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Displacement of Piezo Actuators (Stack & Contraction Type)
Long Travel Flexure Piezo Actuators Long Travel Piezo Benders Long Travel Piezo Linear MotorsCreep (Drift) (open loop PZTs)
For perodic motion, creep does not affect repeatability
Creep only occurs with open loop PZTs. Like hysteresis, creep is related to the effect of the applied voltage on the remanent polarization of the piezo ceramics. Creep decreases logarithmically with time. If the operating voltage of a (open loop) Piezo is increased (decreased), the remanent polarization (piezo gain) continues to increase (decrease), manifesting itself in a slow creep (positive or negative) after the voltage change is complete. The following equation describes the effect:
DL(t) » DL(1+g*lg*(t/0.1)) (4-2)
Creep of Piezo motion as a function of time.
where
DL = displacement 0.1 seconds after the voltage change is complete [m].
g = creep factor which is dependent on the properties of the actuator (on the order of 0.01 to 0.02).
Maximum creep (after a few hours) can add up to a few % of the commanded motion (click here for drift elimination).
Creep of open loop Piezo motion after a 60 µm change in length as
a function of time. Creep on the order of 1 % of the last commanded motion per time decade.
Aging
Aging refers to reduced piezo gain as a result of the depoling process. Aging can be an issue for sensor or charge generation applications (direct piezo effect), but with actuator applications it is negligible, because repoling occurs every time a higher electric field (in the poling direction) is applied to the element.
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