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Capture Point Control in Thruster-Assisted Bipedal Locomotion

Shreyansh Pitroda, Aditya Bondada, Kaushik Venkatesh Krishnamurthy, Adarsh Salagame, Chenghao Wang, Taoran Liu, Bibek Gupta, Eric Sihite, Reza Nemovi, Alireza Ramezani, Morteza Gharib

TL;DR

This work addresses the fragility of dynamic bipedal locomotion on rough terrain by augmenting a biped with thrusters and developing a capture point-based controller. It derives a VLIP-based reduced-order model (HROM) using an energy-based Lagrangian framework and integrates thruster forces into the capture point control, introducing the concept of virtual buoyancy when torso thrust aligns with the ground. The controller is designed in the sagittal plane and validated in high-fidelity Simscape simulations, demonstrating rapid convergence to a stable limit cycle and reduced actuation effort for fallover prevention. The results highlight a promising direction for thruster-assisted dynamic terrestrial locomotion and set the stage for hardware experiments and perception-enabled robustness on complex terrains.

Abstract

Despite major advancements in control design that are robust to unplanned disturbances, bipedal robots are still susceptible to falling over and struggle to negotiate rough terrains. By utilizing thrusters in our bipedal robot, we can perform additional posture manipulation and expand the modes of locomotion to enhance the robot's stability and ability to negotiate rough and difficult-to-navigate terrains. In this paper, we present our efforts in designing a controller based on capture point control for our thruster-assisted walking model named Harpy and explore its control design possibilities. While capture point control based on centroidal models for bipedal systems has been extensively studied, the incorporation of external forces that can influence the dynamics of linear inverted pendulum models, often used in capture point-based works, has not been explored before. The inclusion of these external forces can lead to interesting interpretations of locomotion, such as virtual buoyancy studied in aquatic-legged locomotion. This paper outlines the dynamical model of our robot, the capture point method we use to assist the upper body stabilization, and the simulation work done to show the controller's feasibility.

Capture Point Control in Thruster-Assisted Bipedal Locomotion

TL;DR

This work addresses the fragility of dynamic bipedal locomotion on rough terrain by augmenting a biped with thrusters and developing a capture point-based controller. It derives a VLIP-based reduced-order model (HROM) using an energy-based Lagrangian framework and integrates thruster forces into the capture point control, introducing the concept of virtual buoyancy when torso thrust aligns with the ground. The controller is designed in the sagittal plane and validated in high-fidelity Simscape simulations, demonstrating rapid convergence to a stable limit cycle and reduced actuation effort for fallover prevention. The results highlight a promising direction for thruster-assisted dynamic terrestrial locomotion and set the stage for hardware experiments and perception-enabled robustness on complex terrains.

Abstract

Despite major advancements in control design that are robust to unplanned disturbances, bipedal robots are still susceptible to falling over and struggle to negotiate rough terrains. By utilizing thrusters in our bipedal robot, we can perform additional posture manipulation and expand the modes of locomotion to enhance the robot's stability and ability to negotiate rough and difficult-to-navigate terrains. In this paper, we present our efforts in designing a controller based on capture point control for our thruster-assisted walking model named Harpy and explore its control design possibilities. While capture point control based on centroidal models for bipedal systems has been extensively studied, the incorporation of external forces that can influence the dynamics of linear inverted pendulum models, often used in capture point-based works, has not been explored before. The inclusion of these external forces can lead to interesting interpretations of locomotion, such as virtual buoyancy studied in aquatic-legged locomotion. This paper outlines the dynamical model of our robot, the capture point method we use to assist the upper body stabilization, and the simulation work done to show the controller's feasibility.
Paper Structure (6 sections, 18 equations, 10 figures)

This paper contains 6 sections, 18 equations, 10 figures.

Table of Contents

  1. Introduction and Motivation
  2. Harpy Reduced-Order Model (HROM) Derivations
  3. Energy-based Lagrange Formalism
  4. Capture Point Control
  5. Results and Discussion
  6. Conclusion

Figures (10)

  • Figure 1: Illustrates the CAD model of Harpy platform, a bipedal robot with two electric ducted fans attached to its torso.
  • Figure 2: Illustrates Harpy reduced-order model parameters. A variable-length inverted pendulum model with thruster force is projected to the sagittal, frontal and transversal planes of locomotion.
  • Figure 3: Snap shots of Harpy's thruster-assisted walking on flat ground.
  • Figure 4: Capture point obtained for various $g'=g-\bm u_{t,c}/m$.
  • Figure 5: Illustrates Harpy state variable trajectories evolution obtained using the Simscape/Matlab model.
  • ...and 5 more figures