Q u ick Facts (cont.)

ff. Open- and Closed-Loop Operation (> 10 kN capacity) would use several In contrast to many other types ofdrive systems, piezo actuators canbe operated without servo-control. The displacement is approximately equal to the drive voltage. Hysteresis, nonlinearity and creep effects limit the absolute accuracy. For positioning tasks which require high linearity, long-term stability, hundred watts under the same con- ditions. For more information, see pp. 4-27 Protection from MechanicalDamage PZT ceramics are brittle and cannotwithstand high pulling or shear forces. The mechanical actuatordesign must thus isolate theseundesirable forces from the ceram- ic. This can be accomplished by measures such as spring preloads, use of ball tips, flexible couplings, etc. (for more mounting guidelines, see p. 4-48). In addition, the ceram- ics must be protected from mois-ture and the intrusion of foreignparticles. Close contact between the piezo mechanics manufacturer and the user facilitates finding an optimal match between the piezo system and the application envi-ronment. repeatability and absolute accuracy, closed-loop (servo-controlled) pi-ezo actuators and systems areused (see p. 4-31). With suitable controllers, closed-loop operation enables reproducibilities in the sub-nanometer range. High-Resolution Sensors forClosed-Loop Operation LVDT (linear variable differentialtransformer), strain gauge and capacitive sensors are the most common sensor types used forclosed-loop operation. Capacitivesensors offer the greatest accuracy. For more information, see p. 4-19 ff. ff. Dynamic Behavior A piezo actuator can reach its nom-inal displacement in approximatelyone third of the period of its reso-nant frequency. Rise times on the order of microseconds and acceler- ations of more than 10,000 g are possible. This feature makes piezo actuators suitable for rapid switch- ing applications such as controllinginjector nozzle valves, hydraulicvalves, electrical relays, optical switches and adaptive optics. For more information, see pp. 4-24 Power Requirements Piezo actuators behave as almostpure capacitive loads. Static opera- tion, even holding heavy loads, consumes virtually no power. In dynamic applications the energyrequirement increases linearly withfrequency and actuator capaci- tance. At 1000 Hz with 10 µm amplitude, a compact piezo trans- lator with a load capacity of approx. 100 N requires less than 10 W, while a high-load actuator
Fig. 4. Piezo actuator with water-proof case and connection for flushing/cooling air. © PI 1998-2005. Subject to change w/o notice. Cat 118 05/09.17 4-12