Environ m ental Con d itions an d Infl u ences
Lifetime of Piezo actuators The lifetime of a piezo actuatoris not limited by wear and tear. Tests have shown that PI piezo actuators can perform billions(10 Example: An P-842.60 LVPZT actuator(see p. 1-36in the “Piezo Actuators” section) is to oper- ate a switch with a stroke of 100 µm. Of its operating time, itis to be open for 70 % andclosed for 30 %.Optimum solution: The actua-tor should be linked to the switch in such a way that the open position is achieved with the lowest possible operatingvoltage. To reach a displace-ment of 100 µm, a voltage amplitude of approximately110 volts is required (nominal displacement at 100 V is only 90 µm). Since the P-842.60 can be oper-ated down to -20 volts, the closed position should be achieved with 90 V, and the open position with -20 volts. When the switch is not in use at all, the voltage on the piezo actuator should be 0 volts.Statistics show that most fail-ures with piezo actuators occur because of excessive mechani- cal stress. Particularly destruc- tive are tensile and shearforces, torque and mechanicalshock. To protect the ceramic from such forces PI offers a variety of actuators with pre- loads, ball tips, flexible tips as well as custom designs. Failures can also occur whenhumidity or conductive materi- als such as metal dust degrade the PZT ceramic insulation, leading to irreparable dielectric breakdown. In environmentspresenting these hazards,PICMA 9 ) of cycles without anymeasurable wear.As with capacitors, however,the field strength does have aninfluence on lifetime. The aver- age voltage should be kept as low as possible. Most PI piezo actuators and electronics are designed for semi-bipolar operation.There is no generic formula todetermine the lifetime of a piezo actuator because of the many parameters, such as tem- perature, humidity, voltage, acceleration, load, preload,operating frequency, insulationmaterials, etc., which have (nonlinear) influences. PI piezo actuators are not only opti- mized for maximum travel, but also designed for maximum lifetime under actual operatingconditions. The operating voltage rangevalues in the technical data tables are based on decades of experience with nanomech-anisms and piezo applicationsin industry. Longer travel can only be obtained with higher voltages at the cost of reduced reliability. Fig. 39 c. PICMA ® piezo actuators (lower curve) compared with conventional multi-layer piezo actuators with polymer insulation. PICMA ® actuators are insensitve tohigh humidity in this test. In conventional actuators, the leakage current begins to rise after only a few hours—an indication of degradation of the insulation and reduced lifetime. Test conditions: U = 100 VDC, T = 25 °C, RH = 70%. ® actuators with theirceramic-only insulation are strongly recommended. PI alsooffers hermetically sealed actu-ators and stages. Fig. 39 d. P-885.50 PICMA ® actuators with 15 MPa preload in dynamic motion test at 116 Hz. No observable wear after 1.2 billion (10 9 ) cycles. © PI 1998-2005. Subject to change w/o notice. Cat 118 05/09.17 4-38