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BITSPilaniPilani Campus Swapna Kulkarni

Instrumentation And Control

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BITSPilaniPilani Campus

Pressure Sensors(p>1 atmosphere)

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BITS Pilani, Pilani campus

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.




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.




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.




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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.




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.


Copyright 2006 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458


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.



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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BITS Pilani, Pilani Campus

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




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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.




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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.




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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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BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

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




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.



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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)]

67

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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




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.




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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BITS Pilani, Pilani campus73

Figure: The Op Amp summing Amplifier




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




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




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




Integrator

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IntegratorFigure An integrator circuit using an

op amp.

81

0dt

dVC

R

Voutin




dtVRC

Vinout

1

Differentiator

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Differentiator

82

Figure : This circuit takes the time derivative of the

input voltageCopyright 2006 by Pearson Education, Inc.



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




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


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.




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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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.


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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

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