User Manual

Page 34
Figure 32. Folded piezo film actuator
Figure 33. Piezo film ultrasound transducers
Compared to mechanical or piezo ceramic actuators, multilayer
piezo film actuators have fewer ringing problems due to their
lower Q. Applications of multilayer actuators are
micropositioners for industrial equipment, acoustic wave
generators and ink jet printers.
Ultrasonic Actuators
Ultrasonic actuators, as discussed in this section, exclude very
high frequency (> 1 MHz) transmitter applications. The use of
piezo film in these very high frequency applications, like medical
ultrasound imaging and nondestructive testing, use thickness
mode operation, d
33
.This section deals with low frequency
ultrasound (20-100 KHz) where the piezo film can be used in
the length change (d
31
) mode.
The advantage of piezo film in low frequency ultrasound can be
found from the flexibility of the material. Piezo film can be
easily curved or formed to make circular transducers as shown
in Figure 33. The beam pattern is determined by the number of
half circular elements and their diameter. The operating
frequency is determined by the diameter of the half circular
elements. Note that the difference between Figures 33(a) and
33(b) is their number of active elements and diameters. To
widen the beam coverage, the number of active elements should
be reduced. With a cylindrical transducer, a 360
o
beam pattern is
obtained.
In ultrasound applications, a narrow beam with minimum side
lobes is required for remote distance measurements. On the
other hand, a wide beam, as wide as 180
o
or more, is required
for applications like automobile rear bumper proximity sensing.
Figure 33 shows design configurations for both narrow beam
and wide beam ultrasound transducers. The applications for
piezo film in through-air ultrasonic actuators include distance
ranging for air pen, air mouse, white board digitizer, collision
avoidance, physical security systems, air flow velocity (doppler)
sensors, and inter-object communications. Similar constructions
can be produced for underwater or fluid sensing, including flow
sensors, level sensors, and communications.
PYROELECTRIC BASICS
Piezoelectric polymers, such as PVDF and its copolymers of VF
2
/VF
3
, are also pyroelectric. Pyroelectric
sensor materials are normally dielectric materials with a temperature-dependent dipole moment. As these
materials absorb thermal energy, they expand or contract, thereby inducing secondary piezoelectric signals.
As piezo film is heated, the dipoles within the film exhibit random motion by thermal agitation. This causes
a reduction in the average polarization of the film, generating a charge build up on the film surfaces. The
output current is proportional to the rate of temperature change (∆T). The amount of electrical charge
produced per degree of temperature increase (or decrease) is described by the pyroelectric charge
coefficient, ρ.