# High-Precision Roller Supported by Active Magnetic Bearings
## 5 Automatic Alignment Function Test of Precision Roller with Magnetic Bearings
### Improved Automatic Alignment Test of Roller  
The error in the previous method is due to the difference between the position of the magnetic bearing shaft core, which performs position control, and the location of the magnetic bearings which perform movements of the smaller. From an alignment perspective, reducing the difference in the contact force between the left and right ends of the roller is more important than the reducing the error between the contact forces and the reference force. To reduce this difference, the automatic alignment  
Figure 14: Test results under the reference force of 5 N and movement of 10 \(\upmu\)m.  
Figure 13: Test results under the reference force of 5 N and movement of 5 \(\upmu\)m.  
procedure was modified, as shown in Figure 15. First, both bearings move at a given speed (coarse step). When either end of the roller reaches the reference force first, the roller end that reached the reference force moves in the opposite direction of the printing surface at a reduced speed (fine step) to compensate for the lever effect and prevent further increase of the contact force. Movement of the roller stops when the contact forces at both ends of the roller are higher than the reference force. An estimate for the ratio of coarse step to fine step can be obtained using the principle of lever using the distance between the bearings and the distance from the bearing to the load cell. Through this method, the time spent for the contact force to reach the reference force can be reduced, the error with the reference force can be reduced, and the difference in the contact force between the left and right ends can be reduced.  
Figure 16 shows the results of the automatic alignment test conducted based on the improved automatic alignment procedure for a reference force of 5 N. Here, step movement of coarse mode and fine mode was selected as 10 \(\upmu\)m and 2 \(\upmu\)m, respectively. In the previous section, the roller was evaluated to be capable of step movement of 1-\(\upmu\)m resolution, thus it can perform fine mode of 2-\(\upmu\)m step movement. 1 The right load cell started to detect force from 22 \(\upmu\)m based on the origin, and 2 started moving in the opposite direction in steps of 2 \(\upmu\)m. 3 The left load cell started to detect force from 60 \(\upmu\)m, and 4 alignment was completed once the left core moved by 130 \(\upmu\)m. This kept the contact force of the right-end load cell nearly constant throughout the movement of the other bearing. After approximately 9 s, the alignment was completed as the measured forces on both ends of the roller exceeded the reference force. After alignment completion, the measured forces on both ends of the roller were 5.96 N and 6.12 N. The difference was 0.16 N, indicating significantly improved alignment compared to the previous automatic alignment method. Table 3 shows the results of five automatic alignment tests for the reference forces of 5 N and 10 N under random misalignment conditions. In all cases, the difference in the measured force between each end of the roller after alignment was less than 0.3 N, confirming the reliable automatic alignment function of the improved algorithm.  
Figure 15: Flow chart for improved automatic alignment.