More Like this

1. Optimally Biomimetic Passivity-Based Control of a Lower-Limb Exoskeleton over the Primary Activities of Daily Life

   https://doi.org/10.1109/OJCSYS.2022.3165733  

   Lin, J. ; Divekar, N. ; Thomas, G. ; Gregg, R. ( January 2022 , IEEE Open Journal of Control Systems)

   Task-specific, trajectory-based control methods commonly used in exoskeletons may be appropriate for individuals with paraplegia, but they overly constrain the volitional motion of individuals with remnant voluntary ability (representing a far larger population). Human-exoskeleton systems can be represented in the form of the Euler-Lagrange equations or, equivalently, the port-controlled Hamiltonian equations to design control laws that provide task-invariant assistance across a continuum of activities/environments by altering energetic properties of the human body. We previously introduced a port-controlled Hamiltonian framework that parameterizes the control law through basis functions related to gravitational and gyroscopic terms, which are optimized to fit normalized able-bodied joint torques across multiple walking gaits on different ground inclines. However, this approach did not have the flexibility to reproduce joint torques for a broader set of activities, including stair climbing and stand-to-sit, due to strict assumptions related to input-output passivity, which ensures the human remains in control of energy growth in the closed-loop dynamics. To provide biomimetic assistance across all primary activities of daily life, this paper generalizes this energy shaping framework by incorporating vertical ground reaction forces and global planar orientation into the basis set, while preserving passivity between the human joint torques and human joint velocities. We present an experimental implementation on a powered knee-ankle exoskeleton used by three able-bodied human subjects during walking on various inclines, ramp ascent/descent, and stand-to-sit, demonstrating the versatility of this control approach and its effect on muscular effort. 

   more » « less
2. Optimal Energy Shaping Control for a Backdrivable Hip Exoskeleton

   Zhang, J. ; Lin, J. ; Peddinti, V. ; Gregg, R. ( January 2023 , Proceedings of the American Control Conference)

   Task-dependent controllers widely used in exoskeletons track predefined trajectories, which overly constrain the volitional motion of individuals with remnant voluntary mobility. Energy shaping, on the other hand, provides task-invariant assistance by altering the human body's dynamic characteristics in the closed loop. While human-exoskeleton systems are often modeled using Euler-Lagrange equations, in our previous work we modeled the system as a port-controlled-Hamiltonian system, and a task-invariant controller was designed for a knee-ankle exoskeleton using interconnection-damping assignment passivity-based control. In this paper, we extend this framework to design a controller for a backdrivable hip exoskeleton to assist multiple tasks. A set of basis functions that contains information of kinematics is selected and corresponding coefficients are optimized, which allows the controller to provide torque that fits normative human torque for different activities of daily life. Human-subject experiments with two able-bodied subjects demonstrated the controller's capability to reduce muscle effort across different tasks. 

   more » « less
3. A Potential Energy Shaping Controller with Ground Reaction Force Feedback for a Multi-Activity Knee-Ankle Exoskeleton

   https://doi.org/10.1109/BioRob49111.2020.9224341  

   Divekar, Nikhil V. ; Lin, Jianping ; Nesler, Christopher ; Borboa, Sara ; Gregg, Robert D. ( November 2020 , 2020 8th IEEE RAS/EMBS International Conference for Biomedical Robotics and Biomechatronics (BioRob))

   This paper presents the design and implementation of a novel multi-activity control strategy for a backdrivable knee-ankle exoskeleton. Traditionally, exoskeletons have used trajectory-based control of highly geared actuators for complete motion assistance. In contrast, we develop a potential energy shaping controller with ground reaction force (GRF) feedback that facilitates multi-activity assistance from a backdrivable exoskeleton without prescribing pre-defined kinematics. Although potential energy shaping was previously implemented in an exoskeleton to reduce the user’s perceived gravity, this model-based approach assumes the stance leg is fully loaded with the weight of the user, resulting in excessive control torques as weight transfers to the contralateral leg during double support. The presented approach uses GRF feedback to taper the torque control output for any activity involving multiple supports, leading to a closer match with normative joint moments in simulations based on pre-recorded human data during level walking. To implement this strategy, we present a custom foot force sensor that provides GRF feedback to the previously designed exoskeleton. Finally, results from an able-bodied human subject experiment demonstrate that the exoskeleton is able to reduce muscular activation of the primary muscles related to the knee and ankle joints during sit-to-stand, stand-to-sit, level walking, and stair climbing. 

   more » « less
4. Energy Shaping Control with Virtual Spring and Damper for Powered Exoskeletons

   https://doi.org/10.1109/CDC40024.2019.9029624  

   Lin, Jianping ; Divekar, Nikhil ; Lv, Ge ; Gregg, Robert D. ( December 2019 , IEEE Conference on Decision and Control)

   Task-invariant feedback control laws for powered exoskeletons are preferred to assist human users across varying locomotor activities. This goal can be achieved with energy shaping methods, where certain nonlinear partial differential equations, i.e., matching conditions, must be satisfied to find the achievable dynamics. Based on the energy shaping methods, open-loop systems can be mapped to closed-loop systems with a desired analytical expression of energy. In this paper, the desired energy consists of modified potential energy that is well-defined and unified across different contact conditions along with the energy of virtual springs and dampers that improve energy recycling during walking. The human-exoskeleton system achieves the input-output passivity and Lyapunov stability during the whole walking period with the proposed method. The corresponding controller provides assistive torques that closely match the human torques of a simulated biped model and able-bodied human subjects’ data. 

   more » « less
5. Contact-Invariant Total Energy Shaping Control for Powered Exoskeletons

   https://doi.org/10.23919/ACC.2019.8815003  

   Lin, Jianping ; Lv, Ge ; Gregg, Robert D. ( July 2019 , Proceedings of the ... American Control Conference)

   Energy shaping methods can be used to design task-invariant feedback control laws for the powered exoskeletons (i.e., orthoses). In order to achieve a desired closed-loop energy, certain matching conditions must be satisfied, which are sets of nonlinear partial differential equations. In this paper, we solve the matching conditions and come up with a new solution for under-actuated systems by using Auckly’s method.We find a unified feedback control law that is task-invariant with respect to human inputs and different contact conditions. We propose assistive and resistive shaping strategies to alter the mass/inertia matrix and simulate on a powered knee-ankle exoskeleton. The simulation results show the reduction and increment of the human model’s metabolic cost of generating muscular forces in human walking. The interchange between the kinetic and potential energy and the changes in acceleration of the center of mass are also investigated in the simulation. 

   more » « less