Abreu, Manuel A.,1959-Alves, David Miguel Ventura de Castro,1975-Gonçalves, Nuno Miguel Cabecinhas2021-05-192021-05-1920202020http://hdl.handle.net/10451/48028Tese de mestrado integrado em Engenharia Física, Universidade de Lisboa, Faculdade de Ciências, 2020Optical measuring systems offer new ways to determine absolute or relative distance measurements, dubbed as ADM, or RDM, respectively. These systems allow the determination of distances, deformations, or vibrations through accurate and high resolution optical techniques. These techniques are non-contact processes that can have a wide array of applications, from industrial measuring devices to aerospace equipment. Therefore, the goal of the present dissertation was to develop and test a high accuracy absolute distance sensor, based on optical processes that could perform an ADM within a [10, 20] m range with an expanded uncertainty, UD< 100 µm. In this work, an assessment of the state of the art of the different optical measuring techniques was conducted to evaluate which method allow reaching the established requirements. After this analysis, the phase shift continuous wave time of flight technique was chosen. To measure a distance using this method, a coherent light source is continuously amplitude modulated by a sinusoidal signal. The difference in the phase angle of the signal in the source and its reflection on a target contains the time of flight information. Therefore, the ADM between source and target can be obtained by a phase shift measurement. Due to the cyclical properties of the phase angle, the position information is contained within an ambiguity interval. To perform distance measurements, we must consider the number of times the ambiguity interval is repeated, N, plus the fraction part of this interval given by the unwrapped phase shift. It has been already reported by several authors referenced in this document that this technique can achieve distance measurement accuracies in the order of 100 µm for modulation frequencies in the GHz range. However, to perform an ADM with these frequencies, the ambiguity integer, N, in a phase measurement has to be obtained. In this context, we developed an adaptation of the Vernier method to remove the ambiguity in a phase shift measurement. It consists of performing a dual phase shift measurement for two different modulation frequencies. However, to measure its ambiguity integer correctly, one must determine it with an expanded uncertainty of UN < 0.5. We performed Monte Carlo and analytical computational simulations to evaluate the maximum magnitude of the uncertainty contributions for an ambiguity integer and ADM in a [10, 20] m range. To verify the validity of the proposed Vernier approach for ADM an experimental setup was devised and built whilst satisfying the established uncertainty requirements. Finally, the method was tested by performing ADM at a short and mid range, [0, 100] mm and [4.808, 4.818] m, respectively and with a ”blind measurement”, where the target was placed at ≈ 2 m. We were able to verify that for the different test ranges, the developed sensor was able to correctly measure most of the ambiguity integer correspondent to the tested positions. Additionally, the N measurements in the short range prove that the presented method can detect a change in the ambiguity integer. However, these tests reveal that in some situations the sensor was not able to measure the correct ambiguity integer for the tested position, which leads to an incorrect ADM. By considering the erroneous N measurements as an outlier in our evaluation, it is possible to extract a calibration curve that corrects the scale and offset in our results. When performing a correction of the measured values, for the short and mid range tests, we obtained an average experimental absolute error of 126 µm and 256 µm, respectively, computed for a 1.8 GHz modulation. These results are within an expanded uncertainty of ≈ 600 µm with a coverage factor of k = 2, this means a confidence interval of 95 %.engMedição de distâncias absolutasTempo de vooDiferença de faseMetrologia ópticaHeterodinagemTeses de mestrado - 2020master thesis202604438