To solve the problem that the body of quadruped robot always rotates about the body diagonal line during trotting, a simple and effective attitude control method is proposed through theoretically analyzing the essential cause of the body rolling. Firstly, the kinematic model of the quadruped leg is established to run the numerical analysis, hence the underlying reason for body rolling is obtained. That is, the reaction torques of the forward joints at the hips of standing legs cause the rolling motion about the body diagonal line. Based on the analysis, an attitude control method, which uses the torques of the sideward joints at the hips of the standing legs to resist the body rolling, is proposed, and then the potential lateral movement of the robot is discussed. At the end, dynamical simulations are conducted. The results show that the robot is easy to lose balance without attitude control. However, with the proposed attitude control method, it can achieve stable trotting and keep the roll angle changing within a small range. The dynamical simulations verify that the proposed approach can effectively control the body rolling motion and maintain dynamic balance of the robot.
 McGhee R B. Finite state control of quadruped locomotion[J]. Simulation, 1967, 9(3): 135-140.  Hirose S. A study of design and control of a quadruped walking vehicle[J]. International Journal of Robotics Research, 1984, 3(2): 113-133.  Kalakrishnan M, Buchli J, Pastor P, et al. Learning, planning, and control for quadruped locomotion over challenging terrain[J]. International Journal of Robotics Research, 2011, 30(2): 236-258.  Raibert M, Blankespoor K, Nelson G, et al. BigDog, the rough-terrain quadruped robot[C]//Proceedings of the 17th World Congress, The International Federation of Automatic Control. Kidlington, UK: Elsevier, 2008: 10822-10825. Boston Dynamics. LS3—Legged squad support systems[N/OL]. [2014-08-27]. http://www.bostondynamics.com/robot_ls3.html. Kimura H, Fukuoka Y, Cohen A H. Adaptive dynamic walking of a quadruped robot on natural ground based on biological concepts[J]. International Journal of Robotics Research, 2007, 26(5): 475-490.  Inagaki K, Kobayashi H. Dynamical motion control for quadruped walking with autonomous distributed system[C]// IEEE/RSJ/GI International Conference on Intelligent Robots and Systems. Piscataway, USA: IEEE, 1994: 1004-1010. 何冬青,马培荪,曹曦,等.四足机器人对角小跑起步姿态对稳定步行的影响[J].机器人,2004,26(6):529-532.He D Q, Ma P S, Cao X, et al. Impact of initial stance of quadruped trotting on walking stability[J]. Robot, 2004, 26(6): 529-532. 张秀丽,曾翔宇,郑浩峻.四足机器人高速动态步行中后腿拖地问题研究[J].高技术通讯,2011, 21(4): 404-410.Zhang X L, Zeng X Y, Zheng H J. Resolution of the hind leg dragging problem of a quadrupedal robot in high-speed dynamic walking[J]. Chinese High Technology Letters, 2011, 21(4): 404-410. Raibert M H. Legged robots that balance[M]. Cambridge, USA: MIT Press, 1986.