Two-dimensional shape photomask is widely used to calibrate and correct optical precision measurement system like image measurement instrument and to calibrate or distortion correction of camera probe with CCD/CMOS sensor. Among all 2-D shapes, circle is adopted frequently because of its information such as size, form and position. Standard photomask with circle shape is multi-parameter calibrated by high precision laser two-coordinate standard device based on coordinate measurement method. Roundness error is assessed by least square circle method. How center coordinate, radius and roundness of a circle affected by number of measurement points, incident light intensity and optical magnification of micro probe are analyzed. Test results indicate than that standard with circle shape could be calibrated with high precision.
National working standard of two-dimensional line scale based on laser two-coordinate standard device was set up to solve the problem of the calibration and traceability of 2-D line scale optical standard and high precision photomask. The operating principle and system composition of the working standard device were introduced. The characteristics were test in special experiments. A high precision differential laser interferometer system was used for a length standard, a high magnification optical microvision system was used for precision optical positioning feedback. In order to improve the measuring accuracy, several high precision sensors were installed to measure environmental parameters for compensating the laser wavelength in atmosphere according to the empirical Edlén equation. High resolution CCD modeling and calibrating based on two-dimensional nanoscale positioning movable platform and laser interferometer were adopted to improve the pointing accuracy. Two-dimensional line scale working standard could be used to measure line spacing, point spacing, and coordinates of 2-D optical standard or photomask, with measurement range 300mm × 300mm, measurement uncertainty U=(0.1~0.3)μm, k=2. Some experiments were carried out to identify the characteristics of length measurement error, probing error, measurement repeatability and measurement reproducibility of the working standard, and measurement uncertainty was validated by the measurement experiments.
An angle interferometer was set up using concept ‘ratio of two lengths’ and an angle encoder was set up using concept ‘subdivision of full circle (2π rad=360°)’ at the National Institute of Metrology, China (NIM). For the analysis of the systematic errors of each device, two autocollimator calibration systems were separately set up based on the angle interferometer and the angle encoder with a similar measuring uncertainty (around 0.1″). An autocollimator was calibrated using two systems in the same measurement range (±1000″) and the same measurement step (10″). The systematic errors of each system were found through comparison between their original calibration results. The compensation curves were calculated using the analysis results, and two systems’ original calibration results were compensated according to two systems’ compensation curves. The maximum difference between the compensated calibration results of two systems was 0.05″ which is lower than measuring uncertainty of each system.
In order to calibrate a high precision rotary table, a calibration system was established to measure the position error and repeatability of rotary table. The position error was measured with a polygon, an index table and an autocollimator to separate the angular error of the polygon from the position error of the rotary table, and the position error of rotary table was calculated using least square method. The rotary table was compensated and calibrated with the position error measured. The repeatability of the rotary table established through 10 times full circle rotations was 0.02 arc second. The measurement results indicated that the combination calibration method was suitable for the calibration of a high precision rotary table. It was found through the analysis that the angular measurement uncertainty was 0.08 arc second.
Gear measuring machine is a specialized device for gear profile, helix or pitch measurement. The classic method for gear measurement and the conventional gear measuring machine are introduced. In this gear measuring machine, the Abbe errors arisen from the angle error of guideways hold a great weight in affection of profile measurement error. For minimize of the Abbe error, a laser measuring system is applied to develop a high accurate gear measuring machine. In this laser measuring system, two cube-corner reflectors are placed close to the tip of probe, a laser beam from laser head is splited along two paths, one is arranged tangent to the base circle of gear for the measurement of profile and pitch, another is arranged parallel to the gear axis for the measurement of helix, both laser measurement performed with a resolution of 0.3nm. This approach not only improves the accuracy of length measurement but minimize the Abbe offset directly. The configuration of this improved measuring machine is illustrated in detail. The measurements are performed automatically, and all the measurement signals from guide rails, rotary table, probe and laser measuring system are obtained synchronously. Software collects all the data for further calculation and evaluation. The first measurements for a gear involute artifact and a helix artifact are carried out, the results are shown and analyzed as well.
Probe is the kernel component of the precision measuring instrument for the system accuracy which is determined by the probe characteristic. Three-dimensional scanning probe is an ideal choice for gear helical error measurement because it has both space coordinates points detecting capacity and scan capacity on the space of curves and surfaces. In order to make full use of the probe’s capacity and improve the measurement accuracy, characteristic evaluation of the probe is necessary before used. The static calibration equipment for the sensor has been established based on the high precision PZT micro displacement platform. Linear characteristic analysis and compensate of the ultra-high precision three-dimensional scanning probe has been done by this equipment, which greatly improved the accuracy of the probe. Finally, probe characteristic under working status is analyzed and experimentally verified which will be very helpful to compensate the probe errors.
A new principle of processing asphere with locus compensation method has been presented. The mathematical model for principle of machining aspherical surface with locus compensation method was created based on the equidistant line formula. The feasibility and practicability was analyzed and the corresponding processing program was made by MATLAB and Visual basic. The overall structure of corresponding machine tools was designed. The program can figure out the compensation error of quadratic curves and the designed machine tools can perform the functions of interception and compensation of quadratic curves , which meets the basic motion requirements of ultra-precision machining. All quadric and high order aspherical surface will be achieved and the surface form accuracy also will be improved. The structure of machine tools is simple and processing range is wide which will reduce the cost of machining greatly.
Photomask is a kind of 2-D optical standard with etched orthogonal coordinates made of a glass substrate chrominged or
filmed with other metal. In order to solve the problems of measurement and traceability of ultra precision photomasks
used in advanced manufacturing industry, 2-D photomask optical standard was calibrated in high precision laser two
coordinate standard device. A high precision differential laser interferometer system was used for a length standard, a
high magnification optical micro vision system was used for precision optical positioning feedback. In this paper, a
image measurement model was purposed; A sampling window auto identification algorithm was designed. Grid stripe
image could be identified and aimed at automatically by this algorithm. An edge detection method based on bidirection
progressive scanning and 3-sigma rule for eliminating outliers in sampling window was found. Dirty point could be
removed with effect. Edge detection error could be lowered. By this means, the measurement uncertainty of 2-D optical
standard's ruling span was less than 0.3 micrometer (k=2).