Simulation and experiment on obstacle-surmounting performance of four swing arms and six tracked robot under unilateral step environment

Abstract: In the agricultural environment, in order to realize the replacement of human by robots and reduce the burden of human labor, the 4-swing-arm6-track robot(4SA-6TR)was designed after studying the structure of tracked robot. The maximum size of the robot is 830 mm ×790 mm ×360 mm, its swing arm is 295 mm, and its weight is about 120 kg. The robot adopts the structure of the sleeve shaft, and the rotation motion of swing arm is independent from that of the tracked wheel, so that the robot's 4 arms can rotate freely, and robots can achieve multi-posture changes. When the robot faces an obstacle, the robot can adjust its posture through changing the rotation angle of the 4 swing arms, so the robot can maintain the higher stability during the obstacle course. Kinematics matrix equations of 4SA-6TR were established. And the center of gravity(CoG) coordinates of the robot were gotten in the global coordinate system with the help of the CoG theorem after analysis of motion attitude of 4SA-6TR robot. The CoG can be used to study the stability of the robot. Sometimes the 2 tracks of the robot don't touch obstacles at the same time. When one side of the robot meets an obstacle, the roll angle, pitch angle and position of the CoG of the robot will change, which will affect the stability of the robot. The obstacle-surmounting mechanism of the robot under the condition of unilateral obstacle was studied. The robot is considered as a connecting rod consisting of a free joint and 4 joints, and the mathematical model was established between the rotation angle of robot's swinging arms and the robot's roll angle. The equation of the rotation angle of robot's swinging arms and the height of the step can be obtained by the derivation of the formula. When robot climbs a unilateral step, its pitch angle and roll angle will change due to its CoG. In the unilateral obstacle environment, when the step height is less than L1+r-R(L1is the length of the robot's swing arm, r is the radius of the arm wheel, and R is the radius of the driving wheel),the robot can achieve the horizontal posture to overcome the obstacle. The robot will be inclined but it is still able to move steadily without rolling, when the step height meets the conditions of L1+r-R≤H≤Hmax(H is the step height, and Hmax is the maximum step height a robot can cross). Therefore, the robot can be balanced by adjusting its swing arm's angle. The 3D (three-dimensional) model of 4SA-6TR robot and terrain was completed by using Solid Works software, then its simplified model was introduced into Adams virtual prototype simulation software, and the virtual prototype simulation environment of the robot and terrain was obtained. A combination of the steps with height of 150 and 100 mm was established in the simulation experiments. The height of the step is less than 247 mm, and the robot can keep its body level during the obstacle course. From the simulation experiment, it can be concluded that, the robot could adjust the posture of its 4 swing arms in real time according to the height and orientation of the obstacle and the speed of the robot in the obstacle-surmounting process. Through indoor test experiments, it can be found that the robot's pitch angular deviation is bigger, and the maximum deviation is 2.4°, but in the end its pitch angle can be stabilized at about 0.1°; the roll angle deviation is small, and finally the roll angle can be stabilized at about 0.3°. The results of the test experiments show that the robot could complete the obstacle-surmounting movement very smoothly, which verified the correctness and feasibility of the control strategy. The attitude control scheme of the unilateral obstacle of the robot can provide the theoretical basis for the automatic obstacle surmounting.