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TRANSCRIPT
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Radar level measurement
- The users guide
Peter Devine
VEGA Controls / P Devine / 2000
All rights reseved. No part of this book may reproduced in any way, or by any means, without prior
permissio in writing from the publisher:
VEGA Controls Ltd, Kendal House, Victoria Way, Burgess Hill, West Sussex, RH 15 9NF England.
British Library Cataloguing in Publication Data
Devine, Peter
Radar level measurement - The users guide
1. Radar
2. Title621.3848
ISBN 0-9538920-0-X
Cover by LinkDesign, Schramberg.
Printed in Great Britain at VIP print, Heathfield, Sussex.
written by
Peter Devine
additional informationKarl Griebaum
type setting and layout
Liz Moakes
final drawings and diagrams
Evi Brucker
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Foreword ix
Acknowledgement xi
Introduction xiii
Part I
1. History of radar 1
2. Physics of radar 13
3. Types of radar 33
1. CW-radar 33
2. FM - CW 36
3. Pulse radar 39
Part II
4. Radar level measurement 47
1. FM - CW 48
2. PULSE radar 54
3. Choice of frequency 62
4. Accuracy 68
5. Power 74
5. Radar antennas 771. Horn antennas 81
2. Dielectric rod antennas 92
3. Measuring tube antennas 101
4. Parabolic dish antennas 106
5. Planar array antennas 108
Antenna energy patterns 110
6. Installation 115
A. Mechanical installation 115
1. Horn antenna (liquids) 115
2. Rod antenna (liquids) 117
3. General consideration (liquids) 120
4. Stand pipes & measuring tubes 127
5. Platic tank tops and windows 134
6. Horn antenna (solids) 139
B. Radar level installation cont. 141
1. safe area applications 1412. Hazardous area applications 144
Contents
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The benefits of radar as a level mea-surement technique are clear.
Radar provides a non-contact sensorthat is virtually unaffected by changesin process temperature, pressure or thegas and vapour composition within avessel.
In addition, the measurement accu-racy is unaffected by changes in densi-ty, conductivity and dielectric constantof the product being measured or by air
movement above the product.The practical use of microwaveradar for tank gauging and process ves-sel level measurement introduces aninteresting set of technical challengesthat have to be mastered.
If we consider that the speed of lightis approximately 300,000 kilometresper second. Then the time taken for
a radar signal to travel one metreand back takes 6.7 nanoseconds or
0.000 000 006 7 seconds.How is it possible to measure this
transit time and produce accurate ves-sel contents information?
Currently there are two measure-ment techniques in common use forprocess vessel contents measurement.They are frequency modulated continu-ous wave (FM - CW) radar and PULSE
radarIn this chapter we explain FM - CWand PULSE radar level measurementand compare the two techniques. Wediscuss accuracy and frequency consid-erations and explore the technicaladvances that have taken place inrecent years and in particular two wire,loop powered transmitters.
47
4. Radar level measurement
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The FM - CW radar measurementtechnique has been in use since the
1930's in military and civil aircraftradio altimeters. In the early 1970's thismethod was developed for marine usemeasuring levels of crude oil in super-tankers. Subsequently, the same tech-nique was used for custody transferlevel measurement of large land basedstorage vessels. More recently, FM -CW transmitters have been adapted for
process vessel applications.FM - CW, or frequency modulatedcontinuous wave, radar is an indirectmethod of distance measurement. Thetransmitted frequency is modulatedbetween two known values, f1 and f2,and the difference between the trans-mitted signal and the return echosignal, fd, is measured. This differencefrequency is directly proportional to the
transit time and hence the distance.(Examples of FM - CW radar level
transmitters modulation frequencies are8.5 to 9.9 GHz, 9.7 to 10.3 GHz and 24to 26 GHz).
The theory of FM - CW radar issimple. However, there are many prac-tical problems that need to beaddressed in process level applications.
An FM - CW radar level transmitterrequires a voltage controlled oscillator,
VCO, to ramp the signal between thetwo transmitted frequencies, f1 and f2.It is critical that the frequency sweep iscontrolled and must be as linear as pos-sible. A linear frequency modulation isachieved either by accurate frequencymeasurement circuitry with closed loopregulation of the output or by carefullinearisation of the VCO output includ-ing temperature compensation.
48
FM-CW, frequency modulated continuous wave
frequency
f2
f1t1
t
fd
time
Transmitted signal
Receivedsignal
Fig 4.1 The FM - CW radar technique is an indirect method of level measurement.
fd is proportional to t which is proportional to distance
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49
4. Radar level measurement
Fig4.2Typical
blockdiagramofFM-CWradar.Averyaccuratelinear
sweepisrequired
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FM - CW block diagram (Fig 4.2)The essential component of a fre-
quency modulated continuous waveradar is the linear sweep control cir-cuitry. A linear ramp generator feeds avoltage controller which in turn rampsup the frequency of the VoltageControlled Oscillator. A very accuratelinear sweep is required. The output
frequency is measured as part of theclosed loop control.
The frequency modulated signal isdirected to the radar antenna and
hence towards the product in the ves-
sel. The received echo frequencies aremixed with a part of the transmission
frequency signal. These difference fre-quencies are filtered and amplifiedbefore Fast Fourier Transform (FFT)analysis is carried out. The FFTanalysis produces a frequency spec-trum on which the echo processing andecho decisions are made.
Simple storage applications usuallyhave a large surface area with very lit-tle agitation, no significant false echoesfrom the internal structure of the tankand relatively slow product movement.These are the ideal conditions forwhich FM - CW radar was originallydeveloped.
However, in process vessels there is
more going on and the problemsbecome more challenging.
Low amplitude signals and falseechoes are common in chemical reac-tors where there is agitation and lowdielectric liquids.
Solids applications can be trouble-some because of the internal structureof the silos and undulating product sur-faces which creates multiple echoes.
An FM - CW radar level sensor
transmits and receives signals simulta-neously.
50
Pic 2 Typical glass lined
agitated process
vessel. A radar
must be able to
cope with variousfalse echos from
agitatior blades
and baffles
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4 . Radar level measurement
In an active process vessel, the vari-ous echoes are received as frequencydifferences compared with the frequen-cy of the transmitting signal. These fre-quency difference signals are receivedby the antenna at the same time. Theamplitude of the real echo signals aresmall compared with the transmittedsignal. A false echo from the end of theantenna may have a significantly high-er amplitude than the real level echo.The system needs to separate and iden-tify these simultaneous signals beforeprocessing the echoes and making anecho decision.
The separation of the variousreceived echo frequencies is achievedusing Fast Fourier Transform (FFT)
analysis. This is a mathematical proce-
dure which converts the jumbled arrayof difference frequencies in the timedomain into a frequency spectrum inthe frequency domain.
The relative amplitude of each fre-quency component in the frequencyspectrum is proportional to the size ofthe echo and the difference frequencyitself is proportional to the distancefrom the transmitter.
The Fast Fourier Transform requiressubstantial processing power and is arelatively long procedure.
It is only when the FFT calculationsare complete that echo analysis can becarried out and an echo decision can bemade between the real level echo and anumber of possible false echoes.
Fig 4.3a FM - CW radar level transmitters in an active process vessel
Transmitted signal
f2
fd1, -f-f d2d2, -fd3, -fd4, -fd5
f1 t1
Real echo signal
False echo signals
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52
Fig 4.3b combined echo frequencies are received simultaneously
Fig 4.3c The individual frequencies must be separated from
the simultaneously received jumble of frequencies
Signalamplitude
Signalamplitude
Mixture of frequencies received by FM - CW radar
Combination of mixed difference frequencies received by FM - CW radarIndividual difference frequencies fd1, ffd2d2, fd3, are shown
fd1, fd2, fd3, fd4, fd5 etc combined
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Complex process vessels and solidsapplications can prove too difficult forsome FM - CW radar transmitters.Even a simple horizontal cylindricaltank can pose a serious problem. Thisis because a horizontal tank producesmany large multiple echoes that arecaused by the parabolic effect of thecylindrical tank roof. Sometimes theamplitudes of the multiple echoes are
higher than the real echo. The proces-sors that carry out the FFT analysis areswamped by different amplitude sig-nals across the dynamic range all at thesame time. As a result, the FM - CWradar cannot identify the correct echo.
As we shall see, this problem doesnot affect the alternative pulse radartechnique.
53
4 . Radar level measurement
Fig 4.4 FM - CW frequency spectrum after Fast Fourier transform. The Fast Fourier
transform algorithm converts the signals from the time domain into the frequencydomain. The result is a frequency spectrum of the difference frequencies. The
relative amplitude of each frequency component in the spectrum is proportional to
the size of the echo and the difference frequency itself is proportional to the
distance from the transmitter. The echoes are not single frequencies but a span
of frequencies within an envelope curve
Frequency spectrum echoesEach echo is within an envelope curve of frequencies
amplitude
frequency
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PULSE radar level transmitters
54
Pulse radar level transmitters pro-vide distance measurement based on
the direct measurement of the runningtime of microwave pulses transmittedto and reflected from the surface of theproduct being measured.
Pulse radar operates in the timedomain and therefore it does notrequire the Fast Fourier transform(FFT) analysis that characterizes FM -CW radar.
As already discussed, the runningtime for a distance of a few metres ismeasured in nanoseconds. For this rea-son, a special time transformation pro-
cedure is required to enable these shorttime periods to be measured accurately.
The requirement is for a slow motionpicture of the transit time of themicrowave pulses with an expandedtime axis. By slow motion we meanmilliseconds instead of nanoseconds.
Pulse radar has a regular and period-ically repeating signal with a high pulserepetition frequency (PRF). Using amethod of sequential sampling, the
extremely fast and regular transit timescan be readily transformed into anexpanded time signal.
Fig 4.5 Pulse radar operates purely within the time domain. Millions of pulses are
transmitted every second and a special sampling technique is used to produce a
time expanded output signal
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A common example of this principleis the use of a stroboscope to slowdown the fast periodic movements ofrotating or reciprocating machinery.
Fig 4.7 shows how the principle of
sequential sampling is applied topulse radar level measurement. Theexample shown is a VEGAPULS trans-mitter with a microwave frequency of5.8 GHz.
55
4 . Radar level measurement
To illustrate this principle, considerthe sine wave signal in Fig 4.6. It is a
regular repeating signal with a periodof T1. If the amplitude (voltage value)of the output of the sine wave is sam-pled into a memory at a time period T2
which is slightly longer than T1, then atime expanded version of the original
sine wave is produced as an output.The time scale of the expanded outputdepends on the difference between thetwo time periods T1 and T2.
Fig 4.6 The principle of sequential sampling with a sine wave as an example.
The sampling period, T2, is very slightly longer than the signal period, T1. The
output is a time expanded image of the original signal
Fig 4.7 Sequential sampling of a pulse radar echo curve. Millions of pulses per secondproduce a periodically repeating signal. A sampling signal with a slightly longer
periodic time produces a time expanded image of the entire echo curve
PeriodicSignal(sine wave)
Periodic
Signal(radar echoes)
Samplingsignal
Samplingsignal
Expandedtime signal
T1
T2
T1
Emission
pulse
Echo
pulse
T2
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ExampleThe 5.8 GHz, VEGAPULS radar level transmitter has the following pulse repeti-tion rates.
Transmit pulse 3.58 MHz T1 = 279.32961 nanosecondsReference pulse 3.58 MHz - 43.7 Hz T2 = 279.33302 nanoseconds
56
Therefore the time expansion factoris 81920 giving a time expanded pulserepetition period of 22.88 milliseconds.
There is a practical problem in sam-pling the emission / echo pulse signalsof a short (0.8 nanosecond) pulse at 5.8GHz. An electronic switch would needto open and close within a few picosec-onds if a sufficiently short value of the5.8 GHz sine wave is to be sampled.
These would have to be very specialand expensive components.
The solution is to combine sequen-tial sampling with a cross correlationprocedure.
Instead of very rapid switch sam-pling, a sample signal of exactly thesame profile is generated but with aslightly longer time period between thepulses.
Fig 4.9 compares sequential sam-pling by rapid switching with sequen-
tial sampling by cross correlation witha sample pulse.
This periodically repeating signalconsists of the regular emission pulse
and one or more received echo pulses.These are the level surface and anyfalse echoes or multiple echoes. Thetransmitted pulses and therefore thereceived pulses have a sine wave formdepending upon the pulse duration. A5.8 GHz pulse of 0.8 nanosecond dura-tion is shown in Fig 4.8.
The period of the pulse repetition is
shown as T1 in Fig 4.7. Period T1 is
the same for the emission pulse repeti-tion as for any echo pulse repetition as
shown.However, the sampling signal
repeats at period of T2 which is slight-ly longer in duration than T1. This isthe same time expansion procedure bysequential sampling that has alreadybeen described for a sine wave. Thefactor of the time expansion is deter-mined by T1 / (T2-T1).
Fig 4.8 Emission pulse (packet).
The wave form of the 5.8
GHz pulse with a pulse
duration of 0.8
nanoseconds
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Instead of taking a short voltagesample, cross correlation involves mul-tiplying a point on the emission or echosignal by the corresponding point onthe sample pulse. The multiplicationleads to a point on the resultant signal.All of these multiplication results, oneafter the other, lead to the formation ofthe complete multiplication signal.
Fig 4.10 shows a short sequence ofmultiplications between the receivedsignal (E) and the sampling pulsesignal (M). The resultant E x M curvesare shown on page 58.
Then the E x M curve is integratedand represented on the expanded curveas a dot. The sign and amplitude of the
signal on the time expanded curvedepends on the sum of the area of theE x M curve above and below the zeroline. The final integrated value corre-sponds directly to the time position ofthe received pulse E relative to thesample pulse M.
The received signal E and samplesignal M in Fig 4.10 are equivalent tothe periodic signal (sine wave) andsample signal in Fig 4.6. The result ofthe integration of E x M in Fig 4.10 isdirectly analogous to the expandedtime signal in Fig 4.6.
57
4 . Radar level measurement
Fig 4.9 Comparison of switch sampling with cross correlation sampling. The pulse
radar uses cross correlation with a sample pulse. This means that rapid picosec-
ond switching is not required
Sampling with picosecond switching
Sample signal
Emission / Echo pulse
Sampling by cross correlation with asample pulse
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The pulse radar sampling procedureis mathematically complicated but atechnically simple transformation toachieve. Generating a reference signalwith a slightly different periodic time,multiplying it by the echo signal andintegration of the resultant product areall operations that can be handled easi-ly within analogue circuits. Simple, but
good quality components such as diodemixers for multiplication and capaci-tors for integration are used.
This method transforms the highfrequency received signal into an accu-rate picture with a considerablyexpanded time axis. The raw valueoutput from the microwave module isan intermediate frequency that is simi-lar to an ultrasonic signal. For examplethe 5.8 GHz microwave pulse becomesan intermediate frequency of 70 kHz.
The pulse repetition frequency (PRF)of 3.58 GHz becomes about 44 Hz.
58
Fig 4.10 Cross correlation of the received signal E and the sampling M.
The product E x M is then integrated to produce the expanded time curve. The
technique builds a complete picture of the echo curve
E
IntegralE x M
max
min
0
M
E x M
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4 . Radar level measurement
amplitude
transmit pulse
t1 tt22 t3 t4 t5
time
Pulse radar operates entirely within
the time domain and does not need thefast and expensive processors thatenable the FM - CW radar to function.There are no Fast Fourier Transform(FFT) algorithms to calculate. All ofthe pulse radar processing is dedicatedto echo analysis only.
Part of the pulse radar transmissionpulse is used as a reference pulse thatprovides automatic temperature com-pensation within the microwave mod-ule circuits.
The echoes derived from a pulseradar are discrete and separated in time.This means that pulse radar is betterequipped to handle multiple echoes andfalse echoes that are common inprocess vessels and solids silos.
Pulse radar takes literally millions of
shots every second. The return echoesfrom the product surface are sampledusing the method described above. Thistechnique provides the pulse radar withexcellent averaging which is particular-ly important in difficult applicationswhere small amounts of energy arebeing received from low dielectric andagitated product surfaces.
The averaging of the pulse techniquereduces the noise curve to allow small-er echoes to be detected. If the pulseradar is manufactured with welldesigned circuits containing good qual-ity electronic components they candetect echoes over a wide dynamicrange of about 80 dB. This can makethe difference between reliable andunreliable measurement.
Fig 4.12 With a pulse radar, all echoes (real and false) are separated in time. This allows
multiple echoes caused by reflections from a parabolic tank roof to be easily
separated and analysed
Pulse echoes in a process vessel are separated in time
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60
F
ig4.11Blockdiagramo
fPULSEradarmicrowavemod
ule
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4 . Radar level measurement
Pulse block diagram - (Fig 4.11)The raw pulse output signal (inter-
mediate frequency) from the pulseradar microwave module is similar, in
frequency and repetition rate, to anultrasonic signal.
This pulse radar signal is derived inhardware. Unlike FM - CW radar,PULSE does not use FFT analysis.Therefore, pulse radar does not needexpensive and power consuming
processors.The pulse radar microwave modulegenerates two sets of identical pulseswith very slightly different periodictimes. A fixed oscillator and pulse for-mer generates pulses with a frequencyof 3.58 MHz. A second variable oscil-lator and pulse former is tuned to a
frequency of 3.58 MHz minus 43.7 Hzand hence a slightly longer periodic
time. GaAs FET oscillators are used toproduce the microwave carrier fre-quency of the two sets of pulses.
The first set of pulses are directedto the antenna and the product beingmeasured. The second set of pulses arethe sample pulses as discussed in the
preceding text.The echoes that return to the anten-
na are amplified and mixed with thesample pulses to produce the raw, timeexpanded, intermediate frequency.
Part of the measurement pulse sig-nal is used as a reference pulse that
provides automatic temperature com-pensation of the microwave moduleelectronics.
Pic 3 Two wire pulse radar level transmitter mounted in a process reactor vessel
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Process radar level transmittersoperate at microwave frequencies
between 5.8 GHz and about 26 GHz.Manufacturers have chosen frequenciesfor different reasons ranging fromlicensing considerations, availability ofmicrowave components and perceivedtechnical advantages.
There are arguments extolling thevirtues of high frequency radar, low
frequency radar and every frequencyradar in between.
In reality, no single frequency isideally suited for every radar levelmeasurement application. If we com-pare 5.8 GHz radar with 26 GHz radar,we can see the relevant benefits of highfrequency and low frequency radar.
62
Choice of frequency
Fig 4.14 Comparison of 5.8 GHz and 26 GHz radar antenna sizes. These instruments
have almost identical beam angles. However this is not the full picture when it
comes to choosing radar frequencies
2.6 GHz
5.8 GHz
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The higher the frequency of a radarlevel transmitter, the more focused the
beam angle for the equivalent sizeantenna.
With horn antennas, this allowssmaller nozzles to be used with a morefocused beam angle.
For example, a 1" (40 mm) hornantenna radar at 26 GHz has approxi-mately the same beam angle as a 6"(150 mm) horn antenna at 5.8 GHz.
However, this is not the completepicture. Antenna gain is dependent onthe square of the diameter of the anten-na as well as being inversely propor-tional to the square of the wavelength.
Antenna gain is proportional to:-
Antenna gain also depends on the aper-ture efficiency of the antenna.Therefore the beam angle of a smallantenna at a high frequency is notnecessarily as efficient as the equiva-lent beam angle of a larger, lower fre-quency radar. A 4" horn antenna radar
at 6 GHz gives excellent beam focus-ing.A full explanation of antenna gain
and beam angles at different frequen-cies is given in Chapter 5 on radarantennas.
63
4 . Radar level measurement
Focusing at different frequencies
5 GHz 10 GHz 15 GHz 20 GHz 25 GHz
Fig 4.13 For a given size of antenna, a higher frequency gives a more focused beam
Antenna size - beam angle
diameter
wavelength
2
2
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A 26 GHz beam angle is morefocused but, in some ways, it has to be.
The wavelength of a 26 GHz radar isonly 1.15 centimetres compared with awavelength of 5.2 centimetres for a5.8 GHz radar.
The short wavelength of the 26 GHzradar means that it will reflect off many
small objects that may be effectivelyignored by the 5.8 GHz radar. Without
the focusing of the beam, the high fre-quency radar would have to cope withmore false echoes than an equivalentlower frequency radar.
Antenna focusing and false echoes
Fig 4.15 a Low frequency radar has a wider beamangle and therefore, if the installation
is not optimum, it will see more false
echoes. Low frequencies such as
5.8 GHz or 6.3 GHz tend to be more
forgiving when it come to false echoes
from the internal structure of a vessel
or silo
Fig 4.15 b High frequency radar has a much
narrower beam angle for a given
antenna size. The narrower beam angle
is important because the short
wavelength of the higher frequencies,
such as 26 GHz, reflect more readily
from the internal structures such as
welds, baffles, and agitators.
The sharper focusing avoids this
problem
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High frequency radar transmittersare susceptible to signal scatter from
agitated surfaces. This is due to the sig-nal wavelength in comparison to thesize of the surface disturbance.
The high frequency radar willreceive considerably less signal than anequivalent 5.8 GHz radar when the liq-
uid surface is agitated. The lowerfrequency transmitters are less affected
by agitated surfaces.It is important that, whatever the fre-
quency, the radar electronics and echoprocessing software can cope with verysmall amplitude echo signals. As dis-cussed, pulse radar has an advantage inthis area no matter what the frequency.
65
4 . Radar level measurement
Agitated liquids and solid measurement
Fig 4.16 High frequency radar transmitters aresusceptible to signal scatter from
agitated surfaces. This is due to the
signal wavelength in comparison to the
size of the surface disturbance. It is
important that radar electronics and
echo processing software can cope with
very small amplitude echo signals.
By comparison, 5.8 GHz radar is not as
adversely affected by agitated liquid
surfaces. Lower frequency radar is
generally better suited to solid level
applications
Condensation and build upHigh frequency radar level transmit-
ters are more susceptible to condensa-tion and product build up on the anten-na. There is more signal attenuation atthe higher frequencies, such as 26 GHz.Also, the same level of coating or con-densation on a smaller antenna natural-ly has a greater effect on the perfor-mance.
A 6" horn antenna with 5.8 GHz fre-quency is virtually unaffected by con-densation. Also, it is more forgiving of
product build up.
Steam and dustLower frequencies such as 5.8 GHz
and 6.3 GHz are not adversely affectedby high levels of dust or steam. Thesefrequencies have been very successfulin applications ranging from cement,flyash and blast furnace levels to steamboiler level measurement.
In steamy and dusty environments,higher frequency radar will suffer fromincreased signal attenuation.
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4 . Radar level measurement
Fig 4.19 Signal strength from agitated and undulating surfaces and radar frequency
reduced
signalcausedbydamping
reflectionfromm
edium
Reduced signal strength caused by
damping at higher emitting frequency
caused by:
. condensation. build - up. steam and dust
Higher damping caused by agitated
product surface
. wave movement. material cones with solids. signal scattered
5 GHz 10 GHz 15 GHz 20 GHz 25 GHz
frequency range
5 GHz 10 GHz 15 GHz 20 GHz 25 GHz
frequency range
67
Fig 4.18 Signal damping and radar frequency
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68
AccuracyThere is no inherent difference in
accuracy between the FM - CW andPULSE radar level measurement tech-niques.
In this book, we are concernedspecifically with process level mea-surement where process accurate andcost effective solutions are required.
The achievable accuracy of aprocess radar depends heavily on the
type of application, the antenna design,the quality of the electronics and echoprocessing software employed.
The niche market for custody trans-fer level measurement applications isoutside the scope of this book. Thesecustody transfer radar systems areused in bulk petrochemical storagetanks. Large parabolic or planar arrayantennas are used to create a finely
focused signal. A lot of processingpower and on site calibration time isused to achieve the high accuracy.Temperature and pressure compensa-tion are also used.
Range resolution andbandwidth
In process level applications, bothFM - CW and PULSE radar work withan envelope curve. The length of thisenvelope curve depends on the band-width of the radar transmitter. A widerbandwidth leads to a shorter envelopecurve and therefore improved rangeresolution. Range resolution is one of anumber of factors that influence theaccuracy of process radar level trans-mitters.
Pulse radar bandwidthThe carrier frequency of a pulse
radar varies from 5.8 GHz to about26 GHz.
The pulse duration is importantwhen it comes to resolving two adja-cent echoes. For example, a onenanosecond pulse has a length of about300 mm. Therefore, it would be diffi-cult for the radar to distinguish betweentwo echoes that are less than 300 mm
apart. Clearly a shorter pulse durationprovides better range resolution.An effect of a shorter pulse duration
is a wider bandwidth or spectrum offrequencies.
For example, if the carrier frequencyof a pulse is 5.8 GHz and the durationis only 1 nanosecond, then there is aspectrum of frequencies above andbelow the nominal carrier frequency.
The amplitude of the pulse spectrum offrequencies changes according to a
curve.The shape of this curve is shown in
Fig 4.21.The null to null bandwidth BWnn of
a pulse radar is equal to
where is the pulse duration.It is clear from the curve that the
amplitude of frequencies reduces sig-nificantly away from the main pulsefrequency.
sin xx
2
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4 . Radar level measurement
Fig 4.20 Pulse radar range resolution.The guaranteed range
resolution is the length of thepulse. A shorter pulse has awider bandwidth and betterrange resolution
shorter pulse
better range resolution
bandwidth BW nn,
equal to
pulse frequency
5.8 GHz
6.8 GHz4.8 GHz
Fig 4.21 The null to null bandwidthBWnnof a radar pulse is equal
to 2 / where is the pulseduration. Example a 5.8 GHzradar with a pulse duration ofone nanosecond has a null tonull bandwidth of 2 GHz
Fig 4.22 Envelope curve with pulse radar
Pulse radar envelope curveFig 4.22 shows how a pulse radar
echo curve is used in process levelmeasurement.
A higher frequency pulse with ashorter pulse duration will allow betterrange resolution and also better accura-cy because the leading edge of theenvelope curve is steeper.
2
Fig 4.23 A shorter pulse duration gives better range resolution. The combination ofshorter pulse duaration and higher frequency allows better accuracy because theleading edge of the envelope curve is steeper
High frequency, short
duration pulse
Lower frequency pulse with
longer duration
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70
FM-CW radar bandwidthThe bandwidth of an FM - CW radar is
the difference between the start andfinish frequency of the linear frequencymodulation sweep.Unlike pulse radar, the amplitude of theFM - CW signal is constant across therange of frequencies.
A wider bandwidth produces narrowerdifference frequency ranges for each
echo on the frequency spectrum. Thisleads to better range resolution in thesame way as with shorter duration puls-es with pulse radar.This is explained in the following dia-grams and equations.
frequency
amplitude
fd
fd
fd
frequency
timeT
s
F
Fast Fourier Transform
fd
= F x 2R
Ts x c [Eq. 4.1]
F bandwidth
Ts sweep time
R distance
fd difference frequency
c speed of light
The FAST FOURIER
TRANSFORM produces a
frequency spectrum of all echoes
such as that at fd.There is an ambiguity fd for each
echo fd.
fd
[Eq. 4.2]
=2Ts
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4 . Radar level measurement
From equation 4.3, it can be seenthat with an FM - CW radar the rangeresolution R is equal to:-
Therefore, the wider the bandwidth, thebetter the range resolution.Examples:
A linear sweep of 2 GHz has a rangeresolution of 150 mm whereas a 1 GHzbandwidth has a range resolution of300 mm.
In process radar applications, eachecho on the frequency spectrum isprocessed with an envelope curve. Theabove equations (Equations 4.1 to 4.3)show that the Fast Fourier Transforms(FFTs) in process radar applications donot produce a single discrete differencefrequency for each echo in the vessel.Instead they produce a difference fre-quency range f
dfor each echo within
an envelope curve. This translates intorange ambiguity.
amplitude
distanceR
R
Fig 4.24 to 4.26 - FM - CW range resolution
The ambiguity of the distance R,
is R
R
R=
fdfd
R
R=
2
F x 2 R
Ts
Ts x c
RR
= cF x R
R =cF
[Eq. 4.3]
cF
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Other influences on accuracyAs we have demonstrated, FM - CW
and PULSE process radar transmittersuse an envelope curve for measure-ment. A wider bandwidth produces bet-ter range resolution. The correspond-ingly short echo will have a steep slopeand therefore a more accurate measure-ment can be made. Other influences onaccuracy include signal to noise ratioand interference.
A high signal to noise ratio allowsmore accurate measurement whileinterference effects can cause a distur-bance of the real echo curve leading toinaccuracies in the measurement.
Choice of antenna and mechanicalinstallation are important factors inensuring that the optimum accuracy isachieved.
FM - CW frequency spectrum -bandwidth and range resolution
Frequency spectrum - narrow bandwidth of linear sweep
envelope curvesaround echoes
envelope curvesaround echoes
frequency
amplitude
amplitude
Frequency spectrum - wide bandwidth of linear sweep
Fig 4.28 Illustration of envelope curve around the frequency spectram of FM - CW
radars. The same four echoes are shown for radar transmitters with different
bandwidths. An improvement in the range resolution is achieved with a wider
bandwidth of the linear sweep
frequency
72
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73
4 . Radar level measurement
Fig 4.29 Higher accuracy of pulse radar
level transmitters can be
achieved by looking at the phase
of an individual wave within the
envelope curve. This is only
practical in slow moving storage
tanks
High accuracy radarHigh accuracy of the order of
+ 1 mm is generally meaningless in anactive process vessel or a solids silo.For example, a typical chemical reactorwill have agitators, baffles and otherinternal structures plus constantlychanging product characteristics.
Although custody transfer levelmeasurement applications are not in thescope of this book, this section discuss-
es how a higher accuracy can beachieved.
Pulse radarFor most process applications, mea-
surement relative to the pulse envelopecurve is sufficient. However, if the liq-uid level surface is flat calm and theecho has a reasonable amplitude, it ispossible to look inside the envelope
curve wave packet at the phase of anindividual wave.
However, the envelope curve of ahigh frequency radar with a short pulseduration is sufficiently steep to producea very accurate and cost effective leveltransmitter for storage vessel applica-tions.
FM - CW radarThe fundamental requirement for an
accurate FM - CW radar is an accuratelinear sweep of the frequency modula-tion.
As with the pulse radar, it is possibleto look inside the envelope curve of thefrequency spectrum if the applicationhas a simple single echo that is charac-teristic of a liquids storage tank. This isachieved by measuring the phase angle
of the difference frequency. However,this is only practical with custodytransfer applications where fast andexpensive processors are used withtemperature and pressure compensa-tion.
Fig 4.30 It is essential that the linear
sweep of the FM - CW radar isaccurately controlled
frequency error
f2
f2
t1
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74
Microwave powerRadar is a subtle form of level mea-
surement. The peak microwave powerof most process radar level transmittersis less than 1 milliWatt. This level ofpower is sufficient for tanks and silosof 40 metres or more.
The average power depends on thesweep time and sweep repetition rate ofFM - CW radar and on the pulse dura-tion and pulse repetition frequency of
pulse radar transmitters.An increase in the microwave powerwill produce higher amplitude echoes.However, it will produce higher ampli-tude false echoes and ringing noiseas well as a higher amplitude echoesoff the product surface. The averagemicrowave power of a Pulse radar canbe as little as 1 microWatt.
Processing powerFM - CW radars need a high level of
processing power in order to function.This processing power is used to calcu-late the FFT algorithms that producethe frequency spectrum of echoes.The requirement for processing powerhas restricted the ability of FM - CWradar manufacturers to make a reliabletwo wire, intrinsically safe radar trans-mitter.
Pulse radar transmitters work in thetime domain without FFT analysis andtherefore they do not need powerfulprocessors for this function.
SafetyThe low power output from
microwave radar transmitters means
that they are an extremely safe methodof level measurement.
Pulse radarThe low energy requirements of
pulse radar enabled the first ever twowire, loop powered, intrinsically saferadar level transmitter to be introducedto the process industry in mid-1997.The VEGAPULS 50 series of pulseradar transmitters have proved to bevery capable in difficult process condi-tions. The performance of the two wire,4 to 20 mA, sensors is equal to the four
wire units that preceded them.The pulse microwave module onlyneeds a 3.3 volt power supply witha maximum power consumption of50 milliWatts. This drops down to5 milliWatts when it is in stand-bymode. The difference between the twowire pulse and the four wire pulse isthat the two wire radar sends out burstsof pulses and updates the output about
once every second. The four wire sendsout pulses continuously and updatesseven times a second.
With high quality electronics, thecomplete 24 VDC, 4 to 20 mA trans-mitter is capable of operating at only14 VDC. This allows it to directlyreplace existing two wire sensors.
Pulsed FM - CWThe low power requirements of
pulse radar have allowed two wireradar to become sucessful. FM - CWradar requires processing power andtime for the FFT's to be calculated.Power saving has been used to producea pulsed FM - CW radar. However,this device is limited to simple storageapplications because the update time is
too long and the processing too limitedfor arduous process applications.
Power Two wire,loop powered radar
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4 . Radar level measurement
Summary of radar level techniques
FM - CW (frequency modulated continuous wave) radar
Indirect method of level measurement Requires Fast Fourier Transform (FFT) analysis to convert signals into a fre-quency spectrum
FFT analysis requires processing power and therefore practical FM - CWprocess radars have to be four wire and not two wire loop powered
FM - CW radars are challenged by large numbers of multiple echoes (causedby the parabolic effects of horizontal cylindrical or dished topped vessels)
PULSE radar Direct, time of flight level measurement Uses a special sampling technique to produce a time expanded intermediatefrequency signal
The intermediate frequency is produced in hardware and does not require FFTanalysis
Low processing power requirement mean that practical and very capable twowire, loop powered, intrinsically safe pulse radar can be used in some of themost challenging process level applications