User Manual

Page 47
Figure 56. Multidirectional Sensor
Figure 57. Shielding piezo film
"New Jersey barriers", the modern
concrete barriers that separate opposing
lanes on highways, introduce problems
for multilane sensing of four lane
highways. Piezo cable can solve this
problem with a single sensor that has
opposite polarities corresponding to
each lane. Vehicles crossing the near
lane produce a signal of opposite sign
from vehicles in the far lane (Figure 56).
This ability to provide lane activity in a
single sensor is a significant
development.
Traffic sensors can monitor vehicle
speed, count axles, weigh vehicles,
provide direction, and vehicle
classification. Recently, these sensors have also proven valuable on airport taxiways. From the output,
one can discern the ground speed of an aircraft (time lag between two sensors), its direction, weight
(fueled), number of axles, and the span of the aircraft (determined from the speed and the known fixed
distance between sensors). This information can be used to classify the aircraft and provides taxiway
traffic control and safety information at airports.
VIBRATION SENSING
One of the first applications for piezo film was as an acoustic pickup for a violin. Later, piezo film was
introduced for a line of acoustic guitars as a saddle-mounted bridge pickup, mounted in the bridge. The
very high fidelity of the pickup led the way to a family of vibration sensing and accelerometer
applications.
Music Pickups
Piezo film is used today in several guitar pickup designs; one is a thick film, compressive (under the
saddle) design; another is a low cost accelerometer, while another is an after market pickup design that
is taped to the instrument. Because of the low Q of the material, these transducers do not have the self-
resonance of hard ceramic
pickups. Shielding can be
achieved by a foldover design
as shown in Figure 57. The
hot side is the slightly
narrower electrode on the
inside of the fold. The
foldover technique provides a
more sensitive pickup than
alternative shielding methods
because the shield is formed
by piezoelectric material.
Conventional shielding
laminates can be easily
fabricated by a multilayer
laminate of piezo film,
adhesive and shielding foil.