Initialization of a Position Measurement System

摘要

This disclosure relates to a method of absolute phase capturing in a position measurement system. In particular, it relates to absolute phase capture interferometers in a measurement system with multiple degrees of freedom (DOF). A multi-DOF measurement system typically has a geometrical position model which converts signals of multiple sensors into a multi-DOF target position. For the purpose of finding a reference position ("zeroing" or "initializing") an incremental measurement system may be assisted by an absolute position measurement. An incremental system typically has a faster update rate, lower data delay, a better resolution and/or uses less computational power to derive a displacement than an absolute measurement system. Once the absolute position is known typically the incremental position measurement system can take over and the absolute position sensor is no longer used. The zeroing accuracy is limited by the discrepancy between the geometrical position model of an incremental measurement system and the geometrical model of an absolute measurement system. If the discrepancy is larger than half the increment of the incremental measurement system, then absolute phase capturing of the incremental measurement system is not possible at an arbitrary absolute position. In such a case one must first bring the absolute position of the target object within a range where the uncertainty of the geometrical position model of the absolute position system and the uncertainty of the geometrical position model of the incremental position measurement system is less than a fraction of the period of the incremental measurement system.