Piezoelectric Actuation
imaging is performed in small areas that are subsequently patched together in soſtware. Ultrasonic piezomotors are also attractive for transverse
sample positioning in coarse/fine microscopy applications. Tey provide the long travel over many millimeters necessary for quickly accessing the full span of a microscope slide or well plate, and they also provide an inherently stiff and stable platform at rest. In fact, data published in a recent peer-reviewed paper [2] reveals nanoscale stabilities over many minutes for a combined coarse/fine stack based on an ultrasonic piezomotor coarse stage and a PZT-stack-based multiaxis nanopositioner. Tis performance is roughly an order of magnitude more stable than a high-quality screw-driven manual positioning stage.
Sensors Several types of position sensors are employed in closed-loop
nanopositioning mechanisms. Te highest-resolution, highest- bandwidth, highest-accuracy, and highest-stability sensor is the two-plate capacitive sensor, in which one highly polished and complexly configured metallic plate is mounted on the moving element of a stage, with a corresponding plate on the fixed structure of the stage. As the stage moves, the actual position of the workpiece is measured in real time. Tese sensors may be arrayed around the stage’s moving platform to monitor its position in multiple degrees of freedom. Tis facilitates parallel-kinematic configurations, in which a specimen can be actuated simultaneously in several directions. Tis increases system bandwidths and improves trajectory performance over less-costly stacked multi-axis designs. For applications where the resolution, accuracy, bandwidth,
and stability of capacitive sensors are not needed, various types of strain gauges have been used. Tin-film strain gauge sensors, for example, mount on the piezo stack and measure its expansion and contraction, allowing the servo-controller to linearize its actuation and eliminate hysteresis. Tese sensors span a significant area, so they are comparatively insensitive to local effects. Silicon piezoresistive sensors, by comparison, are very small, ideal for incorporation into a flexure element of a stage’s structure. Tese are the foundation for a new wave of high-quality, but cost-effective, U.S.-made nanopositioners targeted at research and commercial microscopy applications (Figure 4).
Software and Interfacing Define the Microscope Imaging soſtware suites that control the microscope,
camera, focusing mechanism, and sample positioning stages are gaining in importance. Tey define the microscope’s capabilities and that of the application, and the suites are growing increasingly vertical in their focus. Examples include Metamorph and MetaFluor, Micro-Manager, ScanImage, and Intelligent Imaging Innovations’ SlideBook™ suite, all of which support an impressive array of hardware from manufacturers committed to users’ productivity. Tis has increased market awareness of soſtware as not only an enabler for industrial and academic research but also as a pacing item for instrument manufacturers. Generally speaking, microscopy users must prioritize productivity and ease of use, so the majority of today’s microscopy applications are well-served by imaging suites that offer plug-and-play support of popular subsystems, and microscope manufacturers and distributors offer configuration services (and sometimes their own proprietary
34 Figure 4: PInano Microscope Stage family (PI).
platforms) that allow users to devote their attention to their real work. In this way, subsystems have taken on a role reminiscent of common office peripherals like printers and scanners: users expect them to “just work” without a lot of low-level engineering on their part.
Technology Advances Pace the Field John Hanks, VP for Life Sciences Segment at National
Instruments, says “We see the field-programmable gate array (FPGA) and the GPU as hot topics: in the application and integration of lasers, for galvo control, and for inline FPGA processing for optical coherence tomography (OCT); for new medical devices; and for ophthalmic, cardiac, and dental applications. Understanding tradeoffs between FPGAs/GPUs/ CPUs is key. FPGAs are good for IO and inline processing, GPUs for rendering and post-processing (large datasets and cache); host computer processing has the advantage of the most mature development environments and ability to manage heterogeneous/parallel processing.” Vijay Iyer, Senior Soſtware Engineer at HHMI/Janelia
Farm, states, “Light microscopists in the life sciences are increasingly seeking to interact with entire volumes of tissue— imaging, stimulating, disrupting points throughout those volumes. Tis demands fast, precise automated motion of the sample or the microscope optics, particularly when interacting with living tissue.” Te challenge for subsystem manufacturers is now to
support both the plug-and-play applications suites and the scientists immersed in writing their own code. At the same time, subsystem manufacturers must keep pace with the increasing importance of high-speed data acquisition and processing. Tis has driven a cascade of developments that have included: (a) advanced interface techniques that offer microsecond- scale synchronization between motion and optical processes; (b) novel motion devices with extended travels, resolution capabilities and stabilities, new sensors, and command sets with backward and forward compatibility as new controls are developed; (c) digital nanopositioning controllers of extraordinary capability but that undercut traditional analog controls in price, thanks to semiconductor developments; and (d) innovative control techniques that address fundamental limitations in motion system bandwidths, for more accurate scanning and acquisition.
www.microscopy-today.com • 2011 July
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