Measurements of DB supporting systems Fiducialisation

1. Measurements of DB supporting systemsFiducialisation Measurements performed in February and early March 2011

2. Plan Measurements objectives CMM measurements, first session Calculations, fist session Checking of girders Checking of cradles Pin holes locations CMM measurements, second session Calculations, second session Checking of girders and cradles Pin holes locations Conclusions

3. Measurements objectives After measuring cradles and girders separately late January, to determine the shims that are necessary (EDMS n°1112165 and 1112030), the two DB supporting systems have been assembled in the building 169. Early February, these two assemblies were brought in metrology to be measured for the following reasons: • To verify that shimming ensures a correct positioning of the pin holes (articulation points) with respect to the beam axis, • To add targets on the girder 3079 to allow us to develop the Micro-Triangulation method, • And finally, to measure during the same time all fiducials and all sensors plates in the same coordinates system, that is to say, carry out the fiducialisation of the supporting system (All fiducials are available in the EDMS document n°1131579).

4. CMM measurements, first session Measurements were performed with Olivetti CMM within 6 µm MPE (Maximal Permissible Error). The most accurate CMM, Leitz Infinity, was out of order early February. Moreover, its capacity and its technology (measuring table that moves but not the portal above) would have complicated the measurements. • Measurements with Olivetti took a lot of time because of the following reasons: • The CMM is a little too short, that requires to use and calibrate a lot of different probes, • The fiducials positioning is not systematic, that requires to perform the measurements once manually before automatically.

5. Calculations, first session Checking of girders The goal is to check that results obtained are consistent with girders measurements performed in January. For this, the cloud of 10 points (B3069-1 to B3069-10 for the girder 3069) measured in January is put into the coordinate system of points measured in February using a 3D similitude. Thanks to this fast check for each girder, we have controlled that all points are in the tolerances and we detected that a point of girder 3069 was not measured again.

6. Calculations, first session • Checking of cradles • Like for girders, the goal is to check that results obtained are consistent with cradles measurements of January. • The two clouds of 13 points for each cradles (4 prism holders and 3 sensor plates composed by 3 balls for each) are expressed in the same coordinate system thanks to a 3D similitude. • From the beginning of the calculations for cradles 2 and 4 (assembled with girder 3069), we found offsets about 20 to 40 µm, there were 3 possible explanations: • Problem of Ferranti CMM (MPE of ± 3 µm) used for cradles measurements, • Problem of Olivetti CMM (MPE of ± 6 µm) used for girders measurements and supporting systems measurements, • Deformation of cradles between the two sessions of measurements. • To confirm one of these explanations, we processed data from the second girder and we found the same problems. • We can see in the next slide an example of the "deformations" seen on the cradle 3.

8. Calculations, first session Checking of cradles As we can see in the graph, the “deformations” are on the same direction for each type of CMM probe. On the other hand, Nicolas Chritin has made simulations of deformation and he has never been able to obtain such deformation forms. Finally Dominique Pugnat, from metrology, has made different tests on probes and he identified calibration failures. By a post-treatment of data using the errors found by Dominique we significantly improved results. Unfortunately, to have identified the problem will not allow us to remove it: The configuration of DB supporting system can’t be measured by another mean now, to improve measurements we would need larger CMM .

9. Calculations, first session • Pin holes locations • The analysis of the location of pin holes with respect to their theoretical location allowed us to identify problems concerning the assembly of the cradles on the girders. The faults, for 3 cradles on 4, are several tenths of millimeter. • To remove faults on each of these 3 cradles, we had followed this method: • Calculation of theoretical locations of all points in the beam coordinate system, • Laser tracker measurements of prisms and pin holes, • Disassembly and reassembly of cradle, • Check with laser tracker measurements. • When necessary, we repeated the 3 last points until to obtain offsets better than the laser tracker accuracy, about 40 µm in that case. All results are available in EDMS n°1134701. • The explanation of these faults was determinate during this work: the plates glued on girder, which allow to fix cradles to girder, are misaligned, so the screwing generates stress and bad positioning.

10. CMM measurements, second session Measurements were performed from the 10th to the 16th of March with Olivetti CMM. The Leitz CMM was efficient but it was necessary to redo all the measurements program without being sure of getting good results (because of movements of the measurement table and the CMM’s capacity) and thus lose much time. Results are available in the EDMS reports n°1132046.

11. Calculations, second session Checking of girders and cradles The measurements method is the same, therefore we again obtained offsets between measurements performed in January and March, but not exactly with the same distribution. To summarize, on the cradles 1 and 2 the offsets average is about 20 µm whereas on the girders and on the cradles 3 and 4 the offsets average is about 10 µm. Assuming that the measures of January are better quality, we have expressed these ones in a common coordinate system that constitute the final results. Micro-Triangulation targets, glued on the girder 3079 and the cradles 1 and 3, were not measured during the session of January, therefore the results could not be crossed-checked : the accuracy on these points is about ±6 µm MPE on the girder but on the cradles we expect an accuracy from ±10 to 15 µm MPE. We will have to be careful during our future measurements.

12. Calculations, second session Pin holes locations The new assemblies are successful: the pin holes are better than 20 µm with respect to their theoretical locations for 3 cradles and better than 10 µm on the cradle 1. These values ​​include measurement errors, we could not expect better results using shimming method. The pin holes locations defined the articulation point. The articulation point location between the two girders depends on the cradles 2 and 3: we can see that the shimming offset will be partly compensate vertically but not laterally.

13. Conclusions • The usefulness of checking: • The systematic check for each step of the measurement results allowed us to detect CMM probe problem, to detect a problem in the assembly of DB supporting systems and then to properly resume the assemblies of cradles on girders. • What we will have to improve: • To check the locations of plates which allow to fix cradles to girder, • To follow the assembly of each cradle on girder by Laser Tracker measurements, • To improve CMM measurements by a new protocol for Epucret girders and by the use of a larger CMM in the future. • To conclude: • The fiducialisation of the DB supporting system is finally over, there will be no additional dismantling to be done. The fiducialisation accuracy is generally within CMM tolerances, ± 6 µm MPE, by integrating the January results more reliable in an overall coordinate system. However, we will have to be careful about Micro-Triangulation targets fixed on cradles. • All results and analysis are available on EDMS n°1134701.