DecARt Leg: Design and Evaluation of a Novel Humanoid Robot Leg with Decoupled Actuation for Agile Locomotion

TL;DR

The paper introduces the DecARt Leg, a decoupled-actuation humanoid leg with motors placed above the knee that preserves an anthropomorphic appearance while enabling simple modeling and control. It proposes a quasi-telescopic pantograph-like kinematic design with a novel multi-bar ankle transmission and derives an analytic inverse kinematics solution to facilitate straightforward control. A new metric, FAST, quantifies agility by the fastest achievable swing time, enabling cross-design comparisons independent of upper-body effects; FAST is demonstrated against several decoupled and coupled designs and provided as an open-source tool. Simulation in PyBullet and preliminary hardware experiments show the DecARt Leg can achieve rapid swing, agile walking, push recovery, and basic loco-manipulation tasks, supporting the viability of decoupled actuation in humanoid applications and guiding future development toward full upper-body integration and energy-efficient actuation strategies.

Abstract

In this paper, we propose a novel design of an electrically actuated robotic leg, called the DecARt (Decoupled Actuation Robot) Leg, aimed at performing agile locomotion. This design incorporates several new features, such as the use of a quasi-telescopic kinematic structure with rotational motors for decoupled actuation, a near-anthropomorphic leg appearance with a forward facing knee, and a novel multi-bar system for ankle torque transmission from motors placed above the knee. To analyze the agile locomotion capabilities of the design numerically, we propose a new descriptive metric, called the `Fastest Achievable Swing Time` (FAST), and perform a quantitative evaluation of the proposed design and compare it with other designs. Then we evaluate the performance of the DecARt Leg-based robot via extensive simulation and preliminary hardware experiments.

Figures (8)

• Figure 1: The concept of DecARt Leg design: decoupled actuation, all motors above the knee, anthropomorphic appearance with a forward-facing knee.
• Figure 2: Concept of the DecARt quasi-telescopic leg design: a) Pantograph-like design, b) DecARt Leg design, c) DecARt Leg CAD drawing and its intrinsic telescopic actuator.
• Figure 3: DecARt Leg multi-bar ankle torque transmission structure: different rods are active during different periods of motion. Front rods (marked blue) and rear rods (marked red) are active during different poses of the leg: a) Fully stretched, b) Half-sitting, c) Fully crouched.
• Figure 4: DecARt Leg motors placement and kinematic structure. All motors are placed above the knee. Angles of joints $j'_3$ are equal to $j_3$ due to mechanical constraints of the quasi-telescopic design.
• Figure 5: Fastest Achievable Swing Time (FAST) metric variables description (a), and screenshots of different robots doing the test: DecARt (b), Cassie (c), Fourier GR1T2 (d), DecARt-Serial emulation (e), Unitree G1 (f), and Booster T1 (g)