383 research outputs found
Optimal Control of Legged-Robots Subject to Friction Cone Constraints
A hierarchical control architecture is presented for energy-efficient control
of legged robots subject to variety of linear/nonlinear inequality constraints
such as Coulomb friction cones, switching unilateral contacts, actuator
saturation limits, and yet minimizing the power losses in the joint actuators.
The control formulation can incorporate the nonlinear friction cone constraints
into the control without recourse to the common linear approximation of the
constraints or introduction of slack variables. A performance metric is
introduced that allows trading-off the multiple constraints when otherwise
finding an optimal solution is not feasible. Moreover, the projection-based
controller does not require the minimal-order dynamics model and hence allows
switching contacts that is particularly appealing for legged robots. The
fundamental properties of constrained inertia matrix derived are similar to
those of general inertia matrix of the system and subsequently these properties
are greatly exploited for control design purposes. The problem of task space
control with minimum (point-wise) power dissipation subject to all physical
constraints is transcribed into a quadratically constrained quadratic
programming (QCQP) that can be solved by barrier methods
Dynamic whole-body motion generation under rigid contacts and other unilateral constraints
The most widely used technique for generating wholebody motions on a humanoid robot accounting for various tasks and constraints is inverse kinematics. Based on the task-function approach, this class of methods enables the coordination of robot movements to execute several tasks in parallel and account for the sensor feedback in real time, thanks to the low computation cost.
To some extent, it also enables us to deal with some of the robot constraints (e.g., joint limits or visibility) and manage the quasi-static balance of the robot. In order to fully use the whole range of possible motions, this paper proposes extending the task-function approach to handle the full dynamics of the robot multibody along with any constraint written as equality or inequality of the state and control variables. The definition of multiple objectives is made possible by ordering them inside a strict hierarchy. Several models of contact with the environment can be implemented in the framework. We propose a reduced formulation of the multiple rigid planar contact that keeps a low computation cost. The efficiency of this approach is illustrated by presenting several multicontact dynamic motions in simulation and on the real HRP-2 robot
An Efficiently Solvable Quadratic Program for Stabilizing Dynamic Locomotion
We describe a whole-body dynamic walking controller implemented as a convex
quadratic program. The controller solves an optimal control problem using an
approximate value function derived from a simple walking model while respecting
the dynamic, input, and contact constraints of the full robot dynamics. By
exploiting sparsity and temporal structure in the optimization with a custom
active-set algorithm, we surpass the performance of the best available
off-the-shelf solvers and achieve 1kHz control rates for a 34-DOF humanoid. We
describe applications to balancing and walking tasks using the simulated Atlas
robot in the DARPA Virtual Robotics Challenge.Comment: 6 pages, published at ICRA 201
Stability of Surface Contacts for Humanoid Robots: Closed-Form Formulae of the Contact Wrench Cone for Rectangular Support Areas
Humanoid robots locomote by making and breaking contacts with their
environment. A crucial problem is therefore to find precise criteria for a
given contact to remain stable or to break. For rigid surface contacts, the
most general criterion is the Contact Wrench Condition (CWC). To check whether
a motion satisfies the CWC, existing approaches take into account a large
number of individual contact forces (for instance, one at each vertex of the
support polygon), which is computationally costly and prevents the use of
efficient inverse-dynamics methods. Here we argue that the CWC can be
explicitly computed without reference to individual contact forces, and give
closed-form formulae in the case of rectangular surfaces -- which is of
practical importance. It turns out that these formulae simply and naturally
express three conditions: (i) Coulomb friction on the resultant force, (ii) ZMP
inside the support area, and (iii) bounds on the yaw torque. Conditions (i) and
(ii) are already known, but condition (iii) is, to the best of our knowledge,
novel. It is also of particular interest for biped locomotion, where undesired
foot yaw rotations are a known issue. We also show that our formulae yield
simpler and faster computations than existing approaches for humanoid motions
in single support, and demonstrate their consistency in the OpenHRP simulator.Comment: 14 pages, 4 figure
A Model-based Hierarchical Controller for Legged Systems subject to External Disturbances
Xin G, Lin H-C, Smith J, Cebe O, Mistry M. A Model-based Hierarchical Controller for Legged Systems subject to External Disturbances. In: IEEE/RSJ Int. Conf. on Robotics and Automation. 2018.Legged robots have many potential applications
in real-world scenarios where the tasks are too dangerous for
humans, and compliance is needed to protect the system against
external disturbances and impacts. In this paper, we propose a
model-based controller for hierarchical tasks of legged systems
subject to external disturbance. The control framework is
based on projected inverse dynamics controller, such that the
control law is decomposed into two orthogonal subspaces,
i.e., the constrained and the unconstrained subspaces. The
unconstrained component controls multiple desired tasks with
impedance responses. The constrained space controller maintains
the contact subject to unknown external disturbances,
without the use of any force/torque sensing at the contact
points. By explicitly modelling the external force, our controller
is robust to external disturbances and errors arising from
incorrect dynamic model information. The main contributions
of this paper include (1) incorporating an impedance controller
to control external disturbances and allow impedance shaping
to adjust the behaviour of the motion under external disturbances,
(2) optimising contact forces within the constrained
subspace that also takes into account the external disturbances
without using force/torque sensors at the contact locations. The
techniques are evaluated on the ANYmal quadruped platform
under a variety of scenarios
- âŚ