Creep And Drift; Force And Displacement; Force Versus Displacement Characteristics - Aerotech QNP3 Series Hardware Manual

Piezo Engineering Tutorial
C.3.3 Creep and Drift
The response time for a piezo ceramic subjected to an electric field is much faster than the reorientation time
of the individual dipoles. This phenomenon causes undesirable behavior in open-loop position control. When
an electric field is applied, the piezo stack will make a corresponding displacement almost instantaneously.
If the field is then held constant, the piezo stack will continue to move slowly as the dipoles reorient, a
phenomenon known as creep. It can take many minutes or even hours to reach steady state, with strain
increasing by as much as 1% to 5% past the initial strained position. There is a similar effect called zero
point drift. When an electric field is removed, the dipoles will gradually relax and motion will continue slowly
until a steady state is reached. Operating the piezo actuator or stage in closed-loop control eliminates this
drift because the controller is compensating for this movement in real time to keep the output motion at the
desired position.
C.3.4 Force and Displacement
C.3.4.1 Force Versus Displacement Characteristics
The force generated by an actuator acting in the polarization direction is completely independent of the
overall length of the actuator and is only a function of the cross-sectional area of the actuator and the applied
electric field. An illustration of the force versus displacement output of a piezoelectric actuator with various
applied voltages is shown in
Figure C-3: Force vs. displacement output of a piezo actuator at various applied voltages
A few interesting characteristics become evident upon inspection of
and displacement increase as the applied voltage is increased. The maximum force output of a piezo
actuator, or blocking force, occurs when the rated voltage is applied across the actuator and the output of the
actuator is "blocked" or not allowed to move. As the actuator expands, the force production capability
reduces until the force output reaches zero at the maximum rated displacement of the actuator.
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Figure C-3.
Appendix C
QNP3 Hardware Manual
Figure C-3. The piezo actuator's force
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