Research On A Biped Robot With Variable Stiffness Ankle Joint

Compared with wheeled and tracked robots,biped robots have better adaptability to various terrains,higher mobility,and lower energy consumption.Owing to their anthropomorphic features,they can be better integrated into human society,replacing or assisting humans to complete high-load and high-risk tasks.Up to the present,how to ensure and improve the stability in motion,especially at the moment when foot-ground impact occurs,has always been a focus in the research of biped robots.Therefore,in this paper,the issue of how to reduce the impact force in the foot-ground collision process is investigated in three aspects,that is,design of the variable stiffness ankle joint,gait planning,and control strategy.Firstly,the effects of the impact in the collision process to the stability of biped robots is analyzed.The kinematics and dynamics of a biped robot with double support phase are modeled.With different assumptions,the seven-link biped robot model is simplified,and the foot-ground impact models of the biped robot with rigid ankle joint and compliant ankle joint are built respectively.It is verified by the analysis of the two impact models that the ankle joint with compliance can reduce the impact force in the foot-ground collision process and ruduce the energy consumption.Then,gait planning of the biped robot is performed based on the linear inverted pendulum model and Zero Moment Point(ZMP)stability criterion.In single support phase,the ZMP stays at the center of the stance foot;while in double support phase,the ZMP moves smoothly to the center of the new stance foot.Considering the characteristics of human gait and the working principle of the variable stiffness ankle joint,the foot of the swing leg is planned to rotate around its toe when lifting off the ground,and rotate around its heel when striking with the ground.Since the ankle joint can absorb impact,the velocities of the ankle joint before and post foot-ground impact are set to be continious,making the movement of the swing leg more human-like.Afterwards,a stiffness controller of the ankle joint is designed according to the footground impact model.The position of the ankle joint of stance leg is controlled based on extended inverted pendulum model and ZMP preview control,while the positions of the other joints are controlled by PID controllers.Co-simulations in Adams and Matlab are performed to test the walking performance of the biped robot with different stiffness at the ankle joint.Finally,the structure of the ankle joint with pneumatic variable stiffness element is designed and tested.With this variable stiffness ankle joint,a biped robot prototype platform and its control system are built.On this basis,walking experiments are performed using the biped robot in two conditions,i.e.,with rigid ankle joint,and with variable stiffness ankle joint.Experimental results show that the variable stiffness ankle joint can reduce the impact of heel strike and improve the walking stability.In addition,the effectiveness of gait planning and control strategy based on variable stiffness ankle joint proposed in this paper is also verified.