Tham khảo tài liệu 'mobile robots book 2011 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 166 Mobile Robots - State of the Art in Land Sea Air and Collaborative Missions 5. Multi-sensor Fusion for 3D SLAM A mobile robot is usually equipped with many sensors which will work together. Fusing the measurements from more than one sensor will provide a more accurate estimation than by using only one sensor s measurement. An example of multi-sensor fusion is shown in Figure 7 where an underwater mobile robot is equipped with a stereo camera and communicated with a set of buoys on the surface. Fig. 7. A sensor fusion example for a stereo camera and a set of buoys. This kind of system structure has two advantages for the underwater robot. The buoys can provide a long range of measurements while the measurements can be applied to estimate the robot position in a global frame. The stereo camera can provide detailed information of the immediate environment which will be used for SLAM in a local frame. Integration of both can solve the SLAM problem in a large area for the underwater robot. The buoy system has been proposed using acoustic sensor by Liu Milios 2005 . . Synchronization for Multi-sensor Fusion All the sensors in a system cannot work at the same speed or frequency such as in Figure 7. Synchronization for multi-sensor fusion is an important issue. Usually measurement frequency for each sensor is different and their measurement time will not coincide. An instance of the estimated robot position from stereo camera and buoys with time stamp is General Concept of 3D SLAM 167 shown in Figure 8. The buoys will provide a long range estimate in a global frame which has less accumulated errors than that with a camera. Therefore when the estimate from buoys at time tb k is obtained the robot position from buoys at tk should be estimated by interpolation as Xb tk Xb tb k_2 - Xb tb k_ 6 Xb tb k 26 where b means buoy and k means time stamp X b fb k-2 X b t robot position estimated by buoys at time tb k-2 tb k-1 and tb k 1 and 3 are coefficients related to the .