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Vega radar book

  1. 1. Peter Devine Peter Devine Radar level measurement Radar level measurement Radar level measurement The user's guide The user's guide
  2. 2. Radar level measurement - The users guide Peter Devine written by Peter Devine additional information Karl Grießbaum type setting and layout Liz Moakes final drawings and diagrams Evi Brucker © 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 user´s guide 1. Radar 2. Title 621.3´848 ISBN 0-9538920-0-X Cover by LinkDesign, Schramberg. Printed in Great Britain at VIP print, Heathfield, Sussex.
  3. 3. Contents Foreword Acknowledgement Introduction ix xi xiii Part I 1. History of radar 2. Physics of radar 3. Types of radar 1. CW-radar 2. FM - CW 3. Pulse radar 1 13 33 33 36 39 Part II 4. Radar level measurement 1. FM - CW 2. PULSE radar 3. Choice of frequency 4. Accuracy 5. Power 47 48 54 62 68 74 5. Radar antennas 1. Horn antennas 2. Dielectric rod antennas 3. Measuring tube antennas 4. Parabolic dish antennas 5. Planar array antennas Antenna energy patterns 77 81 92 101 106 108 110 6. Installation A. Mechanical installation 1. Horn antenna (liquids) 2. Rod antenna (liquids) 3. General consideration (liquids) 4. Stand pipes & measuring tubes 5. Platic tank tops and windows 6. Horn antenna (solids) B. Radar level installation cont. 1. safe area applications 2. Hazardous area applications 115 115 115 117 120 127 134 139 141 141 144
  4. 4. Foreword To suggest that any one type of level transmitter technology could be regarded as 'universal' would be unrealistic and potentially irresponsible due to the variation and complexity of available applications when liquids, powders and solids are all considered. However, the rate at which radar based level transmitters have established themselves over the last couple of years would tend to suggest that this technology is closer to that definition that any principle has ever been. I have personally been involved in the development, applications, sales and marketing of level transmitters, controllers and indicators of most types over the last twenty years. In that time nothing has, in my opinion, come close to matching the significance of radar in terms of its overall suitability, for not just conventional but extreme process conditions applications for the vast majority of substances in vessels of virtually any size or complexity. This unique principle combined with current reflections processing software, materials of construction, simplicity of installation and transmitter digital communications allows this to be considered as a day to day 'first consideration' for level, whereas only a very short time ago it was regarded as expensive and specialised - this is no longer the case. The purpose of this publication is quite specific, and that is to explain some of the principles involved, and to show that by applying some simple guidelines, what is obviously a sophisti-cated technology can be simple and reliably used in an enormously wide range of industrial and process applications. We make no apology for including a chapter on Vega specific products, and hope this guide stimulates a radar user, or some greater depth of knowledge if you have some experience, we look forward to hearing from you. Mel Henry Managing Director Vega Controls Ltd. ix
  5. 5. Acknowledgements In writing and compiling this book I had the invaluable assistance of several colleagues from VEGA in Schiltach both in the developing department and within the product management. Particular thanks must go to Karl Griessbaum for his lucid explanations of the 'secrets' of pulse radar; his insight into the workings of FM - CW radar and the drawings to accompany the explanations. Thanks also to Juergen Skowaisa and Juergen Motzer for their technical contributions to the book. The publication of 'radar level measurement - the user´s guide' is a reflection of the wealth of product knowledge of radar level application experience in the VEGA group of companies and our agents and distributors world wide. This experience has accelerated since the advent of the VEGAPULS 50 series two wire, loop powered radar. I would like to thank all those who contributed to the section on radar applications. This in-cludes Doug Anderson, Dave Blenkiron, Chris Brennan, Graeme Cross and John Hulme in the UK, Paal Kvam of Hyptech in Norway, Dough Groh and his colleagues at Ohmart VEGA in the USA, and Juergen Skowaisa and Roger Ramsden from VEGA Germany. Thank also to the VEGA marketing department in Germany and the UK for their assistance in producing and collating pictures and photographs. Thank to all the other unnamed contributors. Finally, the most important contributors to this book are all VEGA radar users world wide without whom our high level of expertise in process radar measurement applications would not be possible. Peter Devine Technical manager Vega Controls Ltd. xi
  6. 6. Introduction The technical benefits of radar as a level measurement technique are clear. Radar provides a non-contact sensor that is virtually unaffected by changes in process temperature, pressure or the gas and vapour composition within a vessel. In addition, the measurement accuracy is unaffected by changes in density, conductivity and dielectric constant of the product being measured or by air movement above the product. These benefits have become more significant to the process industry since the advent of low costs, high performance, two wire loop powered radar level transmitters. This breakthrough, in the summer of 1997, produced an unprecedented boom in the use of non-contact microwave radar transmitters for liquid and solids process level application. 'Radar level measurement - the user´s guide' is offered as a reference book for all those interested in the technology, the application, and the practical installation of radar level sen-sors. We cover many practical process level applications rather than the closed niche market of custody transfer measurement. Radar history, physics and techniques are presented as well as descriptions of types of ra-dar antenna and mechanical and electrical installations. Now radar is an affordable option for process level measurement. We compare it closely with all of the other process level techniques and give many examples of the myriad applications of radar across all industries. Radar level measurement has come of age. We hope that this book will be invaluable in helping you to see the potential of this latest and almost universal level measurement technology. More than anything, we hope that you enjoy delving into the pages of this book. Peter Devine Technical manager Vega Controls Ltd xiii
  7. 7. 1. History of radar James Clerk Maxwell predicted the existence of radio waves in his theory of electromagnetism as long ago as 1864. He showed mathematically that all electromagnetic waves travel at the same velocity in free space, independent of their wavelength. This velocity is of the order of 300,000 kilometres per second, the speed of light. Heinrich Rudolf Hertz, verified Maxwell’s theory by experiments carried out in 1886-87 at Karlsruhe Polytechnic. He used a spark gap transmitter producing bursts of high frequency electromagnetic waves at about 455 MHz, or a wavelength of 0.66 metres. Hertz confirmed that these electromagnetic radio waves had the same velocity as light and could be reflected by metallic and dielectric bodies. In addition to their reflective properties, Hertz demonstrated that radio waves exhibit refraction, diffraction, polarization and interference in the same way as light. These early experiments in reflecting radio waves off metal plates were the first manifestations of radar as we know it today. The first practical form of radar was produced by a German engineer, Christian Hülsmeyer. Patented in various countries in 1904 as the ‘Telemobiloscope’, Hülsmeyer’s apparatus was described as ‘A Hertzian wave projecting and receiving apparatus adapted to indicate or give warning of the presence of a metallic body, such as a ship or a train, in the line of projection of such waves’. An addition to the patent in the same year described ‘Improvements in Hertzian wave projecting and receiving James Clerk Maxwell predicted the existence of radio waves in his theory of electromagnetism (Pic. 1.1 - J.C.M.F) Heinrich Hertz Hertz confirmed by experiment that electromagnetic radio waves have the same velocity as light and can be reflected by metallic and dielectric bodies (Pic. 1.2 - I.N.T) 1
  8. 8. Prior to World War II, radar was being developed independently in a number of different countries, including Britain, Germany, the United States, Italy, France and the Soviet Union. In 1934, following a series of experiments at the Naval Research Laboratory in the United States, a patent was granted to Taylor, Young and Hyland for a ‘System for detecting objects by radio’. In February 1935, British scientist, Robert Watson-Watt presented a paper on ‘The detection and location of aircraft by radio methods’ to the Tizard Committee for the Scientific Survey of Air Defence. Christian Hülsmeyer produced the first practical radar patented in 1904 (Pic. 1.3 - D.M.M) apparatus for locating the position of distant metal objects’. A successful demonstration of the telemobiloscope was made at the International Shipping Congress in Rotterdam in 1904, and also to the German navy. However, the telemobiloscope was considered to be limited and was not a commercial success. Guglielmo Marconi, is famous for pioneering trans-Atlantic radio communications. In 1922 Marconi had also recognised the potential of using short wave radio for the detection of metallic objects. Marconi envisaged the use of radio for ship to ship detection at night or in fog. However, he did not appear to receive the support or have the resources to carry these ideas further at the time. 2 Guglielmo Marconi recognised the potential of using short wave radio for the detection of metallic objects in 1922 (Pic. 1.4 - GEC Marconi)
  9. 9. 1. History of radar Sir Robert Watson - Watt was a senior figure in the development of British radar in the 1930’s & 40’s (Pic. 1.5 - I.W.M) Subsequently, a practical demonstration was carried out using a BBC radio transmitter at Daventry. About five and a half miles (9 km) away, a separate radio receiver connected to an oscilloscope was used to detect the presence of a Handley Page Heyford aircraft as it flew between the transmitter and receiver. Both the American system and Watson-Watt’s Daventry experiment were types of continuous wave (CW) radar. Called CW wave-interference radar or bistatic CW radar, a continuous single frequency was transmitted from one point and detected by a receiver at a separate location. The receiver also detects doppler shifted echoes from the target object. The interference between the frequency of the direct signal and reflected signals at a slightly different frequency indicated the presence of the target object. If you are unfortunate enough to live on an airport flight path, you may have witnessed this effect on your television screen. As an aircraft approaches, the picture on the screen may flicker with regular horizontal bands scrolling vertically on the screen. These diminish when the aircraft is directly overhead and then continue as the aircraft moves away. Although it proved a point at Daventry, CW wave-interference radar was not a practical device. It could detect the presence but not the position of the target. After Daventry, the British effort continued at Orford Ness and then nearby Bawdsey Manor on the Suffolk coast. It was clear that pulse radar would be needed to provide the required distance and direction information essential for a defensive radio detection system. The British, under the direction of Watson-Watt developed a defensive system of CH (Chain Home) radar stations which eventually covered all of the coastal approaches to Britain. The standard chain home radars had a relatively low frequency of between 22 & 30 MHz (wavelength 10 to 13.5 metres). They had a power of 200 kilowatts and a range of up to 190 kilometres. However, the long range CH radar transmitters were blind to low flying aircraft and therefore they were supplemented by CHL (Chain Home Low) radar transmitters which had a shorter range and covered the lower altitudes that were overlooked by the main CH 3
  10. 10. British Chain Home Radar aerials Radar was instrumental in the defence of Britain during the second world war (Pic. 1.6 - I.W.M) transmitters. They operated on a frequency of 200 MHz (wavelength 1.5 metres). It is well documented that the CH and CHL network of radar stations were a crucial factor during the Battle of Britain in the summer of 1940. It enabled the fighters of the Royal Air Force to be deployed when and where they were needed and rested when the threat receded. The limited resources in men and machines were not wasted in long standing patrols. German radar research was also conducted in secret in the late 1930’s. Whereas the development effort in Britain was focused on air defence, in Germany separate radar developments were carried out for the Navy, Army and Luftwaffe. Companies involved in German naval research produced a range of ship 4 mounted sea search radar transmitters called Seetakt. These were delivered as early as 1938 with a frequency of 366 MHz (wavelength 82 cm) and were installed on many vessels including the famous battleships, Bismarck and Graf Spee. German Naval developments also produced the Freya range of search radars operating on 125 MHz (wavelength 2.4 metres). These were found to be effective for tracking aircraft at long range, and were subsequently supplied to the Luftwaffe for early warning. However, they could not provide altitude information. Other German radars in wide use were the parabolic antenna Würzburg and Würzburg Riese (Giant Würzburg) transmitters. The standard Würzburgs were generally used for directing searchlights and flak batteries and the Würzburg Riese for tracking individual intruders and directing night fighters to intercept them. In a similar fashion to the British Chain Home system, the Germans built a defensive network of ‘Himmelbett’ radar stations. The literal translation of Himmelbett is four poster bed. The four ‘posts’ of the bed consisted of a Freya early warning radar, a Würzburg radar for tracking the intruding aircraft, a Würzburg radar to guide the night fighter to the intruder and a Seeburg plotting table (Seeburgtisch) to monitor the interception. This defensive radar system became known by the British as the ‘Kammhuber Line’ after the German general in charge of night fighters.
  11. 11. 1. History of radar Above - The famous aerial reconnaissance photograph of a German Würzburg radar antenna at Bruneval in northern France. This image alerted the British to the presence and advanced state of German defensive radar which led to a commando action in which components from the radar were taken back to Britain for analysis (Pic. 1.7 - I.W.M) Right - The German Würzberg radar was used for directing searchlights and flak batteries and for tracking individual targets and directing interceptors to them (Pic. 1.8 - P.D) 5
  12. 12. Both Britain and Germany developed airborne radar for fighter interception by night. British airborne radar trials started in 1937 with the production AI Mark 1 taking to the air in May 1939. The first practical British Airborne Interception radar was the AI Mark IV which was first tested in August 1940. In Germany the Lichtenstein airborne radar was available in mid 1941. The characteristic external radar aerial array of the Lichtenstein caused significant aerodynamic drag. This could reduce the aircraft speed by as much as 40 kilometres per hour. By 1943 the range had been extended to 6000 metres. It became clear to radar researchers that a shorter ‘centimetric’ wavelength would be more useful for a number of applications. This would enable a more focused airborne radar that would not suffer from the ground returns that restricted capabilities of the first airborne radars. The higher frequency could be used for a ground mapping radar unit to locate towns and other geographic features. The problem was how to find a method of generating sufficient power at the desired wavelength of 10 centimetres. British Airborne Radar - AI Mark IV developed for fighter interception by night in 1940 German Airborne Radar ‘Lichtenstein’ available in mid 1941 - the external aerial radar caused significant aerodynamic drag (Pic. 1.9 - I.W.M) (Pic. 1.10 - I.W.M) 6
  13. 13. 1. History of radar In late February 1940, an historic breakthrough was made by John Randall and Harry Boot, researchers at the University of Birmingham, when they tested their world changing invention the Cavity Magnetron. The heart of this cavity magnetron was a simple solid copper block with six cavities machined into it. In the centre was the cathode. When a strong magnetic field and high voltage was applied between the copper block and the cathode, the stream of electrons resonated in unison within the cavities instead of passing directly to the copper block anode. The frequency of oscillation was calculated to be about 3 GHz (10 centimetre wavelength). The theoretical calculations of the prototype cavity magnetron were correct. The actual wavelength was found to be 9.87 centimetres and the all important power of the prototype was 400 Watts. Cavity Magnetron the world changing invention by John Randall and Harry Boot invented in 1940 (Pic. 1.11 - GEC) Production of cavity magnetrons followed very quickly and the power output was significantly increased. Britain developed microwave airborne interception AI radar sets for night fighters which had a vastly improved long and near range. The British microwave airborne interception radar was the AI Mark VII which was introduced in mid 1942. The improved AI Mark VIII was mass produced and in wide use by early 1943. The Cavity Magnetron was used in centrimetric ‘microwave’ airborne radar and duced a quantum leap in performance. The radar dish was protected inside a plastic nose assembly pro- (Pic. 1.12 & 1.13 - H.R.A) 7
  14. 14. Britain also used the cavity magnetron in the development of a ground mapping radar called H2S. This device enabled aircraft to be accurately navigated to their destinations without the aid of ground based beacons or beams. Britain shared this secret microwave technology with the United States where additional development took place at the Radiation Laboratory within the Massachusetts Institute of Technology. From the work carried out at MIT, further airborne interception radars and gun laying radars were mass produced and delivered to the allied forces. The American SCR-720 (known as AI Mark X in Britain) was first delivered to the USAAF by late 1942. This radar unit became a standard device long after the war had finished. War time secrecy meant that radio detection devices were given coded names. In Britain, the early chain home radar was called RDF after the existing Radio Direction Finding systems in the hope that it would mislead their real function. In the same way in Germany, radar was disguised as ‘Dezimeter Telegraphie’ or ‘De-Te’, translated as decimetric telegraphy It was the Americans who introduced the now universally used palindrome, RADAR or RAdio Detection And Ranging. The history of the development of radar during the course of the Second World War is a huge subject in itself. Many devices were developed. Measures and counter measures were taken in the radar war. Since 1945, radar has been used for an increasing number of peaceful applications. The giant Würzburg parabolic radar transmitters of the Second World War became post war radio telescopes. The basic designs were developed and enlarged and can be seen at the well known Jodrell Bank Observatory near Manchester which has a dish diameter of 75 metres. Viewed from Earth, the planet Venus Modern radar systems are exemplified by this ‘AWAC’ airborne early warning aircraft. Multiple targets can be detected at extreme range (Pic. 1.14 - P.D) 8
  15. 15. 1. History of radar is one of the brightest celestial bodies. However, the mysteries of our close neighbour in the Solar System were only uncovered with the assistance of radar. The surface of Venus is shrouded in dense clouds of vapour including carbon dioxide gas at pressures of 90 bar and an average temperature of 750 K. Earth bound pulse radar measurements over an extended period of time were used to calculate the radius of the orbit of Venus. Doppler shift measurements from the surface were used to calculate the rate of rotation of the shrouded planet. The Venus ‘day’ was found to be 243 Earth days. During the 1970’s, radar mapping of the planet’s surface by space probe uncovered surface features such as craters. Jodrell Bank - the observatory near Manchester which has a 75 metre dish diameter (Pic. 1.15 - P.D) Detection by radar is not always desirable. Huge sums of money have been spent reducing the radar signature of the F117 stealth fighter (Pic. 1.16 - P.D) 9
  16. 16. Radar technology is part of our everyday lives. The cavity magnetron is used in microwave ovens. Continuous wave (CW) radars are used in automatic door detection and vehicle speed measurement. Other well known civilian radar applications include air traffic control, shipping and weather radar. Radar altimeters developed in the 1930’s use a form of radar called FM - CW or Frequency Modulated Continuous Wave radar. In the 1970’s, the same FM - CW measurement technique was used in the production of the first radar level tank gauge. Initially these radar level transmitters were used to measure petroleum products in supertankers. Further developments of FM - CW level transmitters led to their use on shore based storage tanks in the mid 1980’s. Originally these were expensive, high accuracy systems for fiscal measurement of petroleum products. Later, lower accuracy FM - CW radar transmitters became available for the process industry. In the late 1980’s, pulse radar level transmitters were developed for process measurement applications. The availability of suitable crystals and solid state components such as GaAs FET oscillators enabled cost effective radar level transmitters to enter the market. In 1997 a significant improvement in the specification of radar level transmitters was achieved. VEGA produced the world’s first two wire, loop powered, intrinsically safe radar level transmitter. For the first time low cost, high specification radar level transmitters became available. It is likely that these advances will continue into the new millennium and that radar level transmitters will become a commodity item in the same way as differential pressure transmitters. In the field of radar level measurement, technological advances have resulted in two wire, intrinsically safe transmitters (Pic. 1.17 - Vega) 10
  17. 17. 1. History of radar Comparing the old with the new A raw oscilloscope echo trace had to be interpreted by skilled operators using the British war time Chain Home Low radar (Pic. 1.18 & 1.19 - I.W.M) Comprehensive information is available on the PC echo trace of the latest two wire loop powered radar level transmitters (Pic. 1.20 - Vega Pic. 1.21 - Vega) 11
  18. 18. Inhalt Foreword Acknowledgement Introduction ix xi xiii Part I 1. History of radar 2. Physics of radar 3. Types of radar 1. CW-radar 2. FM - CW 3. Pulse radar 1 13 33 33 36 39 Part II 4. Radar level measurement 1. FM - CW 2. PULSE radar 3. Choice of frequency 4. Accuracy 5. Power 47 48 54 62 68 74 5. Radar antennas 1. Horn antennas 2. Dielectric rod antennas 3. Measuring tube antennas 4. Parabolic dish antennas 5. Planar array antennas Antenna energy patterns 77 81 92 101 106 108 110 6. Installation A. Mechanical installation 1. Horn antenna (liquids) 2. Rod antenna (liquids) 3. General consideration (liquids) 4. Stand pipes & measuring tubes 5. Platic tank tops and windows 6. Horn antenna (solids) B. Radar level installation cont. 1. safe area applications 2. Hazardous area applications 115 115 115 117 120 127 134 139 141 141 144
  19. 19. 2. Physics of radar Electromagnetic waves Th e velocity of light in free space is 299,792,458 metres per second, but who is timing? For the purposes of the calculations in this book, we will call it 300,000 kilometres per second or 3 x 108 metres per second. Maxwell’s theories of electromagnetism were confirmed by the experiments of Heinrich Hertz. These show that all forms of electromagnetic radiation travel at the speed of light in free space. This applies equally to long wave radio transmissions, microwaves, infrared, visible and ultraviolet light plus X-rays and Gamma rays. Maxwell showed that the velocity of light in a vacuum in free space is given by the expression : Examples :- 1 co = µo εo o x εo) [Eq. 2.1] velocity of electromagntic wave in a vacuum in metres / second the permeability of free space (4 π x 10 -7 henry / metre) the permittivity of free space (8.854 x 10 -12 farad / metre) c = f xλ [Eq. 2.2] c velocity of electromagnetic waves in metres / second f λ frequency of wave in second -1 wavelength in metres The original cavity magnetron had a wavelength of 9.87 centimetres. This corresponds to a frequency of 3037.4 MHz (3.0374 GHz). The frequency of a pulse radar level transmitter may be 26 GHz or 26 x 108 metres per second. The wavelength is 1.15 centimetres. The electromagnetic waves have an electrical vector E and a magnetic vector B that are perpendicular to each other and perpendicular to the direction of the wave. This will be discussed and illustrated further in the section on polarization. The electrical vector has the major influence on radar applications. λ direction of wave amplitude co (µ The velocity of an electromagnetic wave is the product of the frequency and the wavelength. Fig 2.1 13
  20. 20. The Electromagnetic spectrum 10 8 10 7 10 6 10 5 10 4 10 3 10 2 electric waves 10 1 10 2 10 1 10 0 10 -1 10 -2 10 -3 10 -4 radio waves 10 3 10 4 10 5 10 6 10 7 10 8 infra 10 9 10 10 10 11 10 12 3m 0.3 m 3 cm 3 mm 100 MHz 1 GHz 10 GHz 100 GHz The microwave frequencies of the electromagnetic spectrum. Radar level transmitters range between 5.8 GHz (5.2cm) and 26 GHz (11.5mm) 14
  21. 21. 2. Physics of radar 10 -5 10 -6 red 10 13 10 -7 10 -8 ultra violet 10 14 10 15 10 16 10 -9 10 -10 10 -11 10 -12 10 -13 10 -14 10 -15 10 -16 X rays 10 17 10 18 m gamma rays 10 19 10 20 10 21 10 22 10 23 10 24 Hz Fig 2.2 Electromagnetic spectrum. All electromagnetic waves travel at the speed of light in free space. This spectrum shows the range of frequencies and wavelengths from electric waves to gamma rays 15
  22. 22. Permittivity In electrostatics, the force between two charges depends upon the magnitude and separation of the charges and the composition of the medium between the charges. Permittivity ε is the property of the medium that effects the magnitude of the force. The higher the value of the permittivity, the lower the force between the charges. The value of the permittivity of free space (in a vacuum) εo, is calculated indirectly and empirically to be: 8.854 x 10-12 farad / metre. Relative permittivity or dielectric constant εr The ratio of the permittivity of a medium to the permittivity of free space is a dimensionless property called ‘relative permittivity’ or ‘dielectric constant’. For example, at 20° C the relative permittivity of air is close to that of a vaccum and is only about 1.0005 whereas the relative permittivity of water at 20° C is about 80. (Dielectric constant is also widely known as DK.) The value of the dielectric constant of the product being measured is very important in the application of radar to level measurement. In non-conductive products, some of the microwave energy will pass through the product and the rest will be reflected off the surface. This feature of microwaves can be used to advantage or, in some circumstances, it can create a measurement problem. 16 Permeability µ and relative permeability µr The magnetic vector, B, of an electromagnetic wave also has an influence on the velocity of electromagnetic waves. However, this influence is negligible when considering the velocity in gases and vapours which are non-magnetic. The relative permeability of the product being measured has no significant effect on the reflected signal when compared with the effects of the relative permittivity or dielectric constant. For the non-magnetic gases above the product being measured, the value of the relative permeability, µr = 1. Frequency, velocity and wavelength As we have already stated, the frequency (f), velocity (c) and wavelength (λ) of the electromagnetic waves are related by the equation c = f x λ. The frequency remains uninfluenced by changes in the propagation medium. However, the velocity and wavelength can change depending on the electrical properties of the medium in which they are travelling. The speed of propagation can be calculated using equation 2.3. c = co (µ x ε ) r c co µr εr r [Eq. 2.3] velocity of electromagnetic wave in the medium in metres/second velocity of electromagnetic waves in free space the relative permeability (µ medium / µo) the relative permittivity
  23. 23. 2. Physics of radar Changes in the wavelength and velocity of microwaves are apparent in certain radar level applications. Changes in temperature, pressure and gas composition have a small effect on the running time of microwaves because the dielectric constant of the propagation medium is altered to a greater or lesser extent. This is discussed in detail later. Radar level transmitters can be used to measure conductive liquids through low dielectric ‘windows’ such as glass, polypropylene and PTFE. The optimum thickness of the low dielectric window is a half wavelength or multiple of half wavelength. For example, polypropylene has a dielectric constant εr of 2.3 and the half wavelength at a frequency of 5.8 GHz is 17 mm compared with a half wavelength of about 26 mm in a vacuum. It follows that the speed of Empty vessel: large echo from metal bottom microwaves in polypropylene is about two thirds of the speed in air. As with low dielectric windows, non-conductive, low dielectric constant liquids may absorb more power than they reflect from the surface. The velocity of the microwaves within the liquid is slower than in the vapour space above. For example, if there is about 0.5 metres of solvent in the bottom of a metallic vessel, a radar level transmitter may see a larger echo from the vessel bottom than from the product. This large echo will appear to be further away than it really is because the running time within the solvent is slower. For this reason, special considerations must be made within the echo processing software to ensure that the radar follows the solvent level and does not follow the vessel bottom as it apparently moves away! As the vessel fills with solvent two echoes are received. The echo from the vessel bottom appears further away because the running time of the microwaves in solvent is slower solvent echo Fig 2.3 - Effect of dielectric constant on the running time of a microwave radar 17
  24. 24. The same effect can be experienced when looking at interface detection using guided microwave level transmitters to detect oil and water or solvent and aqueous based liquids. Fig 2.4 Oil/water interface detection using a guided microwave level transmitter. Note that the water echo has a reduced amplitude and appears to be further away. The running time of microwaves in oil is slower than in air reference echo (water without oil) oil echo water echo Effects on the propagation speed of microwaves Microwave radar level transmitters can be applied almost universally because, as a measurement technique, they are virtually unaffected by process temperature, temperature gradient, vacuum and normal pressure variations, gas or vapour composition and movement of the propagation medium. However, changes in these process conditions do cause slight variations in the propagation speed because the dielectric constant of the propagation medium is altered. Calculating the propagation speed of microwaves The temperature, pressure and the gas composition of the vapour space all have an effect on the dielectric constant of the propagation medium through which the microwaves must travel. This in turn affects the propagation speed or running time of the instrument. 18 The dielectric constant or relative permittivity can be calculated as follows : εr = 1 + (εrN - 1) x θN x P θ x PN [Eq. 2.4] εr εrN calculated dielectric constant (relative permittivity) dielectric constant of gas/vapour under normal conditions (temperature 273 K, pressure 1 bar absolute) θN PN θ P temperature under normal conditions, 273 Kelvin pressure under normal conditions, 1 bar absolute process temperature in Kelvin process pressure in bar absolute
  25. 25. 2. Physics of radar From equation 2.4 and equation 2.3, we can calculate the percentage error caused by variations in the dielectric constant of different gases and vapours and the relative effects of changes in process temperature and pressure. Gases and vapours By definition, the dielectric constant in a vacuum is equal to 1.0. The dielectric constants of the gases and vapours that may be present above the product differ but they have only a very small effect on the accuracy of radar. Radar level transmitters are usually calibrated in air. For this reason, the following tables show 1. Dielectric constant of different gases at normal temperature and pressure (273K, 1 Bar A) 2. Percent error in the running time in the gases compared with air Table 2.1 The dielectric constants under normal conditions, εrN and the error caused by the dielectric constant of typical process gases under normal conditions % Error from air (at normal temperature and pressure) Vacuum Air Argon Ammonia / NH 3 Hydrogen Bromide HBr Hydrogen Chloride HCl Carbon Monoxide / CO Carbon Dioxide / C0 2 Ethane / C 2 H6 Ethylene / C 2H4 Helium ε rN (dielectric constant at normal conditions) 1.0000 1.000633 1.000551 1.006976 1.002994 1.004078 1.000692 1.000985 1.001503 1.001449 1.000072 Hydrogen / H 2 Methane / CH 4 Nitrogen / N 2 Oxygen / O 2 1.000275 1.000878 1.000576 1.000530 + 0.0179 - 0.0122 + 0.00285 + 0.0052 Gas / Vapour + 0.0316 0.0 + 0.0041 + 0.3154 - 0.1178 - 0.1717 - 0.00295 - 0.0176 - 0.0434 - 0.0407 + 0.0280 19
  26. 26. Temperature High temperature or large temperature gradients have very little effect on the transit time of microwaves within an air or vapour space. At a temperature of 2000° C the variation is only 0.026% from the measurement value at 0° C. Radar level transmitters with air or nitrogen gas cooling are used on molten iron and steel applications. 0.03 0.025 % error 0.02 0.015 0.01 0.005 0.0 0 250 500 750 1000 1250 1500 1750 2000 Temperature in ° C Fig 2.5 Temperature effect on radar measurement of air at a constant pressure of 1 BarA 20
  27. 27. 2. Physics of radar Pressure Pressure does have a small but more significant influence on the velocity of electromagnetic waves. At a pressure of 30 Bar, the error is only 0.84%. However this becomes more significant and at a pressure of 100 Bar there is a velocity change of 2.8%. If the pressure is varying constantly between atmospheric pressure and 100 Bar, the velocity variations can be compensated using a pressure transmitter. 10 % error 8 6 4 2 0 0 50 100 150 200 250 300 350 400 Pressure in Bar (absolute) Fig 2.6 The influence of pressure on radar measurement in air at a constant temperature of 273 K 21
  28. 28. Waveguides, stilling tubes & bypass tubes In the preceding equations, we have assumed that the microwaves are travelling in ‘free space’ in a vacuum. However, in practice the proximity of metallic vessel walls and other structures will have an influence on the propagation velocity of the microwaves. This is particularly true when microwave radar level transmitters are fitted inside bypass tubes or stilling tubes or when a horn antenna is fitted with a waveguide extension. When microwaves are propagating within a metallic tube the running time appears to slow down because the microwaves travel further bouncing off the inside wall of the tube and currents are set up on the inside surface of the tube. This effect is discussed in more detail in the chapters on antennas and mechanical installations. The waveguide effect can be compensated during calibration and the use of stilling tubes and bypass tubes can be beneficial in some level applications. Electromagnetic waves exhibit the same properties as light. · · · Reflection Polarization Diffraction · · Refraction Interference Reflection of electromagnetic waves Conductive products Using a spark gap transmitter, Heinrich Hertz demonstrated that electromagnetic waves could be reflected off metallic objects and objects with a relatively high dielectric constant. In the same way, radar can easily measure conductive aqueous liquids such as acids and caustic and other conductive products ranging from molten metal to saturated spent grain in the brewing process. When microwaves from a radar hit a conductive surface the electrical field E is short circuited. The resultant current in the conductive product causes the microwaves to be re-transmitted or reflected from the surface. 22 Radar level transmitters have no problem in measuring conductive liquids and solids because the microwaves with frequencies between 5.8 GHz and 26GHz are readily reflected off a conductive surface producing relatively large echoes. Non-conductive products If a liquid or solid is non-conductive, the value of the dielectric constant (relative permittivity εr) becomes more important. The theoretical amount of reflection at a dielectric layer can be calculated using equation 2.5
  29. 29. 2. Physics of radar W1 Transmitted power: W2 Reflected power: Dielectric constant: εr Then the percentage of reflected power at the dielectric layer, Π = 1- 4 x εr (1 + ε ) 2 r W2 Π = [Eq. 2.5] W1 Typical examples are as follows: Acetone Solvent with a dielectric constant, εr = 20 Toluene Solvent with a low dielectric consta t n, εr = 2.4 Π = 1- (2.4) 4x (1 + Π = 1- 2 (2.4)) 4.46% power is reflected 4x (1 + ( 20 ) (20) 2 ) 40 % power is reflected Π x 100% power reflected 100 80 60 40 20 0 0 10 20 30 40 50 60 70 80 Dielectric constant, εr Fig 2.7 Reflected radar power depends upon the dielectric constant of the product being measured 23
  30. 30. In radar level measurement the reflected energy from a product surface becomes more critical at a dielectric constant (εr) of less than 5. The following graph shows this important region. Π x 100% power reflected 20 15 10 5 0 1.0 1.5 2.0 3.0 2.5 3.5 Dielectric constant, εr 4.0 4.5 5.0 Fig 2.8 Reflected radar power depends upon the dielectric constant of the product being measured. This graph shows the critical region where care must be taken over choice of radar antenna 0 Loss L, dB - 10 - 20 - 40 - 60 3.0 3.5 2.5 Dielectric constant, εr Fig 2.9 Reflection loss in dB: loss L = 10 log Π 1.0 1.5 2.0 4.0 4.5 5.0 Most electrically conductive products or products with a dielectric constant of more than 1.5 can be measured using microwave radar level transmitters. Stilling tubes can be used to concentrate the microwaves for lower dielectric constant products. 24
  31. 31. 2. Physics of radar Polarization Electromagnetic waves have an electrical vector E and magnetic vector B that are in phase but perpendicular to each other. The direction of propagation of the waves is perpendicular to the electrical and magnetic vectors as shown in the diagram below. Polarization defines the orientation of the electromagnetic waves and refers to the direction of the electrical vector E. Most process radar level transmitters exhibit linear polarization as in the dia- gram. The direction of the linear polarization is set by the orientation of the signal coupler from the microwave module. The properties of the polarization of microwaves can be important in the application of radar to level measurement. In television and microwave communications, linear polarization is also referred to as horizontal or vertical polarization depending on the relative orientation of the aerials or antennas. E direction of wave B Fig 2.10 Diagram showing linear polarization and the relative orientation of the electric vector E, the magnetic vector B and the direction of propagation of the microwaves 25
  32. 32. Another form of polarization is elliptical polarization. A specific form of elliptical polarization is circular polarization where the electrical vector E and magnetic vector B rotate through 360° within the space of a single wavelength, when a linear or circular polarized signal is reflected the direction of polarization is reversed. With circular polarization it is possible to use the reversal of polarization to distinguish between a direct echo and an echo that has made two reflections. Circular polarization can also be used in search radars to separate the reflections from aircraft or ships from interference echoes from rain. The almost spherical shape of the rain drops causes a definite reversal of polarization which can be easily rejected by the receiving antenna. However, the scattered reflections from the ship or aircraft provide roughly equal amounts of reversed and un-reversed energy that enables detection. λ Fig 2.11 Circular polarization involves rotation of the electrical and magnetic vectors through 360° within a wavelength 26
  33. 33. 2. Physics of radar The linear polarization that is common with process radar level transmitters can be used to minimise the effects of false echo returns from the internal structure of a process vessel. These false echoes could be reflected from probes, welds, agitators and baffles. In some applications, the effect of false echoes within a vessel can be significantly reduced by rotating the radar in the connection flange or boss. The principle is illustrated below and detailed in the section on mechanical installations in Chapter 6. Polarization can be used to reduce the amplitude of false echoes E Direction of wave B Large echo Fig 2.12 If a metallic or high dielectric object is orientated in the same plane as the electrical vector of the polarized microwaves, the radar level transmitter will receive a large amplitude echo E Direction of wave Small echo B Fig 2.13 If the same object is orientated at right angles to the plane of the electrical vector, the received echo will have a smaller amplitude 27
  34. 34. Diffraction Beam angle is often discussed in relation to radar transmitters. This can give the impression that the radar antenna can direct a finely focused beam towards the target. Unfortunately this is not the case. In practice, although they are designed to produce a directed beam, a radar antenna radiates some energy in all directions. As well as the main lobe which accounts for most of the radiated power, there are also weaker side lobes of energy. This phenomenon is caused, in part, by diffraction. In addition to this, destructive interference causes the null points or notches that form the characteristic side lobes. Chapter 5 provides a detailed explanation of beam angles, side lobes and types of antennas. side lobes main lobe antenna Fig 2.14 The lobe structure of antenna beams is caused by diffraction and destructive interference Refraction In the same way as light is refracted at an air/glass or air/water interface, microwaves are refracted when they encounter a change in dielectric. This could be a low dielectric window (PTFE/glass/polypropylene) or a nonconductive low dielectric liquid such as a solvent. reflected energy The angle of refraction depends on the angle of the incident wave and also on the ratio of the dielectric constants at the interface. It is possible to utilise the refractive properties of electromagnetic waves to construct a dielectric lens that will focus microwaves. a a Fig 2.15 Refraction & reflection microwave interface dielectric window / product refracted energy 28 B
  35. 35. 2. Physics of radar Interference - Phase Problematic interference effects are caused primarily by the inadvertent mixing of signals that are out of phase. The microwave signals have a sinusoidal waveform. Phase angle 45° Fig 2.16 In this illustration both of the sine waves have an identical frequency and amplitude but the second wave has a 45° phase lag Interference can be ‘constructive’ where in-phase signals produce a signal with a higher amplitude or it can be destructive where signals that are 180° out of phase effectively cancel each other out. signals in-phase constructive interference 180° out of phase destructive interference Fig 2.17 Illustration of constructive and destructive interference 29
  36. 36. Interference Microwaves can manifest interference effects in exactly the same way as light. Potentially this can cause measurement problems. The causes of interference should be understood and avoided by design and installation considerations. The wrong choice of antenna, installation of an antenna up a nozzle, positioning transmitters too close to vessel walls or other obstructions can all lead to interference of the signal. The chapter on mechanical installation should help a radar level user to avoid this potential problem. However, we use destructive interference to our advantage when we apply pulse radar level measurement through a low dielectric ‘window’ to measure conductive or high dielectric liquids. A + = C B’ B B” Fig 2.18 Interference caused by positioning an antenna too close to the vessel wall. If a radar level transmitter is installed too close to the vessel wall it is possible that interference will occur. With indirect reflection A B’ B’’ C, the phase may be altered by 180° when compared with the direct reflection A B C. For this reason the microwaves may partially cancel out due to destructive interference 30
  37. 37. 2. Physics of radar The thickness of the dielectric window must be a half wavelength of the window material. When the half wavelength is used, there is destructive interference between the reflection off the top surface of the window and the reflection off the internal second surface of the window. There is a 180° phase shift between these reflections and they cancel each emitted wave plastic vessel ceiling other out. This type of installation is explained more fully in Chapter 6 on the mechanical installations of radar level transmitters together with a table showing the optimum thickness of most important plastics and glasses which are suitable for penetration with radar sensors. reflection with phase shift from top surface reflection without phase shift from internal surface D emitted wave reflection with phase shift off top surface of window reflection without phase shift off internal face of window Fig 2.19 Destructive interference is a benefit when using pulse radar to measure through a low dielectric window. The reflection from the top surface and the reflection from the internal second surface cancel each other if the thickness is a half wavelength 31
  38. 38. Contents Foreword Acknowledgement Introduction ix xi xiii Part I 1. History of radar 2. Physics of radar 3. Types of radar 1. CW-radar 2. FM - CW 3. Pulse radar 1 13 33 33 36 39 Part II 4. Radar level measurement 1. FM - CW 2. PULSE radar 3. Choice of frequency 4. Accuracy 5. Power 47 48 54 62 68 74 5. Radar antennas 1. Horn antennas 2. Dielectric rod antennas 3. Measuring tube antennas 4. Parabolic dish antennas 5. Planar array antennas Antenna energy patterns 77 81 92 101 106 108 110 6. Installation A. Mechanical installation 1. Horn antenna (liquids) 2. Rod antenna (liquids) 3. General consideration (liquids) 4. Stand pipes & measuring tubes 5. Platic tank tops and windows 6. Horn antenna (solids) B. Radar level installation cont. 1. safe area applications 2. Hazardous area applications 115 115 115 117 120 127 134 139 141 141 144
  39. 39. 3. Types of radar 1a. CW, continuous wave radar In continuous wave or CW Radar, a continuous unmodulated frequency is transmitted and echoes are received from the target object. If the target object is stationary, the frequency of the return echoes will be the same as the transmitted frequency. The range of the object cannot be measured. However, the frequency of the return signal from a moving object is changed depending on the speed and direction of the object. This is the well known ‘doppler effect’. The doppler effect is apparent when the siren note of an emergency vehicle changes as it speeds past a pedestrian. The pitch of the siren note is higher as it approaches the listener and lower as it recedes. The doppler effect is also used by astronomers to monitor the expansion of the Universe. By measuring the ‘red shift’ of the spectrum of distant stars and galaxies the rate of expansion can be measured and the age of distant objects can be estimated. In the same way, when an object that has been illuminated by a CW Radar approaches the transmitter, the frequency of the return signal will be higher than the transmitted frequency. The echo frequency will be lower if the object is moving away. yv elocit rece requ ived f tv targe f + f dp ency t itted m trans ave yf w uenc t, lengt hλ freq Fig 3.1 CW radar uses doppler shift to derive speed measurement 33
  40. 40. In Fig 3.1, the aircraft is travelling towards the CW radar. Therefore the received frequency is higher than the transmitted frequency and the sign of fdp is positive. If the aircraft was travelling away from the radar at the v = λ x fdp 2 = same speed, the received frequency would be ft - fdp. The velocity of the target in the direction of the radar is calculated by equation 3.1 c x fdp c v ft 2 x ft fdp [Eq. 3.1] ft+fdp is the velocity of microwaves is the target velocity is the frequency of the transmitted signal is the doppler beat frequency which is proportional to velocity is received frequency. The sign of fdp depends upon whether the target is closing or receding 1b. CW wave-interference radar or bistatic CW radar We have already mentioned that CW radar was used in early radar detection experiments such as the famous Daventry experiment carried out by Robert Watson - Watt and his colleagues. In this case, the transmitter and receiver were separated by a considerable distance. A moving object was detected by the receiver because there was interference between the fre- quency received directly from the transmitter and the doppler shifted frequency reflected off the target object. Although the presence of the object is detected, the position and speed cannot be calculated. In essence, this is what happens when a low flying aircraft interferes with the picture on a television screen. See Fig 3.2. 1c. Multiple frequency CW radar Standard continuous wave radar is used for speed measurement and, as already explained, the distance to a stationary object can not be calculated. However, there will be a phase shift between the transmitted signal and the return signal. If the starting position of the object is known, CW radar could be used to detect a change in position of up to half wavelength (λ/2) of the transmitted wave by measuring the phase shift of the echo signal. Although further movement could be detected, the range 34 would be ambiguous. With microwave frequencies this means that the useful measuring range would be very limited. If the phase shifts of two slightly different CW frequencies are measured the unambiguous range is equal to the half wavelength (λ/2) of the difference frequency. This provides a usable distance measurement device. However, this technique is limited to measurement of a single target. Applications include surveying and automobile obstacle detection.
  41. 41. transmitter transmitted signal direct television interference reflected signal (doppler shift) Fig 3.2 The effect of low flying aircraft on television reception is similar to the method of detection by CW wave-interference radar transmitted signal indirect target 3. Types of radar 35
  42. 42. 2. FM-CW, frequency modulated continuous wave radar If the distance to the target is R, and c is the speed of light, then the time taken for the return journey is:- 2xR c ∆t = [Eq. 3.2] We can see from Fig. 3.3 that if we know the linear rate of change of the transmitted signal and measure the difference between the transmitted and received frequency fd, then we can calculate the time ∆t and hence derive the distance R. frequency Single frequency CW radar cannot be used for distance measurement because there is no time reference mark to gauge the delay in the return echo from the target. A time reference mark can be achieved by modulating the frequency in a known manner. If we consider the frequency of the transmitted signal ramping up in a linear fashion, the difference between the transmitting frequency and the frequency of the returned signal will be proportional to the distance to the target. cy en u eq r df itte m ns tra re ∆t fd c e eiv df re e qu nc y ∆t = 2xR c time Fig 3.3 The principle of FM - CW radar 36
  43. 43. 3. Types of radar In practice, the FM - CW signal has to be cyclic between two different frequencies. Radio altimeters modulate between 4.2 GHz and 4.4 GHz. Radar level transmitters typically modulate between about 9 GHz and 10 GHz or 24 GHz and 26 GHz. The cyclic modulation of FM - CW radar transmitter takes different forms. These are sinusoidal, saw tooth or triangular wave forms. FM - CW wave forms transmitted frequency received frequency frequency Fig 3.4 Sine wave Commonly used on aircraft radio altimeters between 4.2 and 4.4 GHz 4.4GHz time 4.2GHz frequency Fig 3.5 Triangular wave Used on FM - CW radar transmitters time frequency Fig 3.6 Saw tooth wave 10 GHz 9 GHz time Most commonly used on most FM - CW process radar level transmitters 37
  44. 44. If we look at a triangular wave form we can see that there is an interruption in the output of the difference frequency , fd. In practice, the received signal is heterodyned with part of the transmitted frequency to produce the difference frequency which has a posi- tive value independent of whether the modulation is increasing or decreasing. The diagram below makes the assumption that the target distance is not changing. If the target is moving, there will be a doppler shift in the difference frequency. frequency time difference frequency fd time Fig 3.7 & 3.8 The change in direction between the ramping up and down of the frequency creates a short break in the measured value of the difference frequency. This has to be filtered out. The transmitted frequency is represented by the red line and the received frequency is represented by the dark blue line. The difference frequency is shown in light blue on the bottom graph 38
  45. 45. 3. Types of radar 3. Pulse radar a. Basic pulse radar Pulse radar is and has been used widely for distance measurement since the very beginnings of radar technology. The basic form of pulse radar is a pure time of flight measurement. Short pulses, typically of millisecond or nansecond duration, are transmitted and the transit time to and from the target is measured. The pulses of a pulse radar are not discrete monopulses with a single peak of electromagnetic energy, but are in fact a short wave packet. The number of waves and length of the pulse depends upon the pulse duration and the carrier frequency that is used. These regularly repeating pulses have a relatively long time delay between them to allow the return echo to be received before the next pulse is transmitted. t τ 3rd pulse 2nd pulse 1st pulse Transmitted pulses Fig 3.9 Basic pulse radar The inter pulse period (the time between successive pulses) t is the inverse of the pulse repetition frequency fr or PRF. The pulse duration or pulse width, τ, is a fraction of the inter pulse period. The inter pulse period t effectively defines the maximum range of the radar. Example The pulse repetition frequency (PRF) is defined as fr = 1 t If the pulse period t is 500 microseconds, then the pulse repetition frequency is two thousand pulses per second. In 500 microseconds, the radar pulses will travel 150 kilometres. Considering the return journey of an echo reflected off a target, this gives a maximum theoretical range of 75 kilometres. If the time taken for the return journey is T, and c is the speed of light, then the distance to the target is R= Txc 2 [Eq. 3.3] 39
  46. 46. b. Pulse doppler radar The pulses transmitted by a standard pulse radar can be considered as a very short burst of continuous wave radar. There is a single frequency with no modulation on the signal for the duration of the pulse. If the frequency of the waves of the transmitted pulse is ft and the target is moving towards the radar with velocity v, then, as with the CW radar already described, the frequency of the return pulse will be ft + fdp , where fdp is the doppler beat frequency. Similarly, the received frequency will be ft - fdp if the target is moving away from the radar. Therefore, a pulse doppler radar can be used to measure speed, distance and direction. The ability of the pulse doppler radar to measure speed allows the system to ignore stationary targets. This is also commonly called ‘moving target indication’ or MTI radar. In general, an MTI radar has accurate range measurement but imprecise speed measurement, whereas a pulse doppler radar has accurate speed measurement and imprecise distance measurement. 40 The velocity of the target in the direction of the radar is calculated in equation 3.4: c = c x fdp λ x fdp = 2 x ft 2 [Eq. 3.4] This is the same calculation as for CW radar. The distance to the target is calculated by the transit time of the pulse, equation 3.3. R = Txc 2 [Eq. 3.3] As well as being used to monitor civil and military aircraft movements, pulse doppler radar is used in weather forecasting. A doppler shift is measured within storm clouds which can be distinguished from general ground clutter. It is also used to measure the extreme wind velocities within a tornado or ‘twister’.
  47. 47. R Fig 3.10 Pulse doppler radar provides target speed, distance and direction f t + f dp ft Pulse doppler radar 3. Types of radar 41
  48. 48. c. Pulse compression and ‘Chirp’ radar frequency With pulse radar, a shorter pulse duration enables better target resolution and therefore higher accuracy. However, a shorter pulse needs a significantly higher peak power if the range performance has to be maintained. If there is a limit to the maximum power available, a short pulse will inevitably result in a reduced range. With limited peak power, a longer pulse duration, τ , will provide more radiated energy and therefore range but (with a standard pulse radar) at the expense of resolution and accuracy. Pulse compression within a ‘Chirp’ radar is a method of achieving the accuracy benefits of a short pulse radar together with the power benefits of using a longer pulse. Essentially, Chirp radar is a cross between a pulse radar and an FM - CW radar. f1 f2 time t2 t1 amplitude τ time Fig 3.11 Chirp radar wave form. Chirp is a cross between pulse and FM - CW radar 42
  49. 49. 3. Types of radar Each pulse of a Chirp radar has linear frequency modulation and a constant amplitude. The echo pulse is processed through a filter that compresses the echo by creating a time lag that is inversely proportional to the frequency. Therefore, the low frequency that arrives first is slowed down the most and the subsequent higher frequencies catch up producing a sharper echo signal and improved echo resolution. Time lag Filter Frequency Long frequency modulated echo pulse Compressed signal Fig 3.12 Pulse compression of chirp radar echo signal Pulse compression of chirp radar echo signal Another method of echo compression uses binary phase modulation where the transmitted signal is specially encoded with segments of the pulse either in phase or 180° out of phase. The return echoes are decoded by a filter that produces a higher amplitude and compressed signal. The name ‘Chirp’ radar comes from the short rapid change in frequency of the pulse which is analogous to the chirping of a bird song. The above methods of radar detection are used widely in long range distance or speed measurement. In the next chapter we look at which of these methods can be applied to the unique problems involved in measuring liquid or solid levels within process vessels and silos. 43
  50. 50. Part II Radar level measurement Radar antennas Radar level installations 45
  51. 51. Contents Foreword Acknowledgement Introduction ix xi xiii Part I 1. History of radar 2. Physics of radar 3. Types of radar 1. CW-radar 2. FM - CW 3. Pulse radar 1 13 33 33 36 39 Part II 4. Radar level measurement 1. FM - CW 2. PULSE radar 3. Choice of frequency 4. Accuracy 5. Power 47 48 54 62 68 74 5. Radar antennas 1. Horn antennas 2. Dielectric rod antennas 3. Measuring tube antennas 4. Parabolic dish antennas 5. Planar array antennas Antenna energy patterns 77 81 92 101 106 108 110 6. Installation A. Mechanical installation 1. Horn antenna (liquids) 2. Rod antenna (liquids) 3. General consideration (liquids) 4. Stand pipes & measuring tubes 5. Platic tank tops and windows 6. Horn antenna (solids) B. Radar level installation cont. 1. safe area applications 2. Hazardous area applications 115 115 115 117 120 127 134 139 141 141 144
  52. 52. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 47 4. Radar level measurement The benefits of radar as a level measurement technique are clear. Radar provides a non-contact sensor that is virtually unaffected by changes in process temperature, pressure or the gas and vapour composition within a vessel. In addition, the measurement accuracy is unaffected by changes in density, conductivity and dielectric constant of the product being measured or by air movement above the product. The practical use of microwave radar for tank gauging and process vessel level measurement introduces an interesting set of technical challenges that have to be mastered. If we consider that the speed of light is approximately 300,000 kilometres per second. Then the time taken for a radar signal to travel one metre and back takes 6.7 nanoseconds or 0.000 000 006 7 seconds. How is it possible to measure this transit time and produce accurate vessel contents information? Currently there are two measurement techniques in common use for process vessel contents measurement. They are frequency modulated continuous wave (FM - CW) radar and PULSE radar In this chapter we explain FM - CW and PULSE radar level measurement and compare the two techniques. We discuss accuracy and frequency considerations and explore the technical advances that have taken place in recent years and in particular two wire, loop powered transmitters. 47
  53. 53. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 48 FM-CW, frequency modulated continuous wave The FM - CW radar measurement technique has been in use since the 1930's in military and civil aircraft radio altimeters. In the early 1970's this method was developed for marine use measuring levels of crude oil in supertankers. Subsequently, the same technique was used for custody transfer level measurement of large land based storage vessels. More recently, FM CW transmitters have been adapted for process vessel applications. FM - CW, or frequency modulated continuous wave, radar is an indirect method of distance measurement. The transmitted frequency is modulated between two known values, f1 and f2, and the difference between the transmitted signal and the return echo signal, fd, is measured. This difference frequency is directly proportional to the transit time and hence the distance. (Examples of FM - CW radar level transmitters modulation frequencies are 8.5 to 9.9 GHz, 9.7 to 10.3 GHz and 24 to 26 GHz). The theory of FM - CW radar is simple. However, there are many practical problems that need to be addressed in process level applications. An FM - CW radar level transmitter requires a voltage controlled oscillator, VCO, to ramp the signal between the two transmitted frequencies, f1 and f2. It is critical that the frequency sweep is controlled and must be as linear as possible. A linear frequency modulation is achieved either by accurate frequency measurement circuitry with closed loop regulation of the output or by careful linearisation of the VCO output including temperature compensation. f2 frequency Transmitted signal fd ∆t Received signal f1 t1 time 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 48
  54. 54. f(t) Directional Coupler Signal sampling and Fast Fourier transforms (FFT) Frequency Measurement Intermediate frequency Amplifier Filter Mixer f (t + Dt) Directional Coupler f (t + Dt) Fig 4.2 Typical block diagram of FM - CW radar. A very accurate linear sweep is required Signal Microprocessor Front end control function Linear ramp generator Voltage Control V(t) Linear sweep control loop Voltage Controlled Oscillator VCO f(t) 4. Radar level measurement 49
  55. 55. radar_applied_to_level_rb.qxd 15.01.2007 FM - CW block diagram (Fig 4.2) The essential component of a frequency modulated continuous wave radar is the linear sweep control circuitry. A linear ramp generator feeds a voltage controller which in turn ramps up the frequency of the Voltage Controlled Oscillator. A very accurate linear sweep is required. The output frequency is measured as part of the closed loop control. The frequency modulated signal is directed to the radar antenna and 18:46 Seite 50 hence towards the product in the vessel. The received echo frequencies are mixed with a part of the transmission frequency signal. These difference frequencies are filtered and amplified before Fast Fourier Transform (FFT) analysis is carried out. The FFT analysis produces a frequency spectrum on which the echo processing and echo decisions are made. Pic 2 Typical glass lined agitated process vessel. A radar must be able to cope with various false echos from agitatior blades and baffles Simple storage applications usually have a large surface area with very little agitation, no significant false echoes from the internal structure of the tank and relatively slow product movement. These are the ideal conditions for which FM - CW radar was originally developed. However, in process vessels there is more going on and the problems become more challenging. 50 Low amplitude signals and false echoes are common in chemical reactors where there is agitation and low dielectric liquids. Solids applications can be troublesome because of the internal structure of the silos and undulating product surfaces which creates multiple echoes. An FM - CW radar level sensor transmits and receives signals simultaneously.
  56. 56. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 51 4. Radar level measurement fd1, -f d2 , -fd3, -fd4, -fd5 f2 f1 t1 Transmitted signal Real echo signal False echo signals Fig 4.3a FM - CW radar level transmitters in an active process vessel In an active process vessel, the various echoes are received as frequency differences compared with the frequency of the transmitting signal. These frequency difference signals are received by the antenna at the same time. The amplitude of the real echo signals are small compared with the transmitted signal. A false echo from the end of the antenna may have a significantly higher amplitude than the real level echo. The system needs to separate and identify these simultaneous signals before processing the echoes and making an echo decision. The separation of the various received echo frequencies is achieved using Fast Fourier Transform (FFT) analysis. This is a mathematical proce- dure which converts the jumbled array of difference frequencies in the time domain into a frequency spectrum in the frequency domain. The relative amplitude of each frequency component in the frequency spectrum is proportional to the size of the echo and the difference frequency itself is proportional to the distance from the transmitter. The Fast Fourier Transform requires substantial processing power and is a relatively long procedure. It is only when the FFT calculations are complete that echo analysis can be carried out and an echo decision can be made between the real level echo and a number of possible false echoes. 51
  57. 57. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 52 Mixture of frequencies received by FM - CW radar Signal amplitude fd1, fd2, fd3, fd4, fd5 etc combined Fig 4.3b combined echo frequencies are received simultaneously Signal amplitude Combination of mixed difference frequencies received by FM - CW radar Individual difference frequencies fd1, f d2 , fd3, are shown Fig 4.3c The individual frequencies must be separated from the simultaneously received jumble of frequencies 52
  58. 58. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 53 4. Radar level measurement amplitude Frequency spectrum echoes Each echo is within an envelope curve of frequencies frequency 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 frequency domain. 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 Complex process vessels and solids applications can prove too difficult for some FM - CW radar transmitters. Even a simple horizontal cylindrical tank can pose a serious problem. This is because a horizontal tank produces many large multiple echoes that are caused by the parabolic effect of the cylindrical tank roof. Sometimes the amplitudes of the multiple echoes are higher than the real echo. The processors that carry out the FFT analysis are swamped by different amplitude signals across the dynamic range all at the same time. As a result, the FM - CW radar cannot identify the correct echo. As we shall see, this problem does not affect the alternative pulse radar technique. 53
  59. 59. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 54 PULSE radar level transmitters Pulse radar level transmitters provide distance measurement based on the direct measurement of the running time of microwave pulses transmitted to and reflected from the surface of the product being measured. Pulse radar operates in the time domain and therefore it does not require the Fast Fourier transform (FFT) analysis that characterizes FM CW radar. As already discussed, the running time for a distance of a few metres is measured in nanoseconds. For this reason, a special time transformation pro- cedure is required to enable these short time periods to be measured accurately. The requirement is for a ‘slow motion’ picture of the transit time of the microwave pulses with an expanded time axis. By slow motion we mean milliseconds instead of nanoseconds. Pulse radar has a regular and periodically repeating signal with a high pulse repetition frequency (PRF). Using a method of sequential sampling, the extremely fast and regular transit times can be readily transformed into an expanded 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 54
  60. 60. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 55 4. Radar level measurement To illustrate this principle, consider the sine wave signal in Fig 4.6. It is a regular repeating signal with a period of T1. If the amplitude (voltage value) of the output of the sine wave is sampled into a memory at a time period T2 which is slightly longer than T1, then a time expanded version of the original sine wave is produced as an output. The time scale of the expanded output depends on the difference between the two time periods T1 and T2. T1 Periodic Signal (sine wave) Sampling signal T2 Expanded time signal 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 A common example of this principle is the use of a stroboscope to slow down the fast periodic movements of rotating or reciprocating machinery. Fig 4.7 shows how the principle of Periodic Signal (radar echoes) sequential sampling is applied to pulse radar level measurement. The example shown is a VEGAPULS transmitter with a microwave frequency of 5.8 GHz. T1 Emission pulse Echo pulse T2 Sampling signal Fig 4.7 Sequential sampling of a pulse radar echo curve. Millions of pulses per second produce a periodically repeating signal. A sampling signal with a slightly longer periodic time produces a time expanded image of the entire echo curve 55
  61. 61. radar_applied_to_level_rb.qxd 15.01.2007 18:46 This periodically repeating signal consists of the regular emission pulse and one or more received echo pulses. These are the level surface and any false echoes or multiple echoes. The transmitted pulses and therefore the received pulses have a sine wave form depending upon the pulse duration. A 5.8 GHz pulse of 0.8 nanosecond duration is shown in Fig 4.8. The period of the pulse repetition is shown as T1 in Fig 4.7. Period T1 is Seite 56 the same for the emission pulse repetition as for any echo pulse repetition as shown. However, the sampling signal repeats at period of T2 which is slightly longer in duration than T1. This is the same time expansion procedure by sequential sampling that has already been described for a sine wave. The factor of the time expansion is determined 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 Example The 5.8 GHz, VEGAPULS radar level transmitter has the following pulse repetition rates. Transmit pulse 3.58 MHz Reference pulse 3.58 MHz - 43.7 Hz Therefore the time expansion factor is 81920 giving a time expanded pulse repetition period of 22.88 milliseconds. There is a practical problem in sampling the emission / echo pulse signals of a short (0.8 nanosecond) pulse at 5.8 GHz. An electronic switch would need to open and close within a few picoseconds if a sufficiently short value of the 5.8 GHz sine wave is to be sampled. These would have to be very special and expensive components. 56 T1 = 279.32961 nanoseconds T2 = 279.33302 nanoseconds The solution is to combine sequential sampling with a ‘cross correlation’ procedure. Instead of very rapid switch sampling, a sample signal of exactly the same profile is generated but with a slightly longer time period between the pulses. Fig 4.9 compares sequential sampling by rapid switching with sequential sampling by cross correlation with a sample pulse.
  62. 62. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 57 4. Radar level measurement Emission / Echo pulse Sample signal Sampling with picosecond switching Sampling by cross correlation with a sample pulse 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 ‘picosecond’ switching is not required Instead of taking a short voltage sample, cross correlation involves multiplying a point on the emission or echo signal by the corresponding point on the sample pulse. The multiplication leads to a point on the resultant signal. All of these multiplication results, one after the other, lead to the formation of the complete multiplication signal. Fig 4.10 shows a short sequence of multiplications between the received signal (E) and the sampling pulse signal (M). The resultant E x M curves are shown on page 58. Then the E x M curve is integrated and represented on the expanded curve as a dot. The sign and amplitude of the signal on the time expanded curve depends on the sum of the area of the E x M curve above and below the zero line. The final integrated value corresponds directly to the time position of the received pulse E relative to the sample pulse M. The received signal E and sample signal M in Fig 4.10 are equivalent to the periodic signal (sine wave) and sample signal in Fig 4.6. The result of the integration of E x M in Fig 4.10 is directly analogous to the expanded time signal in Fig 4.6. 57
  63. 63. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 58 E M ExM max Integral ExM 0 min 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 The pulse radar sampling procedure is mathematically complicated but a technically simple transformation to achieve. Generating a reference signal with a slightly different periodic time, multiplying it by the echo signal and integration of the resultant product are all operations that can be handled easily within analogue circuits. Simple, but good quality components such as diode mixers for multiplication and capacitors for integration are used. 58 This method transforms the high frequency received signal into an accurate picture with a considerably expanded time axis. The raw value output from the microwave module is an intermediate frequency that is similar to an ultrasonic signal. For example the 5.8 GHz microwave pulse becomes an intermediate frequency of 70 kHz. The pulse repetition frequency (PRF) of 3.58 GHz becomes about 44 Hz.
  64. 64. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 59 4. Radar level measurement amplitude Pulse echoes in a process vessel are separated in time t1 t 2 t3 t4 t5 time transmit pulse 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 radar operates entirely within the time domain and does not need the fast and expensive processors that enable the FM - CW radar to function. There are no Fast Fourier Transform (FFT) algorithms to calculate. All of the pulse radar processing is dedicated to echo analysis only. Part of the pulse radar transmission pulse is used as a reference pulse that provides automatic temperature compensation within the microwave module circuits. The echoes derived from a pulse radar are discrete and separated in time. This means that pulse radar is better equipped to handle multiple echoes and false echoes that are common in process vessels and solids silos. Pulse radar takes literally millions of ‘shots’ every second. The return echoes from the product surface are sampled using the method described above. This technique provides the pulse radar with excellent averaging which is particularly important in difficult applications where small amounts of energy are being received from low dielectric and agitated product surfaces. The averaging of the pulse technique reduces the noise curve to allow smaller echoes to be detected. If the pulse radar is manufactured with well designed circuits containing good quality electronic components they can detect echoes over a wide dynamic range of about 80 dB. This can make the difference between reliable and unreliable measurement. 59
  65. 65. 15.01.2007 18:46 Seite 60 Fig 4.11 Block diagram of PULSE radar microwave module radar_applied_to_level_rb.qxd 60
  66. 66. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 61 4. Radar level measurement Pulse block diagram - (Fig 4.11) The raw pulse output signal (intermediate frequency) from the pulse radar microwave module is similar, in frequency and repetition rate, to an ultrasonic signal. This pulse radar signal is derived in hardware. Unlike FM - CW radar, PULSE does not use FFT analysis. Therefore, pulse radar does not need expensive and power consuming processors. The pulse radar microwave module generates two sets of identical pulses with very slightly different periodic times. A fixed oscillator and pulse former generates pulses with a frequency of 3.58 MHz. A second variable oscillator and pulse former is tuned to a frequency of 3.58 MHz minus 43.7 Hz and hence a slightly longer periodic time. GaAs FET oscillators are used to produce the microwave carrier frequency of the two sets of pulses. The first set of pulses are directed to the antenna and the product being measured. The second set of pulses are the sample pulses as discussed in the preceding text. The echoes that return to the antenna are amplified and mixed with the sample pulses to produce the raw, time expanded, intermediate frequency. Part of the measurement pulse signal is used as a reference pulse that provides automatic temperature compensation of the microwave module electronics. Pic 3 Two wire pulse radar level transmitter mounted in a process reactor vessel 61
  67. 67. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 62 Choice of frequency Process radar level transmitters operate at microwave frequencies between 5.8 GHz and about 26 GHz. Manufacturers have chosen frequencies for different reasons ranging from licensing considerations, availability of microwave components and perceived technical advantages. There are arguments extolling the virtues of high frequency radar, low frequency radar and every frequency radar in between. In reality, no single frequency is ideally suited for every radar level measurement application. If we compare 5.8 GHz radar with 26 GHz radar, we can see the relevant benefits of high frequency and low frequency radar. 2.6 GHz 5.8 GHz 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 62
  68. 68. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 63 4. Radar level measurement Antenna size - beam angle The higher the frequency of a radar level transmitter, the more focused the beam angle for the equivalent size antenna. With horn antennas, this allows smaller nozzles to be used with a more focused beam angle. For example, a 1½" (40 mm) horn antenna radar at 26 GHz has approximately the same beam angle as a 6" (150 mm) horn antenna at 5.8 GHz. However, this is not the complete picture. Antenna gain is dependent on the square of the diameter of the antenna as well as being inversely proportional to the square of the wavelength. Antenna gain is proportional to:2 diameter wavelength 2 Antenna gain also depends on the aperture efficiency of the antenna. Therefore the beam angle of a small antenna at a high frequency is not necessarily as efficient as the equivalent beam angle of a larger, lower frequency radar. A 4" horn antenna radar at 6 GHz gives excellent beam focusing. A full explanation of antenna gain and beam angles at different frequencies is given in Chapter 5 on radar antennas. 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 63
  69. 69. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 64 Antenna focusing and false echoes A 26 GHz beam angle is more focused but, in some ways, it has to be. The wavelength of a 26 GHz radar is only 1.15 centimetres compared with a wavelength of 5.2 centimetres for a 5.8 GHz radar. The short wavelength of the 26 GHz radar means that it will reflect off many small objects that may be effectively ignored by the 5.8 GHz radar. Without the focusing of the beam, the high frequency radar would have to cope with more false echoes than an equivalent lower frequency radar. Fig 4.15 a Low frequency radar has a wider beam angle 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 64
  70. 70. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 65 4. Radar level measurement Agitated liquids and solid measurement High frequency radar transmitters are susceptible to signal scatter from agitated surfaces. This is due to the signal wavelength in comparison to the size of the surface disturbance. The high frequency radar will receive considerably less signal than an equivalent 5.8 GHz radar when the liq- uid surface is agitated. The lower frequency transmitters are less affected by agitated surfaces. It is important that, whatever the frequency, the radar electronics and echo processing software can cope with very small amplitude echo signals. As discussed, pulse radar has an advantage in this area no matter what the frequency. Fig 4.16 High frequency radar transmitters are susceptible 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 up Steam and dust High frequency radar level transmitters are more susceptible to condensation and product build up on the antenna. There is more signal attenuation at the higher frequencies, such as 26 GHz. Also, the same level of coating or condensation on a smaller antenna naturally has a greater effect on the performance. A 6" horn antenna with 5.8 GHz frequency is virtually unaffected by condensation. Also, it is more forgiving of product build up. Lower frequencies such as 5.8 GHz and 6.3 GHz are not adversely affected by high levels of dust or steam. These frequencies have been very successful in applications ranging from cement, flyash and blast furnace levels to steam boiler level measurement. In steamy and dusty environments, higher frequency radar will suffer from increased signal attenuation. 65
  71. 71. radar_applied_to_level_rb.qxd 15.01.2007 Foam The effect of foam on radar signals is a grey area. It depends a great deal on the type of foam including the foam density, dielectric constant and conductivity. However, low frequencies such as 5.8 GHz and 6.3 GHz cope with low density foam better than higher frequencies such as 26 GHz. For example, a 26 GHz radar signal will be totally attenuated by a very thin detergent foam on a water surface. A 5.8 GHz radar signal will see through this type of foam and continue to see the liquid surface as the foam thickness increases to 150 mm or even 250 mm. 18:46 Seite 66 However, the thickness of foam will cause a small measurement error because the microwaves slow down slightly as they pass through the foam. When foam is present, it is important to provide the radar manufacturer with as much information as possible on the application. Minimum distance Higher frequency radar sensors have a reduced minimum distance when compared with the lower frequencies. This can be an additional benefit when measuring in small vessels and stilling tubes. Summary of the effects of radar frequency Better focusing at higher emitting frequency means: higher antenna gain (directivity) less false echoes reduced antenna size focusing . . . 5 GHz 10 GHz 15 GHz frequency range Fig 4.17 Focusing and radar frequency 66 20 GHz 25 GHz
  72. 72. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 67 4. Radar level measurement reduced signal caused by damping Reduced signal strength caused by damping at higher emitting frequency caused by: . . . 5 GHz 10 GHz condensation build - up steam and dust 15 GHz 20 GHz 25 GHz frequency range Fig 4.18 Signal damping and radar frequency Higher damping caused by agitated product surface reflection from medium . . . 5 GHz 10 GHz wave movement material cones with solids signal scattered 15 GHz 20 GHz 25 GHz frequency range Fig 4.19 Signal strength from agitated and undulating surfaces and radar frequency 67
  73. 73. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 68 Accuracy Pulse radar bandwidth There is no inherent difference in accuracy between the FM - CW and PULSE radar level measurement techniques. In this book, we are concerned specifically with process level measurement where ‘process accurate’ and cost effective solutions are required. The achievable accuracy of a process radar depends heavily on the type of application, the antenna design, the quality of the electronics and echo processing software employed. The niche market for custody transfer level measurement applications is outside the scope of this book. These custody transfer radar ‘systems’ are used in bulk petrochemical storage tanks. Large parabolic or planar array antennas are used to create a finely focused signal. A lot of processing power and on site calibration time is used to achieve the high accuracy. Temperature and pressure compensation are also used. The carrier frequency of a pulse radar varies from 5.8 GHz to about 26 GHz. The pulse duration is important when it comes to resolving two adjacent echoes. For example, a one nanosecond pulse has a length of about 300 mm. Therefore, it would be difficult for the radar to distinguish between two echoes that are less than 300 mm apart. Clearly a shorter pulse duration provides better range resolution. An effect of a shorter pulse duration is a wider bandwidth or spectrum of frequencies. For example, if the carrier frequency of a pulse is 5.8 GHz and the duration is only 1 nanosecond, then there is a spectrum of frequencies above and below the nominal carrier frequency. The amplitude of the pulse spectrum of frequencies changes according to a Range resolution and bandwidth In process level applications, both FM - CW and PULSE radar work with an ‘envelope curve’. The length of this envelope curve depends on the bandwidth of the radar transmitter. A wider bandwidth leads to a shorter envelope curve and therefore improved range resolution. Range resolution is one of a number of factors that influence the accuracy of process radar level transmitters. 68 sin x x 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 2 τ where τ is the pulse duration. It is clear from the curve that the amplitude of frequencies reduces significantly away from the main pulse frequency.
  74. 74. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 69 4. Radar level measurement pulse frequency 5.8 GHz 4.8 GHz shorter pulse better range resolution Fig 4.20 Pulse radar range resolution. The guaranteed range resolution is the length of the pulse. A shorter pulse has a wider bandwidth and better range resolution 6.8 GHz bandwidth BW nn, 2 equal to τ Fig 4.21 The null to null bandwidth BWnn of a radar pulse is equal to 2 / τ where τ is the pulse duration. Example a 5.8 GHz radar with a pulse duration of one nanosecond has a null to null bandwidth of 2 GHz Pulse radar envelope curve Fig 4.22 shows how a pulse radar echo curve is used in process level measurement. A higher frequency pulse with a shorter pulse duration will allow better range resolution and also better accuracy because the leading edge of the envelope curve is steeper. Fig 4.22 Envelope curve with pulse radar High frequency, short duration pulse Lower frequency pulse with longer duration Fig 4.23 A shorter pulse duration gives better range resolution. The combination of shorter pulse duaration and higher frequency allows better accuracy because the leading edge of the envelope curve is steeper 69
  75. 75. radar_applied_to_level_rb.qxd 15.01.2007 18:46 FM-CW radar bandwidth The bandwidth of an FM - CW radar is the difference between the start and finish frequency of the linear frequency modulation sweep. Unlike pulse radar, the amplitude of the FM - CW signal is constant across the range of frequencies. Seite 70 A wider bandwidth produces narrower difference frequency ranges for each echo on the frequency spectrum. This leads to better range resolution in the same way as with shorter duration pulses with pulse radar. This is explained in the following diagrams and equations. frequency fd = ∆F x 2R Ts x c fd ∆F Ts R fd c ∆F [Eq. 4.1] bandwidth sweep time distance difference frequency speed of light time Ts Fast Fourier Transform 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. amplitude fd ∆fd = 2 Ts [Eq. 4.2] ∆fd 70 frequency
  76. 76. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 71 4. Radar level measurement The ambiguity of the distance R, is ∆ R ∆fd fd ∆R R = 2 Ts ∆F x 2 R Ts x c ∆R R = c ∆F x R ∆R R ∆R = = c ∆F ∆R R amplitude distance Fig 4.24 to 4.26 - FM - CW range resolution [Eq. 4.3] From equation 4.3, it can be seen that with an FM - CW radar the range resolution ∆R is equal to:- c ∆F Therefore, the wider the bandwidth, the better the range resolution. Examples: A linear sweep of 2 GHz has a range resolution of 150 mm whereas a 1 GHz bandwidth has a range resolution of 300 mm. In process radar applications, each echo on the frequency spectrum is processed with an envelope curve. The above equations (Equations 4.1 to 4.3) show that the Fast Fourier Transforms (FFTs) in process radar applications do not produce a single discrete difference frequency for each echo in the vessel. Instead they produce a difference frequency range ∆fd for each echo within an envelope curve. This translates into range ambiguity. 71
  77. 77. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 72 FM - CW frequency spectrum - bandwidth and range resolution Frequency spectrum - narrow bandwidth of linear sweep amplitude envelope curves around echoes frequency Frequency spectrum - wide bandwidth of linear sweep amplitude envelope curves around echoes frequency 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 Other influences on accuracy As we have demonstrated, FM - CW and PULSE process radar transmitters use an envelope curve for measurement. A wider bandwidth produces better range resolution. The correspondingly short echo will have a steep slope and therefore a more accurate measurement can be made. Other influences on accuracy include signal to noise ratio and interference. 72 A high signal to noise ratio allows more accurate measurement while interference effects can cause a disturbance of the real echo curve leading to inaccuracies in the measurement. Choice of antenna and mechanical installation are important factors in ensuring that the optimum accuracy is achieved.
  78. 78. radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 73 4. Radar level measurement High accuracy radar FM - CW radar High accuracy of the order of + 1 mm is generally meaningless in an active process vessel or a solids silo. For example, a typical chemical reactor will have agitators, baffles and other internal structures plus constantly changing product characteristics. Although custody transfer level measurement applications are not in the scope of this book, this section discusses how a higher accuracy can be achieved. The fundamental requirement for an accurate FM - CW radar is an accurate linear sweep of the frequency modulation. As with the pulse radar, it is possible to look inside the envelope curve of the frequency spectrum if the application has a simple single echo that is characteristic of a liquids storage tank. This is achieved by measuring the phase angle of the difference frequency. However, this is only practical with custody transfer applications where fast and expensive processors are used with temperature and pressure compensation. Pulse radar For most process applications, measurement relative to the pulse envelope curve is sufficient. However, if the liquid level surface is flat calm and the echo has a reasonable amplitude, it is possible to look inside the envelope curve wave packet at the phase of an individual wave. However, the envelope curve of a high frequency radar with a short pulse duration is sufficiently steep to produce a very accurate and cost effective level transmitter for storage vessel applications. frequency error f2 f2 t1 Fig 4.30 It is essential that the linear sweep of the FM - CW radar is accurately controlled 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 73