Variable stiffness actuators were introduced to decouple an otherwise stiff actuator from the load by an adjustable elasticity. This variable elastic element can be used as torque sensor, acts as an energy storage, decouples the actuator for exogenous high frequency excitation inputs and contributes towards shock resistance and safety in human-robot interaction scenarios. However, the variable element complicates the design of torque and impedance controllers which have to be synthesized by employing contradicting design objectives, such as minimisation of the output impedance, robust stability and performance. Moreover, the system to be controlled consists of an additional control-loop to set-up the stiffness of the elastic element in real-time. To overcome these synthesis problems, we present a new controller design procedure that imposes a positive-real constraint on the load output port function to guarantee a stable interaction with respect to a passive, yet otherwise unknown environments. Additional design requirements are subsequently cast into a generalised plant. Ultimately, a H ∞ - nonsmooth design procedure is employed to design a torque controller under these constraints and is tested in in silico experiments with the Mechanical Rotational Impedance Actuator (MeRIA).
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