Motion Planning of Emergency Stop for Humanoid Robot by State Space Approach Mitsuharu Morisawa, Kenji Kaneko, Fumio Kanehiro, Shuuji Kajita, Kiyoshi Fujiwara, Kensuke Harada, Hirohisa Hirukawa National Institute of Advanced Industrial Science and Technology(AIST) 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan Email:{m.morisawa, k.kaneko, f-kanehiro, s.kajita, k-fujiwara, kensuke.harada, hiro.hirukawa }@aist.go.jp Abstract— A motion planner of emergency stop must make an operating humanoid robot to a stationary state under the emergency signal. It plays an important role in prevention of falling over because humanoid robots fall over easily. Immedi- ately after the emergency signal, it must generate the emergency stop motion in real-time. We modeled a humanoid robot as a simple dynamic system consists of ZMP (Zero-Moment Point) and COG (Center of Gravity) as its states. The emergency stop motion is generated by a state feedback. We determined optimal feedback gains in terms of the initial conditions and the pole assignment. The proposed method realized a reliable emergency stop with low computational cost. Furthermore, it can easily predict the possibility of the successful emergency stop at any time. The validity of the proposed method is confirmed by an experiment using humanoid robot HRP-2. I. I NTRODUCTION Humanoid robots are expected to deal with the complicated tasks in not only the structured environment like industrial manipulator, but also an unstructured environment like human. Especially, since humanoid robots have a high affinity with human, they will assist human in our future. The humanoid robots that have the same size as a human have actively researched and developed [1]-[5]. ASIMO that HONDA R&D Co., Ltd. developed especially achieved a usual walking of 2.7[km/h] and running of 6[km/h], and has physical ability enough for the light labor in such office. However, the biped humanoid robots generally have a fatal weak point. It is easy to fall over because the stability margin is small according to a restriction of support polygon and a high COG position. Therefore, it is necessary to improve the fail-safe technology such as a prevention of falling over before happens like in Fig.1, a control of falling over in case of unavoidableness[6], and a standing by themselves in any state[7] in addition to the walking mobility for working in a living environment. If a human-size humanoid robot falls over, the robot will have suffer physical damage. As preliminary step toward practical application, the hu- manoid robots has tried to digitalize the traditional arts or technique such as fork dance and utilize in the entertainment field. Nakaoka[8] et al. achieved Japanese traditional dance with a dynamic whole body motion using HRP-2. Our group developed the biped dinosaur robots and demonstrated in EXPO 2005 Aichi Japan held for 185 days[9]. For long term Fig. 1. Emergency stop. demonstration, the motion suspension system was introduced in order to ensure safety of spectator and prevent a falling over[10], [11]. This paper proposes the motion planning of emergency stop in motion suspension system. This algorithm forces operating humanoid robot to stop without falling over by emergency stop signal from human operator, or detector of abnormal signals of sensors. The rest of this paper is organized as follows. The related works about real-time gait planning is overviewed in Section II. The precondition of stop motion is shown in Section III. In Section IV, the details of the motion generation of emergency stop will be explained. How to return home position after emergency stop will be shown in Section V. Experimental results are shown in Section.VI. Finally, we conclude in Section VII. II. RELATED WORKS Firstly, let us consider the difference between the traditional real-time gait planning and the motion planning of emergency stop. The traditional real-time biped gait can be generated from the preplanned ZMP[12] according to the future footprint. However, the motion planning of emergency stop can not be preplanned because a trigger time from emergency stop signal is unpredictable. At the stop motion, the trajectories of ZMP and COG must be connected to original trajectories continu- ously and terminal velocity of COG must be 0 explicitly. Therefore, a motion planning of emergency stop is for- mulated as an initial and a terminal value mixed problem. 1-4244-0259-X/06/$20.00 ©2006 IEEE 2986 Proceedings of the 2006 IEEE/RSJ International Conference on Intelligent Robots and Systems October 9 - 15, 2006, Beijing, China