MECHANICS/CONTROL

Under-Actuated Tendon-Driven Robot

We present a novel design framework for general under-actuated tendon-driven (UATD) robots to mimic desired free motion while maintaining posture during contact operations. The key enabler is stiffness decomposition, which allows us to decompose UATD robot into actuated and un-actuated space, thereby, allowing us to attain compliant free motion, while minimizing un-actuated space deformation. Design optimization is also proposed, which automatically provides active/passive tendon routing, joint spring, pre-tension, etc.  (ICRA2018 (Accepted))

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Arm-Stage System on Vertical Beam

Simultaneous trajectory tracking and vibration stabilization control of manipulator-stage system on vertical flexible beam, which will be used for operation in height. Euler-Bernoulli and Lagrange dynamics formulation are used with certain boundary conditions to model the system. Coordinate transformation and passive decomposition are used to decompose the total 7-DOF dynamics (4-DOF for arm+stage and 3-DOF for vibration) into vibration+stage and tracking objective. Passivity-based control with damping injection control is then design to these decomposed dynamics. EF-tracking experiment is performed to show the efficacy, for which the typical null-space based tracking control becomes unstable.  (IROS2017)

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PBC of Nonholonomic Systems

Passivity-based control has been widely used for robotic manipulators with its superior robustness as compared to feedback linearization due to its exploitation of open-loop nonlinear dynamics rather than cancel them out.  This passivity-based approach, yet, has been surprisingly missing for nonholonomic mechanical systems.  In this work, we present passivity-based stabilization control for a certain class of nonholonomic systems.  For this, we establish passive configuration decomposition (PCD) and propose passivity-based time-varying and switching controls. We also manifest when PCD is possible; and also establish equivalence of the proposed controls with kinematic controllability. (To Appear in IEEE TRO 2017)

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Passive Decomposition Theory

Passivity is a fundamental property of mechanical systems with close connection with Lyapunov control synthesis.  At the same time, task can often be described by holonomic map h(q) (e.g., grasping shape).  For this, we develop the theory of passive decomposition, i.e., we can decompose open-loop Lagrange robot dynamics into: (1) shape system, describing h(q)-dynamics; (2) locked system, describing system behavior with h(q) fixed; and (3) passive coupling between them. Locked and shape systems individually inherit Lagrange structure and passivity, greatly facilitating their control design. Passive decomposition has also been extended to Riemannian manifold and nonholonomic systems. (IEEE TRO2010, IEEE TAC2013)

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Control of Under-Actuated UAVs

Quadrotor UAV is an under-actuated system, that is, its-DOF is 6 evolving in SE(3), yet, its actuation is only 4-DOF (i.e., 4 rotors).  We investigated the issue of control of this under-actuated quadrotor UAVs. In particular, we utilize backstepping control technique to overcome its under-actuation and also combine it with adaptation to address inertial uncertainty. We also extend this backstepping framework to distributed control of multiple quadrotors with their information flow constrained by a balanced graph. (Automatica2012RAS2014, Best Paper Award IAS2012)

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