User Manual

Page 27
Figure 27a. Sensitivity vs. load Figure 27b. Piezo cable linearity
Cable Typical Properties
The output sensitivity of piezo cable in response to increasing impact load is shown in Figure 27a. The
linearity in output for increasing force as shown in Figure 27b is typical of all piezo cable gages.
PIEZOELECTRIC BASICS
Mechanical to Electrical
Like water from a sponge, piezoelectric materials generate charge when squeezed. The amplitude and
frequency of the signal is directly proportional to the mechanical deformation of the piezoelectric material.
The resulting deformation causes a change in the surface charge density of the material so that a voltage
appears between the electroded surfaces. When the force is reversed, the output voltage is of opposite
polarity. A reciprocating force thus results in an alternating output voltage.
Piezo film, like all piezoelectric materials, is a dynamic material that develops an electrical charge
proportional to a change in mechanical stress. Piezoelectric materials are not suitable static measurements
(true dc) due to their internal resistance. The electrical charges developed by piezo film decay with a time
constant that is determined by the dielectric constant and the internal resistance of the film, as well as the
input impedance of the interface electronics to which the film is connected. Practically speaking, the lowest
frequency measurable with piezo film is in the order of 0.001Hz. There are methods to achieve true dc
response, but these require using the piezo film as both an actuator and sensor, monitoring change in the
actuation resulting from the dc event.
The fundamental piezoelectric coefficients for charge or voltage predict, for small stress (or strain) levels,
the charge density (charge per unit area) or voltage field (voltage per unit thickness) developed by the piezo
polymer.