Semi-Active Actuators for Gravity Compensation with Application to Exoskeletons

Gravity compensation is a well-established technique in robotic design to maintain balance throughout the range of motion while reducing actuator loads. Existing solutions can be classified into three categories: passive, active, and semi-active systems. Passive systems rely on mechanical components such as springs or counter-weights to counteract gravitational forces, offering a simple and energy-efficient solution but lacking adaptability to dynamic loads. Active systems, on the other hand, use sensors and actuators to provide highly precise and versatile compensation at the cost of increased power consumption. A promising alternative is semi-active or semi-active systems, which combine both approaches to achieve high energy efficiency while ensuring high-performance gravity compensation. Surprisingly, semi-active solutions have received little attention from the research community. To bridge this gap, the research presented in this dissertation analyzes semi-active gravity compensation mechanisms aimed at reducing power consumption in robotic systems. Specifically, the study investigates applications in upper-limb exoskeletons to balance variable payloads and enhance usability in industrial and medical contexts. The first part of this work evaluates the Agadexo Shoulder, a commercial semi-active upper-limb exoskeleton. This device integrates a semi-active gravity compensation system that combines elastic elements and motors to achieve an optimal torque curve and assistance level, supporting workers in their tasks. Experimental studies conducted in a simulated industrial environment demonstrated reductions in muscle activation of upper-body muscles. Additionally, subjective assessments reported decreased perceived effort and physical demand. Building on these insights, the second part of the study introduces a novel semi-active actuation system: AGtuator, the Anti-Gravity actuator. Unlike traditional approaches, the AGtuator integrates actuation and gravity compensation into a single system. The active and passive elements work in series and are managed by a low-level torque control module. This allows the AGtuator not only to compensate for variable gravitational forces but also to generate customizable torque profiles. Experimental validation demonstrated that the AGtuator enables dynamic compensation of payload variations and the generation of customized torque profiles while maintaining low power consumption, addressing the primary limitations of existing systems. To demonstrate its applicability, the AGtuator was used to develop a prototype upper-limb exoskeleton with shoulder and elbow joints. The results highlight the versatility of the AGtuator and, more broadly, semi-active gravity compensation systems, paving the way for more efficient, adaptable, and sustainable robotic solutions across a wide range of applications.