Traditional precision positioning systems (based on ballscrew drives or electromagnetic linear motors) have been used by engineers and researchers for many years and have been gradually improved over time. They provide many advantages for applications that require the positioning of heavy loads over long distances, such as gantry systems and multi-axis machining centers. The recent miniaturization of motors has also helped with the design of small positioning systems, that are well adapted to small-footprint motion control applications. However for the most demanding applications in optics, semiconductor, bio/nanotechnology fields, the traditional approach is often not the answer. As new applications demand faster and more precise motion control systems, manufacturers had to dig deeper than ever before to keep pace.
Piezoelectric Positioners
Piezoelectric based positioners have always been known for their ability to provide sub-nm precise motion, however, at very short travel ranges. Recent progress in piezo ceramic actuator and motor design as well as control technologies has solved the travel distance / precision conundrum. Users can now select from a number of piezo systems with unlimited travel, together with exceptional speeds, high force generation, compactness, fieldlessness and extremely stable power-off position-hold. Let's look at some motion control applications that have spawned the development of these new technologies:
Non-Magnetic Applications in Semiconductor, SEM Microscopy, Medical Design
Positioning and alignment systems in e-beam lithography systems and SEM can be equipped with electromagnetic drive mechanisms. However the expense to shield them and / or position them outside the action is very high, along with the increase in size. Fieldless piezo ceramic motors are significantly smaller, and can be positioned anywhere inside these machines, without causing negative effects. Medical technology can also benefit from piezo solutions that were optimized over many years for applications in semiconductor manufacturing & testing and biotechnology research. Active ceramic components such as piezo ceramic sensors and actuators have already been used in medical design technology, e.g. in micro pumps, ultrasonic transducers, fast valves for nano dispensing applications and for laser beam control in the eye and skin surgery. For imaging applications, such as 3-D-optical microscopy, MRI (Magnetic Resonance Imaging) systems or Optical Coherence Tomography (OCT), piezo drives offer substantial advantages due to their high-efficiency, direct-acting linear motion, high-resolution, fast response and non-magnetic characteristics.
Nanometer Precision in Scanning Microscopy: Small Ranges, but Very High Throughput
In modern drug discovery applications, a multitude of samples has to be examined in the shortest possible time. Techniques such as fluorescence imaging are employed and require precise focusing on small amounts of liquid usually held in multiwell plates. For long range, well-to-well positioning, conventional electric motors or voice-coil drives typically can provide the required speed and precision. However, the focusing is best achieved with frictionless, piezo flexure stages or objective positioners. Response times on the order of a few milliseconds allow extremely fast focusing and thus rapid data acquisition. The fast response also reduces the risk of photo bleaching caused by long term exposure.
Similar speed / resolution requirements are prevalent in nearfield scanning microscopy. Here, small samples are scanned, typically 100x100 µm to 500x500µm with nanometer lateral resolution. To minimize the scanning time and achieve the high resolution required, flexure guided piezo stages are the only option. The latest designs employ a parallel-kinematic motion principle, with all actuators acting on one moving central platform, greatly reducing inertia for much improved dynamics. Capacitive sensors integrated into the stage take multi-axis measurements against a common fixed reference (parallel metrology). This approach allows drift free positioning with nanometer straightness, not available with classical stacked / nested multi-axis designs.
The same approach yields superior surface metrology results in atomic force microscopes (AFM). Progress in semiconductor development heavily relies on materials testing and an AFM's output data is only as good as the out-of-plane motion (OOPM) of the XY scanning stage it employs. Here, traditional bearings are totally out of the question and the requirements have been pushing the mechanical limitations of the best flexure designs. Active trajectory control approaches (compensating minute off-axis errors with integrated piezo transducers) now provide OOPM in the sub-nanometer realm, over large scanning areas to hundreds of microns.
Long Travel Ranges Combined with Very High Resolution and Extremely High Dynamics
In today's industrial production and testing processes, dynamics and throughput are paramount. In disk-drive head/media metrology applications for example, sub-nm steps need to be executed and settled to nanometer tolerances in a matter of milliseconds. In order to test an entire disk, travel ranges in the cm range are often required. In the past, these could not be achieved with piezo positioning systems. Recently, new controller technologies have made it possible to devise hybrid nanopositioning systems combining the frictionless flexure technology and the piezoelectric properties of unlimited resolution and very fast and crisp response with the long travel ranges and high holding forces of a servo-motor / ballscrew arrangement. A specialized hybrid controller reads the stage position from an integrated, nanometer-class linear encoder and continuously coordinates both the piezoelectric and servo motor drives in a way to provide the best possible overall performance, with rapid pull-in, nanometer-scale bi-directional repeatability and inherent axial stiffness. The result is a fast, long-travel system with extraordinary repeatability and resolution.
High-Force Long Travel Actuators for High-Energy Physics & Optical Alignment
Many optical alignment applications in astronomy and high energy physics would benefit from actuators that provide millimeters of travel with nanometer resolution, free of backlash, yet capable of holding a position against large forces without drift and heat dissipation over long periods of time. These requirements were impossible to meet in the past. Now, a novel robust piezo linear motor based on the Piezo Walk effect is available.
The Piezo Walk effect combines lateral and longitudinal actuation of piezo elements that are preloaded in arrays about a central ceramic runner. A digital controller sequences their operation, providing high-force, long travel step-mode actuation plus picometer resolution high-bandwidth actuation. As all piezo motors, PiezoWalk drives are inherently vacuum-compatible and fieldless, and provide forces to 60 kg with nanoscale power-off position stability for months and years. When arranged in a parallel kinematic Hexapod design, they can provide motion in 6 degrees of freedom.
The hexapod approach with its virtual pivot point and available large central aperture is crucial for optical alignment problems as large as secondary mirrors in the latest generation earthbound telescopes and as small as fiber-to-photonic-component-alignment in telecommunication chips.
Smaller versions of the piezo walk motor replace motorized linear actuators in nanoimprint systems or laser tuning applications providing a very fast short range sweep mode and 20 mm of travel to rotate gratings over long ranges.
High-Speed and Long Travel Ranges Smallest Dimensions, e.g. For Autofocus in Cell Phone Cameras
Another use of piezo ceramics technology is in ultrasonic piezo motors. These are not stacked structures but instead are composed of monolithic piezoceramic slabs electrically stimulated to drive standing waves in the substrate at high frequencies. A hardened contact-point, at a resonant node-point is thereby made to oscillate in a quasi-elliptical fashion; when preloaded against a slide, this confers linear motion. The latest ultrasonic linear motor implementations come in sizes as small as 8mm, achieve velocities to 800 mm/sec and can provide unlimited travel ranges using this approach while providing submicron resolutions.
Conclusion
Piezo ceramic motion systems have long been number one choice for ultra-high precision motion. With ever increasing requirements from the optics, biotech and semiconductor industries in recent years, manufacturers were forced to find ways to overcome limitations such as travel range and linearity while preserving their unmatched speed, acceleration and resolution capabilities. The latest generation piezo systems are now ready to take on the toughest motion control challenges in the world.
This information has been sourced, reviewed and adapted from materials provided by PI (Physik Instrumente) LP, Piezo Nano Positioning.
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