Impedence Control for Variable Stiffness Mechanisms with Nonlinear Joint Coupling

The current discussion on physical human robot interaction and the related safety aspects, but also the interest of neuro-scientists to validate their hypotheses on human motor skills with bio-mimetic robots, led to a recent revival of tendondriven robots. In this paper, the modeling of tendon-driven elastic systems with nonlinear couplings is recapitulated. A control law is developed that takes the desired joint position and stiffness as input. Therefore, desired motor positions are determined that are commanded to an impedance controller. We give a physical interpretation of the controller. More importantly, a static decoupling of the joint motion and the stiffness variation is given. The combination of active (controller) and passive (mechanical) stiffness is investigated. The controller stiffness is designed according to the desired overall stiffness. A damping design of the impedance controller is included in these considerations. The controller performance is evaluated in simulation

Efficient computation of inverse dynamics and feedback linearization for VSA-based robots

We develop a recursive numerical algorithm to compute the inverse dynamics of robot manipulators with an arbitrary number of joints, driven by variable stiffness actuation (VSA) of the antagonistic type. The algorithm is based on Newton-Euler dynamic equations, generalized up to the fourth differential order to account for the compliant transmissions, combined with the decentralized nonlinear dynamics of the variable stiffness actuators at each joint. A variant of the algorithm can be used also for implementing a feedback linearization control law for the accurate tracking of desired link and stiffness trajectories. As in its simpler versions, the algorithm does not require dynamicmodeling in symbolic form, does not use numerical approximations, grows linearly in complexity with the number of joints, and is suitable for online feedforward and real-time feedback control. A Matlab/C code is made available

Design, Assessment, and Comparison of Antagonistic, Cable-Driven, Variable Stiffness Actuators

This thesis presents the designs and test results for two antagonistic, cable-driven, variable stiffness actuator designs. Each of these variable stiffness actuators is compact, has a large range of controllable stiffness, and limits the inertia at the robotic link it is controlling. Each design consists of a cable running through a set of three pulleys. Tension on the cable displaces a linear spring, which moves along a path designed to achieve quadratic spring behavior. One design uses a variable radius path to achieve the nonlinear elastic behavior while the other uses a fixed radius (lever) path.A quasi-static model of each mechanism was developed to assess the performance of each design in matching the desired nonlinear (quadratic) elastic behavior of the ideal system. Eight geometric parameters of each design were optimized to match the desired behavior. Prototypes of the optimized designs were built and tested to evaluate performance.While the results of the parametric optimization predicted that the variable radius design would more closely match the desired elastic behavior, the added complexity of this design resulted in inadequate performance. Test results for the fixed radius design matched the desired behavior well and ultimately proved to be better for achieving controllable linear stiffness at a robotic joint