Note For periodic motion, creep andhysteresis have only a minimal effect on repeatability. Creep / Drift (Open-Loop PiezoOperation) (Equation 3) The same material propertiesresponsible for hysteresis also cause creep or drift. Creep is achange in displacement withtime without any accompany- ing change in the control volt- age. If the operating voltage of a piezo actuator is changed, the remnant polarization (piezo gain) continues to change, manifesting itself in a slowchange of position. The rate ofcreep decreases logarithmical- ly with time (see below). The following equation describes this effect: 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 upto 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 every time a higher electric field is applied to the actuator material in the poling direction.
Creep of open-loop PZT motion after a 60 µm change in length as a function of time.Creep is on the order of 1 % of the last commanded motion per time decade © PI 1998–2006. Subject to change without notice. This data sheet is superseded by any newrelease. The newest release is available for download at www.pi.ws. 06/10/30.0 44