High Precision Optical Instrumentation for Large Structures Position Monitoring: The BCAM System Applied to the CMS Magnet

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1 High Precision Optical Instrumentation for Large Structures Position Monitoring: The BCAM System Applied to the CMS Magnet James Bensinger, Kevan Hashemi, Brandeis University P.O. Box Waltham, MA USA Christoph Amelung, Adriano Garonna, Raphaël Goudard, Friedrich Lackner, Christian Lasseur, CERN, CH-1211 Geneva-23 Switzerland The BCAM (Brandeis CCD Angle Monitor) is a simple optical instrument designed to monitor the geometry of large structures. It consists of one or more electronic cameras and one or more pairs of light sources, all integrated into a single enclosure mounted kinematically onto three steel balls. The angular resolution is better than 5 microrad in transverse directions. Several items have been installed in the Atlas muon spectrometer and the system is used for controlling the closures and the openings of the Atlas detectors and of the 5 magnet yoke wheels of CMS. The paper will present a description of the system, its performances; the application to the CMS magnet will be detailed. 1. INTRODUCTION The BCAM [1] is an optical instrument designed to monitor the geometry of large structures. A single-ended BCAM contains a camera and two light sources. The camera consists of a lens and an image sensor. The light sources are red laser diodes placed on either side of the camera lens. The laser diodes have no collimating lenses, and so act as bright points of light. A double-ended BCAM contains two cameras looking in opposite directions, with two pairs of light sources. Figure 1: Double-Ended BCAM. The enclosure is 91 mm x 53 mm x 41 mm, is made of anodized aluminum, and weights roughly 300 g. It contains two cameras and four red laser diodes. The lid is shown transparent so you can see inside. 1

2 Each BCAM uses its camera to monitor lasers on all other BCAMs in its field of view. The camera measures the position of each laser image on its image sensor. We assume that each laser lies somewhere along the line passing through its image and the camera lens. The camera is sensitive to movement across its field of view, but not to movement towards or away from the camera. The camera measures the bearing of a laser, but not its range. The name BCAM stands for Brandeis CCD Angle Monitor, in which angle refers to the bearing measurement and CCD refers to the image sensor. A BCAM camera has relative accuracy 5 µrad within its field of view, and absolute accuracy 50 µrad with respect to its mounting plate. Its field of view is an angular cone 30 mrad x 40 mrad. At 1 m, its relative accuracy is 5 µm, its absolute accuracy is 50 µm, and its field of view is 30 mm x 40 mm. At 10 m, its relative accuracy is 50 µm, absolute accuracy is 500 µm, and its field of view is 300 mm x 400 mm. Figure 2: A Camera Image. The lasers are 1.3 m distant from the camera, and 16 mm apart. The red and blue rectangles are artifacts of our analysis software. The exact appearance of the lasers in the camera images depends upon the range of the lasers, but for ranges greater than 1 m, they look like points of light as seen in this example. To monitor the deformation of a large structure, we distribute BCAMs and mounting plates throughout the structure. Each mounting plate holds two or more BCAMs. Each BCAM mounts kinematically on three steel balls, and is held down by a single screw. The BCAMs on a plate do not look at one another, but instead look at BCAMs on other plates. The balls upon which a BCAM sits define its mount coordinate system. Each BCAM camera comes with seven calibration constants: the position of its optical pivot point in mount coordinates, the bearing of its optical axis in mount coordinates, the separation of the image sensor and pivot point, and the rotation of the sensor about the optical axis. These calibration constants allow us to transform two-dimensional spot positions on the camera's image sensor into three-dimensional bearing lines in mount coordinates. Before installation, we measure each mounting plate with a CMM (computer measuring machine), and so obtain the locations of the BCAM mounting balls in the plate coordinate system. These locations allow us to transform bearing lines from mount coordinates into plate coordinates. Each BCAM laser comes with its own three calibration constants. These tell us the position of the laser's optical center in mount coordinates. The laser calibration allows us to determine the plate coordinates of each laser. Each plate is now the source of many bearing lines. Each line constrains the location of a laser on another plate. Meanwhile, each plate is also the destination of many bearing lines, each of which constrains the location of one of its lasers. The combination of these constraints in our system of many plates allows us to determine the relative positions of the plates, and so deduce our global geometry. We make sure our BCAM system provides us with many more measurements than are necessary to determine the global geometry. Because the system is over-constrained, and at the same time imperfect, there is no global geometry 2

3 for which each bearing line passes exactly through its corresponding laser. Instead, we choose a global geometry that minimizes the root mean square distance between the bearing lines and their lasers. By comparing this error measure to the distance between individual bearing lines and their lasers, we can judge the performance of each camera and laser in the system. Because our system is over-constrained, we can eliminate poorly performing cameras and lasers from our measurements, and so obtain a more accurate estimate of the global geometry without them. Although the BCAM is conceptually simple, the task of bringing together calibration constants, image analysis results, and plate measurements to create a network of bearing lines and laser points, and then arrive at the global geometry by minimizing a global error measure, is difficult in practice. We use ARAMyS to determine our global geometry, and LWDAQ [2] to acquire and analyze our BCAM images. We describe the application of ARAMyS and LWDAQ to a large system of BCAMs in Reference Bars for the Alignment of the ATLAS Muon Spectrometer [3]. The paper describes a test stand consisting of thirty-six BCAMs arranged throughout a 10 m x 20 m structure. Out of the thirty-six BCAM, we rejected measurements from two cameras, and were left with absolute alignment accuracy of around 200 µm throughout the structure, and resolution of around 20 µm. 2. ARAMYS The program ARAMyS [4] has been developed to reconstruct from a set of alignment sensor measurements the geometry of the setup, i.e. the relative positions and rotations of all the elements. The program can also be used, in the design phase of a detector, to simulate the expected alignment performance of a network of sensors with given resolutions. ARAMyS is a general-purpose alignment reconstruction program, and treats in a unified way information from sources as different as e.g. CCD cameras, temperature sensors, and optical surveys. The reconstruction is performed by comparing measured values from the sensors to expected values for an assumed set of local coordinate system positions and rotations, and by iteratively minimizing the difference in the usual way by variations of this set of numbers using MINUIT [5]. The reconstructed geometry, including the alignment sensors, can be displayed in a dedicated viewer application based on OpenGL. Figure 3: The complete alignment system of one endcap of the ATLAS muon spectrometer as implemented in ARAMyS. 3

4 Figure 4: CMS diameter: m, length: m, weight: tons. 3. THE BCAM SYSTEM IN CMS EXPERIMENT The CMS [6] experiment (Compact Solenoid Muon) on which the BCAM system has been installed is made up of five rings coupled YBs (Yoke Barrel) closed at each end by three discs YEs (Yoke End cap). The whole 15 m in diameter and 21 m length once closed, must be aligned with a precision of the order of 1 mm with respect to the CMS and LHC [7] geometrical reference axis. The five YB rings (15 m in diameter and 2.5 m in thickness) and the six YE discs (15 m in diameter and 0.6 m in thickness) have been regularly measured by photogrammetry during their construction in factory and after remounting at CERN. More than 150 targets per face were measured, their co-ordinates are known within an average RMS (one sigma) of 0.3 mm. These various networks of precise points are used as reference for the positioning of new sub-detectors or BCAM supports or new targets as those which will be aimed by the BCAM. The purpose of this work was to find an easy tool for efficient, economic and accurate enough alignment in such a way that all the YB rings and YE discs can be positioned on line with respect to the central ring YB0. As it will be detailed in this part, the BCAM performances correspond to the described requirements perfectly Description of the System in CMS Eight corridors of 10 cm by 20 cm, four at the bottom periphery and four at the top periphery of the CMS experiment have been reserved for theodolite measurements. All have been materialized with yellow plates in order to avoid the installation of cables through it. Two bottom corridors and two top corridors have been chosen for the closure monitoring system. 4

5 Figuire 5: Bottom corridors alongside CMS Figure 6: Top corridors alongside CMS The following material was needed: eight mounting plates with kinematic mounting balls for the BCAMs and survey holes for photogrammetry targets, eight BCAMs, four retro reflectors (corner cube prisms), one data acquisition electronics, data acquisition cables, LWDAQ, ARAMyS softwares and BCAM calibration parameters (free download). Figure 7: Double ended BCAM ready for acquisition Figure 8: Corner cube prism installed inside a corridor The eight mounting plates are fixed on each side of the stiffest structure of the central ring YB0. Four double ended BCAM installed on one side of YB0 aim at the targets in both directions through the corridors but they also aim at the four resting temporary BCAMs fixed on the other side of YB0 for a calibration process. The position of the four corner cube prisms in the object coordinate system was determined by theodolite measurements within an average RMS (1 sigma) of 0.5 mm. The prisms are installed in the centre of the corridors on each YB and YE to be aligned. The targets are assembled at CERN. Made up of four pieces: a cornercube in glass BK7, a 26mm diameter spherical part, a support with a 8mm g6 pin and a ring to hold the sphere on the support. For the calculation with ARAMyS the prism is considered like a BCAM. This integration has been done carrefully cross-checking the results with measurements in laboratoy since the cornercubes reflect entering light back 180, and parallel to the original beam, while both inverting and reversing the image regardless of its orientation to the beam. Different tests in laboratory with a prism in a distance of 800 mm from a BCAM showed a measurement accuracy of 5 μm in the transverse direction and 250 μm in the longitudinal direction. 5

6 Figure 9: Prisms are glued manually in their spherical support 3.2. Installation The installation of this system was carried out when CMS was fully opened Measurement of the mounting plates on CMM to get the relative position of the kinematic mounting balls and the survey holes Wiring: about 50 m of cross-cables was needed since all of them arrive at the same place near the central ring where the operator performs the monitoring Fixation and prepositionning of the mounting plates Alignment of the BCAMs so that they see one another and all locations of the retro reflectors Fixation and prepositionning of the targets supports (8mm H7 machined hole) Determination of the eight YB0 mounting plate s balls position measuring targets inserted in the survey holes by photogrammetry and using CMM information. Determination of the YB and YE target s position by theodolite Figure 10: Mounting plates with kinematic balls and three survey holes One encountered problem during the installation is related to the field of vision of the BCAM. This system was tested once CMS was closed and aligned by traditional methods with an average RMS (1 sigma) of 1 mm. Although the field of view of a BCAM is 40 mrad x 30 mrad the operation of calibration between BCAM of the central wheel did not succeed the first time since the devices only 2 m distant could not observe reciprocally. An operation of realignment 6

7 using BCAM laser pointers allowed correcting this defect. The BCAM supporting plate s position of each state was determined by photogrammetry within an average RMS (1 sigma) of 0.05 mm. Finally this operation was very useful since it increased the dynamic range of the system by the optimization of the fields of vision of the four BCAM Measurements Process First we calibrate the orientation of the BCAM mounts. We already know the mounting plate s ball position in the YB0 system by digital photogrammetry (DP) within an average RMS (1 sigma) of 0.3mm. This mount orientation calibration process consists in reciprocal measurements of BCAMs (located on each side of YB0) known in the main reference system in order to define with a better accuracy the orientation of the BCAM in the object reference system using ARAMyS and internal calibration parameters of the BCAMS. The result of this process shows an important improvement on the angular uncertainty from 300 µrad before up to 100 µrad after the calibration operation. The closure process consists in taking images with BCAM on the prisms fixed on the ring to be aligned. This acquisition is done in a precise order and takes less than two minutes. Using the parameters from the calibration ARAMyS predicts the prisms location of each element and calculates its center using the survey data in a few seconds. Comparing the results with nominal values the people in charge of the adjustment get the corrections quasi on line and move the elements with hydraulic jacks and shimming. An interface is currently being developed in order to perform this data treatment. This will enable a technician to automatically : acquire data from BCAMs and prisms on a moving barrel (on site or remotely), format the received information, analyze it using the Aramys software and display the live position of the barrel to determine any required adjustements. This User Interface is being written in PVSS, a common framework for user interfaces at CERN. Our BCAM system will thus be fully integrated in all CMS monitoring activities. Figure 11: BCAM on the central ring aiming at a target 3.4. Results One milestone was to define whether the BCAM system will be able or not to replace the traditional methods of alignment which were implemented for the first closure of CMS. Using the results from photogrammetry measurements of the mounting plates and the results from theodolite measurements of the ring s relative alignment one can compare with the results obtained with ARAMyS Photogrammetry vs. BCAM to BCAM The coordinates of the kinematic balls of each mounting plate have been determined using CMM information and result from digital photogrammetry (DP), relative position within an average RMS (1 sigma) of 0.05mm. Since the position of the balls has been also calculated by ARAMyS using the results from the BCAM to BCAM measurements 7

8 Table 1 : Direct coordinates comparison, BCAM vs. DP SPHERE DX (mm) DY (mm) DZ (mm) [BCAM - DP] [BCAM - DP] [BCAM - DP] MIN (mm) MAX (mm) STDEV (mm) for the calibration process one compare both sets of coordinates. Table 1 shows that we obtain an excellent correspondence; the differences are within the range of the DP accuracy Theodolite vs. BCAM to prism The relative alignment of the YB rings has been determined measuring the prisms by theodolite with an average RMS (1 sigma) of 1 mm. We calculated the position of the different element s center using those data. The same positions have also been reconstructed by ARAMyS using the calculated coordinates of the prisms from the BCAM to prism measurements. The comparison between both sets of coordinates shows a good correspondence. The global RMS 0.7mm is in agreement with the required RMS (1 sigma) for the system which is 1 mm Discussion and prospects Table 2 : Comparison Survey vs. BCAM to prism. CENTERS DX (mm) DY (mm) [BCAM - Survey] [BCAM - Survey] YB YB YB YB YB MIN (mm) MAX (mm) Stdev (mm) This first comparison exercise constitutes an excellent cross-checking with the BCAM to BCAM measurements since it shows how accurate this configuration is (average RMS of 0.05 mm). The reasons for which it was decided to monitor the YB rings and YE discs with prisms are: avoiding any additional wiring, facilitating installation and allowing also direct measurement with theodolite or digital photogrammetry. 8

9 It is the very first time that prisms are measured by BCAMs in real industrial working conditions to reconstruct online the center of a large scale objects such as CMS. The reasons for which the results with prisms are not as precise as the BCAM to BCAM configuration are the following: The set of reference coordinates from the survey is only 1 mm accurate (1 sigma), this error dominates the system's performance Some defects on the prisms have been discovered influencing the final accuracy of the system. The prisms are not perfectly centred on the main rotation axis of their support causing some microns (about 8µm) differences on the BCAM readings implying some tenth of mm uncertainty on the 3D reconstruction of the targets, about 0.5mm for 5m distant point There is also an influence from the precision of the parameters obtained by the mount orientation calibration process, about 0.5mm for 5m distant point It is the very first time that prisms are measured by BCAMs in real industrial working conditions to reconstruct online the center of a large scale objects such as CMS The redundancy is poor since only one BCAM is observed in the prism Measuring a prism is like measuring a BCAM twice distant and one drawback of the BCAM is that its precision is getting worse and worse with distance. Finally the error budget for the 3D reconstruction of a point 5m distant can be summarized like that: σtotal = (σprisms² + σsurvey² + σcalibration²) = 1.2 mm Although it was the very first time that prisms and BCAMs are used in these industrial working conditions and in spite of the error budget described above, the alignment of the 15 m diameter YB rings has been controlled and presents at final a precision comparable with that obtained by theodolite. Prisms are convenient and simple; they open nice prospects for other measurement or alignment configuration. Contrary to the BCAM to BCAM system which is purely linear, prisms can be aimed by several BCAMs quasi at the same moment, increasing the accuracy of the target s 3D reconstruction. BCAMs appear to behave as advertised in an industrial environment and fulfil the precision requirements perfectly. However this work has enabled us to imagine means of improving certain points such as the centring of the prisms on their support, the accuracy of the survey to determine better the position of the targets, the marking of the orientation of the prisms in order to reproduce it for each new measurement session. 4. THE SPACEFRAME MONITORING SYSTEM (SMS) In the ALICE [8] (A Large Ion Collider Experiment at CERN) experiment the sub detectors are held in place by a structure called Spaceframe. It has a cylindrical 18 - sided geometry with a length and diameter of 8 m. The weight of all the sub detectors, about 80 tons on the whole, will deform the Spaceframe in radial direction: This weight was simulated with water filled PVC tubes. For obvious reasons these deformations have to be monitored. 9

10 9th International Workshop on Accelerator Alignment, September 26-29, 2006 Figure 12: ALICE Spaceframe diameter: 8.5 m, length: 7 m, weight: 10.5 tons. The 18 corners of the Spaceframe show relative movements within a few millimeters for the entire detector load. The monitoring system has to determine all the positions of the corners to an accuracy of better than 500 μm. The BCAM system is implemented to the Spaceframe by fixing a mounting plate (see 0) with two BCAMs on each corner. By measuring the relative angles of all BCAMs the 18 internal angles of the space frame are monitored. Figure 13: BCAM baseplate In summer 2005 a load test [9] was performed to verify the expected deformations. Different load conditions were measured by the SMS and with external survey done by the CERN survey group [10]. Both results were compared and showed that the SMS could reconstruct the positions of the 36 vertices on both sides of the Spaceframe for every load condition within the expected 500 μm of their actual position. 10

11 Acknowledgments The authors would like to thank especially A. HERVE, D. CAMPI and B. CURE for their advice and support, and also J.F. FUCHS and J.D. MAILLEFAUD for their participation in the installation in CMS. They would also like to thank W. RIEGLER and H. KOPETZ for their advice. References [1] BCAM user s manual. [2] LDWAQ user s manual. [3] Reference Bars for the Alignment of the ATLAS Muon Spectrometer C. Amelung, J.R. Bensinger, F. Cerutti, C.W. Fabjan, K. Hashemi, S. Palestini, J. Rothberg, A. Schricker, I. Trigger. [4] ARAMyS user s guide. [5] F.James and M.Roos, \cpc{10} (1975) 343; MINUIT Reference Manual. [6] CMS experiment at CERN. General presentation. [7] The Large Hadron Collider Project General Information. [8] The ALICE experiment at CERN. [9] Heinrich Kopetz A novel method of precision surveying of large particle detectors Master thesis, CERN and TU-Graz 2005 [10] ALICE Spaceframe load test - measurement of Spaceframe for different load conditions. Survey report