| While the mechanisms were driven on soft terrain, the driving force depended mainly on the friction force and shear force of the soil. Soft soil has the characteristics of low pressure-bearing capacity and low shearing strength. Therefore, the mechanisms being driven on soft soil with low pressure-bearing capacity and low shearing strength would often lead to severe sinkage and slip.Cattle walk frequently on soft soil. Their hooves gradually formed optimum geometric shapes through their evolution upon millions of years. Cattle have the characteristics of stronger passing ability, smaller resistance and higher efficiency while walking on soft soil.The aim of this paper was to describe the postures of the cattle's limbs during walking on paddy field. According to bionic principle, the movement mechanism of the cattle walking on paddy field may provide certain theoretical and practical base to bionic design of improving the properties of the walking mechanisms on soft terrain.The walking process of the cattle, Bostaurus, on paddy field before spring plowing was captured by the high-speed photography. The geometrical configurations and motion postures of the cattle during walking on paddy field were analyzed, and the gait parameters were gained from the high-speed photographs. The conclusions were as follows:(1) While the cattle walking on paddy field at the speed of 1.01m/s, the gaits were symmetrical. The movements of the front and hind leg at left side were earlier half of the cycle than the front and hind leg at right side.(2) Each limb of the cattle during walking repeated its motion in each successive step, a part of its cycle being occupied in swinging forward to a new contact and the remainder consisting of its action as a mechanism for the support and propulsion of the body of the cattle. These cycles of the four limbs of the cattle weren't randomly spaced but take a well regulated order, which ensured the cattle to move steadily.(3) During walking, the changes of the forelimb were more prominent compared with the hind limb, moreover, the landing place of the hind hooves were farther than the front hooves, which made the hind limb larger movement space to prevent the interference of forelimb and hind limb when the strides were increased. But, the changes of the landing place were smaller than the changes of horse, camel and buffalo.The inverse kinetics method was used to establish the kinematics equations of the cattle during walking on paddy field. The changes of displacement, velocity, acceleration and force of the centre of mass (COM) of the cattle were analyzed. Combined with the curve fitting, the angle and angular velocity plots of each leg were drawn, and then the characteristics of the cattle during walking on paddy field were discussed. The conclusions were as follows:(1) During walking, the maximum horizontal velocity of the COM was 1.492m/s, the minimum value was 0.748m/s; the maximum vertical velocity of the COM was 0.786m/s, the minimum value was 0m/s. While the cattle walked on paddy soil, the amplitude of the COM in vertical direction was 0.001～0.034m, the oscillated ratio was 0.08～2.85% relative to the body height 1.19m, which ensured the cattle to move steadily.(2) The changes of hip joint angle and shoulder joint angle were more remarkable, which provided the driving force for the body. The movements of knee joint and elbow joint were very obvious after limb left the ground, moreover, the swing directions of forelimb and hind limb were opposite. Before limb touched down the ground, the movements of hip joint and shoulder joint were steady to reduce the impact of the ground.(3) The maximum horizontal force of the soil acted on the hoof was 14071.2N, the average value was 4531.7N; the maximum vertical force of the soil acted on the hoof was 14284.4N, the average value was 4984.8N.(4) While the cattle walking on paddy soil, the maximum value of the kinetic energy was 372.4J, the minimum value was 78.6J; the maximum value of the gravitational potential energy was 2378.6J, the minimum value was 2213.8J.According to the characteristics of the cattle during walking on paddy field, the mechanism walking on soft ground was designed. The jumping process of the locust, Oedaleus infernalis, was captured by the high-speed photography method.The jumping mechanism, driving mechanism and buffer action of locust's legs were studied. According to the characteristics of the locust during jumping, the buffer mechanism was designed to reduce the impact of the ground. The models of the mechanism walking on soft ground and the buffer mechanism were established by the technology of the virtual prototyping, and then the 3D dynamic simulation was analyzed. The conclusions were as follows:(1) The displacement changes of the crank were steady. The changing rules of the top connecting rod, the middle connecting rod and the bottom connecting rod were the same, in accordance with the sine law. At the time of 9s, the speed of the slide rod reached the maximum value of 148.2mm/s.(2) The force changes of the crank and the top connecting rod were steady, the force changes of the middle connecting rod and the slide rod were obvious. At the time of 9s, the force of the middle connecting rod and the slide rod reached the maximum value of 239.71N. (3) The linear mode of the buffer device was analyzed to obtain the natural frequency of 2.8936Hz. At each time, the forces of buffer spring and assistant buffer spring were equal, but the direction of the forces were opposite, in accordance with the sine law. At the time of 2s, the buffer device was achieved the overall balance, and the oscillation of the buffer device stopped.The method reported in this paper would provide certain theoretical and practical references for the bionic design of the mechanisms walking on soft ground, and provide a new method for the bionic study of the mechanical engineering. The method has the significance in shortening research cycle, reducing development cost and raising product performance.
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