EPAIA: Design, modelling and control of a novel electro-pneumatic adaptable impedance actuator

In this contribution we present the novel electro-pneumatic adaptable impedance actuator (EPAIA) consisting of a brushless DC-motor and a rotational pneumatic actuator, applied in series to the gear train. The pneumatic actuator is employed as a pneumatic torsional spring with a continuously adaptable stiffness, controlled by air inflow and pressure control. Thus, a rotary variable stiffness actuator is obtained, for which the stiffness can be expressed as a smooth nonlinear function of chamber pressures. The mechanical design furthermore utilises a planetary and a bevel gear reduction, to provide a torque amplification ratio necessary for human assistive knee joint actuation. Besides large and precise torque generation, the actuator provides a smoothly adjustable output impedance, backdrivability and a low cost, yet quality sensitive torque sensor. Following a detailed description of the mechanical design of the actuator, a dynamical model is derived using the Lagrange formalism. We propose a robust H2-torquecontroller design procedure for the linearised actuator model, for which a constraint for passivity of the load transfer function is employed. The resulting controller is tested in simulations using the nonlinear validated model.