Introducing compliant actuators in robotic systems improves mechanical robustness against rigid impacts as well as unknown contact forces and potentially increases the energetic efficiency. The elastic elements acting between motor and link inertias lead to a dynamical behavior of a low-pass filter on external loads. In contrast to classical flexible-joint robots (FJR) where the linear elasticities are mainly a result of weight reduction, the generally state-dependent stiffness of compliantly actuated robots is in an order of magnitude lower such that singular perturbation assumptions which allow to neglect the motor dynamics definitely do not hold any more. Mechanical compliance provides many benefits, but it also comes at a price; the plant dynamics is under-actuated as the number of dimensions of the configuration space is twice the number of dimensions of the control input space. Moreover, to improve energy storing capabilities and efficiency in general, compliant actuators are often designed such that damping and friction in parallel to the spring is negligible. Thus, unwanted intrinsic oscillatory dynamics may arise. In addition, many variable stiffness robots feature highly nonlinear elasticity. This is what makes control of the link configuration variables a challenging task.
The Video below shows the effectiveness of a novel control approach for motion tracking and damping assignment in compliantly actuated robotic systems with highly nonlinear variable stiffness actuators . The control concept is based on the idea of changing the intrinsic system dynamics only to a minimal extend by solely adding damping terms on the link side and feedforward terms to achieve asymptotic tracking performance. The inherent elastic behavior remains unchanged.
In order to take maximum advantage of the inherent benefits of compliant robots, they are often designed such that damping and friction in parallel to the springs are neglible. As such unwanted, intrinsic oscillatory dynamics arise (see right picture). Active damping control, based on state feedback control, allows us to adjust the convergence behavior of the system to our needs (see left picture).
The combination of active (impedance) control with passive elasticities increases the stiffness range and shape.
The elastic energy storage of the variable stiffness actuators can be exploited to generate explosive and cyclic motions. Thereby, the natural dynamics of the plant is excited.
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