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PI Fast Alignment Comes to ACS Controls
Continued from page 72
case for the decades-old approaches still commonly offered.
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Area scans. A good example of lega- cy approach to an area scan is a clas- sical raster or serpentine scan, which sweeps one axis, then increments its orthogonal axis, and repeats until the area is covered. Variations on this theme are common, including stepwise hill-climbs. But these approaches pose fun-
damental issues today. The stopping- and-starting adds settling time and causes vibration throughout the sys- tem, and the linear acquisitions can lead the system to actually de-align in common situations of asymmetric coupling profiles. By comparison, PI’s firmware-
based area scans use smooth, contin- uous sinusoidal and spiral patterns of selectable frequency. So system resonances can easily be avoided, allowing the nonstop scan to proceed without vibration. The result is con- siderably higher speed. Add built-in
the technology. A small, circular motion causes the coupling signal (or other figure-of-merit) to vary, and this variation can be analyzed in phase and amplitude to determine the instantaneous gradient. This allows a fast and direct path to opti- mum, with tracking possible for appropriate mechanisms. PI’s radical
fifth-generation
approach builds on this classical foundation to enable multiple gradi- ent searches to proceed in parallel. For example, this allows an x/y lock- on to be performed at the same time a theta-z optimization runs — an essential combination for any array- device alignment. This fast, parallel execution replaces the time-consum- ing iterative loop of separate x/y and theta-z alignments that was formerly required: one step instead of dozens.
PI’s Latest Breakthrough Since 2016, firmware-based fast
area scan, gradient search and paral- lel gradient-search technologies have been implemented in PI’s powerful piezo nanopositioner and hexapod controllers. Now fast alignment func- tionality is available for ACS con- trols. Combined with PI’s large industrial stages (including spindle- driven and linear-motor stages, gantries and airbearing assemblies), this forms a foundation for especially high-throughput applications involv- ing large-area processing, such as when devices are processed in trays, or across several stations, or on an indexing platform or conveyor. ACS controls lead the industry
in modularity and performance. Based on an EtherCAT open, distrib- uted architecture, they support absolute encoders, minimizing sys- tem start up times, easing initializa- tion approaches and reducing colli- sion avoidance risks. ACS’ yaw con- trol (combined with PI’s highly opti- mized joint construction) provides industry-leading orthogonality cor- rection and minimizes risk of axis binding — a distressingly common issue for older architectures. True MIMO gantry control plus
Continued on next page
A Deeper Dive Two basic alignment techniques
are most useful today: area scans and gradient searches for fast opti- mization and tracking.
modeling in some controllers, and the system can determine the peak (or even the centroid of a top-hat cou- pling) with good accuracy and speeds down to a few hundred milliseconds. Gradient search. The digital gradi- ent search was first developed in 1987 and, until now, has been mostly unchanged in its implementation through four subtle generations of
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