Research On The Locomotion Mechanism And Control Method Of A Leg-wheel Robot With An Active Spine

Combining the high energy efficiency of the wheeled robot on flat ground and the high mobility of the legged robot on rough terrain,the leg-wheel robot has a great application prospect in tasks,such as reconnaissance and detection,safety inspection and post-disaster search and rescue.However,the existing leg-wheel robot can not give full play to the advantages of wheeled motion and legged motion due to the deficiency of mechanism design and control methods.In this paper,a series of studies are carried out on the leg-wheel robot,focusing on mechanism design,gait planning for legged motion and legged motion control over rough deformable terrain.Because the technology of wheeled motion control is mature,the research on the motion control of the proposed leg-wheel robot in this paper mainly focuses on the legged motion.At first,the design and implementation of a novel leg–wheel robot with an active spine called Transleg are presented.Transleg adopts the wire as the transmission mechanism to simplify the structure and reduce the weight.Transleg possesses four transformable leg–wheel mechanisms,each of which has two active degrees-of-freedom(DOFs)in the legged mode and one in the wheeled mode.Two actuators driving each leg–wheel mechanism are mounted on the body,so the weight of the leg–wheel mechanism is reduced as far as possible,which contributes to improving the stability of the legged locomotion.Inspired by the quadruped mammals,a compliant spine mechanism is designed for Transleg.The spine mechanism is also actuated by two actuators to bend in the yaw and pitch directions.The design and kinematic analyses of the leg–wheel and spine mechanisms are presented in detail.To verify the feasibility of Transleg,a prototype is implemented.The experiments on the motions in the legged and wheeled modes,the switch between the two modes,and the spine motions are conducted.The experimental results demonstrate the validity of Transleg.In addition,a step-climbing maneuver based on the structure of Transleg is proposed.Using this maneuver,Transleg can climb steps with the height of 94 mm which are much higher than the radius of its wheel which is 55 mm.Secondly,the bounding gait combining the pitch motion of the spine and the leg motion is studied.In this gait,the spine moves in phase with the front legs.All the joints of the legs and spine are controlled by cosine signals to simplify the control,and the initial position and oscillation amplitude of the joints can be tuned.To verify the effectiveness of the proposed gait,the spine joints are set with different initial positions and oscillation amplitudes,and the initial position and oscillation amplitude of the leg joints are tuned to make the virtual model locomote as fast as possible while keeping stable in the simulation.The control signals are also used to control a real robot called Transleg.The experimental results show that stable bounding locomotion can be achieved using the proposed gait,and the speed of the bounding locomotion is increased with the spine motion in the pitch direction.In addition,three different turning strategies are proposed,including driving the left and right legs with different strides,swaying the spine in the yaw direction,and combining the above two methods.These strategies are tested on the real robot,and the experimental results show that the turning radius is reduced and the angular velocity is increased with the spine motion in the yaw direction.Thirdly,to solve the unbalance problem caused by the moment around the diagonal supporting axis for the quadruped robot in the trotting gait,two novel methods of trotting gait planning are proposed based on the kinematic modeling and the analysis of the unbalance reason,including trotting gait combined with the abduction and adduction of legs and trotting gait combined with the spine motion in the yaw direction.The former introduces the abduction and adduction of legs to adjust the foot trajectories of the supporting legs.The latter augments the spine motion in the yaw direction to regulate the center of gravity of the robot while keeping the foot trajectories changeless.The two methods both make the center of gravity of the robot locate on the diagonal supporting axis in the whole trotting gait.The simulation results show that compared with the common trotting gait and the trotting gait with the foot trajectory moving backward,the proposed trotting gaits greatly improve the locomotion stability of the robot.In addition,the robustness of the proposed gaits is verified when deviations of the model parameters exist.At last,to achieve the trotting locomotion of the quadruped robot with an active spine on the rough deformable terrain,a CPG(central pattern generator)based control method is proposed based on the kinematic analysis,including six modules: gait planning,ground inclination estimator,posture control,stumbling reflex,step-missing reflex and lateral stepping reflex.The gait planning module use CPG to generate the phase signals for legs and apply the gait planning method to shape the phase signals to obtain the joint signals of legs and spine.The ground inclination estimator calculates the ground inclination based on the contact force on the feet and the posture angles of the body.According to the ground inclination,the planned foot trajectory is adjusted.The posture control module adjusts the joint motion of legs in the stance phase according to the posture angles of the body and the ground inclination,making the body parallel to the ground and controlling the heading angle of the robot.The stumbling reflex adjusts the joint motion of legs in the swing phase when the legs contact with barriers,making them cross barrier and then return to the planned motion trajectory quickly.The step-missing reflex adjusts the joint motion of legs in the stance phase when the legs miss their steps,making them contact with the ground and then return to the planned motion trajectory quickly.The lateral stepping reflex actuates the robot to move along the direction of the external force to eliminate the effect of the force.In the simulation,the quadruped robot with an active spine is controlled to locomote on the rough deformable terrain with the proposed method,and the results demonstrate the effectiveness of this method.