Journal of Bionic Engineering (2023) 20:417–435https://doi.org/10.1007/s42235-022-00278-x

Optimal Variable Stifness Control and Its Applications in Bionic Robotic Joints: A Review

 Yuanxi Sun1  · Yuntao Tang1  · Jia Zheng2  · Dianbiao Dong3  · Long Bai1 

1 State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing 400044, China 

2 School of Advanced Manufacturing Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, China

3 School of Mechanical Engineering, Northwestern Polytechnical University, Xi’an 710072, China

AbstractVariable Stifness Actuation (VSA) is an efcient, safe, and robust actuation technology for bionic robotic joints that have emerged in recent decades. By introducing a variable stifness elastomer in the actuation system, the mechanical-electric energy conversion between the motor and the load could be adjusted on-demand, thereby improving the performance of the actuator, such as the peak power reduction, energy saving, bionic actuation, etc. At present, the VSA technology has achieved fruitful research results in designing the actuator mechanism and the stifness adjustment servo, which has been widely applied in articulated robots, exoskeletons, prostheses, etc. However, how to optimally control the stifness of VSAs in diferent application scenarios for better actuator performance is still challenging, where there is still a lack of unifed cognition and viewpoints. Therefore, from the perspective of optimal VSA performance, this paper frst introduces some typical structural design and servo control techniques of common VSAs and then explains the methods and applications of the Optimal Variable Stifness Control (OVSC) approaches by theoretically introducing diferent types of OVSC mathematical models and summarizing OVSC methods with varying optimization goals and application scenarios or cases. In addition, the current research challenges of OVSC methods and possible innovative insights are also presented and discussed in-depth to facilitate the future development of VSA control.

 Keywords Bionic robotic joint · Variable stifness actuator · Optimal control · Robotics · Controller design