A compact variable stiffness joint for compliant robotics enabled by torsion-spring mean-diameter variation

Area of Science:

• Robotics
• Mechanical Engineering
• Control Systems

Background:

• Robotic applications in rehabilitation, human-robot collaboration, and unstructured environments necessitate compliant and safe joint designs.
• Existing variable-stiffness joints often face limitations in compactness, range of regulation, or control complexity.

Purpose of the Study:

• To propose and validate a novel variable-stiffness joint design method (TSDV) based on torsion spring mechanics.
• To achieve a compact, wide-range, and controllable variable-stiffness joint for robotic systems.

Main Methods:

• Analysis of torsion spring mean coil diameter variation with joint deflection.
• Development of a structural scheme constraining spring inner diameter for stiffness modulation.
• Establishment of a nonlinear stiffness model using an energy-based method.
• Verification through numerical simulations, finite-element analysis, and experimental testing of a TSDV prototype.

Main Results:

• The TSDV joint exhibits tunable nonlinear stiffness and output torque characteristics.
• Experimental results confirm passive and active stiffness regulation capabilities.
• Demonstrated output torque range increase from 0-3.8 Nm to 0-9.2 Nm at 1.2 rad deflection.
• Achieved a locking function, transitioning to a rigid joint when spring diameter variation is fully suppressed.

Conclusions:

• The proposed TSDV variable-stiffness joint offers a compact structure and a large stiffness regulation range.
• It provides a new approach for safe interaction and compliant actuation in robotic systems.
• The joint features high control resolution and tunable stiffness for diverse robotic applications.