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Choose a precision measuring
June 21, 1999
6 Min Read
Here, representatives of companies supplying laser-interferometer and linear-encoder-based systems debate the merits of their technologies
Laser interferometers define accuracy in sub-micron positioning
By David Barker
David Barker, Vice President of R&D, Teletrac, a subsidiary of Axsys Technologies Inc., 137 Aero Camino, Santa Barbara, CA 93117
For design engineers specifying precise measurement and positioning systems, only laser interferometry performs well across all categories of resolution, precision, and accuracy. There are three key reasons for this:
A helium-neon (HeNe) laser is used as the measurement standard.
The measurement plane is co-aligned with the plane of the workpiece to eliminate geometric errors.
A stationary external reference frame exists for error detection and control.
When it comes to resolution, HeNe lasers are the standard in high-accuracy positioning. The National Institute of Standards and Technology (NIST) accepts the wavelength of HeNe light in a vacuum as "a priori traceable," and in most applications it does not require calibration. That is why linear encoders themselves are characterized using laser interferometry. The frequency of light emitted by Axsys Technologies' Teletrac temperature-stabilized HeNe lasers is stable to one part in 108. When incorporated into a simple Michelson interferometer, a single-frequency HeNe yields 79 nm of resolution, and various optical and interpolation techniques can boost this to 0.6 nm.
When it comes to precision, accuracy, and repeatability, however, resolution cannot go it alone--measurement resolution has nothing to do with the geometric and stage-motion errors encountered in sub-micron positioning. For example, the scales used in linear encoders are usually mounted on different planes and offset from the measured object, introducing geometric measurement errors called Abbe errors. Axsys' laser interferometric systems avoid Abbe errors by co-aligning the measurement plane with the object plane. Co-alignment also minimizes the measuring offset distance to the workpiece, which further reduces errors.
For this X-Y stage, two remote laser interferometer/receiver modules provide a stable external reference frame and co-alignment with the workpiece.
The most important advantage of laser interferometric systems, though, is the stable external reference frame. Unlike linear-encoder scales mounted on motion stages, laser interferometers can be rigidly mounted on a base structure. This stationary external reference frame detects and corrects for the inevitable meandering, and deviations in squareness and straightness that positioning stages undergo. When one axis of a two-axis stage wanders from the ideal straight path, the laser interferometer on the opposite axis can detect the error. A feedback servo loop then corrects the error. Novel optical components also can adapt laser interferometers to measure angular positions up to 40 degrees with the same level of precision.
The combination of all these advantages provides the resolution, precision, accuracy, and repeatability needed for high-performance applications. Moreover, for applications requiring multi-axis stages or distances more than a foot, laser-interferometer systems become cost-competitive with--or even cheaper than--linear encoders since the cost of laser interferometers does not depend on length.
Axsys laser interferometric systems are used in semiconductor fabrication, large-area probers, and inspection systems for flat-panel displays, and servo track-writing and test subsystems for the disk-drive industry. From components to complete turnkey systems, these laser-interferometric systems compensate for the environmental and mechanical subtleties of sub-micron positioning.
Linear encoders: most often the best precision platform option
By John Thormodsgard
John Thormodsgard, Product Specialist, Heidenhain Corp., 333 State Parkway, Schaumburg, IL 60173
Design engineers have regularly specified linear-encoder position feedback on motion platforms requiring precise positioning and repeatability. As position control demands tighten, new developments in encoder (linear scale) technology have kept pace.
There are two basic types of linear encoders: exposed and sealed systems, each of which can be produced in a number of mechanical variations for applications ranging from machine tools to semiconductor production, and from woodworking to aerospace.
An aluminum scale housing and a pair of sealing lips protect sealed encoders from the environment, making them ideal for applications where contamination is present. Sealed encoders also allow for loose mounting tolerances--a suspension mechanism exists between the encoder's mounting area and the scanning unit. The scanning unit is also guided along the scale by roller bearings.
Exposed encoders are suited for motion platforms requiring the highest level of precision. They consist of a scanning unit mounted independently from the encoder graduation, with no contact between them. Thus there is zero friction or backlash in the encoder to allow the highest accuracies.
Sealed linear encoders are used in demanding environments where airborne contaminants are found.
Recent advancements in linear encoder technology have made possible their use in applications that were once the domain of laser-interferometer feedback. An example is on machines that require very long traverses. Sealed encoder systems in which a single steel tape rides within an aluminum housing are now possible in measuring lengths up to 120 m. First, the steel tape carrying the incremental positioning track is tensioned, and then clamped to the machine at both ends. The tape rides freely within the housing, permitting it to expand and contract with the machine as the temperature changes. Thus the thermal behavior of the scale is identical to the machine's mounting surface.
The advantage of such a system lies in the nature of the scanning technology. The steel tape graduation is a mechanically stable, fixed instrument--variations in temperature, humidity, or air composition do not affect the graduation. Like all sealed encoders, the scale is protected from contamination, and the suspension allows loose mounting tolerances. The result is affordable, highly reliable, and accurate measuring for long lengths.
New developments in exposed encoder technology have made them very suitable for applications requiring the highest levels of resolution and accuracy, such as in semiconductor production. Resolution down to 1 nm is now possible with encoders that use an interferential scanning principle--a very fine diffraction grating produces an output signal with a period of 0.128 mu. A simple 128-fold interpolation in the encoder results in the 1-nm step.
Previously, very high level interpolation was done on a courser signal to achieve similarly high resolutions, but imperfections in the output signal made the resultant data tenuous. Also, the 0.128-mu encoder features a substrate material that doesn't expand with temperature, for the ultimate thermal stability. This technology measures length up to 270 mm at a significant cost reduction over laser interferometer technology.
From such precision-measurement examples, linear-encoder position feedback should always be considered when designing a linear-motion platform.
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