19-09-2012, 01:29 PM
Response Analyses of Micro-Ultrasonic Sensor Devices for Underwater Robotic Applications
[attachment=34060]
Abstract
The vastness of the ocean offers great challenges
for various underwater exploratory and communication related
applications using multiple robotic platforms such as
remotely operated vehicle (ROV), underwater glider, vertical
profiler and autonomous surface vessel (ASV). Small
in size, higher sensitivities and lower power consumptions
of micro-ultrasonic devices make it a perfect match for
those robotic platforms. This paper proposed two microultrasonic
transducer (MUT) designs for underwater applications.
Two sensing mechanisms were utilized namely,
piezoelectric (pMUT) and capacitive (cMUT). The objective
is not meant for comparison, but it is rather a work carried
to investigate on the option available to cater different
underwater applications. Responses of those micro-devices
were identified using finite element method (FEM). Three
different materials were suggested for each design; which
was ZnO, PZT and quartz for pMUT and Si3N4, Teflon and
PDMS for cMUT. Analyses conducted including mechanical,
piezoelectric and frequency.
Introductions
Various robotic-based underwater platforms have been utilized
in ocean explorations such as remotely operated vehicle
(ROV), underwater glider, vertical profiler (VP) as well
as autonomous surface vessel (ASV). Multiple types of sensor
modules can be found onboard of these platforms involving
acoustic and non-acoustic type. Depth sounding, pressure
measurement, obstacle avoidance, navigation, imaging
and communication are several applications that utilize
acoustic wave. Micro-ultrasonic transducers (MUT),
adopting MEMS and microelectronics fabrication technology
were forecasted to carry huge potential these coming
years in sensing and robotic design [1–3]. In general,
many advantages offered by MEMS-based sensor including
low power consumptions, higher sensitivity and lightweight
with embedded electronics. For terrestrial applications, two
most popular sensing mechanisms of MUT are piezoelectric,
known as pMUT and capacitive or cMUT [4–7].
Theory
Proposed pMUT design is shown in Fig. 1. It consists of
six material layers, with piezoelectric film sandwiched between
0.5 μm of aluminium top and bottom electrodes. Top
electrode is partially sputtered on top of the piezoelectric
film. Three type of materials will be tested as piezoelectric
film namely ZnO, PZT and quartz. Structural wafer layers
consist of bottom-etched silicon-on-insulator (SOI), leaving
silica and silicon layers as part of vibrating membrane after
bottom silicon layer was etched. The sixth layer is a polymer
adhesive, Cytop, placed between functional and structural
wafer layer, bonding those two forming a diaphragm.
Results & Discussions
Linear responses have been observed due to applied voltage
on pMUT as illustrated in Fig. 5. Positive voltage has
resulting maximum upward deflection at the center of the
membrane. At the same thickness, PZT has the highest deflection
with 1.3×10−3 μm/V of response, followed by ZnO
and Quartz with 3.0 × 10−5 μm/V and 5.0 × 10−7 μm/V
of responses respectively. The analyses were extended to
estimate the amount of charge on the top surface of the
piezo, Spzt. The charge response curve is shown in Fig. 6
with PZT carries 2750.2 pC/V. ZnO and Quartz piezo layer
carry slightly different amount of charge responses with
19.45 pC/V and 7.92 pC/V. Based on the voltage applied
across the electrodes, stack capacitance Co of the pMUT according
to BVD model can be calculated using Co = Q/V ,
with assumption that displacement is negligible.
Conclusions
Two typical MUT designs which are piezoelectric type and
capacitive type have been successfully studied. PMUT utilize
nickel aluminium coating while cMUT vacuum gap
were set at 50 μm, and the dimension were tuned so that resonant
frequency occur below 1 MHz, specifically for underwater
applications. For pMUT, three different piezoelectric
materials have been compared namely PZT, ZnO and Quartz
while for cMUT, Si3N4, PDMS and Teflon were shortlisted
for further investigations as membrane layer. As a conclusion,
PZT carry the highest charge response of pMUT, however
mechanical analysis revealed that PZT lack of stiffness
resulting in major deflection when pressure is applied
compare to ZnO and Quartz. By having the least charge response,
quartz piezoelectric crystal also less stiff compared
to ZnO, resulting in higher resonant frequency. In the middle
of it, ZnO produced enough charge amounts and vibrate
moderately due to applied cyclic pressure with reference to
PZT and quartz.
[attachment=34060]
Abstract
The vastness of the ocean offers great challenges
for various underwater exploratory and communication related
applications using multiple robotic platforms such as
remotely operated vehicle (ROV), underwater glider, vertical
profiler and autonomous surface vessel (ASV). Small
in size, higher sensitivities and lower power consumptions
of micro-ultrasonic devices make it a perfect match for
those robotic platforms. This paper proposed two microultrasonic
transducer (MUT) designs for underwater applications.
Two sensing mechanisms were utilized namely,
piezoelectric (pMUT) and capacitive (cMUT). The objective
is not meant for comparison, but it is rather a work carried
to investigate on the option available to cater different
underwater applications. Responses of those micro-devices
were identified using finite element method (FEM). Three
different materials were suggested for each design; which
was ZnO, PZT and quartz for pMUT and Si3N4, Teflon and
PDMS for cMUT. Analyses conducted including mechanical,
piezoelectric and frequency.
Introductions
Various robotic-based underwater platforms have been utilized
in ocean explorations such as remotely operated vehicle
(ROV), underwater glider, vertical profiler (VP) as well
as autonomous surface vessel (ASV). Multiple types of sensor
modules can be found onboard of these platforms involving
acoustic and non-acoustic type. Depth sounding, pressure
measurement, obstacle avoidance, navigation, imaging
and communication are several applications that utilize
acoustic wave. Micro-ultrasonic transducers (MUT),
adopting MEMS and microelectronics fabrication technology
were forecasted to carry huge potential these coming
years in sensing and robotic design [1–3]. In general,
many advantages offered by MEMS-based sensor including
low power consumptions, higher sensitivity and lightweight
with embedded electronics. For terrestrial applications, two
most popular sensing mechanisms of MUT are piezoelectric,
known as pMUT and capacitive or cMUT [4–7].
Theory
Proposed pMUT design is shown in Fig. 1. It consists of
six material layers, with piezoelectric film sandwiched between
0.5 μm of aluminium top and bottom electrodes. Top
electrode is partially sputtered on top of the piezoelectric
film. Three type of materials will be tested as piezoelectric
film namely ZnO, PZT and quartz. Structural wafer layers
consist of bottom-etched silicon-on-insulator (SOI), leaving
silica and silicon layers as part of vibrating membrane after
bottom silicon layer was etched. The sixth layer is a polymer
adhesive, Cytop, placed between functional and structural
wafer layer, bonding those two forming a diaphragm.
Results & Discussions
Linear responses have been observed due to applied voltage
on pMUT as illustrated in Fig. 5. Positive voltage has
resulting maximum upward deflection at the center of the
membrane. At the same thickness, PZT has the highest deflection
with 1.3×10−3 μm/V of response, followed by ZnO
and Quartz with 3.0 × 10−5 μm/V and 5.0 × 10−7 μm/V
of responses respectively. The analyses were extended to
estimate the amount of charge on the top surface of the
piezo, Spzt. The charge response curve is shown in Fig. 6
with PZT carries 2750.2 pC/V. ZnO and Quartz piezo layer
carry slightly different amount of charge responses with
19.45 pC/V and 7.92 pC/V. Based on the voltage applied
across the electrodes, stack capacitance Co of the pMUT according
to BVD model can be calculated using Co = Q/V ,
with assumption that displacement is negligible.
Conclusions
Two typical MUT designs which are piezoelectric type and
capacitive type have been successfully studied. PMUT utilize
nickel aluminium coating while cMUT vacuum gap
were set at 50 μm, and the dimension were tuned so that resonant
frequency occur below 1 MHz, specifically for underwater
applications. For pMUT, three different piezoelectric
materials have been compared namely PZT, ZnO and Quartz
while for cMUT, Si3N4, PDMS and Teflon were shortlisted
for further investigations as membrane layer. As a conclusion,
PZT carry the highest charge response of pMUT, however
mechanical analysis revealed that PZT lack of stiffness
resulting in major deflection when pressure is applied
compare to ZnO and Quartz. By having the least charge response,
quartz piezoelectric crystal also less stiff compared
to ZnO, resulting in higher resonant frequency. In the middle
of it, ZnO produced enough charge amounts and vibrate
moderately due to applied cyclic pressure with reference to
PZT and quartz.