Research On The Pneumatic Muscle Actuated Biped Robot With Metatarsophalangeal Joints

Biped robot has always been a research hotspot and chanllenge in the area of robotics.Realizing stable,natural,and efficient human-like walking is an important objective for research of biped robots.However,there are still some critical issues with most current biped robots,including poor walking stability,stiff and unnatural gait,as well as low energy efficiency.Humans became advanced creatures that walk upright through long-term evolution.They are able to walk stably,naturally,and efficiently in various complex environment under the synergetic effects of their neuromusculoskeletal system.Owing to the similarity between human body and biped robot,it has several benefits to study the mechanisms of the stable,natural,and efficient human walking and then to integrate them into the design and control of biped robots.On one hand,it can contribute to the realization of human-like bipedal robotic walking;on the other hand,it can deepen the understanding of researchers on human body structures and walking mechanisms,and provide some referential significance for limb function rehabilitation and exoskeleton appliance design.Therefore,this dissertation aims to realize human-like dynamic walking on a biped robot based on the biomechanics of human walking.Mechanisms of human walking,bionic structure design of the biped robot,control scheme for human-like dynamic walking of the biped robot,and experiments of dynamic walking of the biped robot are studied in depth.The mechanisms of stable,natural and efficient human walking are analyzed based on biomechanics.Using the OpenSim software,a human musculoskeletal model is built,walking of the model with movements of metatarsophalangeal joints is simulated,and then various representative biomechanics of human is acquired.On this basis,the function mechanisms of lower limb muscles during walking are investigated.The results indicate that the support and propulsion/braking forces and the mechanical power of each lower limb joint during walking are mainly provided by five muscle groups,that is,psoas major,gluteus maximus,vastus medialis,soleus,and gastrocnemius.Whereas,biceps femoris(short head),tibialis anterior,rectus femoris,and biceps femoris(long head)behave like passive springs,assisting to improve the walking energy efficiency by absorbing,releasing and transferring power between joints.The metatarsophalangeal joints interact with the ground directly during walking,but their roles are usually neglected.For this reason,the biomechanics of the metatarsophalangeal joints and their effects to human gait are studied.The results show that the metatarsophalangeal joints,which can be regarded as passive during walking,make human gait present a typical time sequence feature of “heel strike-foot flat-heel off-toe off”.When the movements of metatarsophalangeal joints are constrained,the above feature will also be affected,causing an increase of the total joint mechanical work and decrease of the margin of walking stability.The bionic structures of the biped robot are designed based on the mechanisms of human walking.According to the biomimetic foundation derived from the studies on human walking mechanisms,a pneumatic artificial muscle actuated biped robot which has active and passive combined actuation and passive metatarsophalangeal joints is proposed.The overall structure of the biped robot is designed,and then the specific structures of the hip,knee,ankle,and foot are designed inspired by the anatomy structures of each corresponding part of human lower limbs.Referring to the normal human gait,the complete hybrid dynamics of walking that contains single support phase,double support phase,and impact phase is modelled for the biped robot.By means of the dynamics model,the key structural parameters of the biped robot are optimized.And the validity of the structure design is verified by evaluating the gait performance of the biped robot.The control scheme for human-like dynamic walking of biped robot actuated by pneumatic artificial muscles are designed.In order to solve the difficulties caused by the complicated hysteresis and strong nonlinearity of pneumatic artificial muscles,the modeling of length/pressure hysteresis is studied.A modified Prandtl-Ishlinskii hysteresis model with piecewise play operators is built,which not only can describe the asymmetric length/pressure hysteresis accurately,but also be directly utilized to describe the inversion of length/pressure hysteresis without any complex calculation.Furthermore,a position control method of pneumatic artificial muscles with direct inverse hysteresis compensation is proposed,as capable of achieving high-precision positon control.On the basis,a control scheme for human-like dynamic walking of the biped robot is designed by combining foot placement control,reflexive control,and ankle joint control.The control scheme does not depend on the ZMP stability creteria and simple position tracking control,but takes the patterns of human joint movements and muscle actions during walking as reference.Dynamic walking of the biped robot under different conditions is simulated in MATLAB.Experimental study of the biped robot are performed.The control system that takes an embedded controller as the core is designed for the biped robot.Then the physical prototype of the biped robot is integrated and the experiment paltform is built.Based on this,taking the ankle joint as an example,the joint position control experiments of the biped robot are performed.In order to verify the proposed structure design and walking control scheme,walking experiments under different conditions are carried out on the prototype.Human-like dynamic walking of the biped robot are achieved,including walking on even ground at moderate speed,at changed speed,with changed step length,up the ramp,down the ramp,as well as on uneven ground.