AIDA-2020 Advanced European Infrastructures for Detectors at Accelerators. Milestone Report

Transcription

1 AIDA-2020-MS54 AIDA-2020 Advanced European Infrastructures for Detectors at Accelerators Milestone Report Integrated FBG sensors for monitoring the mechanical tension of MPGD films and meshes Benussi, Luigi (INFN) 31 May 2017 The AIDA-2020 Advanced European Infrastructures for Detectors at Accelerators project has received funding from the European Union s Horizon 2020 Research and Innovation programme under Grant Agreement no This work is part of AIDA-2020 Work Package 13: Innovative gas detectors. The electronic version of this AIDA-2020 Publication is available via the AIDA-2020 web site < or on the CERN Document Server at the following URL: < Copyright c CERN for the benefit of the AIDA-2020 Consortium

2 Grant Agreement No: AIDA-2020 Advanced European Infrastructures for Detectors at Accelerators Horizon 2020 Research Infrastructures project AIDA MILESTONE REPORT INTEGRATED FBG SENSORS FOR MONITORING THE MECHANICAL TENSION OF MPGD FILMS AND MILESTONE: MS54 Document identifier: AIDA-2020-MS54 Due date of milestone: End of Month 24 (April 2017) Report release date: 31/05/2017 Work package: Lead beneficiary: Document status: WP13: Innovative gas detectors INFN Final Abstract: The integration of FBG sensors on large GEM foils for the GE1/1 detectors to be installed at CMS in has been performed. The technique was already in an advanced stage one year ago and it was described in the report concerning the Milestone MS12: Optical system for the quality assessment of MPGD foil/mesh mechanical tensioning. This activity is complemented by the measurements of the foil flatness. In fact, these detectors are formed by stack of foils, which are very fragile and should preserve their flatness in the final assembly. To measure the flatness of the GEM foil, the Moiré interferometry technique is used. Moiré setup arrangements, calibration results and future plan are also presented. AIDA-2020 Consortium, 2017 Grant Agreement PUBLIC 1 / 12

3 AIDA-2020 Consortium, 2017 For more information on AIDA-2020, its partners and contributors please see The Advanced European Infrastructures for Detectors at Accelerators (AIDA-2020) project has received funding from the European Union s Horizon 2020 Research and Innovation programme under Grant Agreement no AIDA began in May 2015 and will run for 4 years. Delivery Slip Name Partner Date Authored by L. Benussi INFN 08/05/17 Edited by L. Benussi INFN 08/05/17 Reviewed by L. Benussi [Task coordinator] S. Dalla Torre, I. Laktineh [WP coordinator] P. Giacomelli [Deputy Scientific coordinator] INFN INFN, CNRS INFN 31/05/17 Approved by Scientific Coordinator 31/05/17 Grant Agreement PUBLIC 2 / 12

4 TABLE OF CONTENTS 1. INTRODUCTION INTEGRATION OF FBG SENSORS IN LARGE GEM FOILS: OVERVIEW AND RESULTS MOIRÉ INTERFEROMETRY: PROJECT OVERVIEW MOIRÉ SETUP AND PROCEDURE OF THE MEASUREMENTS LEVEL 1 CALIBRATION OF THE MOIRÉ SETUP AND THE RESULTS OVERVIEW LEVEL 2 CALIBRATION OF THE MOIRÉ SETUP AND THE RESULTS SUMMARY REFERENCES ANNEX: GLOSSARY Grant Agreement PUBLIC 3 / 12

5 Executive summary Results on the integration of FBG sensors on large GEM foils for the GE1/1 detectors to be installed at CMS in and on the flatness measurements of the GEM foils by Moiré interferometry technique are reported. In both approaches the design resolution has been obtained. 1. INTRODUCTION This WP aims to provide an engineered system able to measure the flatness of GEM foils over most of their surface. The usage of GEM detectors demands that both flatness and parallelism of the foils are controlled with a typical precision of about 30 m, in order to ensure uniform and constant detector gain. The technique used must be fast, reliable, simple, adapt to be used in quality controls of a large-scale GEM chamber production. The use of FBG sensors is novel in the field of gaseous tracker. Moiré interferometry is known to provide the required resolution on a small scale, while application on large GEM foils is a novel concept. 2. INTEGRATION OF FBG SENSORS IN LARGE GEM FOILS: OVERVIEW AND RESULTS The FBG sensors act as low-cost precision spatial and temperature sensing tools for space deformations and are commonly used for strain measurements. In this work FBG sensors are used to measure the planarity and mechanical tension of the GEM foils in a CMS chamber. A FBG is a type of distributed Bragg reflector, constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. The sensitivity of FBG in terms of strain, defined as the relative elongation w.r.t. the initial position, is of the order of 0.1 micron. This sensitivity is obtained by creating a periodic variation in the refractive index of the fiber core, which generates a wavelengthspecific dielectric mirror. Therefore, it can be used as a strain measurement tool, since a variation of the FBG translates into a different frequency of the light response. In order to validate the mechanical stretching technique a network of FBG sensors is affixed on the triple-gem stack as shown in Figure 1(Left). Each sensor is glued on the GEM foil using a very thin layer of epoxy glue. The test is performed by modifying the stretching conditions of the GEM foils stack with real time monitoring and recording of the FBG sensors data. The test starts with the chamber assembled with the standard procedure, where the GEM stack is mechanically stretched to the nominal tensile load. After keeping it steady for some time at the initial nominal conditions, the mechanical stretching is released and then the GEM stack is kept in this condition for several hours. Finally, the GEMs are stretched again up to the nominal tensile load. The trends of the FBG sensors are shown in Figure 1(Right). The steep variations of the strain evident in Figure 1(Right) correspond to the actions of un-screwing and screwing the mechanical stretchers during the test. The initial stretch value is assumed as reference condition with strain = 0. When stretchers are un-screwed the strain goes to the lower value, different strain values apply to different foils as they fold quasi-free and assume unequal conditions. After the stretchers are screwed back, the strain value is similar for all foils, showing that they all experience similar stretching, about the original value of the reference condition. Thus, it can be inferred that at the predetermined tensile load all foils reach a similar stretched level although they started from different values. It can be seen from the plot that all the sensors of the network react at the same moment. Grant Agreement PUBLIC 4 / 12

6 Figure 1 (Left) a scheme showing the FBG sensor network on a GEM foil. The same scheme is repeated on the other two foils used to assemble the GEM chamber. (Right) The measured strain versus time for the test procedure is described in the text. The three regions correspond to mechanically stretched, loosened and again stretched triple GEM foil stack, respectively. 3. MOIRÉ INTERFEROMETRY: PROJECT OVERVIEW Moiré interferometry has a wide range of applications in metrology [1]. Thanks to the development of computer based image processing, the Moiré application spectrum became wider. Among the Moiré techniques, the phase shift technique actually originated from the electromagnetic waves [2-6] and it is of particular importance. Moiré interferometry and phase shift techniques are used for measuring the GE1/1 foil flatness by applying the five buckets phase shift algorithm. The Moiré setup is calibrated in the horizontal position by measuring the flatness of a known specimen which is placed in front of the setup. Figure 2 shows the horizontal arrangement of the setup and the specimen dimensions. Grant Agreement PUBLIC 5 / 12

7 Figure 2: (top) The horizontal arrangement of the Moiré setup: projector, receiving lens, recording camera, stepping motor for the phase shift analysis; (bottom) the specimen used for the calibration. 4. MOIRÉ SETUP AND PROCEDURE OF THE MEASUREMENTS The setup is composed of a projector lamp, a camera, a pair of identical Ronchi gratings (120 lines/mm) and a stepping motor. One grating is used in front of the lamp to project the optical lines onto the specimen and the second grating is placed in front of the camera to view the fringes. Once a clear fringes pattern is observed on the camera screen, a picture is taken and the stepping motor is adjusted to create the 90 o phase shift in the fringe pattern. The optical axis of the projection lens is kept parallel to the viewing lens. With such geometrical arrangement, the Moiré fringes contour lines are generated in the x-y plane [7,8]. The camera records the images at every 90 o phase shift step in the fringes. The translation motion of the stepping motor actually varies the corresponding image intensity at given fixed points, e.g. I1(x, y), I2(x, y), I3(x, y), I4(x, y) and I5(x, y). The combined and clear phase is obtained by applying the five bucket algorithm given in equation (1). φ(x, y) = tan 1 2(I 4 I 2 ) I 1 2I 3 +I 5 (1) The detailed results are described in the following sections LEVEL 1 CALIBRATION OF THE MOIRÉ SETUP AND THE RESULTS OVERVIEW For the level 1 calibration the Moiré setup is arranged on the horizontal table as shown in Figure 2, and the specimen is placed vertically in front of the setup. The aim of the level 1 calibration is to check the correct functioning of the Moiré setup and to determine which surface flatness measurement resolution is achieved. For this step, a specimen with a known edges height difference (1.5 mm) is prepared. By arranging the setup as described in the previous section, five consecutive images are collected by translating the projector over 90 o phase shifts. The selected region of interest (specimen region) is shown in Figure 3. Grant Agreement PUBLIC 6 / 12

8 I1write I1write Figure 3. The specimen region (Left) and the Moiré fringe contrast (Right). The image intensity variations are shown in Figure 4; they are obtained from the selected specimen region in the images. I1 33 I2 33 off I off I off 100 I off I off I Figure 4. Intensity variation along a horizontal line, one pixel width, as obtained by translating the projector over the 90o phase shift. The phase shift algorithm of eq. (1) is finally applied to the data presented in Figure 4. The resulting intensity versus position shows a very clear sawtooth-like pattern (Figure 5) in the specimen region G I 250 Figure 5. The specimen profile after applying the phase shift algorithm. Grant Agreement PUBLIC 7 / 12

9 Number of pixel INTEGRATED FBG SENSORS FOR MONITORING THE Equation 2 is used to determine the resolution in the transverse plane: R = Δh (2) N where R is resolution, h is the height difference between two edges of the specimen and N is the number of fringes. The obtained resolution is 250 m. Furthermore, each sawtooth is resolved well enough to make possible the analysis of three subsets of data separately, each portion having a welldefined slope, as shown in Figure 6. Therefore, the resolution improves to about 83 m. A further improvement on the resolution can be obtained by fitting each sawtooth with a linear function (Figure 7). The linear function well describes the experimental data; the residuals distribution (Figure 8) shows a gaussian shape with =23 m: this is the ultimate resolution obtained with the Moiré method G I Figure 6. Zoom of the specimen profile First Peak from left y = x R² = Number of pixels Experimetnal data Figure 7. Example of a linear fit of the first sawtooth. Grant Agreement PUBLIC 8 / 12

10 Figure 8. Residuals distribution of the experimental points with respect to the linear fit function describing the sawtooth wave. The standard deviation represents the transverse resolution R= 23 μm LEVEL 2 CALIBRATION OF THE MOIRÉ SETUP AND THE RESULTS In the level 2 calibration exercise, the Moiré setup is installed vertically. In the vertical arrangements, extra mechanical arrangement makes possible the camera movement left-right, up-down and forwardbackward. These movements of the camera are very important to easily obtain the fringe images. For the vertical arrangements, an extra lens has been introduced in front of the camera, for an easier focusing procedure. The vertical arrangement schematic view is shown in Figure 9. Figure 9. The vertical arrangement of the Moiré setup. This arrangement is the final engineering setup. The GE1/1 detector is assembled in horizontal position on the optically aligned table. Then, as shown in Figure 10, the detector is moved to the vertical position and inserted together with the calibration specimen in the setup of Figure 9. In the vertical arrangements, it is more difficult to obtain the 90 o phase shift precisely, a requirement, which Grant Agreement PUBLIC 9 / 12

11 is compulsory in order to use the phase shift algorithm. Moreover, a light reflection from the GE1/1 surface problem is present; this is overcome by tilting the GE1/1 detector by about 5 o, later compensated in the data analysis. Figure 10. The level 2 calibration Moiré setup The three major aims of the second level calibration are: firstly to validate the vertical arrangement of the Moiré setup, secondly to create fringes on the GE1/1 chamber and thirdly to compare with the level 1 calibration data. These aims have been fulfilled: we succeeded in demonstrating the fringes formation on the GE1/1 surface as shown in Figure 11. The fringes with phase shift created from the vertical Moiré setup are shown in Figures 12 and 13. The entire procedure is summarized in Figure 14. a Figure 11. The fringes formation on the GE1/1 surface; two details are evidenced: (a) specimen for the calibration; (b) the GE1/1 mechanical arrangement to stretch the GEM foils. b Grant Agreement PUBLIC 10 / 12

12 Figure 12. Phase shift demonstration in the level 2 calibration setup Figure 13. (top) Fringes at the specimen in the level 2 calibration setup before applying the algorithm; (bottom) After applying the five-bucket algorithm the number of fringes doubles and the noise is suppressed by enhancing the contrast as was seen in level 1 calibration. Figure 14. Summary of the image processing and phase shift algorithm. 5. SUMMARY The monitoring of large foil deformations by a proper net of FBG sensors has been confirmed by prototyping. The Moiré interferometry technique is used to measure the flatness of the GE1/1 detector foils by optimizing a novel procedure. The setup has been built and calibrated, the algorithm has been checked and is properly working. The obtained sensitivity (23 m) is better than the requirement (30 m). An engineering demonstrator has been built and successfully operated. Further studies are in progress to characterize wide regions of the GEM foils. Grant Agreement PUBLIC 11 / 12

13 6. REFERENCES [1] H. Takasaki, Moiré topography, Appl. Opt. 9(6), (1970). [2] J. Bruning et. al., Digital wavefront measuring interferometer for testing optical surfaces and lenses, Appl. Opt. 13 (11), (1974). [3] M. Kujawinska, Use of phase stepping automatic fringe analysis in moiré interferometry, Appl. Opt. 26 (22), (1987). [4] J. Dirckx and W. Decraerner, Automatic calibration method for phase shift shadow moiré interferometry, Appl. Opt. 29 (10), (1990). [5] R. Gu and R. Zhou, Talbot projection moiré interferometry topography and its automatic fringe processing using phase shifting methods, Proc. SPIE 1230, (1990). [6] T. Yoshizawa and Tomisawa, Shadow moiré topography by means of the phase shifting method, Opt. Eng. 32(7), (1993). [7] M. Idesawa, T. Yalagai, and T. Soma, Scanning moiré method and automatic measurement of 3-D shapes, Appl. Opt. 16(8), (1977). [8] J. F. Cardenas-Garcia, S. Zheng and F.Z. Shen, Implementation and use of the automated projection moiré experimental setup, Opt. Las. Eng. 21, (1994). ANNEX: GLOSSARY Acronym GEM GE1/1 FBG sensor Definition Gaseous Electron Multiplier GEM Endcap Station 1 Ring 1 (CMS experiment) Fiber Bragg grating sensor Grant Agreement PUBLIC 12 / 12