Appendix C: Piezo Engineering Tutorial; The Direct And Inverse Piezoelectric Effect; Piezo Actuator Materials; Rohs Exemption - Aerotech QNP3 Series Hardware Manual

QNP3 Hardware Manual

Appendix C: Piezo Engineering Tutorial

C.1 The Direct and Inverse Piezoelectric Effect
In 1880, while performing experiments with tourmaline, quartz, topaz, cane sugar and Rochelle salt crystals,
Pierre and Jacques Curie discovered that when mechanical stress was applied to a crystal, faint electric
charges developed on the surface of that crystal. The prefix "piezo" comes from the Greek piezein, which
means to squeeze or press. As a result, piezoelectricity is electrical charge that is produced on certain
materials when that material is subjected to an applied mechanical stress or pressure. This is known as the
direct piezoelectric effect.
The converse or inverse piezoelectric effect, or the application of an electric field to induce strain, was
discovered using thermodynamic principles in 1881 by Gabriel Lippmann. It is the inverse piezoelectric
effect that enables piezoelectric materials to be used in positioning applications.
C.2 Piezo Actuator Materials
Although many materials exhibit the inverse piezoelectric effect, the most popular and widely applicable
piezoelectric material by far is PZT, or lead-zirconium-titanate. The term PZT is generally used to refer to a
wide range of ceramics which display different properties depending on the grain size and mixture ratios of
their main raw materials: lead, zirconium and titanium. The properties of the ceramic can also be
manipulated by adding dopants and making adjustments to the manufacturing process. The recipes for
particular materials are usually proprietary and vary between suppliers.
C.2.1 RoHS Exemption
Despite the presence of lead as a doping material, PZTs are exempt from RoHS directive 2002/95/EC due to
a lack of a suitable replacement material. Although efforts are underway to develop alternative materials, no
suitable alternative is expected in the field for years to come.
C.3 Properties of Piezo Actuators
C.3.1 Displacement Performance
The response of a piezoelectric material to an applied stress or applied electric field depends on the direction
of application relative to the polarization direction. Because of this, most electrical and mechanical
properties that describe piezo materials are direction dependent, as well.
The inverse piezoelectric effect can be described mathematically as:
Equation 1
where, X is strain (m/m), d
j
the applied electric field (V/m). The subscripts i and j represent the strain direction and applied electric field
direction, respectively. Electric field is a voltage across a distance, so large electric fields can be generated
with small voltages if the charge separation distance is very small.
As a general rule, the strain (X
applied electric fields on the order of 2 kV/mm. For example, a 20 mm long active-length PZT actuator will
generate approximately 20-30 μm of maximum displacement. One can easily see that to generate 250 μm, a
PZT stack would be approximately 170 to 250 mm long. Therefore, most piezo flexure stages with >50 μm of
travel use lever amplification to achieve longer travels in a more compact package size. A tradeoff is made
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is the piezoelectric charge coefficient (m/V) and is a material property, and E
ij
) for most PZT materials found on the market is around 0.1 to 0.15% for
j
Appendix C
Piezo Engineering Tutorial
is
i
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