Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR Ag::CURaTE ACOUSTIC DISTANCE MEASlJREMENT
This invention relates to acoustic distance measurement, more specifically, to a method
and apparatus for impreving the accuracy of acoustic distance measurement, for extending
the maximum range, increase versatility, increiase measurement repeat rate, and increase the
reliability of acoustic distance measurement. Furthermore, this invention relates to a method
of eliminating acoustic distance measurement errors due to air or fluid currents.
Commercially available acoustic distance measuring devices generally operate in a
configuration similar to SONAR. The travel ~ime of an acoustic pulse propagating in some
medium ( e.g. air, water ) from the device to a reflective target and back to the point of origin is
measurad and the distance calculated ~rom the known ambient speed of sound. A necessary
criterion for reliabla m~asurements with such devices is a well-defined target, (i.e. Iarge, flat,
smooth, oriented perpendicular to the incidant beam), so as to reflect a siynificant portion of
the incident signal back towards the source. Thus the use of such a distance mster is
severely limited by th~ quality of the target. In addition, the presence of other reflective
objects in the acoustic path can lead to misleading results.
Attempts to correct thes0 faults have resulted in acoustic distance measuremant sys-
t0ms comprised of a transmitting unit and a receiving unit separated by the distance to bs
measured. The acoustic pulse only travels in one direction along the acoustic path, i.e. from
the transmitter unit to the receiver unit. Therefore, such a system is more severaly affected by
motion of the medium of acoustic transport (e.g. wind, current), since Ihere i5 no cancellation
of motion as in echo ranging. Some of the known three-dimensional digitizin~ systems (e.g.
U.S. patents: 5,043,950 and 5,142,506) operate on this principle, and are therefore limited to
measurements in very still air. Alternatively, a master and slave transceiver pair can be
employed to simulate the echo ranging apparatus. The acoustic pulse travels in both
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directions along the acoustic path, from a master transceiver unit to a slave transceiver unit
which transmits a return pulse back to the master. However, many of the known acoustic
distance measurement systems utilizing a master/slave transceiver pair (e.g. U.S. patents:
4,254,478, 4,894,810, and 5,175,695) employ an electronic delay at the master or slave
transceiver to ensura that all spurious echoes have died out. Typical delay times can be of
the order of 200ms, allowing for significant changes in the characteristics of air (e.g. wind,
temperature) between the rnaster and slave, resulting in increased ranging errors. The
rneasurernent repeat rate is also reduced by any electronic delay. A different master/slave
system which does not employ any additional delays is described in U.S. patent: 3,076,519.
In that particular method, tha master and slave transceivers oparate with different transmitting
frequencies to which only the opposite unit is responsive. This typically requires separate
transmitter and receiver elements at both master and slave units. Furthermore, the
performance is unpredictable for m0asurements of shorter distances where strong signals of
the first frequency can erroneously trigg~r the receiver tuned ~o the second frequency.
A disadvantage of the master/slave systems comparod to the ~ransmitter/receiver sys-
tems is a much lower measurement repcat rate. An advantage of the master/slave systems
over the transmitter/receiver systems is its partial current compensation feature. An ideal
acoustic ranging system would combine the fast measurement repeat rate of transmitter/re-
ceiver systems with the wind/current compensation of the master/slave systems.
It is a principal object of the present invention to provide a method and apparatus for
accurate acoustic distance measurement.
It is another object of the present invention to provide a distance measuring system
which is operative for a distance far greater than scho-ranging systems.
It is another objsct of the presant invention ~o provide a distance measuring system
with a measurement repeat rate far greater than known master/slave systems.
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It is yet anoliher object of the present invention to provide a distance measuring system
with wind or current-compensating features which are rnore accurate than any known system,
allowing one to make distaince measurements in environments where the medium of acoustic
transport is subject to currents and other movements which otherwise restrict the use of
acoustic ranging devices.
The present invention operates with the provision of two transcaiver units named'controller' and 'remote' in which the remote is locat0d at a first location, and the controller is
located at a second location. Each transceiver unit is operative to first generate and transmit
an acoustic pulse signal and subsequently receive the acoustic signal from another
transceiver. Timer means are provided by a plurality of timers situated within the controller or
in a separate 'master commander', as are calculation means for computing distance from
timer values and the ambient speed of sound. The measured times of flight between
transceivers ara used along with the known ambient speed of sound to compulie the distance
between the transc~ivers. In addition, each transceiver and master commander is
equipped with an instantaneous trigger m~ans for synchronizing transmission of acoustic
pulses from the controller and remote, and for sending starVstop commands to the timer
means. This trigger mechanism can be one of a number of options such as radio wave,
infrared light, a current pulse through a wire means, or other, provided that such trigger pulses
are instantaneous with respect to th0 speed of sound. Any pulse travelling at the speed of
light satisfies this requiremen~i.
The novel feature of the invention is the provision for transmitting acoustic pulses frorn
the controller to a remote, and at the same instanli, transmitting acoustic pulses from the
remote to the controller. Thus, each distance measurement comprises two simultaneous
acoustic transmissions propagating in opposite directions along the acoustic path between the
controller and ~mote. The resulting distance measurement effectively eliminates errors due
to wind or fluid motion along the acoustic path. Measurement repeat rate is approximately
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double that of echo-ranging systems, and at leas~ double that of systems such as US Patents:
5,175,695; 5,140,859; 4,894,810. Furthermore, the present invention will have improved
accuracy over systems which measure distance in only one direction along the acoustic ,oath
(e.g. US Patent: 5,043,950), with no loss in the high measurement repeat rates offered by
such systems.
In drawings which illustrats the embodiments of the invention:
Fi~. 1 is an illustration describing the effact of air/fluid motion on acoustic distance
measurements.
Fi~. 2 is a general arrangement diagram of the present invention showing the opera-
tion sequences resulting in a one-dimensional distance measurement.
Fig. 3 is an alternate embodiment of the present invention ernploying a master
commander.
Fig. 4 is a diagram showing tho time/signal characteristics for the preferrsd embodi
ment of the present invention.
Fi~. 5 is a diagram showing the measurament protocol for the prsfefred embodiment of
the present invention.
Fig. 6 is an illustration showing the application of the present invention to two-
dimensional distance measurement.
Fig. 7 is an illus~ration showing the application of the prssent invention to three-
dimensional distance measurement.
It is clear that the present inven~ion is operative in several media, e.g. air, other gases,and fluids, and that the instantaneous trigger signal can be realized by wired or wireless
means. For simplicity and convenience, ~he preferred embodiment will be discussed with the
assumption that the rnedium of acoustic transport is air, and the trigger signal is provided by
wireless means. However, we make no restrictions on the other options available.Fig. 1 illustra~s the basic principle behind the present invention. A first acoustic trans-
ceiver 1 and a second acoustic transceiYer 2 are separateci by some unknown dis~ance, and a
wind 5 is blowing between the two transceivers at a given instant in ~ime. An acoustic wave
(shock-wave, sonic, infrasonic or ultrasonic) 3 generated by acoustic transceiver ti is prop-
agated through th~ air (or fluid) at a velocity equal to the spead of sound in still air plus the
vector component of the wind velocity along the acoustic path in the direction of propagation
of the acoustic pulse 3. This results in a faster time of flight for the acoustic pulse 3 which
relates to a shorter perceived distance than is expected. In the case of acoustic transceiver 2
transmitting an acoustic wavc 4 in the opposite dir~ction, the wave 4 also propagates with
velocity equal to the speed of sound in still air plus the vector cornponent of the wind velocity
along ths acoustic path in the dir0ction of propagation of the acoustic pulse 4. This results in
a longer time of flight for the acoustic puls9 4, hence a longer distance rneasurement is
perceiv~d. Thus, if acoustic pulse 3 and acoustic pulse 4 are simultaneously transmitted from
tranæceiver 1 to transceiver 2 and from transceiver 2 to transceiver 1 respectively, the error
contriblution due to air or fiuid motion 5 can be eliminated by averaging the two
rneasurements. It is this wind compensation which is the basis of the presellt inven~ion.
Accuracy irnprovements to distance, velocity, or acceleration measurements in other ~aseous
or fluid media are possible with the application of the present invention.
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Fig. 2 shows a general arrangement of the present invention and its operation protocol.
The controlier transcaiver ~0 comprises acoustic transceiver circuitry ~1 and transducer
means 12, an instantaneous trigger signal communications means 13, and a signal
processing module 14 which contains a plurality of timer means, calculations means for
computing distances, a means for storing results in a memory, and a display means for
displaying the measured distance. The remote transceiver 20 comprises acoustic transceiver
circuitry ~1 and transducer means 22, an instantaneous trigger signal communications means
23, and an indicator signal generating means 24 by which reception of acoustic pulses at the
remote 20 are indicated to a user at a distant location away from the remote 20. The
transducer means 12, 22 comprises a single acoustic transducer for both transmitting and
receiving, or alternatively, separats transmitter and receiver elements.
In the alternate embodiment of the present invention shown in fig. 3, the signalprocessing module 14 may reside in a central master commander 30 removed from the
controller as a separate unit 34 which interacts with tha controller and remote transceivers via
instantaneous trigger signal communication means 31 to perform control, timing, calculating,
storing, displaying, and other processing functions. The controller 10 still retains its own
signal processing module 14 so that results can be communicated trom the mas~er
commander 30 to the controllar 10 and displayad at the controller 10.
As illustrated in fig. 2, ~ig.4, and fig. 5, a typical measurement sequencs for a single
controller/remote transceiver pair consists of the following steps. An acoustic pulse 15 and an
instantaneous trigger signal 16 are sent synchronously from the controller 10 to the remote
20, and timers in tha signal processing module 14 at the controller 10 are started. The remote
20 r~ceives the instantaneous trigger signal 16 and immediately transmits its own acoustic
pulse 25 to the controll2r 10. Since the trigger signal 16 propagatin~ at the speed of light is
effectively instantaneous with respe~ to the acous~ic signal 15, there is no perceived delay
~etween the transmission of acoustic pulse 15 from ~he controller 10 and the transmission of
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acoustic pulse 25 from remote 20, therefore the controller 10 and remote 2û generate
their respective acoustic pulses at the same instant. After a time tll indicated in fig. 4, the
remote 20 receives the controller's acoustic pulsa 15 and irnmediately sends an instantaneous
trigger signal 26 to the controller 10 to halt a timer. At the same instant, the indicator signal
generating maans 2~ in the remo~e 20 generates an indicator signal (e.g. audio signal or
visible light strobe) to confirm detection of the acoustic puise 15. After a time t2 indicated in
fig.4, controller 10 rec0ives the remote's acoustic pulse 25 and immediately stops a second
timer in the signal processing module 14 inside the controller 10. The timer data is then used
with ~he ambient speed of sound value to calculate the avera~e distance by means of the
calculation capabilities of the signal processing module 14, thus giving a wind-compensated
distance measurement. The result is displayed and optionally stored in a memory by means
of the signal processing module 14 at the controller 10. It should be noted that any method
can be used to determine the ambiant speed of sound, provided the method is accurate. If
the speed of sound is measured by a separate acoustical determination, th~n the method of
the present invention should be applied in order to eliminate wind or current errors and
achicvs the most accurate value for the ambient speed of sound.
In the alternate embodiment of the present inven~ion shown in fig. 3, the measurement
sequ~nce for a single controller/remote transceiver pair is initiated by an instantaneous trigger
pulse 16 generated at the controller transceiver. The master commander 30 receives the
instantaneous trigger pulse 16, sends an instantancous trigger pulse 32 to the controller 10
and remote 20, and starts a plurality of timers in the signal processing module 34. Upnn
receipt o~ the instantaneous trigger pulse 32, the controller 10 and remote 20 triansrnit their
respective ~coustic pulses synchronously; the controller 10 transmits its acoustie pulse 15 to
the remote 20, and at the same instant, the rernote 20 transmits its iacoustic pulse 25 to the
controller 10. At the instant the oontroller 10 recaivas the remote's acoustio pulse 25, the
controller 10 transmits an instantaneous trigger pulse 17 to the master commander 30,
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whareby a first ~imer is stopped. Upon receipt of the controller's acoustic puise 15, the remote
20 transmits an instantaneous trigger pulse 27 to the master commander 30, whereby a
second timer is stapped. At the same instant, the indicator signal generating means 24
generates an indicator signal, e.g. an audio pulse or visible light strobe, so as to confirm
detection of the controller's acoustic pulse 15 at the remote 20. The calculation means within
the si~nal processing module 34 uses the timer data and the ambient speed of sound value to
calculata the average distance between the controller 10 and remote 20. Results may be
stored in a memory and displayed by means of the si~nal processing module 34 Also, the
measurement is optionally sent to the controller 10 via coded instantaneous pulse signals so
that measurements are processed and displayed at the controller 10.
As the attenuation of sound is frequency dependent, the frequency of the acoustic
pulses 15, 25 will be chosen to give the desired maximum range of the system. Typical
frequencies of interest could be 25 - 250 kHz for acoustic ranging in air, 150 kHz to 2 MHz for
acoustic ranging in water. The preferred embodiment is able to discriminate against spurious
echoes from nearby obstacles by means of peak detection or signal processing whereby only
the strongest, unreflected signals are recognized. This technology is well known and is not
claimed in this invention.
An additional capability of the invsntion resulting from the high measurement repeat
rate is m~asurement of velocity or acceleration of a remote transceiver 20 with respact to the
controller transceiver 10. A series of distance measurements together with the known elapsed
time ~etween each mcasurement givss the instantaneous velocity and acc~leration of the
remota 20 with respect to the controller 10. Also, the instantanaous velocity of the medium of
acoustic transport can be easily calculated from the times of flight of the first acoustic pulse 15
and the second acoustic pulse 25. This wind or current velocity can then be displayed and
stored along with the corresponding distance measurement.
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Position measurements in higher dimensiorls can be made by employing a plurality o~
remote transceivers placed at known locations. Figure 6 illustrates the application of the
present invention to two-dimensional mapping. At least two remote transceivers 2Ua, 20b are
placed a known distance apart such that Ihey define a coordinate sys~em. A first remote
transceiver 20a reprasents the origin of th~ coordinate system, while a second remote
transceiver 20b represents the y-axis as shown in figure 6. The location of the controll~r
transceiver 10 is easily determined by measuring ~he distance between each controller/remote
transceiver pair sequentially, (i.e. remote transceiver 20a and the controller 10, and remote
transceiver 20b and controller 10), and then triangulating using the wind-compensated
distances between transceiver pairs and the distance separating the remote transceivers 2Ua,
20b. Furthermore, tha high measurement repeat rate of the present invention allows for two-
dimensional wind-compensated velocity and accelera~ion measurements.
Three-dimensional mapping is realized by employing at least thre~ remote transceivers
placed at known locations defining a coordinate system as illustrated in figure 7. A first
remote transceiver 20a represents the origin of the coordinate systern, a second remote
transceivar 20b represents the y-axis, and a third remote transceivar 20c represents the x-
axis. The location of the controller transceiver 10 in three-dimensions is easily determined by
measuring the distance between each controller/remote transceiver pair sequentially, (i.e.
remote transceiver 2Qa and the controller 10, remote transceiver 20b and con~roller 10, and
remote transceivor 20c and the controller 10), and then triangulating using the wind-
compensated distances between transceiver pairs and the distances separa~ing the remote
transceivers 20a, 20b, 20c. As previously statad, the high measurement repeat rate of the
present invention allows for wind-compensated velocity and acceleration measurements in
higher dimensions.
Alternatively, a single con~roll~r/remote transceiver pair may be employed for mapping
in highar dimensions by mounting the controller transceiver 1G on a transit or goniometer and
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aiming at the remote 2û. The resulting wind-compensated distance measurement and the
measured angles on the transit or goniometer give the two or three-dimensional location of the
remote 20 with respect to the controller 10~
While the invention has been described in what is pr~sently considered to be a
preferred embodiment, many variations and modifications will be apparent to those skilled in
the art. Therefore, it is intended that the invention not be limited to this embodiment but be
interpreted within the spirit and scope of ~he appended claims.
