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Characterization and FEM-based Performance Analysis of a Tonpilz Transducer
for Underwater Acoustic Signaling Applications
Bhagya Lakshmi G Venky Vadde
PESIT, Bangalore
5th Nov 2011
Underwater acoustics and transducers
• Underwater acoustics finds its major applications in SONAR.
• Transducers are processes or devices which convert one form of energy to another.
• Sonars use electro-acoustic transducers. Electroacoustic transduction mechanisms are of 6 major types:
1. Piezoelectric
2. Electrostrictive
3. Magnetostrictive
4. Electrostatic
5. Variable Reluctance
6. Moving coil
• Piezoelectric transducers are preferred for their excellent properties
Tonpilz Transducer: overview
Piston Head Mass
Piezo Stack
Stress Rod
Tail Mass
Washer
Insulator
“Pig Tail” Connector
Transformer
Rubber boot
Housing
Materials used for transducer parts
Part of transducer Material used Density (kg/m^3)
Head Alumina 3690
Active element PZT4 Ceramics 7550
Stress rod Beryllium copper 8200
Tail Copper 8800
Tail Steel 7900
• Head material can be alumina or even nylon to lower the weight
• Active material used is PZT4 or a similar ceramic. Ceramics are often preferred to quartz due to their higher coupling coefficient.
Head Mass (Alumina)
Tail Mass (Copper)
Ceramic Stack (PZT4)
Stress bolt (Beryllium Copper)
Solid Head mass/ Gap in head mass
Models showing Tonpilz transducer design built in COMSOL
• Piezoelectric effect : Inverse piezoelectric effect which induces a deformation of crystal for an applied electric potential difference.
• The Pressure acoustics interface designed for the analysis of various pressure acoustics problem in frequency domain, all concerning pressure waves in fluids.
• The interaction of the piezoelectric transducer structure with the aqueous medium and the study of the acoustic wave generated is the main concern.
Multi-physics phenomena studied
Equations solved in COMSOL
Using the values of strain tensor and electric field Stress tensor & Electric displacement values are found using below expression in piezoelectric model:
Pressure is solved in pressure acoustics interface using the Helmholtz equation of form given below:
For 33 mode longitudinal vibrator on expanding the matrix of all the tensors & constants reduces into following four main equations:
Types of head designs and materials
• Transducer heads can be with or without an air-gap. Providing an air-gap helps in lowering head density and mass
• Filling the gap with different materials like water, air, vacuum, mercury etc can also be tried to tweak performance
• Rectangular shape for the air gap at the head portion
• Selecting different material for head portion with different densities like Titanium, Nylon etc.
Effect of head material on performance
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
x 104
-50
0
50
100
150
200
250
Frequency (Hz)
SP
L (d
B)
Air filled gap
Vacuum filled
Water filled
Solid Alumina
Rect. Air Gap
Mercury filled
Solid Titanium
Solid Nylon
nylonvaccuum
Beam pattern for transducer with and without air gap in head portion
10
20
30
40
50
30
210
60
240
90
270
120
300
150
330
180 0
SPL(dB) re1uPa @1m, fR = 27.9KHz
Angular displacement on semi circle of 1m away
from centre of transducer
10
20
30
40
50
30
210
60
240
90
270
120
300
150
330
180 0
SPL(dB) re1uPa @1m, fR = 25.6KHz
Angular displacement on semi circle 1m
away from centre of transducer Beam-pattern of transducers: (a) left: with an air-gap and (b) right: without an air-gap @ 1m away
Effect of head sizing
Increasing head diameter leads to reduction in resonant frequency
0 0.5 1 1.5 2 2.5 3 3.5 4
x 104
0
50
100
150
200
250
Frequency (Hz)
SP
L (d
B)
re 1
uPa
@1m
Head diameter = 70mm
Head diameter = 80mm
Head diameter = 50mm
Head diameter = 60mm
Head diameter = 40mm
Effect of head-sizing on
transducer, no air-gap
80mm70mm 60mm
Effect of voltage across piezo-ceramic
• Power differential of nearly 5dB per octave observed
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4
x 104
120
130
140
150
160
170
180
190
200
210
220
230
Frequency (Hz)
SP
L (d
B)
re 1
Pa
@ 1
m
20V across Piezo
40V across Piezo
80V across Piezo
160V across Piezo
Power -vs- voltage
gain ~5dB /octave
Effect of piezo thickness on transducer response
• Piezo thickness has not much role in tuning resonant frequency
1 1.5 2 2.5 3 3.5 4
x 104
60
80
100
120
140
160
180
200
220
Frequency (Hz)
SP
L (d
B)
re
Pa
@ 1
m
4mm Piezo Thickness
6mm Piezo thickness
8mm Piezo thickness
12mm Piezo thickness
Transducer response
without air-gap
Transducer performance under miniaturization
• Increase in resonant frequency with reduction in size
0 0.5 1 1.5 2 2.5 3 3.5 4
x 104
0
50
100
150
200
250
Frequency (Hz)
SP
L (
dB
) re
1uP
a @
1m
Scale factor = 0.25
Scale factor = 0.50
Scale factor = 0.75
Scale factor = 1.00
Scaling effect on sensors
without an air gap
Conclusions
• We have successfully modeled and simulated in Comsol the performance of a Tonpilz transducer
• Pressure acoustics & piezoelectric phenomena have been explored as a function of sizing and materials
• Key features such as resonant frequency, tunability, beam-pattern and scalability have been investigated
• Factors influencing the value of resonant frequency are:
– Head mass (Presence & absence of air gap)
– Head Diameter
– Size of transducer
– Voltage across piezo ceramics
Thank You!!