Machine tool errors caused by optical linear encoders

Abstract

Position measurement in machine tools is usually carried out through rotary or linear encoders. Quite often, it is considered that part dimensional errors are only due to direct strain, thermal or vibration e ects in the machine tool structure. There is, however, another important source of error: the encoder itself. This paper is intended to recognize, classify and determine the value of the di erent kinds of error in optical linear encoders: strain and displacement errors, thermal errors and errors caused by vibrations. The error identi®cation will also permit a better understanding of some of the limitations that appear in the use of these devices.

Analysis of optical linear encoders’ errors under vibration at different mounting conditions

“…The tests have been performed at a constant temperature and with the encoder in a fixed position to avoid influence of main sources of errors (thermal and deformations) [7].…”

“…Machine tools and CMM errors have traditionally been focused on the analysis and performance of the machine structure [2][3][4][5][6]. Recent works [7] have treated the encoder itself as a source of error but current methods for estimating the encoders' accuracy usually neglect vibration influence [16]. Encoder error quantification has been done for several mounting conditions, such as lack of parallelism between the measurement axis of the encoder and the machine guide, lack of flatness of the surface in which the encoder is to be mounted, scanning head support weakness and relative movement between glass scale and scanning head, to name a few.…”

“…Although clarifying examples allowing identification and evaluation of the error due to mounting conditions in static situations have been done [7], there is a lack of 0263-2241/$ -see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2011.05.004 information regarding the performance of the encoder under dynamic solicitations.…”

Method for the evaluation of optical encoders performance under vibration

“…This accuracy is sharply influenced by external factors. While some attention has been already paid to errors arising from deformation and temperature changes [1][2][3][4][5][6][7], there is a lack of information of detailed encoder behaviour under vibration. Considering that these devices are often present in the field of precision engineering, its contribution to the total error cannot be ignored when trying to reduce the measurement instability to a minimum.…”

Thermal non-linear behaviour in optical linear encoders

“…[6], but their main error source is temperature change. In fact, the scale (usually deposited on glass) just stretches or contracts as any other material.…”