Research Progress on Variable Stiffness Methods Based on Mechanical and Non-Mechanical Adjustments

Robotic physical interaction tasks require both compliance and load-bearing capacity. This review surveys variable stiffness method (VSM) based on mechanical and non-mechanical adjustment, with direct modulation of the equivalent physical stiffness at the output. Mechanical approaches are classified into independent and antagonistic configurations, and their principles are summarized in terms of variable transmission ratio, variable spring pretension, and variable physical properties of springs. Non-mechanical approaches are then reviewed from the smart materials, structural reconfiguration, jamming effects, and multi-mechanism synergy. Representative applications in humanoid robot, exoskeletons and prostheses, soft robots, and special robots are compared in terms of stiffness range, adjustment speed, load capacity, energy consumption, control complexity, and reliability. Mechanical approaches provide higher stiffness, load capacity, and predictability, whereas non-mechanical approaches offer greater potential for lightweight design, compliant integration, and adaptation to large deformation. The two categories involve distinct trade-offs in response speed, energy consumption, hysteresis, lifetime, and system integration. Future research should pursue task oriented integration of mechanisms, materials, and structures, establish unified testing protocols that include auxiliary actuation systems, and strengthen coupled motion stiffness sensing, low-power stiffness retention, and lifecycle reliability validation.