow region, which is rotatedout of the flow as the flow
velocity increases. The angle ofthe vane is a measure of theflow rate. If the rotating vaneshaft is attached to an angle-
measuring sensor, the flow ratecan be measured for use in aprocess-control application.
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Turbine
A turbine type of flowmeter is composed of a
freely spinning turbine
blade assembly in the
flow path. The rate ofrotation of the turbine
is proportional to the
flow rate. If the turbine
is attached to a
tachometer, a
convenient electrical
signal can be produced.
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Magnetic Flow meter
It can be shown that if charged particles moveacross a magnetic field, a potential is establishedacross the flow, perpendicular to the magneticfield. Thus, if the flowing liquid is also a
conductor(even if not necessarily a goodconductor) of electricity, the flow can bemeasured by allowing the liquid to flow through amagnetic field and measuring the transverse
potential produced. The pipe section in whichthis measurement is made must be insulated, anda nonconductor itself, or the potential produces,will be cancelled by currents in the pipe.
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FIGURE 5.42 A magnetic flow meter will work only with conducting
fluids such as blood.
Curtis JohnsonProcess Control Instrumentation Technology, 8e]
Copyright 2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458All rights reserved.
This type of sensor produces an electrical
signal directly and is convenient for processescontrol applications involving conducting fluid
flow.
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Summary
Position, location and displacement sensors, includingthe potentiometric, capacitive, and LVDT. The LVDTconverts displacement linearly into a voltage.
The strain gauge measures deformation of solid objectsresulting from applied forces called stress. The straingauge converts strain into a change of resistance.
Accelerometers are used to measure the accelerationof objects because of rectilinear motion vibration , and
shock. Most of them operate by the spring-mssprinciple, which converts acceleration information intoa displacement.
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Pressure is the force per unit area that a fluidexerts on the walls of a container. Pressuresensors often convert pressure information into adisplacement. Examples include diaphragms,bellows, and the Bourdon tube. Electronicmeasures are often used for low pressures.
For gas pressures less than 1 atom, purely
electrical techniques are used. Ins some cases,the temperature of a heated wire is used toindicate pressure.
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Flow sensors are very important in themanufacturing world. Typically, solid flow is
mass or weight per unit time.
Fluid flow through pipes or channels typicallyis measured by converting the flow
information into pressure by a restriction in
the flow system.
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41
Review Session
06-01-2012 ETZC341 Instrumentation and control
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Introduction
Definitions
Process Control Principles
Human Aided Control
Automatic Control
Servomechanisms
Discrete State Control Systems
Process Control Block Diagram
Process Control
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Controlled variable: The process variable regulatesby process control loop.
Controlling variable: The process variable changes
by the final control element under the commandof controller to effect regulation of controlled
variable.
Setpoint: The desired value of a controlled variable
in process control loop.
Some definitions related to process
control
Instrumentation and control
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Error: The algebraic difference between themeasured value of variable and setpoint.
Controller: The element in a process control loop
that evaluates error of the controlled variableand initiates corrective action by a signal to
controlling variable.
Dynamic variable: The process variable that can
change from moment to moment because ofunknown sources.
Instrumentation and control
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Block Diagram of process Control
Fig. 1.5 This block diagram of a control loop defines allthe basic elements and signals involved
c = controlled variable in the process
b = the measured representation of the
controlled variable
r = controlled variable setpoint
Copyright 2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
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A practical statement of control system objective is bestrepresented by three requirements:
1. The system should be stable
2. The system should provide the best possible steady
state regulation.
3. The system should provide the best possible transient
regulation: If one of them suddenly changes the value ,
the controlled variable may be driven to change also,
so the control system acts to minimize the effect. This iscalled transient response.
Control System Objective
06-01-2012 ETZC341 Instrumentation and control
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Analog and Digital processing
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In electronic circuits , data is represented bythe magnitude of voltage or currents. This is
referred asAnalog processing.
Most modern control systems now employdigital computers to perform controlleroperations.
In computers, data are represented as binarynumbers consisting of a specific number of
bits. This is referred as digital processing.
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There are two approaches to using computers forcontrol:
1) Supervisory Control
Supervisory control emerged as an intermediate
step wherein the computer was used to monitorthe operation of analog control loops and todetermine appropriate setpoints
2) Direct Digital Control(DDC): This direct digital control
system lets the computer perform the errordetection and controller functions.
Digital control
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Using modern integrated circuit technology, thesensor, signal conditioning,ADC, and computer
controller are all contained within the sensor
housing. Networked Control Systems:Each of the process
control computer operates one or more DDC loops.
Bus users can monitor the operations of any of the
plant process control loops, and those withauthorization can modify control characteristics such
as setpoints and gains
Smart sensor
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Need of standards: have universalagreement among process equipment
manufacturers on how data are
represented on the bus line and how dataare transmitted and received.
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Types of discrete control functions were oftenprovided by a system of electrical relays wired
according to a complex diagram. This was
called a relay logic controller.
In recent years, computers have also takenover the operation of such relay logic
controllers, known asprogrammable logiccontrollers(PLCs).
PLCs
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The SI system is progressivelybeing adopted by 'metric'
countries of long standing and
it is the required system for
almost all internationaljournals.
It has seven base units andtwo supplementary units.
All other units whichcomprise the SI system can be
derived.
International System of units
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The centimeter-gram-second system(CGS) English System
Other Units
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Process Control Drawings
LettersNumbers
Essential elements
Instrument line symbols
Instrument symbols
E l f th l tt / b di
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Examples of the letter/number coding
a) P means pressure,R means a recording unit and C means a
controller, so this is a recording pressure controller located in
loop or plant location 103. The unit is accessible to an operator
and is probably in a panel of the control room
b) The interpretation is L for level, C for control, and symbol for
computer, located in the field of loop or plant location 330.
Copyright 2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
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Examples of the letter/number coding
c) Y for event controller
generally means a PLC, herefor Z meaning for positioncontrol, not accessible to anoperator and in the loop orplant location 200
d) As part of a T fortemperature- measurementsystem, the Y means aconverter, converting 4 to 20
mA into 3 to 15 psi. The unitis not accessible to anoperator and is part of loopor plant location 203.
Copyright 2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
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First order response
Second order response
Real time effects
Significant figures
Significance in measurements
Significance in Calculations
Significance in design
Sensor Time Response
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Statistics1) Arithmetic Mean
2) Standard Deviation:A measure of the dispersion of a set
of data from its mean. The more spread apart the data,
the higher the deviation. Standard deviation iscalculated as the square root of variance.
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Analog signal conditioning provides the operationsnecessary to transform a sensor output into a form
necessary to interface with other elements of the
process control loop.
Signal Level and Bias Changes
Linearity
Conversions
Digital interface
Filtering and Impedance MatchingImpedance matching is an important element of signal
conditioning when transducer internal impedance or line
impedance can cause errors in measurement of a
dynamic variable.
Analog Signal processing
Si l l l d Bi h
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We perform the required signal conditioning by firstchanging the zero to occur when the sensor output is
0.2V. This can be done by simple subtracting 0.2 from
the sensor output, which is called a zero shift or a bias
adjustment.If we have voltage that varies from 0 to 0.4V and need to
make it larger, multiply the voltage by 12.5,the new
output will vary from 0 to 5 V as required. This is called
amplificationand 12.5 is called the gain.
Signal level and Bias changes
Signal Level and Bias
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In some cases, we need to make a sensor outputsmaller, which is called attenuation. The circuit that does
either chore is called amplifier.
In designing bias and amplifier circuits, we must be
concerned with issues such as the frequency response,output impedance, and input impedance.
Signal Level and BiasChanges
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Passive circuitsBridge and divider circuits
Wheatstone Bridge
DC bridge and AC bridge
Bridge Applications
Low pass filters
High pass filters
Active Circuits
Op-amp
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Fig. 2.4 The simple voltage divider canoften be used to convert resistancevariation into voltage variation
The elementary voltagedivider often can be used toprovide conversion ofresistance variation into avoltage variation.
The voltage of such divider isgiven byVD=(R2*Vs)/(R1+R2) (2.2)
Where Vs=supplyvoltage;R1,R2=divider resistors
Either R1or R2can be the sensorwhose resistance varies withsome measured variable.
Passive Circuit_Divider Circuits
64
h id
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Fig. 2.5 The basic dcWheatstone bridge
For initial analysis, assume the detectorimpedance is infinite. The potential difference , V=Va-Vb (2.3) where Va= potential of point a with respect to
c
Vb= potential of point b with respect to c.
Va=VR3
/(R1
+R3
) and Vb=VR2
/(R2
+R4
) where V= bridge supply voltage. V= V(R3R2-R1R4)/(R1+R3)(R2+R4) (2.7) A particular combination of resistors can be
found that will result in zero difference andzero voltage across the detector, i.e. , a null.
R3
R2
=R1
R4
(2.8)
The application of Wheatstone bridges toprocess-control applications using high-inputimpedance detectors.
Wheatstone Bridge
65
Copyright 2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
AC Bridge
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Employs an ac excitation, usuallya sine wave voltage signal. Theanalysis of bridge behavior isbasically the same as in theprevious treatment, but
impedances replace resistance.The bridge offset voltage then isrepresented as
voltageoffsetacE
ZZZZ
ZZZZEE
4231
4123
Figure : A general ac bridgecircuit
AC Bridge
66
Where
E = sine wave excitation voltage
Z1,Z2,Z3,Z4= bridge impedances.
Z3Z2=Z1Z4Copyright 2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
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Low pass RC filter
Figure : Circuit for the low-passRC filter It is called low pass because itblocks high frequencies and
passes low frequencies.
The critical frequency is thatfrequency for which the ratio ofo/p to i/p voltage is approx.0.707.
In terms of the resistor andcapacitor, the critical frequencyis given by
fc = (1/ 2RC) The output to input voltage
ratio for any signal frequencyan be determined graphicallyor can be computed by
Vout/Vin= 1/[1+(f/fc)]
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Figure : Circuit for the high-pass RC filter
A highpass filter passeshigh frequencies (no
rejection) and blocks(rejects) low frequencies. A
filter of this type can be
constructed using a resistor
and a capacitor, as shown in
schematic of figure.
High-Pass RC Filter
68
Copyright 2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
O
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Operational amplifiers (op amp) are a special signal-conditioning building block around which many specialfunction circuits can be developed. The device was
demonstrated in applications involving amplifiers,
converters, linearization circuits, integrators, and severalother functions.
Voltage follower
Inverting and Non inverting amplifiers
V to I converter and other applicationsSumming Amplifier,Differential Instrumentation Amplifier
Integrator,Differentiator
circuit design guidelines
Op-amp
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Ideal Inverting Amplifier
I1+I2 = 0 Where I1 = current through R1;I2 = currentthrough R2
70
Figure The opamp inverting amplifier.
Copyright 2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
0
21
R
V
R
Voutin
inout V
R
RV
1
2
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Voltage Follower
Figure : The Op amp voltage follower. This circuithas unity gain but very high input impedance Figure shows an opampcircuit with unity gain and
very high input impedance.
The input impedance is
essentially the inputimpedance of the op amp
itself, which can be greater
than 100M. The voltage
output tracks the input over
a range defined by the plus
and minus saturation
voltage outputs.
71
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Inverting Amplifier
inout VR
RV
1
2
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Figure: The Op Amp summing Amplifier
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2
3
2
1
1
2V
R
RV
R
RV
out
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Figure A noninverting amplifier
I1+ I2= 0Where
I1= Current through R1I2= Current through R2
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inout VR
RV
1
21
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Op Amp
The common-mode input voltageis theaverage applied to the two input terminals,
2ba
cmVVV
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The commonmode rejection (CMR) is theCMRR expressed in dB.
)(log20 10 CMRRCMR
A
ACMRR
cm
76
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Figure : An instrumentation amplifier includes voltage
followers for input isolation.
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Figure: This instrumentation amplifier allows the
gain to be changed using a single resistor
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Figure : A voltage-to-current using an op amp
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Current-to-Voltage Converter
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Figure : A current-to-voltage converter using an op amp. Care must
be taken that the current output capability of the op amp is not
exceeded.
Current-to-Voltage Converter
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Integrator
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IntegratorFigure An integrator circuit using an
op amp.
81
0dt
dVC
R
Voutin
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dtVRC
Vinout
1
Differentiator
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Differentiator
82
Figure : This circuit takes the time derivative of the
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0R
V
dt
dVC
outin
dt
dVRCV in
out
Digital signal conditioning
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Introduction Digital information
PLCs: These devices are particularly suited to thesolution control problems associated with Boolean
equations and binary logic problems in general. Computer interfacing
Digital signal conditioning
Computer Interface
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p
Generic model of acomputer bus system.
The processor is connected to
external equipment via threeparallel sets of digital lines.
The data lines carry data to andfrom the processor.
The address lines allow thecomputer to select externallocations for input and output.
The control linescarryinformation to and from the
computer related to operations,such as reading, writing,
interrupts, and so on.
This collection of lines is calledthe busof the computer.
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Comparators:
o Open Collector Comparator
o Hysteresis Comparator
Digital to analog converters
o Bipolar DACo DAC structure
Comparator
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A basic comparator comparesvoltages and produces a digital
output.
The most elementary form ofcommunication between theanalog and digital is a device(usually an IC) called acomparator.
This device simply compares two
analog voltages on its inputterminals.
Depending on which voltage islarger, the output will be a 1(high) or a 0(low) digital signal.
The comparator is extensivelyused for alarm signals tocomputers or digital processingsystems.
Comparator
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Open Collector comparators
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Open Collector comparators
Many comparators use an open-
collector output.
Fig.(a) shows that the output terminal
of the comparator is connectedinternally to the collector of atransistor in the comparator. This iscalled an open-collector output.
Even if there is base-emitter currentin the transistor, no voltage will show
up on the collector until it isconnected to a supply through somecollector resistor.
Fig (b) shows that an external resistoris connected from the output to anappropriate power supply. This iscalled a collectorpull-upresistor.
Now the output terminal will showeither a 0(0V) if the internal transistoris ON or 1 (Vs) if the internal transistoris OFF. Copyright 2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
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Hysteresis Comparator
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Hysteresis Comparator
The condition for which the output will go high (V) is defined by the condition
Vin>=Vref
Once having been driven high, the condition for the output to drop back to the
low (0 V)
state is given by the relation Vin=
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DAC Characteristics
For modern applications, most DACs are ICassemblies , viewed as a black box having certaininput and output characteristics.
A generic DAC diagram, showing
typical input and output signals.
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Data acquisition systems: Special PCBs called dataacquisition systems (DACs) have been developed for thepurpose of providing for input and output of analog data.
The data acquisition system(DAS) is a modular device
that interfaces many analog signals to computer. Signaladdress decoding, multiplexing, and ADC operations are
included in the device.
DAS h d
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DAS hardwareTypical layout of a data-acquisition board for use in a personal computer expansion
slot.
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Thermal Sensor
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Thermal Energy
RTD: A resistance-temperature detector (RTD) is atemperature sensor that is based on the principles
discussed in the preceding section; that is metal
resistance increasing with temperature
Thermistors: The thermistor represents another class oftemperature sensor that measures temperature through
changes of material resistance
Thermocouple: A thermocouple is a junction of dissimilarmetal wires, usually joined to a third metal wire throughtwo reference junctions.
Seebeck effect and peltier effects
Thermal Sensor
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Criteria Thermocouple RTD Thermistor
Temperature RangeVery wide
-450F +4200F
Wide
-400F +1200F
Narrow
-100F +500F
Interchangeability Good Excellent Poor to fair
Long-term Stability Poor to fair Good Poor
Accuracy Medium High MediumRepeatability Fair Excellent Fair to good
Sensitivity (output) Low Medium Very high
Response Medium to fast Medium Medium to fast
Linearity Fair Good Poor
Self Heating No Very low to low High
Point (end) Sensitive Excellent Fair Good
Lead Effect High Medium Low
Size/Packaging Small to large Medium to small Small to medium
Temperature Sensor Attributes
T t S Ad t d Di d t
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Sensor Advantages Disadvantages
Thermocouple
No resistance lead wire problems
Fastest response
Simple, rugged
Inexpensive
High temperature operation
Point temperature sensing
Non-linear
Low voltage
Least stable, repeatable
Least sensitive
RTD Most stable, accurate
Contamination resistant
More linear than thermocouple
Area temperature sensing
Most repeatable temperature measurement
Current source required
Self-heating
Slow response time
Low sensitivity to small temperature
changes
Thermistor High output, fast
Two-wire ohms measurement
Economical
Point temperature sensing
Non-linear
Limited range
Fragile
Current source required
Self heating
Temperature Sensor Advantages and Disadvantages
Other Thermal Sensors
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Other Thermal Sensors
Bimetal strips: This type of temperaturesensor has the characteristics of being
relatively inaccurate, having hysteresis, having
relatively slow time response, and being lowin cost.
Gas Thermometers: It converts temperature
information directly into pressure signal , it isparticularly useful in pneumatic systems.
Other Thermal Sensors
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Other Thermal Sensors
Vapor-Pressure Thermometers:A vaporpressure thermometer converts temperatureinformation into pressure, as does the gasthermometer, but it operates by a different
process. LiquidExpansion Thermometers: Just as a
solid experiences an expansion in dimensionwith temperature, a liquid also shows anexpansion in volume with temperature.
Solid-state Temperature Sensors.
Design Considerations
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1. Identify the nature of measurement
2. Identify the required output signal
3. Select an appropriate sensor
4. Design the required signal conditioning
Design Considerations
All the best