【正文】 rover is a good choice. However, the Moon is far from Earth, traveling to the Moon costs a lot, we cannot afford maintenance if one or two legs are broken. On the other hand, almost all insects have six legs. According to bionics, sixleg/hexapods robot may be a better choice. There are several benefits for hexapods rover.(a)Hexapod robot is easy to keep balanced. (b)Hexapod technology is a redundant lootion system which increases reliability. It is workable even if one, two or three legs broken. (c)Hexapods makes it possible for the robot to use one, two or three legs to work as hand and perform plex operations. III. The structure of the robot There are two basic architectures of hexapod robots (see ) [6], rectangular and hexagonal. Generally, the hexagonal architecture is axisymmetric. It can have many kind of gaits and can easily change direction. For example, to realize 0, 177。60, 177。120and 177。180 turning with the waving gait, it needs only regroup its legs and/or change the leader leg. The leader leg changes from leg 1 to leg 3 in group ‘1+3+5, 2+4+6’, the direction will change from 0 to 120( see ). In contrast, for the rectangular architecture, a special gait is required for turning action. Generally, it requires four steps for a rectangular robot to realize a turning action(see ).Compared with recetangular structure, a hexagonal chassis with a hemisphere body is better for lunar rover(Fig. 4) IV. Implementation and results Gait analysis and simulation For hexagonal hexapod robot, the wave gaits were studied mostly. However, it can have several different gaits even for straight walking. A. Wave gait Robot with wave gait () is the easiest gait to turn around. But it is very plex to control because every leg has a different gait. For the wave gait, the leg’s structure is as in Fig. 6. There are two revolute joints along axes Y, one along axes Z, its foot, contacting with the ground bees a spherical joint (with three revolute freedoms). During walking, there will be three legs to support the body, and three legs wave ahead (Fig. 5). The whole body’s simple structure is as Fig. 6(b). There are 12 links, 13 revolute joints, two spherical joints in this configuration. The positon is described in a space coordinate frame. The number of degrees of freedom of the robot is puted as follows : F=12*65*123*2=6 In this case, every supporting leg has three freedoms, which makes control very plex.B. Crab gait Another gait for hexagonal robot is ‘crab gait’ or ‘kickup gait’ [8], which is a continuous gait.. Six legs are also grouped into two patterns, 1+3+5 and 2+4+6. There willbe 3 legs for supporting while three legs rise to walk ahead at every time. The track of foot is a parabola ( see ): y=ax^2+b ‘b’: is the maxmimal height that the robot’s feet can raise. While passing small obstacles, ‘b*fh’ is the height of obstacle, ‘2*sqrt(by)*fw’ is the width of obstacle, given that, ‘fh’ and ‘fw’ are factors of obstacle’s height and width, 0‘fh, fw’1. In figure 7(a), legs in solid line are in the supporting phase, legs in dashed line are in the walking phase. From simple structure (see (b)), the number of degrees of freedom of the robot is: F=3*52*6=3. From the above analysis, the crab gait is simpler than the wave gait. However, it also needs special gaits for turning. Turning To realize turning motion, there are two cases. For small angle turning, turning can be realized during walking, the robot does not need to stop. The turning angle must be less than 30 degrees to avoid walking legs colliding with supporting legs. See in . For large angle turning, three steps are needed. There are always four legs standing on the ground to support the body, and the other two legs rise to adjust direction. Fig. 9 and Fig. 10 listed the steps of 60 degrees and 90 degrees turning cases. Quadrangles in the above figures are areas of support。 the white circle is the robots’ mass centre. It can safely turn through 90 degrees using four steps (Fig. 10). Simulation When the robot walks in a straight line, the body should be kept horizontal. Three drivers are needed. The kinematics can be simply denoted by geometric equations. The relationship between joint angles are shown in the following equations: Therefore, the result can be obtained as follows:Figure 11 shows the simulation of the robot walking traight using ADAMS with crab gait.The trajectories of joints are shown in The simulation results for the displacement of the mass centre using MATLAB amp。 ADAMS are shown in . 16. Gaits with sick legs Fault Tolerant Lootion Because of the plex lunar environment, the robot’s legs may be damaged during working. If one or two legs are broken, it still can run with wheels and walk with the other four or five legs with two kinds of gait. Even if three legs are broken, the robot can still walk with a suitable gait [8]. However, if two interphase legs are out of action, the crab gait is impossible. It is still possible for supporting and running, but if three adjoining legs are broken (see Fig. 17) walking is almost impossible. Figure 18 shows how the robot can run with two legs out of action. Gaits with wheels Because wheels can provide higher speed lootion than legs, our robot will run with wheels in the case of a smooth surface on the Moon. All wheels will be grouped into two branches, one on the left, the other on the right. The robot runs like a car. It can realize turning through changing the velocity difference between these two groups of wheels, which had been studied intensively. The ideal velocity for forward motion[9] is, v(t)=(vl(t)+vr(t))/2 (3) vl (t):velocity of left group。 vr (t):velocity of right group. The radius of turning is, p=D*( vl (t)+ vr (t))/(2*( vl (t) vr (t)))