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Analysis under Vibrations of Optical Linear Encoders Based on Different Scanning Methods Using an Improved Experimental Approach


J. López, Departamento de Mecánica, Universidad Nacional de Educación a Distancia, C/Juan del Rosal, 12. Ciudad Universitaria, 28040 Madrid, Spain


Optical linear encoders are high-precision positioning sensors based on the interference patterns produced by the relative movement between two gratings. One of the gratings is impressed on a scale and the other is located in what is known as the scanning head. The rest of the main components are a light source, collimating lens, and a set of photodetectors. The arrangement of these components is mainly conditioned by the scanning method chosen, resulting in a different encoder mechanical design and therefore in dissimilar dynamic behavior if transmission or reflection is selected. In these methods, sealed single field scanning encoders represent the ultimate state-of-the-art becoming the most widespread solution nowadays, but it has to be said that there is a lack of information concerning their dynamic behavior under vibrations in relation to conventional transmission or reflection reading heads. A novel methodology that allows identifying both the frequency and the error due to vibrations, which can be used in compensation procedures to correct the sensor's displacement measurement, is proposed in this work. Additionally, a comparison study based on the simulation results obtained by finite element models of transmission and reflection type encoders is presented. In the second part of the work, experimental performance comparison under vibration of the above-mentioned scanning methods used in optical linear encoders is done. Using the new methodology that includes an experimental technique with an improved mathematical approach that allows not only to assess the loss of accuracy of the sensor due to vibration, but also the error committed by the encoder in a range of excitation frequencies is characterized.