14 research outputs found
ZMP support areas for multi-contact mobility under frictional constraints
We propose a method for checking and enforcing multi-contact stability based
on the Zero-tilting Moment Point (ZMP). The key to our development is the
generalization of ZMP support areas to take into account (a) frictional
constraints and (b) multiple non-coplanar contacts. We introduce and
investigate two kinds of ZMP support areas. First, we characterize and provide
a fast geometric construction for the support area generated by valid contact
forces, with no other constraint on the robot motion. We call this set the full
support area. Next, we consider the control of humanoid robots using the Linear
Pendulum Mode (LPM). We observe that the constraints stemming from the LPM
induce a shrinking of the support area, even for walking on horizontal floors.
We propose an algorithm to compute the new area, which we call pendular support
area. We show that, in the LPM, having the ZMP in the pendular support area is
a necessary and sufficient condition for contact stability. Based on these
developments, we implement a whole-body controller and generate feasible
multi-contact motions where an HRP-4 humanoid locomotes in challenging
multi-contact scenarios.Comment: 14 pages, 10 figure
Multi-contact Walking Pattern Generation based on Model Preview Control of 3D COM Accelerations
We present a multi-contact walking pattern generator based on preview-control
of the 3D acceleration of the center of mass (COM). A key point in the design
of our algorithm is the calculation of contact-stability constraints. Thanks to
a mathematical observation on the algebraic nature of the frictional wrench
cone, we show that the 3D volume of feasible COM accelerations is a always a
downward-pointing cone. We reduce its computation to a convex hull of (dual) 2D
points, for which optimal O(n log n) algorithms are readily available. This
reformulation brings a significant speedup compared to previous methods, which
allows us to compute time-varying contact-stability criteria fast enough for
the control loop. Next, we propose a conservative trajectory-wide
contact-stability criterion, which can be derived from COM-acceleration volumes
at marginal cost and directly applied in a model-predictive controller. We
finally implement this pipeline and exemplify it with the HRP-4 humanoid model
in multi-contact dynamically walking scenarios
Legged Robots
International audienc
Impact-Aware Multi-Contact Balance Criteria
Intentionally applying impacts while maintaining balance is challenging for
legged robots. This study originated from observing experimental data of the
humanoid robot HRP-4 intentionally hitting a wall with its right arm while
standing on two feet. Strangely, violating the usual zero moment point balance
criteria did not systematically result in a fall. To investigate this
phenomenon, we propose the zero-step capture region for non-coplanar contacts,
defined as the center of mass (CoM) velocity area, and validated it with
push-recovery experiments employing the HRP-4 balancing on two non-coplanar
contacts. To further enable on-purpose impacts, we compute the set of candidate
post-impact CoM velocities accounting for frictional-impact dynamics in three
dimensions, and restrict the entire set within the CoM velocity area to
maintain balance with the sustained contacts during and after impacts. We
illustrate the maximum contact velocity for various HRP-4 stances in
simulation, indicating potential for integration into other task-space
whole-body controllers or planners. This study is the first to address the
challenging problem of applying an intentional impact with a
kinematic-controlled humanoid robot on non-coplanar contacts
ZMP Support Areas for Multicontact Mobility Under Frictional Constraints
International audienceWe propose a method for checking and enforcing multi-contact stability based on the Zero-tilting Moment Point (ZMP). The key to our development is the generalization of ZMP support areas to take into account (a) frictional constraints and (b) multiple non-coplanar contacts. We introduce and investigate two kinds of ZMP support areas. First, we characterize and provide a fast geometric construction for the support area generated by valid contact forces, with no other constraint on the robot motion. We call this set the full support area. Next, we consider the control of humanoid robots using the Linear Pendulum Mode (LPM). We observe that the constraints stemming from the LPM induce a shrinking of the support area, even for walking on horizontal floors. We propose an algorithm to compute the new area, which we call pendular support area. We show that, in the LPM, having the ZMP in the pendular support area is a necessary and sufficient condition for contact stability. Based on these developments, we implement a whole-body controller and generate feasible multi-contact motions where an HRP-4 humanoid locomotes in challenging multi-contact scenarios