Piezoelectrics in Positioning - PZT & Piezo Actuators: Sub ...

Piezo • Nano • PositioningQuick Facts (cont.)© Physik Instrumente (PI) GmbH & Co. KG 2008. Subject to change without notice.Cat120E Inspirations2009 08/10.182-180Open- and Closed-Loop OperationIn contrast to many other types ofdrive systems, piezo actuators canbe operated without servo-control.The displacement is approximatelyequal to the drive voltage.Hysteresis, nonlinearity and creepeffects limit the absolute accuracy.For positioning tasks which requirehigh linearity, long-term stability,repeatability and absolute accuracy,closed-loop (servo-controlled) piezoactuators and systems areused (see p. 2-199). With suitablecontrollers, closed-loop operationenables reproducibilities in thesub-nanometer range.High-Resolution Sensors forClosed-Loop OperationLVDT (linear variable differentialtransformer), strain gauge andcapacitive sensors are the mostcommon sensor types used forclosed-loop operation. Capacitivesensors offer the greatest accuracy.For more information, see p. 2-187 ff.Dynamic BehaviorA piezo actuator can reach its nominaldisplacement in approximatelyone third of the period of its resonantfrequency. Rise times on theorder of microseconds and accelerationsof more than 10,000 g arepossible. This feature makes piezoactuators suitable for rapid switchingapplications such as controllinginjector nozzle valves, hydraulicvalves, electrical relays, opticalswitches and adaptive optics. Formore information, see p. 2-192 ff.Power RequirementsPiezo actuators behave as almostpure capacitive loads. Static operation,even holding heavy loads,consumes virtually no power. Indynamic applications the energyrequirement increases linearly withfrequency and actuator capacitance.At 1000 Hz with 10 μmamplitude, a compact piezo translatorwith a load capacity ofapprox. 100 N requires less than10 W, while a high-load actuator(> 10 kN capacity) would use severalhundred watts under the same conditions.For more information, seep. 2-195 ff.Protection from MechanicalDamagePZT ceramics are brittle and cannotwithstand high pulling or shearforces. The mechanical actuatordesign must thus isolate theseundesirable forces from the ceramic.This can be accomplished bymeasures such as spring preloads,use of ball tips, flexible couplings,etc. (for more mounting guidelines,see p. 2-216). In addition, theceramics must be protected frommoisture and the intrusion of foreignparticles. Close contactbetween the piezo mechanics manufacturerand the user facilitatesfinding an optimal match betweenthe piezo system and the applicationenvironment.Fig. 4. Piezo actuator with water-proof case and connection for flushing/cooling air