Variable stiffness methods for robots: a review

Traditional robots with constant stiffness demonstrate reliable output power and positioning precision, which may conversely reduce their flexibility and adaptability or even incur greater damage for accidental collisions with the environment or humans. Here, we review state-of-the-art robots with a variable stiffness mechanism, which is a key design concept that is widely used to improve robot reliability and impart new functionalities. To determine the similarities and differences between variable stiffness methods, we discuss the existing principles for variable stiffness of both rigid and soft robots, such as coupled and uncouple structures, thermal stimuli and magneto-rheological approaches. We hope this paper can help readers better understand these methods with regard to interesting applications. In addition, we also outline challenges and perspectives, where a simpler structure, larger band and faster response of stiffness modulation are required for robots in the future.