Trajectory Optimization for Wheeled-Legged Quadrupedal Robots Driving in Challenging Terrain. Medeiros, S. V.; Jelavic, E.; Bjelonic, M.; Siegwart, R.; Meggiolaro, A. M.; and Hutter, M. IEEE Robotics and Automation Letters, 2020.
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Wheeled-legged robots are an attractive solution for versatile locomotion in challenging terrain. They combine the speed and efficiency of wheels with the ability of legs to traverse challenging terrain. In this paper, we present a trajectory optimization formulation for wheeled-legged robots that optimizes over the base and wheels' positions and forces and takes into account the terrain information while computing the plans. This enables us to find optimal driving motions over challenging terrain. The robot is modeled as a single rigid-body, which allows us to plan complex motions and still keep a low computational complexity to solve the optimization quickly. The terrain map, together with the use of a stability constraint, allows the optimizer to generate feasible motions that cannot be discovered without taking the terrain information into account. The optimization is formulated as a Nonlinear Programming (NLP) problem and the reference motions are tracked by a hierarchical whole-body controller that computes the torque actuation commands for the robot. The trajectories have been experimentally verified on quadrupedal robot ANYmal equipped with non-steerable torque-controlled wheels. Our trajectory optimization framework enables wheeled quadrupedal robots to drive over challenging terrain, e.g., steps, slopes, stairs, while negotiating these obstacles with dynamic motions.
@article{medeiros2020trajectory,
  author    = {Medeiros, S. Vivian and
               Jelavic, Edo and
               Bjelonic, Marko and
               Siegwart, Roland and
               Meggiolaro, A. Marco and
               Hutter, Marco},
  title     = {Trajectory Optimization for Wheeled-Legged Quadrupedal Robots
               Driving in Challenging Terrain},
  journal   = {IEEE Robotics and Automation Letters},
  year      = {2020},
  abstract  = {Wheeled-legged robots are an attractive solution for versatile
               locomotion in challenging terrain. They combine the speed and
               efficiency of wheels with the ability of legs to traverse
               challenging terrain. In this paper, we present a trajectory
               optimization formulation for wheeled-legged robots that
               optimizes over the base and wheels' positions and forces and
               takes into account the terrain information while computing the
               plans. This enables us to find optimal driving motions over
               challenging terrain. The robot is modeled as a single
               rigid-body, which allows us to plan complex motions and still
               keep a low computational complexity to solve the optimization
               quickly. The terrain map, together with the use of a stability
               constraint, allows the optimizer to generate feasible motions
               that cannot be discovered without taking the terrain information
               into account. The optimization is formulated as a Nonlinear
               Programming (NLP) problem and the reference motions are tracked
               by a hierarchical whole-body controller that computes the torque
               actuation commands for the robot. The trajectories have been
               experimentally verified on quadrupedal robot ANYmal equipped
               with non-steerable torque-controlled wheels. Our trajectory
               optimization framework enables wheeled quadrupedal robots to
               drive over challenging terrain, e.g., steps, slopes, stairs,
               while negotiating these obstacles with dynamic motions.},
  keywords  = {legged robots, wheeled robots, motion planning,
               optimization and optimal control},
  url_pdf   = {files/2020_ral_medeiros.pdf},
  url_video = {https://youtu.be/DlJGFhGS3HM},
  url_link  = {https://ieeexplore.ieee.org/document/9079567},
}
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