Study On Bionic Crab Legged Foot Facing Deformable Terrain

Legged walking mechanism has the superiority to wheeled vehicles on the deformable terrain,and there is a great potential for the development.Meanwhile,the rise of bionic technology develops a new direction for the legged mechanism.Chinese mitten crab is excellent in locomotion ability,which can travel for a long distance and walk through the deformable environment such as swamp,wet land and so on.Therefore,the locomotion characteristic and structure of Chinese mitten crab paraeiopod provides a biological prototype and bionic basis for the development of legged walking mechanism and the design of legged foot,which is the soil-engaging component of legs.By using a high speed 3D video recording system,the images of Chinese mitten crab were recorded when they walked on flat floor,slope and sand.The kinetics and dynamics parameters described by joint angle,soil-cutting surface angle and soil-contact angle were analyzed.The behavior of crabs on different terrains was compared and the role of each paraeiopod was discussed.The results showed that when Chinese mitten crab walking on slope,the velocity and acceleration of the center of mass were minimum,and the duty factor was maximum.With the greater fluctuation of soil-cutting surface angle and smaller soil-contact angle,Chinese mitten crab walked more slowly,and the supporting time was longer on slope than other terrains.They adjusted the posture frequently to avoid sliding.While walking on sand,the M-C joint angle and soil-cutting surface angle of paraeiopods were small,as well as concentrated distribution of soil-contact angle,that shows Chinese mitten crab adopted shorter strides,faster conducts and lower body center to prevent sinkage and being trapped.For Chinese mitten crab,the 2nd and 3rd pair of paraeiopods played a major role in the motion,the 1st pair of paraeiopod mainly support the body,while the 4th pair of paraeiopod were rarely used during walking.Using the techniques of stereoscopic microscope,EDS,XRD,FTIR and SEM,biology studies including material composition and microstructure of Chinese mitten crab dactylopodites were carried out.Also,the material mechanical properties,such as flexural strength,were measured by three point bending techniques.By which,the properties of ight weight and high rigidity were tested.The results showed that the dactylopodites of Chinese mitten crab were mainly composed of three elements-C,Ca,O,as well as a few other elements.Chitin and protein were the major organic matter in the exoskeleton of dactylopodites.CaCO3 was identified as a main inorganic matter component,which mainly existed in the form of calcite with a high crystallization degree.There were a small amount of CaCO3 exist in the form of ACP.The exoskeleton of Chinese mitten crab dactylopodites was multi-layer structure which consisted of epicuticle,exocuticle and endocuticle.The thickness of exocuticle was related to the concave and convex extent of profile.?Bouligand? structure was observed in endocuticle: numbers of fibre bundle were arranged in the form of fibrous chitinprotein planes,and the planes displayed a helicoidal stacking sequence,which looked like a twist plywood structure.There were many pore canals and spiral pore canal tubules within the fibous planes used for transport of ions and nutrients to exoskeleton.In addition,the structure is helpful to increase tenacity.The tip of dactylopodites showed calcification,which enhanced surface strength,anti-impact capability and wearability.The results of three point bending test indicated the toughness properties of wet dactylopodite were obvious,and they can better withstand the bending loads,which showed the mechanical characteristics of dactylopodite related to hydration.The cross section structure of Chinese mitten crab dactylopodite were observed under SEM,bionic models were built,and extracted the geometric parameter.Four kinds of bionic legged foot and a cylindrical foot for comparison were designed.A serious of soil bin tests was carried out in six kinds of soil.The results showed bionic legged foot took less energy consumption during insertion and extraction than cylindrical foot,and bionic structure improved the propulsion performance of legged foot.Moisture content of soil was the major factor influence the propulsion,followed by depth and velocity,particle size didn‘t have significant influence on propulsion.When load kept unchange,cone bionic walking feet behaved better than cylindrical foot in propulsion.Therefore,bionic legged feet had better performance than cylindrical foot considering the whole motion,which can be applied to light-weight and lightly loaded walking mechanism to ensure the locomotion flexible and convenient.The drag force in the soil increased slightly with velocity and square of depth of insertion.Meanwhile it nonlinearly increased with square of the cone angle of variable section soil-engaging component.Based on the estimating formula,parameters cone angle,velocity were introduced.A prediction model of drag force for both cone and cylinder soil-engaging component was developed,which can evaluate the propulsion performance of soil-engaging component on soft terrain.Take Chinese mitten crab as a biological prototype,some joints of paraeiopod were simplified,and a six-leg walking mechanism imitating crabs was designed.The legs adopted closed-chain linkage mechanism,gear and linkage were the main transmission,and cam mechanism was utilized to realize alternating step.The simulation analyses proved the duty factors of walking legs were about 54.21%~74.36%,and the return time was short.Thus,the walking mechanism had a good movement continuity and stability.Passing ability tests were carried out on three terrains with different roughness.The stability of walking mechanism on rough ground was better than smooth surface.Reasonable goals such as decreasing the displacement difference,the maximum acceleration,and increasing the average velocity,the maximum height were set,the length of leg linkage were optimized.The optimizated value of comprehensive evaluating function was decreased 11.06%,which indicated the integral performance of walking mechanism was improved effectively.The type and deminson of foot were choosen to match the legs.Then the feet were assembled with the walking mechanism.By three-dimension motion observation and analysis system,the passing ability of crab-like walking mechanism in the dry sand and wet sand were tested.The results showed that under the test condition,prototype B and prototype C beared larger traction load in comparison with prototype A,so they had superior tractive performance.With the increase of traction load,the average velocity of walking mechanism decreased linearly,while the vertical load had a small impact on average velocity.The motion images of walking mechanism were analyzed combining with tractive performance.When the traction load was certain,the sinkage of three prototypes under the same vertical load: A>B>C.Thus the carrying capacity of prototypes with bionic legged feet was higher,which reduced energy consumption and improved locomotion efficiency,and they had a better passing ability.Moreover,the installation angle of legged feet affected the performance of prototype.When the legged feet were assembled with legs at 15°,the traction performance,carrying capacity and passing ability of walking mechanism were superior than that with legged foot assembled at 0°.