Impedance Control of Rehabilitation Robots for Lower Limbs, Review

Impedance control is commonly used in rehabilitation robots to maintain the human-robot interaction force below safe levels for the patient, and simultaneously, to control the limb position according to the desired trajectories determined by the therapy. Impedance control aims to regulate the dynamic relationship between the robot force and the actuator movement. On the other hand, design of rehabilitation robots for lower limbs demands specific features that come from their biomechanical functions as weight bearing and locomotion. This paper aims to review the main features for implementing an impedance control in a lower limb rehabilitation robot. Initially, we present a general formulation of the impedance control starting from the dynamic equations toward the most basic forms of this controller. We emphasize that impedance control enable real-time variations in the amount of assistance via the variation of the impedance parameters (inertia, damping and stiffness) of the controller without significant problems of stability. Control performance does not only depend on the control schemes but also a good selection of all the elements of the control loop, including sensors and actuators. We review on the most commonly used actuators for impedance controlled robots. Finally, we addressed the main challenges and limitations for research and development of most efficient implementations and indicates the most relevant trends that literature evidence.