An experimental verification of the compensation of length change of line scales caused by ambient air pressure

Abstract

Line scales are used as a working standard of length for the calibration of optical measuring instruments such as profile projectors, measuring microscopes and video measuring systems. The authors have developed a one-dimensional calibration system for line scales to obtain a lower uncertainty of measurement. The scale calibration system, named Standard Scale Calibrator SSC-05, employs a vacuum interferometer system for length measurement, a 633 nm iodine-stabilized He-Ne laser to calibrate the oscillating frequency of the interferometer laser light source and an Abbe's error compensation structure. To reduce the uncertainty of measurement, the uncertainty factors of the line scale and ambient conditions should not be neglected. Using the length calibration system, the expansion and contraction of a line scale due to changes in ambient air pressure were observed and the measured scale length was corrected into the length under standard atmospheric pressure, 1013.25 hPa. Utilizing a natural rapid change in the air pressure caused by a tropical storm (typhoon), we carried out an experiment on the length measurement of a 1000 mm long line scale made of glass ceramic with a low coefficient of thermal expansion. Using a compensation formula for the length change caused by changes in ambient air pressure, the length change of the 1000 mm long line scale was compensated with a standard deviation of less than 1 nm.

Long-term dimensional stability and longitudinal uniformity of line scales made of glass ceramics

“…The scale calibration system used in this experiment aims to achieve a measurement uncertainty of as low as a few tens of nanometers for a 1000 mm measurement [10]. Table 1 shows the specifications of the scale calibration system.…”

“…Another method is the line scale measurement. Since the turn of the century, very high precision calibration systems for line scales and linear encoders have been developed by several laboratories [7][8][9][10]. Interlaboratory comparisons of line standards have also been conducted to verify the reliability of measurement [11][12][13].…”

Optical Sensors for Multi-Axis Angle and Displacement Measurement Using Grating Reflectors

“…For such a purpose, optical diffractometers can be employed; by detecting the diffraction angle of the diffracted beams from the scale grating in the Littrow configurations, the grating pitch can be obtained based on the grating equation [141,142]. Another candidate for the evaluation of planar scale gratings is the linear scale comparator [143,144,145]. In the linear scale comparator, an edge of each grating pattern structure is detected by using focused light, while the displacement of the grating pattern structure is measured by using a laser interferometer.…”

Measurement technologies for precision positioning

“…A few 1D high precision measurement comparators using vacuum interferometry have been developed, which achieve measurement uncertainties of a few nm over length ranges of about 500 mm [57,184,204]. Fig.…”

“…To obtain small uncertainties of only a few nm over measurement ranges of about 500 mm requires careful analysis of all relevant influences, including alignment errors, Abbe errors [116], dimensional sensitivity of substrates to variations of ambient pressure [204], ambient temperature [190] and mounting of the substrate [115].…”