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Fig. 8 Decomposition of wall thickness pro le
By applying a low-pass lter on the thickness pro le, one may isolate the slow cycle. From investigation of possible sources, we have found that the origin of the cycle is from non-uniform heating. A non-uni- form heating will result in zones which are “softer”, slightly “pulling” the mandrel in the direction of the softer material.
By applying a high-pass lter on the thickness pro le, one may isolate the fast cycle. As the fast cycle is almost in-sync with the rotation fre- quency of the processing mill, one may link to origin of the eccentricity to tools of the process. In the present case shown, the fast cycle has been demonstrated to be linked to plug wear.
Finally, by applying an intermediate band-pass lter on the thickness pro le, one my isolate the intermediate cycle. The intermediate cycle is generally mill related, in the sense that it is speci c to a mill or plant process. In the case shown, we have been able to link the intermediate eccentricity cycles to a problem with the hollow.
As shown in Figure 8, the full wall thickness pro le may be decom- posed into three wall thickness pro les, each providing valua- ble information the operator. By tracking individually each cycle, the operator may react on the heating furnace, on the plug or on the hollow depending on which signal has reached a threshold requiring a remedial action.
The previous example is for a case of a manufacturing process where the tube is rotating. In many modern mills, the manufacturing process does not rotate the tube and eccentricity is observed with a more or less a constant feature throughout the length of the tube. The same analysis as with the previous case may be applied by imposing a rotation to the tube when passing under the wall thick- ness gauge. However, this may not
always be feasible due to limited available space on the plant oor.
An alternative is then to rotate the wall thickness probe. The Lut has a con guration with two probes, one on top and one on the bottom, mounted on a swiveling device providing data acquisition over the
ITAtube Journal No2/July 2018
28