Piezo mechanisms: making advances in laser technology.
Not all Piezo Drives are Created Equal
Piezo actuators and motors are all based on the same physical principle, molecular level motion with intrinsically unlimited resolution. However, there are a number of commercially available basic designs, specialized for different applications. Piezo stack actuators, for example are short travel devices (<0.2mm travel), providing picometer resolution and high acceleration to 10,000 g's for microsecond responsiveness. They are used for laser cavity tuning/switching and also often incorporated in more complex positioning mechanisms, such as flexure guided tip-tilt steering mirrors, or piezo scanning stages, such as used in laser scanning microscopy. For longer travel ranges, a variety of different piezo motors is available, from high-force, high resolution "piezo-walk" type designs, to high-speed ultrasonic, travelling wave motors, and compact, low cost stick-slip (inertial) drives.
Maintenance Free, Mars-Rover-Tested Technology
Piezo devices are maintenance-free, vacuum compatible, lubricant-free, and do not have gears and moving components such as found in conventional positioning devices. This fact allows for very high lifetime and reliability, as well as speed and precision, ideal conditions for the use in laser control applications. Modern digital servo algorithms have further increased the performance of such piezo-based positioning systems making it easy to tune and quickly adapt the mechanics to changing requirements.
Laser Scanning in Fluorescence Microscopy
In laser scanning microscopy, the laser beam needs to be controlled and focused to excite fluorescence in the sample. Galvo scanners have traditionally been employed in the steering part, but piezo scanners have the advantage of being more precise, with faster response, and provide multiple motion axes integrated into a single device. This reduces size and also eliminates polarization rotation effects. Piezo devices are used in most fields of super resolution microscopies (Figure 2) where resolution can be far below the diffraction limit. Piezo-driven tip/ tilt mirrors and platforms demonstrate their advantages here. The S-334 tip/tilt scanner (Figure 3), for example, allows highly dynamic and precise steering motions of an integrated mirror in two orthogonal axes around a common pivot point. The frictionless piezo drives and flexure guides allow higher accelerations than conventional drives. A compact controller was specially developed for this type of mechanism and combines the functions of a multi-channel servo and driver in one.
Its internal hardware also carries out the coordinate transformation for different mechanical designs to provide pure orthogonal motion of the platform.
Modern telecommunication mostly relies on fiber optic data transmission via laser-light. Bandwidth requirements keep going up, and the need to speed up the production of fiber optical components is on the rise. To align most single-mode fiber optic components, nanometer resolution is required, which is why piezo technology is key. A compact multi-axis piezo mechanism, such as the NanoCube[R] positioner, together with a fast controller and modern align and tracking algorithms can align fiber optic components in a matter of a few seconds, a task that could have taken several minutes in the past. The piezo scanner provides 100um of fine travel and can be combined with motorized positioning stages for coarse positioning such as shown in Figure 4. Piezo scanners are also used in free-space optical communication to keep laser transmitters and receivers aligned dynamically.
Typical applications for laser materials processing are found in electronics manufacturing, e.g. the production of templates used for PCB coating with solder paste. The precision and speed requirements here are very high; the material must be removed very precisely with exactly defined edges. Time is money and high throughput is essential. Traditional laser-beam deflection techniques such as galvo scanners are suitable, but for two axis motion, two single-axis systems have to be "stacked". This results in different, variable pivot points, beam polarization rotation and increased space requirements. Piezo-driven tip/tilt mirror systems are more compact and provide higher acceleration and bandwidth. Due to their parallel kinematics design (Figure 5) there is only one moving optic with a fixed pivot point, leading to reduced inertia and higher dynamics in a smaller package, while achieving superior accuracy.
Medical Technology: Ophthalmology Benefits from Piezo Drives
Ultrasonic ceramic piezo motors also open up new possibilities for laser applications. They are characterized by extremely high velocities and acceleration. A patented drive principle makes them self-locking at rest without power dissipation. Linear motors for the integration in OEM devices as well as completely packaged and guided systems such as the M-663 miniature linear stage (Figure 6) are both available. This piezo positioner combines compact dimensions with high performance motion. It provides closed-loop velocities of up to 250 mm/sec and 3/4 inch travel range with 0.1 [micro]m resolution and very fast settling (Figure 7). A non-contact optical linear encoder guarantees high linearity and repeatability. Its compact dimensions of 15x30x35 mm allow for easy integration into the respective application.
Set-and-Forget Laser Alignment Components
In addition to the highly dynamic applications mentioned above, there are a number of "set-and-forget" applications where an optic or mirror in a laser system needs to be tuned occasionally. The focus here is on high stability and very smooth motion. PiezoMike type inertial motor actuators combine resolutions of 30 nanometers with holding forces of 100 N and reliable startup, even after longer downtimes (Figure 9).
The field of piezo motion devices for laser applications has expanded rapidly in recent years and today's optical engineers have a wealth of different piezo technologies at their disposal, from small piezo stacks for cavity tuning to tip/tilt platforms and long travel motors with high dynamics and precision. Piezo actuators can even be found on Mars where they help researchers answer the question if there once was life on the Red Planet, based on laser spectroscopy.
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|Title Annotation:||DESIGN TALK|
|Author:||Vorndran, Stefan; Jordan, Scott; Arnold, Steffen|
|Publication:||ECN-Electronic Component News|
|Date:||Dec 1, 2015|
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