Leading nanometer-level motion control technology innovator, ALIO Industries, continues to advocate the examination of the impact of 6D repeatability testing and performance on single and multi-axis motion control systems. Advancements in manufacturing processes and metrology sensors along with the continuing demand from industry to create innovative technologies and products is driving a greater need for motion systems that are both highly accurate and repeatable at the nanometer level.
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. ALIO suggests that repeatability performance for metrology inspection and manufacturing systems must now 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.
Traditional Systems and Test Methods — Plane Repeatability
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.