SPIBOT: A Drone-Tethered Mobile Gripper for Robust Aerial Object Retrieval in Dynamic Environments

TL;DR

SPIBOT introduces a drone-tethered hexapod gripper designed for robust aerial object retrieval in dynamic environments such as moving ship decks. A lightweight hexapod under a carrier UAV, connected via a winch tether, is controlled by a real-time action selection policy that combines a goal proximity weight with action affordance, yielding adaptive behavior $a_t = \arg\max_{i \in N} (g_i \alpha_i(S_t))$. The hardware employs 12 servos (two per leg) plus a tripod gait, a sensor suite including ultrasonic, pressure, IMU, and AprilTags, and an onboard Nvidia Jetson Nano for control, enabling rapid recovery via a retry mechanism when instability is detected. Experimental validation across pontoon, grass, and rubber-mat environments demonstrates reliable target convergence and retrieval of approximately $1\text{ kg}$ objects within roughly $13$ seconds on the ground, indicating practical viability for maritime and other dynamic applications.

Abstract

In real-world field operations, aerial grasping systems face significant challenges in dynamic environments due to strong winds, shifting surfaces, and the need to handle heavy loads. Particularly when dealing with heavy objects, the powerful propellers of the drone can inadvertently blow the target object away as it approaches, making the task even more difficult. To address these challenges, we introduce SPIBOT, a novel drone-tethered mobile gripper system designed for robust and stable autonomous target retrieval. SPIBOT operates via a tether, much like a spider, allowing the drone to maintain a safe distance from the target. To ensure both stable mobility and secure grasping capabilities, SPIBOT is equipped with six legs and sensors to estimate the robot's and mission's states. It is designed with a reduced volume and weight compared to other hexapod robots, allowing it to be easily stowed under the drone and reeled in as needed. Designed for the 2024 MBZIRC Maritime Grand Challenge, SPIBOT is built to retrieve a 1kg target object in the highly dynamic conditions of the moving deck of a ship. This system integrates a real-time action selection algorithm that dynamically adjusts the robot's actions based on proximity to the mission goal and environmental conditions, enabling rapid and robust mission execution. Experimental results across various terrains, including a pontoon on a lake, a grass field, and rubber mats on coastal sand, demonstrate SPIBOT's ability to efficiently and reliably retrieve targets. SPIBOT swiftly converges on the target and completes its mission, even when dealing with irregular initial states and noisy information introduced by the drone.

Figures (6)

• Figure 1: Drone retrieving the target box using SPIBOT
• Figure 2: SPIBOT is fitted beneath the drone, with a winch positioned above it.
• Figure 3: SPIBOT exploded view: The main body box construction, bearing the operation loads, accommodates the attachments for detection (top plate and AprilTags), sensing (platelet - pressure sensor and ultrasonic sensors), communication (telemetry radio) and gait. The red arrow denotes the connection to the carrier UAV through a steel wire.
• Figure 4: The drone-tethered mobile gripper retrieves a target object on the ground. Each image shows an action: (A) Stabilizing, (B) Aligning, (C) Approaching, (D) Forward Approach, (E) Grasping, and (F) Retrieving.
• Figure 5: Target distance and heading difference relative to the robot. The orange, yellow, and green regions represent the timeline of the robot’s stabilizing, aligning, and approaching actions, respectively. The blue and dark blue regions indicate the forward approach and grasping actions. The red area represents the retreat action triggered when the robot became unstable.