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

Abstract

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.

Modeling, analysis and control of robot–object nonsmooth underactuated Lagrangian systems: A tutorial overview and perspectives

“…The framework is that of object's control through pre-impact velocities (see section 4.2.1), and it extends the well-known passivity+zero-state detectability output feedback stabilization result [97, section 5.5]. The IDA-PBC method is applied in [358,359,360]. It uses the Hamiltonian framework.…”

“…The IDA-PBC is applied to the stance phace without impacts in [359]. The footground contact is divided into three phases in [358], but transitions (impacts and constraints deactivation) are not taken into account. In both [359,358] the contact multiplier is calculated assuming independent bilateral constraints, and a dynamics similar to (6) or (17) (but without taking into account constraints) is used to design the IDA-PBC input.…”

“…The footground contact is divided into three phases in [358], but transitions (impacts and constraints deactivation) are not taken into account. In both [359,358] the contact multiplier is calculated assuming independent bilateral constraints, and a dynamics similar to (6) or (17) (but without taking into account constraints) is used to design the IDA-PBC input. It is noteworthy that PBC has been used in [145,147] to facilitate the analysis at impact times, being the closed-loop's Lyapunov function close to the mechanical energy.…”

“…But the switching can also stem from multicontact sole/ground model, which implies that ( 6) or (10) or (17) are changing along the foot/ground contact process: heel, flat, toes successively activate/deactivate unilateral constraints (the index set α(t) in ( 26) increases and then decreases). The passivity-based (IDA-PBC) controllers designed in [359,358] switch along such foot/ground contact evolution (since the Delassus matrix changes), but the input constraints and the collisions are not incorporated in the study. Switching controllers seem mandatory for some clearance systems as in Figure 14 (b) [318].…”

Estimating human joint moments unifies exoskeleton control, reducing user effort

“…Instead, instantaneous estimates of the user’s underlying joint moments could replace conventional high-level states ( 29 – 39 ). Because lower-limb joint moments naturally vary across ambulation modes and conditions ( 40 ), lower-limb joint moments could serve as a single, continuous high-level state for modulating exoskeleton assistance.…”