Design of azimuth servo control system for the 14.5-metre optical-infrared telescope

To meet the ultra-low-speed high-precision tracking requirements of Asia's first 14.5-m optical-infrared telescope, the azimuth-axis servo system is designed. The drive scheme and segmented arc motor configuration are explained, and a lumped-mass model is built. Based on modal analysis, a Notch filter suppresses mechanical resonance, achieving a velocity loop bandwidth of 8.4 Hz. Following the tracking-error RMS ≤0.07″ criterion, velocity, position-loop, and dual feedforward controllers are developed to improve dynamic tracking and disturbance rejection. Wind-load simulations verify performance: under the site's annual mean wind speed, with sinusoidal guidance at 1.3°/s and 0.05°/s², the turntable maximum error is 0.0442″ (RMS 0.0076″), and the structural load error is 0.0944″ (RMS 0.0118″). Tests under various wind speeds confirm resistance to Force-8 gale conditions. Under abrupt extreme disturbances, step responses settle within 5-10 s with small oscillations and errors within allowable limits. The design meets the accuracy and robustness requirements, serving as a reference for large-aperture telescope servo systems.Abbreviations: CIOMP: Changchun Institute of Optics, Fine Mechanics and Physics; E-ELE: European Extremely Large Telescope; GTC Gran: Telescopio Canarias; IOE: Institute of Optics and Electronics; LAMOST: The Large Sky Area Multi-Object Fiber Spectroscopic Telescope; NIAOT: Nanjing Institute of Astronomical Optics and Technology; PWM: Pulse-Width Modulation; RMS: Root mean square; TMT: Thirty Metre Telescope; UMAC: Universal Motion and Automation Controller; VLT: Very Large Telescope