Dynamically Extensible and Retractable Robotic Leg Linkages for Multi-task Execution in Search and Rescue Scenarios

TL;DR

This work addresses the dual challenge of rapid terrain traversal and high-force rescue in SAR robotics by introducing a morphing leg built from a parallel five-bar mechanism whose effective link lengths are dynamically adjusted via capstan-driven actuators on $B$ and $C$ and a linear-stage ground link $N$. The authors develop forward and inverse kinematics, perform workspace and static-force analyses, and use a Jacobian-based condition-number metric to guide configuration choices, validating the approach on a planar testbed and a boom-mounted bipedal prototype. Results show that extending the ground link $N$ or retracting the passive links $B$ and $C$ increases horizontal pulling forces, while elongating $B$/$C$ expands the reachable workspace, enabling deliberate transitions between a fast-traveling 'search' mode and a high-force 'rescue' mode. The findings demonstrate a practical pathway toward SAR robots capable of both efficient terrain negotiation and effective casualty extraction in debris-filled environments, with potential for real-world deployment and further hardware-scale optimizations.

Abstract

Search and rescue (SAR) robots are required to quickly traverse terrain and perform high-force rescue tasks, necessitating both terrain adaptability and controlled high-force output. Few platforms exist today for SAR, and fewer still have the ability to cover both tasks of terrain adaptability and high-force output when performing extraction. While legged robots offer significant ability to traverse uneven terrain, they typically are unable to incorporate mechanisms that provide variable high-force outputs, unlike traditional wheel-based drive trains. This work introduces a novel concept for a dynamically extensible and retractable robot leg. Leveraging a dynamically extensible and retractable five-bar linkage design, it allows for mechanically switching between height-advantaged and force-advantaged configurations via a geometric transformation. A testbed evaluated leg performance across linkage geometries and operating modes, with empirical and analytical analyses conducted on stride length, force output, and stability. The results demonstrate that the morphing leg offers a promising path toward SAR robots that can both navigate terrain quickly and perform rescue tasks effectively.

Figures (12)

• Figure 1: Bipedal proof-of-concept SAR robot with adaptable legs switching between 'search' and 'rescue' modes. Upper images show the robot in search mode with legs augmented for fast travel; the lower image shows rescue mode with legs augmented for load-dragging.
• Figure 2: Diagram of 5-bar linkage. The six links are modeled as rigid bodies $\{A, B, C, D, E, N\}$. Links $A$ and $D$ are actively controlled (red and blue). Links $B$ and $C$ are passive (yellow and green). $N$ is the ground link (gray). $E$ (purple) is an ankle extension rigidly attached to link $C$ with the foot at point $E_F$. Each link has an angle $q_{(A, B, C, D, E)}$ measured relative to the world frame with positive sense. All links have an origin point $\{A_o, B_o, C_o, D_o, E_o, N_o\}$ with $\hat{x}$ components along the link length and $\hat{z}$ out of the page.
• Figure 3: Testbed setup with vertical rail-mounted leg, horizontal force measurement platform, and three-motor actuation system for evaluating adaptive linkage performance.
• Figure 4: Capstan-driven actuator mechanism showing motor-driven spool system for varying effective link length through cable tension adjustment. The overall length change is 64:273 mm.
• Figure 5: System architecture for the robotic leg testbed. Two AK60-6 V3.0 motors are simultaneously controlled by a remote desktop over CAN bus, shown in green and yellow dashed lines. The remote desktop converts simulated position, velocity, and torque metrics into MiT commands sent to the motors via a USB to CAN converter. The crane scale is used to record the pulling force of the leg to validate simulation results.