Article

Application of laser tracker used in the measuring and the adjusting of the workbench for SAR antenna

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Abstract

In order to accurately position the nodes of a SAR(synthetic aperture radar), a special assemble workbench is designed and a laser tracker named LTS-1100 from the API is used to measure it. The two stations layout of laser tracker along the lengthways center line of the workbench are adopted. The six common points used when the laser tracker moving from one station to the other are selected based on the stability experiment under the working environment. The laser tracker measures the positions of all the points real time and directs their adjustments. The distance error of the points from the ideal on the workbench is less than +/-0.105mm. The close range photogrammetric method is then used to measure the nodes position of the SAR antenna assembled on the workbench. The result indicates the coincidence with that obtained by the laser tracker.

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... Applications for laser tracker-based systems are very wide. These systems are used in large-volume measurements [5] and alignments involved in levelling machine tools, process part inspection, final part inspection, inspection of moulds and dies, prototyping, reverse engineering [6], monitoring structure movement [7] and analysis of deformations of industrial elements [8]. In Gu et al. [1], a laser tracker (LT) is used in high-precision measurement of large antenna surface shape, and this system is used in measuring large-scale dimensional metrology in [3]. ...
... Beam tilt projection on a perpendicular to the beam x 5 Transit tilt x 6x X component of the horizontal encoder eccentricity x 6y Y component of the horizontal encoder eccentricity x 7n N component of the vertical encoder eccentricity x 7z Z component of the vertical encoder eccentricity x 8 Bird-bath error x 9 Error in the encoder scale in horizontal encoder x 10 Error in the encoder scale in vertical encoder ...
Article
A new kinematic model for a laser tracker is presented in this paper. This model obtains the kinematic parameters by the coordinate transformation matrices between successive reference systems based on the Denavit-Hartenberg method. The ASME B89.4.19 Standard provides some ranging tests, length measurement system tests and two-face system tests that can be performed to analyze the performance of the laser tracker. However, these tests take a lot of time and require specialized equipment. Another problem is that the end user cannot apply the manufacturer’s model because he cannot measure physical errors. The kinematic model developed has been compared with a geometric model based on modelling physical errors. To do this, the laser tracker kinematic model has been presented and validated using a mesh with synthetic reflector coordinates and known error parameters. The laser tracker has then been calibrated, in an easy and fast way, with experimental data using the measurements obtained by a coordinate measuring machine as nominal values. The calibration has been performed with both the kinematic model presented in this work and the geometric model based on physical errors. A comparison of both calibrations has been made, analyzing the performance of both models. Finally, a sensitivity analysis of the length measurement system tests is presented to recommend the more suitable positions to perform the calibration procedure.
... Other studies show the use of LTs to calibrate systems such as the calibration of a Stewart platform [7] or the calibration of an articulated arm coordinate measuring machine [8]. A LT is used in the measuring and the adjusting of the workbench for SAR antenna in [9]. An optical surface in a telescope is measured in [10], obtaining high accuracy. ...
Article
This paper presents a new kinematic model, a parameter identification procedure and a sensitivity analysis of a laser tracker having the beam source in the rotating head. This model obtains the kinematic parameters by the coordinate transformation between successive reference systems following the Denavit–Hartenberg method. One of the disadvantages of laser tracker systems is that the end-user cannot know when the laser tracker is working in a suitable way or when it needs an error correction. The ASME B89.4.19 Standard provides some ranging tests to evaluate the laser tracker performance but these tests take a lot of time and require specialized equipment. Another problem is that the end-user cannot apply the manufacturer’s model because he cannot measure physical errors. In this paper, first the laser tracker kinematic model has been developed and validated with a generator of synthetic measurements using different meshes with synthetic reflector coordinates and known error parameters. Second, the laser tracker has been calibrated with experimental data using the measurements obtained by a coordinate measuring machine as nominal values for different strategies, increasing considerably the laser tracker accuracy. Finally, a sensitivity analysis of the length measurement system tests is presented to recommend the more suitable positions to perform the calibration procedure.
... Laser trackers have been used in different applications such as inspection, large-volume measurements [3], analysis of deformations [4], measurement compensation of geometric errors of machine tools [5][6][7], and calibration of an industrial robot [8]. ...
Article
Full-text available
Different calibration strategies based on network measurements have been studied to improve the accuracy of a laser tracker having the beam source in the rotating head, thus allows us to determine if nominal distances are needed. Moreover, the minimum gauge needed to ensure a calibration valid result is characterised. First, the laser tracker calibration performance, using only network measurements without any nominal data known, has been studied. Different strategies have then been carried out, using reflector gauges as nominal data in the calibration procedure to determine the more suitable gauge in terms of accuracy and efficiency. The reflectors have been measured from different positions of the laser tracker. The gauge reflectors have been measured too with a coordinate measuring machine for obtaining the nominal data. The objective function to be minimised in the identification parameter procedure has been developed for every strategy for the distance criterion (distances between every pair of reflectors must be constant regardless of the laser tracker position from which they are measured). Then, two criteria, distance criterion and coordinate criterion (the reflector positions measured by the laser tracker are expressed in the same reference system and are then compared), have been used to evaluate the calibration performance. The analysis developed shows the improvement accuracy of every strategy studied.
... Recently, owing to the advantages of higher accuracy, large range and high-speed measurement, laser trackers are being used extensively in large-scale industrial and scientific metrology, for example, in the aerospace, automotive and nuclear physics domains. [10][11][12][13]. In addition, laser trackers play an important role in ALMA, LMT (Large Millimeter Telescope) and other international well-known large antenna metrology and alignment systems [14,15]. ...
Article
A large compact range (CR) having a width of 23 m and height of 16 m that will generate a Φ15 m quiet zone is presented. The antenna consists of 30 blocks and 76 serrated reflectors. Its mechanical accuracy is reflected in two aspects: surface precision and gap precision. In addition, the root-mean-square (RMS) surface accuracy should be less than or equal to 0.075 mm for achieving the highest operating frequency of 40 GHz, and the gaps between two segments should be controlled strictly to the tolerance of 0.4 ± 0.2 mm for avoiding gap diffraction and compensating for inter-block interference due to thermal deformation. The surface accuracy in terms of mechanical structure, metrology and alignment approach is very tight. First, a high-accuracy honeycomb sandwich panel, anisotropic back structure and spatial parallel adjustment mechanism are introduced, and the error contributions of these three mechanisms are 0.03 mm, 0.01 mm and 0.005 mm, respectively. Second, a measurement network based on laser tracker metrology was established, and the RMS error of the measurement system is controlled to 0.025 mm through the optimization of the measuring stations and weighted coordinate regression. Third, an original alignment approach that divides the entire assembly into three key phases by marked point edge-constrained surface is proposed. By performing a few iterations of onsite adjustment, the reflectors were aligned in the prescribed positions, and the gap quality was controlled effectively. Finally, the on-site alignment of the large CR is introduced. The final antenna surface RMS accuracy was up to 0.054 mm, and the gaps achieved the desired design index.
Article
The geometric accuracy of the surface shape of large antenna is an important indicator of antenna’s quality. Currently, high-precision measurement of large antenna surface shape can be performed in two ways: photogrammetry and laser tracker. Photogrammetry is a rapid method, but its accuracy is not enough good. Laser tracker can achieve high precision, but it is very inconvenient to move the reflector (target mirror) on the surface of the antenna by hand during the measurement. So, a smart car is designed to carry the reflector in this paper. The car, controlled by wireless, has a small weight and a strong ability for climbing, and there is a holding bracket gripping the reflector and controlling reflector rise up and drop down on the car. During the measurement of laser tracker, the laser beam between laser tracker and the reflector must not be interrupted, so two high-precision three-dimensional miniature electronic compasses, which can real-time monitor the relative angle between the holding bracket and the laser tracker’s head, are both equipped on the car and the head of laser tracker to achieve automatic alignment between reflector and laser beam. With the aid of the smart car, the measurement of laser tracker has the advantages of high precision and rapidity.
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