Decoupling Torque and Stiffness: A Unified Modeling and Control Framework for Antagonistic Artificial Muscles

TL;DR

This paper introduces a real-time, unified control framework for antagonistic artificial muscles that enables independent torque and stiffness control across various muscle types, improving adaptive interaction capabilities.

Contribution

It presents a novel, generalizable control framework with a separable force model and bio-inspired stiffness scheduling, enhancing independent torque and stiffness regulation in artificial muscles.

Findings

Controller runs in under 1 ms per control cycle.

Demonstrates independent torque and stiffness tracking.

Reveals a tradeoff between shock absorption and stability in contact protocols.

Abstract

Antagonistic artificial muscles can decouple joint torque and stiffness, but contact transients often degrade this independence. We present a unified real-time framework applicable across pneumatic, electrohydraulic, and dielectric elastomer artificial muscle families: a separable Pad\'e force model with a minimal two-state dynamic wrapper, a cascaded inverse-dynamics controller in co-contraction/bias coordinates, and a bio-inspired depth-adaptive interaction policy that schedules stiffness based on penetration depth. The controller runs in under 1 ms per control tick and demonstrates independent torque and stiffness tracking, including a fixed-torque stiffness-step test that preserves torque regulation through stiffness transitions. In a coupled impedance contact protocol simulated across soft-to-rigid environments, comparing depth-adaptive stiffness to fixed-stiffness baselines reveals a shock/load versus stability tradeoff. These results provide a control-oriented foundation for musculoskeletal antagonistic robots to execute adaptive impedance behaviors in dynamic interactions.