Piezoelectric Force Transducer

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Piezoelectric force transducer

Physical measuring principle

The physical measuring principle behind this sensor technology is based on the experimental evidence of the relationship between mechanical stress and the electrical charge generated, which was discovered in 1880 by brothers Pierre (https://en.wikipedia.org/wiki/Pierre_Curie) and Jacques Curie (https://en.wikipedia.org/wiki/Jacques_Curie). This so-called direct piezoelectric effect (see also: piezo ceramic) was first demonstrated in tourmaline and quartz (SiO₂), although today numerous polycrystalline ceramics (BaTiO₃, PbTiO₃, PbZrO₃ or PZT) and polymers such as polyvinylidene difluoride (abbreviation: PVDF) are used in the construction of force transducers, pressure sensors and acceleration transducers [1]. In contrast to the piezoelectric effect, the pyroelectric effect involves a correlation between temperature and electrical charge, which is why thermal drift can occur in single-crystal or polycrystalline sensors used in technical applications, and this drift cannot be easily compensated for [2].

Fig. 1:

Scheme of the piezoelectric effect using the example of the trigonal structure of quartz, a) longitudinal, b) shear or thrust, and c) transverse according to [2]

Under load, the deformation of the crystal lattice in piezoelectric sensors causes a charge to occur, which is generated by the electric dipole moment of the displaced or distorted negative and positive lattice components. Depending on the type of stress applied, distinction is made between the longitudinal effect, the shear or thrust effect and the transverse piezoelectric effect (Fig. 1). The three types of associated sensors are produced by different types of cuts depending on the lattice structure, e.g. on the quartz crystal, resulting in piezo discs with the desired properties as shown in Fig. 1. The charge released depends only on the deformation or the force F generating the deformation of the respective crystal and not on the acceleration of the deformation or the geometric dimensions of the crystal discs, i.e. the decisive factor is the change in thickness or length, which is, however, very small. As a result, piezoelectric sensors exhibit very little deformation under load, i.e. they have high stiffness or low compliance (see also: specimen compliance). As a result, their resonance frequency is very high, which is generally very favourable for dynamic applications. However, the entire measuring chain (mass, amplifier) is decisive for the dynamic properties of the force sensor, especially its limit frequency. Metallic contact surfaces are applied to the upper and lower parts of the crystal discs, which serve as electrodes for tapping the charge. To increase the charge efficiency, several such crystal discs are mechanically coupled in series and electrically connected in parallel, usually using charge amplifiers. The charge Q generated in this way is calculated as follows (Fig. 2):

$Q=d\ F_{N}\ n\!$

(1)

with

Q		charge
d		piezoelectric coefficient (dQuartz = -2.3 pC/N)
Fn		normal force
n		number of crystal discs

Fig. 2:

Increase in charge efficiency by cascading the discs according to [2]

Piezo sensors

As already noted, piezoelectric force sensors exhibit drift in principle, even when the measuring chain is operating stably, because the charges in the sensors dissipate over time. This value is independent of the measured force, which means that the relative measurement error is particularly unfavourable when small forces are to be measured over a longer period of time (static or quasi-static). Especially when used in tensile applications (see: tensile test), these sensors, which are based on the longitudinal piezoelectric effect, must operate with a mechanical preload that mechanically represents a force shunt. Shear-sensitive piezo sensors, on the other hand, are used, for example, as acceleration or vibration sensors in accordance with NEWTON's second law, or as subcomponents in multi-axial force sensors.

Types of load cells

These load cells, some of which are also referred to as measuring discs or measuring washer discs, are manufactured in a compact or ring-shaped design and can then be used, for example, as force sensors in high-speed testing machines (see: high-speed tensile test) or as ring load cells in instrumented pendulum impact testers for tensile stress (Fig. 3) (see also: impact loading pendulum impact tester). The ring-shaped cover or pressure plate is used to achieve the required preload condition. Due to their high resonance frequency and wide frequency range, these load cells can also be used for a wide range of measurement tasks as a result of their high signal dynamics.

Fig. 3:

Schematic diagram and construction types of piezoelectric load cells

See also

• Piezoelectric ceramic
• Piezoelectric ceramic transducer
• Ultrasonic composite sensors
• Ultrasonic immersion bath sensors
• Servo-hydraulic testing machine

References

[1]	Schrüfer, E.: Elektrische Messtechnik. Carl Hanser, Munich (2007); 9th Edition (ISBN 978-3-446-40904-0)
[2]	Laible, M., Müller, R. K., Bill, B., Gehrke, K.: Mechanische Größen elektrisch gemessen – Grundlagen und Beispiele zur technischen Ausführung. Expert Verlag, Renningen (2009); 7th Edition (ISBN 978-3-8169-2892-8)