Bipedal walking with continuously compliant robotic legs

Abstract

In biomechanics and robotics, elasticity plays a crucial role in enhancing locomotion efficiency and stability. Traditional approaches in legged robots often employ series elastic actuators (SEA) with discrete rigid components, which, while effective, add weight and complexity. This paper presents an innovative alternative by integrating continuously compliant structures into the lower legs of a bipedal robot, fundamentally transforming the SEA concept. Our approach replaces traditional rigid segments with lightweight, deformable materials, reducing overall mass and simplifying the actuation design. This novel design introduces unique challenges in modeling, sensing, and control, due to the infinite dimensionality of continuously compliant elements. We address these challenges through effective approximations and control strategies. The paper details the design and modeling of the compliant leg structure, presents low-level force and kinematics controllers, and introduces a high-level posture controller with a gait scheduler. Experimental results demonstrate successful bipedal walking using this new design.

Figures (4)

• Figure 1: This paper demonstrates successful bipedal walking on continuously compliant legs. The hardware platform used for the experiments is based on the robot RAMonesmit2017ramone in which the series compliance was removed from the actuators and the lower legs were replaced with a compliant semicircular structure made from spring steel. Bota Systems Rokubi F/T-sensors at the proximal end of the springs allow for measurement of the elastic forces. The robot is constrained to motion in the sagittal plane using a planarizer green2016design.
• Figure 2: Model of a robotic leg with a deformable shank. During the stance phase, the continuously compliant structure (shown in dark red) is deflected from its undeformed configuration (shown in light red). The elastic response is approximated by a linear elastic element which acts in response to the deflection of the foot from its nominal position (at $\mathbf x_{F}^{o}$) to its actual position (at $\mathbf q_F$). The underlying linear-elastic characteristics are constant in a spring-fixed coordinate system $K$, leading to restoring forces $f_x^k$ and $f_y^k$. (To improve readability, the sub-indices $i$ for left/right leg have been omitted in this figure)
• Figure 3: Shown are 10 seconds of the vertical and horizontal positions of the main body of the bipedal robot during a steady state walking experiment is shown. Desired values are shown in black and measured ones in blue.
• Figure 4: Snapshots of a half-stride of a walking experiment with RAMone . The sequence starts just before lift-off of the right leg.