Piezoelectric transducers

Piezoelectricity is the other transduction technology used in transducers intended to measure time-varying forces, pressures and accelerations. As the technology of piezoresistive materials has developed since the 1960s, a similar effort in the years 1920 to 1950 was accomplished in piezoelectric technology. The crystals belonging to those classes that do not have a center of symmetry of the structure of the lattice are piezoactive, i. e. the material develops a varying electric potential difference when subjected to a change in stress (pressure) and, conversely, undergoes a variable deformation if subjected to the action of a variable voltage (oscillator). Twenty of the 32 crystal classes lack this symmetry, and all crystals belonging to these classes, except one, exhibit piezoelectric characteristics.

The resulting charge is distributed on certain crystal faces in a manner determined by the symmetry of the crystal and the nature of the efforts applied. Some materials, such as tourmaline, lithium sulfate and barium titanate, are sensitive to hydrostatic pressure; other materials, such as quartz and Rochelle salt, develop a charge only under the unilateral application of force, the charge is spread through the influence of a normal component of the stress, such as quartz, or tangential stress, such as Rochelle salt. Some ferroelectric ceramic materials can be made artificially piezoelectric.

The piezoelectric constant of a material expresses the charge generated per unit of applied force or the deflection per unit of electrical voltage applied. The constant is typically given in tensor notation, such as d33, with the first subscript identifying electrical direction and the second subscript identifying mechanical direction. Typical units are Coulombs/ Newton or meters/volt.

A piezoelectric transducer is effectively a capacitor that produces a charge proportional to the force applied to it, the load can be applied in a unilateral direction by a piston or a diaphragm or the material may be subject to a hydrostatic load. Unlike piezoresistive transducers, piezoelectric probes do not have response to zero frequency: they are therefore not suitable for measuring stationary or slowly varying pressures and can only be used for the measurement of fast transient ones. This shortcoming can be avoided by making transducers in which the pressure, even when constant, can be measured indirectly by measuring the frequency of the oscillation of the crystal, which is a function of the pressure.

As the crystals grow polarization due to the movement of the “center of gravity” of positive and negative charges under the influence of elastic deformations, we expect that the expansion and contraction under the influence of temperature changes will produce similar effects. This type of pyroelectric effect is frequently a source of interference, especially when the measured pressure transients are accompanied by violent fluctuations in temperature. Hydrostatically sensitive materials such as tourmaline are especially susceptible to this interference and are much more sensitive than non-hydrostatic crystals, such as quartz. In the latter case, the pyroelectric charge will not grow at all if the temperature changes uniformly through the crystal.

Of course, it is impossible to precisely define the limits of application of piezoelectric pressure transducers but to give a rough idea of the magnitude of the practical limits of pressure and frequency, it can be said that these sensors have been used to measure transient changes in pressure from 10-2 to 103 atm. It is relatively easy to measure transients lasting up to one-fifth of a second, but it becomes difficult to measure slower phenomena because of pyroelectric interference. The high frequency response is limited, in most applications, by the time taken by the pressure impulse to propagate in the transducer rather than by the natural frequency of the crystal. So the upper limit is determined by the size of the transducers (the smallest transducers are 1 mm in diameter). Their usable frequency response can extend to 300 kHz with an accuracy of 0.01% FS.