Self-calibration for Air-bearing Rotary Encoders

INTRODUCTION High-accuracy rotary encoders are required for rotational axis position measurement in ultraprecision manufacturing and measurement equipments. A typical rotary encoder is made of a glass disk with a fine grating pattern and a scanning read head unit. Digital pulse trains are output from the read head to represent the disk rotary position. The angular measurement accuracy of such a rotary encoder is highly dependent on the grating pattern manufacturing error (uniformity and eccentricity of the graduation), glass disk installation on the measured rotary axis, the scanning head alignment, and signal processing circuits. All these elements unavoidably contribute to rotary encoder errors. If repeatable components of rotary encoder errors can be calibrated accurately, then they can be corrected through software mapping. Calibration of a rotary encoder needs to be conducted after the encoder is installed on the spindle end product, because this installation process may contribute to a significant amount of encoder errors. Therefore, a simple and accurate calibration method is highly desired by end users. Most existing calibration methods are based on a secondary angle sensor of higher accuracy mounted on the same rotary axis as the encoder to be calibrated. Usually that secondary angle sensor installation and setup relies on sophisticated instrument to ensure accuracy [1], and thus cannot be easily performed by end users. Self-calibration encoders have been developed to eliminate the use of a secondary angle sensor for calibration. Watanabe et al has demonstrated a self-calibration encoder using five identical read heads evenly distributed around a single optical disk [2]. However, that specially constructed encoder cannot calibrate error components in the 5 order spindle rotation frequencies and its harmonics. Moreover, the calibration accuracy also depends on the accuracy of these read heads installation and alignment. Most importantly, this calibration method cannot be applied to most existing encoders. This work presents the principle, analysis, and experimental results of a novel self-calibration method for rotary encoders on air-bearing supported spindles. By recording the time of each encoder signal edge during spindle free response, calibration curve can be derived by processing the recorded data. Two distinctive features of this method are: 1) it does not need a secondary sensor as a calibration reference; 2) it can be applied to commercial rotary encoders without hardware modification. The experimental results indicate that the calibration repeatability is within 0.002 RMS grating line of the encoder.