Piezoelectric Sensorics

A measurement is the experimental determination of the magnitude of a physical quantity by comparing it with the corresponding unit of measurement. However, most measurements are made indirectly by exploiting physical effects in suitable measuring devices such as transducers or sensors. Most often the quantity to be measured is converted by a sensor into an output quantity of different nature that then can be amplified and recorded as required. A particularly convenient type of output signal is an electric quantity, and one speaks of an electric measuring method.

Piezoelectricity is understood as a linear electromechanical interaction between the mechanical and the electrical state in crystals without a center of symmetry. The direct piezoelectric effect is present when a mechanical deformation of the piezoelectric material produces a proportional change in the electric polarization of that material, i.e. electric charge appears on certain opposite faces of the piezoelectric material when it is mechanically loaded. The converse piezoelectric effect means that mechanical stress proportional to an acting external electric field is induced in the piezoelectric material, i.e. the material is deformed when an electric voltage is applied.

Out of the increasing number of piezoelectric materials only a rather restricted number have been proven suitable for transduction elements in piezoelectric sensors. Basically, natural and synthetic single crystals, piezoelectric ceramics, textures and thin films can be used.

The field of measuring — like all types of technical activities — calls for well and clearly defined terms. Only if specifications are given in an universally understandable way, leaving no ambiguities whatsoever about their meanings and implications can misunderstandings and misinterpretations be avoided.

Piezoelectric sensors are characterized by having a transduction element made of a piezoelectric material. They are called active sensors because, in principle, no external energy is needed to obtain an output signal.

The SI (Système International) unit of measurement for force F is the „Newton“ (N), which is one of the 7 base units of that system. In practice, large forces are usually expressed in kN (kilonewton) or MN (meganewton) while for small forces, the mN (millinewton) is used.

The quantity “strain” — in this context meant to be “linear strain” — in an elastic body is defined as the ratio ε=Δl/l₀, where Δl is the increase in length and /0 is the length in a reference state to be specified. The unit for strain in the SI (Système International) is “one” i.e. 1 ε= 1 = 1 m/m. In practice, the “unit” for strain is called “strain” and the symbol e is used. Usually, strain is in the order of um/m, i.e. 10^-6, and therefore, the unit “µε” (microstrain) is most commonly used.

The quantity “pressure” in a fluid (a liquid or a gas) is defined as “force divided by area”. The unit for pressure in the SI (Système International) is the “Pa” (Pascal), defined as 1Pa=1N/m². Unfortunately, lPa is a very small pressure compared with the barometric pressure, which is about lOOkPa. Pressure encountered in most practical applications, especially in hydraulics and pneumatics, is always related to barometric (atmospheric) pressure. Because the unit “Pa” is not very convenient to use in these fields, the coherent unit “bar”, defined as 1bar= 1OOkPa (exactly) is widely used and also accepted by IS031-3:1992(E), item 3-15.1, for use in these fields. Therefore, in this book, the “bar” is used as the working unit for pressure.

The quantity “acceleration” is denned as the time rate of change in velocity and represented by the symbol a. The unit for acceleration in the SI (Système International) is m/s². In general engineering practice, especially for measuring vibration, often the acceleration of free fall g is preferred as a working unit. Because the acceleration of free fall (also called “acceleration due to gravity”) slightly varies with the geographical location, the “standard acceleration of free fall”, defined as g_n=9,80665m/s² (exactly), is generally used. The working unit g must not be combined with decimal prefixes.

The quantity “acoustic emission” can only be defined in a qualitative way. An excellent formal definition can be found in [Miller and Mclntire 1987]: “Acoustic emission is the class of phenomena where transient elastic waves are generated by the rapid release of energy from localized sources within a material, or the transient waves so generated”. Acoustic emission (often just called “AE”) can be looked at as a vibration with an extremely small amplitude (in the order of nm) and of very high frequency (in the order of 10 kHz to over 1 MHz) of particles in a solid material. It can also be described as a sound wave originating in and traveling through a solid (the term “structure-born sound” to describe AE is also employed by some authors). The units usually used for measuring vibration or acceleration (see 9.1) are sometimes used in AE work, when acceleration sensors with a high sensitivity are used to capture AE in certain applications. Units used in measuring (acoustic or sound) pressure (see 8.1) are also suitable to quantify AE, especially if the sensor — a pressure sensor of high sensitivity, such as a hydrophone — is coupled by a fluid to the test object or used in underwater investigations.

The output of piezoelectric sensors is electric charge. The unit for electric charge in the SI (Système International) is the “Coulomb” (C), defined as 1 C= 1 A • s. The quantity of electric charge yielded by piezoelectric sensors is usually in the order of pC only, which is the reason why in practice the “pC” has become the commonly accepted working unit.