On controllable stiffness bipedal walking
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Date
2008-05-28T16:17:17Z
Authors
Ghorbani, Reza
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Abstract
Impact at each leg transition is one of the main causes of energy dissipation in most
of the current bipedal walking robots. Minimizing impact can reduce the energy loss.
Instead of controlling the joint angle profiles to reduce the impact which requires significant
amount of energy, installing elastic mechanisms on the robots structure is
proposed in this research, enabling the robot to reduce the impact, and to store part
of the energy in the elastic form which returns the energy to the robot. Practically,
this motivates the development of the bipedal walking robots with adjustable stiffness
elasticity which itself creates new challenging problems. This thesis addresses some of
the challenges through five consecutive stages. Firstly, an adjustable compliant series
elastic actuator (named ACSEA in this thesis) is developed. The velocity control mode
of the electric motor is used to accurately control the output force of the ACSEA. Secondly,
three different conceptual designs of the adjustable stiffness artificial tendons
(ASAT) are proposed each of which is added at the ankle joint of a bipedal walking
robot model. Simulation results of the collision phase (part of the gait between
the heel-strike and the foot-touch-down in bipedal walking) demonstrate significant
improvements in the energetics of the bipedal walking robot by proper stiffness adjustment
of ASAT. In the third stage, in order to study the effects of ASATs on reducing
the energy loss during the stance phase, a simplified model of bipedal walking is introduced
consisting of a foot, a leg and an ASAT which is installed parallel to the ankle
joint. A linear spring, with adjustable stiffness, is included in the model to simulate the generated force by the trailing leg during the double support phase. The concept
of impulsive constraints is used to establish the mathematical model of impacts in
the collision phase which includes the heel-strike and the foot-touch-down. For the
fourth stage, an energy-feedback-based controller is designed to automatically adjust
the stiffness of the ASAT which reduces the energy loss during the foot-touch-down.
In the final stage, a speed tracking (ST) controller is developed to regulate the velocity
of the biped at the midstance. The ST controller is an event-based time-independent
controller, based on geometric progression with exponential decay in the kinetic energy
error, which adjusts the stiffness of the trailing-leg spring to control the injected energy
to the biped in tracking a desired speed at the midstance. Another controller is also
integrated with the ST controller to tune the stiffness of the ASAT when reduction in
the speed is desired. Then, the local stability of the system (biped and the combination
of the above three controllers) is analyzed by calculating the eigenvalues of the linear
approximation of the return map. Simulation results show that the combination of the
three controllers is successful in tracking a desired speed of the bipedal walking even
in the presence of the uncertainties in the leg’s initial angles.
The outcomes of this research show the significant effects of adjustable stiffness artificial
tendons on reducing the energy loss during bipedal walking. It also demonstrates
the advantages of adding elastic elements in the bipedal walking model which benefits
the efficiency and simplicity in regulating the speed. This research paves the way
toward developing the dynamic walking robots with adjustable stiffness ability which
minimize the shortcomings of the two major types of bipedal walking robots, i.e., passive
dynamic walking robots (which are energy efficient but need extensive parameters
tuning for gait stability) and actively controlled walking robots (which are significantly
energy inefficient).
Description
Keywords
Bipedal walking, Adjustable Stiffness, Control, Robot