ff. ). Example: Piezoelectrically driv-en fiber aligners and tracking systems derive the control sig- nal from an optical power meter manufacturer in the system. There, thegoal is to maximize the opticalsignal level as quickly as possi- ble, not to attain a predeter- mined position value. An open- loop piezo system is sufficient for such applications. Ad- vantages like unlimited resolu-tion, fast response, zero back-lash and zero stick/slip effect are most welcome, even with- out position control. Hysteresis (Open-Loop Piezo Operation) Note: Hysteresis is observable inopen-loop operation; it can be reduced by charge control and virtually eliminated by closed- PI piezo actuators and stagesare designed for high reliability in industrial applications. The travel, voltage and load rangesin the technical data tables canactually be used in practice. They have been collected over many years of experience in piezo actuator production and in numerous industrial applica- tions.In contrast to many other piezosuppliers, PI has its own piezo ceramic development and pro- duction facilities together with the necessary equipment and knowhow. The goal is alwaysreliability and practical useful-ness. Maximizing isolated parameters, such as expansion or stiffness, at the cost of piezo lifetime might be interesting to an experimenter, but has no place in practical application. When selecting a suitablepiezo actuator or stage, consid- er carefully the fact that “maxi- mum travel” may not be theonly critical design parameter.
15 mode.For example, if the drive volt-age of a 50 µm piezo actuator is changed by 10%, (equivalent to about 5 µm displacement) the position repeatability is still on the order of 1% of fulltravel or better than 1 µm. The smaller the move, thesmaller the uncertainty. Hyste- resis must not be confused with the backlash of conven-tional mechanics. Backlash isvirtually independent of travel, so its relative importance increases for smaller moves. For tasks where it is not theabsolute position that counts,hysteresis is of secondary importance and open-loop actuators can be used, even if high resolution is required. In closed-loop piezo actuatorsystems hysteresis is fully compensated. PI offers thesesystems for applications re- quiring absolute position infor- mation, as well as motion with high linearity, repeatability and accuracy in the nanometer and sub-nanometer range (see PImain catalog page4-31

Piezo · Nano · Positioning

L loop operation ( see PI main catalog page 4-31 ff. ). Open-loop piezo actuatorsexhibit hysteresis in theirdielectric and electromagneticlarge-signal behavior. Hys- teresis is based on crystalline polarization effects and molec- ular effects within the piezo- electric material.The amount of hysteresisincreases with increasing volt- age (field strength) applied to the actuator. The “gap” in the voltage/displacement curve typically begins around 2%(small-signal) and widens to a maximum of 10% to 15% under large-signal conditions. The highest values are attain- able with shear actuators in d
V Hysteresis curves of an open-loop piezo actuator for various peakvoltages. The hysteresis is related to the distance moved, not to thenominal travel range 43