MPTC – Modular Passive Tracking Controller for stack of tasks based control frameworks

Johannes Englsberger (German Aerospace Center (DLR)); Alexander Dietrich (DLR); George Mesesan (German Aerospace Center (DLR)); Gianluca Garofalo (German Aerospace Center (DLR)); Christian Ott (DLR); Alin Albu-Schaeffer (Robotics and Mechatronics Center (RMC), German Aerospace Center (DLR))


This work introduces the so-called Modular Passive Tracking Controller (MPTC), a generic passivity-based controller, which aims at independently fulfilling several subtask objectives. These are combined in a stack of tasks (SoT) that serves as a basis for the synthesis of an overall system controller. The corresponding analysis and controller design are based on Lyapunov theory. An important contribution of this work is the design of a specific optimization weighting matrix that ensures passivity of an overdetermined and thus conflicting task setup. The proposed framework is validated through simulations and experiments for both fixed-base and free-floating robots.

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The paper describes a control approach for fixed and floating base articulated robots building on a hierarchical control structure composed of multiple tasks. It reuses concepts from passivity based task space control formulations together with a deliberately chosen weighing matrix to maintain passivity of the complete hierarchical control framework. The paper addresses different use cases of fully actuated fixed based robots, underactuated robots and robots with constraint actuation. II.C.: "For such cases, further analysis may become necessary." It is not entirely clear what this sentence should tell the reader. Is the stability in the regulation case (unexpected perturbations) completely out of the scope of this paper? Then this should be clearly stated. III.B.: Eqn 26: Why are there weights for the different V_k or how are these weights defined? They seem to be the same as the task weights, is there a reason for this? III.D.: Eqn 39: If the conflict resolution is time variant, does this still work? One stylistic remark: use less emph or italic text, this does not increase readability.