Design and experimental validation of an ultra-precision Abbe-compliant linear encoder-based position measurement system

Abstract

The design and development of an Abbe-compliant linear encoder-based measurement system for position measurement with a targeted 20nm uncertainty (k=2) in machine tools and CMMs is presented. It consists of a linear scale and a capacitive sensor, mounted in line on an interface which is guided in the scale's measurement direction and driven by a linear motor based on the output signal of the capacitive sensor. The capacitive sensor measures the displacement of a target surface on the workpiece table. The functional point, which is the center of a tool or touch probe, is always aligned with the scale and capacitive sensor such that this configuration is compliant with the Abbe principle. Thermal stability is achieved by the application of a thermal center between the scale and capacitive sensor at the tip of the latter, which prevents both components to drift apart. Based on this concept, a prototype of a one-DOF measurement system was developed for a measurement range of 120mm, together with an experimental setup aimed at verifying the reproducibility of the system for changing ambient conditions of ±0.5°C and ±5%rh and the repeatability during tracking of a target surface over a short period of time. These experiments have shown that the measurement uncertainty of the one-DOF system is below 29nm with a 95% confidence level.