Harnessing compliance in the design and control of running robots
Date
2017
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Legged robots have the potential to extend our reach to terrains that challenge
the traversal capabilities of traditional wheeled platforms. To realize this potential,
diverse legged robot designs have been proposed, and a number of these robots achieved
impressive indoor and outdoor terrain mobility. However, combining mobility with
energy efficiency is still a challenging task due to the inherently dissipative nature of
legged locomotion. Furthermore, legged robots typically operate in regimes where the
natural dynamics of the mechanical system imposes strict limitations on the capability
of the actuators to regulate its motion. This is especially the case for running, during
which the magnitude of the ground reaction force is several times of the body weight
due to the prominent dynamic effects of the motion. ☐ Biological systems demonstrate the great potential of utilizing compliant elements
in legged locomotion. During running, part of the mechanical energy is recovered
by the elastic deformation of muscles and tendons and returned back to the system
when it is needed. In addition, by storing muscle work slowly and releasing it rapidly,
compliance alleviates the requirement for powerful actuators. Introducing compliance
into legged robots, however, is not a straightforward task. Compliance might lead to
high frequency oscillations or impede the free motion of the joints. In addition, due
to the relatively large stiffness, the behavior of the system is largely governed by the
natural dynamics of the spring-mass system. Careful analysis of the natural dynamics
is necessary to fully exploit the benefits of compliant elements. ☐ With the objective to close the gap between mobility and efficiency, this thesis
explores the applications of both active and passive compliant elements in the design
and control of running robots. The thesis begins with reduced-order running models with massless springy legs before delving into higher-dimensional models that constitute
more faithful representation of robotic systems. Although these models do not
incorporate energy losses due to impacts or damping effects, they can predict important
aspects of running, including ground reaction force profiles, center of mass trajectories,
and the change of stance duration with respect to speed. Using time-reversal symmetries
of the underlying dynamics of these reduced-order models, this thesis states
analytic conclusions on the stability of periodic running gaits, which can be used to
facilitate controller design. Next, a detailed model with segmented leg and inelastic
impact is adopted to study the periodic bounding of quadrupedal robot HyQ. Mimicking
the reduced-order models, the controller introduces active compliance into the
robot. Stable periodic bounding gaits emerge as the interaction results between the
robot and its environment. ☐ Inspired by the complementary benefits of passive and active compliance in energy
efficiency and control authority, respectively, we propose in this thesis a novel
actuation concept: the switchable parallel elastic actuator (Sw-PEA). This concept
relies on adding compliance in parallel with the actuator to reduce both the energy
consumption as well as the torque requirement related to running robots. In addition,
a mechanical switch is used to disengage the spring when it is not needed to
facilitate control of joint movement. The effectiveness of the concept is demonstrated
experimentally by monopedal robot SPEAR which is actuated by a Sw-PEA.
Overall, this thesis explores the application of active and passive compliant
elements in the control and design of running robots, using both numerical simulations
as well as experimental evaluations. The result of this thesis points out a promising
direction on how to use passive compliant elements in combination with actuators for
the development of running robots with both good mobility and energy efficiency.
Description
Keywords
Applied sciences, Control, Design, Legged locomotion, Robotics, Simulation, Symmetry