Design And Research Of Nonlinear Stiffness Actuator Based On The Given Stiffness Characteristic Curve

Both safe human-robot interaction and bionic joints’ adaptability to dynamic environments require robot joints to have compliance.The passive compliance generated by the inherent flexibility of mechanical structure is economical and reliable.In the actuators with passive compliance,the series elastic actuator(SEA)with constant stiffness have weak adaptability to different external load conditions.Variable stiffness actuator(VSA)usually need additional motor to adjust the stiffness,which makes the structure and control complex,increases the energy consumption,and may produce unreasonable matching relationship between stiffness and load.The nonlinear stiffness actuator(NSA)which selects stiffness based on load can improve shortcomings of the former two kinds of actuators,but the existing research achievements still have some limitations,such as the stiffness adjustment angles are too large,and the achievable stiffness function types are limited.More importantly,there are few cases of NSA structural design based on the given stiffness characteristic curve for a wide range of stiffness types.In order to solve these problems,a new type of nonlinear stiffness actuator is designed in this paper.Combining with cam mechanism,torsion spring and multilated gears,the given bidirectional symmetric nonlinear stiffness can be realized.Firstly,the generation mechanism of nonlinear stiffness is analyzed.By comparing different types of nonlinear transmission mechanisms,the design scheme of nonlinear stiffness mechanism with cams,multilated gears and torsion spring as main parts is selected.Then,the flow of solving the effective profile curve of cam by using “reversal method” and force and motion analyses is introduced in detail,as well as the optimization design method of the related mechanism parameters.Then,a prototype of nonlinear stiffness actuator is designed,including the design and selection of key components such as motor,torsion spring and sensors.Through the mechanical simulations and experiments,it is verified that the designed actuator prototype can achieve the given nonlinear stiffness characteristic curve well.By comparing with the similar actuators,the nonlinear stiffness actuator designed in this paper is indicated to have good comprehensive characteristics.Finally,the dynamic model of the actuator and the torque control system and experimental hardware platform are built.Through the torque step response experiments and sinusoidal torque tracking experiments,it is proved that the actuator has good torque control performances from three aspects of response rapidity,stationarity and accuracy.The comprehensive research results show the effectiveness of the design method of nonlinear stiffness actuator proposed in this paper.