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ECEg535/EE507:-Instrumentation Eng’g 1

By Sintayehu Challa

CHAPTER 5: OUTPUT PRESENTATION UNIT

5.1 INTRODUCTION

All analogue electrical and electronic instruments can be broadly classified into two categories,

namely:

(1) Instruments with pointer movements, and

(2) Instruments with graphical displays.

All analogue electrical and electronic meters belong to the first category. Included in this very

wide group category are the basic electromechanical instruments used as panel board or

industrial instruments, and the portable electronic instruments. There are four types of these

meters built around the following principles of operations:

1. The permanent magnet moving coil (PMMC) movement mechanism, which responds

to average or direct current only;

2. The moving iron vane (MIV), fixed coil movement mechanism, which responds to d-c

or a-c currents.

3. The moving magnet fixed coil (MMFC) movement, which again responds to direct

current only; and

4. The electrodynamometer (EDM) movement with one fixed coil and one moving coil,

which again responds to direct or alternating currents.

We shall next examine briefly the principle of operation of each of the above type of

instruments by emphasising the input-output response relationships as discussed in the

generalised static and dynamic performance characteristics of instruments. Particular attention

will be devoted to the PMMC as this meter movement is still the heart of many simple and

complex electronic instrumentation systems.


By Sintayehu Challa

5.2 DYNAMICS OF POINTER METER MOVEMENTS

Electromechanical instruments have a number of common features in that they basically

involve pointer movements rotated against fixed scales. As illustrated in Fig. 5-1, the moving

elements of these instruments are mounted on rigid supports (called spindles) which rest

pivoted on jewel cups or bronze bearings. To minimise friction losses, the pivots and bearings

are polished as thoroughly as possible. The force giving rise to deflecting torques is typically

exerted on current carrying conductors, and in principle these can be determined using the laws

of electromagnetic dynamics. Thus starting from the instantaneous force acting on a charge of

q columb moving with a velocity V in an electric field of strength E and magnetic field of flux

density B, the basic force law yields

F = q (E + v x B) newtons

If E is neglected in comparison to the cross product vxB, we can set

F = q ( v x B )

Fig.5.1


By Sintayehu Challa

We consider first the movements of the PMMC. If two parallel conductors (carrying equal but

oppositely direct currents) are placed in a magnetic field, then two force couples giving rise to

a torque will result as shown in Fig. 5-2, which represents the top view of a PMMC.

The deflecting torque will then be given by

Fdi =τ

where d is the distance between the lines of action of the two couple forces.

This deflecting torque will be opposed by three other torque acting on the moving assembly,

namely:

1. A restoring or control torque which will be directly proportional to the angular

displacement θ, and is also directly proportional to the spring constant K of the spiral

spring, and thus can be set equal to K;

2. An accelerating torque which is dependent on the moment of inertial J of the moving

assembly, and with the angular acceleration 2

2

dt

d θ

3. A damping torque, which is dependent on the angular, speeddt

Cdθ C being an effective

damping coefficient.

Fig. 5.2


By Sintayehu Challa

Hence, the torque equation

∑ = 0τ

Can be written as

θθθ

τ

θθθ

τ

kdt

dC

dt

dJ

elyequivalentor

kdt

d

dt

dJ

i

i

++=

=−−−

2

2

2

2

0

The above equation represents the dynamic performance of a second order instruments.

5.3 THE PERMANENT MAGNET MOVING COIL (PMMC)

The permanent magnet moving coil (PMMC) movement mechanism, which responds to

average or direct current only; the dynamic performance of second order instruments of

PMMC is given by

θτ ki =

5.4 THE MOVING IRON VANE (MIV)

While the above procedure for obtaining the dynamic equation of the PMMC is in principle

valid for all electromechanical instruments, it could pose difficulties in determining the

magnetic forces, which give rise to the deflecting torque. An alternative procedure for

determining τi from energy relations is, however, found helpful for practically all

electromechanical and electrostatic pointer movements. The principle behind this method is

that torque can be related to rotational systems. Consider the top-view representation of a

repulsion type of moving iron vane movement shown in Fig. 5-2.


By Sintayehu Challa

Let θ, i, di, L, and dL denote, respectively, the angular displacement of the moving vane, the

current applied to the fixed coil, a small increase in current, the self-inductance of the coil, and

a small variation in L. During the time that the moving vane carrying the pointer is being

deflected, an emf will be induced in the coil, which can be expressed by

dt

dLi

dt

diL

Lidt

de

+=

= )(

Which are due to changes in i and L, respectively. The electrical work done against this

opposing emf will be given by

dLidiLi

dteiWdone

2+=

= ∫

This work must be equal to the change of stored energy in the whole system. This comprises

of the change in potential energy of the spring which is Kθ, and the change in energy stored in

the inductance of the coil due to current and inductance changes such that

LidLLdiiWstored

22

2

1)()(

2

1=+++= τθ

Fig.5.3


By Sintayehu Challa

Equating the above Equations, it can thus be shown that the deflecting torque will be

θθ

θθτ

d

dLiK

orWWd

d

d

dWdonestoredi

2

2

1

,)(

=

−==

Where K is the spring constant. which is generally valid for any rotating system. Simply

states that the deflection torque is proportional to the product of the square of the current and

the change in inductance due to the rotation of the moving vane from its equilibrium position.

5.5 THE ELECTRODYNAMOMETER (EDM)

The same procedure can be followed to describe the principle of operation of an EDM. The

basic construction of an electrodynamic instrument consists of a moving coil mounted the

magnetic field of two series-connected stator coils. Let il and i2 denote the current in the stator

and moving coil, respectively. The conditions relating the deflecting torque to the magnetising

currents il and i2 is very similar to the torque relationship applying to the moving-iron vane

instrument, but the important variable here is the mutual inductance M between the fixed and

moving coils, and the stored magnetic energy i1 i2 M. Starting from the expression for the

induced emf

dt

dMi

dt

diM

dt

dMi

dt

diM

eee i

22

11

2

+++=

+=

where the first and second terms resepresent the emfs induced in the moving coil, and the third

and fourth terms give the emfs induced in the fixed coil. By considering the change in stored

energy, we obtain

dMiiMdiiW istored 2112 ++= θτ

It can thus be shown that the deflecting torque for the EDM movement is given by

θτ

d

dM

K

iii

2

21=


By Sintayehu Challa

Ignoring the transient parts of the solutions of the basic equations for the three-meter

movements, the steady state solutions (θss) for the three basic electromechanical meter

movements can then be summarised by the following relationships:

(i) For the PMMC,

KIxinputtConsC

iNBldss === tanθ

(ii) For the MIV,

22

2

)(tan2

KIinputxtConsC

ddli

SS ===θ

θ

(iii) for the EDM,

2121

21

)()(tan IKIinputxinputxtConsC

ddMii

SS ===θ

θ

where C is stiffness of the suspension spring.

5.6 ANALOG ELECTRONIC INSTRUMENTS

All analogue electronic instruments are built around the PMMC as the basic display

mechanism. Such instruments are capable of measuring a-c signals (i.e voltages, currents and

non-electrical quantities) which cannot be measured by other standard electrical instruments.

As shown in Fig. 5-5, the input signal qi (t) is fed to a signal conditioning circuit before it is

applied to a PMMC instrument which contains different scales for different quantities on a

single plate.


By Sintayehu Challa

The signal conditioning circuit involves the use of a suitable range selector or attenuator as

well as an interconnection of active (i.e amplifying) and non-linear passive (i.e. rectifying)

devices. Depending on the electronic circuitry, an analogue electronic meter belongs to one of

the following instruments:

1. Average responding meter

2. RMS responding meter;

3. peak responding rectifier

4. peak-to-peak responding rectifier

5.6.1 AVERAGE RESPONDING METER

The deflection of the PMMC meter is proportional to the average value for this purpose the

instrument are used for dc responding either voltages or currents to safe values. This can be

realised in practice by using very large series of resistors (for voltage ), and very small shunt

paths (for current ).

Fig.5.4


By Sintayehu Challa

5.6.2 RECTIFIER METERS

As shown in Fig. 5-5, rectifier meters can be built with the use of half-wave or full-wave

rectifying devices. In both circuits, currents can only flow in one direction. The deflection of

the PMMC meter is proportional to the average value of the rectified current (or voltage).

Assuming that the input current is sinusoidal, then the average current Iav will be

WAVEFULLFORII

WAVEHALFFORII

RMSAVE

RMSAVE

9.0

45.0

=

=

Fig.5.5


By Sintayehu Challa

Typically, the meter scale can also be calibrated in terms of rms values of sinusoidal voltages

(or currents). However, the meter indications will be incorrect when non-sinusoidal voltages

are measured.

5.6.3 PEAK AND PEAK-TO-PEAK RESPONDING METERS

As shown in Figs 5-6, the peak-responding meter is built with the use of a capacitor and a

diode.

Fig.5.6


By Sintayehu Challa

It can be seen that

Vo(t) = Vin + Vc

where Vin can be taken as a sinusoidal voltage with

Vin(t) = Vm sin ωt

and Vc = Vm is the voltage to which the capacitor C will be charged when the diode D is

conducting during the negative half of the input voltage. The primary difference between the

peak-responding meter and the average responding meter is the use of a storage capacitor with

the rectifying meter. The average voltage read by the meter is given by

m

T

oavO VdttVT

tV == ∫ )(1

)(0

Which is equal to the peak amplitude of Vin.

The peak-responding voltmeter is able to measure signals of frequencies up to hundreds of

megahertz. However for unsymmetrical waveforms, the reading will be in error. To overcome

this problem, a capacitor and a diode can be added as shown in Fig. 5-7 to obtain a peak-to-

peak response.

Fig.5.7


By Sintayehu Challa

Fig.5.8


By Sintayehu Challa

INSTRUMENTS WITH GRAPHICAL DISPLAYS.

5.7 GENERAL PURPOSE CATHODE RAY OSCILLOSCOPES

The oscilloscope, or more generally the cathode ray oscilloscope (CRO), is the most versatile

electronic measuring instrument. The CRO can mainly be regarded as a true analog instrument

in that it serves both as a voltmeter, and as an electrical-to-optical transducer showing a

waveform display of the signal under measurement. Still more importantly, the CRO is an

indispensable instrument for the measurements of frequency, time duration, and phase

differences at audio and higher frequencies The basic structures of a general-purpose cathode

ray tube (CRT) are illustrated in Fig. 5-9 and 5-10. It consists of five major parts, namely: (i)

power supply, (ii) vertical amplifier, (iii) triggering circuit, (iv) time base circuit, (v) gate

amplifier, (vi) horizontal amplifier, and (vii) the cathode ray tube(CRT). The power supply

unit indicated in Fig. 5-9 provides d-c voltages to the various electrodes inside the CRT, and

also to the amplifier and time-base circuits.

Laboratory oscilloscopes are classified in many ways. Usually, the distinctions are based

either on frequency-response capability or on CRT characteristics. Thus there are low-

frequency oscilloscopes (DC to 10 MHz for vertical amplifier response), high frequency

Oscilloscopes (sometimes capable of capturing and displaying single-shot phenomena of less

Fig.5.9


By Sintayehu Challa

than 1-n sec rise time), and sampling oscilloscopes which reconstruct very high frequency

signals up to 18 GHz - repetitive waveforms on a dot-sample basis. In terms of the

characteristics of the display screen, there are standard refreshed phosphor oscilloscopes, and

storage oscilloscopes,

Depending upon the type of CRT used. With the foregoing general introduction, we shall next

try to examine in some detail important features and principles of operation of the CRO under

the following major topics:

- Beam Generation, Focusing and Deflection inside a CRO,

- Saw-tooth Sweep Waveform Generation and Waveform Display

- Large Input Signal Attenuation and Attenuators

- Bandwidth and Rise-Time features of CRO's

- Applications.

Without going too much into details, we shall thus attempt to understand firmly the underlying

principles and techniques which explain the basic operations and working conditions of

general-purpose CRO's in particular, and other similar devices.

Fig.5.10


By Sintayehu Challa

5.8 BEAM GENERATION, FOCUSING AND DEFLECTION

Generation and focusing of an electron beam inside a CRT is illustrated in Fig. 5-11. An

electron gun assembly mounted at the base of the CRT produces the electron beam. In this

section of the tube, continuous beams of electrons emitted by a heated cathode subsequently

pass through a series of electrodes. Starting from the control grid, followed by the accelerating

grid, the intensity of the beam of the beam is adjusted to produce a very thin line that will

strike the CRT screen with sufficient energy. In a laboratory type CRO, the diffused beam is

first narrowed into a pencil-sharp ray using an electrostatic method of focusing as illustrated in

Fig. 5-11.

Fig.5.11


By Sintayehu Challa

Fig 5.12 Electrostatic focusing in a CRT

Further, the focused beam spot striking the CRT screen can be deflected vertically and

horizontally by means of voltages applied to pairs of vertical and horizontal deflection plates

that are placed at right angles to each other, and the focused beam.

There are two methods employed in the deflections of electron beams in CRTs:

A) In laboratory oscilloscopes, the beam passes through two pairs of orthogonal mounted

deflection plates, which deflect the beam vertically up and downwards, and horizontally

sideways. These are called, respectively, vertical and horizontal deflection plates. Time

varying and direct current potentials are applied across each plate pair, and the deflection

method is called electrostatic deflection.

B) Another method of beam deflection, which is called magnetic deflection, uses cross-aligned

coils. This method of deflection is practically used in the CRTs found in television and word-

processing terminals.

The CROs used in measurement systems are built with electrostatic deflection mechanisms. As

illustrated in Fig. 5-13, the important parameters in the deflection mechanisms are

D = Separation or spacing of deflection plates at right angles to the beam;

Lp = Effective length of deflection plates int he direction of the focused electron beam;

S = Length along beam from center of deflecting plates to the CRT screen;


By Sintayehu Challa

Va = Beam voltage or the potential to which the electron beam has been accelerated

when it enters the deflection region,

Vd = Deflecting voltage between plates,

D = final deflection distance on the CRT screen, either horizontally or vertically;

Z = Coordinate along central axis of the CRT


By Sintayehu Challa

Fig. 5.13 Principle of electrostatic deflection in a CRT

Detailed analyses on the electrodynamics of the fields and accelerating voltages finally lead

))((a

dP

V

V

d

LSD =

From the above Equation, the deflection sensitivity is defined as the ratio of D to Vd in

millimeters per volt (or centimeters per volt)

De = D/Vd (millimeters/volt)

Which is independent of both the deflecting voltage Vd and the ratio e/m, but varies inversely

with the accelerate voltage Va. Inversely the deflection factor Va/D of a CRT

Is defined by

G = 1/De

Can thus be defined in volts/centimeter for standard measurements, or in volts/millimeter for

very sensitive deflections. Typical values of deflection sensitivities range from 1.0mm per volt

to 0.1 mm per volt, and with corresponding deflection factors ranging from 10 volts per

centimeter to 100 volts per centimeter.

5.9 SWEEP WAVEFORM GENERATION AND DISPLAY

To use an oscilloscope properly, it is first very essential to know when or why the various

selector switches are operated. The functions of the four important circuits (i.e-vertical input

circuits, horizontal amplifier circuits, triggering circuits, and sweep generator circuits) are

therefore discussed below.


By Sintayehu Challa

5.9.1 VERTICAL INPUT CIRCUITS

A typical functional diagram of vertical input circuits is illustrated in Fig. 5-14

The AC/DC switch short-circuits the blocking capacitor when the oscilloscope is to be used to

display signals from d-c to an upper frequency limit determined by the bandwidth of the

vertical amplifier. A calibrated attenuator placed before the vertical amplifier controls the

vertical sensitivity or deflection factor in VOLTS/DIVISION.

Fig.5.14Block – diagram representation of vertical, input circuits

5.9.2 TIME BASE CIRCUIT AND WAVEFORM DISPLAY

The most frequent application of an oscilloscope is to display a changing signal y(t), as a

function of time. Thus oscilloscopes are provided with time base or sweep-generator circuit

which produce saw-tooth voltages as illustrated in Fig. 5-15.

The waveform illustrated rises linearly from "A" to "B" during the active sweep period T1,

during which time the electron beam moves across the CRT screen with constant horizontal

velocity. the slope of this waveform and hence the horizontal beam velocity is adjusted by

setting the time base to the desired TIME DIVISION. During the retrace period, T2, the

electron beam is returned to its starting point and the CRT is "blanked out" during this flyback

time. To obtain a stable display on the CRT it is necessary that the start of each sweep by

synchronized to the signal being displayed. In other words, the frequency of the signal (y-

input) must be equal to or be integer multiples of the sweep frequency (i.e. l/Tl). Otherwise a

stable display will not be obtained.


By Sintayehu Challa

In conjunction with the sweep circuits, there is also a STABILITY control mechanism, which

controls the sensitivity of the sweep circuit to an input pulse. In TRIG (i.e. triggered)

operation, this control is set so that the sweep will not operate unless a triggering pulse is

received. The synchronized (SYNC) operation for stability control is adjusted such that

repetitive sweeps are generated. The function of the sweep circuit is shown in Fig.5-16. In

many respects, the horizontal amplifier circuits are similar to the vertical amplifier circuits.

However, the gain and bandwidth of the horizontal amplifier are usually lower than those of

the vertical amplifier. Just as the vertical amplifier is provided with different gain settings for

different input signal levels, the horizontal amplifier gain can also be adjusted to magnify or

expand the horizontal sweep display. This magnification is obtained by increasing the gain of

the amplifier by a variable setting or by a certain factor, which in effect increases the display

area width by the same factor.

Fig.5.16 Functional diagram of a sweep circuit

Fig. 5.15 Typical output of a time-base circuit


By Sintayehu Challa

5.9.3 TRIGGERING CIRCUITS

These circuits enable the sweep generator circuits to start the sweep generation in coincidence

with selected time spots. Triggering signals can be selected from one of the following three

sources:

a) Internal source from the vertical amplifier;

b) External source from an externally applied signal;

c) Line supply, which is a line frequency of 50 or 60 HZ, and usually obtained

from a transformer internal to the oscilloscope unit.

These are shown abbreviated as INT, EXT, and LINE on the front panel of a CRO. The

internal source, whereby the triggering signal is derived from one of the vertical inputs

is most frequently used, while line triggering is used to display signals which contain

components at line frequency or multiples of line frequency.

When the two triggering sources are not satisfactory, external-triggering sources will be

required. In standard service or laboratory oscilloscopes, one frequently finds triggering

controls or MODE selector switches for +VE or -Ve slope TRIGGERING TRIG LEVEL,

AUTO, TV LINE, and TV FRAME. The selection of the triggering point is illustrated in Fig.

5-17.


By Sintayehu Challa

Fig. 5.17 Selection of triggering level.

Consider the Y-input signal (i.e. vertical amplifier input) to be a triangular waveform. Then,

the triggering point can be chosen either from the + ve going or -ve going parts of the signal.

Additionally, the TRIG LEVEL required to obtain the pulse, which triggers the sawtooth

waveform generator can also, be adjusted. In the AUTO mode, the TRIG LEVEL is not used

and the circuit slope selector still operates as indicated above. The AUTO mode is usually

preferred as this minimizes the number of adjustments, which must be made to obtain a stable

display on the CRT screen.

The triggering modes TV LINE and TV FRAME are only found in oscilloscopes used

additionally for servicing television equipment, and they are used for synchronizing a

composite video waveform display with the horizontal or vertical synchronizing pulses,

respectively. It should be mentioned here that more complex triggering switches are also used

in advanced oscilloscopes.


By Sintayehu Challa

5.9.4 SAW TOOTH VOLTAGE GENERATION

The sweep waveform illustrated in Fig. 5-17 is generated with the use of circuits, which can be

represented as simple RC circuits as shown in Fig. 5-18.

The switch SW represents an electronic switching circuit which can alternatively connect the

capacitor to position 1 for charging, and to position 2 for discharging through resistors R1 and

R2, respectively. If the electronic switch is running free, then the sweep also becomes free-

running. Typical output waveforms are presented in Figs. 5-19(a) and (b) for different duration

of sweep or scan times.

Actual saw-tooth waveforms can be described by

Vc = V(1-e-t/R1C

)

in which the voltage levels Vm, Vm1 and Vm2 are determined by the values of the capacitor C

for fixed values of R1 and R2. To obtain T2<<T1, then R2C<<R1C. For different sweep rates,

different capacitors are then connected to the switch SW, which is physically built from a fast

active electronic switch such as unijunction transistor or a combination of bistable

multivibrators.

Fig. 5.18 Equivalent circuit for a

saw-tooth waveform generator


By Sintayehu Challa

5.9.5 CALIBRATED SWEEPS

To obtain reasonably accurate displays of applied waveforms, it is necessary to maintain a

precise relation between the electron beam and the sweep generation. This means that the

sweep voltage must be highly linear and adjustable between exactly known limits as illustrated

in Fig. 5-20. The one important use of such a calibrated sweep is for time calibrations during

measurements.

Very dependable oscilloscopes actually have two calibrated sweeps. One is produced by the

sawtooth voltage generator, and is called the main sweep. The second sweep is produced by a

similar but delayed circuit, thus giving a waveform, which is behind that of the main sweep.

The speed of the delayed waveform is usually arranged to be 2,5, and 10 times that of the main

Fig.5.20 Linearized sweep waveform needed for calibration

Fig.5.19


By Sintayehu Challa

sweep. Consequently, two time axes are produced in advanced oscilloscopes, each using a

time scale of its own.

5.9.6 MULTIPLE TRACE DISPLAYS

In many CRO measurements, it is necessary to compare one waveform (signal) with another

waveform simultaneously. To facilitate such applications, dual (and multiple) trace displays

are obtained using one of the following three methods: a) dual beam oscilloscope, (b) alternate

mode, and (c) chopped mode. These are illustrated in Fig.5-21.

Fig 5.21 Examples of multiple traces : (a) dual beam oscilloscope; (b) dual traces with alternate mode;

(c) dual traces with chopped mode

The use of two electron guns complete with two independent sweep generators can produce an

instrument known as a dual beam oscilloscope. The same effect may also be produced by a

single electron gun, but with the output being split into two independent controllable electron

beams. Employing a single electron gun, a double trace can also be produced by switching the

Y deflection plates from one input signal to another in an alternate mode of operation

(Fig. 5-21(b). By building dot traces of two (or more) channel inputs as shown in Fig. 5-21(c),

the instrument can also be operated in a chopped mode. In such a mode, the Y deflection

plates are switched from one input signal amplifier to another at a rate, which is faster than the

standard sweep rate.


By Sintayehu Challa

APPENDIX FOR CHAPTER 5

5.9.7 APPENDIX A 5.1 CHARACTERISTICS OF CRT SCREEN PHOSPHORS

Type

Trace Type

or

Fluorescence

PER-

SISTENCE

Relative

Writing

Speed

Application

P1

P2

P4

P7

P11

P31

P33

P39

Yellow-green

Blue-green

White

Blue

Blue-Violet

Yellow-green

Orange

Green

Medium

Medium

Medium to

Medium short

Medium

Medium

Medium short

Very long

Medium to

medium long

35%

70%

75%

95%

100%

75%

7%

4 0%

CROs, Radar

CROs

Black & white television

Radar, Medical

Photographic recording

General purpose

replacement for P1

Radar

Medical, graphics


By Sintayehu Challa

Note:- 1. By Fluorescence of a phosphor is meant the light emission observed when an

electron beam hits the phosphor.

2. By Phosphorescence is meant the colour of the light left after the phosphor has

been stimulated into light emission and the excitation has been removed.

While the phosphorescences of most phosphor screens are the same as their

fluorescences types P2 and p7 have yellow-green phosphorescences.

3. By Persistence is meant an approximate measure of the time it takes for the

phosphorescence to decay to l.e of the excitation level during fluorescence.

For short-persistence, decay time is less than 1 msec. If the decay-time is

less than 2 sec., the phosphor is said to have medium-persistence.

Phosphors with decay times of the order of a minute or more are said to have

long-persistence.

4. Writing-speed is a measure of the fastest deflection rate of a single beam trace

(or single-short wave-form display) that is visible on a film when a

photograph is taken of a waveform display with P11 taken as a reference

phosphor.


By Sintayehu Challa

DATA ACQUISITION SYSTEMS

In transducer instrumentation, data acquisition systems are used to measure and record

information for later study and analysis. The recorded data takes the form of signals with either

continuous or pulse waveforms. Accordingly, data acquisition systems are divided into analog

and digital recorders. The important analog data storing devices are: (1)X-Y recorders, (2)

graphical recorders with moving coil movements, and (3) analog magnetic tape recorders.

Digital recording, reproduction and processing of massive data, which is the standard feature

of computing systems, is commonly made with instrumentation magnetic tapes and disks,

5.9.8 X-Y RECORDERS

X-Y recordings are similar to CRT waveform displays, with the x-coordinate provided by a

ramp generator. The Y- input receives low-frequency analog signals. Accuracy and resolution,

functions of an X-Y recorder’s electronic and mechanical characteristics, determine the static

performance of such an instrument. The slew speed and acceleration in response to the capture

of rapid and transient signal input determine the dynamic response. Other important features,

which are also common to other graphical recorders, include chart size, number of pens, time

base capability, preamplifiers and filters.

5.9.9 GRAPHICAL RECORDERS

Graphical recorders measure variations of electrical or non-electrical quantities with respect to

time taking place over many seconds, minutes, hours, or days. In many instances, the recording

is needed to check the performance of industrial processes or electrical power generation and

distribution systems. Mostly, these recorders are built around a moving coil instrument where

Fig.5.22


By Sintayehu Challa

the indicating pointer is replaced by a writing pen movement on a graduated paper moving at a

constant speed. As shown in Fig. 5-22, the tip of the pointer leaves marks on the paper thereby

forming a permanent record of the amplitude of the input signal with respect to time.

Maximum sensitivity is of the order of 4mV/cm within a narrow bandwidth of d-c to about

10Hz. Important features of the basic graphical recorders are: (1) input impedance, (2) time

scale, (3) event markers, and (4) writing mechanism. The use of amplifier ensures the input

impedance to the recorder is maintained relatively high. The writing mechanism can be an ink

pen with a capillary feed system, or heated stylus recording the variations of the input (Vin) on

a heat sensitive paper. Other recorders, commonly known as photographic or ultraviolet (UV)

recorders, use a light beam as a pointer leaving traces on photographic papers. Recorders

known as multi-channel recorders contain a number of writing pens in all making marks

simultaneously on a wide roll of paper thereby permitting easy comparison of several

simultaneous functions.

5.9.10 MAGNETIC TAPE RECORDERS

The principle of recording with instrumentation magnetic tapes is illustrated in Fig. 5-23.

There are three main components: (1) a core with a small nonmagnetic gap, (2) a coil wound

on the core, and (3) a thin magnetic coating sitting on a base. The latter can be a wire, but in

most application it consists of a flexible plastic tape.

Fig.5.23


By Sintayehu Challa

The magnetic coating is a thin layer of iron oxide(Fe2°3) particles, and the core is made of

laminated steel alloys. The core assembly carrying the winding is called a tape recording head.

With current (1) flowing in the coil, a magnetic flux will bridge cross the non-magnetic gap,

thus magnetizing the iron oxide particles as they pass the gap. Because the magnetic coating is

purposely selected for its high remanence, the iron particles will remain magnetized in the

direction of tape travel with the magnitude of flux impressed upon the tape as it moves in front

of the non-magnetic gap. Hence, a recording of the applied signal will have been realized.

When the same tape is passed through the front gap of a similar playback head, it will cause

variations in the reluctance of the winding, and thereby inducing a voltage which ideally is

required to be a faithful reproduction of the recorded signal. A functional diagram of a

complete magnetic tape-recorder is shown .in Fig. 5-24. While two different heads are needed

for accurate instrumentation recording and reproduction, only one combined head is used for

coarse instrumentation works as well as for audio recordings. With one head, one needs only to

switch from a record to reproduce (playback) mode. At the same time the direction of tape

travel has accordingly to is set by the sense of rotation of the motor moving the tape transport.

Fig.5.24


By Sintayehu Challa

Four factors contribute to the accuracy of magnetic tape recording, particularly for

instrumentation and data processing purposes. These are: (1) the magnetization characteristics

of the magnetic recording medium, (2) the tape speed, (3) the bandwidth of the recorded signal,

and (4) the gap width. All these factors are considered together to minimize errors and

distortions.

Fig.5.25


By Sintayehu Challa

5.9.11 DIGITAL RECORDING

Magnetic tapes are also commonly used for digital recording of binary data encoded. in terms

of "1s" or "0s", both in instrumentation and data processing systems. Three broad methods of

digital recording can be briefly described as follows:

a) Return to zero (RZ) systems which employ one direction of saturation for" 1" ,

while magnetization to saturation in the opposite direction then represents "0" .

b) Non-return to zero (NRZ) systems which regard changes in the direction of

magnetization as "1", while no changes in magnetization are taken as "0".

c) Phase encoding (FE) systems in which one sense of magnetization is regarded

as" 1" , while the opposite sense is regarded as "0" .

For each method, the density .of recording is measured in terms of so many bits per tape

length. The unit often used in bits per inch (BPI), typical values being 8000 BPI, and 1600

BPI for NRZ and phase encoded systems, respectively. The recorded binaries represent

decimals or alphanumeric characters using one of the standard binary codes. Also, the record

and reproduce amplifiers are very simple, and the only requirements are maintenance of

accurate tape and head alignments. For computer application, recording is mostly made in a

block form, meaning that a block of information is recorded at one time, in place of

conventional or incremental recording where data is continuously stored on the tape. As the

recorded binaries are typically obtained by digitizing input analog signals, the latter can be

recovered by use of digital to analog converters. Alternative recording systems which are

gaining wide applications use the same principles as already described for magnetic recording,

but with the flexible tapes replaced by rigid disk packs, diskettes or floppy disks, all operated

with use of movable heads to scan magnetically coated surfaces. Storage capacities measured

in kilo (i.e. thousand), millions, and possibly

billions of bytes (i.e 8-bit words), and access times ranging from 35 to 100 ms are the two

important features of these recording devices. Data recorded on these devices is therefore

much more voluminous than those recorded in tapes, and can also be accessed much


By Sintayehu Challa

faster. Further, magnetic tapes are also commonly used for very large digital data

recording and processing.

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