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!!