Tadakuma pided each wheel of the robot into three contin- uous parts connected by two joints. Then  he  added  motors  to each joint and built the transformable wheel-legged robot named Armadillo-Inspired Wheel–Leg Robot [26]. The parts are expanded because of the actuations. And the wheel is transformed into a legged-like structure. Similarly, Chen split the wheel into two halves and converted into a leg. He called  it  TurboQuad  [27]. Note that extra actuations had to be inserted in these two robots, the whole structures are not compact and more energy will be consumed. To reduce the energy consumption, Kim achieved the transformation between wheeled mode and  wheel-legged  mode by a special trigger device without motors. The novel robot  is called Wheel Transformer Robot [28,29]. Different from the radius expansion mentioned above, some  other  researchers  utilize  ax- ial push–pull to achieve transformation between working modes. Shen and Chen split each circle wheel into C-type leg by folding two semicircles along axial direction and invented Quattroped [30,31]. Through folding or refolding the magic balls, Lee designed an origami wheel transformer robot [32]. Instead of changing wheel structures, Chou derived four semicircular legs from double circle wheels by transforming the whole robot body and he named it Claw-Wheel Transformable Robot [33]. Another important factor is the maintenance of working status after the transformation. She proposed a transformable wheel robot [34] to keep the working mode via physical  contacts.

From the above literature review, it is summarized that radial expansion is easy to achieve but the driving and transferring forces are mixed due to the same circumferential rotation. Comparatively, the axial push–pull can easily separate the forces thanks to the different actuation directions. However, it occupies more space in axial direction. In term of the transformation actuation, less motors are used, less energy consumption will be achieved. For the maintenance of working modes, it is found that forces provided by transformation actuation are mainly used. But this is not econom- ical to add extra motors for the transformation. Therefore, a more practical way to keep the working modes is by utilizing special features of transformation mechanism, such as self-locking, dead point or partial physical contacts.

Having outlined the state of current transformable wheel- legged mobile robots, a new transformable wheel-legged mobile robot is proposed in this paper. It is able to perform both wheeled and legged motions. The robot under wheeled mode can achieve smooth, high-speed and flexible locomotion on the flat terrains, and the robot under legged mode can adapt to rough terrains flexibly through the T-shape spokes. With the wheel-legged trans- formable mechanism, the proposed robot can easily change work- ing modes according to surroundings. This paper is organized as follows. Section 2 carries out the mechanical design and the control hardware scheme of the proposed robot. Motion analysis for the transformable process and different working modes are imple- mented in Section 3. Section 4 introduces the strategies of obstacle avoidance under wheeled mode and legged mode respectively, whereas simulation and experiment validation are conducted in Section 5. Conclusions are drawn in Section 6.

2. Design of transformable wheel-legged mobile robot

2.1. Concept design

Fig. 1 presents the mechanical structure of the proposed trans- formable wheel-legged robot. It consists of two wheel-legged transformable mechanisms, a rectangular body and a replaceable empennage. Two working modes of the robot are shown in Fig. 1(a) and Fig. 1(b) respectively. The change of working mode is achieved by the transformable mechanism, thus the robot can roll with wheels or walk with legs. The rectangular body is designed to sup- port motors and control hardware. And three motors are included. Two of them are DC servo motors which make robot moving with a constant speed. The other one is a steering gear providing actuation force to the transformation. The empennage can be redesigned according to the shape of loads.