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Abstract - Stability and traction are critical to mobile robots cruising in uneven terrains. A small track-wheel-legged mobile robot was developed. Wheel-legged mode which combined wheeled and legged body with their respective merits of institutions became the most important one; correspondingly, three basic non-isolated control problems such as drive traction control, stability control and robot posture control were produced. In order to have good adaptability, high maneuverability of obstacle negotiation and high locomotion security, coupled optimization function was built and coupled optimization control was realized based on the comprehensive analysis of the stability and traction characteristics with wheel-legged locomotion, which can supply the coupled optimization criteria for stability and traction, and realize the posture optimization control of mobile robots, so that the robot has the abilities of maintaining relatively stability of the body over uneven ground and diminishing its slide, which make sure the robot have stable posture and good traction characteristics. And these abilities are of great significance to stable operation and vision control of the robot. Index Terms - Wheel-Legged robot, Stability, Traction, Coupled optimization. 42491
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I. INTRODUCTION Because of the traction of application background such as dangerous environment operations, space exploration, military operations, self-driving vehicle, the various kinds of mobile robots, important branches of robotics, are emerging. The truth drawn from the existing small ground mobile robot is that using two differential-drive form of a balanced body cannot adapt to the real motion environment. Basically, there are three types of locomotion mechanisms, wheeled, tracked and legged styles, and many researchers have studied these mechanisms [1-4]. Compared to wheeled robots, the track body has a better ability to adapt to the environment [5-7], but while climbs stairs up and down, the size of robot appears too large. In theory, the foot robot provides an option to solve the locomotion problem under the three-dimensional terrain [4], but its kinematic mechanism and control complexity have still not been resolved effectively. The robot is designed to perform semi-autonomous, and teleoperated tasks. It is an integration of sophisticated mechanical, electrical, high and low level electronic components. It belongs to a family of mobile robots which cannot only traverse on plane surfaces, but also has the capability to climb and descent stairs and steps etc., it is designed to perform task in wheel, leg and track locomotion modes of operations in indoor and outdoor environments. Fig. 1 shows the mechanical structure of the robot, which is a 3-type structure, composed of four wheels, four arms and robot body. It includes four independent legs attached with a rectangular frame. Each leg can rotate 360 degrees around the y-axis and tracks rotate around the legs. The overall structure of the robot has been described in detail in reference [13], Each locomotion unit consists of a track leg body and a driving wheel body, four track legs are configured inside the wheel tracks, not only to achieve self-rotation drive track, but also to swing around the central axis of driven wheel. Each leg and track is provided with an independent motor. Rear wheels and the rear tracks are attached with same motor. The front tracks are actuated by the independent motors whereas front wheels are free supporting wheels. Its three-dimensional and physical figures are shown in Fig. 1.
Fig.1 Robot platform Ⅱ. STABILITY ANALYSIS The control of robotic systems under stability margin conditions was mainly addressed in the field of legged locomotion. Research on stability control of walking machines was first considered by McGhee and Frank (1968). A first static stability criterion was developed to evaluate the stability of an ideal machine walking at constant speed on flat, even terrain. It simply considers that the vehicle is stable if the projection of the center of gravity (CoG) lies inside the support polygon. Different mechanical stability margins were defined during past research on walking machines. The control method presented in this paper considers the vehicle motion on irregular terrain without discontinuities. Thus, the tipover stability margin is mainly constrained by terrain geometry. The robot coordinate system (, ,,) R GXYZ = was introduced in the section of legged mode coordinate system shown in Fig.2. Robot stability analysis principle is shown in Fig.3, G—center of gravity(CoG) , ,( 1,2,3,4) iPi = — wheel-ground contact point of robot Fig.2 Simplified model of wheel-legged mode Fig.3 Stability analysis principle The line joining two consecutive terrain-contact points iP , define tipover axises 12 P P − ， 23 P P − ， 34 P P − ，41 P P − . The unit vector (1,2,3,4) iIi = of the axis joining the vehicle CoG to the center of each tipover axis is computed. Then, angle (1,2,3,4) ii θ = between each (1,2,3,4) iIi = and the total external force vector applied to the robot gives the stability angle over the corresponding tipover axis. if iθ is smaller , the robot will be more instable, when iθ is less than zero，the robot will be turnover. The overall vehicle stability margin is defined as the minimum of all stability angles: min{ , 1, 2,3,4} sii ϕθ == （1）