Publication: Prescribing Cartesian Stiffness of Soft Robots by Co-Optimization of Shape and Segment-Level Stiffness

09 August 2023

Inspired by the dexterous environmental interactions shown by animals, the inclusion of soft or compliant structure is being mirrored in robotic systems.

Animals' performance and adaptability in behaviour not only arises from their softness, but from their ability to vary their compliance through postural or physical changes. For example, an elephant trunk shows low stiffness when compliantly exploring the environment, but it stiffens when pulling branches from a tree. It is therefore not surprising that much attention has been given at the design of soft segments with the ability to vary the physical stiffness.

However, simply equipping a robot with softness will not generate intelligent behaviours. Indeed, most interaction tasks require careful specification of the compliance at the interaction point; some directions must be soft and others firm (e.g., while drawing, entering a hole, tracing a surface, assembling components). On the contrary, without careful planning, the preferential directions of deformation of a soft robot are not aligned with the task.

In this work, EMERGE partners from Delft University of Technology and collaborators develop and experimentally test for the first time a model-based algorithm that solves such a challenge. The algorithm takes advantage of soft robot’s redundancy and of novel variables stiffness capabilities by co-optimizing null-space motions and physical stiffness.

They propose a strategy to prescribe variations of the physical stiffness and the robot's posture so to implement a desired Cartesian stiffness and location of the contact point. The authors validate the algorithm in simulation and with experiments. To perform the latter, they also present a new tendon-driven soft manipulator, equipped with variable-stiffness segments and proprioceptive sensing and capable to move in three dimensional. They show that, combining the intelligent hardware with the proposed algorithm, we can obtain the desired stiffness at the end-effector over the workspace.

Read the paper: https://doi.org/10.1089/soro.2022.0025

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