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September, 2016
Precision Motion Control with Piezoelectric Technology
By Birgit Bauer, Scott Jordan, and Stefan Vorndran, Physik Instrumente L.P. I
n motion control systems with movement re- quirements of mere microns, piezoelectric mo- tion control technology is unparalleled in its
precision. The piezoelectric effect is found in mate- rials such as quartz, and converts electrical energy into mechanical energy and vice versa. First dis- covered in 1880, it was found that compressing quartz crystals generates an electric potential, which was followed by the discovery that applying an electric current to a piezoelectric material actu- ally changes its shape. Piezo actuator and piezo motor technologies
are used in a variety of industries including nan- otechnology, microelectronic chip manufacturing, metrology, data communications, and photonics. With their ability to finely position devices, includ- ing laser systems, piezo mechanisms are useful for a wide range of ultra-precise applications. Piezo motion-based devices provide a number
of features that are highly sought after in fields such as medical device manufacturing, which often require lubricant-free, sterile and ceramic designs. Ceramics are also useful because they are non- magnetic, an advantage in high-energy imaging/ scanning based on strong magnetic fields.
Seeing the Light In the medical field, ametropia —a condition in
which images do not come to focus properly on the human retina — can be corrected up to high diopter ranges with refractive laser surgery techniques. For this purpose, the cornea is slightly reshaped by re- moving small particles with laser energy, so that the resulting refractive power of the cornea is once again proportionate to the length of the eyeball. Two different laser types are used for these
Piezo-driven steering mirror systems are a
better alternative. In addition to providing com- pact, superior position accuracy, high dynamics and acceleration, they are much more compact. They provide optical deflection up to 120 mrad, ex- tremely fast response, and position resolutions into the nanoradian range. Piezoceramic actuators are the driving force
behind these scanning and positioning systems. Actuators exploiting the piezoelectric effect can move with atomic resolution, can respond to an electric control signal in microseconds, and provide scanning bandwidth with frequencies up to sever- al thousand hertz. The motion is based on crys- talline effects, so there are no traditional mechan- ical parts that can cause wear and create friction. The steering mirrors also do without mechan-
Compact piezo laser beam steering mirror unit.
refractive operations: excimer lasers and femtosec- ond lasers. The latter work in the infrared range and send light pulses with durations of femtosec- onds. One femtosecond is equal to 10-15 of a second — that is, one quadrillionth. Excimer lasers emit UV light and discharge the energy directly in beam widths of approximately 125 to 225 nm, de- pending on the types of gases used. Common deflection techniques, such as galvo
scanners, are generally suitable for high-precision applications, but have limitations. To be able to po- sition in two axes, two systems have to be stacked. This results in a pivot point shift, polarization ro- tation and additional space and alignment re- quirements for integration.
The material matters in material handling
ical bearings. This means they also do not require lubricants, are sterile and can be baked out at high temperatures, ideal prerequisites for medical ap- plications. Motion is guided by flexure joints, di- rectly driven by the solid-state actuators. There is nothing to wear and nothing to cause backlash, or play. Piezo mechanisms present capacitive loads to the control electronics and dissipate no power in steady state operation.
Piezo Ultrasonic Motors In addition to piezo flexure mechanisms, for
short travel ranges, ultrasonic ceramic piezo mo- tors open up new possibilities for laser beam con- trol. Ultrasonic motors provide basically unlimited travel with acceleration to 10-g, velocity to hun- dreds of millimeters per second, and do so in a very compact package.
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