Application of anamorphic optics system and a high-speed line scan photodetector in an open-type relative encoder

Optical encoders based on imaging optical systems and two-dimensional sensor arrays have shown substantial potential for reducing measurement errors through the use of modern image processing algorithms. However, the application of two-dimensional photodiode array in optical encoders significantly reduces the update rate of positional data. This limitation is critical for wide industrial application. This study presents an optical encoder design that incorporates a high-speed linear photodiode array and an anamorphic optical system utilizing a cylindrical lens, allowing for increased update rates and reduced positional error. A Renishaw RTLC40 tape, with a grating period of 40 μm, fabrication accuracy of ±5 μm/m, and a length of 300 mm, was employed as the encoding structure. The prototype optical encoder was developed using a GL3504-BVM-NCN-AU1 linear photodetector array, a custom designed and manufactured objective (linear field in object plane of 0,84 mm, magnification 10×) with an integrated cylindrical lens and 5CEFA9F23 programmable logic device. The calculation of the object displacement is based on determining the energy centroid of the images of grating scale, which tightly mounted on the object. The error analysis of the proposed encoder prototype was determined using an XD6 LS interferometer and an LTS300/M motorized stage. Positional update frequency was measured with an MSO5074 oscilloscope. The use of a cylindrical lens amplified the irradiance projected onto the photodetector array, achieving a maximum gain factor of three. Update frequency and measurement error of the proposed optical encoder were experimentally determined and equals 10 kHz and 0,94 μm over 290 mm range, respectively. The proposed design of the optical encoder can be used as a recommendation for the development of encoders that employ cylindrical lens-based optical systems. Field-programmable gate array is recommended for grating scale displacement computation. The results obtained may prove valuable for specialists in precision displacement measurement and machine tool construction.