Whole-Body MPC and Online Gait Sequence Generation for Wheeled-Legged Robots

Our paper proposes a model predictive controller as a single-task formulation that simultaneously optimizes wheel and torso motions. This online joint velocity and ground reaction force optimization integrates a kinodynamic model of a wheeled quadrupedal robot. It defines the single rigid body dynamics along with the robot's kinematics while treating the wheels as moving ground contacts. With this approach, we can accurately capture the robot's rolling constraint and dynamics, enabling automatic discovery of hybrid maneuvers without needless motion heuristics. The formulation's generality through the simultaneous optimization over the robot's whole-body variables allows for a single set of parameters and makes online gait sequence adaptation possible. Aperiodic gait sequences are automatically found through kinematic leg utilities without the need for predefined contact and lift-off timings, reducing the cost of transport by up to 85%. Our experiments demonstrate dynamic motions on a quadrupedal robot with non-steerable wheels in challenging indoor and outdoor environments. The paper's findings contribute to evaluating a decomposed, i.e., sequential optimization of wheel and torso motion, and single-task motion planner with a novel quantity, the prediction error, which describes how well a receding horizon planner can predict the robot's future state. To this end, we report an improvement of up to 71% using our proposed single-task approach, making fast locomotion feasible and revealing wheeled-legged robots' full potential.Comment: 8 pages, 6 figures, 1 table, 52 references, 9 equation

Suspension effect in tip-over stability and steerability of robots moving on terrain discontinuities

[ES] En este artículo se estudia el efecto que produce el sistema de suspensión sobre la estabilidad al vuelco y la capacidad de direccionamiento en un robot móvil Skid Steer, cuando este se enfrenta a distintas discontinuidades del terreno: descenso (frontal y lateral) y ascenso sobre escalones, además del desplazamiento sobre zanjas. Específicamente, se estudió el instante cuando se generan cargas de impacto producto del movimiento del robot sobre la irregularidad del terreno. En cada caso se hizo un análisis correlacional del efecto sobre la estabilidad al vuelco y el direccionamiento (cuantificadas con métricas fundamentadas en las fuerzas de reacción de las ruedas con el suelo), al variar cuatro parámetros que definen el sistema de suspensión: constante de rigidez en los resortes, constante de amortiguamiento en los amortiguadores y las constantes de rigidez y amortiguamiento en las ruedas. Por último se estimó para cada caso, qué magnitudes deberían adquirir estos parámetros para garantizar una mejor estabilidad y direccionamiento del robot.[EN] This article studies the effect produced by the suspension system in tip-over stability and steerability of a Skid Steer mobile robot, when it faces different terrain discontinuities: descent (front and side) and ascent on steps, plus displacement over ditches. Specifically, the moment was studied when impact loads producted by the robot's movement on the irregularity of the terrain are generated. In each case, a correlational analysis was made about the effect in tip-over stability and steerability (quantified with metrics based on the reaction forces of the wheels with the ground), by varying four parameters that define the suspension system: stiffness constant in the springs, damping constant in the dampers and the stiffness and damping constants in the wheels. 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