  Variable impedance control is advantageous for physical human-robot
interaction to improve safety, adaptability and many other aspects. This paper
presents a gain-scheduled variable stiffness control approach under strict
frequency-domain constraints. Firstly, to reduce conservativeness, we
characterize and constrain the impedance rendering, actuator saturation,
disturbance/noise rejection and passivity requirements into their specific
frequency bands. This relaxation makes sense because of the restricted
frequency properties of the interactive robots. Secondly, a gain-scheduled
method is taken to regulate the controller gains with respect to the desired
stiffness. Thirdly, the scheduling function is parameterized via a nonsmooth
optimization method. Finally, the proposed approach is validated by
simulations, experiments and comparisons with a gain-fixed passivity-based PID
method.
