Research On Foot Force Optimization And Slippage Suppression Of Hexpod Robot Under Unknown Terrain

The multi-legged walking robot is characterized by its stable and flexible motion,non-continuous support and discrete foothold,making it capable of performing tasks that cannot be performed by conventional wheeled and tracked robots.It has high adaptability and flexibility to unknown and rugged terrain.As a typical representative of legged robots,hexapod robots have more demanding applications in the fields of transportation,disaster relief and extreme operations under extreme working conditions.The problems caused by the hexapod robots during the rugged terrain walking include: the flexible of motion planning algorithms in the process of foot terrain interaction;the state of the robot under the body sensor framework is not clear;large internal force and foot terrain slippage affect the walking efficiency;the motion control based on multi-sensor fusion is complex,which poses a higher challenge to the sensing fusion,locomotion control of the hexapod robot.The research in the subject promotes the locomotion control for the legged robots,which provides theoretical and technical support for the research of legged robot.The electrical driven hexapod robot is studied in this paper.The foot trajectory planning with passive elastic foot is studied.In order to ensure smooth contact for the foot terrain interaction,a passive spring structure is added between the leg and foot,so that the foot of hexapod robot become elastic structure with retractable.The foot terrain dynamic interaction process of a robot with deformable leg is analyzed.Aiming at the foot terrain impact force for normal and tangential directions caused by traditional planning methods,a low impact force and energy consumption foot trajectory based on passive elastic legs is presented.The method solves the foot terrain impact problem during the lifting and touching process by the passive spring.And the increase of the energy consumption caused by the frequent acceleration and deceleration of the robot is also solved.The method is verified on the hexapod robot prototype.The accurate of the velocity and posture data of hexapod robot can not be obtained by sensors only due to the influence of the error of the sensor itself and the installation error,as well as the foot slip or mechanical error during the foot terrain interaction.The data fusion method based on Extended Kalman Filter is adopted,and the data are from the kinematics and IMU information.The accurate velocity and posture of trunk body is obtained.When the slippage occurs,it is difficult to detect the occurrence of slip through the body sensors.This paper establishes foot terrain slip acceleration model for crab and insect walking based on leg dynamics model with foot slip,and presents foot terrain slip estimation method based on state estimation.The method can detect the slip rate of the foot and the state of the foot slip is determined.The estimated value is compared with robot state data obtained by the external motion capture system(Mocap system),and the effectiveness of the proposed method is verified.In order to realize the autonomous walking of the hexapod robot under the unknown terrain,the coupling characteristics of the multi-leg support are analyzed.The mechanical transformation matrix based on terrain estimation is obtained and is used to correct the foot force direction on the sloped terrains.The foot force optimization method for steady walking under large friction terrain is proposed,which solves the internal force generated by the unsynchronized locomotion in the traction and tangential directions.The foot slippage caused by the deformation of the elastic foot,deformation of unknown terrain and the large internal force will cause the offset of the walking trajectory and affect the locomotion efficiency.The effective displacement of the stance foot has significantly different from the pre-planned displacement.The foot force optimization method considering terrain estimation and foot slippage is proposed.The method reduces the internal force in the tangential direction,and the occurrence of foot slippage due to internal force confrontation is suppressed.The effectiveness of the method is verified by experimental data under complex and flat-slope transition unknown terrains.In order to prevent the joint from reaching the limit position under the unknown terrain,this paper presents mathematical model for the specific structural topography and joint space,which ensures the safety in the joint space.The pitch and roll target angles are adjusted according to the influence of the geometric constraint.The autonomous obstacle avoidance and the foothold selection strategy of the hexapod robot are studied based on external visual perception fusion.An autonomous obstacle avoidance strategy based on artificial potential field method is presented.The terrain height,safety landing area and the terrain inclination are analyzed and the area of the landing point is selected.The posture and leg control of the robot are studied.For the blind and visual guidance walking state,the periodic and continuous tracking gaits are adopted to realize the walking coordination under the complexed terrain.Finally,the experimental on unknown field terrains for the locomotion control of the hexapod robot are carried out,which verifies the correctness and practicability of the proposed method.