User Manual

Page 30
Dynamic Range
Piezo film has a vast dynamic range. The sensor has been used to detect the impact of high speed particles
in space having a mass of 10
-12
grams, and at the other extreme, measures shock waves at 300,000
atmospheres produced during weapons testing. A recent study was conducted to determine the maximum
output energy of a 52µm thick film, having an area of 155.5 mm x 18.5 mm. The film was subjected to
approximately 350 MPa (in the stretch or "n = 1" direction) without failure. The charge generated was
found to be very linear, with the following measurements made at maximum applied stress:
Maximum Charge Observed: 20µC, giving 6.95 mC/m
2
Maximum Voltage Observed: 1600 V, giving 30.8 x 10
6
V/m
Maximum Energy Converted: 30.9 mJ, giving 207 kJ/m
3
Later experiments showed that about 10% of the above energy levels can be sustained for long periods of
time without measurable damage to the piezo film element.
Electrical to Mechanical
When a voltage is applied to a sheet of piezo film, it causes the film to change dimensions due to the
attraction or repulsion of internal dipoles to the applied field.. With one voltage polarity is applied, the piezo
film becomes thinner, longer and wider. The opposite polarity causes the film to contract in length and
width and become thicker. An ac voltage causes the film to "vibrate".
The amount of deformation is given by the piezoelectric "d
3n
" constant:
for length change
∆l  ld
31
V
/
t
where
= change in film length in meters
∆ l
l = original film length in meters
d
31
= piezoelectric coefficient for length ("n=1" direction) change in meters per volt
V = applied voltage across the thickness (t)
for width change
∆w  wd
32
V/t
where
d
32
= piezoelectric coefficient for width ("n=2" direction) change
for thickness change
∆t  td
33
V/t  d
33
V
where
d
33
= piezoelectric coefficient for thickness ("n=3" direction) change