Baloo: A Large-Scale Hybrid Soft Robotic Torso for Whole-Arm Manipulation

TL;DR

Baloo addresses the challenge of open-world, whole-arm manipulation by leveraging a large-scale hybrid soft-rigid torso. The approach integrates compliant soft components with rigid structure and pneumatic actuation to realize two 2-meter arms mounted on a rigid torso, achieving end-effector payloads up to $19$ kg per arm and demonstrating 30 whole-body grasping trials with a simple open-loop control strategy. A single pressure trajectory enabled successful grasps of six diverse objects, illustrating mechanical intelligence where passive compliance shapes contact dynamics and stabilizes interactions without complex sensing or planning. The work highlights the practical potential of soft-rigid designs for robust, contact-rich manipulation and outlines directions for sensing, materials optimization, and closed-loop control to further enhance performance.

Abstract

Soft robotic actuators can simplify the design of controllers when operating in contact-rich environments. Importantly, their passive compliance fundamentally alters contact mechanics by smoothing impacts and distributing forces over large areas. By integrating soft actuators, we can perform high-impact, dynamic, and contact-rich tasks that are challenging or impossible for traditional rigid robots. In order to explore the benefits of passive structural compliance and learn to utilize it effectively, we present a prototype robotic torso named Baloo. Baloo's hybrid soft-rigid design incorporates both adaptability from soft components and strength from rigid components with two meter-long, pneumatic robot arms mounted on a rigid torso. The hybrid design is capable of lifting end effector payloads of up to 19 kg, far exceeding many hybrid robot designs. Such payloads are competitive with similar-sized rigid robots, but with a much higher strength-to-weight ratio. Through 30 physical whole-body grasping experiments, we also demonstrate how a simple control strategy can generalize for effective lifting across six challenging objects with diverse shapes, sizes, and weights. A 100% success rate across all objects--achieved with the simple control strategy--underscores the potential of our hybrid soft-rigid robot design for contact-rich, whole-body tasks.

Figures (12)

• Figure 1: Photo of Baloo with one arm lifting a 19 kg weight. HTC Vive trackers are mounted on the end plate of each joint for joint configuration estimation.
• Figure 2: Step-by-step assembly of a small compliant joint. The medium and large joints are assembled in a similar fashion.
• Figure 3: Exploded assembly view of a single small joint. Three additional plastic actuation chambers (#4) are hidden to simplify the view.
• Figure 4: Steel ball embedded in aramid rope braid and inserted into end fitting.
• Figure 5: All three sizes of the passively compliant joints. Each size was selected based on torque requirements as discussed in Section \ref{['sec:compliant_joints']}. The subscripts $s$, $m$, $l$ refer to the small, medium, and large joint sizes listed in Table \ref{['tab:joint_params']}.