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ECE 4902 Spring 2008
Prototyping and Real Time Testing of
Underwater Acoustic Modem
Final Report
Jason Thomas Electrical Engineering
Abbas Zaidi Electrical Engineering
Juny Thengumthyil Electrical Engineering
Advisor Shengli Zhou
Sponsor:
University of Connecticut, ECE Department
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Abstract:
The aim of our senior design project is to prototype and test a stand-alone underwater
acoustic modem, based on existing multicarrier modem designs tested mainly in a lab
environment. Important aspects are signal amplification and system modularization. With
signal amplification, we were able to transmit the signal for a range of almost 200 meters
as opposed to the previous tests of distance about 1 meter. With system modularization,
we were able to perform real time tests of the stand-alone modem in the lakes, rather than
in the tanks. In the demonstration, transmitter is implemented in two ways. The first one
uses a laptop with GUI interface and second one uses a DSP board which is programmed
to send data in every 20sec. There are two receivers to be demonstrated, the first one uses
Matlab programs on a laptop, while the second one is based on a DSP board containingall the system software. The DSP board, a power amplifier, batteries, and an underwater
transducer/hydrophone are placed in a waterproof casing.
Statement of need:
Electromagnetic waves do not propagate in an effective manner through water.
The use of acoustics for transmission provides an alternative method to efficiently
transmit data under water. Sound waves were chosen because they propagate well in
water and travel at a speed of about 1500m/s. Data transmission via waves also provides
isolation between portions of a data transmission system. Much like a transformer, a
failure on one side of the system would not have an effect on the other. This system also
allows for minimal hardware to be in contact with the water media, and also provides a
cost reduction by eliminating hard lines over large distances.
Previous work:
This is an ongoing project that has currently been implemented in two ways. The Fall
2006 groups method used code, programmed in Matlab, to transmit data between two
laptops: one laptop was used in conjunction with a transmitter to send the signal, and the
other laptop was connected to a receiver. The first laptop converted the digital data into
sound signals. The transmitted signals were then received by the second laptop which
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then converted the signal back into the original digital file. The system used Orthogonal
Frequency Division Multiplexing for the process. For underwater testing, a speaker and
hydrophone were placed in a water tank. Instructions were then fed to the transmitting
laptop, and the receiving laptop successfully acquired and decoded the data. This design
allowed for the transmission of data, but had a limited range of about one meter.
The Fall 2007 group modified the first project by implementing a three node
relay using the same principles as that of the first design. The three nodes acted like a
daisy chain, such that the communication between the end computers should be
through the middle one .This modification provided an increase in transmission distance
due to the middle computer acting as a relay unit. Another modification was the
incorporation of a motion sensor. Whenever motion occurred, the computer attached to
the sensor would wake up and record the data and send the data automatically to the
other nodes. Further modifications are currently being implemented to make a four node
relay/network.
Approach:
Our group had three major tasks: 1) Amplification of the transmitted signal
2) Build a transmitter module and a receiver module 3) Real time testing at Mansfield
Hollow Lake and Mirror Lake.
In order to amplify the signal, we had to purchase a power amplifier. Part of the
research involved determining which class of amplifier to use for our application. The
choices we looked at were class A, class B, class A/B, and class D. Class A required a lot
more power, was less efficient and produced more heat. We decided against class B since
it only amplifies 1800 of the signal. Class B amplifiers are also subjected to crossover
distortion if a complementary transistor is added to obtain the other half of the signal.
Class D amplifiers, though they are the most efficient, use pulse width modulation and
current technology limits this class of amplifier to low frequency applications. Since our
application uses a center band frequency of 12 KHz, this class cannot be used. Class A/B
amplifiers incorporate the benefits and minimize the limitations of class A and class B
amplifiers. Due to these findings, we decided to use a class A/B power amplifier.
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There were two versions of the transmitter module as shown in phase two and
phase three figures. The first version consisted of a transducer connected to a power
amplifier which was connected to both to a 12V DC battery as a power supply, and a
laptop which provided the input data for our system. The second version consisted of the
same components, but used a programmed DSP board to provide the system input. These
components were then placed in a water proof case, with the transducer placed in the
water.
Project Plan:
Our project plan was divided into three phases
Phase 1 Interface/ Testing Housing
Battery Testing
Real Time Tests
Phase 2 PC based tests Amplification
Signal Acquirement / Processing
Phase 3 DSP based tests Amplification
Signal Acquirement / Processing
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Phase 2 schematic:
In this phase we used laptop sound cards to send and receive the data.
Phase 3 schematic:
In phase 3, we assembled the prototypes using the DSP boards as the transmitter and
receiver data encoders/decoders.
Components:
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The following are the components used, with specifications given below each figure.
DSP Board TMS320C6713 (Used in phase 3)
Stand Alone Product
Includes Audio Encoder and Decoder
Includes Stereo IO
Presonus TUBE Pre Mono Tube Pre Amplifier (DSP Pre Amp, used in phase 3)
Switchable phantom power
80 Hz low cut filter
Up to 20 dB of tube drive48V Phantom power
Pelican 1650 Waterproof Case
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Outside Dimensions: 32.50"L x 20.50"W x 11.31"D
Inside Dimensions: 29.00"L x 17.88"W x 10.50"D
Weight: 37 lbs
Valve regulated- Gelled Electrolyte Battery
Dimensions: 8.31 x 5.13 x 7.25
Nominal Voltage: 12V
Capacity: 36.5Ah
Weight: 23.4 Lbs.
Deepwater Omni directional Transducer
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Freq Range: 1.00 Hz 45KHZ
Input Power: 800 watts
Resonance Freq: 33 kHz
Oceanears DRS-8 Transducer
Freq Range: 200 Hz 32 kHz
Input Power: 200 Watts (max)
MOSFET Bridgeable Power Amplifier
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Peak Power: 200 W 2 channel
THD @ 4: 0.01%
S/N ratio : 102 dB
High pass crossover
Variable Low Pass Crossover
Dimensions: 11.8 x 7.2 x 2.8
Weight: 7.8 pounds
Miniature Reference Hydrophone
Freq Range: 1Hz to 170KHZ
Sensitivity(dB re V/Pa): 211
High sensitivity, broad banded, and Omni directional
Aquarian AQ-3 Hydrophone
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Freq Range: 20Hz - 100 kHz
Low self noise
Power Output: 300 mWatts
Input Voltage: 6 15 Vdc
VP 1000 Voltage Preamp
VP 1000 Voltage Preamp
Input Impedance : 100MOhm
Output Impedance: 10Ohm
Operating Freq Range:
0.5 Hz to 1 MHz
Preliminary Experimental results:
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We used a Sony Xplode 300 Watt 2 channel car audio amplifier to amplify a
signal sent to a transducer. The input to the amplifier was music from a laptop. We did
this preliminary testing out of water to test whether our wiring and amplification idea
would qualitatively work. The results of the experiment showed that the sound was in
fact amplified using the car audio amplifier. But due to the high power of the amplifier
(the transducer was rated for 200 Watt maximum input) the sound coming out of the
transducer was distorted. Lowering the gain of the amplifier did correct the problem
however. Taking this into account, we have concluded that we can use a smaller
amplifier, which will also be a cost reduction, to begin testing of our project.
Final Results (Phase 2):
The following plots give the results obtained via laptop using Matlab coding. The top left
graph gives the received data. The first packet is used to ensure proper signal
synchronization and the second packet contains the actual transmitted data. The top right
graph gives the impulse response of the channel. The bottom left graph is the Correlation
Plateau, which shows an analysis of the first packet. It checks the first half of the packet
against the second half. Both halves should be identical which would be indicated by a
plateau equal to one. The code searches for the best correlation which is shown by the red
line in the graph. The bottom right graph shows the data point acquisition. This graph is
separated into four quadrants. The blue circles represent two bits of data:
Quadrant 1: 00
Quadrant 2: 01
Quadrant 3: 10
Quadrant 4: 11
Deviation from the centre of the quadrants is due to noise interference. If the noise is
high, it may cause the transferred two bits to appear in the wrong quadrant giving
erroneous data, causing the signal to not decode properly.
Plot at 10m
SNR: 15 dB
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Plot at 20 m
SNR: 12dB
Plot at 30m
SNR: 16.7 dB
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Plot at 50m
SNR: 19.7dB
Plot at 100m
SNR: 25dB
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Plot at 150m
SNR: 21.6
Timeline
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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Research
Purchasing
Construction
Testing
Demo
Budget:
Product Quantity Price
MOSFET Bridgeable
Power Amplifier
1 62
Valve regulated-
Gelled Electrolyte
Battery
2 170
12 Volt BatteryCharger
1 100
Pelican 1650
Waterproof case
2 380
Miscellaneous
Expenses
200
Total 912
Problems Encountered:
One of the problems we encountered was extraneous noise. When we first went
out for testing we used the Aquarian AQ-3 hydrophone which did not have a signal filter.
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This lead to triggering by any noise present in the water. To counter this problem, we
had to turn up our trigger value which lowered the transmission distance. This happened
because our signal had to be sent at higher gain values. At larger distances this lead to
being at the saturation level of the speaker. As a solution to this problem, our advisor
ordered the Miniature Reference hydrophone which included a high-pass filter and built
in pre-amp. This hydrophone eliminated the motor boat noise and wildlife noise present
in the lake. This dramatically increased our transmission distance. We did note that on a
windy day the choppy water still caused some false triggers. But most of the other
interference was eliminated.
Another problem we encountered was the power supplies of the DSP boards.
While reading the product manual in the first semester, we understood that a part could
be ordered providing a 5V input to the DSP board from a 12V DC source. When we
ordered this part, we realized that this connector was only to be used with a desktop
computers power supply, so we could not use our original plan of using it with our 12V
battery. To remedy this, we made 4.5V battery packs using battery holders and AA
batteries, but their life span was approximately 30 minutes.
Another issue was the power supply of the Presonus TUBE Pre Mono Tube Pre
Amplifier used for the DSP board. It required a 16V AC input, which we were unable to
provide in time for project completion deadline. We ordered a 12V DC pre-amp but its
limitations proved inadequate for proper system operation. This was mainly due to this
pre-amps inability to control its gain level. This was proved when we took this
configuration out for testing in the lake. We were unable to receive any successful
messages using this configuration.
Conclusion:
We met our goal of increasing the transmission distance from 1m to 100m. We
got reliable transmission and reception from up to 200m. In fact we were able to get a
successful decoded message from 700m using the phase 2 setup with the Oceanears
DRS-8 Transducer and the Miniature Reference Hydrophone. But at this distance the
reception is unreliable using our components because only about 1 in 10 messages get
decoded correctly. We desired to do further testing with the Deepwater Omni directional
Transducer, but weather and time constraints did not allow us to do so. From our
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observation of using the higher quality hydrophone and in lab testing of using the
superior DSP board, we believe that the higher quality transducer will significantly
improve our results.
Our other goal was to provide an independent prototype using off the shelf
components. We were semi-successful in this case. Our phase 2 setup was implemented
and provided the majority of our testing. But the phase 3 setup had power supply
problems. The 12V DC DSP board pre-amp we purchased was inadequate. The phase 3
setup worked in the lab trials in the water tank. But data could not be properly received
and decoded in the real time testing environment on the lake. We believe this is due to
the pre amplifier.
We think this was an excellent project and could be used in many applications
including wireless unmanned submersible communication. We would like to thank Janny
Liao and Sean Mason for helping us in the underwater acoustic modem lab. They
provided all the code changes and DSP programming and helped us understand the
system and designs of previous teams. We would also like to thank our advisor Dr.
Shengli Zhou for the support of providing us with all the high-end components and
helping us organize our research and testing efforts.