Piezoelectric Actuation
automatically compensated. In particular, laser autofocus sensors coupled with fast nanopositioning controllers can now acquire and lock in perfect focus in milliseconds, even starting from many hundreds of microns out of focus (Figure 3).
Scanning Applications Transverse motion of the sample across the image field is
another near-universal requirement in microscopy applica- tions. Depending on the application, the required motions may be coarse, over the many mm of a slide or well plate, or they may be fine, with nanoscale precision over just a few dozen µm of range. Tey can be point-to-point, or they can be linear scans of constant velocity, or even with specific position-versus-time waveforms, as in the case of calibrating optical tweezers. Very oſten, there is a requirement for both coarse and fine actuation, so that a region of interest can be brought into the field of view and then scanned at high resolution. Scanning can be accomplished by optical means such as
using beam-steering mirrors or acousto-optic deflectors, but moving the sample using a piezo-flexure stage offers benefits for applications requiring the highest high-resolution and flattest scanning and positioning. Piezo-flexure stages can provide up to 2 mm of scanning range, but more typically a less costly piezo stage of ~50–200 µm is stacked on top of a coarse positioning stage. Such coarse/fine applications have seen benefits from piezo-based technical developments in recent months. In particular, coarse-positioning mechanisms using a third type of piezoelectric actuation (besides the layered-stack actuators used in nanoscale positioning, and the walking-type actuators described for long- travel focusing mechanisms above) employ matchbook- sized slabs of piezoelectric ceramic, which are stimulated at their physical resonant fre- quency, typically in the ultra- sonic range above 100 kHz. Tis stimulation causes a rhythmic fluttering of the slab, and a friction tip placed at the resonance’s anti-node performs a quasi-elliptical nanoscale motion. When preloaded against a driven element, this confers a motive force. Te stimulus-to-velocity profile of this actuation is similar to that of familiar DC servomotors, but the pro- minent deadband in the actuation profile means that the device holds the driven element with nanoscale stabilities when quiescent. In addition, lubricant flow mechanisms, which cause long-term driſt and settling behaviors in conventional screw-driven coarse-positioner mechanisms, are eliminated.
32
Figure 3: Laser Autofocus Sensor (MotionX Corp.).
Te resulting long-term nanoscale stability and innate position-hold force have improved resolutions and repeatabilities. Tis development also promises significant benefits in stitching applications, where high-resolution
Figure 3a: A 60 msec capture of focus demonstrated by laser vibrometer independently measuring PIFOC objective position versus time.
www.microscopy-today.com • 2011 July
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