LONG-TERM STABILITY OF A MOVING-SCALE MEASUREMENT SYSTEM WITH NANOMETER UNCERTAINTY Niels Bosmans 1 , Jun Qian 1 , and Dominiek Reynaerts 1 1 Department of Mechanical Engineering KU Leuven Heverlee, Belgium ABSTRACT To improve the accuracy of normal machine tools, a new concept for placing linear encoders in an Abbe-offset-free configuration is presented. A design for a 1-DOF system has been made and a prototype has been developed. Changes of the environment’s temperature and humidity contribute most to the measurement uncertainty. A dedicated measurement setup was built to determine the reproducibility of the moving-scale system for these environmental changes. The uncertainty budget of the setup indicates that the long-term stability can be determined with an uncertainty of 21 nm. INTRODUCTION Improving machine tool accuracy has been one of the main drivers of technological evolution for centuries. Taniguchi [1] predicted that machine tools for ‘normal machining’ will reach accuracies below 0.1 μm by 2020. The largest error sources in current normal machine tools are thermo-mechanical [2] and kinematic errors [3]. Extensive application of two precision design rules, namely Abbe- or Bryan-based metrology [4] and functional separation of the structural and the metrology frame [5] could be used to reduce these errors significantly. Examples of machine tools employing these principles with laser interferometers for position measurement of the stages are [6-8]. A typical, more simplified configuration for coordinate measurement machines with laser interferometers is shown in Figure 1 for two degrees of freedom. Here, two laser interferometers are attached to a stationary metrology frame together with a touch probe. While the workpiece table moves in x-z- direction, the laser beams always cross at the centre of the touch probe, in this way eliminating the Abbe errors. To integrate laser interferometers into the harsh environment of normal machine tools however poses a great challenge since interferometry is highly sensitive to changes in the index of refraction of air. Linear encoders, on the other hand, are far less susceptible to environmental changes, as was already indicated by [9]. FIGURE 1. Abbe-offset-free configuration of laser interferometers in a CMM. FIGURE 2. Abbe-offset-free configuration of moving linear encoders in a CMM. Therefore, a new concept for using linear encoders in a similar configuration as laser interferometers was proposed by KU Leuven [10] (Figure 2). In this concept, a linear scale is mounted on a carrier which is driven by a linear motor. A capacitive sensor is located at the end of the carrier and in-line with the scale. The capacitive sensor measures the gap between the sensor tip and a target surface and the linear motor is controlled in such a way that this gap stays within the limits of the capacitive sensor’s range. Consequently, the scale follows the Reading head x z Moving-scale Stationary metrology frame Touch probe x z Moving Workpiece table Laser interferometer