Variable impedance joint module for force controlled manipulation

The work of Series Elastic Actuator (SEA) showed that robots with compliant joint could overcome the drawback due to large impedance of the conventional stiff manipulator. Inspired by SEA, many variable stiffness mechanisms were built to further improve force control performance and to resolve the limitation of SEA due to the fixed spring compliance. These types of variable stiffness joints have one property in common: stiffness is dependent of the output load. This property creates non-linear load-displacement relationship and therefore complicates the system and controller design with non-linear stiffness.