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A Laser Tracker Calibration System

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A Laser Tracker Calibration System A Laser Tracker Calibration System Daniel Sawyer Bruce Borchardt Steve Phillips NIST NIST NIST 100 Bureau Drive 100 Bureau Drive 100 Bureau Drive Stop 8211 Stop 8211 Stop 8211 Gaithersburg, MD Gaithersburg, MD Gaithersburg, MD 20899-8211 20899-8211 20899-8211 (301) 975-5863 (301) 975-3469 (301) 975-3565 Charles Fronczek Tyler Estler Wadmond Woo NIST NIST Naval Surface Warfare 100 Bureau Drive 100 Bureau Drive Center, Corona Division Stop 8211 Stop 8211 2300 Fifth Street Gaithersburg, MD Gaithersburg, MD Norco, CA 92860-1915 20899-8211 20899-8211 (909) 273-5544 (301) 975-4079 (301) 975-3483 Robert W. Nickey Naval Surface Warfare Center, Corona Division 2300 Fifth Street Norco, CA 92860-1915 (909) 273-4371 these instruments to ensure the integrity of their Abstract - We describe a laser tracker measurement results is increasingly critical. The calibration system developed for frameless task is complicated by the fact that the measuring coordinate metrology systems. The system envelope of these instruments is quite large (> 35 employs a laser rail to provide an in-situ meters). Such large measurement volumes often calibrated length standard that is used to test require the use of large length standards, e.g., a tracker in several different configurations. greater than two meters, to characterize the The system is in service at the National instrument. Physical standards such as large Institute of Standards and Technology (NIST) calibrated artifacts, commonly referred to as scale and at the Naval Surface Warfare Center, bars, may suffer the problems of being unwieldy Corona Division (NSWC, Corona Division). to use, difficult to construct and quite often costly The system description, calibration procedure, to maintain. and uncertainty budget are presented. This paper describes the design of the NIST Laser Introduction Rail Calibration System (Larcs) which is deployed Laser trackers are becoming the tool of choice for at the NSWC, Corona Division. The system was large scale coordinate measuring needs. The developed to provide users with an easy to use requirement to rapidly validate the performance of and easy to maintain tool for performing length measurement tests for characterizing the Commercial equipment and materials are identified in order to adequately specify certain procedures. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. U.S. Government work not protected by U.S. copyright performance of laser tracker measurement the extensive performance tests is not typically systems [1]. The system employs a laser performed. As a result, any drift in the artifact interferometer with environmental sensors, a rail dimensions due to a change in its temperature system and support structure that can be may mask or exaggerate tracker measurement configured to generate a long reference length, errors. For artifact materials such as steel and typically four meters, in any arbitrary orientation in aluminum, the thermal drift due to temperature the tracker measurement volume. The detailed change is typically 10 to 25 micrometers per design of the Larcs and the advantages of its use meter per degree Celsius. for performing length measurement testing are discussed in this paper. Even if the temperature of the artifact is accurately known and compensation for the thermal Background expansion applied, the uncertainty in the artifact’s Fixed length artifacts, such as calibrated scale coefficient of thermal expansion may degrade the bars, having target nests at each end are accuracy of the performance evaluation. For frequently used to perform length measurement example, the uncertainty in the nominal coefficient testing. Alternatively, rigid structures, such as of thermal expansion for common artifact tripods, which hold a single target nest fixed materials is estimated at one to two micrometers relative to another fixed nest may also be per meter per degree Celsius. For a four meter employed. The latter construction is called a artifact that changes temperature by two degrees virtual length since there is no material standard during measurement, the uncertainty in the between the nests and the metrology loop is calibrated length due to the uncertainty in the connected through the tripods and the floor. coefficient of thermal expansion is approximately Limitations exists for both practices which may 16 micrometers for just this single source of make them either impractical or, more seriously, uncertainty. For some measurement tasks this incapable of assessing the manufacturer’s stated uncertainty may be unacceptably large. measurement specifications. What follows is a discussion of both the physical and virtual length To support physical artifacts during measurement, methodologies for establishing a calibrated length operators frequently clamp them to a single tripod artifact as well as a brief description of the or rigid stand. The position of the target is then advantages of the Larcs design. recorded in each of the target nests. Because of the long cantilevered lengths, the weight of the In practice, the ranging system of the laser tracker spherically mounted retroreflector targets (SMRs) is used to perform an in-situ calibration of physical can substantially bend the tripod or support length artifacts used during testing [2]. This is structure. The change in static loading when the carried out by aligning the interferometer of the SMR is placed in each of the nests results in tracker directly along the length of the artifact movement of the target between measurements. such that only the interferometer is employed in The effect is that the origin of the reference length the measurement and not the angular encoders shifts during the measurement which leads to an (which are less accurate) of the laser tracker. In apparent length error. To compensate for this order to satisfy the requirement for traceability [3], behavior, operators frequently use two SMRs. if the tracker is used to calibrate the artifact’s One is used as the measurement target while the length, the laser tracker interferometer system other is simply placed in the opposite target nest itself must be independently calibrated. This and used as a dummy static load during the usually necessitates a back-to-back comparison measurement process. The coordinate of the of the tracker’s interferometer with a second target SMR is recorded in one nest and then the calibrated interferometer. measurement and dummy load SMRs are switched. Using this technique, the static loading Once this requirement has been met, the newly on the support structure, during the measurement, calibrated artifact is then used to determine the remains unchanged at the cost of additional measurement errors for lengths that are operator work and consequently lower efficiency. particularly sensitive to errors in the tracker’s Although this procedure effectively eliminates the angular encoders and optical and mechanical problem caused by non-symmetric loading of the geometrical alignment. Due to time and economic support structure, increased manual intervention constraints, re-calibration of the scale bar during provides opportunity for the creation of operator U.S. Government work not protected by U.S. copyright blunders. In particular, the magnets which Division and NIST. However, the Larcs hardware typically hold the SMR in the target nests are quite can be used independently of the software to strong, making removing and replacing the SMRs characterize any laser tracker.) The results of the without moving the test setup quite difficult. scripted measurement tasks provide a fairly Increasing the number of times the operator complete characterization of the laser tracker removes and replaces the SMR creates the measurement system capability. The software is opportunity for inadvertently bumping or moving designed to provide careful checks to guarantee the reference length, resulting in an erroneous that the measurement procedures are consistent performance evaluation. for all measurements and that operator influence quantities are minimized. Although at the time of Another way of creating a reference length is to fix this writing the draft standard “B89.4.19 a single target nest to a rigid structure such as a Performance Evaluation of Laser Based Spherical tripod or stand. Two of these structures are then Coordinate Measurement Systems” [5] is not yet positioned relative to one another to create a complete, it is believed that these tests will be virtual length. Again, the tracker’s interferometer consistent with the evaluation procedures of the is typically used to measure the separation of the standard. targets, thereby establishing a virtual artifact. If the environment is not thermally stable, as is often System Description the case for large work volumes, the tripods The design of the system was driven, in large thermally distort causing relative motion between part, by the functional requirement of the the two target nests. During the measurement measurement tasks. In order to characterize the process, this motion may be incorrectly point-to-point length measurement capability of interpreted as laser tracker measurement error. laser trackers, a reference length must be Even if the operator correctly identifies this error measured in enough orientations to examine a source, characterizing or compensating for this
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