A motion of a robot manipulator under environment for human and robot to coexist should be able to quickly sense external force and react for minimizing a damage due to physical contacts. In the absence of sensing external force, relative motions between robot and human are not predictable and unexpected collisions may occur at some position during motion of the robot arm. This paper proposes a real-time collision detection method and a compliance control based on detecting abnormal current value to minimize an impact force at the moment of collision between service robot arm and unknown obstacle. In the introduced method, the extra sensors such as the Force/Toque sensor or the visual sensor to detect the collision are not necessary. Instead of these sensors, the collision detection and the safety motion are achieved by a simple method based on the current characteristics in according to operating of each joint motor of robot manipulator. In order to evaluate the performance for minimizing an impact force, the proposed method is applied to the developed light weight robot arm for a service robot.