Publications

Prosthesis Design and Control

Exoskeleton Design and Control

Modeling and Measuring Human Locomotion

Control of Autonomous Legged Robots

Actuator Design and Optimization

Other Control Topics


Prosthesis Design and Control


  • Design and Validation of a Powered Knee-Ankle Prosthesis with High-Torque, Low-Impedance Actuators. T. Elery, S. Rezazadeh, C. Nesler, and R. Gregg. IEEE Transactions on Robotics, under review. (Abstract)

  • A Phase Variable Approach to Improved Volitional and Rhythmic Control of Powered Knee-Ankle Prostheses. S. Rezazadeh, D. Quintero, N. Divekar, E. Reznick, L. Gray, and R. Gregg. IEEE Transactions on Robotics, under review. (Abstract, PDF)

  • Extremum Seeking Control for Model-Free Auto-Tuning of Powered Prosthetic Legs. S. Kumar, A. Mohammadi, D. Quintero, S. Rezazadeh, N. Gans, and R. Gregg. IEEE Transactions on Control Systems Technology, under review. (Abstract)

  • Variable Impedance Control of Powered Knee Prostheses Using Human-Inspired Algebraic Curves. A. Mohammadi and R. Gregg. ASME Journal of Nonlinear and Computational Dynamics, under review. (Abstract)

  • Intuitive Clinician Control Interface for a Powered Knee-Ankle Prosthesis: A Case Study. D. Quintero, E. Reznick, D. Lambert, S. Rezazadeh, L. Gray, and R. Gregg. IEEE Journal of Translational Engineering in Health and Medicine, 6(1): 1-9, 2018, doi: 10.1109/JTEHM.2018.2880199. (Abstract, PDF, Video)

  • A Phase Variable Approach to Volitional Control of Powered Knee-Ankle Prostheses. S. Rezazadeh, D. Quintero, N. Divekar, and R. Gregg. In IEEE Int. Conf. Intelligent Robots & Systems, Spain, 2018. (Abstract, PDF, Video)

  • Design and Benchtop Validation of a Powered Knee-Ankle Prosthesis with High-Torque, Low-Impedance Actuators. T. Elery, S. Rezazadeh, C. Nesler, J. Doan, H. Zhu, and R. Gregg. In IEEE Int. Conf. Robotics & Automation, Brisbane, Australia, 2018. (Abstract, PDF, Video)

  • Continuous-Phase Control of a Powered Knee-Ankle Prosthesis: Amputee Experiments Across Speeds and Inclines. D. Quintero, D. Villarreal, D. Lambert, S. Kapp, and R. Gregg. IEEE Transactions on Robotics, 34(3): 686-701, 2018, doi: 10.1109/TRO.2018.2794536. (Abstract, PDF, Video)

  • Automatic Tuning of Virtual Constraint-Based Control Algorithms for Powered Knee-Ankle Prostheses. S. Kumar, A. Mohammadi, N. Gans, and R. Gregg. In IEEE Conference on Control Technology and Applications (Invited Session on Robotic Locomotion Control), Hawaii, 2017. Best Student Paper Award Finalist. (Abstract, PDF, Video)

  • Piecewise and Unified Phase Variables in the Control of a Powered Prosthetic Leg. D. Villarreal, D. Quintero and R. Gregg. In IEEE International Conference on Rehabilitation Robotics, London, United Kingdom, 2017. (Abstract, PDF, Experiment Video)

  • Stable, Robust Hybrid Zero Dynamics Control of Powered Lower-Limb Prostheses. A. Martin and R. Gregg. IEEE Transactions on Automatic Control, 62(8): 3930-3942, 2017, doi: 10.1109/TAC.2017.2648040. (Abstract, PDF)

  • Toward Unified Control of a Powered Prosthetic Leg: A Simulation Study. D. Quintero, A. Martin, and R. Gregg. IEEE Transactions on Control Systems Technology, 26(1): 305-312, 2018, doi: 10.1109/TCST.2016.2643566. (Abstract, PDF)

  • Preliminary Experiments with a Unified Controller for a Powered Knee-Ankle Prosthetic Leg Across Walking Speeds. D. Quintero, D. Villarreal, and R. Gregg. In IEEE Int. Conf. Intelligent Robots & Systems, Daejeon, Korea, 2016. (Abstract, PDF, Experiment Video)

  • A Haptic Feedback System for Phase-Based Sensory Restoration in Above-Knee Prosthetic Leg Users. A. Plauche, D. Villarreal, and R. Gregg. IEEE Transactions on Haptics, 9(3): 421-426, 2016, doi: 10.1109/TOH.2016.2580507. (Abstract, PDF)

  • Prosthetic Leg Control in the Nullspace of Human Interaction. R. Gregg and A. Martin. In Invited Session on Control Theory in Legged Locomotion, American Control Conf., Boston, MA, 2016. (Abstract, PDF)

  • Evaluation of Transradial Body-Powered Prostheses Using a Robotic Simulator. R. Ayub, D. Villarreal, R. Gregg, and F. Gao. Prosthetics & Orthotics International, 2016, doi: 10.1177/0309364616650077. (Abstract, Full Text)

  • Unifying the Gait Cycle in the Control of a Powered Prosthetic Leg. D. Quintero, A. Martin, and R. Gregg. In IEEE Int. Conf. Rehabilitation Robotics, Singapore, 2015. (Abstract)

  • Hybrid Invariance and Stability of a Feedback Linearizing Controller for Powered Prostheses. A. Martin and R. Gregg. In American Control Conference, Chicago, IL, 2015. (Abstract, PDF)

  • 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, 30(6): 1455-1471, 2014, doi: 10.1109/TRO.2014.2361937.
    (Abstract, PDF, Experiment Video)

Exoskeleton Design and Control


  • Contact-Invariant Total Energy Shaping for Powered Exoskeletons. J. Lin, G. Lv, and R. Gregg. Submitted to American Control Conference, 2019. (Abstract)

  • A Powered Knee-Ankle Orthosis with Quasi-Direct Drive Actuation for Partial Gait Assistance. H. Zhu, J. Doan, C. Stence, G. Lv, T. Elery, and R. Gregg. IEEE/ASME Transactions on Mechatronics, under review. (Abstract)

  • On the Design and Control of Highly Backdrivable Lower-Limb Exoskeletons. G. Lv, H. Zhu, and R. Gregg. IEEE Control Systems Magazine, 38(6): 88-113, 2018, doi: 10.1109/MCS.2018.2866605. (Abstract, PDF)

  • Towards Total Energy Shaping Control of Lower-Limb Exoskeletons. G. Lv and R. Gregg. In American Control Conference, Seattle, WA, 2017. (Abstract, PDF)

  • Design and Validation of a Torque Dense, Highly Backdrivable Powered Knee-Ankle Orthosis. H. Zhu, J. Doan, C. Stence, G. Lv, T. Elery, and R. Gregg. In IEEE Int. Conf. Robotics & Automation, Singapore, 2017. (Abstract, PDF, Experiment Video)

  • Underactuated Potential Energy Shaping with Contact Constraints: Application to a Powered Knee-Ankle Orthosis. G. Lv and R. Gregg. IEEE Transactions on Control Systems Technology, 26(1): 181-193, 2018, doi: 10.1109/TCST.2016.2646319. (Abstract, PDF)

  • Experimental Implementation of Underactuated Potential Energy Shaping on a Powered Ankle-Foot Orthosis. G. Lv, H. Zhu, T. Elery, L. Li, and R. Gregg. In IEEE Int. Conf. Robotics & Automation, Stockholm, Sweden, 2016. (Abstract, PDF, Experiment Video)

  • Orthotic Body-Weight Support Through Underactuated Potential Energy Shaping with Contact Constraints. G. Lv and R. Gregg. In IEEE Conf. Decision & Control, Osaka, Japan, 2015. Best Student Paper Award. (Abstract, PDF)

Modeling and Measuring Human Locomotion


  • Modeling the Kinematics of Human Locomotion over Continuously Varying Speeds and Inclines. K. Embry, D. Villarreal, R. Macaluso, and R. Gregg. IEEE Trans. Neural Systems Rehabilitation Eng., 26(12): 2342-2350, 2018, doi: 10.1109/TNSRE.2018.2879570 (Abstract, PDF, Data)

  • Human-Inspired Algebraic Curves for Wearable Robot Control. A. Mohammadi and R. Gregg. In ASME Dynamic Systems & Control Conference, 2018. (Abstract, PDF)

  • Real-Time Continuous Gait Phase and Speed Estimation from a Single Sensor. D. Quintero, D. Lambert, D. Villarreal, and R. Gregg. In IEEE Conf. on Control Technology and Applications (Invited Session on Robotic Locomotion Control), Hawaii, 2017. (Abstract, PDF)

  • A Robust Parameterization of Human Gait Patterns Across Phase-Shifting Perturbations. D. Villarreal, H. Poonawala, and R. Gregg. IEEE Trans. Neural Systems & Rehabilitation Engineering, 25(3): 265-278, 2017, doi: 10.1109/TNSRE.2016.2569019. (Abstract, PDF, Video)

  • Unified Phase Variables of Relative Degree Two for Human Locomotion. D. Villarreal and R. Gregg. In IEEE Engineering in Medicine and Biology Conference, Orlando, FL, 2016. (Abstract, PDF)

  • A Unified Parameterization of Human Gait Across Ambulation Modes. K. Embry, D. Villarreal, and R. Gregg. In IEEE Engineering in Medicine and Biology Conference, Orlando, FL, 2016. (Abstract, PDF)

  • A Perturbation Mechanism for Investigations of Phase-Dependent Behavior in Human Locomotion. D. Villarreal, D. Quintero, and R. Gregg. IEEE Access, 4: 893-904, 2016, doi: 10.1109/ACCESS.2016.2535661. (Abstract, Open Access, Data)

  • Incorporating Human-like Walking Variability in an HZD-Based Bipedal Model. A. Martin and R. Gregg. IEEE Transactions on Robotics, 32(4): 943-948, 2016, doi: 10.1109/TRO.2016.2572687. (Abstract, PDF)

  • Characterizing and Modeling the Joint-level Variability in Human Walking. A. Martin, D. Villarreal, and R. Gregg. Journal of Biomechanics, 49(14): 3298-3305, 2016, doi: 10.1016/j.jbiomech.2016.08.015. (Abstract, PDF)

  • A Perturbation Mechanism for Investigations of Phase Variables in Human Locomotion. D. Villarreal, D. Quintero, and R. Gregg. In IEEE Int. Conf. Robotics & Biomimetics (ROBIO), Zhuhai, China, 2015. (Abstract, PDF, Experiment Video)

  • A Survey of Phase Variable Candidates of Human Locomotion. D. Villarreal and R. Gregg. In IEEE Engineering in Medicine and Biology Conference, Chicago, IL, 2014. (PDF)

Control of Autonomous Legged Robots


  • Decentralized Passivity-Based Control with a Generalized Energy Storage Function for Robust Biped Locomotion. M. Yeatman, G. Lv, and R. Gregg. ASME Journal of Dynamic Systems, Measurement, and Control, under review. (Abstract)

  • Hybrid Zero Dynamics of Bipedal Robots Under Nonholonomic Virtual Constraints. J. Horn, A. Mohammadi, K. Hamed, and R. Gregg. IEEE Control Systems Letters, 2018, in press. (Abstract, PDF)

  • Hybrid Nonlinear Disturbance Observer Design for Underactuated Bipedal Robots. A. Mohammadi, S. Fakoorian, J. Horn, D. Simon, and R. Gregg. To appear in IEEE Decision & Control Conference, 2018. (Abstract, PDF)

  • Decentralized Event-Based Controllers for Robust Stabilization of Hybrid Periodic Orbits: Application to Underactuated 3D Bipedal Walking. K. Hamed and R. Gregg. IEEE Transactions on Automatic Control, 2018, doi: 10.1109/TAC.2018.2863184. (Abstract, PDF)

  • Passivity-Based Control with a Generalized Energy Storage Function for Robust Bipedal Walking. M. Yeatman, G. Lv, and R. Gregg. In American Control Conference, Milwaukee, WI, 2018. (Abstract, PDF)

  • Observer-Based Feedback Controllers for Exponential Stabilization of Hybrid Periodic Orbits: Application to Underactuated Bipedal Walking. K. Hamed, A. Ames, and R. Gregg. In American Control Conference, Milwaukee, WI, 2018. (Abstract, PDF)

  • Exponentially Stabilizing Controllers for Multi-Contact 3D Bipedal Locomotion. K. Hamed, R. Gregg, and A. Ames. In American Control Conference, Milwaukee, WI, 2018. (Abstract, PDF)

  • Removing Phase Variables from Biped Robot Parametric Gaits. A. Mohammadi, J. Horn, and R. Gregg. In IEEE Conference on Control Technology and Applications (Invited Session on Robotic Locomotion Control), Hawaii, 2017. (Abstract, PDF)

  • Decentralized Feedback Controllers for Robust Stabilization of Periodic Orbits of Hybrid Systems: Application to Bipedal Walking. K. A. Hamed and R. Gregg. IEEE Transactions on Control Systems Technology, 25(4): 1153-1167, 2017, doi: 10.1109/TCST.2016.2597741. (Abstract, PDF)

  • A Control Framework for Anthropomorphic Biped Walking Based on Stabilizing Feedforward Trajectories. S. Rezazadeh and R. Gregg. In ASME Dynamic Systems & Control Conference, Minneapolis, MN, 2016. (Abstract, PDF)

  • Decentralized Feedback Controllers for Exponential Stabilization of Hybrid Periodic Orbits: Application to Robotic Walking. K. Hamed and R. Gregg. In Invited Session on Control Theory in Legged Locomotion, American Control Conf., Boston, MA, 2016. (Abstract, PDF)

Actuator Design and Optimization


  • Robust Optimal Design of Series Elastic Actuators: Application to a Powered Prosthetic Ankle. E. Bolivar, S. Rezazadeh, T. Summers, and R. Gregg. IEEE Robotics & Automation Letters, under review. (Abstract)

  • Mechanical Simplification of Variable Stiffness Actuators Using Dielectric Elastomer Systems. D. Allen, E. Bolivar, S. Farmer, W. Voit, and R. Gregg. IEEE Transactions on Robotics, under review. (Abstract)

  • Minimizing Energy Consumption and Peak Power of Series Elastic Actuators: a Convex Optimization Framework for Elastic Element Design. E. Bolivar, S. Rezazadeh, and R. Gregg. IEEE/ASME Transactions on Mechatronics, under review. (Abstract)

  • Stretchable Conductive Fabric Simplifies Manufacturing of Low-Resistance Dielectric-Elastomer-System Electrodes. D. Allen, S. Farmer, R. Gregg, and W. Voit. In SPIE Electroactive Polymer Actuators and Devices, 2018. (Abstract, PDF)

  • A General Framework for Minimizing Energy Consumption of Series Elastic Actuators with Regeneration. E. Bolivar, S. Rezazadeh, and R. Gregg. In ASME Dynamic Systems & Control Conference, Virginia, 2017. Best Student Robotics Paper. (Abstract, PDF)

  • Towards a Series Elastic Actuator with Electrically Modulated Stiffness for Powered Ankle-Foot Orthoses. E. Bolivar, D. Allen, G. Ellson, J. Cossio, W. Voit, and R. Gregg. In IEEE Conf Automation Science & Engineering, Fort Worth, TX, 2016. (Abstract, PDF)

Other Control Topics


  • Limit Cycle Minimization by Time-Invariant Extremum Seeking Control. S. Kumar, A. Mohammadi, R. Gregg, and N. Gans. Submitted to American Control Conference, 2019. (Abstract)

Unreviewed Magazine Articles


  • Challenges for Control Research: Control of Powered Prosthetic Legs. R. Gregg, L. Hargrove, and J. Sensinger. The Impact of Control Technology, 2nd ed., T. Samad and A.M. Annaswamy (eds.), IEEE Control Systems Society, 2014, available at www.ieeecss.org. (PDF)


Visit the PI's Google Scholar page for past publications.