Underwater Explorers: Becoming an Ambassador for Underwater Archaeology
An ultrasonic transducer array for velocity measurement in underwater vehicles
Transcript of An ultrasonic transducer array for velocity measurement in underwater vehicles
P. Boltryk et al. / Ultrasonics 42 (2004) 473–478 475
pendent narrow beamwidth projectors are often ar-
ranged in a Janus configuration. The Janus configura-
tion arranges the four projectors to point towards the
seafloor in the forwards, aft, port and starboard direc-
tions; differences in the Doppler shift in the four direc-tions can thus be used to estimate velocity. To achieve
the necessary directionality, the ultrasonic transmitters
operate at a relatively high frequency, typically between
100 kHz and 1 MHz.
Fig. 2. Comparing the correlation coefficient distribution as a function
of distance from peak, upper plot using the empirical model to gen-
erate the predicted distribution and lower plot using actual trials data.
4. AUV applications
An important advantage of the CVL system is that
the velocity estimate is independent of sonic velocity
because it can be assumed that during the short inter-
pulse interval separating the pair of transmitted sig-
nals the ocean environment conditions remain constant.
The CVL therefore does not need to make tempera-
ture or salinity profiling measurements to maintain
precision.Whilst DVLs require narrow beams the ideal trans-
mitter for the CVL has a wide beam width, typically
about 60�. Reducing the directionality criterion awayfrom narrow beams allows for smaller transducers than
DVLs for the same operating frequency, or the potential
to use lower frequency operation for the same size of
transducer. This presents two significant advantages for
the application of CVLs to AUV platforms. Firstly,smaller and lighter transducers integrate more easily
with the existing AUV structure. Secondly, operation at
a lower frequency results in lower attenuation of the
acoustic signals. The CVL is either able to operate in
deeper water for the same transmitted power, or is able
to operate with lower power requirements than the DVL
operating in the same conditions. Both CVLs and DVLs
are able to extend their operating range in deeper waterby referencing waterborne scatterers rather than by
being limited to use fixed scatterers on the seabed.
However, this mode of operation is unfavourable be-
cause the velocity estimation is subject to current effects
and therefore the velocity estimate is not necessarily
velocity over ground.
The short-duration pulses transmitted into the water
by CVLs further reduce projector power requirements.Combined with a broadband acoustic signature, the
short pulses are less easily detected and therefore make
the CVL attractive for covert operation.
Another drawback with the DVL is that it requires
there to be a relative velocity between the acoustic tar-
gets and source. It is therefore not well suited to low
speeds or hover situations, which are common charac-
teristic flight profiles for AUVs. In contrast the CVLmaintains accuracy at low speeds and the prototype
system that has been developed is capable of operating
during hover manoeuvres.
5. Development of COVELIA
A spatial CVL system named COVELIA aimed spe-
cifically for AUVs has been developed. Its acoustical
and signal processing design has been optimised throughextensive sea trials and computer modelling of the sound
field.
5.1. Computer modelling and simulation
A detailed model, described in depth in Ref. [5],
investigated the characteristics of the sound field as de-
tected by receivers in the CVL array on a pulse-by-pulsebasis. This detailed model simulates a random distri-
bution of acoustic scatterers on the seabed, each scat-
terer acting as an acoustic source. Using in-depth
models of the acoustic reflectivity characteristics of the
scatterers, the attenuation of the signals through the
seawater and the sensitivity characteristics of the trans-
ducers based on transducer test data, the detailed model
predicts the waveform as detected by each receiver in agiven receiver array configuration. Thorough investiga-
tions into behaviour of the correlation coefficient dis-
tribution across the velocity vector map have developed
an empirical model that is used to predict the distribu-
tion of the correlation coefficient surface as a function of
the distance from the peak position on the velocity
vector map. Parametric tests have investigated the effect
on this correlation coefficient surface distribution causedby changes in receiver array configuration, seabed con-
ditions, vertical velocity, background noise and trans-
ducer properties such as the Q-factor and centre
frequency.
Fig. 2 compares the correlation coefficient surface
distribution as predicted by the empirical model with the
distribution obtained using sea trials of the prototype
476 P. Boltryk et al. / Ultrasonics 42 (2004) 473–478
system. Considering first the trials data in Fig. 2, the
magnitude of the correlation coefficients as calculated
for each receiver are plotted as dots as a function of the
distance from the estimated peak position for each sea
trials dataset. For the modelled data, a peak position isassigned manually at random throughout the velocity
vector map and correlation coefficients assigned to the
measurement points using a binomial distribution
between the limits as defined by the detailed model.
Calculating the absolute distances between the mea-
surement points and the corresponding peak position,
the magnitude of the correlation coefficients can be
plotted as a function of the distance from the peak po-sition. The distribution of the modelled correlation
coefficient data can be illustrated using a large number
of repeat tests. The modelled data here uses parameters
such as depth that are equivalent to the sea trials con-
ditions.
The detailed model has shown that it is preferable to
sample a specific window of the incoming acoustic
waveform rather than calculating correlation coefficientsbased on the entire time history. This correlation time
window (CTW) affects the definition and width of the
correlation coefficient surface’s peak. The earliest re-
turns resulting from specular reflections immediately
below the CVL are found to contain little spatial
information for the CVL. Referring to Fig. 3, progres-
sively delaying the CTW though the entire received
waveform tends to make the correlation peak narrower.However, this also causes the characteristic side lobes of
the correlation surface to become more prominent.
Increasing depth insonifies a wider annulus on the sea-
bed for a given beamwidth and reduces the variance on
the surface.
Fig. 3. The influence of CTW and depth on correlation coefficient
distribution as a function of distance from peak position in element
spacings. Upper three plots vary the CTW for a fixed depth of 40 m.
Lower plots vary depth with CTW set as 20–30% of complete time
history.
A macroscopic model [6] is used to simulate the
performance of COVELIA in the long term using
parameters found using the detailed model. The macro-
scopic model sets up a simulated test run around a user-
defined track, and the AUV is subjected to randomcrosscurrents and waves. Correlation coefficient data is
generated using the empirical model according to the
environmental conditions and the chosen CTW. The
model then predicts velocity and position using peak
finding routines on the simulated correlation data.
5.2. Acoustical development
COVELIA uses a two-dimensional receiving array
that facilitates velocity estimation throughout the for-
wards, backwards, starboard and port directions. It is
additionally possible to infer vertical velocity although
in practice it may be easier to calculate change in depth
using static pressure measurements. The Tonpiltz
transducers are housed in impedance matching oil and
operating at 60 kHz permits operation up to an altitudeof 500 m above the seabed. COVELIA’s 60 kHz oper-
ating frequency is a compromise between a deeper
propagation of the acoustic pulses due to the reduction
in attenuation with decreasing frequency whilst avoiding
the background noise at lower frequencies.
The acoustic power requirements have been mini-
mised to make integration into an AUV favourable. The
transmitted acoustic pulses are very short, typicallyabout 10 acoustic cycles at 60 kHz. Additionally, the
maximum repetition rate is intended to be only about 4
measurement cycles per second in shallow water. Since
the maximum repetition rate is dependent on the height
above the seafloor it is necessary to operate at a lower
repetition frequency in deeper water.
5.3. Onboard signal processing
To ensure that the incoming signals used within the
correlation calculation result from bottom reflections
rather than volume reverberation or background noise,
COVELIA first determines the height of the instrument
above the seabed and uses time of flight calculations to
neglect signals detected before this depth threshold. The
CVL can therefore operate as a depth sounder to verifyother onboard instrumentation. The transmitted power
is then automatically adjusted to take into account the
attenuation of the returns from the seabed caused by
transmission losses and absorption losses by the sea-
floor. After a predetermined post-trigger period, COV-
ELIA samples the incoming received waveforms for a
sample length equal to the required CTW.
The complex form of the correlation coefficient (2)between two complex acoustic signals v1ðtÞ and v2ðtÞ isused by COVELIA [4]. In usual notation, v�2ðtÞ is the
Fig. 4. Comparison of raw COVELIA speed estimate versus Raystar
120 GPS speed prediction.
P. Boltryk et al. / Ultrasonics 42 (2004) 473–478 477
complex conjugate time history of the complex signal
v2ðtÞ.
q ¼Rþ1�1 v1ðtÞv�2ðtÞdt
Rþ1�1 jv1ðtÞj2 dt
Rþ1�1 jv2ðtÞj2 dt
h i1=2 ð2Þ
This allows phase information to be calculated and
the magnitude of the correlation coefficient is found
to be robust to the phase effects caused by vertical
velocity. A signing algorithm is used to improve the
clarity of the correlation surface. The phase of the cor-
relation data at a given velocity vector point is com-pared with the phase at the highest point in the
correlation coefficient dataset. If the difference in phase
exceeds 1=2p then the correlation coefficient magnitudeat that point is multiplied by )1. Referring to Fig. 3,rather than being bounded to lie within the range 0 to 1,
the signed correlation coefficient magnitude varies be-
tween )1 and +1.The spatial vectors separating all combinations of the
receivers are plotted onto a grid called a velocity vector
map. By superimposing the correlation coefficient data
onto this map, a correlation surface is produced whose
peak position represents the optimum estimate for the
separation vector D. The velocity estimate therefore re-quires COVELIA to estimate the peak position of the
surface using the available correlation data. The reso-
lution of the instrument is improved by using peakfinding methods. Various peak estimation methods have
been investigated [7], including those based on highest
point (HP), least squares (LS), and maximum likelihood
estimation (MLE). Current work concentrates on
exploiting the learning behaviour of Gaussian processes
(GP) [8] and radial basis functions (RBFs) [9] for peak
finding.
HP is the most rudimentary peak estimation method,whereby the indices of the highest point in the correla-
tion coefficient dataset are selected as the peak position.
Whilst this method provides no interpolation between
measurement points it has proven to be a reliable fall-
back method when more complicated methods fail. An
axisymmetric quadratic model is fitted to the correlation
coefficient data using both LS and MLE methods. The
correlation surface peak is estimated by finding the peakposition of the fitted model. MLE is implemented in a
non-linear iterative process, which results in weighted
LS, where the weighting function is the expected vari-
ance at the given measurement points. Both GP and
RBF models are trained offline using sample data, where
the inputs to the model are the correlation coefficients
and the output is the predicted peak position. The
trained GP and RBF models make peak predictions inreal-time, and a quality factor may be output from the
model, which is useful for integrating the output data
with other navigational devices using processes such as
Kalman filtering. Simultaneous models are used for
estimating the peak position in the x and y directions.LS is currently the favoured peak estimation method.
It achieves good results throughout the measurement
area, and avoids the iterative process necessary for MLEand the complexity of GPs and RBFs. Preliminary re-
sults suggest that GPs are extremely well suited to peak
finding in this context, although inclusion of this peak
estimation technique could affect the maximum repeti-
tion rate of the measurement cycle due to the additional
computational load over LS.
6. Trials data
COVELIA has undergone a comprehensive test pro-
gramme that has included tests in different conditions. A
reservoir environment has been used extensively, to-
gether with tests offshore. The trials data in this paper
represents a short period during a recent sea trial on a
test boat that is equipped with precision navigationalequipment including a Raystar 120 GPS system.
The peak finding algorithm used during these tests is
LS with HP fallback. Whilst COVELIA estimates
velocity directly, it is more useful here to compare its
performance with respect to the reference GPS system
by considering the speed and the position estimated by
each system. Referring to Fig. 4, the raw COVELIA
speed estimate tracks the GPS speed reliably, althoughthe COVELIA data has a significantly greater variation.
Note that whilst the GPS system uses filtering tech-
niques the COVELIA speed data presented here is
unfiltered. Fig. 5 plots position using GPS data versus
the estimated position using COVELIA over the same
period of about 36 min.
Fig. 5. Comparison of position estimation using Raystar 120 GPS
system and COVELIA.
478 P. Boltryk et al. / Ultrasonics 42 (2004) 473–478
7. Conclusions
DVLs have traditionally been the instrument of
choice for interfacing with INS in the AUV navigation
system. An alternative ultrasonic-based navigational aid
that is aimed specifically for AUVs has been developed
and tested. Rather than detecting Doppler shifts in the
ultrasonic echoes from the seabed, the CVL searches forsimilarities in the signals detected across a receiver
array, and estimates velocity of the vessel based on the
spatial separation vector corresponding to maximum
correlation between receivers, and the time interval
separating a pair of ultrasonic pulses directed at the
seafloor.
COVELIA has been developed using a combination
of computer modelling of the sound field, and a com-prehensive sea trials programme. The computer model-
ling has been influential in the selection of design
parameters such as the optimum correlation time win-
dow. The empirical model of how the correlation surface
distribution varies with distance from peak position has
been used to test a variety of peak finding methods for
improving the precision of the instrument. Whilst LS
combined with capability to revert to highest point has
currently been selected for use in COVELIA, numerical
studies suggest that Gaussian processes offer promisingcapabilities for peak finding on the correlation data.
Sea trials data benchmarked against GPS demon-
strate that the prototype COVELIA is a reliable and
accurate instrument. Current work seeks to fine-tune the
RBF and GP models to make their inclusion into the
device more attractive.
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