Abstract: The transition motion from biped standing to ladder climbing was investigated with a humanoid robot under the changed environmental boundary condition. To improve the stability in the process of ladder climbing motion， the robot catches the rung firstly， and then adjusts the location of body with respect to the supporting foot based on the reaction forces between the rung and robot hands. Finally， the balance of robot body was maintained in the proposed transition motion. Experimental results prove that the proposed control strategy can prevent the falling accident， which may be caused by environmental parameter errors， external force disturbances， initial joint angle errors， and so on. It is effective to guarantee the transition motion of robot from biped standing to ladder climbing smoothly and successfully.

Key words: changed environmental boundary, humanoid robot, biped standing, ladder climbing, falling