# Bionic Walking Foot and Mechanical Performance on Soil
## 1 Introduction  
Severe sinkage and slipping of terrain vehicles often happens on soft terrain with low pressure-bearing capacity and low shearing strength. The discrete footfalls of a legged machine allow it to overcome obstacles with greater ease and to travel over soft terrain with less energy loss due to soil work [1]. Achieving good traveling behavior has long been an important issue. Compared with wheeled vehicles, legged mechanisms have superiority on unstructured terrain such as loose, muddy roads. The leg can pierce the soil to gain a grip [2] and pass through the places that other forms of vehicles are harder to reach. Therefore, much more research focuses on the traffic ability of legged vehicles on various terrains and the interaction between the leg and its environments.  
As the contact part with soil, the walking foot plays a fundamental role in legged vehicles. Many factors contribute to the performance of the walking foot, including the shape, dimension, material and so on [3]. However the shape of the conventional walking foot for legged vehicles (robots) are not varied. [PERSON] et al. [3] summarized the main shapes of foot: (1) flat, for increasing the contact area, such as TITAN [4]; (2) cylindrical, spherical or cambered, including crab-like robot Bigdog [5], SILO 6 [6], fits for multi-freedom legs; and (3) irregular shape. After millions of years of adaptation in the natural environment, animal legs have evolved and diversified their leg morphology into various forms [7], with feet of various shapes and functions to adjust to complex terrains [8]. Thus, more andmore feet of legged vehicles are beyond the traditional shape, such as the \"C\" leg of RHex [9] and Sandbot [10; 11]. So far, much research [12; 13; 14] has regarded the leg and foot as a whole, and there is little attention to the influence of the structure of the walking foot on a legged mechanism.  
It is well known Chinese mitten crabs (_Ericheir sinensis_) live in soft, muddy conditions, such as shore and wetland. They move fast and freely: the speed reaches greater than 0.4 m/s in trotting [15; 16]. By the support of stomach, the crab can walk even though its legs sink into the mire, which is meaningful for the bionic research of a legged mechanism. The second and third paraeiopods contribute most to the walking performance [17; 18; 19]. Further, the dactylopodites are particularly significant during contact with soil. Surface structure plays a key role in the walking performance of the legged mechanism. Thus, the dactylopodite of the third paraeiopod was chosen as a subject and the surface structure was studied. Based on the bionics coupling theory, three bionic walking feet and a cylinder foot used for comparison on the aluminum alloy were designed and fabricated successfully. The performance of bionic samples and the cylinder foot on three kinds of soil were investigated through comparison experiments, which was evaluated by intrusion force, extraction force; energy consumption and propulsion force. The influence of the depth on propulsion was discussed. Meanwhile, the optimal depth, balancing the sinkage and propulsion, was determined further. The results are helpful to optimize the structure and improve the performance of the walking foot.