A survey of phase variable candidates of human locomotion

Abstract

Studies show that the human nervous system is able to parameterize gait cycle phase using sensory feedback. In the field of bipedal robots, the concept of a phase variable has been successfully used to mimic this behavior by parameterizing the gait cycle in a time-independent manner. This approach has been applied to control a powered transfemoral prosthetic leg, but the proposed phase variable was limited to the stance period of the prosthesis only. In order to achieve a more robust controller, we attempt to find a new phase variable that fully parameterizes the gait cycle of a prosthetic leg. The angle with respect to a global reference frame at the hip is able to monotonically parameterize both the stance and swing periods of the gait cycle. This survey looks at multiple phase variable candidates involving the hip angle with respect to a global reference frame across multiple tasks including level-ground walking, running, and stair negotiation. In particular, we propose a novel phase variable candidate that monotonically parameterizes the whole gait cycle across all tasks, and does so particularly well across level-ground walking. In addition to furthering the design of robust robotic prosthetic leg controllers, this survey could help neuroscientists and physicians study human locomotion across tasks from a time-independent perspective.

Unifying the gait cycle in the control of a powered prosthetic leg

“…Real-time estimation methods using inertial measurement units (e.g., [22]) could be used to estimate the hip position. Alternatively, other phase variables could be measured from other sensors on-board the prosthetic leg [21]. …”

“…To find the virtual constraint, we sample the desired angular trajectories of the knee and ankle over the entire gait cycle at n discrete, equally-spaced values of the phase variable. For this work, the phase variable was chosen as the hip x -position q x , measured relative to a coordinate frame created at the transition from the contralateral stance to the prostheses stance period, although other options may exist [21]. The phase variable was normalized between 0 and 1 using $$s_{P}false({\theta }_{P}false(q_{P}false)false)=\frac{{\theta }_P-{\theta }_P^{+}}{{\theta }_P^{-}-{\theta }_P^{+}},$$ where the ‘+’ signifies the start of the stance period for the prosthetic leg and the ‘−’ indicates the end of its swing period.…”

Virtual Constraint Control of a Powered Prosthetic Leg: From Simulation to Experiments With Transfemoral Amputees

“…Alternatively, a definition of effective shape for the swing period would allow the use of virtual constraints, resulting in a unified swing period to further reduce the number of control switches and hand-tuned parameters. This development may require a new phase variable that is measurable from the prosthesis during swing (see initial work in [48]). Only after these promising directions are investigated will the virtual constraint approach be mature enough for clinical comparison with state-of-the-art impedance control methods [4]–[6].…”

“…The significance of effective shape begs the question as to whether human locomotion might employ a phase variable [42], [48]. Phase-based virtual constraints could also be applied to powered exoskeletons (e.g., [11]), motivating future investigation of hybrid zero dynamics for wearable robots.…”

A Perturbation Mechanism for Investigations of Phase-Dependent Behavior in Human Locomotion

“…The largest perturbation considered for this purpose would cause a 5 degree change in the global leg angle (the angle between vertical and the vector going from the hip joint to the ankle), which normally has a 60 degree range of motion [37]. Assuming the hip position remains stationary during the perturbation, a 10 cm displacement would cause approximately a 5 degree change in the global leg angle.…”