A new scheme of obstacle avoidance based on the self-motion of a null space was proposed to control 7-degree-of-freedom(DOF) redundant manipulators. By introducing an arm plane and an obstacle avoidance plane, the representation of the null space motion was parameterized. Based on this formulation, the collisions were detected by the artificial potential field method . With computing virtual repulsion forces, an equation of the null space motion was derived. Then, the inverse dynamics control with an inner position loop was modified to allow the manipulators to show physical meaningful behaviors of a mass-damper system, by which the control of end-effector motion and the obstacle avoidance of manipulator could be achieved at the same time. An experiment on a self-servicing experimental platform was carried out to demonstrate the performance of the proposed scheme. The experimental results show that the closest distance between the manipulator and the obstacle is greater than 40 mm, the dynamic error of the end-effector position is less than 10 mm, and the steady-state error is less than 2 mm. These results indicate that the proposed scheme realizes the obstacle avoidance by proper behaviors of the null space motion, and minimizes the effect on the end-effector during the avoidance.