# A Master-Slave Separate Parallel Intelligent Mobile Robot Used for Autonomous Pallet Transportation
## 4 Experiment and Analysis
### Analysis of Lifting and Laying
#### 4.3.2 Analysis of Pallet Offset Error  
During the lifting process, the offset errors of the position of the pallet include longitudinal offset error and transverse offset error. They are both caused by uneven ground and jittering of sub-robot's motion. It is difficult to guarantee the level of the ground, so changing or optimizing the robot motion is the most effective way.  
* Analysis of pallet longitudinal offset errors  
The longitudinal offset errors of the pallet cannot be completely eliminated. It is unable to measure the absolute accurate deviation between pallet position and robot position, so no correction can be made. There must be cumulative error between them. What we should do is to ensure the absolute stop position between lifting and laying as consistent as possible. The two sub robots should be controlled to stop accurately and synchronously while height position of the pallet changes. The control strategy of accurate stop is shown in Figure 11.  
* Analysis of pallet transverse offset errors  
The transverse offset of the pallet refer to errors in the left and right direction. In general, the mobile robot moves underneath of the pallet and then lift the pallet up. But once there are some transverse offset errors, the mobile robot may be stuck under the free entry of the pallet. It may be caused by uneven ground or jitters of the sub robot's motions. In order to reduce the transverse offset errors, dynamic position adjustment is required during lifting and laying. The dynamic adjustment strategy is shown in Figure 12. The rotation angle of each driving unit is divided into 2 areas. (1) Angle adjustment areas. If the angle of rear driving unit is in the range of angle that needs to be adjusted, the rear driving unit will follow the front one, and they will be perpendicular. Within the angle adjustment areas, the center of gravity of the sub robot can always maintain a relatively large area, which will prevent the sub robot to roll over. (2) Position adjustment areas. If the angle of driving unit is in the  
Figure 11: Control strategy of accurate stop.  
Figure 10: The initial relative angle of front and rear driving units before lifting and laying.  
range of position that needs to be adjusted, the speed of the driving motor will be changed according to the position error collected by the camera or navigation sensor. It ensures that the sub robot performs left and right position feedback during the procedure of lifting and laying. According to the position adjustment areas, it is possible to optimize the transverse offset errors between the sub robot and the pallet.  
When the front or rear driving unit lies in the angle adjustment area, we adjust the speeds of the two motors of this driving unit synchronously. The two motors may accelerate or decelerate at the same speeds. But when the front or rear driving unit lies in the position adjustment area, we adjust the speeds of the two motors of this driving unit asynchronously. The two motors may accelerate or decelerate at the different speeds.