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Towards an air sonar
lULTRASONICS FOR INDUSTRY 1967 1conference paper
Towards an air sonar D. S. Dean*
INTRODUCTION Ultrasonic position sensing devices have been little used in air despite their apparent attractions and despite the fact that animals have used them successfully. The purpose of this paper is to examine critically the factors involved in the construction of an air sonar from a theoretical point of view. No practical work on the construction of such a sonar is described although the author’s work on air coupled inspection systems provides some background information on which to draw.
FUNDAMENTAL DIFFICULTIES Let us consider first of all the fundamental difficulties facing the designer of an equipment. Some of these can be overcome to a sufficient extent to allow a workable system to be constructed, but others seem insoluble. The possible solutions will be discussed later.
that if V is the sound velocity in air and v the velocity of the air, then the time taken will be
t=d.__ v-v
d v+v
2dV vz - va
The ratio of recorded distance d’ to true distance d will therefore be d’ -=d
1 1-s
Taking our will be the expression transducer
previous bad example, the measured distance true distance multiplied by 1.007. Since the only involves va, the error is the same if the is directed up or down wind.
A further difficulty is that the frequency of the radiation may be changed between transmission and reflection. Under steady wind conditions the wavelength when transmitting into wind will be
Acoustic mismatch - The first difficulty is the mismatch between almost any transducer medium and air. The transmission of acoustic energy across an interface is given by 4
T= (Z,
ZlZ2 +
2212
where T = Transmission = Z,Z,
coefficient
1
The acoustic impedance of piezoelectric materials varies between 11 X 106 and 30 X 106 kg me2 s-1, whilst that of air is only 330kg mm2s-l. The energy crossing the interface is therefore between 1.3 X 10m4 and 0.44 X 10e4 of that incident upon it. Wind effects Since the energy travels in a material medium which may itself be in motion, this will modulate the signals returning to the receiver. It is possible to derive general formulae for this, but it is probably more instructive to consider particular instances. To take a bad example (by no means the worst possible), let us assume a wind moving at 30ms-1 (about 60mph) at right angles to the direction of propagation of the ultrasound, which is itself travelling at 330ms-1. This will introduce a bearing error of tan-r 30/330, or about 5”, and range readings will all be multiplied by a factor of 1.004, which is perhaps an acceptable range error, but rather a large bearing error. Consider now the case where the same wind is blowing directly towards the transducer. Bearing error will be zero, but the pulse will have to travel to the target and back, so * Rocket Propulsion Establishment, Westcott, near Aylesbury, Buckinghamshire, England
f
v+v
AR = -
Incident energy in medium 1
where p is the density of the medium and V the velocity of sound in the medium.
=v--v
where f is the transmission frequency. At the target the frequency will appear the same since the velocity of the ultrasound, V - v, will just compensate for the compression of the wavelength. Similarly the wavelength during the return path will be
Transmitted energy in medium 2
= Acoustic impedances of media (= pV)
0-
(5)
f
and the frequency at the receiver will be unchanged. In all the above we have considered steady wind conditions and stationary targets, conditions which rarely apply. In fact winds usually change their direction and strength quite rapidly, and it would not be impossible for transmission to occur when the wind velocity was v as before, and for the velocity to drop almost to zero on reception. Assume that the waves are returning at a wavelength X = (V + v)/f when the wind drops to zero. The wavelength will not change, but the waves will meet the receiver travelling at velocity V, so that the frequency it sees will be
Or
f’ --f -v!v
Using the same figures as before we have fl-0 f
*
92
(‘3)
In a practical system it is also likely that the target at least will be moving, resulting in a Doppler change of frequency which can be shown to be the same as in the example just considered, so that a 60mph target will result in an 8% frequency change. The change may he in either sense, and the bandwidth of the equipment must be wide enough to cope with this. ULTRASONICS January 1968
89
1
IULTRASONICS FOR INDUSTRY 1967 1conference paper Whether this is a handicap or not depends upon the range resolution required of the equipment. Let us assume that we can resolve targets ,separated by one pulse width 7 and that a resolution of 33mm is required in range. For relatively undistorted amplification of such a pulse the bandwidth must be sufficient to pass the tenth harmonic of the frequency given by f = l/r. Our minimum pulse width will be 10-d set, so that the tenth harmonic will be 10sHz. To obtain adequate range it is unlikely that a carrier frequency greater than 10sHz can be used. so that a bandwidth of 2 x 105Hz is necessary, and frequency changes due to target motion are of little consequence. If we reduce the range resolution required to 0.33m and keep the same carrier frequency, the bandwidth becomes only 20% of the centre frequency and the target motion doubles the bandwidth required. If we are forced to use a wide bandwidth we must have a transducer with an equally flat response, and this can only be obtained with piezoelectric transducers by heavy damping which reduces their sensitivity and is difficult to achieve. In addition to this the noise in a system is given by e=JKat
(9)
where e is the rms voltage K is a constant so that the minimum detectable signal becomes greater as the bandwidth increases. Thermal effects A further fundamental difficulty is due to changes in air temperature which will affect the acoustic properties of the air. At constant pressure the density and velocity changes are in opposition, so that a change of 40°C will only result in a 1% change in acoustic impedance. The velocity, V, of sound however changes according to the relationship (10)
so that a 40°C rise in temperature will reduce the apparent range of a target by 6. 5%. The possibility must also be considered of temperature gradients in the air such as might be caused by vehicle exhausts, radiators or even differential heating by the sun. Although the gradients would probably be fairly gradual in most cases, it is easier to consider the case where there is a step change in temperature of, say, 10°C. The refraction at such a boundary is given by Snell’s law Vi -=_ sin i
Vr sin r
(11)
where the subscripts refer to the incident and refracted beams. Substituting figures shows that we can expect a deviation of 32min of arc when i is 30”, or 2” when i is 60”. Fortunately the variation in velocity with atmospheric changes in pressure is negligible, so that no correction needed for this.
The low propagation velocity of ultrasound means that scanning speeds of the beam cannot be high. Let us assume a range of 33m so that the out and back time of the pulse will be 0.2sec, and a scanned sector 45” in bearing and elevation. If we assume that our picture must consist of a matrix of 100 X 100 separate measurements, this will require 2000sec, i.e. over half an hour to build up. If we are content with a very coarse display of 10 X 10 points, the total time will still be 2Osec per picture, and our 6Omph target could pass through the scanned area without detection. ADVANTAGES OF AN AIR SONAR After the formidable list of obstacles has been considered, what are the features which would make an air sonar worth using rather than a more conventional radar system ? ULTRASONICS January 1968
To achieve the same wavelength with a radar system we would need to transmit at 9OGHz and using the same pulse length the circuits would need rise times approaching lO_llsec. Devices for transmitting at these frequencies are available commercially now, although they are considered rather specialised items, but the best resolution that can be obtained so far is about 0.3m. If ranges up to 10m are adequate the ultrasonic frequency can be raised and we have ourselves used our air -coupled inspection apparatus, which works at 5OOkH2,as a rudimentary sonar in this range. The wavelength is reduced to O.‘7mm, but since we were using high Q transducers the range resolution was not as good as could be obtained with specially designed equipment.
Partly for the reasons stated above, an ultrasonic apparatus would be cheaper than the equivalent radar, and would generally be more robust, although the further development of the Gunn diode to higher frequencies would result in a radar system almost as robust as the equivalent sonar. The life of a sonar system would be indefinite if properly designed, whereas existing microwave generators usually have a guaranteed life of 500hr. Again the Gunn diode should eventually overcome this handicap. High reflectivity The acoustic mismatch between any solid or liquid material and air has already been stated to be very great, so that almost 100% of the incident energy would be reflected from any object. Presumably some surfaces might absorb to some extent, but this is not likely to be a problem as anyone can confirm who has tried to prevent echoes from the walls of an ultrasonic immersion test tank, where the mismatch is not nearly so great. It would be rather like a conventional radar set used in an all metal world. Small source size
It is comparatively easy to obtain a narrow radiated beam, because the velocity in air is low. The beam width is given by sin 0 = 0.61 x/a
(12)
where 6 is the half angle of the beam, and a is the radius of the circular source (Fig 1). If the source is operating at lOOHz, X = 3.3mm, so that for an angle f3of only 1” a source size of 115mm is all that is required.
is
Scan speed
30
resolution
The biggest advantage is probably in range resolution, since the velocity of ultrasonic waves is so much lower than that of electromagnetic waves. The wavelength of a 1OOkHzultrasonic signal is only 3. 3mm, so that a pulse consisting of ten cycles would only be 33mm long, allowing resolution in range to about half this figure. The time period involved is only 10m4sec, so that cheap timing circuits working to 1MBz would be more than adequate.
Cheapness and long life
Af is the bandwidth of the system
v=vo+0.54t
Range
1
POSSIBLE SOLUTIONS Let us now take the difficulties in turn and see if there are solutions which can reasonably be applied. Overcoming acoustic mismatch lf we intend to use piezoelectric disks or magnetostrictive transducers to generate and detect the ultrasound, there is little we can do to avoid large interface losses. The losses can be reduced by using a sandwich construction in which layers of material half a wavelength thick are firmly attached to the front surface of the disk in descending order of acoustic impedance (Fig 2). It can be shown that maximum transmission occurs across the boundaries between three media when 2,
=
Jz,z,
(13)
~t_XTRASOlWCSFOR IADUSTRY 1967 1conference paper (22,
T123
-=
l/2
T13
z;
f 42,
+ 2Z,)2 2
2/3
z,
]
+ z,z,
+ 5212,
3 + 42, 3/2Z1/2
+ z”, (14)
Fig 1 Idealized beam pattern produced by air transducer. Sin. 0 = 0.61 x/a
Fiezoelectric material(Z,) Intermediate
layer(s=vj
Outer face(Z3) Silvered layers Live connection Insulator
Fig 2 Sandwich transducer
0.0065mm thick mylar film outer surface silvered Etched conducting back plate
Fig 3 Mylar film transducer
The subscripts refer to the media in the order in which they are assembled. The ratio between the energy transmitted through such a layer and that transmitted across an interface between media 1 and 3 is given by If we assume that the last layer will be nylon, which has one of the lowest figures for acoustic impedance of any solid material (2kg rnV2s-l) and the intermediate material attached to the lead zirconate disk is one with an acoustic impedance m the improvement over the case where the intermediate layer is omitted will be a factor of two. The simpler example of a lead-zirconate/air interface compared with a lead-zirconate/nylon/air system shows that the nylon face should increase the transmitted energy by a factor of about 15. Increasing the number of layers of the sandwich is therefore probably not worthwhile when the additional losses are taken into account. We are still left with the large loss at the nylon/air interface and here we seem to reach a limit, since there is no material available with an acoustic impedance in the range between nylon and air. We can improve the position if we can use a thin diaphragm vibrated in air in the same way as a loudspeaker, but at the frequencies we wish to use the cone of a speaker would vibrate in complex modes and produce little output. This can be overcome by making the diaphragm from a thin nylon film (0.0065mm thick) silvered on one side, and stretching it across a conducting plate (Fig 3). If a potential is applied between the film and the plate the force produced between them will move the silvered surface. In this case the nylon dielectric will have to be compressed, and a more sensitive transducer can be made by finely etching the conducting plate so that the film forms a large number of small diaphragms each with its own air spring behind it. The natural frequency of each diaphragm will be high and these devices can be used up to 25OkHz at least. The loss can be reduced to a minimum by generating the waves directly in air, and we have carried out some work in this respect. An instrument is available commercially called an ionophone (Fig 4) which generates a high frequency (27MHz) corona discharge between an internal electrode and a quartz tube. The discharge is modulated at the required sonic or ultrasonic frequency and the resultant noise from the end of the quartz tube is fed to the surroundings via an exponential horn. The device is capable of working at ultrasonic frequencies, since there are no moving parts; the upper limiting factors are likely to be electrical, the limit being quoted as ‘above 1OOkHz’. The response of this instrument is flat over a wide range-a great advantage over piezoelectric transducers. We have been able to generate greater power by using an open spark fed by a 4MHz transmitter modulated as before. Control of beam shape can be by suitable reflectors, or multiple spark sources can be used. The heat generated by the sources can cause refraction of the waves from the sources, so that the interference pattern between them changes and the combined signal fluctuates. Correction
Insulating
bush
Correction
Fig 4 Diagram of ionic loudspeaker
for wind speed
Some degree of compensation is possible by measuring wind speed with any suitable transducer and applying corrections automatically to the measured ranges and bearings, but this would assume that the wind velocity measured at one point could be applied to all others. Any sailor of small boats can confirm that this is hardly ever so, and widely differing wind velocities and directions are encountered over distances of a few tens of feet. Fortunately winds tend to become steadier as velocity increases so that correction would probably be worthwhile. It does not seem possible to correct for effects due to motion of the target. for thermal effects
The general ambient temperature can always be sensed and corrections applied comparatively easily, but the effects of temperature gradients are always likely to be troublesome, ULTRASONICS January 1668
31
k_JL7’RmASOlvrCS FOR INLXJSTRY 1967 I conference paws Overcoming scan speed problems We have so far been thinking in terms of one or more ultrasonic pulses for each unit of the target area, but there are techniques used in underwater sonar which could well be applied to speed up the scan. One technique uses a long duration signal to illuminate ultrasonically a sector of the target area and a narrow beam receiver transducer which scans from one side of the sector to the other in a time equal to the pulse duration, one scan immediately following the previous one. If a target is present the receiver will pick it up at some point in one of its scans. A complete sector can thus be scanned in the time taken for one pulse to travel to maximum range and back. The power required per pulse is obviously much greater than with a narrow beam scan, although the mean power is not. The range resolution, which is already good, and signal to noise ratio can be still further improved by the use of pulse compression, an example of which is shown in Fig 5. The frequency of the carrier is increased during each pulse and sweeps through a suitable frequency range. On reception the frequency components of the pulse are separated by a dispersive grating, so that the earlier, lower frequencies travel a longer path to the detector than the later, higher frequencies. It is arranged that all frequencies arrive together at the detector to combine into a larger narrower pulse which can be more easily separated from noise. The pulse length becomes approximately l/B whilst the amplitude increases in the ratio 1: &, where B is the swept frequency range and T is the uncompressed pulse duration. APPLICATIONS Air sonars have already been used in some applications with varying degrees of success. Crane operations have been controlled by ultrasonic systems, although the control system only needs to be fairly coarse. Radar simulators have also been constructed: a recent one was used to demonstrate the application of pulse compression. Blind aids are a successful application, and sonars could be effective as intruder alarms, either looking for the disturbance of an ultrasonic diffraction pattern, or simply the interruption of an ultrasonic beam. Perhaps the largest use of sonars is in the detection of the presence or absence of moving objects without the need for a detailed scan. Trucks in factories can be controlled in
I
this fashion, and primary information for the Munich computer-controlled traffic system is obtained by ultrasonic sensors. Suitably spaced transducers Received
wave of progressive decreasing frequency ,
;
I
I
I
I
-.
,,I,,,
Position of wave for additive effect
Fig 5 An example of pulse compression. The transmitted pulse is a long one of progressively decreasing frequency (typically a 10% change, e.g. from lOO-9OkBz). The returning pulse is received on the end of a rod of acoustically transmitting material (e.g. quartz). A train of compressional waves of progressively increasing wavelength thus travels through the rod to piezoelectric transducers cemented to it. The taper ensures that reflected waves do not travel up and down the rod. The transducers are spaced so that when the wave train lies completely within the bounds of the end transducers a compressional wave is located at each transducer. These are connected in parallel and hence the outputs due to the Poisson’s ratio effect are additive. At any other position of the wave complete addition will not take place and in fact the outputs of some transducers will be in the opposite sense to the rest. The result is an output pulse considerably shorter in time than the travelling wave in the air and of greater amplitude than that which any single part of the wave would produce. CONCLUSIONS For applications in still air over distances of a few tens of feet the air sonar can show worthwhile advantages over other detection systems. In disturbed air it may still be the cheapest way of detecting objects provided a detailed scan is not required. For greater distances, and for applications where a detailed picture is required in a reasonable time, it seems most unlikely that air sonar systems will be developed.
Animal sonar in air J. D. Pye* Active sonar is now known to be used for orientation by several groups of animals, all of which move in a three-dimensional medium with poor visibility. Under water, whales and dolphins have been shown to emit brief click-like pulses of wide bandwidth, while similar pulses, partly audible to man, are used in air by two groups of birds and by Megachiropteran bats of the genus Rousettus. Terrestrial shrews, when forced to find a jumping platform in complete darkness emit longer pulses, up to 30ms, of high frequency, 30-6OKBzi. But the first such systems to be discovered, those of Microchiropteran bats,213 remain-the best known, the most sophisticated and, in terms of radar/sonar theory, the most interesting.
from 54 species by oscillographic recording. The author has continued this work by tape-recording and subsequent sound spectrographic analysis of 66 species (in preparation) of which only 12 had been examined before. The equipment has been described previously* but will be reviewed here to inelude some recent improvements.
In an attempt to define the considerable variability which exists among the ‘IOO-odd species of Microchiroptera and often within a single species under different conditions, Griffin and Novicks and Novick5-7 analysed the pulse signals
The diaphragm material now used is Mylar grade C (Vacuum Developments Ltd) at a thickness of 3.5pm. Backplates made of watch hairsprings, cemented to perspex discs, were first used as an expedient to simplify microphone construction. They are still retained, however, since they appear to be rather more sensitive than grooved metal discs, although the reasons for this have not yet been analysed.
* King’s College, University of London, England 32
ULTRASONICS January 1968
INSTRUMENTATION The most useful microphone is still the solid dielectric pacitance microphone described by Kuhl et alg.
ca-
Towards an air sonar D. S. Dean*
INTRODUCTION Ultrasonic position sensing devices have been little used in air despite their apparent attractions and despite the fact that animals have used them successfully. The purpose of this paper is to examine critically the factors involved in the construction of an air sonar from a theoretical point of view. No practical work on the construction of such a sonar is described although the author’s work on air coupled inspection systems provides some background information on which to draw.
FUNDAMENTAL DIFFICULTIES Let us consider first of all the fundamental difficulties facing the designer of an equipment. Some of these can be overcome to a sufficient extent to allow a workable system to be constructed, but others seem insoluble. The possible solutions will be discussed later.
that if V is the sound velocity in air and v the velocity of the air, then the time taken will be
t=d.__ v-v
d v+v
2dV vz - va
The ratio of recorded distance d’ to true distance d will therefore be d’ -=d
1 1-s
Taking our will be the expression transducer
previous bad example, the measured distance true distance multiplied by 1.007. Since the only involves va, the error is the same if the is directed up or down wind.
A further difficulty is that the frequency of the radiation may be changed between transmission and reflection. Under steady wind conditions the wavelength when transmitting into wind will be
Acoustic mismatch - The first difficulty is the mismatch between almost any transducer medium and air. The transmission of acoustic energy across an interface is given by 4
T= (Z,
ZlZ2 +
2212
where T = Transmission = Z,Z,
coefficient
1
The acoustic impedance of piezoelectric materials varies between 11 X 106 and 30 X 106 kg me2 s-1, whilst that of air is only 330kg mm2s-l. The energy crossing the interface is therefore between 1.3 X 10m4 and 0.44 X 10e4 of that incident upon it. Wind effects Since the energy travels in a material medium which may itself be in motion, this will modulate the signals returning to the receiver. It is possible to derive general formulae for this, but it is probably more instructive to consider particular instances. To take a bad example (by no means the worst possible), let us assume a wind moving at 30ms-1 (about 60mph) at right angles to the direction of propagation of the ultrasound, which is itself travelling at 330ms-1. This will introduce a bearing error of tan-r 30/330, or about 5”, and range readings will all be multiplied by a factor of 1.004, which is perhaps an acceptable range error, but rather a large bearing error. Consider now the case where the same wind is blowing directly towards the transducer. Bearing error will be zero, but the pulse will have to travel to the target and back, so * Rocket Propulsion Establishment, Westcott, near Aylesbury, Buckinghamshire, England
f
v+v
AR = -
Incident energy in medium 1
where p is the density of the medium and V the velocity of sound in the medium.
=v--v
where f is the transmission frequency. At the target the frequency will appear the same since the velocity of the ultrasound, V - v, will just compensate for the compression of the wavelength. Similarly the wavelength during the return path will be
Transmitted energy in medium 2
= Acoustic impedances of media (= pV)
0-
(5)
f
and the frequency at the receiver will be unchanged. In all the above we have considered steady wind conditions and stationary targets, conditions which rarely apply. In fact winds usually change their direction and strength quite rapidly, and it would not be impossible for transmission to occur when the wind velocity was v as before, and for the velocity to drop almost to zero on reception. Assume that the waves are returning at a wavelength X = (V + v)/f when the wind drops to zero. The wavelength will not change, but the waves will meet the receiver travelling at velocity V, so that the frequency it sees will be
Or
f’ --f -v!v
Using the same figures as before we have fl-0 f
*
92
(‘3)
In a practical system it is also likely that the target at least will be moving, resulting in a Doppler change of frequency which can be shown to be the same as in the example just considered, so that a 60mph target will result in an 8% frequency change. The change may he in either sense, and the bandwidth of the equipment must be wide enough to cope with this. ULTRASONICS January 1968
89
1
IULTRASONICS FOR INDUSTRY 1967 1conference paper Whether this is a handicap or not depends upon the range resolution required of the equipment. Let us assume that we can resolve targets ,separated by one pulse width 7 and that a resolution of 33mm is required in range. For relatively undistorted amplification of such a pulse the bandwidth must be sufficient to pass the tenth harmonic of the frequency given by f = l/r. Our minimum pulse width will be 10-d set, so that the tenth harmonic will be 10sHz. To obtain adequate range it is unlikely that a carrier frequency greater than 10sHz can be used. so that a bandwidth of 2 x 105Hz is necessary, and frequency changes due to target motion are of little consequence. If we reduce the range resolution required to 0.33m and keep the same carrier frequency, the bandwidth becomes only 20% of the centre frequency and the target motion doubles the bandwidth required. If we are forced to use a wide bandwidth we must have a transducer with an equally flat response, and this can only be obtained with piezoelectric transducers by heavy damping which reduces their sensitivity and is difficult to achieve. In addition to this the noise in a system is given by e=JKat
(9)
where e is the rms voltage K is a constant so that the minimum detectable signal becomes greater as the bandwidth increases. Thermal effects A further fundamental difficulty is due to changes in air temperature which will affect the acoustic properties of the air. At constant pressure the density and velocity changes are in opposition, so that a change of 40°C will only result in a 1% change in acoustic impedance. The velocity, V, of sound however changes according to the relationship (10)
so that a 40°C rise in temperature will reduce the apparent range of a target by 6. 5%. The possibility must also be considered of temperature gradients in the air such as might be caused by vehicle exhausts, radiators or even differential heating by the sun. Although the gradients would probably be fairly gradual in most cases, it is easier to consider the case where there is a step change in temperature of, say, 10°C. The refraction at such a boundary is given by Snell’s law Vi -=_ sin i
Vr sin r
(11)
where the subscripts refer to the incident and refracted beams. Substituting figures shows that we can expect a deviation of 32min of arc when i is 30”, or 2” when i is 60”. Fortunately the variation in velocity with atmospheric changes in pressure is negligible, so that no correction needed for this.
The low propagation velocity of ultrasound means that scanning speeds of the beam cannot be high. Let us assume a range of 33m so that the out and back time of the pulse will be 0.2sec, and a scanned sector 45” in bearing and elevation. If we assume that our picture must consist of a matrix of 100 X 100 separate measurements, this will require 2000sec, i.e. over half an hour to build up. If we are content with a very coarse display of 10 X 10 points, the total time will still be 2Osec per picture, and our 6Omph target could pass through the scanned area without detection. ADVANTAGES OF AN AIR SONAR After the formidable list of obstacles has been considered, what are the features which would make an air sonar worth using rather than a more conventional radar system ? ULTRASONICS January 1968
To achieve the same wavelength with a radar system we would need to transmit at 9OGHz and using the same pulse length the circuits would need rise times approaching lO_llsec. Devices for transmitting at these frequencies are available commercially now, although they are considered rather specialised items, but the best resolution that can be obtained so far is about 0.3m. If ranges up to 10m are adequate the ultrasonic frequency can be raised and we have ourselves used our air -coupled inspection apparatus, which works at 5OOkH2,as a rudimentary sonar in this range. The wavelength is reduced to O.‘7mm, but since we were using high Q transducers the range resolution was not as good as could be obtained with specially designed equipment.
Partly for the reasons stated above, an ultrasonic apparatus would be cheaper than the equivalent radar, and would generally be more robust, although the further development of the Gunn diode to higher frequencies would result in a radar system almost as robust as the equivalent sonar. The life of a sonar system would be indefinite if properly designed, whereas existing microwave generators usually have a guaranteed life of 500hr. Again the Gunn diode should eventually overcome this handicap. High reflectivity The acoustic mismatch between any solid or liquid material and air has already been stated to be very great, so that almost 100% of the incident energy would be reflected from any object. Presumably some surfaces might absorb to some extent, but this is not likely to be a problem as anyone can confirm who has tried to prevent echoes from the walls of an ultrasonic immersion test tank, where the mismatch is not nearly so great. It would be rather like a conventional radar set used in an all metal world. Small source size
It is comparatively easy to obtain a narrow radiated beam, because the velocity in air is low. The beam width is given by sin 0 = 0.61 x/a
(12)
where 6 is the half angle of the beam, and a is the radius of the circular source (Fig 1). If the source is operating at lOOHz, X = 3.3mm, so that for an angle f3of only 1” a source size of 115mm is all that is required.
is
Scan speed
30
resolution
The biggest advantage is probably in range resolution, since the velocity of ultrasonic waves is so much lower than that of electromagnetic waves. The wavelength of a 1OOkHzultrasonic signal is only 3. 3mm, so that a pulse consisting of ten cycles would only be 33mm long, allowing resolution in range to about half this figure. The time period involved is only 10m4sec, so that cheap timing circuits working to 1MBz would be more than adequate.
Cheapness and long life
Af is the bandwidth of the system
v=vo+0.54t
Range
1
POSSIBLE SOLUTIONS Let us now take the difficulties in turn and see if there are solutions which can reasonably be applied. Overcoming acoustic mismatch lf we intend to use piezoelectric disks or magnetostrictive transducers to generate and detect the ultrasound, there is little we can do to avoid large interface losses. The losses can be reduced by using a sandwich construction in which layers of material half a wavelength thick are firmly attached to the front surface of the disk in descending order of acoustic impedance (Fig 2). It can be shown that maximum transmission occurs across the boundaries between three media when 2,
=
Jz,z,
(13)
~t_XTRASOlWCSFOR IADUSTRY 1967 1conference paper (22,
T123
-=
l/2
T13
z;
f 42,
+ 2Z,)2 2
2/3
z,
]
+ z,z,
+ 5212,
3 + 42, 3/2Z1/2
+ z”, (14)
Fig 1 Idealized beam pattern produced by air transducer. Sin. 0 = 0.61 x/a
Fiezoelectric material(Z,) Intermediate
layer(s=vj
Outer face(Z3) Silvered layers Live connection Insulator
Fig 2 Sandwich transducer
0.0065mm thick mylar film outer surface silvered Etched conducting back plate
Fig 3 Mylar film transducer
The subscripts refer to the media in the order in which they are assembled. The ratio between the energy transmitted through such a layer and that transmitted across an interface between media 1 and 3 is given by If we assume that the last layer will be nylon, which has one of the lowest figures for acoustic impedance of any solid material (2kg rnV2s-l) and the intermediate material attached to the lead zirconate disk is one with an acoustic impedance m the improvement over the case where the intermediate layer is omitted will be a factor of two. The simpler example of a lead-zirconate/air interface compared with a lead-zirconate/nylon/air system shows that the nylon face should increase the transmitted energy by a factor of about 15. Increasing the number of layers of the sandwich is therefore probably not worthwhile when the additional losses are taken into account. We are still left with the large loss at the nylon/air interface and here we seem to reach a limit, since there is no material available with an acoustic impedance in the range between nylon and air. We can improve the position if we can use a thin diaphragm vibrated in air in the same way as a loudspeaker, but at the frequencies we wish to use the cone of a speaker would vibrate in complex modes and produce little output. This can be overcome by making the diaphragm from a thin nylon film (0.0065mm thick) silvered on one side, and stretching it across a conducting plate (Fig 3). If a potential is applied between the film and the plate the force produced between them will move the silvered surface. In this case the nylon dielectric will have to be compressed, and a more sensitive transducer can be made by finely etching the conducting plate so that the film forms a large number of small diaphragms each with its own air spring behind it. The natural frequency of each diaphragm will be high and these devices can be used up to 25OkHz at least. The loss can be reduced to a minimum by generating the waves directly in air, and we have carried out some work in this respect. An instrument is available commercially called an ionophone (Fig 4) which generates a high frequency (27MHz) corona discharge between an internal electrode and a quartz tube. The discharge is modulated at the required sonic or ultrasonic frequency and the resultant noise from the end of the quartz tube is fed to the surroundings via an exponential horn. The device is capable of working at ultrasonic frequencies, since there are no moving parts; the upper limiting factors are likely to be electrical, the limit being quoted as ‘above 1OOkHz’. The response of this instrument is flat over a wide range-a great advantage over piezoelectric transducers. We have been able to generate greater power by using an open spark fed by a 4MHz transmitter modulated as before. Control of beam shape can be by suitable reflectors, or multiple spark sources can be used. The heat generated by the sources can cause refraction of the waves from the sources, so that the interference pattern between them changes and the combined signal fluctuates. Correction
Insulating
bush
Correction
Fig 4 Diagram of ionic loudspeaker
for wind speed
Some degree of compensation is possible by measuring wind speed with any suitable transducer and applying corrections automatically to the measured ranges and bearings, but this would assume that the wind velocity measured at one point could be applied to all others. Any sailor of small boats can confirm that this is hardly ever so, and widely differing wind velocities and directions are encountered over distances of a few tens of feet. Fortunately winds tend to become steadier as velocity increases so that correction would probably be worthwhile. It does not seem possible to correct for effects due to motion of the target. for thermal effects
The general ambient temperature can always be sensed and corrections applied comparatively easily, but the effects of temperature gradients are always likely to be troublesome, ULTRASONICS January 1668
31
k_JL7’RmASOlvrCS FOR INLXJSTRY 1967 I conference paws Overcoming scan speed problems We have so far been thinking in terms of one or more ultrasonic pulses for each unit of the target area, but there are techniques used in underwater sonar which could well be applied to speed up the scan. One technique uses a long duration signal to illuminate ultrasonically a sector of the target area and a narrow beam receiver transducer which scans from one side of the sector to the other in a time equal to the pulse duration, one scan immediately following the previous one. If a target is present the receiver will pick it up at some point in one of its scans. A complete sector can thus be scanned in the time taken for one pulse to travel to maximum range and back. The power required per pulse is obviously much greater than with a narrow beam scan, although the mean power is not. The range resolution, which is already good, and signal to noise ratio can be still further improved by the use of pulse compression, an example of which is shown in Fig 5. The frequency of the carrier is increased during each pulse and sweeps through a suitable frequency range. On reception the frequency components of the pulse are separated by a dispersive grating, so that the earlier, lower frequencies travel a longer path to the detector than the later, higher frequencies. It is arranged that all frequencies arrive together at the detector to combine into a larger narrower pulse which can be more easily separated from noise. The pulse length becomes approximately l/B whilst the amplitude increases in the ratio 1: &, where B is the swept frequency range and T is the uncompressed pulse duration. APPLICATIONS Air sonars have already been used in some applications with varying degrees of success. Crane operations have been controlled by ultrasonic systems, although the control system only needs to be fairly coarse. Radar simulators have also been constructed: a recent one was used to demonstrate the application of pulse compression. Blind aids are a successful application, and sonars could be effective as intruder alarms, either looking for the disturbance of an ultrasonic diffraction pattern, or simply the interruption of an ultrasonic beam. Perhaps the largest use of sonars is in the detection of the presence or absence of moving objects without the need for a detailed scan. Trucks in factories can be controlled in
I
this fashion, and primary information for the Munich computer-controlled traffic system is obtained by ultrasonic sensors. Suitably spaced transducers Received
wave of progressive decreasing frequency ,
;
I
I
I
I
-.
,,I,,,
Position of wave for additive effect
Fig 5 An example of pulse compression. The transmitted pulse is a long one of progressively decreasing frequency (typically a 10% change, e.g. from lOO-9OkBz). The returning pulse is received on the end of a rod of acoustically transmitting material (e.g. quartz). A train of compressional waves of progressively increasing wavelength thus travels through the rod to piezoelectric transducers cemented to it. The taper ensures that reflected waves do not travel up and down the rod. The transducers are spaced so that when the wave train lies completely within the bounds of the end transducers a compressional wave is located at each transducer. These are connected in parallel and hence the outputs due to the Poisson’s ratio effect are additive. At any other position of the wave complete addition will not take place and in fact the outputs of some transducers will be in the opposite sense to the rest. The result is an output pulse considerably shorter in time than the travelling wave in the air and of greater amplitude than that which any single part of the wave would produce. CONCLUSIONS For applications in still air over distances of a few tens of feet the air sonar can show worthwhile advantages over other detection systems. In disturbed air it may still be the cheapest way of detecting objects provided a detailed scan is not required. For greater distances, and for applications where a detailed picture is required in a reasonable time, it seems most unlikely that air sonar systems will be developed.
Animal sonar in air J. D. Pye* Active sonar is now known to be used for orientation by several groups of animals, all of which move in a three-dimensional medium with poor visibility. Under water, whales and dolphins have been shown to emit brief click-like pulses of wide bandwidth, while similar pulses, partly audible to man, are used in air by two groups of birds and by Megachiropteran bats of the genus Rousettus. Terrestrial shrews, when forced to find a jumping platform in complete darkness emit longer pulses, up to 30ms, of high frequency, 30-6OKBzi. But the first such systems to be discovered, those of Microchiropteran bats,213 remain-the best known, the most sophisticated and, in terms of radar/sonar theory, the most interesting.
from 54 species by oscillographic recording. The author has continued this work by tape-recording and subsequent sound spectrographic analysis of 66 species (in preparation) of which only 12 had been examined before. The equipment has been described previously* but will be reviewed here to inelude some recent improvements.
In an attempt to define the considerable variability which exists among the ‘IOO-odd species of Microchiroptera and often within a single species under different conditions, Griffin and Novicks and Novick5-7 analysed the pulse signals
The diaphragm material now used is Mylar grade C (Vacuum Developments Ltd) at a thickness of 3.5pm. Backplates made of watch hairsprings, cemented to perspex discs, were first used as an expedient to simplify microphone construction. They are still retained, however, since they appear to be rather more sensitive than grooved metal discs, although the reasons for this have not yet been analysed.
* King’s College, University of London, England 32
ULTRASONICS January 1968
INSTRUMENTATION The most useful microphone is still the solid dielectric pacitance microphone described by Kuhl et alg.
ca-