In the core components of intelligent control systems, absolute angular displacement sensors play a crucial role. Their measurement accuracy and long-term operational reliability directly determine the performance limits of the entire control system. This article provides a comprehensive review of absolute angular displacement sensors based on different physical principles (inductive, photoelectric, electric field, magnetoelectric). It delves deeply into their core sensing mechanisms and elaborates on the working principles of each type of absolute angular displacement sensor. Sensing electrode shapes (special encoding patterns, rings, sectors, sinusoidal patterns, petal shapes, etc.) are designed to adapt to different sensing mechanisms. Configuration characteristics of sensing units (evolving from grating lines to grating planes and further to arrays) are utilized to enhance sensing accuracy. Sensing media are shifting towards multi-media coupling to improve adaptability in complex environments. Typical structural features of absolute measurement and current research status are included. Their respective advantages are analyzed while also explicitly identifying their inherent limitations. It includes a comparative benchmarking of performance indicators of mainstream products currently on the market and their typical application domains, offering valuable references for engineering selection. This article dissects the absolute time-grating angular displacement sensor. This sensor fuses spatiotemporal modulation technology with diverse sensing mechanisms, achieving high precision and interference resistance through the spatial periodic distribution of sensing units coupled with temporal signal scanning and demodulation Finally, this article outlines future development trends for absolute angular displacement sensors, including improving sensor accuracy and resolution while enhancing robustness and reliability in extreme environments, and advancing multi-sensor information fusion technology to meet the increasing demand for multi-dimensional sensing in complex intelligent control systems.