In piezoelectric stress sensors, a piezoelectric effect is applied. This effect occurs when an electric charge Q is created in the dielectric crystal, or when a mechanical force (e.g., stress, pressure) is applied to the crystal. Such stress causes a mechanical deformation of the crystal. The electric voltage V is proportional
to the charge: V = Q/C, where C is the electrical capacity of the sensor with metallic plates, and the dielectric crystal is an insulator between them. The sensor is an electric condensator with capacity C [5, 6]. A piezoelectric sensor can be modeled as a voltage generator with high internal resistance. Amplification of voltages from this sensor requires an amplifier with very high input resistance, for example amplifiers with MOSFET transistors at their input. Another option is the use of a charge amplifier: an operational amplifier with a capacitor in the feedback path. The advantage of this approach is a transfer that is independent of cable impedance and input impedance of the amplifier. Quartz, Rochelle salt, and certain ceramic materials are used as dielectric crystals in piezoelectric gauges. Piezoelectric gauges are used only for dynamic measurements of pressure. Piezoelectric microphones are a similar application. Be- cause of resistance leakage between two terminals of a piezoelectric gauge, meas- urements of static pressure are not carried out. The electric condensator Cs (sensor) is discharged by the leakage resistance, so the voltage caused by static pressure is not static itself. The upper limit of measurement by piezoelectric gauges is 30 MPa. The frequency range is from 1 Hz to approximately 50 kHz. Piezoelectric sensors are also applied to measure acceleration a. In an acceler- ometer sensor, a known mass m is attached to a piezoelectric crystal. As the sensor moves, the mass creates a force (Fy = m · a) acting on the piezoelectric crystal. The force Fy generates an electrical charge Q and voltage Va, proportional to the acceleration value a. Of course, to measure the voltage Va, we need a charge amplifier with a very high input impedance. There are passive and active accelerometer sensors. Passive accelerometers generate a small electrical voltage that must be amplified by a charge amplifier. An active accelerometer is more complicated. It is composed of an accelerator sensor, a charge amplifier, a source of an excitation constant current, and a lowpass filter. In fact, the active accelerator is a signal conditioner with an accelerator sensor. The well-known inverse piezoelectric effect consists of mechanically deforming (e.g., to the extension) the piezoelectric crystal, on which is applied the electric voltage. For example, the piezoelectric actuator manufactured by Physik Instrumente becomes 2 µm longer with the applied voltage of 1,500V.
