A robust parameterization approach for impedance control

We present an actuator system and an advanced impedance control strategy, designed for the interaction with human limbs. Relying on the inherent stiffness, the connection between actuator and joint is considered to improve safety and guarantee performance. The control approach consists of a cascaded structure. In an outer loop, impedance control provides limb motion support with variable stiffness. The inner linear-quadratic regulator (LQR) pressure control allows for separate control over chamber pressures and mean pressure. A new method has been applied to prove stability for the three-dimensional range of impedance parameters with Kharitonov´s stability criterion for robust positive polynomials. Furthermore robust stability for the perturbed control system has been shown with structured singular value μ. The impedance of the setup is studied in simulation as well as on a test bench. Effects of different mean pressures lead to results for energy efficiency and compressed air energy storage.