The wheeled mode can also be seen in Fig. 2(a). Rubber bars are evenly embedded in the rim of the grooved wheel to increase friction between wheels and terrain. Legged mode is as shown in Fig. 2(b). The legs are expanded by three T-shape spokes. These spokes are folded by the transformable mechanism when wheeled mode is on. Fig. 2(c) shows the detail of the T-shape spoke. Rmax denotes the maximum length of the spoke, r is the radius of the wheel and α is the extreme arc of each spoke.

Based on the traditional offset slider-crank mechanism, the wheel-legged transformable mechanism is designed as shown in Fig. 3. The wheel-legged transformable mechanism consists of seven components: a grooved wheel, a spindle, a shifting bearing, a circular framework, a gear-and-rack part, three connecting rods

and three T-shape spokes. The grooved wheel is controlled by the spindle. Through receiving power from the servo motor, the wheeled mode is switched on. When changing wheeled mode to legged mode, steering gear in the rectangular body would provide power to the gear-and-rack part. And the input rotation is turned into translation. Shifting bearing realizes axial movement by re- ceiving the translation through circular framework. Thus, the con- necting rods can make the T-shape spoke expand and the legged mode is realized. The change of legged mode to wheeled mode is achieved by reversing the rotations of steering gear. This simple and compact structure of transformable mechanism enable    the Bluetooth module, an Arduino UNO microcontroller and a motor control module. The upper android app is for operators to give commands. And the commands are transferred to the Bluetooth module through the USART ports. The Arduino UNO microcon- troller deals with the events and the commands. It communicates with the motor control module through the I2C ports. Then the motor control module can simultaneously drive two servo motors through the PWM method. The Arduino UNO microcontroller can also transmit the commands to the steering gear through the PWM method directly. Due to the modularized design of the control panel, each motor functions independently without affecting other controlling circuits.

3. Motion analysis

3.1. Transformable process

Kinematics and statics analysis for the transformable process are discussed in this part. Fig. 5 shows the dimensions and force analysis of the transformable process. The steering gear provides power to the transformation. The power converts into actuation force Fin via a gear-and-rack part. Reaction force Fn and friction force Ff are generated at the point where the T-shape spoke is touching the terrain. Some dimensional parameters are defined to analyze the forces. r0 is the base diameter of the spindle header in wheel-legged transformable mechanism, e is the distance between torus of the circular framework and torus of the spindle header, a is length of the connecting rod, b is length of the rear part of the T-shape spoke and c is the length of the front part. When the steering gear is actuated, the expansion height q of the T-shape spoke, the angle β of expansion, and the axial displacement of the shifting bearing s change accordingly. Rmin is each spoke min effective length during transformable process.

For the transformation from wheeled mode to legged mode, there are two special occasions as shown in Fig. 6. The first one is single spoke touching terrain and the second one is double spokes touching terrain.

摘要：本文提出了一种新型的可变形轮足式移动机器人，可用于平坦地形和崎岖地形。它通过轮-腿转换装置，将轮式机器人的稳定性、可操作性和腿式机器人的越障能力结合起来。通过让轮子的两个辐条接触地面，机器人可以在两种模式轻易地实现切换。本文在简要介绍机器人的概念和控制系统设计的基础上，对机器人在轮式模式、腿式模式和转换模式下分别进行了运动分析，然后得到了机器人在轮式模式和腿式模式下的障碍物攀登策略。最后，在仿真分析的基础上，设计并制造了该机器人的原型。实验结果验证了所提出的可转换轮式移动机器人的可行性。