Fast Online Trajectory Optimization for the Bipedal Robot Cassie

Apgar, Taylor; Clary, Patrick; Green, Kevin; Fern, Alan; Hurst, Jonathan

doi:10.15607/rss.2018.xiv.054

Cited by 118 publications

(82 citation statements)

References 20 publications

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“…Our controller provides stability against small disturbances through passive CoP modulation and recovers from stronger disturbances dominantly through an active foot-stepping strategy. The proposed algorithm can be suitable for taller robots such as Atlas or robots with even smaller feet sizes such as Marlo (Buss et al, 2014), Cassie (Apgar et al, 2018), or Humo (Kim et al, 2016). This is an advantage for our model-based controller that encodes robot and gait properties in the 3LP model.…”

Section: Discussionmentioning

confidence: 99%

Bipedal walking and push recovery with a stepping strategy based on time-projection control

Faraji

Razavi

Ijspeert

2019

The International Journal of Robotics Research

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In this paper, we present a simple control framework for on-line push recovery with dynamic stepping properties. Due to relatively heavy legs in our robot, we need to take swing dynamics into account and thus use a linear model called 3LP which is composed of three pendulums to simulate swing and torso dynamics. Based on 3LP equations, we formulate discrete LQR controllers and use a particular time-projection method to adjust the next footstep location on-line during the motion continuously. This adjustment, which is found based on both pelvis and swing foot tracking errors, naturally takes the swing dynamics into account. Suggested adjustments are added to the Cartesian 3LP gaits and converted to joint-space trajectories through inverse kinematics. Fixed and adaptive foot lift strategies also ensure enough ground clearance in perturbed walking conditions. The proposed structure is robust, yet uses very simple state estimation and basic position tracking. We rely on the physical series elastic actuators to absorb impacts while introducing simple laws to compensate their tracking bias. Extensive experiments demonstrate the functionality of different control blocks and prove the effectiveness of time-projection in extreme push recovery scenarios. We also show self-produced and emergent walking gaits when the robot is subject to continuous dragging forces. These gaits feature dynamic walking robustness due to relatively soft springs in the ankles and avoiding any Zero Moment Point (ZMP) control in our proposed architecture.

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Section: Discussionmentioning

confidence: 99%

Bipedal walking and push recovery with a stepping strategy based on time-projection control

Faraji

Razavi

Ijspeert

2019

The International Journal of Robotics Research

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“…When prescribing behaviors in terms of purely task space objectives, this is commonly referred to as task-or operational-space control (OSC) [16]. In many recent works, variations of these approaches have been shown to allow for high-level tasks to be encoded with intuitive constraints and costs in optimization based controllers, some examples being [7], [10], [14], [15], [18]- [20].…”

Section: Optimization Based Controllersmentioning

confidence: 99%

An Inverse Dynamics Approach to Control Lyapunov Functions

Reher

Kann

Ames

2020

2020 American Control Conference (ACC)

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With the goal of moving towards implementation of increasingly dynamic behaviors on underactuated systems, this paper presents an optimization-based approach for solving full-body dynamics based controllers on underactuated bipedal robots. The primary focus of this paper is on the development of an alternative approach to the implementation of controllers utilizing control Lyapunov function based quadratic programs. This approach utilizes many of the desirable aspects from successful inverse dynamics based controllers in the literature, while also incorporating a variant of control Lyapunov functions that renders better convergence in the context of tracking outputs. The principal benefits of this formulation include a greater ability to add costs which regulate the resulting behavior of the robot, in addition, the model error-prone inertia matrix is used only once, in a non-inverted form. The result is a successful demonstration of the controller for walking in simulation, and applied on hardware in real-time for crouching.

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“…The literature presents two main approaches to motion planning: the first applies trajectory optimization with whole-body dynamics (Koenemann et al, 2015). The second approach optimizes for centroidal dynamics of a reduced-order model (Kajita et al, 2003;Apgar et al, 2018). Postural stability is crucial in motion planning; the trunk is underactuated, and its motions can be controlled only indirectly.…”

Section: Introductionmentioning

confidence: 99%

Virtual Point Control for Step-Down Perturbations and Downhill Slopes in Bipedal Running

Drama

Badri-Spröwitz

2020

Front. Bioeng. Biotechnol.

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Bipedal running is a difficult task to realize in robots, since the trunk is underactuated and control is limited by intermittent ground contacts. Stabilizing the trunk becomes even more challenging if the terrain is uneven and causes perturbations. One bio-inspired method to achieve postural stability is the virtual point (VP) control, which is able to generate natural motion. However, so far it has only been studied for level running. In this work, we investigate whether the VP control method can accommodate single step-down perturbations and downhill terrains. We provide guidelines on the model and controller parameterizations for handling varying terrain conditions. Next, we show that the VP method is able to stabilize single step-down perturbations up to 40 cm, and downhill grades up to 20–40° corresponding to running speeds of 2–5 ms−1. Our results show that the VP approach leads to asymmetrically bounded ground reaction forces for downhill running, unlike the commonly-used symmetric friction cone constraints. Overall, VP control is a promising candidate for terrain-adaptive running control of bipedal robots.

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Fast Online Trajectory Optimization for the Bipedal Robot Cassie

Cited by 118 publications

References 20 publications

Bipedal walking and push recovery with a stepping strategy based on time-projection control

Bipedal walking and push recovery with a stepping strategy based on time-projection control

An Inverse Dynamics Approach to Control Lyapunov Functions

Virtual Point Control for Step-Down Perturbations and Downhill Slopes in Bipedal Running

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