handbook of modern sensors

320 8 Velocity and Acceleration
(A)
(B)
Fig. 8.15. Piezoelectric cable sensors: (A) construction of Vibracoax; (B) polymer film as a
voltage generating component. (Adapted from [13].)
in turboshaft aircraft engines. Other applications include the detection of insects in
silos and automobile traffic analysis. In these applications, the cables are buried in the
highway pavement, positioned perpendicular to the traffic. When properly installed,
they last for at least 5 years [12]. The sensors are designed to be sensitive primarily
to vertical forces. A piezoelectric cable consists of a solid insulated copper sheath
having a 3-mm outer diameter, piezoelectric ceramic powder, and an inner copper
core (Fig. 8.15A). The powder is tightly compressed between the outer sheath and
the core. Usually, the cable is welded at one end and connected to a 50 extension
cable at the other end.
Another method of fabrication of the piezoelectric cables is to use a polyvinylidene
fluoride (PVDF) polymer film as a component in the cable insulation (Fig. 8.15B).
The PVDF can be made piezoelectric, thus giving the cable sensing properties. When
a mechanical force is applied to the cable, the piezoelectric film is stressed, which
results in the development of electric charges of the opposite polarities on it surfaces.
The inner copper wire and the braided sheath serve as charge pickup electrodes.
For the cable to possess piezoelectric properties, its sensing component (the ceramic
powder or polymer film) must be poled during the manufacturing process; that
is, the cable is warmed up to near the Curie temperature, and subjected to high voltage
to orient ceramic dipoles in the powder or polymer dipoles in the film, then cooled
down while the high voltage is maintained. When the cable sensor is installed in the
pavement (Fig. 8.16), its response should be calibrated, because the shape of the signal
and its amplitude depend not only on the properties of the cable but also on the type
of the pavement and subgrade. The electrical output is proportional to the stress imparted
to the cable. The long, thin piezoelectric insulating layer provides a relatively
low output impedance (600 pF/m), unusual for a piezoelectric device. The dynamic
range of the cable is substantial (>200 dB), sensing distant, small-amplitude vibrations
caused by rain or hail, yet responding linearly to the impacts of heavy trucks.
The cables have withstood pressures of 100 MPa. The typical operating temperature
range is −40 ◦ Cto+125 ◦ C. Table 8.2 lists typical properties for piezo cable.