Note
For periodic motion, creep andhysteresis have only a minimal
effect on repeatability.
F u n d a m entals of Piezo m echanics
Creep / Drift (Open-Loop PiezoOperation) The same material propertiesresponsible for hysteresis also
cause creep or drift. Creep is a
change in displacement with
time without any accompany-
ing change in the control volt-age. If the operating voltage ofa piezo actuator is changed,
the remnant polarization (piezo
gain) continues to change,
manifesting itself in a slow
change of position. The rate of
creep decreases logarithmical-
ly with time (see Fig. 12). Thefollowing equation describesthis effect:(Equation 2)Creep of PZT motion as a func-tion of time. Where:t = time [s] L(t) = change in position as a function of time L
t=0.1 = displacement 0.1 seconds after the voltage change is complete [m]. = creep factor, which is depend- ent on the proper- ties of the actua-
tor (on the order of 0.01 to 0.02, which is 1 % to 2 % per time decade). In practice, maximum creep(after a few hours) can add up
to a few percent of the com-manded motion. Aging Aging refers to reduction inremnant polarization; it can be
an issue for sensor or charge-
generation applications (direct
piezo effect). With actuatorapplications it is negligible,because repoling occurs everytime a higher electric field is
applied to the actuator material
in the poling direction.
Fig. 12. Creep of open-loop PZT motion after a 60 µm change in length as a functionof time. Creep is on the order of 1 % of the last commanded motion per time decade. © PI 1998-2005. Subject to change w/o notice. Cat 118 05/09.17 4-18