Abstract: For collaborative robots, a real-time method is proposed estimate the single contact information of the whole robotic arm according to the information of control system and motion state based on the momentum observation and optimization algorithm. The proposed method transforms the estimation problem of contact position into bounded searching problem of the optimization algorithm. The searching range is determined by the length of contact robotic arm, which is obtained through momentum observation. The combination of the above two methods can guarantee the precision and speed of estimation. The results of simulation and experiment based on UR robotic arm test platform show that the proposed method can accurately estimate single contact information in real time which occurs for the first time at anywhere of the robotic arm except the first joint. Based on this research, the robotic arm can perceive external forces without sensors.