[Music]
our next speaker is an engineering major
at Harvey muddy mud uh College whose
technical interests include single
processing and digital design his talk
today represents an acoustic phased
array that is open source and easy to
use so please welcome our hackaday
supercon at our hackaday supercon stage
Alec fairchusi
foreign
[Applause]
today I'll be presenting on a low-cost
underwater ultrasonic phased array this
work was done by
um at Harvey Mudd College with professor
Matthew Spencer and fellow students
Tejas Rao and riazaverchan
so for a bit of motivation as to why we
want to do Ultrasonics underwater is
that traditional electromagnetic waves
don't propagate well underwater at radio
frequencies because water is a conductor
radio waves don't propagate just the
same way that a radio wave doesn't
propagate well through metal it's not
going to propagate well through water at
um past non-trivial deaths at the same
time light doesn't propagate that well
through water because you know of uh
scattering in the water often there's
particles that just cause it make it
hard to see
sound does propagate extremely well
through water however and we can see an
example of that this is a sonar image in
the figure to the right taken of a
shipwreck over 200 feet down
um so we want to take advantage of
acoustic propagation underwater to do
fish tracking with an array and as an
intermediate step towards that we built
an underwater 3D imager we're leaving
with the results here because we think
they're pretty cool and the figure to
the left you can see the array we built
pointed away from the camera this is an
underwater test tank
in the background you see a steel Target
test plate and in the figure to the
right we see the point Cloud generated
by our Imager
so the question is how do we use sound
to create an imager and the answer is
that if we're able to transmit a beam of
sound in a specific Direction and we're
able to steer that beam of sound all we
have to do is steer at different angles
in Azimuth and elevation send out a
pulse and if we get an echo from that
direction we know there's something
there we can use the time that it took
the echo to get back to figure out the
round trip distance to the Target and
create a point cloud from that
the figure to the right shows the
envelope of a received signal bouncing
off of the steel test plate so the test
plate is around 1.2 meters away so we
can see a big spike here and this
actually this the small Spike here is
due to multi-path it bounces off the
test plate and then probably off the
back wall behind the array back to the
test plate again effectively doubling
the distance
um so I'm going to talk about the whole
system from the bottom up to describe
this imager and how we do this I've kind
of got three sections first we need an
element that can create sound we need to
be able to control that sound with the
circuit and then we need to make use
those circuits to create a directional
beam then I'll talk a bit about the
results
so first we need um
we need an element that creates sound
and so to create sound underwater you
need something that vibrates and appears
to Electric material can do that
a piezoelectric material when voltage is
applied deformed slightly to create uh
acoustic propagation and we can
understand the converse is true as well
if you deform a Piezo like material a
material it'll create a voltage and so
this piezoelectric material can act as a
transducer which is both a speaker and a
microphone we can intuitively understand
how this works by you looking at the
unit cell of pzt which is an extremely
common piezoelectric material we see in
this unit cell that we have a positively
charged atom that's slightly off center
so there's going to be a bit of a charge
distribution a positive charge at the
top of the unit cell and a negative
charge at the bottom of the unit cell
when an electric field is applied the
positive charge wants to move with the
field the negative charge wants to move
away from the field and so the unit cell
is going to deform
practically RP is a transducer is shown
on the figure to the right so for some
sense of context this is about a 1.6
centimeters in diameter
apologies the
the electric field is applied between
two terminals um this is a hollow sphere
the electric field is applied between
two terminals um on the outer ring and
on the inner ring of the transducer the
non-central symmetric Titanium or the
non-setro symmetric positively charged
atom is kind of pointing inwards here so
when we apply this electric field the
hollow
cylinder is going to expand and contract
um
which will push water kind of in and out
of the transducer creating a sound wave
um so now we have the transducers but to
let them to make them work underwater we
need to do two things which is
waterproof them and do acoustic
impedance matching
so the way we accomplish both of these
things both of these things is by
dipping them into a thin layer of
silicone this accomplishes the
waterproofing by just covering the
transducers in a thin layer of silicone
um
Sorry by by covering the terminals of
the transducers and this also
accomplishes acoustic impedance matching
it turns out that pressure wave carries
a lot about the the ratio between the
speed of sound of a material and the
density of that material and that's
called specific acoustic impedance this
is analogous to electrical impedance
which cares about voltage to current
um
the specific acoustic impedance of
silicone is very similar to that of
water so at the boundary between
silicone and water
um to a pressure wave it seems like
there's no boundary at all the specific
acoustic impedance of PCT material
however is much higher than that of
water so it's kind of a hard to cross
boundary we improve the coupling the
power transfer between the water and our
transducers by
um
by having our boundary kind of between
this be the silicone to water boundary
instead of the water to PCT boundary
uh so now we have the transducers
figured out I'll talk a bit about the
circuits that drives a pzt element
oh and uh quickly this is the array
we've made it's these nine um silicone
dipped elements mounted on a foam plate
okay
um so now I'll get into the electronics
here
um basically each there's a channel
board a single Channel board that drives
a single PCT element times nine
on the transmit side we want to be able
to generate a um a wave with an
arbitrary amplitude an arbitrary a
pulsed sine wave with an arbitrary
amplitude arbitrary frequency arbitrary
phase shift and arbitrary like length of
the pulse as well and the way we can do
that using a microcontroller and a
simple analog front end is by taking
advantage of a couple of the peripherals
on that microcontroller so here we have
an
here we have an analog multiplexer that
uses a pwm peripheral as the select line
and it selects between the static output
of a digital to analog converter and
ground this produces a square wave with
an arbitrary amplitude selected by the
static digital analog converter and with
frequency and phase parameters basically
selected by the pwm peripheral
we can filter out the harmonics of the
square wave to create a sine wave and we
do this with a second order low pass
filter a sound key topology we then AC
couple the signal to zero Center it and
it's Amplified by a linear audio power
amplifier which drives the transducer
on the receive side the signal is uh
filtered with the 60 hertz rejection
filter so this filters out things like
electromagnetic interference from the
wall sockets for example then we have a
couple op-amp stages to do front end
amplification and it's sampled directly
by the microcontroller the
microcontroller samples at one Mega
sample per second and it uses direct
memory access to write this into
internal into internal memory of the
microcontroller
samples are then offloaded on an i
squared C bus to a host computer to do
the processing
okay uh you'll note that we kind of have
nine different elements here and that's
ultimately how we're able to create
um
it sound traveling in a very specific
Direction how we can create a
directional beam of sound and the
technique we use to do that as with a
phased array
um so I'm only going to introduce kind
of a phased array very briefly here
because it's been done in the past
um in 2018 at supercon Hunter Scott gave
a great presentation about phased arrays
kind of in the context of RF this is in
the context of Acoustics so it's a lot
lower frequency but if we take a look
at a single element we can see that a
single element emanates like a very
spherical wavefront but if we have
multiple elements
if we have multiple elements uh
emanating wavefronts and we look at a
specific Direction constructive
interference is going to make it look
like there's one single plane wave
emanating from that direction and the
key takeaway here is that we can control
by by varying the phase shift
um or you know the time that each
element fires we can control a steering
angle so in this case this element is
firing slightly before this one this
one's slightly before this one and vice
versa and so that causes that steering
angle to be in this direction but you
can imagine if all elements were firing
at the exact same time we'd have zero
steering angle the plane wave would
basically be traveling exactly away from
the Tran of the array
uh so the figure to the right shows us
practically implementing this
we have the measured beam pattern here
so this is kind of power versus angle of
our array this is steering directly away
from the array
steering directly away from the array
and then we we slightly vary the
steering angle to the right more and
more
and it does match pretty closely with
the theoretical measurements uh the
theoretic the theory which which is good
we'll note that this bottom figure here
shows that we have to kind of limit our
maximum steering angle and that's due to
grading lobes
which is due to the physical setup of
the array itself
okay so now I'll talk a little bit about
the results
I'll talk a little bit about the results
uh
this figure here shows the traces at
different parts of the transmit chain we
produce this red signal
which drives the PCT and again to do
that we have this analog multiplexer the
blue tray shows the output of the static
DAC the orange tray shows the output of
the analog multiplexer so we have this
arbitrary amplitude Square wave after
filtering out the harmonics to produce a
sine wave that that shows this that's
shown by this green Trace here so we can
see it's a sine wave there's a little
bit of weirdness here on the front and
that's the settling time of the filter
and then this is zero centered using the
AC coupling and then Amplified by the
power amplifier
okay
um so
this figure at the bottom here shows the
microcontroller sampling the received
signal and so this is a um
and a signal sorry this is an echo
that's coming in at an angle relative to
the array so we see that it's a planar
wavefront and it's because it's coming
in at an angle relative to the array
it's got a slightly different travel
time to each element in the array and
that produces this phase shift here so
if we're looking in a very specific in
software if we're looking at a very
specific if we're looking straight out
from the array
and we just sum these signals directly
that's going to produce destructive
interference and so the magnitude of the
envelope of the signal isn't going to be
very high but if we're looking at a
um if we're looking at the direction
that this signal came in we essentially
reverse the delays to have these signals
you know have zero phase shift to each
other if we sum them then that produces
constructive interference that creates
an echo with a large magnitude so that's
how we kind of do the receive beam
forming
now I'll show the test setup so these
are the pcbs we have apologies for all
the cabling here
foreign
so this is the motherboard it houses a
central
um multi-channel transmit and receive
switch and the piezos are connected to
it via twisted pair
wires and that just goes into the tank
uh then the motherboard is connected to
each of the nine Channel boards which
has the microcontroller and the analog
front end on it I'm using a coax cable
for the receive side which is sensitive
signals and then for the transmit side
just this twisted pair wire
one issue we had in the uh physically
building the system was with
synchronization of the sampling
um the internal oscillators on the
microcontroller which we were originally
using as the sample clock for the adcs
vary by like plus or minus five percent
even after calibration from the factory
and this was completely unacceptable for
you know sampling these signals and
trying to do beam forming with them um
so we have a one megahertz uh kind of
clock distribution to act as the sample
clock for the ADC we also do another
synchronization technique as well where
we have a signal that basically on the
rising Edge triggers an interrupt on the
microcontroller to start the transmit
sequence and on the falling Edge
triggers the receive sequence so using
these two techniques we can basically
have synchronous sampling
uh so now for the results here so this
is the same set of images shown at the
beginning
again with the array here and the test
set up here showing this point Cloud
it's kind of hard to see here but this
point Cloud isn't actually rectangular
unfortunately it's kind of more of a
sphere and this has to do with the point
spread function of the imager itself
basically the beam produced by this
array isn't infinitely small so if we
you know if we shoot in a specific
direction we're going to get Echoes
coming back from another direction that
affects our measurement uh the reason
this array the the width of the beam is
so large ultimately has to do with the
amount of elements we're using and also
kind of how far apart they're spaced but
um and that's that's kind of future work
for the project
this image here shows a kind of a 2d
slice of that uh of that setup so we're
just scanning an Azimuth now not in
elevation and so we can see the steel
plate here
uh okay uh so in conclusion we've open
sourced the design files for this and
they're available on GitHub and we
invite you to check it out
um also feel free to email me at the
email address on this slide
and thank you guys very much for
listening
[Applause]