In order to enhance the locomotion adaptability of legged robots, a novel compliant rotational joint with continuously adjustable stiffness is designed. To achieve this, the relation between transmission ratio and output stiffness of lever mechanism is analyzed, and the lever mechanism with variable transmission ratio is utilized as a core functional module in the design of the compliant joint with adjustable stiffness. The joint is designed to be light weight and compact in size, and the joint's mechanisms as well as driving module are specified for practical applications to legged robots. The relationship between joint structure parameters and output stiffness is analyzed, and the range of the relevant parameter is selected so that the output stiffness can be sensitive to the parameter change. The natural stiffness characteristics of the joint are further determined based on kinematic study. Tests on the prototype joint show that the output stiffness can be adjusted and controlled effectively by the adjusting-mechanism, and the joint can be applied to robotic legs with respect to its structure design and functions.
 Blickhan R. The spring-mass model for running and hopping[J]. Journal of Biomechanics, 1989, 22(11/12): 1217-1227. Blickhan R, Seyfarth A, Geyer H, et al. Intelligence by mechanics[J]. Philosophical Transactions of the Royal Society, A: Mathematical, Physical and Engineering Sciences, 2007, 365(1850): 199-220.  Hutter M, Remy C D, Hoepflinger M A, et al. High compliant series elastic actuation for the robotic leg scarleth[C]//14th International Conference on Climbing and Walking Robots and the Support Technologies for Mobile Machines. Singapore: World Scientific Publishing, 2012: 507-514. Seyfarth A, Iida F, Tausch R, et al. Towards bipedal jogging as a natural result of optimizing walking speed for passively compliant three-segmented legs[J]. International Journal of Robotics Research, 2009, 28(2): 257-265.  李哲,苏兴旺,阎宏伟,等.气动弹跳腿垂直方向稳定跳跃研究[J].机器人,2013,35(3):306-312.Li Z, Su X W, Yan H W, et al. Stable hopping of a pneumatically actuated leg in vertical direction[J]. Robot, 2013, 35(3): 306-312. Hobara H, Inoue K, Muraoka T, et al. Leg stiffness adjustmentfor a range of hopping frequencies in humans[J]. Journal of Biomechanics, 2010, 43(3): 506-511.  Ferris D P, Louie M, Farley C T. Running in the real world: Adjusting leg stiffness for different surfaces[J]. Proceedings of the Royal Society, B: Biological Sciences, 1998, 265(1400): 989-994.  Arampatzis A, Bruggemann G P, Metzler V. The effect of speed on leg stiffness and joint kinetics in human running[J]. Journal of Biomechanics, 1999, 32(12): 1349-1353.  Riese S, Seyfarth A. Stance leg control: Variation of leg parameters supports stable hopping[J]. Bioinspiration & Biomimetics, 2012, 7(1): 016006. Galloway K C, Clark J E, Koditschek D E. Variable stiffness legs for robust, efficient, and stable dynamic running[J]. Journal of Mechanisms and Robotics, 2013, 5(1): 011009. Hurst J W, Rizzi A A. Series compliance for an efficient running gait[J]. IEEE Robotics & Automation Magazine, 2008, 15(3): 42-51.  van Ham R, Vanderborght B, van Damme M, et al. MACCEPA, the mechanically adjustable compliance and controllable equilibrium position actuator: Design and implementation in a biped robot[J]. Robotics and Autonomous Systems, 2007, 55(10): 761-768.  Wolf S, Hirzinger G. A new variable stiffness design: Matching requirements of the next robot generation[C]//IEEE International Conference on Robotics and Automation. Piscataway, USA: IEEE, 2008: 1741-1746. Jafari A, Tsagarakis N G, Vanderborght B, et al. A novel actuator with adjustable stiffness (AwAS)[C]//IEEE/RSJ International Conference on Intelligent Robots and Systems. Piscataway, USA: IEEE, 2010: 4201-4206. Tonietti G, Schiavi R, Bicchi A. Design and control of a variable stiffness actuator for safe and fast physical human/robot interaction[C]//IEEE International Conference on Robotics and Automation. Piscataway, USA: IEEE, 2005: 526-531. Koganezawa K, Nakazawa T, Inaba T. Antagonistic control of multi-DOF joint by using the actuator with non-linear elasticity[C]//IEEE International Conference on Robotics and Automation. Piscataway, USA: IEEE, 2006: 2201-2207. 马洪文,赵朋,王立权,等.刚度和等效质量对SEA能量放大特性的影响[J].机器人,2012,34(3):275-281.Ma H W, Zhao P, Wang L Q, et al. Effect of stiffness and equivalent mass on energy amplification characteristics of SEA[J]. Robot, 2012, 34(3): 275-281. Braun D J, Petit F, Huber F, et al. Robots driven by compliant actuators: Optimal control under actuation constraints[J]. IEEE Transactions on Robotics, 2013, 29(5): 1085-1101.