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ASME Endorses The Concept of Point Precision

It was recently announced that the draft of the ASME B5.64 standard ‘Methods for the Performance Evaluation of Single Axis Linear Positioning Systems’ has been completed and passed to the ASME for balloting. This suggests that a new method for defining the precision of advanced motion control solutions is being considered for the motion control sector which is similar to suggestions made by ALIO Industries — an Allied Motion company.

Bill Hennessey, President of ALIO stated, “The language used by suppliers of technology solutions aimed at precision engineering applications has for a long time been vague and, in some instances, confusing. ALIO is a provider of the some of the most accurate and repeatable motion control solutions available in the market today and has always found itself constrained by the use of traditional language and words such as ‘precision’ and ‘resolution’, which without any degree of qualification are basically meaningless.

When analyzing motion control solutions that provide sub-micron and nanometer-level accuracy, ALIO contended  that a new language was necessary, and new standards were required to indicate the real levels of precision that different motion control solutions can achieve. The ASME work on the as yet to be published B5.64 standard indicates that bodies other than ALIO are also showing an interest in redefining the way precision motion control solutions are measured. Any initiatives or activity in this area will ultimately be of enormous benefit for end-users who should be able to specify solutions that can truly achieve what they need in respect of precision and repeatability. “ALIO has been advocating the use of Point Precision for over 15 years.” comments Hennessey.

6-D point precision incorporates all sources of error as a meaningful 3 dimensional value

All motion systems operate in 3-dimensional space and have errors in 6 degrees of freedom (6-DOF). However, motion systems are often only characterized by performance data of a single or subset of these 6-DOF. This practice leaves several error sources unaccounted for in performance data and specifications. For more than 15 years, ALIO has contended that repeatability performance for metrology inspection and manufacturing systems must be analyzed and specified using a “point repeatability” method that accounts for 6D spatial errors in order to provide true representation of nanometer-precision performance. This can now be seen as an area that standards organizations also see as worthy of further consideration.

Hennessey continues, “Many traditional stage and motion systems specify repeatability as a single number representing the variation in linear displacement along an axis of travel, i.e. plane repeatability. Historically, this practice was valid as the repeatability specifications were large enough that other error factors were only a small percentage of the total error and could be ignored.  The repeatability of the plane position along the axis is effectively measured over many cycles at a target position. The intersections of this plane with the axis is a point on the axis line and the collection of these points results in 1D repeatability performance.”

This test method makes a critical assumption, namely that the plane only moves in one dimension and the axis is perfectly straight. At the nanometer-level, this assumption is not realistic. In nanometer-level precision systems, “other” errors that were previously ignored in less accurate systems often become equal to or greater contributors to the 6D repeatability performance. At the nanometer-level, the axis of travel should actually be shown as bending and twisting through three-dimensional space and thus plane visualization becomes meaningless as it will tip, tilt, and twist as the stage moves along the axis. The stage moves in 6D space, therefore neglecting these additional error sources can result in a misrepresentation of actual stage repeatability performance.

Each linear (or angular) direction the stage moves (or rotates) in results in a positional error in that direction. That motion, which must not be neglected when nanometer-precision is desired, will have an associated repeatability of that error motion. Each point on a stage mounting surface will move in 3D space as of a result of this error motion in 6-DOF. It is the point repeatability of an infinite number of points attached to a stage that must be characterized by testing and specification data. Thus, each point repeatability will result in a spherical repeatability range. 

Hennessey concludes, “To accurately characterize repeatability, X, Y, and Z components must be measured in a systematic process to characterize the point repeatability of a stage along the entire axis. Additionally, a process must be implemented to test the influence of pitch, yaw, and roll errors of the axis and their influence on repeatability. To have a high confidence in integrated system performance, the motion subsystems must be correctly characterized for 6D performance accounting for all error components of stage motion.”

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