Trajectory planning verification and gait analysis of wheel-legged hybrid robot with variable degree of freedom

Abstract: With the continuous promotion of agricultural modernization in China, new requirements for agricultural robots are put forward. The agricultural robots are developing in the direction of automatic walking and unmanned operation, and the working environment is extended from structured environment to unstructured environment. In the future, the agricultural robots should be able to change DOF (degree of freedom) through mechanical structure changes and carry different end effectors to adapt to different types of crops, which can achieve one machine multipurpose and improve the utilization rate. In order to adapt to the new requirements of modern agriculture for robots, according to the principle of bionics, this paper presents a novel quadruped robot with 3-DOF leg mechanism based on the serial-parallel and wheel-legged mechanism, which consists of 2-universal joint-prismatic joint-spherical joint plus universal joint, and revolute joint ((2-UPS+U) &R). Firstly, based on the principle of bionics, the leg mechanism and the whole structure of the robot are developed. The walking of human and animal is realized by the contraction and relaxation of the skeletal muscle to drive the bone, and through the analysis of the distribution of muscle structure, the mechanism uses 2 electric push rods to mimic the muscle distribution of the leg to drive the swing of the robot's upper leg in 2 directions. The mechanism has the combining advantages of series mechanism and parallel mechanism. It overcomes small work space and can achieve high precision and strong bearing capacity. The robot not only can walk in four-legged mode, but also can curl up the lower leg and become the wheeled mode to achieve rapid movement. According to the analysis of the robots with high speed and heavy load at home and abroad, the desired maximum load of the robot studied is 200 kg, the maximum speed in the legged mode is 1 m/s, and the maximum speed in the wheeled mode is 5 m/s. According to the design objective, the parameters of each part are optimized. Then the forward and inverse kinematics analysis of position for swing leg is carried out. Secondly, according to the general terrain and obstacle terrain in agriculture, the constraints for trajectory planning are put forward. Then the general trajectory planning functions of the robot's foot-end in forward step and side step are obtained, and the obstacle negotiating trajectory of the robot foot-end is planned by spline interpolation curve, which provides the basis for robot gait planning. The obstacle trajectory planning is simulated by using the ADAMS software, results showed that the lifting height is 98.2 mm with error of 1.8 mm, and the step length is 101.3 mm with error of 1.3 mm, witch show that the robot can overcome small obstacles, and the trajectory planning is correct. The velocity and acceleration of the leg mechanism are continuous without distortion points, and the leg's jerking movement and concussion are eliminated. All the results above show that the foot-end can achieve soft landing, and the kinematics and foot-end trajectory planning are correct. Thirdly, according to the requirement of robot gait planning, the static stability criterion and the ZMP (Zero Moment Point) theory of dynamic stability criterion are analyzed. Under the premise of ensuring the stability, the diagonal trot gait planning of robot is completed, and the fast walking of the robot is achieved. Based on the diagonal trot gait planning of the robot, the simulation is carried out. The simulation results show that the robot moves steadily and fast, with the height of the robot's center of gravity changing only about 0.9 mm, and the gait planning is correct. Finally, the trajectory planning experiment of the robot's single leg prototype is carried out. The leg prototype can move according to the given trajectory, and the feasibility of the mechanism design is verified. The trajectory tracking experiment shows that there are some errors between the theoretical trajectory and the actual trajectory. The maximum error of y-axis is 2.5 mm and the maximum error of z-axis is 5.3 mm, but the errors are less than 10 mm, within the allowable range of error. The errors mainly come from the manufacturing errors and assembly errors. In the future, the turn gait, pivot steering gait and climbing stair gait will be studied. The research on stability analysis and gait planning will provide a theoretical basis for establishing control system of the quadruped robot.