Passive gravity compensation in mechanisms and robots: Methods, advances, and challenges

• Reviews passive gravity compensation for mechanisms and robotic systems. • Proposes a new classification based on energy storage structure types. • Highlights advances in variable payloads and multi-DOF compensation. • Identifies open challenges, including friction, coupling, and design trade-offs. • Bridges mechanical theory with real-world robotic system implementation. Passive gravity compensation plays a crucial role in reducing actuator effort, improving energy efficiency, and enhancing ergonomics in mechanisms and robotic systems. This review presents a comprehensive overview of passive gravity compensation, focusing on fundamental methods, recent advances, and emerging challenges. Gravity compensation mechanisms are systematically classified based on the types of structures used to implement energy storage elements. Distinct from prior reviews, this work highlights three major research frontiers in the field: compensation for variable payloads, innovations in multi-degree-of-freedom (multi-DoF) compensation, and the integration of novel energy storage elements. Additionally, the review also provides future research directions and challenges in the field of gravity compensation, including self-regulated gravity compensation for variable payloads, the multi-DoF coupling problem in spatial manipulators, the impact of non-ideal components such as friction and hysteresis, multi-objective design trade-offs, and emerging synergies with artificial intelligence. By bridging theoretical modeling with practical implementation, this review provides a roadmap for developing scalable, adaptive, and deployable gravity-compensated systems for industrial, wearable, and collaborative robotic applications.