Gregg, Robert D.

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Robert Gregg is an Assistant Professor in Bioengineering and was named a Fellow, Eugene McDermott Professor in 2018. He also serves as the head of the Locomotor Control Systems Laboratory. Professor Gregg investigates the control mechanisms of human locomotion for the development of high-performance wearable control systems (e.g., robotic prostheses and orthoses) to enable mobility in persons with disabilities. Learn more about Dr. Gregg on his Home, Departments of Mechanical Engineering and Bioengineering pages, as well as his Research Explorer and Locomotor Control Systems Lab pages.

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Now showing 1 - 6 of 6
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    Decentralized Passivity-Based Control with a Generalized Energy Storage Function for Robust Biped Locomotion
    (American Society of Mechanical Engineers (ASME), 2019-06-13) Yeatman, Mark; Lv, Ge; Gregg, Robert D.; Yeatman, Mark; Lv, Ge; Gregg, Robert D.
    This paper details a decentralized passivity-based control (PBC) to improve the robustness of biped locomotion in the presence of gait-generating external torques and parametric errors in the biped model. Previous work demonstrated a passive output for biped systems based on a generalized energy that, when directly used for feedback control, increases the basin of attraction and convergence rate of the biped to a stable limit cycle. This paper extends the concept with a theoretical framework to address both uncertainty in the biped model and a lack of sensing hardware, by allowing the designer to neglect arbitrary states and parameters in the system. This framework also allows the control to be implemented on wearable devices, such as a lower limb exoskeleton or powered prosthesis, without needing a model of the user's dynamics. Simulations on a six-link biped model demonstrate that the proposed control scheme increases the convergence rate of the biped to a walking gait and improves the robustness to perturbations and to changes in ground slope. © 2019 by ASME.
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    Mechanical Simplification of Variable-Stiffness Actuators Using Dielectric Elastomer Transducers
    (MDPI AG, 2019-05-20) Allen, David P.; Bolívar, Edgar; Farmer, Sophie; Voit, Walter E.; Gregg, Robert D.; 0000-0002-9740-1278 (Allen, DP); 0000-0001-7697-4387 (Bolívar, E); 0000-0002-5898-0449 (Farmer, S); 0000-0003-0135-0531 (Voit, WE); 0000-0002-0729-2857 (Gregg, RD); Allen, David P.; Bolívar, Edgar; Farmer, Sophie; Voit, Walter E.; Gregg, Robert D.
    Legged and gait-assistance robots can walk more efficiently if their actuators are compliant. The adjustable compliance of variable-stiffness actuators (VSAs) can enhance this benefit. However, this functionality requires additional mechanical components making VSAs impractical for some uses due to increased weight, volume, and cost. VSAs would be more practical if they could modulate the stiffness of their springs without additional components, which usually include moving parts and an additional motor. Therefore, we designed a VSA that uses dielectric elastomer transducers (DETs) for springs. It does not need mechanical stiffness-adjusting components because DETs soften due to electrostatic forces. This paper presents details and performance of our design. Our DET VSA demonstrated independent modulation of its equilibrium position and stiffness. Our design approach could make it practical to obtain the benefits of variable-stiffness actuation with less weight, volume, and cost than normally accompanies them, once weaknesses of DET technology are addressed. © 2019 by the authors.
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    Passivity-Based Control with a Generalized Energy Storage Function for Robust Walking of Biped Robots
    (Institute of Electrical and Electronics Engineers Inc.) Yeatman, Mark R.; Lv, Ge; Gregg, Robert D.; Yeatman, Mark R.; Lv, Ge; Gregg, Robert D.
    This paper offers a novel generalization of a passivity-based, energy tracking controller for robust bipedal walking. Past work has shown that a biped limit cycle with a known, constant mechanical energy can be made robust to uneven terrains and disturbances by actively driving energy to that reference. However, the assumption of a known, constant mechanical energy has limited application of this passivity-based method to simple toy models (often passive walkers). The method presented in this paper allows the passivity-based controller to be used in combination with an arbitrary inner-loop control that creates a limit cycle with a constant generalized system energy. We also show that the proposed control method accommodates arbitrary degrees of underactuation. Simulations on a 7-link biped model demonstrate that the proposed control scheme enlarges the basin of attraction, increases the convergence rate to the limit cycle, and improves robustness to ground slopes. © 2018 AACC.
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    Observer-Based Feedback Controllers for Exponential Stabilization of Hybrid Periodic Orbits: Application to Underactuated Bipedal Walking
    (Institute of Electrical and Electronics Engineers Inc.) Hamed, K. A.; Ames, A. D.; Gregg, Robert D.; Gregg, Robert D.
    This paper presents a systematic approach to design observer-based output feedback controllers for hybrid dynamical systems arising from bipedal walking. We consider a class of parameterized observer-based output feedback controllers for local exponential stabilization of hybrid periodic orbits. the properties of the Poincaré map are investigated to show that the Jacobian linearization of the Poincaré map takes a triangular form. This demonstrates the nonlinear separation principle for periodic orbits. In particular, the exponential stabilization of hybrid periodic orbits under dynamic output feedback control can be achieved by solving separate eigenvalue placement problems for the nonlinear state feedback and the observer. the paper then solves the state feedback and observer design problems by employing an iterative algorithm based on a sequence of optimization problems involving bilinear and linear matrix inequalities. the theoretical results are confirmed by designing a nonlinear observer-based output feedback controller for underactuated walking of a 3D humanoid model with 18 state variables, 54 state feedback parameters, and 271 observer parameters.
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    Exponentially Stabilizing Controllers for Multi-Contact 3d Bipedal Locomotion
    (Institute of Electrical and Electronics Engineers Inc.) Hamed, K. A.; Gregg, Robert D.; Ames, A. D.; Gregg, Robert D.
    Models of bipedal walking are hybrid with continuous-time phases representing the Lagrangian stance dynamics and discrete-time transitions representing the impact of the swing leg with the walking surface. The design of continuous-time feedback controllers that exponentially stabilize periodic gaits for hybrid models of underactuated 3D bipedal walking is a significant challenge. We recently introduced a method based on an iterative sequence of optimization problems involving bilinear matrix inequalities (BMIs) to systematically design stabilizing continuous-time controllers for single domain hybrid models of underactuated bipedal robots with point feet. This paper addresses the exponential stabilization problem for multi-contact walking gaits with nontrivial feet. A family of parameterized continuous-time controllers is proposed for different phases of the walking cycle. The BMI algorithm is extended to the multi-domain hybrid models of anthropomorphic 3D walking locomotion to look for stabilizing controller parameters. The Poincaré map is addressed and a new set of sufficient conditions is presented that guarantees the convergence of the BMI algorithm to a stabilizing set of controller parameters at a finite number of iterations. The power of the algorithm is ultimately demonstrated through the design of stabilizing virtual constraint controllers for dynamic walking of a 3D humanoid model with 28 state variables and 275 controller parameters.
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    Evidence for a time-invariant phase variable in human ankle control
    (Public Library Science) Gregg, Robert D.; Rouse, Elliott J.; Hargrove, Levi J.; Sensinger, Jonathon W.
    Human locomotion is a rhythmic task in which patterns of muscle activity are modulated by state-dependent feedback to accommodate perturbations. Two popular theories have been proposed for the underlying embodiment of phase in the human pattern generator: a time-dependent internal representation or a time-invariant feedback representation (i.e., reflex mechanisms). In either case the neuromuscular system must update or represent the phase of locomotor patterns based on the system state, which can include measurements of hundreds of variables. However, a much simpler representation of phase has emerged in recent designs for legged robots, which control joint patterns as functions of a single monotonic mechanical variable, termed a phase variable. We propose that human joint patterns may similarly depend on a physical phase variable, specifically the heel-to-toe movement of the Center of Pressure under the foot. We found that when the ankle is unexpectedly rotated to a position it would have encountered later in the step, the Center of Pressure also shifts forward to the corresponding later position, and the remaining portion of the gait pattern ensues. This phase shift suggests that the progression of the stance ankle is controlled by a biomechanical phase variable, motivating future investigations of phase variables in human locomotor control.

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