Ultra-precise on-axis encoder self-calibration for fast rotary platforms

• Main Author: Amin-Shahidi, Darya
• Format: Others
• Language: English
• Published: University of British Columbia 2009
• Online Access: http://hdl.handle.net/2429/7573

This thesis presents the design, implementation, and experimental testing of a rotary platform and its use for encoder calibration research. So far, the experimental results have demonstrated a calibration repeatability of better than 200 nano-radians (0.04 arc-second), which is much better than the state of the art precision angular encoder error of about 1 arc-second. The major contributions of this thesis include achieving ultra-accurate encoder calibration results as well as the design of an ultra-precise rotary table and a set of high-speed electronics. Others have developed many encoder calibration techniques. This thesis aims to identify, improve, and experimentally demonstrate the accuracy limit of an encoder self-calibration method developed at UBC. The work can be categorized into the following three parts: 1) Ultra Precise Rotary Platform Design, Manufacturing, and Assembly: A precision rotary platform was designed for encoder calibration and other spindle metrology research. The rotary platform uses a precision airbearing. Two different rotary encoders are included for rotation measurement. Three displacement probes monitor the motion of a target ball to identify and compensate for rotor vibrations. The platform is driven by an ultra-low cogging torque motor for minimum velocity ripple. The setup’s versatility makes it an excellent spindle metrology research tool. 2) High-Speed Electronics for Signal Processing: Fast processing is required for encoder calibration and precise stage control. Since no commercial controller met our fast signal detection and processing requirements, a set of electronics was designed: an FPGA-based high-speed digital controller (Avalanche) and a precision analog processing board (NanoRAD). Avalanche incorporates a fast-Virtex4 FPGA chip, a novel 1.2-GHz timer, five fast ADCs, four fast DACs, Gigabit Ethernet, High-Speed USB, and DDR RAM. NanoRAD incorporates fast comparators and precision buffers for analog processing and filtering. 3) Self-Calibration Algorithm Development and Experiments: After integration of the electronics with the mechanical assembly, the encoder calibration technique was examined. The method was adapted for the new setup, the damping estimation portion of the algorithm was improved, and the whole method was tested experimentally and in simulation. The encoder accuracy has been experimentally enhanced by 25-times from 5,000 (1 arc-second) to 200 (0.04 arc-second) nano-radians.