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Open and Closed Loop Operation
Piezo actuators can be operated in open and closed loop. In open loop, displacement roughly corresponds to the drive voltage. This mode is ideal when the absolute position accuracy is not critical or when the position is controlled by data provided by an external sensor (interferometer, CCD chip etc.). Open loop piezo actuators exhibit hysteresis and creep behavior (like other open loop positioning systems).Closed loop actuators are ideal for applications requiring high linearity, long-term position stability, repeatability and accuracy. PI Closed Loop Piezo Actuators & Systems are equipped with position measuring systems providing sub-nanometer resolution and bandwidth up to 10 kHz. A servo controller (digital or analog) determines the output voltage to the Piezo by comparing a reference signal (commanded position) to the actual sensor fed back position signal (for more information click here).
PI has designed multi-axis Closed Loop Piezo positioners that offer the possibility of repeatedly locating a point within a 1 x 1 x 1 nanometer cube (see "Piezo Flexure NanoPositioners" section for more information). It is important to remember that such accuracy is obtainable only if the surrounding environment is controlled, since temperature changes and vibrations will cause changes in position at the nanometer level.
Dynamic Behavior
A piezo actuator can reach its nominal displacement in approximately 1/3 of the period of the resonant frequency. Rise times on the order of microseconds and accelerations of more than 10,000 g's are possible. This feature permits rapid switching applications. Injector nozzle valves, hydraulic valves, electrical relays, adaptive optics and optical switches are a few examples of fast-switching applications.
Resonant frequencies of industrial reliability piezo actuators range from a few tens of kHz for actuators with total travel of a few microns to a few kHz for actuators with travel more than 100 microns. These figures are valid for the piezo itself; an additional load will decrease the resonant frequency as a function of the square root of the mass (quadrupling the mass will halve the resonant frequency).
Piezo actuators are not designed to be driven at resonant frequency (with full stroke and load), as the resulting high dynamic forces might endanger the structural integrity of the ceramic material.
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