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Claims  |
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What is claimed is:
1. A system for measuring a component of the velocity of a flow of liquid
and for providing an output signal representative of the velocity
component, comprising:
at least two transducers positioned so that acoustic energy emitted by each
of the transducers travels along an acoustic path therebetween and
impinges upon the other transducer, each transducer including terminals
for applying a signal thereto to cause the transducer to emit acoustic
signals and for providing a transducer output signal representative of an
acoustic signal received by the transducer;
a transmit oscillator for providing an output signal of a first frequency;
means for periodically applying the transmit oscillator output signal to
each of the transducers to cause the transducers to emit a burst of
acoustic energy;
a reference oscillator for producing an output signal at a second frequency
different from the first frequency by a difference frequency;
first and second signal processing channels respectively associated with
the first and second transducers for processing output signals produced by
the associated transducer in response to acoustic signals received
thereby, each signal processing channel including:
a summing point to which is applied the output signal from the associated
transducer;
means for applying the reference oscillator output signal to the summing
point to produce a composite signal at the summing point representative of
the sum of the reference oscillator output signal and the output signal
from the associated transducer; and
means, responsive to the composite signal, for producing an intermediate
signal at the difference frequency having a phase which is representative
of the phase of the acoustic signal received by the associated transducer;
and
means for comparing the phases of the intermediate signals from the first
and second processing channels and for producing a signal representative
of the phase difference therebetween to provide a representation of the
velocity component being measured.
2. The system of claim 1 wherein
the summing point is located in the signal processing channel so as to be
prior to signal processing stages which would tend to introduce a phase
shift in the reference oscillator and transducer output signals, so that
any phase shift introduced by subsequent signal processing stages will
tend to affect both the transducer and reference oscillator output
signals, thereby reducing any differential phase shift therebetween.
3. The System of claim 1 or 2 wherein each of the signal processing
channels includes:
a non-linear detector, to which is applied the composite signal, for
producing an output signal including difference frequency components.
4. The system of claim 3 wherein each of the signal processing channels
includes means for selecting the difference frequency signals from the
output of the detector and for rejecting other signals produced by the
detector.
5. The system of claim 3 wherein each channel includes a filter, having a
pass-band including the difference frequency, to which is applied the
detector output signal.
6. The system of claim 5 wherein each of the signal processing channels
includes a gating means for selecting a portion of the output signal from
the associated transducer, for processing, the portion being taken from
the output signal subsequent to the beginning thereof so as to reduce the
effects of any residual oscillations resulting from prior emission of
acoustic signals by the associated transducer.
7. The system of claim 5 further including:
third and fourth transducers disposed so that acoustic signals emitted by
each of the third and fourth transducers travels along an acoustic path
therebetween and impinges upon the other of the third and fourth
transducers, and positioned so that the acoustic path between the third
and fourth transducers is orthogonal to the acoustic path between the
first and second transducers;
the means for periodically applying being further operative to periodically
apply the transmit oscillator output signal to the third and fourth
transducers;
third and fourth signal processing channels respectively associated with
the third and fourth transducers for processing output signals from the
associated transducer, the third and fourth signal processing channels
including the same aforementioned elements included in said first and
second signal processing channels.
8. The system of claim 2 wherein the impedance presented to each transducer
during periods when it is emitting acoustic signals is substantially equal
to the impedance presented to that transducer during periods when it is
receiving acoustic signals.
9. The system of claim 8 wherein each of the transducers are piezoelectric
transducers.
10. The system of claim 1 including means for phase locking the frequencies
of the transmit and reference oscillators to provide a stable difference
frequency there between.
11. The system of claim 1 wherein the frequency of the transmit oscillator
is approximately 1.6 MHz.
12. The system of claim 9 wherein the difference frequency is approximately
34 Hz.
13. The systems of claim 1 including an acoustic mirror wherein the
transducers and acoustic mirror are positioned so that acoustic signals
emitted by each of the transducers are reflected by the acoustic mirror in
a V-shaped acoustic path so as to impinge upon the receiving transducer.
14. The system of claim 8 or 13 wherein the acoustic mirror is
substantially square in shape and is positioned so that a diagonal of the
square is parallel to the plane of the V-shaped acoustic path between the
first and second transducers.
15. The system of claim 14 wherein the acoustic mirror is formed of a flat,
thin tungsten plate.
16. An acoustic current meter for measuring a component of a velocity of a
flow of liquid, comprising:
two transducers positioned so that acoustic energy emitted by each of the
transducers travels along an acoustic path therebetween and impinges upon
the other transducer, each transducer including terminals for applying a
signal thereto to cause the transducer to emit acoustic signals and for
providing a transducer output signal representative of an acoustic signal
received by the transducer;
means for periodically applying an A.C. signal of a first frequency to both
transducers concurrently to cause the transducers to periodically emit
bursts acoustic energy of the predetermined frequency; and
means, responsive to output signals produced by each transducer in response
to acoustic energy emitted by the other transducer, for comparing the
phase of the transducer output signals and for providing an output signal
representative of the phase difference therebetween, including:
means for heterodyning each transducer output signal with a signal of a
second frequency to provide two difference frequency signals; and
means for comparing the phases of the difference frequency signals, and
wherein the impedance presented to each transducer during periods when it
is emitting acoustic signals is substantially equal to the impedance
presented to that transducer during periods when it is receiving acoustic
signals.
17. The acoustic current meter of claim 16 wherein the means for comparing
includes gating means for selecting only a portion of the transducer
output signals from each burst for comparing, the portion being taken from
the transducer output signal subsequent to the beginning of each burst of
acoustic energy received thereby so as to reduce the effects of any
residual oscillations resulting from prior emission of acoustic signals by
the associated transducer.
18. The acoustic current meter of claim 16 or 17 wherein each of the
transducers are piezoelectric transducers.
19. The acoustic current meter of claim 18 wherein the first frequency is
approximately 1.6 MHz.
20. In an acoustic current meter for measuring the velocity of a flow of
liquid of the type having a first transducer, a second transducer, an
acoustic mirror, the first and second transducers and acoustic mirror
being positioned so that acoustic signals emitted by each of the
transducers are reflected by a top surface of the acoustic mirror so as to
impinge upon the other transducer in a V-shaped acoustic path, the
improvement comprising:
an acoustic mirror constructed so as to eliminate peripheral edges of the
top surface which are aligned with respect to the path of the acoustic
signals emitted by the transducers so as to reflect a portion of the
acoustic signals impinging thereon back toward the transducer from which
the signals were emitted.
21. The acoustic current meter of claim 20 wherein the top surface of the
acoustic mirror has at least two corners pointing in different directions,
each corner being formed by the intersection of two sides, the mirror
being positioned so that each of the two corners is pointing in the
general direction of a respective one of the two transducers so as to
reduce reflections from the sides which impinge upon the transducers.
22. The acoustic current meter of claim 20 wherein the acoustic mirror is
substantially square in shape.
23. The acoustic current meter of claim 22 wherein the mirror is positioned
so that a diagonal of the square is aligned with the plane in which the
acoustic path between the two transducers lies.
24. The acoustic current meter of claims 20, 21, 22 or 23 wherein the
acoustic mirror is formed of a flat, thin tungsten plate.
25. The acoustic current meter of claim 20 wherein the acoustic current
meter includes third and fourth transducers positioned so that the signals
emitted from each of the third and fourth transducers are reflected by the
acoustic mirror so as to impinge upon the other transducer in a V-shaped
acoustic path, the third and fourth transducers being positioned so that
the plane of the acoustic path therebetween is perpendicular to the plane
of the acoustic path between the first and second transducers; and
wherein the acoustic mirror top surface is substantially square in shape
and is positioned so that the diagonals of the square are parallel with
the planes of the acoustic path between the first and second transducers
and between the third and fourth transducers.
26. The acoustic current meter of claim 25 wherein the mirror is formed of
a thin, flat plate of tungsten.
27. The acoustic current meter of claim 26 wherein the plate is formed of
an annealed tungsten, approximately 0.01 inches thick.
28. The acoustic current meter of claim 20 wherein the acoustic mirror top
surface is polygonal in shape and positioned so that a line running from
one apex of the polygon to another apex of the polygon is substantially
parallel to the plane of the acoustic path.
29. The acoustic current meter of claim 20 wherein the acoustic mirror top
surface is in the shape of a regular polygon having an even number of
sides and is positioned so that a line from one apex of the polygon to
another apex of the polygon bisecting the polygon is substantially
parallel to the plane of the acoustic path.
30. In an acoustic current meter for measuring a component of the velocity
of a flow of liquid of the type having at least two transducers positioned
so that acoustic signals emitted by each transducer impinge on the other
transducer, means for periodically applying a signal to each of the
transducers to cause the transducers to emit acoustic signals, and means,
connected to the transducers and responsive to output signals produced in
response to acoustic signals received by the transducers, for detecting
the relative phase of the transducer output signals and for producing a
signal representative of a component of the liquid velocity,
the improvement wherein the means for applying includes means for
presenting a first impedance to each transducer during periods when the
transducer is emitting acoustic signals, wherein the means for detecting
includes means for presenting a second impedance to each transducer during
periods when the transducer is receiving acoustic signals, and wherein the
first and second impedances are substantially equal.
31. The acoustic current meter of claims 16 or 30 wherein the transducers
are piezoelectric transducers.
32. A system for measuring a component of the velocity of a flow of liquid
and for providing an output signal representative of the component of the
velocity, comprising:
at least two transducers positioned so that acoustic energy emitted by each
of the transducers travels along an acoustic path therebetween and
impinges upon the other transducer, each transducer including terminals
for applying a signal thereto to cause the transducer output signal
representative of an acoustic signal received by the transducer;
a transmit oscillator for providing an output signal of a first frequency;
means for periodically applying the transmit oscillator output signal to
each of the transducers to cause the transducers to emit a burst of
acoustic energy;
a reference oscillator for producing an output signal at a second frequency
different from the first frequency by a difference frequency;
first and second signal processing channels respectively associated with
the first and second transducers for processing output signals produced by
the associated transducer in response to acoustic signals received
thereby, each signal processing channel including;
means, responsive to the output signal from the associated transducer and
to the reference oscillator output signal, for producing an intermediate
signal at the difference frequency having a phase which is representative
of the phase of the acoustic signal received by the associated transducer;
and
means for comparing the phases of the intermediate signals from the first
and second processing channels and for producing a signal representative
of the phase difference therebetween to provide a representation of the
velocity component of the liquid flow being measured; and
wherein the impedance present to each transducer during periods when it is
emitting acoustic signals is substantially equal to the impedance
presented to that transducer during periods when it is receiving acoustic
signals.
33. The system of claim 32 wherein each of the transducers are
piezoelectric transducers.
34. A system for measuring a component of the velocity of a flow of liquid
and for providing an output signal representative of the velocity
component, comprising:
at least two transducers positioned so that acoustic energy emitted by each
of the transducers travels along an acoustic path therebetween and
impinges upon the other transducer, each transducer including terminals
for applying a signal thereto to cause the transducer to emit acoustic
signals and for providing a transducer output signal representative of an
acoustic signal received by the transducer;
a transmit oscillator for providing an output signal of a first frequency;
means for periodically applying the transmit oscillator output signal to
each of the transducers during successive transmit intervals to cause each
of the transducers to emit bursts of acoustic energy, the burst occurring
at a repetition frequency;
a reference oscillator for producing an output signal at a second frequency
different from the first frequency by a difference frequency, the
difference frequency being such that one cycle of the difference frequency
extends over a plurality of transmit intervals;
first and second signal processing channels respectively associated with
the first and second transducers for processing output signals produced by
the associated transducer in response to acoustic signals received
thereby, each signal processing channel including:
means, responsive to the output signal from the associated transducer and
to the reference oscillator output signal, for producing an intermediate
signal at the difference frequency having a phase which is representative
of the phase of the acoustic signal received by the associated transducer;
and
means for comparing the phases of the intermediate signals from the first
and second processing channels and for producing a signal representative
of the phase difference therebetween to provide a representation of the
velocity component of the liquid flow being measured.
35. The system of claim 34 wherein the means for producing includes a band
pass filter having a center frequency equal to the difference frequency.
36. The system of claim 34 wherein the means for producing includes;
combining means responsive to the transducer output signal and the
reference oscillator signal to produce an output signal which includes a
series of pulses occurring at the repetition frequency, the amplitude of
the pulses being modulated at the difference frequency with a phase
representative of the phase of the acoustic signal received by the
acoustic transducer; and
a band pass filter having a center frequency equal to the difference
frequency to which is applied the series of pulses from the combining
means for producing an output signal at the difference frequency having a
phase which is representative of the acoustic signal received by the
associated transducer.
37. The system of claim 36 further including gating means for selecting a
portion of each signal received by the associated transducer and for
applying the selected portion to the combining means.
38. The system of claim 37 wherein the combining means includes a
non-linear detector to which is applied the reference oscillator output
signal and the associated transducer output signal for producing an output
signal including difference frequency components.
39. The system of claims 34, 36, or 38 wherein the ratio between the first
frequency and the difference frequency is at least approximately 50,000. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention is related to instruments for measuring the velocity
of a moving liquid and more particularly to acoustic current meters for
measuring ocean current velocities.
BACKGROUND OF THE INVENTION
Until recently, the measurement of ocean currents and other liquid velocity
measurements have been performed using mechanical meters equipped with
Savonious rotors and vane followers. These methods of measuring currents
have significant problems, including poor reliability due to the direct
exposure of mechanical moving parts to the marine environment resulting in
corrosion and fouling by extraneous matter, and the non-ideal hydrodynamic
properties of these mechanisms which result in inaccurate measurements.
Various attempts have been made in the past to design better instruments.
These designs have utilized acoustic, electromagnetic, and various other
electronic sensing techniques which have eliminated some of the problems
associated with mechanical current measuring devices. However, these
recent systems have significant problems with their use including zero
drift, high power consumption, inoperability in clear water (for acoustic
backscatter current meters), and low sensitivity.
Acoustic current meters have been implemented in a number of different
ways, including: (1) direct measurement of the propagation time of a pulse
emitted by a first transducer and received by a second transducer; (2)
dual "sing-around" sound velocimeters with straight line sound paths in
opposite directions, the difference in "sing-around" frequency being a
linear function of the current; (3) continuous wave systems using two
widely different high frequency carriers (e.g., 1.1 and 1.6 MHz) but
modulated with an identical signal of lower frequency (e.g. 20 kHz) where
the phase difference of the modulating signal on the received carriers is
a linear function of current velocity; and (4) continuous wave bursts
using a single frequency on a single pair of transducers, the burst
interval being approximately equal to the acoustic travel time between the
two transducers.
SUMMARY OF THE INVENTION
The present invention uses a continuous wave burst technique which has
advantages over previous methods of measuring current velocity. The
acoustic current meter includes two identical channels having acoustic
paths oriented at right angles to each other to measure orthogonal
components of a current velocity. Each channel includes two piezoelectric
or other type transducers aimed at an acoustic mirror in such a way that
the reflected acoustic signal from each transducer impinges upon the
opposite transducer. Periodically, a burst of high frequency acoustic
energy is simultaneous emitted by each transducer. The burst length is
shorter than the acoustic travel time, and each transducer receives the
acoustic waves from the opposite transducer after it has finished
transmitting acoustic waves. If a current is flowing through the acoustic
path having a component parallel to the axis between the transducers, the
acoustic signals going in opposite directions will be respectively
advanced and delayed in time by the current to produce a relative phase
shift between the received signals. The phase shift is representative of
the current velocity.
The output signals produced by each of the transducers is combined with a
common reference signal having a frequency very close to the transmitted
frequency. For each transducer, the reference and received signals are
then combined in a non-linear device which generates product terms, such
as a multiplier or a square law detector. The difference or beat frequency
components in the output of each multiplier is selected by an appropriate
filter to provide two beat frequency signals having a phase difference
proportional to the current velocity. Due to the reduction in frequency,
the phase shift is expanded in time with respect to the phase shift of the
transmitted signals and may be easily measured by means of conventional,
low-power circuitry.
DESCRIPTION OF THE DRAWINGS
The advantages and operation of the present invention will become more
clear upon reading the following description of the preferred embodiment
in conjunction with the accompanying drawings, of which:
FIG. 1 shows the mechanical arrangement of the acoustic current meter of
the present invention;
FIG. 2 is a diagram showing the acoustic propagation path useful in
describing the operation of the present invention;
FIG. 3 is a block diagram of the electronic circuitry of the present
invention;
FIG. 4 shows the circuitry associated with the transducers for measuring
the propagation time difference;
FIG. 5 shows waveforms occurring in the circuitry of FIG. 4 useful in
explaining the operation thereof;
FIG. 6a and 6b are equivalent circuit diagrams of the transducer circuits
useful in explaining one aspect of the present invention;
FIG. 7 shows the orientation of two current measuring channels;
FIG. 8a, 8b, and 8c illustrate the advantages of a new acoustic mirror
design; and
FIG. 9 is a detailed circuit diagram of one embodiment of FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, there is shown a drawing of the acoustic current meter
(ACM) as it would be deployed in an ocean or other body of water where
currents are to be measured. The various subassemblies of the current
meter are positioned between a top plate 10 and an X-shaped lower support
section 12 which are maintained in a fixed relationship by means of four
titanium rods 14 (only two of which are shown in FIG. 1.) Rods 14 pass
through holes in top plate 10 and bottom part 12 and are fastened by means
of bolts 16 or other suitable fasteners. The lower support 12 has a
mooring ring 18 formed therein, and a second mooring ring 20 extends from
top plate 10.
Beneath top plate 10 is a hollow cylindrical housing 22 which contains the
electronics of the acoustic current meter, a fluxgate magnetometer, and a
battery for powering the current meter. An electrical connector 24 is
located on top plate 10 to allow calibration of the current meter
electronics and for readout of data from the current meter. The bottom of
the cylindrical body section 22 is formed of an aluminum plate 26 through
which the four rods 14 pass.
Extending below body section 22 is a probe assembly 28 which is of a
smaller cross-section than the body section 22 to reduce turbulence in the
area where current measurements are to be taken. Extending outwardly and
downwardly at an angle from transducer support 28 are four transducer
probes 30 which are orthogonally aligned. Within each probe 24 is a
transducer 32 which transmits and receives acoustic signals having a
frequency of approximately 1.6 MHz. Transducers 32 are made of a
piezoelectric material in the preferred embodiment described, but may also
be electrostrictive, magnetostrictive, electrodynamic, or other type of
transducer. These transducers 32 are oriented so as to transmit and
receive acoustic waves along the paths shown by dotted lines 34. The
acoustic waves are reflected off an acoustic mirror 36 which is positioned
by a mirror support rod 38 extending upwardly from lower support section
12. Acoustic signals travel between transducers 32 located on opposed
pairs of probes 30 so that the acoustic waves from opposed pairs of
transducers travel along two V-shaped paths which are at 90 degrees to one
another.
When the current meter shown in FIG. 1 is moored, currents which are
present in the water cause the water to flow through the volume traversed
by acoustic paths 34. As described below, this flow results in a
differential propagation time and a corresponding phase shift in the
acoustic signals transmitted between opposing pairs of transducers 32. By
measuring such phase shifts, the two pairs of transducers measure the
components of the current along two orthogonal axes. Signals
representative of these components and signals from the fluxgate
magnetometer within current meter body 22 are processed by electronics,
also located within body 22, to provide signals representative of the
north-south and east-west components of the ocean currents. This data may
be immediately used or may be stored for later retrieval and analysis.
Referring to FIG. 2, the following is a brief explanation of the manner in
which a current velocity is measured by the acoustic current meter shown
in FIG. 1. FIG. 2 is a side view showing two of the four transducers 32
and their relationship to acoustic mirror 36. An electric signal is
applied to both of the transducers 32 by means of electronics described in
more detail below. In response, each transducer vibrates causing acoustic
waves to propagate through the water medium along a path shown by dotted
line 34. The acoustic wave travels down from the vibrating transducer 32
and impinges upon acoustic mirror 36. The angle between the propagation
path 34 and the normal to the mirror is denoted as .theta.. The acoustic
waves impinging upon acoustic mirror 36 are reflected and propagate
upwardly toward the opposite transducer. The angle of reflection is equal
to the angle of incidence, .theta..
If the water medium is moving and has a horizontal velocity component in
the direction of the propagation path 34, denoted by arrow 44 in FIG. 2,
the time of propagation between the two transducers 32 is affected. The
total propagation velocity of an acoustic wave is equal to the acoustic
sound velocity in the medium plus the current velocity component in the
direction of propagation. Referring again to FIG. 2, if a current flows
from right to left as shown by arrow 44, sound waves emitted by transducer
32a and received after reflection from mirror 36 by transducer 32b travel
in the same direction as the current and have a shorter propagation time
than sound waves transmitted from transducer 32b to transducer 32a against
the current. The difference in propagation times .DELTA.T, is given by the
following equation:
.DELTA.T=2 vd/c.sup.2 (1)
where v is the current velocity component parallel to a line between the
transducers, d is the transducer spacing, and c is the velocity of sound.
It should be noted that the time difference .DELTA.T depends only on the
separation, d, of the transducers and is independent of the distance
between the mirror 36 and the transducers 32.
In the preferred embodiment described herein, typical dimensions for the
transducer configuration shown in FIG. 2 are d=11 cm, .theta.=30 degrees,
and s=11 cm. For these dimensions and for a sound velocity c of 1500 m/s,
the change in propagation time, .DELTA.T is approximately 1 nanosecond per
cm/s of current velocity. Such time differences can be measured by modern
high-speed circuitry, and prior art current meters are available which use
such circuitry. However, directly measuring such small time differences
requires expensive and relatively high-power electronics. Frequently, it
is desirable to leave a current meter deployed for long periods of time in
remote ocean locations where the current meter must be powered by a
self-contained battery pack. Especially in such applications, the high
power required by the very high-speed electronics makes such techniques
for measuring current velocity impractical.
Referring to FIG. 3, the electronic section of the present current meter is
shown in block diagram form. As described above, two transducers 32a and
32b transmit signals between each other which measure one component of the
current velocity. The output signals from transducers 32 are each applied
to respective continuous wave burst processors 21. One exemplary
embodiment of continuous wave burst processor 21 is shown in detail below
in FIGS. 4 and 9. The output from each of the processors 21 is a signal
having a much lower frequency than the 1.6 MHz acoustic signals
transmitted by transducer 32. These low frequency signals have phase
shifts which are representative of the phase of the acoustic signals
received by each of transducer 32. Briefly, the continuous wave burst
processors heterodyne the output signal from each of the transducers to a
much lower frequency, and the result is that the propagation delay
produced by the measured current and represented by the phase of the
output signals from the transducers is expanded in time. In the present
embodiment, the frequency of the output signal from processors 21 is 34 Hz
resulting in an increase in the propagation delay of a factor of
approximately 50,000. The expanded propagation delay is then easily
measured via conventional digital circuitry drawing extremely low power
such as CMOS. Since the continuous wave burst processor described below
also requires a minimal amount of current, the novel technique disclosed
herein results in an acoustic current meter which draws very low power and
can therefore be deployed for long periods of time using a battery power
supply.
The outputs from processors 21 consist of two 34 Hz signals having a phase
shift there between proportional to the current velocity. The outputs from
processors 21 are applied to phase meter circuitry 23 which measures the
phase difference therebetween to provide an output signal representative
of the current velocity. Although phase meter circuitry 23 may be
implemented using many different circuits known by those in the art, one
particular circuit especially suited for the presently disclosed current
meter is described in a co-pending application of Kenneth D. Lawson and
Neil L. Brown Ser. No. 947,255 entitled Phase Meter Circuit and filed
concurrently with the present application.
The above-described circuitry, shown in FIG. 3 enclosed with dotted box 25,
is duplicated for the two orthogonal transducers, and this circuitry is
represented by dotted box 25' in FIG. 3. A magnetometer 31 is included as
part of the acoustic current meter described herein and provides output
signals representative of the orientation of the acoustic current meter
with respect to the magnetic field of the earth. The output signal from
magnetometer 31 is also applied to digital processor 29. Although many
known circuits may be used to implement magnetometer 31, one particular
circuit suitable for use with the present invention is disclosed in a
co-pending application by Kenneth D. Lawson and Neil L. Brown Ser. No.
947,254 entitled Low Power Magnetometer Circuit and filed concurrently
herewith.
A coordinate conversion circuit 29 receives the outputs from circuitry 25
and 25' representative of the orthogonal components of a measured current
velocity in a reference frame which is dependent upon the horizontal
alignment of the current meter. Conversion circuit 29 also receives
signals from magnetom-31 which are representative of the current meter
alignment with respect to the earth axes. Using well-known trigonometric
transformations, conversion circuit 29 provides output signals
representative of the north-south and east-west component of the current
velocity. Generally, the acoustic current meter also contains a
temperature sensor 33 which measures the temperature of the surrounding
water. The velocity of sound in a liquid is a function of the temperature
of the liquid, and the data from temperature sensor 33 may be used to
provide correction to the measured velocity.
The output data from conversion circuit 29 and temperature sensor 33 is
generally recorded on magnetic tape or stored in some other manner, as
shown by block 35, so that data over a long period of time may be
collected for later analysis. Alternatively, it should be clear that the
signals applied to circuitry 29 may be stored directly and later
processed.
The continuous wave burst processors 21 measure the above-described time
differences in a novel manner which provides a current meter having
numerous advantages over prior art devices. In the present invention, both
transducers of an opposed pair are simultaneously excited for a
predetermined interval of time and emit a burst of continuous wave
acoustic energy. Typically, these waves are at a frequency at
approximately 1.6 MHz. The two bursts of acoustic waves emitted by
transducers 32a and 32b are reflected by acoustic mirror 36 and received
by the opposite transducer. If there is a current component flowing along
the propagation axis, the different propagation times of the waves emitted
by the two transducers moving in opposite directions, as given by equation
(1), produces a phase shift between the two acoustic wave bursts. By
measuring the phase difference between the waves received by each of the
transducers 32a and 32b, the time difference and hence current velocity
may be determined.
To measure the phase difference between the two received waves, the output
signal from each of the transducers 32 is heterodyned with a reference
frequency to produce two difference frequency signals. The heterodyning
process results in two difference frequency signals having the same
relative phase difference as the higher frequency signals received by the
transducers 32 but at a lower frequency. By chosing the reference
frequency so that the difference frequency is a very low frequency, the
small propagation time difference may be converted into a much longer time
difference represented by the phase shift of the two difference frequency
signals.
FIG. 4 is a block diagram of processors 21 which drive transducers 32 with
the above-described continuous wave burst signals and which process the
received acoustic waves to provide a measurement of current velocity. This
circuitry will be described with reference to FIG. 5 which shows
wavesforms occurring at various points of FIG. 4.
A crystal controlled transmit oscillator 50 is periodically connected to
drive transducers 32 via a transmit switch 52. In the preferred embodiment
described herein, transmit oscillator 50 has a frequency of 1.605 MHz. The
transmit switch is connected between the output of oscillator 50 and two
autotransformers 55a and 55b, respectively connected to transducers 32a
and 32b. Autotransformers 55 are used to match the high output impedance
of the driving circuitry of oscillator 50, which is typically 100 kilohms
or more, to the lower impedance of the piezoelectric transducers 32. In
the preferred embodiment described herein, transformers 55 have a turns
ratio of 4 to 1. In transmit mode, transducers 32 are connected to ground
through resistors 60 and the low output impedance of reference oscillator,
represented by resistor 59. Since the output signal from reference
oscillator 58 is much smaller than the signal from transmit oscillator 50,
and since the high impedance of oscillator 50 blocks any flow of current
from oscillator 58, its effect on the transmitted signal may be ignored.
Transmit switch 52 is periodically closed in response to a transmit control
signal, designated as T in FIG. 4. When transmit switch 52 is closed, the
transmit oscillator frequency is applied to opposed transducers 32a and
32b, and in response, these transducers emit a burst of acoustic waves at
the transmit oscillator frequency. This is shown in FIG. 5 by waveform A
which represents the signals from transmit oscillator 50 applied to
transducers 32 by transmit switch 52. In the preferred embodiment
described, switch 52 is closed and transducers 32 are excited for
approximately 91.5 microseconds repeating every 610 microseconds, as shown
in FIG. 5. When switch 52 opens, a second switch 54 closes in response to
a T signal which is the inverse of the T signal. Switch 54 is connected
between autotransformers 55 and ground and provides a return for the
signal produced by transducers 32 when they are operating as receiving
transducers during receive mode.
The burst of acoustic waves simultaneously transmitted by transducers 32
when transmit switch 52 is closed are each received by the opposite
transducer. The propagation time between transducers is determined by the
dimensions of the acoustic current meter in question, and in the
embodiment described herein, the propagation time is approximately 150
microseconds. Referring to FIG. 5, waveform B illustrates the output
signals from transducers 32 in response to the received acoustic waves. As
can be seen from FIG. 5, the output signal B is delayed in time from the
transmitted signal A by approximately 150 microseconds, Although the
output signals from transducer 32a and transducer 32b are illustrated by a
single waveform in FIG. 5, if there is a current velocity component
flowing along the direction of propagation, the signals from the
individual transducers will be shifted slightly in phase with respect to
one another, as described above.
The signals from transducers 32a and 32b are processed in an essentially
identical manner to provide lower frequency signals whose phase is
ultimately compared to measure the current velocity. Accordingly, only the
"channel a" circuitry connected to transducer 32a is described below. The
operation of the correspondingly numbered "channel b" circuitry is
essentially identical.
The output from transducer 32a is applied to a first terminal of a switch
56a. The reference oscillator 58 is connected by a resistor 60 to the
first terminal of switch 56a. The signal present at the first terminal of
switch 56a is the sum of the output signal from transducer 32a and the
output signal of reference oscillator 58. When switch 56a is closed, these
summed signals are applied via a buffer amplifier 62a to a non-linear
device which generates product terms, such as a demodulator or mixer 64a
which produces sum and difference frequency signal components. In the
preferred embodiment described, demodulator 64 is a square law detector.
The output signal from detector 64a is applied to a filter 66a whose
frequency response is chosen to pass the difference frequency component
and attenuate the other signal components in the output signal from
detector 64a, such as a low-pass or bandpass filter. The output signal
from filter 66b is applied to a limiting amplifier 67b.
The output signals produced by transducers 32 in receive mode are much
smaller in magnitude then the signals applied to the transducers from
transmit oscillator 50 during transmit mode. This is primarily due to the
spreading of the acoustic wave which results in only a fraction of the
transmitted energy received by each transducer. The signal magnitude from
reference oscillator 58 should be approximately the same as the output
signal from the transducers 32 to maximize the output signal from detector
64, and thus the signal level from reference oscillator 58 is much smaller
than the signal level from transmit oscillator 50, as described above.
The frequency of reference oscillator 58 is chosen to be slightly offset
from the frequency of transmit oscillator 50. In the preferred embodiment
described herein, the difference between the frequencies of the transmit
and the reference oscillators is 34 Hz. Thus, the output from bandpass
filter 66a is a signal having a frequency of 34 Hz and having a phase
which is determined by the phase of the signal received by transducer 32a
and by the phase of reference oscillator 58. In the preferred embodiment,
the transmit and reference oscillators are crystal controlled oscillators
which are phase locked to ensure a stable 34 Hz difference frequency.
The above-described operation can be more clearly seen by referring to the
waveforms in FIG. 5. As described above, the output signals from
transducers 32a and 32b are composed of essentially simultaneous bursts of
continuous 1.605 MHz signals which are slightly out of phase, the phase
difference being determined by the current velocity. Referring to waveform
C, the receive gate signal R which operates switches 56 is shown. It can
be seen that switches 56 are only closed during a portion of the
continuous wave burst received by the transducers. In the preferred
embodiment described herein, there is a 91.5 microsecond delay following
the end of the transmitted burst to allow any residual oscillations in the
transducers to decay to a negligible level. Following this delay, switches
56 are closed for approximately 30.5 microseconds.
The transducer output signal and reference oscillator signal are mixed by
detector 64a, and the output from detector 64a, when averaged over the
receive gate interval, is a DC signal having a magnitude proportional to
the phase difference between the transducer 32a output signal and the
reference oscillator signal. Thus, during each receive gate interval, a
30.5 microsecond pulse having an average DC value proportional to this
phase difference is produced at the output of detector 64a.
Due to the 34 Hz difference in frequency between the signal received by
transducer 32 and the reference oscillator frequency, the phase difference
between the signals will slowly vary at a constant 34 Hz frequency. Thus,
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