electronic instrumentation & control systems

5/27/2018 Electronic Instrumentation & Control Systems

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Unit-1

Indicating & Recording Instruments

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It is often necessary to have a permanent record of a

phenomenon being investigated, e.g., in industrial & researchprocesses, it is required to monitor continuously the condition,

state, or value of the process variables such as flow, pressure,

temperature, current, & voltage etc.

A recorder, thus, records electrical/non-electrical quantities asa function of time

This record may be written/printed and later on can be

examined & analyzed to obtain a better understanding and

control of the process

Currents/voltages can be recorded directly while non-electrical

quantities are recorded indirectly by first converting them to

equivalent currents/voltages with the help of sensors or

transducers

Introduction to Recorders

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Classification of Recorders

Depending on how the data acquired is recorded? The

recorders are classified as follow:

(A) Analog Recorders:

When we are dealing with a wholly analog system

Broadly, analog recorders are classified into:(1) Graphic Recorders

(2) Oscillographic Recorders

(3) Magnetic-Tape Recorders

(B) Digital Recorder:

For the system whose output is in digital form

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(1) Graphic Recorders

Generally, graphic recordersare devices which display &

store pen-and-ink record of the history of some physicalevent on a chart

Basic Elements of Graphic Recorder:

Chartfor displaying & storing the recorded information

A stylus or penmoving in proper relationship topaper/chart

A suitable means of Interconnectionor Interfaceto

couple the stylus to the source of information

Graphic recorders may be classified into:

(a) Strip chart recorders

(b) X-Y recorders

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Strip Chart Recorders

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A strip chart recorder records one or more variables w.r.t. time

It is an X-t recorder, which consists of:

(i) A long roll of graph paper(ii) A system for driving the paper at some selected speed (usually 1-100

mm/s) by means of speed selector switch

(iii) A stylus, moving horizontally in proportion to the quantity being recorded

(iv) A stylus driving mechanism which moves the stylus in nearly exact

replica of the quantity being recorded

The mechanismsused for marking the marks on the paper are asfollow:

Marking with ink filled stylus

Marking with heated stylus Chopper Bar

Electronic Stylus

Electrostatic Stylus

Optical Marking Method

Strip Chart Recorders (-contd.)

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Marking with Ink Filled Stylus

The stylus is filled with ink by gravity or capillaryactions

The pointer supports an ink reservoir & a pen, orcontain a capillary connection between the pen &pen reservoir

This method is most commonly employed becauseordinary paper can be used and thus the cost is low

Other advantages:

Operation over a very wide range ofrecording speeds is possible

Little friction between the stylus tip & thepaper

Disadvantages: Ink splatters at high speeds, batches at low

speeds, & clogs when the stylus is at rest

The frequency limit of recorders incorporatingthis method of writing is only few Hz

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Marking with Heated Stylus

Some recorders use a heated styluswhich writes on a special paper (a

thin white wax like coating on a blackpaper)

This overcomes the difficultiesencountered in ink writing system

Quite reliable & offers high contrast

traces Sophisticated records, having a

frequency response upto 40 Hz, areavailable

Since the paper is a special one, the

cost is high This method cant be used for

recording certain processes whichproduce heat & indirectly affect therecordings

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Chopper Bar

If a chart made of a pressure sensitive paper is used then a

V-shaped pointer is passed under a chopper bar whichpresses the pen into the paper once per sec (or any other

selected interval) thus marking a series of marks on the

paper

As this system is not purely continuous, thus it is suitable forrecording some slowly varying quantities (e.g. having a

variation of 1 cycle per hour)

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Electronic Stylus

This method uses a paper

with a special coatingwhich is sensitive to current

When the current is

conducted from the stylus

to the paper, a trace

appears on the paper

The electronic stylus

marking method has a wide

range of marking speeds,

low stylus friction, & a long

stylus life

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Electrostatic Stylus

This method uses a stylus which produces a high voltage

discharge, thereby, producing a permanent trace on an electro-sensitive paper

This arrangement has been incorporated in a recorder having a

50 mm wide chart with 9 voltage ranges (10 mV/mm5 V/mm),

8 chart speeds (300600 mm/s), & a frequency response of 60Hz at maximum amplification of 1 dB

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Optical Marking Method

This method uses a beam of light to write on a photosensitive

paper This method allows higher frequencies to be recorded and

permits a relatively large chart speed with good resolution

Disadvantages: Paper cost is very high

Photographic paper must be developed before a record

is available

Not suitable for processes where instantaneousmonitoring is to be done

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Strip Chart Recorders: Tracing Systems

To produce graphical

representation of any quantity, two

types of tracing systemsusedare:

Curvilinear Tracing System

Rectilinear Tracing System

Curvilinear Tracing System:

In this system, the stylus is

mounted a central pivot & moves

through an arc which allows a full-

width chart marking If the stylus makes a full range

recording, the line drawn across

the chart will be curved & the time

intervals will be along these curved

segments 13


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Curvilinear Tracing System

This type of system is

used on many recorders

with PMMC

galvanometers actuating

the stylus filled with ink as

shown in fig. (28.50)

The disadvantage of thismethod of tracing is the

charts are difficult to

analyze because of

curved time base lines

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Rectilinear Tracing System Fig. (28.51) shows rectilinear system of tracing

This system produces a straight line across the width of the chart

Here, the stylus is actuated by a drive cord over pulleys to produceforward/reverse motion as determined by drive mechanism

The stylus may be actuated by a self balancing potentiometer

system, a

photoelectric

deflection system,

a photoelectric

potentiometer

system, or a

bridge balancesystem

This is usually

used with thermal

or electric writing

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Types of Strip Chart Recorders

There are two different types of strip chart recorders:

Galvanometer Type

Null Type

Galvanometer Type Strip Chart Recorder:

The deflection in this recorder is produced by a galvanometer

(dArsonval) which produces a torque on account of a current passingthrough its coil

The current is proportional to the quantity being measured

When the pointer comes to rest because of controlling torque exertedby springs, the stylus also comes to rest and thus the value of the

quantity is recorded This type of recorder is not useful for recording fast variations in

current/voltage/power and only records the average values

The range of operation is from few mA/mV to several mA/mV

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These moving galvanometer type recorder is comparativelyinexpensive instrument having a narrow bandwidth of 0-10 Hz and

a sensitivity of 0.4 mV/mm For measurement of smaller voltage, linear amplifiers are used

Null Type Strip Chart Recorder:

Many recorders operate on the principle whereby a change in its

input upsets the balance of the measuring circuit of the recorder As a result of this unbalance, an error signal is produced that

operates some device which restores balance or brings the system

to null condition

The amount of movement of this balance restoring device is anindication of the magnitude of the error signal and the direction of

the movement is an indication of the direction of the quantity being

measured has deviated from normal

The signal from the transducer/sensor may take any of the forms: voltage

(ac or dc), current (ac or dc), resistance, inductance, or capacitance

Types of Strip Chart Recorders (-contd.)

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Null Type Recorders

There are a number of null type strip chart recorders:

(i) Potentiometric Recorders(ii) Bridge Recorders

(iii) LVDT (Linear Variable Differential Transformer) Recorder

Note: The principle of operation for all these recorders is same, i.e.,

to obtain null condition

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Potentiometric Recorders

The basic disadvantage of a galvanometer type of recorder is

that it has a low input impedance

A simple method of overcoming the input impedance problem

is to use an amplifier between the input terminals & the

display/indicating instrument

However, this technique, while producing a high input

impedance (to reduce loading effect) & improved sensitivity(4V/mm), results in an instrument having low accuracy

The accuracy can be improved if the input signal is compared

with a reference voltage by using a potentiometer circuit

The error signal (difference between the input signal & thepotentiometer voltage) is amplified and is used to energize

the field coil of a dc motor, which moves over the

potentiometer in the appropriate direction to reduce the

magnitude of the error signal and to obtain balance 19


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The wiper comes to rest when the unknown signal voltage is

balanced against the voltage of the potentiometer The chart drive for most potentiometer recorders is obtained

from a motor synchronized to power line frequency

Different speeds may be obtained by using a gear train that

uses different gear ratios The most common application of the potentiometric recorders

is for recording & control of process temperature

Potentiometric Recorders (-contd.)

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Si l P i t & M lti i t R d


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Single Point & Multipoint Recorders

Most of the graphic records show variation of one variable (e.g.

temperature) with time

The instruments that record changes of only one measured variable

w.r.t. time are called single-pointrecorders and the trace of such

instruments is usually in the form of single continuous curve

However, in process industry, it becomes necessary to record

simultaneously variables like pressure, temperature, flow rate, liquidlevel etc.

This feature may be performed either by having several pens

(maximum four) which overlap each other & record the inputs

simultaneously, or by replacing the pen by print wheel geared to a

selector switch so that when a particular input is connected to thepotentiometer balance circuit, a point plus its identifying character is

printed on the chart

This form of recorder is called a multi-point recorder and may have

as many as 24 inputs, with traces displayed in 6 colors 21

X Y R d


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X-Y Recorders

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X Y R d ( td )


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An X-Y recorder is an instrument which gives a graphic record of the

relationship between two variables, i.e., one emf is plotted as a

function of another emf Here, one self-balancing potentiometer circuit moves the recording

pen in x-direction while another self-balancing potentiometer circuit

moves the recording pen in y-direction and the paper remains

stationary

An X-Y recorder consists of a pair of servo-systems, driving a stylus

in two axes through a proper sliding pen & moving arm arrangement,

with reference to a stationary paper chart

Attenuators are used to bring the input signals to the levels (full scale

range: 0.5 mV) acceptable by the recorder As shown in fig (28.53), error signals (the difference between the

input signals & reference voltage) are fed to a choppers which

convert dc signals to ac signals

X-Y Recorders (-contd.)

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X Y R d ( td )


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The signals are then amplified in order to actuate servomotors

which are used to balance the system & hold it in balance asthe value of the quantity being recorded changes

Thus, we get a record of one variable w.r.t. another

In some X-Y recorders, one self-balancing potentiometer

controls the position of the rolls (i.e. the paper) while anotherself-balancing potentiometer controls the position of the

recording pen

An X-Y recorder may have a sensitivity of 10 V/mm, a

slewing speed of 1.5 m/s, a frequency response about 6 Hz forboth axes, chart size of 250 x 180 mm, and accuracy of about

0.3%


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X Y Recorders ( contd )


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Uses:

(i) Speed-torque characteristics of motors

(ii) Plotting of characteristics of zener diodes, rectifiers, transistors

(iii) Regulation curve (output versus input voltage) of power

supplies

(iv) Electrical characteristics of materials such as resistance

versus temperature

(v) Plotting stress versus strain curves, hysteresis curves etc.


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O ill hi R d


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Oscillographic Recorders

Ultraviolet Recorders:

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Ultraviolet Recorders ( contd )


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Ultraviolet Recorders (-contd.) These are basically electromechanical oscillographic recorders

consist of a number of galvanometer (moving coil) elements

mounted in a single magnet block as shown in figure

The galvanometer uses a source of ultraviolet (u.v.) light in

place of white light and a paper sensitive to u.v. light is used

for producing a trace for recording

The u.v. light is projected on the paper with the help of mirrors

attached to the moving coils (pencil galvanometers)

As shown in figure, when a current is passed through the

galvanometer coil, it deflects under the influence of magnetic

field of permanent magnet

The u.v. light falling on the mirrors is reflected & projected on

the u.v. light sensitive paper through a lens & mirror system

The paper is driven past the moving light spot & thus a trace of

variation of current w.r.t. time is produced27



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The u.v. sensitive paper may be processed & photo-developed,

permanized or photocopied

The u.v. recorders are multichannel & have as many as 25

channels

Since, each channel may produce a 10 cm wide p-p trace on a

30 cm wide paper, there will be considerable overlapping of

traces produced by different channels

Therefore, it is essential to provide an identification mark for

each trace so as to avoid confusion

A simplified identification process is to interrupt each trace

momentarily in turn & to coincide this interruption with a

character marked on the side of record by passing u.v. light

through cutouts of the character

Ultraviolet Recorders (-contd.)

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The u.v. recorder may be used for dc & ac signals having a

fundamental frequency upto 400 to 500 Hz

The frequency range depends on the recorder being used and

paper driving speeds

The recording of high frequency inputs is possible if recorders

with high paper speeds of about 10 m/s are available

These recorders are also used for recording the magnitude of

low frequency signals which cant be measured with analog

(pointer) type instruments

Applications:

Typical applications of u.v. recorders are in recording:

(i) output of transducers, (ii) control system performance, &

(iii) regulation transients of generators

Ultraviolet Recorders (-contd.)

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Magnetic Tape Recorders


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Magnetic Tape Recorders

Take up

Real

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Magnetic Tape Recorders ( contd )


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It is frequently desirable & in many cases necessary to record

data in such a way that they can be retrieved/reproduced inelectrical form again

The most common & useful way of achieving this is through

the use of magnetic tape recording

Generally, the magnetic tape recorders are used at highfrequency

A magnetic tape recorder consists of the following basic

components:

(i) Recording Head, (ii) Magnetic Tape, (iii) Reproducing Head,(iv) Tape Transport Mechanism, & (v) Conditioning Devices

Magnetic Tape Recorders (-contd.)

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(i) Recording Head:

This device responds to an electrical signal in such a manner

that a magnetic pattern is created in a magnetizable medium A fine air gap of length 5-15 m is shunted by passing the

magnetic tape

A current in the coil causes a flux of the same shape to

bridge the air gap & hence to pass through the magnetictape, thereby, magnetizing the iron oxide (Fe2O3) particles as

they pass the air gap

The state of magnetization of the oxide as it leaves the air

gap is retained, thus the actual recording takes place at thetrailing edge of the gap

Any signal recorded on the tape appears as a magnetic

pattern dispersed in space along the tape similar to the

original coil current variation with time


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(ii) Magnetic Tape:

Magnetic tape (12.7 mm wide & 25.4 m thick) is composed

of a coating of fine magnetic iron oxide particles (Fe2O3) on aplastic ribbon

The magnetic particles conform to the magnetic pattern

induced in them & retain it

(iii) Reproducing Head:

The reproducing head detects the magnetic pattern stored in

them & converts it back to original electrical signal

Reproducing is similar in appearance to that of a recording

head

(iv) Tape Transport Mechanism:

This mechanism moves the tape along the recording of the

reproducing heads at constant speed


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The tape mechanism must be capable of handling the tape

during various modes of operation without straining,

distortion, or wearing out the tape

This requires that the mechanism must use arrangements to

guide the tape past the magnetic heads with great precision,

maintain proper tension, & obtain sufficient tape to magnetic

head contact Arrangement for fast winding & reversing are also provided

A capstan & pinch roller are used to drive the tape

(v) Conditioning Devices:

These devices consist of amplifiers & filters required for

modifying the signal to a format that can be properly recorded

on a tape


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Working Principle:

When a magnetic tape is

passed through a recordinghead, any signal recorded on

the tape appears as magnetic

pattern dispersed in space

along the tape, similar to the

original coil current variation

with time

The same tape when passed

through reproduce/playback

head produces variations in the

reluctance of the winding,

thereby, inducing a voltage in

the winding dependent upon

the direction of magnetization &

its magnitude on the magnetic

tape


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The induced voltage is proportional to rate of change of flux

linkages

Therefore, the emf induced in the winding of reproducing head isproportional to rate of change of the level of magnetization on the

rate , where Nis number of turns of the winding

put on the reproduce head

Since, the voltage in the reproduce head is proportional to , thereproduce head acts as a differentiator

For example, let the original signal beA sin(t)

The current in the record head winding & the flux produced will be

proportional to this voltage, i.e., = K1A sin(t) (1)

where K1 is constant

Assuming that the tape retains this flux pattern & regenerates it in

the reproduce head, the voltage induced in the reproduce head

winding is ... (2)


dt

dNe.,e.i rep

dt

d

)tcos(NAK

dt

dKNe 2rep

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Thus the output signal from the reproduce head is derivative

of the input signal The magnitude of the output signal is not only proportional to

the flux recorded on the tape but also the frequency of the

recorded signal

Since, the recording is done at constant current level for allfrequencies, and therefore, in order to have an overall flat

frequency response (i.e., a high fidelity), the reproduce head

characteristics and the characteristics of the amplifier

connected in the reproduce head circuit must be

complementary (i.e., the amplitude must have a response of -6dB/octave as shown in figure)

This process of compensation is known as equalization


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Magnetic Tape Recorders (-contd )


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Advantages:

1. Magnetic tape recorders have a wide frequency range from dc to

several MHz2. They have a wide dynamic range which exceeds 50dB, this

permits the linear recording from full scale signal level to

approximately 0.3% of full scale

3. They have a low distortion

4. The magnitude of the electrical input signal is stored in magnetic

memory and this signal can be reproduced whenever desired

5. The recorded signal is immediately available with no time loss in

processing, the recorded signal can be reproduced (played back)

as many times as desired without loss of signal6. When the information has been processed, the tape can be erased

& reused to record a new set of data


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Magnetic Tape Recorders (-contd )


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7. Magnetic tape recording permits multi-channel recording, i.e., a

tape facilitates the continuous record of a number of signals to be

made simultaneously, which is a great advantage especially when

recording transient & once only signals

8. The use of magnetic tape recorders provides a convenient method

of changing the time base, i.e., the data may be recorded at very

fast speeds (1.52 or 3.05 m/s) and played back at speeds (2.38 or4.76 cm/s) slow enough to be recorded with low frequency

recorders like graphic recorders


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Digital Tape Recorders

Digital magnetic tapes are often used as storage devices in

digital processing applications Digital tape units are of two types:

(a) Incremental digital tape recorders

(b) Synchronous Digital Tape Recorders

(a) Incremental Digital Tape Recorders:

They are commanded to step ahead (increment) for each

digital character to be recorded

Input data may be at a relatively slow, or even discontinuous

rate

In this way, each character is equally & precisely spaced

along the tape40

Digital Tape Recorders ( contd )


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Digital Tape Recorders (-contd.)

(b) Synchronous Digital Tape Recorders:

In these, the tape moves at a constant speed (about 75 cm/s)while a large number of data characters are recorded

The data inputs are at precise rates upto 10s of thousand

characters/second

The tape is brought to speed, recording takes place, and tapeis brought to a fast stop

In this way, a block of characters (a record) is written with

each character spaced equally along the tape

Blocks of data are usually separated from each other by anerased area on the tape called the record gap

The synchronous tape unit starts & stops the tape for each

block of data to be recorded

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Digital Recording Method:

In the above two methods, the characters are represented on

magnetic tape by the coded combination of symbols (bits) 0 & 1 bythe IBM-NRZ format

This format uses the change in flux direction on the tape to indicate

symbol 1 & no change in flux direction to indicate symbol 0

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Digital Tape Recorders (-contd )


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In digital data recording, a recording field of sufficient amplitude

to produce magnetic saturation through the complete tape layer

thickness is reversed to record 1 & kept constant to record 0

Reproduction of this recording is achieved by using a timing

signal obtained from a separate clock track corresponding to the

time when a 1 or 0 symbols are recorded

In practice, larger fields are usually employed to ensure morereliable recordings on a coated thicker tape & also to minimize

the effect of dropouts (due to some random surface

inhomogenities in tape coatings because of dirt or poor

manufacture, some portions of the tape may not be perfectly

recorded, which is called dropout)

As a check on dropout errors, most tape systems include a parity

check


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Advantages of Digital DataTape Recording:

High accuracy

Insensitivity to tape speed

Use of simple conditioning equipment

The information is fed directly to a digital computer for

processing & control

Disadvantages of Digital DataTape Recording:

Poor tape economy

The information from transducers is in analog form, hence anADC is required

A high quality tape & tape transport mechanism are required


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Digital Display Devices


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Display devices provide a visual display of numbers, letters, &

symbols in response to electrical input, and serve as

constituents of an electronic display system The basic element in a digital display device is the display for a

single character because a multiple character display is

nothing but a group of single character displays

Classification of Displays:

In general, the displays are classified in a number of ways:

1. According to methods conversion of electrical data into visible light:

(i) Active displays(Light Emitters)CRTs, LEDs, Gas Discharge Plasma, etc.

(ii) Passive Displays (Light Controllers)LCDs

2. According to the Applications:

(i) Analog DisplaysBar-graphic displays (CRT)

(ii) Digital DisplaysNixies tubes, Alphanumeric, LEDs

Digital Display Devices

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Digital Display Devices ( contd )


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3. According to display size & physical dimensions:

(i) Symbolic DisplaysAlphanumeric, Nixie tubes, etc.(ii) Console DisplaysCRTs, LEDs, etc.

(ii) Large Screen DisplayEnlarged Projection Systems

4. According to the display format:

(i) Direct View Type (Flat Panel Planar)Segmental, Dotmatrix, CRTs

(ii) Stacked Electrode (Non-planar Type)Nixie Tubes

5. In Terms of Resolution & Legibility of Characters

(i) Simple Single Indicator

(ii) Multi-element Displays

Digital Display Devices (-contd.)

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Segmental Displays


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Segmental Displays The segmental displays may be either 7 or 14 segmental ones

depending upon whether numeric or alphanumeric displays are

required

(i) Seven Segmental Display:

This is used for numeric display, and consists of 7-segments a, b, c,

d, e, f, & g

It displays the digits (0 to 9) by illuminating proper segments from the

group The display is incandescent & operates on low voltages ( 512 V)

and requires about 550 mA current when using LEDs

LCDs are also used for segmental displays

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Segmental Displays (-contd.)


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(ii) Fourteen Segmental Display:

For display of alphanumeric characters (both numerals as

well as alphabets) a 14-segmental display unit is used byilluminating the proper combination of segments

Segmental Displays ( contd.)

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Dot Matrices


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Dot matrices may be used for display of numeric & alpha-numeric

characters

(i) A 3x5 Dot Matrix:

A 3x5 dot matrix may be used for display of numeric characters(ii) Dot Matrix Utilizing 27 dots:

Another system using 27 dots displays the numeric characters

The dots may be square or round with 0.4 mm side or diameter

LEDs or LCDs are used for display of dots

(iii) 5x7 Dot Matrix:

For display of alphanumeric characters a 5x7 matrix is used

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Dot Matrices (-contd.)


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( )

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Rear Projection Display


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j p y A typical block diagram of a projection display system is shown in figure

In a rear projection display system, the projector is on one side of a

translucent screen & the viewer is on the other

The image signal source & screen are not the part of the display system

Each of the 12 incandescent lamps inside the projector light source when

energized by the input signal illuminates a different part of the filmstrip

The lens system (optics) projects the illuminated part of the film onto a

viewing screen

Image Signal Source Controller

Image Engine Optics Screen

Light Source Optics Projector

Fig. Projector Schematic51

Rear Projection Display (-contd.)


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j p y ( )

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Nixie Tube


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NIXI = Numeric Indicator

eXperimental No. I

A nixie tube is an electronicdevice (non-planar) for displaying

numbers or other information, in

the form of a glass tube

containing multiples cathodes and

a wire mesh anode, filled withneon & often a little mercury

and/or argon at small fraction of

atmospheric pressure

The most common form of nixie

tube has 10 cathodes (thin wires)

in the shapes of the numerals 0-9

(and occasionally a decimal point,

but there are also types that show

various letters, signs, & symbols53

Nixie Tube (-contd.)


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( )

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http://www.decadecounter.com/vta/articleview.php?item=568http://www.decadecounter.com/vta/articleview.php?item=568http://www.decadecounter.com/vta/articleview.php?item=567


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( )

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In its normal operation, the anode is returned to +ve dc supply

(150-220 V) through a suitable current limiting resistor, the

value of the supply being greater than the worst-casebreakdown voltage of the gas within the tube

The gas in the vicinity of the appropriate cathode glows when

the cathode is switched to ground potential

Since 10 cathodes have to be associated with single anodeinside the glass bulb, they have necessarily to be stacked in

different planes

This requires different voltages for different cathodes to enable

the glow discharge

Nixie tubes have the following characteristics:

(1) The numerals are usually large (typically 15-30 mm high) and

appear in the same base line for in-line read-out

( )

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(2) Nixie tubes are single digit devices with or without a decimal

point

(3) The selected cathode carries current in the range of 1-5 mA(4) The Nixie tube can be pulse operated & hence can be used in

multiplexed displays

Applications:

Nixies were used as numeric displays in early digital frequency

counters, voltmeters, & many other types of technical

equipments

They also appeared in costly digital time displays in research

and military establishments

Later, alphanumeric versions in 14-segment display format

found use in airport arrival/departure signs & stock-ticker

displays

( )

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Light Emitting Diode (LED)


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g g ( )

LED is a diode that gives off visible light when it is biased properly & the

mechanism of electromagnetic radiation (injection luminescence) takes

place This occurs in two steps:

(i) injection of minority carriers across the junction, and

(ii) the radiative recombination of minority carriers

Two types of radiative recombination mechanisms are commonly

encountered in LEDs depending upon the band-gap characteristics of the

semiconductor material used, i.e.,

(a) direct recombination, & (b) indirect recombination

The emission of photons as a result of recombination of electrons & holes is

possible only when both energy & momentum are conserved

The simplest & most probable recombination process will be that where the

electrons & holes have the same (i.e., zero) value of momentum which is

the case of direct band-gap materials (e.g., GaP, GaN, GaAsP, Si2N3etc)

and the released light energy (photon in visible spectrum)

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Light Emitting Diode (LED) (-contd.)
http://en.wikipedia.org/wiki/File:LED_symbol.svg


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g g ( ) ( )

In indirect band-gap materials (e.g., Si, Ge, SiC, etc.) the conduction

band minima and valence band maxima occur at different values of

momentum and the recombination of electrons & holes result intoemission of energy into heat (phonons, which conserve momentum)

and light (photons, not in visible range)

The probability of recombination in this case is obviously much lower

than that in direct band-gap semiconductors

GaP is mainly used for emissions in the visible spectrum, it has a

gap energy of 2.26 eV and can be doped with ZnO to give red light

or with N2to give green light

LEDs are also housed in plastic bulb (the round end) that

concentrate the light in a particular direction LEDs are used extensively in segmental & dot matrix displays of

numeric & alphanumeric characters

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g g ( ) ( ) Advantages of LEDs in Electronic Displays:

(i) LEDs are miniature in size & they can be stacked

together to form numeric & alphanumeric displays in

high density matrix

(ii) The light output from an LED is a function of current

flowing through it, therefore, intensity of light emitted

can be smoothly controlled

(iii) LEDs require moderate power (1.2 V of emf & 20 mA

of current) for their operation (full brightness)

(iv) LEDs are very fast as they have switching time less

than 1 ns

(v) LEDs are rugged & can therefore withstand shocks

and vibrations, they can also operate over a wide

range of temperature (0-700C)

(vi) LEDs dont have a filament that will burn out, so they

last much longer (more than 100000 hrs)

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g g ( ) ( )

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g g ( ) ( )

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g g ( ) ( )

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Liquid Crystal Display (LCD)


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q y p y ( )

A liquid crystal is a material (normally organic for LCDs) that flows like a

liquid but whose molecular structure has some properties associated with

solids, i.e., liquid crystals are substances that exist in an odd state: sort of

like a liquid & sort of like a solid

Thus, their molecules tend to maintain their orientation (like the molecules

in a solid) but also move around to different positions (like the molecules in

a liquid)

Depending on the temperature & particular nature of a substance, liquidcrystals can be in one of the several distinct phases, whose nematicphase

makes LCDs possible

Liquid crystals are affected by electric current

A particular type of nematic liquid crystal, called twisted nematic (TN), is

naturally twisted and applying a varying electric current to these liquidcrystals will untwist them to varying degrees

LCDs use these liquid crystals because they react predictably to electric

current in such a way as to control light passage

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Liquid Crystal Display (LCD) (-contd.)


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y y ( ) ( )

Types of LCDs:

(1) Dynamic Scattering Type (Nematic Crystal)

(2) Field Effect Type (Twisted Nematic Crystal)

(1) Dynamic Scattering Type (Nematic Crystal):

As shown in fig. 1 (a) & (b), the individual molecules have a rod like

appearance

The liquid crystals are layered between glass sheets with transparentelectrodes (Indium Oxide) deposited on the inside faces

Under no bias condition, the incident light will pass through & the liquid

crystal structure appears clear

If a voltage (6-20 V) is applied across the conducting surfaces, the

molecular arrangement is disturbed and regions of different refractiveindices are established which reflect the incident light in different directions

at the interface of different refractive indices

This phenomenon is referred to as dynamic scattering and causes a

frosted-glass (bright) appearance67



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The LCD does not generate its own light but depends on a external or

internal light source

Therefore, during the day or in lighted areas, a reflector can be put behindLCD to reflect light back (reflective mode) & under dark condition, the LCD

must have its own internal light source either behind or side of LCD

(transmissive mode)

(2) Field Effect Type (Twisted Nematic Crystal):

The construction of field effect LCD is similar to that of dynamic scattering

type with the exception that two thin polarizing optical filters are placed at

the outside of each glass sheet

Similar to dynamic scattering LCD, the field effect can be operated in the

reflectiveor transmissive mode

In case of transmissive field effect LCD, the internal light source is on the

right & the viewer on the left as shown in fig. 2(a)

Only the vertical component of the incident light can pass through the

vertical polarizer68



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In between the two walls of the liquid crystal, there is a general drift from

one polarization to the other

The left-hand vertical light polarizer only permits the passage of verticalpolarized light

If there is no applied voltage then the viewer sees a uniformly dark pattern

across the entire display but when a threshold voltage is applied (2-8 V), the

rod like molecules align themselves with the field (perpendicular to the wall)

and the light passes directly through without the 900

shift and can passthrough the second vertically polarizer and the light area is seen by the

viewer

Through proper excitation of the segments of each digit, the pattern will

appear

In case of reflective type field effect LCD, the horizontally polarized light atthe far left encounters a horizontally polarized filter and passes through to

the other vertical polarization, & returned to the observer

If there is no applied voltage, there is a uniformly lit display

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The application of a voltage results in a vertically polarized incident light

encountering a horizontally polarized filter at the left which will not be able

to pass through and be reflected which results a dark area on the crystal

and the pattern

Field effect LCDs are normally used when a source of energy is prime

factor (e.g. in watches, cell phones, portable instruments, etc.), since they

consume very small power (in W range) as compared to the dynamic

scattering types (in mW range)

The cost is higher for field effect LCDs and their height is limited to about 2,

while dynamic scattering LCDs are available upto 8 in height

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Advantages of LCDs:

They have low power (in W) consumption as compared to LEDs (in

mW) They have low cost

Since the color generated by LCD units is dependent on the source

of illumination, so there is greater range of color choice (16.8 million

colors)

Disadvantages of LCDs:

They are very slow devices, having response time in the range of

100-300 ms

They occupy a large area When operated on dc, their life-span is quite small because LCDs

degrade chemically, therefore, they are used with ac supplies having

a frequency less than 500 Hz

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Segmental Gas Discharge Displays


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Segmented gas discharge displays work on the principle of gas discharge

glow similar to Nixie tubes

They are mostly available in 7 or 14-segment form to display numeric oralphanumeric characters

As shown in figure, each segment (including decimal point) of the 7-

segment display formed on a base has a separate cathode

The anode is common to each member of 7-segment group, which is

deposited on the covering face plate The space between the anode & cathodes contains the gas

For each group of segments, a keep alive cathode is also provided, which

improves the switching speed of the display by passing a small constant

current (few A) through it and acts as source of ions

Pins are connected to the electrodes at the rear of the base plate with thehelp of which external connections can be made

Figure shows the structure of a 7-segment display making the use of gas

discharge plasma

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Segmental Gas Discharge Displays (-contd.)


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Back electrodes of the thick film type serve as cathode segments & front

electrodes of the thin film type serve as transparent anodes

A gas (typically Neon) is filled in the discharge space between the cathode& anode segment

The gas is struck between the cathode & anode of a chosen segment so

that the cathode glow provides the illumination & characters can be

displayed by activating the appropriate segments

Advantages & Disadvantages:

The power requirements of such devices are more or less in the same

range as those for Nixie tubes (i.e., the power consumed is extremely small)

The major disadvantage of this gas discharge tube is that high voltage (150-220 V) is required for operating

Therefore, high voltage transistors (special ICs) are required as switches for

the cathodes

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