In large-scale metrology, when multiple instruments are used, the measuring field space can be large and the targets can be scattered. However, fixed-pointing distance measurement can limit flexibility. Therefore, there is an urgent need to improve automation and efficiency. This paper proposes a beam-guided laser detection module for absolute ranging that combines camera recognition and position-sensitive detection. The module uses a dual-axis fast steering mirror (FSM) driven by a compact field-programmable gate array (FPGA) to change the beam direction. The angular range for beam guiding is larger than ±20°. The mechanical structure and software are detailed introduced in the paper. This module can be directly combined with an absolute ranging system, such as frequency scanning interferometry or dual-comb ranging. It is expected that these systems will find broader applications in the industrial and geodetic survey fields.
In order to solve the dynamic synchronization problem of two laser trackers, two laser trackers are used in this paper, equipped with two measurement target balls placed on a large indoor standard device platform. By moving the platform at a constant speed at a certain measurement distance, the dynamic measurement distance between the two target balls and the speed of the platform during the dynamic condition are analyzed. The following conclusions are drawn: In synchronous measurement, when the sampling frequency of the two laser trackers is set to 50Hz, the maximum value of the speed difference between the platforms during the uniform speed stage is 3.1μm/s. When the sampling frequency of the two laser trackers is set to 10Hz, 20Hz, and 50Hz, the maximum relative distance between the two target balls is 1.778mm, 1.765mm, and 1.764mm respectively. The measurement method used in this paper can be further studied in dynamic synchronous measurement through certain transformations, and up to five laser trackers can be connected for synchronous measurement, which has great application prospects.
KEYWORDS: Repetition frequency, Ranging, Distance measurement, Switching, Interferometers, Signal processing, Metrology, Frequency combs, Signal detection, Scientific research
Absolute distance measurement based on the dual-comb system is a promising tool for the large-scale measurement in the industrial field. There are two combs with slightly different repetition rates in the dual-comb system, and the distance information could be obtained by the asynchronous optical sampling (ASOPS). However, the measured range could be limited within the pulse-to-pulse interval range by the non-ambiguity range (NAR) without the extension. This paper proposes a method to extend the unambiguous range by changing the repetition range. Firstly, the method of dual-comb ranging and the principle of extending the non-ambiguity range were introduced. Secondly, the two implementations of changing single repetition frequency and switching repetition frequency were compared through simulation. Then, according to the simulated results, the effective conditions and theoretical maximum range of extended ranging were clarified. Finally, a long absolute distance measurement was designed and the ranging results of switching repetition frequency were compared with the conventional incremental interferometer, which proves that this method is reliable for extending the non-ambiguous range.
A multilateration system is a 3-dimensional coordinate measurement system, through measuring multiple distances between a target point and laser interferometry measuring instruments, with high precision and large measuring scare, which consists of three laser trackers or tracers at least. A quantifiable analysis method is proposed, for the effect of stations’ mutual position relationship on the measurement accuracy of the multilateration system. The method is based on a mathematical model which linearizes the relationship between the coordinate measurement error of the system and the distance measurement error of laser trackers. Some methods, optimizing the mutual position relationship of stations, are derived from the model, which can improve the measurement accuracy. The rationality of the methods is verified by simulations and experiments. The results show that the max error of the multilateration system improved is only 0.020mm, which is better than a laser tracker. The multilateration system, with the optimized layout of stations, has higher precision.
High-precision and long-range absolute distance measurement is a vitally important topic in large-scale metrology, and it has broadened applications in the manufacturing industry. Especially, the dual-comb time-of-flight distance measurement is a promising method combining with the advantages of fast speed and high accuracy. The measurand would be determined by asynchronous optical sampling (ASOPS) methods with a slight difference in repetition rates. Here, the home-built optical platform of the Er-fiber femtosecond frequency combs would be introduced with a repetition rate of around 200 MHz and carrier-envelope offset frequency of 20 MHz. Besides, the strategies of precision enhancement for dual-comb time-of-flight distance measurement with nonlinear intensity detection are discussed by the numerical simulation. The sampling interval could be optimized by choosing a proper range of repetition rate and difference of repetition rates. A fine curve fitting method is also proposed for further precision improvement. The results of numerical simulation would provide a valuable reference for the experimental process.
The laser multi-coordinate measuring system has the advantages of high precision and wide measuring range and has wide application prospects in the fields of large-scale high-precision coordinate measurement. The positional relationship between the measuring equipment and the measured point is the key factor affecting the accuracy of the system. To quantify the influence of the positional relationship between the measuring equipment and the measured point on the uncertainty of coordinate measurement, the partial derivative operation of the coordinate solving formula is obtained. The coordinate of the measuring point and the mathematical model for measuring the distance from the base station to the measured point. Firstly, the mathematical model is simulated. The results of the simulation experiment show that the nonlinearity of the measurement uncertainty of the system increases significantly when the measured target point is close to the measurement plane composed of multiple stations. Finally, the simulation results are verified by experiments. When the distance between the retroreflector and the measuring plane is reduced from 1010.7 mm to 509.6 mm, the measurement deviation increases by 4 times. When the distance continues to decrease to 10.9 mm, the measurement deviation increases to 76 times before. The results show that the mentioned error model can accurately describe the relationship between the position of the measured point and the error of the measurement accuracy, and can provide theoretical support for the layout optimization method of the laser multi-coordinate measuring system.
The position error on the end of the industrial robot is an important specification for evaluating its accuracy performance. In the ISO 9283 standard, the laser trackers and the binocular vision measurement methods are recommended to calibrate the positioning error. The calibration accuracy measured by using the laser tracker method is superior to that by using the binocular vision measurement method. Thus, this paper emphasized to study how to improve the calibration accuracy of the end position error by using the binocular vision measurement system. The high precision lengths as reference are introduced to combine with a conventional binocular vision system to make a new measurement system. The system errors of the binocular vision system can be corrected. In order to verify the correction effect, taking the industrial robot as an experiment example, the experimental results show that the absolute position error on the end of the industrial robot by using the binocular vision system with reference length constraint can reduce from 0.491 mm to 0.330 mm, and the repetitive positioning error reduced from 0.116 mm to 0.023 mm. The accuracy improved by about 33% and 80% respectively. Compared with the laser tracker measurement results under the same experimental conditions, the difference between the two are 0.34 μm and 7.80 μm. It can be considered that the corrected binocular vision calibration method can achieve the same accuracy with the laser tracker. It can be widely used for high-precision calibration of industrial robots as a means of balancing economics and precision.
It is difficult for the traditional self-calibration algorithm to achieve high precision calibration when measuring over a
wide range. To overcome this shortcoming, an improved self-calibration algorithm for multilateration coordinates
measuring system is proposed. Different from the traditional self-calibration algorithm, the improved algorithm preselect
certain calibration points which is fixedly mounted on stable base, the lengths between specific points are
measured precisely by laser interference method. Then the precise lengths are taken as a part of optimization objective
function, and then the system parameters are determined by iterative computation. The effectiveness of the proposed
algorithm is verified by simulations and experiments. The results show that by using the improved algorithm, the max
error is only 8.9μm, the improved algorithm helps to improve the accuracy of the multilateration system significantly.
To measure the spatial coordinate accurately and efficiently in large size range, a manipulator automatic measurement system which based on multilateral method is developed. This system is divided into two parts: The coordinate measurement subsystem is consists of four laser tracers, and the trajectory generation subsystem is composed by a manipulator and a rail. To ensure that there is no laser beam break during the measurement process, an optimization function is constructed by using the vectors between the laser tracers measuring center and the cat's eye reflector measuring center, then an orientation automatically adjust algorithm for the reflector is proposed, with this algorithm, the laser tracers are always been able to track the reflector during the entire measurement process. Finally, the proposed algorithm is validated by taking the calibration of laser tracker for instance: the actual experiment is conducted in 5m × 3m × 3.2m range, the algorithm is used to plan the orientations of the reflector corresponding to the given 24 points automatically. After improving orientations of some minority points with adverse angles, the final results are used to control the manipulator's motion. During the actual movement, there are no beam break occurs. The result shows that the proposed algorithm help the developed system to measure the spatial coordinates over a large range with efficiency.
Large-scale laser comparator is main standard device that providing accurate, reliable and traceable measurements for high precision large-scale line and 3D measurement instruments. It mainly composed of guide rail, motion control system, environmental parameters monitoring system and displacement measurement system. In the laser comparator, the main error sources are temperature distribution, straightness of guide rail and pitch and yaw of measuring carriage. To minimize the measurement uncertainty, an equivalent common optical path scheme is proposed and implemented. Three laser interferometers are adjusted to parallel with the guide rail. The displacement in an arbitrary virtual optical path is calculated using three displacements without the knowledge of carriage orientations at start and end positions. The orientation of air floating carriage is calculated with displacements of three optical path and position of three retroreflectors which are precisely measured by Laser Tracker. A 4th laser interferometer is used in the virtual optical path as reference to verify this compensation method. This paper analyzes the effect of rail straightness on the displacement measurement. The proposed method, through experimental verification, can improve the measurement uncertainty of large-scale laser comparator.
Abbe error is the inherent systematic error in all large-scale laser comparators because the standard laser axis is not in line with measured optical axis. Any angular error of the moving platform will result in the offset from the measured optical axis to the standard laser axis. This paper describes to an algorithm which could be used to calculate the displacement of an equivalent standard laser interferometer and to eliminate an Abbe error. The algorithm could also be used to reduce the Abbe error of a large-scale laser comparator. Experimental results indicated that the uncertainty of displacement measurement due to Abbe error could be effectively reduced when the position error of the measured optical axis was taken into account.
A large-scale laser interferometric measurement standard device was designed and developed to improve the quantity transmission and calibration capabilities for linear measuring tools and large-scale high precision measurement instruments such as laser trackers, laser scanners and electronic total stations, etc. It consisted of an 80 meters granite guiding rail system, a length measuring system composed of three interferometers, a coarse-fine composite motion platform, environmental parameter (air pressure, temperature and humidity) automatic measuring system, and an image aiming system. The uncertainty of the standard device was analyzed, and a comparative experiment was made to determine the calibration capability of the built standard device. Experiment result indicated that the measurement uncertainty of this standard device to calibrate the other interferometers was better than 0.1μm + 1.0 × 10-7L(k = 2).
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