Broadband EMFi Ultrasonic Transducer
for Bat Research
A. Streicher, M. Kaltenbacher, R. Lerch, H.Peremans∗
Department of Sensor Technology, Friedrich-Alexander University Erlangen-Nuremberg, Germany
∗
Antwerp-University Faculty St. Ignatius, Belgium
Email: [email protected]
Abstract— By utilizing the EMFi material, ultrasonic transmitters with a diameter of 1.5 cm were developed for emitting
a chirp signal with a sound pressure level up to 90 dB at
a distance of 1m for the whole frequency range of 20-200
kHz. With the same material, a broadband ultrasonic receiver
with a sensitivity of 500 µV /P a and a low equivalent acoustic
noise level of 45 dB was set up. For an optimization of the
transmitter and the receiver we need a deeper understanding
of the physical behavior of the polymer material. Therefore, we
applied a 3D finite element simulation by using a piezoelectric
material model for a macroscopic description of the EMFi
material. However, vibration measurements of the transducer
surface show a nonlinear inhomogeneous vibration behavior
at and above resonance frequency. One reason for this is the
inhomogeneous structure of the foil. Inside the polymer film,
the number of cavities as well as their size strongly varies.
Because the resonance frequency of each point of the surface
depends on the average cavity size at this point, the whole surface
vibrates inhomogeneously. Therefore, the EMFi material cannot
be described with a homogeneous piezoelectric material model
and we developed a more complex microscopic model for the
precise numerical simulation. To solve this problem we computed
the electrostatic and mechanical partial differential equation
coupled by the electrostatic forces (Coulomb forces) including
the complex geometric structure of the cellular ferroelectric film.
To investigate the influence of the geometric structure on the
vibration behavior, models with different void shapes and sizes
have been taken into account.
Hence, a new transducer material is required to build up
broadband ultrasonic transducers with a good adaptation to air.
The most promising transducer material for this is a cellular
polymer film called electro mechanical film (EMFi). EMFi is
a polypropylene film, thickness 30−70 µm, that has a cellular
structure, which results from its manufacturing process [1]. By
using corona discharges, the voids inside the polymer material
are polarized. This results in the formation of macroscopic
dipoles. The cellular structure and the macroscopic dipoles
result in a relatively high piezoelectric constant d33 up to
800 pC/N. A resonance frequency at 300 kHz, a broadband
response covering the frequency range of interest (20-200 kHz)
and excellent impedance matching to air make it possible to
construct efficient broadband transducers.
The acoustic and vibration behavior of different transducer
assemblies will be described in more detail in this paper.
Further on, for a better understanding of the physical behavior
of the EMFi we applied the finite element method to simulate
the contraction and the vibration behavior of the material. In
our paper, we will discuss the simulation results for various
parameters of the transducer material as well as corresponding
measurement results.
I. I NTRODUCTION
By utilizing the EMFi material, different transducers were
assembled and their performance measured. At first, thickness
resonators were built with EMFi (thickness = 70 µm, d33 =
200 µm) of different circle area sizes ranging from a diameter
of 1 cm up to a diameter of 1.5 cm. The polymer was fixed
on one side using a glue making the piezo material oscillate
in the thickness mode. To contact the top electrode of the
polymer, very flexible and thin bond wires were used (Fig. 1
a)). In addition to the single foil transducers we set up different
The goal of the project CIRCE (Chiroptera Inspiered
Robotic CEphaloid) was the reproduction of the echolocation
system of bats by constructing a bionic bat head that can
then be used to investigate how the world is actively explored
by bats. This bionic bat head must be of similar size to a
real bat head to reproduce the relevant physics, i.e. wave
phenomena, and must be capable of generating/processing
realistic pinnae movements. The bionic head will be used
to gain more insight into neural sensory-data encoding from
using it in echolocation task routinely executed by bats. In
the field of bat research and robot navigation, broadband air
ultrasonic transmitters and receivers are of great importance.
In order to investigate the sound as produced by a wide variety
of bat species, the transducers have to operate with a frequency
range of 20 - 200 kHz and have to match sensitivity (receiver)
as well as transmit efficiency (transmitter) of living bats. These
requirements can not be achieved by commercially available
in-air ultrasonic transducers due to their small bandwidth.
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II. U LTRASONIC T RANSDUCER S ETUP
Fig. 1. (a) Schematic representation of an one foil ultrasonic emitter. (b)
Schematic representation of a stack actuators .
stack actuators. Here, two EMFi foils with opposite directions
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of polarization are stuck together and are jointly excited. (Fig.
1 b)) By assembling the stack actuators we expect, that on the
one hand the resonance frequency is shifted down and that
on the other hand the total deflection of the active surface
and, therewith, the produced sound pressure level (SPL) is
increasing by using such a stack transducer.
A. Vibration Measurement
In order to get the frequency response of the various
transducer setups we drive them with a broadband (f =
20 − 500kHz) chirp signal. For the hole frequency range
the displacement of 100 points on the transducer surface
was measured by using a ”Laser Scanning Vibrometer” .
Figure 2 shows the average displacement of 100 measuring
points of a single foil and a stack actuator. For the singe
Fig. 2.
Displacement of an single foil and an stack emitter.
foil transducer we get a very broad resonance peak from
200 kHz up to 300 kHz. The resonance frequency of the
stack actuator is shifted down to 75 kHz. Below resonance
frequency almost all points of the surface oscillate with the
same phase and amplitude (Fig. 3 a). But in the frequency band
between 200 and 300 kHz the vibration measurements show
an inhomogeneous vibration behavior. Amplitude and phase
of the displacement are different for each measured surface
point (Fig. 3 b). One reason for this is the inhomogeneous
Fig. 4. Resonance frequency (top) and maximum Displacement(bottom) as
function of the y postion of the measuring points.
frequency varied from 250 kHz up to 300 kHz depending
on the geometrical position of the measuring point. Therefore
we use a single foil transducer as emitter for the artifical bat
head.
B. Acoustic Measurement of the emitter
The SPL of the EMFi transducers was measured with a
Brüel & Kjaer 1/8 inch condenser microphone. In order to
avoid the build-up of a standing wave field between microphone and transducer, the foil was exited by a sine burst
voltage signal lasting 5 periods with a (maximum) amplitude
of U = 600 V pp. The driving voltage is limited to a maximum
voltage of 1000 V pp, because higher voltage would result in
discharges occurring at the edges of the polymer. Points of
measurements were placed on the principal axis at a distance
of 1 m. Fig. 5 presents the result of this measurements. It
Fig. 5.
Fig. 3.
Displacement and phase of the transducer surface (a): Below
resonance frequency f=200 kHz(b): at resonance frequency f= 250 kHz
structure of the foil. Inside the polymer film the number of
cavities as well as their size strongly varies. Because the
resonance frequency of each point of the surface depends on
the average cavity size below this point, the whole surface
vibrates inhomogeneously near the resonance frequency. Figure 4 shows the resonance frequency and the displacement
of 185 different points along the diameter of the transducer
surface as function of the postion. The measured resonance
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Sound pressure level of different emitter designs.
can be seen, that similar to the vibration measurement the
resonance frequency is shifted down to about 75 kHz for the
two layer stack. Due to the shift of the resonance frequency
to lower frequencies a gain of about 5 dB (compared to the
single foil transducer) is obtained within the frequency range
40 to 90 kHz. Beyond the resonance peak the influence of the
stack design decreases, so that the two curves approach each
other. It can also be seen, that due to the frequency spectrum
of the Brüel & Kjaer 1/8 inch condenser microphone the SPL
is decreasing beyond 150 kHz. By using a special glue to fix
the foil on a printed circuit board (PCB) we get an single foil
transducer with a SPL of 80 − 100 dB for a frequency range
between 20 − 200 kHz in a distance of 1 m.
C. Ultrasonic Receiver
The reciprocal nature of the transduction mechanism allows
to use the same material for designing broadband receivers.
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In order to minimise the size of the receiver, a low noise
preamplifier circuit is implemented with SMD technology on
the bottom side of a multi layer PCB shown in Fig.6. The
Fig. 6.
resonance frequency, an complex 3D simulation modell is
needed. To simplify the simulation problem, our cavity model
of investigation is based on a simplified lens shape. Fig. 8
shows the 3D model of one cavity with the diameter dg and a
maximum hight of hg . By arranging additional cavity around
and above the basis element and subtracting all cavity from
a larger polymer cylinder, we get a three dimensional cellular
structure of the film.
Picture of ultrasonic receiver.
EMFi transducer material with a diameter if 1 cm was fixed
on an electrode pad on the top of the PCB with paste. To
calibrate the receivers the SPL, which arises at the location
of the receiver, was measured with a 1/8 inch Brüel &
Kjaer microphon mounted right to the receiver. For the noise
measurement, the whole receiver was covered in aluminum
foil to minimise the influence of electromagnetic disturbance.
The noise level was measured by a sensitive spectrum analyser
at a frequency range between 20-200 kHz. With an equivalent
acoustic noise level of < 45 dB and a sensitivity of about 0.5
mV/Pa, the receiver meets the required specifications.
Fig. 8. 3D Model of one single gas void. hg = cavity hight and dg is the
diameter of the cavity
In a next step we preload the surface between the air voids
and the polymer with a positive and negative electrical charge
as shown in Fig. 9. By preloading not all nodes of the surface
III. F INITE E LEMENT S IMULATION
For an optimization of the transmitter and the receiver we
need a deeper understanding of the physical behavior of the
polymer material. In a first step we, therfore, applied a finite
element simulation by using a piezoelectric material model for
a macroscopic description of the EMFi material. But, because
of the inhomogeneous vibration behavior of the EMFi, it can
not be described with a homogeneous piezoelectric material
model. To solve these problem we developed a more complex
microscopic model for the finite element simulation.
A. Model of the EMFi
As can be seen in Fig. 7 a the film consists of randomly
distributed gas cavities in the polypropylene material [1].
Inside the film the number of cavities as well as their size
strongly varies, but the amount of gas and polypropylene
in the cross section remains roughly constant, when looked
through the film from any point on the surface. To calculate
Fig. 7. (a) Scanning electron micrograph of the cross section of a charged
piezoelectric polymer foam. (b) Schematic representation of the nonsymmetric
charge distribution in the foam. [1]
the displacement of the foil for frequencies much smaller
than the resonance frequency, an 2D Finite Element Modell
is sufficient [4]. But to simulate the vibration behavior near
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Fig. 9. Electrical Model of one single gas void. εp = relative dielectric
constant of polypropylene and εg is the vacuum permativity
and concentrating the charge to the center of the surface of the
gas void we get an inhomogeneous charge density σi and −σi
along the surface. This leads to an electrical field Eg inside the
gas voids and Ep inside the polymer layers and an mechanical
force Fg and Fp depending on the electrostatic force.
S IMULATION OF THE EMF I FOIL
At a first static step, we calculate the displacement of the foil
caused by the charge density along the surfaces of the voids.
Therefore, we compute the electrostatic and mechanical partial
differential equation coupled by the electrostatic forces. For
the first step no driving voltage is plugged to the electrodes.
In a next step, we compute the dynamic response to an applied
driving voltage U taking into account the static solution.
The foil is oscillating around the static displacement. To
optimize the operating performance of the EMFi - material,
we are investigating the influence of design parameters on
the piezoelectric behavior and the frequency response of the
polymer film. In a first parameter study, we are changing the
charge density |σi | and compute the displacement of the model
as function of the driving voltage U . Figure 10 shows the displacement dstatic of the static and the maximum displacement
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dmax of the transient simulation for different |σi |. It can be
seen, that for a low charge density the displacement of the foil
is dominated by the driving voltage U . Whereas for a high
charge density the influence of the charge to the displacement
increase.
After the investigation of varying the charge density of the
−4
structure on the frequency response of models with different
void shapes and sizes. Therefore, we use a step function with
sharp edges as inputfunction for the top an bottom electrode
to get a broad bandwith. We get the step response for the
different models by computing a transient simulation with
1000 timesteps. Figure 12 shows the displacement as function
Dispalcement (m)
10
−6
10
−8
10
d
static
−10
10
dstatic+ddynamic
−12
10
ddynamic
−14
10
0
1
10
2
10
3
10
4
10
10
Normelized charge density σi / σ0
Fig. 12.
hg .
Frequency response for different cavity shapes and sizes dg and
Fig. 10. Mechanical Displacement of the film structure as a function of
applied driving Voltage and the normaliced charge density σi /σ0
material, we perform simulations to study the influence of
changing the void shape. Therefore, we change the proportion
of diameter dg to hight hg of the cavity. The simulation
results are compared with two measurements [3] using the
same material parameters as shown in Tab. I. Fig. 11 shows
TABLE I
P HYSICAL PARAMETER OF TWO EMF I FOILS
O01
HS01
Y (GPa)
yg (µm)
yp (µm)
yg /yp
εp
1.2
1.2
9
42
28
28
0.32
1.5
2.2
2.2
of frequency and different fractions dg /hg . It can be seen, that
the resonance frequency strongly depends on the cavity shape.
For bg > hg the stiffness of the material decreases and this
leads to a lower resonance frequency.
IV. C ONCLUSION
We presented a broadband ultrasonic emitter and receiver.
Both were realized with a new ferroelectric material called
EMFi, which combines a very high d33 constant and a
good adaption to air. With the EMFi material a broadband
ultrasonic receiver with a sensitivity of 500µV /P a and a low
equivalent acoustic noise level of 40 dB was built up. The
acoustic and vibration behaviour of different transducer set ups
were measured. The vibration measurements of the transducer
surface show an inhomogeneous vibration behavior at and
above resonance frequency. For a better understanding of the
vibration behavior, 3D models of the cellular material have
been developed and simulated by solving the electrostaticmechanical problem include the geometric structure of the
cellular polymer. We showed, that there is a strong dependency
between the geometrical parameter the displacement and the
frequency response of the material. For practical simulation of
the whole transducer behaviour the microscopic model is not
applicable due to computer resources. Therefore, we currently
investigate in the development of a macroscopic model for this
purpose.
R EFERENCES
Fig. 11. Mechanical Displacement of the film structure as a function of
applied driving Voltage and rate between dg and hg . (HS01-EMFi: dg /dh =
2.67)
the displacement of the material for different driving voltages
and different fractions dg /hg . It can be clearly seen, that the
displacement can be increased by increasing dg /hg up to an
optimum. If the cavity is too flat the thickness variation decreases. Finally, we investigate the influence of the geometric
0-7803-9383-X/05/$20.00 (c) 2005 IEEE
[1] S. Bauer, R. Gerhard-Multhaupt, G. M. Sessler Ferroelectrets: Soft
Electroactive Foams for Transducers,
Physics Today, pp. 37 - 43,
(Februay 2004)
[2] M. Paajanen, ElectroMechanical Film (EMFi) - a new multipurpose
electret material, Sensors and Actuators, vol. 84, pp. 95 - 102, 2000
[3] M. Paajanen, H. Vlinmki, J. Lekkala Modeling the sensor and actuator
operations of the ElectroMechanical Film EMFi,
10th International
Symposium on Electrets, pp. 735 - 738, 1999
[4] A. Streicher, R. Mueller, H. Peremans, M. Kaltenbacher, R. Lerch
Ultrasonic Transducer for a Biomimetic Sonar Sysem. In Proceedings
of the 2004 IEEE Ultrasonic Sympsoium, pages 1142-1145, 2004. 24.28.08.2004, Montreal, Canada.
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