Home               Research               Teaching               Publications               Awards

For more recent publications, please visit my lab webpage.

Journal Articles
Conference Proceedings
Book Chapters
Invited Presentations
  • Geometric Control and Motion Planning for Three-Dimensional Bipedal Locomotion. Ph.D. Dissertation, Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, 2010. (Abstract, PDF)

    This thesis presents a hierarchical geometric control approach for fast and energetically efficient bipedal dynamic walking in three-dimensional (3-D) space to enable motion planning applications that have previously been limited to inefficient quasi-static walkers. In order to produce exponentially stable hybrid limit cycles, we exploit system energetics, symmetry, and passivity through the energy-shaping method of controlled geometric reduction. This decouples a subsystem corresponding to a lower-dimensional robot through a passivity-based feedback transformation of the system Lagrangian into a special form of controlled Lagrangian with broken symmetry, which corresponds to an equivalent closed-loop Hamiltonian system with upper-triangular form. The first control term reduces to mechanically-realizable passive feedback that establishes a functional momentum conservation law that controls the "divided" cyclic variables to set-points or periodic orbits. We then prove extensive symmetries in the class of open kinematic chains to present the multistage application of controlled reduction. A reduction-based control law is derived to construct straightahead and turning gaits for a 4-DOF and 5-DOF hipped biped in 3-D space, based on the existence of stable hybrid limit cycles in the sagittal plane-of-motion. Given such a set of asymptotically stable gait primitives, a dynamic walker can be controlled as a discrete-time switched system that sequentially composes gait primitives from step to step. We derive "funneling" rules by which a walking path that is a sequence of these gaits may be stably followed by the robot. The primitive set generates a tree exploring the action space for feasible walking paths, where each primitive corresponds to walking along a nominal arc of constant curvature. Therefore, dynamically stable motion planning for dynamic walkers reduces to a discrete search problem, which we demonstrate for 3-D compass-gait bipeds. After reflecting on several connections to human biomechanics, we propose extensions of this energy-shaping control paradigm to robot-assisted locomotor therapies.


Peer-Reviewed Journal Articles

  • Virtual Constraint Control of a Powered Prosthetic Leg: From Simulation to Experiments with Transfemoral Amputees. R. Gregg T. Lenzi, L. Hargrove, and J. Sensinger. IEEE Transactions on Robotics, 2014, doi: 10.1109/TRO.2014.2361937. (Abstract, PDF, Experiment Movie)

    Recent powered (or robotic) prosthetic legs independently control different joints and time periods of the gait cycle, resulting in control parameters and switching rules that can be difficult to tune by clinicians. This challenge might be addressed by a unifying control model used by recent bipedal robots, in which virtual constraints define joint patterns as functions of a monotonic variable that continuously represents the gait cycle phase. In the first application of virtual constraints to amputee locomotion, this paper derives exact and approximate control laws for a partial feedback linearization to enforce virtual constraints on a prosthetic leg. We then encode a human-inspired invariance property called effective shape into virtual constraints for the stance period. After simulating the robustness of the partial feedback linearization to clinically meaningful conditions, we experimentally implement this control strategy on a powered transfemoral leg. We report the results of three amputee subjects walking overground and at variable cadences on a treadmill, demonstrating the clinical viability of this novel control approach.



  • Evidence for a Time-Invariant Phase Variable in Human Ankle Control. R. Gregg, E. Rouse, L. Hargrove, and J. Sensinger. PLoS ONE 9(2):e89163, 2014, doi:10.1371/journal.pone.0089163.
    (Abstract, Full Text Open Access)

    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.



  • Towards Biomimetic Virtual Constraint Control of a Powered Prosthetic Leg. R. Gregg and J. Sensinger. IEEE Transactions on Control System Technology, vol. 22, no. 1, 2014. (Abstract, PDF)

    This brief presents a novel control strategy for a powered prosthetic ankle based on a biomimetic virtual constraint. We first derive a kinematic constraint for the "effective shape" of the human ankle-foot complex during locomotion. This shape characterizes ankle motion as a function of the Center of Pressure (COP)--the point on the foot sole where the resultant ground reaction force is imparted. Since the COP moves monotonically from heel to toe during steady walking, we adopt the COP as a mechanical representation of the gait cycle phase in an autonomous feedback controller. We show that our kinematic constraint can be enforced as a virtual constraint by an output linearizing controller that uses only feedback available to sensors onboard a prosthetic leg. Using simulations of a passive walking model with feet, we show that this novel controller exactly enforces the desired effective shape whereas a standard impedance (i.e., proportional-derivative) controller cannot. This work provides a single, biomimetic control law for the entire single-support period during robot-assisted locomotion.



  • Controlled Reduction with Unactuated Cyclic Variables: Application to 3D Bipedal Walking with Passive Yaw Rotation. R. Gregg and L. Righetti. IEEE Transactions on Automatic Control, vol. 58, no. 10, 2013. (Abstract, PDF, Straight-Ahead Movie, Steering Movie)

    This paper shows that viscous damping can shape momentum conservation laws in a manner that stabilizes yaw rotation and enables steering for underactuated 3D walking. We first show that unactuated cyclic variables can be controlled by passively shaped conservation laws given a stabilizing controller in the actuated coordinates. We then exploit this result to realize controlled geometric reduction with multiple unactuated cyclic variables. We apply this underactuated control strategy to a five-link 3D biped to produce exponentially stable straight-ahead walking and steering in the presence of passive yawing.



  • The Difference Between Mechanical Stiffness and Quasi-Stiffness in the Context of Biomechanical Modeling. E. Rouse, R. Gregg, L. Hargrove, and J. Sensinger. IEEE Transactions on Biomedical Engineering, vol. 60, no. 2, pp. 562-568, 2013. (Abstract)

    The ankle contributes the majority of mechanical power during walking and is a frequently studied joint in biomechanics. Specifically, researchers have extensively investigated the torque-angle relationship for the ankle during dynamic tasks, such as walking and running. The slope of this relationship has been termed the “quasi-stiffness.” However, over time, researchers have begun to interchange the concepts of quasi-stiffness and mechanical stiffness. This is an especially important distinction as researchers currently begin to investigate the appropriate control systems for recently developed powered prosthetic legs. The quasi-stiffness and mechanical stiffness are distinct concepts in the context of powered joints, and are equivalent in the context of passive joints. The purpose of this paper is to demonstrate the difference between the mechanical stiffness and quasi-stiffness using a simple impedance controlled inverted pendulum model and a more sophisticated biped walking model, each with the ability to modify the trajectory of an impedance controller’s equilibrium angle position. In both cases, mechanical stiffness values are specified by the controller and the quasi-stiffness are shown during a single “step.” Both models have widely varying quasi-stiffness but each have a single mechanical stiffness value. Therefore, from this simple modeling approach, the differences and similarities between these two concepts are elucidated.



  • On the Mechanics of Functional Asymmetry in Bipedal Walking. R. Gregg, Y. Dhaher, A. Degani, and K. Lynch. IEEE Transactions on Biomedical Engineering, vol. 59, no. 5, pp. 1310-1318, 2012. (Abstract, PDF)

    This paper uses two symmetrical models, the passive compass-gait biped and a five-link 3D biped, to computationally investigate the cause and function of gait asymmetry. We show that for a range of slope angles during passive 2D walking and mass distributions during controlled 3D walking, these models have asymmetric walking patterns between the left and right legs due to the phenomenon of spontaneous symmetry-breaking. In both cases a stable asymmetric family of gaits emerges from a symmetric family of gaits as the total energy increases (e.g., fast speeds). The ground reaction forces of each leg reflect different roles, roughly corresponding to support, propulsion, and motion control as proposed by the hypothesis of functional asymmetry in able-bodied human walking. These results suggest that body mechanics, independent of neurophysiological mechanisms such as leg dominance, may contribute to able-bodied gait asymmetry.



  • Control and Planning of 3D Dynamic Walking with Asymptotically Stable Gait Primitives. R. Gregg, A. Tilton, S. Candido, T. Bretl, and M. Spong. IEEE Transactions on Robotics, vol. 28, no. 6, pp. 1415-1423, 2012. (Abstract, PDF, Movie 1, Movie 2, Movie 3)

    In this paper we present a hierarchical framework that enables motion planning for asymptotically stable 3D bipedal walking in the same way that planning is already possible for ZMP walking. This framework is based on the construction of asymptotically stable gait primitives for a class of hybrid dynamical systems with impacts. Each primitive corresponds to an asymptotically stable hybrid limit cycle that admits a priori rules for sequential composition with other primitives, reducing a high-dimensional feedback motion planning problem into a low-dimensional discrete tree search. As a constructive example, we develop this planning framework for the 3D compass-gait biped, where each primitive corresponds to walking along an arc of constant curvature for a fixed number of steps. We apply a discrete search algorithm to plan a sequence of these primitives taking the biped stably from start to goal in three-dimensional workspaces with obstacles. We finally show how this framework generalizes to more complex models by planning walking paths for an underactuated five-link biped.



  • Reduction-Based Control of Three-Dimensional Bipedal Walking Robots. R. Gregg and M. Spong. International Journal of Robotics Research, vol. 26, no. 6, pp. 680-702, 2010.
    (Abstract, Full Text, Supplemental Equations, Straight Walking Movie, Turning Movie)

    In this paper we develop the concept of reduction-based control, which is founded on a controlled form of geometric reduction known as functional Routhian reduction. We prove a geometric property of general serial-chain robots termed recursive cyclicity, identifying the inherent robot symmetries that we exploit with the Subrobot Theorem. This shows that any serial-chain robot can be decomposed for arbitrarily lower-dimensional analysis and control. We apply this method to construct stable directional three-dimensional walking gaits for a four-degree-of-freedom hipped bipedal robot. The controlled reduction decouples the biped’s sagittal-plane motion from the yaw and lean modes, and on the sagittal subsystem we use passivity-based control to produce known planar limit cycles on flat ground. The unstable yaw and lean modes are separately controlled to 2-periodic orbits through their shaped momenta. We numerically verify the existence of stable 2-periodic straight-walking limit cycles and demonstrate turning capabilities for the controlled biped.


Peer-Reviewed Conference Proceedings

  • Experimental Effective Shape Control of a Powered Transfemoral Prosthesis. R. Gregg, T. Lenzi, N. Fey, L. Hargrove, and J. Sensinger. In the 2013 IEEE Int. Conf. Rehabilitation Robotics, Seattle, WA. (Abstract, PDF, Experiment Movie)

    This paper presents the design and experimental implementation of a novel feedback control strategy that regulates effective shape on a powered transfemoral prosthesis. The human effective shape is the effective geometry to which the biological leg conforms--through movement of ground reaction forces and leg joints--during the stance period of gait. Able-bodied humans regulate effective shapes to be invariant across conditions such as heel height, walking speed, and body weight, so this measure has proven to be a very useful tool for the alignment and design of passive prostheses. However, leg joints must be actively controlled to assume different effective shapes that are unique to tasks such as standing, walking, and stair climbing. Using our previous simulation studies as a starting point, we model and control the effective shape as a virtual kinematic constraint on the powered Vanderbilt prosthetic leg with a custom instrumented foot. An able-bodied subject used a by-pass adapter to walk on the controlled leg over ground and over a treadmill. These preliminary experiments demonstrate, for the first time, that effective shape (or virtual constraints in general) can be used to control a powered prosthetic leg.



  • Biomimetic Virtual Constraint Control of a Transfemoral Powered Prosthetic Leg. R. Gregg and J. Sensinger. In the 2013 American Control Conference, Washington, DC. (Abstract, PDF, Movie)

    This paper presents a novel control strategy for a powered knee-ankle prosthesis based on biomimetic virtual constraints. We begin by deriving kinematic constraints for the "effective shape" of the human leg during locomotion. This shape characterizes ankle and knee motion as a function of the Center of Pressure (COP)--the point on the foot sole where the ground reaction force is imparted. Since the COP moves monotonically from heel to toe during steady walking, we adopt the COP as the phase variable of an autonomous feedback controller. We show that our kinematic constraints can be enforced virtually by an output linearizing controller that uses only feedback available to sensors onboard a prosthetic leg. This controller produces walking gaits with human-like knee flexion in simulations of a 6-link biped with feet. Hence, both knee and ankle control can be coordinated by one simple control objective: maintaining a constant-curvature effective shape.



  • Towards Formal Verification Methods for Robotic Lower-Limb Prostheses and Orthoses. R. Gregg and U. Topcu. In the 2013 Medical Cyber Physical Systems Workshop, CPS Week, Philadelphia, PA. (Abstract, PDF)

    Populations including lower-limb amputees, stroke survivors, and the elderly have an increased risk for falls, which significantly limits mobility and quality of life. Recently developed robotic prostheses and orthoses are capable of actively assisting mobility in these populations. New risk management and design control tools are needed to meet the evolving regulatory demands for these increasingly complex systems, which can impart large torques and forces that could potentially harm the user. This paper attempts to introduce formal verification methods to this rapidly developing field by discussing some topical problems, the motivating example of a prosthetic leg, and potential verification metrics that could be used to certify the safety of these robotic medical devices in a pragmatic and cost-effective manner.



  • Stable Open-Loop Brachiation on a Vertical Wall. N. Rosa, A. Barber, R. Gregg, K. Lynch. In the 2012 IEEE International Conference on Robotics and Automation, St. Paul, MN. (Abstract)

    This paper presents a hybrid mechanical model for the Gibbot, a robot that dynamically climbs along a flat vertical wall in a manner analogous to gibbons swinging between branches in the forest canopy. We focus on one particular style of gait, continuous-contact brachiation, which always has one handhold in contact with the wall. We use zero cost, unstable solutions corresponding to horizontal brachiation, originally found by Gomes and Ruina, as templates to generate open-loop stable gaits in arbitrary directions. The first case considered is passive brachiation down a shallow slope, roughly corresponding to upside-down locomotion of the well-studied compass-gait biped. We then consider underactuated brachiation with a constant forcing term at the elbow to produce open-loop stable descending and ascending gaits.



  • Functional Asymmetry in a Five-Link 3D Bipedal Walker. R. Gregg, Y. Dhaher, and K. Lynch. In the 2011 International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), Boston, MA. (Abstract, PDF)

    This paper uses a symmetrical five-link 3D biped model to computationally investigate the cause, function, and benefit of gait asymmetry. We show that for a range of mass distributions, this model has asymmetric walking patterns between the left and right legs, which is due to a phenomenon known as period-doubling bifurcation. The ground reaction forces of each leg reflect different roles, roughly corresponding to support, propulsion, and motion control as proposed by the hypothesis of functional asymmetry in human walking. These results suggest that natural mechanics could be responsible for asymmetry in able-bodied walking, rather than neurophysiological mechanisms such as leg dominance.



  • Controlled Reduction of a Five-Link 3D Biped with Unactuated Yaw. R. Gregg. In the 2011 IEEE Conference on Decision and Control, Orlando, FL. (Abstract, PDF, Movie)

    This paper presents a formulation of controlled geometric reduction with one degree of underactuation for mechanical systems with an unactuated cyclic variable subject to passive damping. We show that the first control term in the fully actuated case reduces to passive joint-velocity feedback, which can be equivalently provided by viscous friction. The underactuated control strategy is applied to a five-link 3D biped with a hip, torso, knees, and unactuated yaw at the foot contact point. We show asymptotically stable walking in the presence of passive yawing for realistic friction coefficients.



  • The Simplest Parkour Model: Experimental Validation and Stability Analysis. A. Long, R. Gregg, and K. Lynch. In the 2011 International Conference on Climbing and Walking Robots, Paris, France.
    (Abstract, Experiment Video)

    We describe and experimentally validate the Simplest Parkour Model (SPM) for the ParkourBot, a planar dynamic climbing robot equipped with two springy BowLegs. By controlling the leg angles and injected energy at impact, the ParkourBot is capable of climbing up and down in a rigid chute on an inclined air table. The SPM consists of a point mass and two massless legs. The legs are assumed to be infinitely stiff, resulting in an instantaneous stance phase and a closed-form solution of the hybrid dynamics. In this paper, we show that the SPM is a good predictor of the actual experimental behavior. Using the SPM we compute the fixed points, stability and basins of attraction of period-1 limit cycles.



  • The Basic Mechanics of Bipedal Walking Lead to Asymmetric Behavior. R. Gregg, A. Degani, Y. Dhaher, and K. Lynch. In the 2011 IEEE International Conference on Rehabilitation Robotics, Zurich, Switzerland. (Abstract, PDF, Poster, Fast Forward Presentation)

    This paper computationally investigates whether gait asymmetries can be attributed in part to basic bipedal mechanics independent of motor control. Using a symmetrical rigid-body model known as the compass-gait biped, we show that changes in environmental or physiological parameters can facilitate asymmetry in gait kinetics at fast walking speeds. In the environmental case, the asymmetric family of high-speed gaits is in fact more stable than the symmetric family of low-speed gaits. These simulations suggest that lower extremity mechanics might play a direct role in functional and pathological asymmetries reported in human walking, where velocity may be a common variable in the emergence and growth of asymmetry.



  • A Control Theoretic Approach to Robot-Assisted Locomotor Therapy. R. Gregg, T. Bretl, and M. Spong. In the 2010 IEEE Conference on Decision and Control, Atlanta, GA. Reprint. (Abstract, PDF, Experiment 1 Video, Experiment 2 Video)

    This paper proposes a control theoretic strategy for human walking gait assistance based on underactuated potential energy shaping. We design a simple control law that lessens the perceived weight of the patient’s center of mass through a robotic ankle-foot orthosis (AFO) with one actuated degree-of-freedom. We then adopt a passive “compass-gait” bipedal walker as an implicit model of human locomotor behavior, which we simulate to draw beneficial implications for rehabilitation such as energy regulation, improved stability, and progressive training by Lyapunov funneling. Given current challenges in developing effective robot-assisted locomotor therapies, this paper offers a novel systematic approach to control strategy design for gait training and at-home assistance.



  • Constrained Accelerations for Controlled Geometric Reduction: Sagittal-Plane Decoupling in Bipedal Locomotion. R. Gregg, L. Righetti, J. Buchli, S. Schaal. In the 2010 IEEE International Conference on Humanoid Robots, Nashville, TN. (Abstract, PDF, Movie)

    Energy-shaping control methods have produced strong theoretical results for asymptotically stable 3D bipedal dynamic walking in the literature. In particular, geometric controlled reduction exploits robot symmetries to control momentum conservation laws that decouple the sagittal-plane dynamics, which are easier to stabilize. However, the associated control laws require high-dimensional matrix inverses multiplied with complicated energy-shaping terms, often making these control theories difficult to apply to highly-redundant humanoid robots. This paper presents a first step towards the application of energy-shaping methods on real robots by casting controlled reduction into a framework of constrained accelerations for inverse dynamics control. By representing momentum conservation laws as constraints in acceleration space, we construct a general expression for desired joint accelerations that render the constraint surface invariant. By appropriately choosing an orthogonal projection, we show that the unconstrained (reduced) dynamics are decoupled from the constrained dynamics. Any acceleration-based controller can then be used to stabilize this planar subsystem, including passivity-based methods. The resulting control law is surprisingly simple and represents a practical way to employ control theoretic stability results in robotic platforms. Simulated walking of a 3D compass-gait biped show correspondence between the new and original controllers, and simulated motions of a 16-DOF humanoid demonstrate the applicability of this method.



  • Asymptotically Stable Gait Primitives for Planning Dynamic Bipedal Locomotion in Three Dimensions. R. Gregg, T. Bretl, and M. Spong. In the 2010 IEEE International Conference on Robotics and Automation, Anchorage, AK. (Abstract, PDF, Slides, Random Walk Movie, Planned Walking Movie)

    This paper applies geometric reduction-based control to derive a set of asymptotically stable dynamic walking gaits for a 3-D bipedal robot, each corresponding to walking along a nominal arc of constant curvature for a fixed number of steps. We show that any such set of asymptotically stable gait primitives may be composed in arbitrary order without causing the robot to fall, so any walking path that is a sequence of these gaits may be followed by the robot. This result enables motion planning for bipedal dynamic walkers, which are fast and energetically efficient, in a similar manner to what is already possible for biped locomotion based on Zero Moment Point (ZMP) equilibrium constraints.



  • Reduction-Based Control of Branched Chains: Application to Three-Dimensional Bipedal Torso Robots. R. Gregg and M. Spong. In the 2009 IEEE Conference on Decision and Control, Shanghai, China.
    (Abstract, PDF, Slides, Supplemental Equations, Straight Walking Movie, Turning Movie)

    This paper revisits the concept of controlled reduction, a symmetry-based method of decomposing the control of mechanical systems into lower-dimensional problems. Founded on a geometric property of serial kinematic chains termed recursive cyclicity, the present work extends this property to branched chains by defining the minimal irreducible tree structure. We then generalize the Subrobot Theorem to branched chains, showing that we can use control to reduce any robot’s dimensionality to that of its irreducible tree structure. This method is applied to a 5-DOF bipedal robot with a hip and torso – we construct stable straight-ahead and turning gaits in 3-D space based on known sagittal-plane limit cycles.



  • Bringing the Compass-Gait Bipedal Walker to Three Dimensions. R. Gregg and M. Spong. In the 2009 IEEE International Conference on Intelligent Robots and Systems, St. Louis, MO.
    (Abstract, PDF, Slides, 4-DOF Eqns, 5-DOF Eqns, 4-DOF Movie, 5-DOF Movie)

    The planar compass-gait biped has been extensively studied in the dynamic walking community, motivated by the gravity-based pendular efficiencies of human walking. These results can be extended to three dimensions using controlled geometric reduction for open-chain robots, by which stable 3- D walking gaits are built from known sagittal-plane limit cycles. We apply this method to the standard and with-torso compassgait (hipless) bipeds, showing straight-ahead walking gaits (i.e., stable 1-step periodic limit cycles) as well as h-step turning in full circles (i.e., stable h-periodic limit cycles). These constantcurvature maneuvers are composed of stable 1-periodic turning gaits modulo heading change, demonstrating two types of gaits for directional dynamic walking in three dimensions



  • Reduction-based Control with Application to Three-Dimensional Bipedal Walking Robots. R. Gregg and M. Spong. In the 2008 American Control Conference, Seattle, WA. Reprint.
    (Abstract, PDF, Supplemental Equations, Straight Walking Movie, Turning Movie)

    This paper develops the concept of reduction-based control, which is founded on a controlled form of geometric reduction known as functional Routhian reduction. We introduce a geometric property of general serial-chain robots termed recursive cyclicity, leading to our presentation of the Subrobot Theorem. This shows that reduction-based control can arbitrarily reduce the dimensionality of any serial-chain robot, so that it may be controlled as a simpler “subrobot” while separately controlling the divided coordinates through their conserved momenta. This method is applied to construct stable directional 3-D walking gaits for a 4-d.o.f. hipped bipedal robot. The walker’s sagittal-plane subsystem can be decoupled from its yaw and lean modes, and on this planar subsystem we use passivity-based control to construct limit cycles on flat ground. Due to the controlled reduction, the unstable yaw and lean modes are separately controlled to 2-periodic orbits. We numerically verify the existence of stable 2-periodic limit cycles and demonstrate turning capabilities for the controlled biped.



  • A Geometric Approach to Three-Dimensional Hipped Bipedal Robotic Walking.
    A. Ames, R. Gregg, and M. Spong. In the 2007 IEEE Conference on Decision and Control, New Orleans, LA. (Abstract, PDF)

    This paper presents a control law that results in stable walking for a three-dimensional bipedal robot with a hip. To obtain this control law, we utilize techniques from geometric reduction, and specifically a variant of Routhian reduction termed functional Routhian reduction, to effectively decouple the dynamics of the three-dimensional biped into its sagittal and lateral components. Motivated by the decoupling afforded by functional Routhian reduction, the control law we present is obtained by combining three separate control laws: the first shapes the potential energy of the sagittal dynamics of the biped to obtain stable walking gaits when it is constrained to the sagittal plane, the second shapes the total energy of the walker so that functional Routhian reduction can be applied to decoupling the dynamics of the walker for certain initial conditions, and the third utilizes an output zeroing controller to stabilize to the surface defining these initial conditions. We numerically verify that this method results in stable walking, and we discuss certain attributes of this walking gait.



  • Stably Extending Two-Dimensional Bipedal Robotic Walking to Three Dimensions.
    A. Ames and R. Gregg. In the 2007 American Control Conference, New York City, New York. (Abstract, PDF)

    In this paper we develop a feedback control law that results in stable walking gaits on flat ground for a threedimensional bipedal robotic walker given stable walking gaits for a two-dimensional bipedal robotic walker. This is achieved by combining disparate techniques that have been employed in the bipedal robotic community: controlled symmetries, geometric reduction and hybrid zero dynamics. Controlled symmetries are utilized to obtain stable walking gaits for a two-dimensional bipedal robot walking on flat ground. These are related to walking gaits for a three-dimensional (hipless) bipedal robot through the use of geometric reduction. Finally, these walking gaits in three dimensions are made stable through the use of hybrid zero dynamics.



  • Towards the Geometric Reduction of Controlled 3D Bipedal Walking Robots.
    A. Ames, R. Gregg, E. Wendel, S. Sastry. In the IFAC 3rd Workshop on Lagrangian and Hamiltonian Methods for Nonlinear Control, Nagoya, Japan. (Abstract)

    The purpose of this paper is to apply methods from geometric mechanics to the analysis and control of bipedal robotic walkers. We begin by introducing a generalization of Routhian reduction, functional Routhian Reduction, which allows for the conserved quantities to be functions of the cyclic variables rather than constants. Since bipedal robotic walkers are naturally modeled as hybrid systems, which are inherently nonsmooth, in order to apply this framework to these systems it is necessary to first extend functional Routhian reduction to a hybrid setting.We apply this extension, along with potential shaping and controlled symmetries, to derive a feedback control law that provably results in walking gaits on flat ground for a three-dimensional bipedal walker given walking gaits in two-dimensions.



  • Is there Life after Zeno? Taking Executions past the Breaking (Zeno) Point. A. Ames, H. Zheng, R. Gregg, S. Sastry. In the 2006 American Control Conference, Minneapolis, MN. (Abstract)

    Understanding Zeno phenomena plays an important role in understanding hybrid systems. A natural—and intriguing—question to ask is: what happens after a Zeno point? Inspired by the construction of (Filippov, 1999), we propose a method for extending Zeno executions past a Zeno point for a class of hybrid systems: Lagrangian hybrid systems. We argue that after the Zeno point is reached, the hybrid system should switch to a holonomically constrained dynamical system, where the holonomic constraints are based on the unilateral constraints on the configuration space that originally defined the hybrid system. These principles are substantiated with a series of examples.


Book Chapters

  • On the Geometric Reduction of Controlled Three-Dimensional Bipedal Robotic Walkers. A. Ames, R. Gregg, E. Wendel, and S. Sastry. Lagrangian and Hamiltonian Methods for Nonlinear Control, ser. Lecture Notes in Control and Information Sciences, vol. 366. Nagoya, Japan: Springer, 2007, pp. 183-196. (Abstract, Full Text)

    The purpose of this paper is to apply methods from geometric mechanics to the analysis and control of bipedal robotic walkers. We begin by introducing a generalization of Routhian reduction, functional Routhian Reduction, which allows for the conserved quantities to be functions of the cyclic variables rather than constants. Since bipedal robotic walkers are naturally modeled as hybrid systems, which are inherently nonsmooth, in order to apply this framework to these systems it is necessary to first extend functional Routhian reduction to a hybrid setting. We apply this extension, along with potential shaping and controlled symmetries, to derive a feedback control law that provably results in walking gaits on flat ground for a three-dimensional bipedal walker given walking gaits in two dimensions.


Invited Presentations

  • “The Hypothesis of Feedback Pattern Generation in Human Locomotion,” Dynamic Walking Conference, Pittsburgh, PA, June 2013. (Abstract, Video)


  • Departmental Seminar, Department of Electrical and Computer Engineering, University of Illinois at Chicago, Chicago, IL, February 2013.


  • Controls Seminar, Coordinated Science Laboratory, University of Illinois, Urbana, IL, February 2013.


  • “From Machine to Biomimetic Control of Powered Prosthetic Legs,” Dynamic Walking Conference, Pensacola, FL, May 2012. (Video)


  • “Translational Control Design for Lower-Limb Prosthetics and Orthotics,” Departmental Seminar, Department of Bioengineering, University of Texas, Dallas, TX, February 2012.


  • “From Bipedal Robotic Walking to Robot-Assisted Locomotor Therapy,” Departmental Seminar, Institute of Automatic Control Engineering, Technical University of Munich, Germany, March 2011.


  • “Progress in Translating Control Theoretic Principles into Robot-Assisted Locomotor Therapy,” Departmental Seminar, Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL, January 2011.


  • “Control and Planning with Asymptotically Stable Gait Primitives: 3D Dynamic Walking to Locomotor Rehabilitation?,” Dynamic Walking Conference, Massachusetts Institute of Technology, Cambridge, MA, July 2010. (Abstract, Video, Slides, Planned Walking Movie)


  • "Passivity, Symmetry, and Reduction in the Control of Bipedal Locomotion," Workshop on 20 Years of Passivity-Based Control: Theory and Applications, IEEE Conference on Decision and Control, Shanghai, China, December 2009. (Slides)

© 2009 Robert D. Gregg, IV