Precisión en sistemas de posicionamiento

ACCURACY IN POSITIONING SYSTEMS
Kevin McCarthy New England Affiliated Technologies
The state of the art in precision positioning systems has undergone continuing improvement, with the result that modern positioning systems can now achieve unprecedented levels of accuracy. These gains have come about due to specific technical advances (most notably, the availability of coherent light sources) aswell as inexorable pressure from high-tech applications which depend on dimensional accuracy for their existence. Notwithstanding the gains that have been made, there are gaps between levels of accuracy which are perceived as achievable, and those levels which can actually (and/or affordably) be met. This paper will attempt to address the realistic accuracy levels which various positioningtechnologies can meet, as well as the nature of the limitations which restrict accuracy. accuracy claims. Positioning system purchasers prefer that accuracy be summarized in a single, easily digestible number (and the smaller, the better). Positioning system vendors, in turn, comply; the unfortunate results include a recent full page ad which claimed to extract "tenth micron accuracy" from an open loopstepper based system. When questioned, an applications engineer responded that they were using a 1 mm leadscrew, and a divide-by-50 microstepper; hence, "tenth micron accuracy". Examples such as these reflect either a profound lack of awareness of the meaning and limitations of high accuracy systems ("fuzz"), or an overly aggressive marketing of "small numbers" for competitive advantage ("bunk").We regularly find that our tables improve dramatically (were the literature to be believed) upon their incorporation into other firms' products. Common practices include defining table accuracy as equal to that of the purchased leadscrew incorporated in the table, ignoring thermal factors and Abbé error; mentioning the accuracy of multi-axis systems without a "per axis" qualifier; providingaccuracy values which reflect only the no-load value, etc. The fact of the matter is that accuracy is a global parameter, which is affected by a combination of positioning table attributes; control and feedback systems; application specific details (e.g., the height above the table of the point of interest); as well as the operating environment. A meaningful characterization of system accuracy is betterachieved by a complete analysis than by an attention grabbing "number".

WHAT IS "ACCURACY"? Dimensional accuracy is simply the degree to which displacements executed by a positioning system match agreed upon standards of length. Ultimately, all length measurements are tied to the meter, as defined by the Committee Consultif pour Definition du Meter. Its current value is the distance whichlight in a vacuum travels in 1/299,792,458 of a second. When describing accuracy, we employ a variety of units considerably smaller than a meter. These include the familiar millimeter (10-3 meter), micron (10-6 meter), nanometer (10-9 meter), Angstrom (10-10 meter) and picometer (10-12 meter). For comparison purposes, a human hair is about 100 microns in diameter, semiconductor line widths are about 1THE PRIOR ART micron, and an atom is about 1 Angstrom. Many of today's applications for high accuracy positioning systems are tied to the requirements of the "FUZZ" vs. "BUNK" The heading, while somewhat jocular in nature, semiconductor industry and inspection systems for ultrareflects a widespread lack of seriousness with respect to precise machined parts. Over a hundred years ago, how-ever, scientists and technicians were busy creating X-Y tables with surprising accuracy, given the tools at their disposal. At that time, the challenge was the ruling of large precise diffraction gratings for spectroscopy, and the positioning tables were referred to as ruling engines. The design and fabrication of these ruling engines was a herculean effort, and the history of their development is...