Safe fall strategies for humanoid robots
Although fall is a rare event in the life of a humanoid robot, we
must be prepared for it because its consequences are serious.
We present a fall strategy which rapidly modifies the
robot's fall direction in order to avoid hitting a person or an
object in the vicinity. Our approach is based on the key observation
that during "toppling" the rotational motion of a robot necessarily
occurs at a leading edge or a leading corner of its support base
polygon. To modify the fall direction the robot needs to change the
position and orientation of this edge or corner vis-a-vis the
prohibited direction. We achieve it through intelligent stepping as
soon as a fall is detected. We compute the optimal stepping location
which results in the safest fall. Additional improvement to the fall
controller is achieved through inertia shaping techniques
aimed at controlling the centroidal inertia of the robot.
We demonstrate our results through the simulation of an Asimo-like
humanoid robot. To our knowledge, this is the first implementation
of a controller that attempts to change the fall direction of a
Simple humanoid models for balance and control
If you could reduce the instantaneous inertia of the entire humanoid
robot to that of a single rigid body, what would it be?
Simple models of complex dynamic systems are often instrumental in
our understaning of their essential behavior. Such models must
possess simplicity and compactness while not over-simplifying
The linear inverted pendulum model and a number of its variations
are frequently used in the gait and balance study of human
and humanoid robots. By focusing attentionto the fundamental
aspects of humanoid dynamics, such models open the way to new
classes of control laws, which would otherwise be difficult
or impossible to conceive.
While useful in their own right, a limitation of the above models is
that they represent the entire humanoid body only as a point mass
and do not characterize the significant centroidal moment of inertia
of the humanoid body. The centroidal moment
of inertia is a property of the distributed masses of the robot
limbs (head, arms, legs, etc) away from the CoM.
We study the Reaction Mass Pendulum (RMP), a reduced model version of the
complete humanoid robot. This model compactly captures the centroidal angular
momentum of the humanoid robot as a spinning ellipsoid which continuously
changes its shape, size and orientation.
We also introduce Inertia Shaping, a high-level approach to
modify the kinodyanmic properties of a humanoid.
Last Revised November 3, 2013