Development of Atmospheric Compensator to Reduce the Laser Measurement Error for Ultra-Precision Machine Tools

The form error, which is generated by the ultra-precision machine tool with laser interferometer, is due primarily to the measurement error caused by the change in laser wavelength that is affected by the refractive index of the air, which depends on its temperature, humidity and pressure. This paper describes a new wavelength tracking compensator that can continuously update the precise refractive index and the performance of its application to the machine tool. This compensator is made up of environmental sensors mounted on the machine tool, a PC based analyzer and a compensation board added to the interferometer processor. The analyzer calculates the refractive index using Edlen's formula from atmospheric temperature, humidity and pressure measured by the sensors. The compensation board, which corrects the measurement wavelength using the obtained refractive index, provides the accurate position update for the CNC controller. The compensator can also correct the error of deadpath length in optical path as well as that of refractive index. The measurements of the displacement of the tool relative to a workpiece and of the form error performed by raster flycutting are executed to evaluate the performance of the compensator. Both measurements that imply the highly corrected accuracy of less than 100nm show a remarkable improvement compared with the case without compensator.