Objective The reference beam Laser Doppler Velocimeter (LDV) boasts advantages such as high accuracy, wide range, rapid response, and non-contact measurement. It is extensively employed in the measurement of physical quantities like solid surface velocity, vibration, displacement, as well as in integrated navigation systems. The reference beam type LDV measures sensitive velocity components parallel to the direction of the outgoing light, where the angle between the outgoing light of the LDV and the moving surface constitutes the emission inclination. Evidently, the magnitude of the velocity component to which the LDV is sensitive will hinge on the magnitude of the launch inclination, thereby influencing the system bandwidth. Additionally, the transmission angle also impacts the signal-to-noise ratio of the Doppler signal and the velocity measurement accuracy of the LDV. It can be discerned that the launch angle will have a significant impact on the performance of the LDV. To enable the LDV to select a rational launch angle, the effect of the launch angle on the performance of the LDV is analyzed and discussed in this paper.
Methods The influence of the launch angle on the performance of LDV is analyzed through theory and simulation. The relationship between the emission inclination angle and the signal frequency is obtained by the principle formula of LDV (Fig.2). Based on the irradiance formula when the laser incident on a surface with different roughness, the signal-to-noise ratio variations corresponding to different emission angles are acquired (Fig.3). Additionally, the impacts of launch inclination on velocity measurement errors were analyzed, including errors caused by velocity measurement resolution (Fig.5), the finite aperture of detector error (Fig.2), principle formula approximation error, finite transit time error and laser divergence angle error, etc. The errors caused by velocity measurement resolution were identified as the main errors (Fig.7).
Results and Discussions According to the relationship between the frequency of the Doppler signal and the transmission angle (Fig.2), to reduce the signal bandwidth of the LDV, a large transmission angle should be selected as far as possible. Meanwhile, the signal-to-noise ratio of the Doppler signal also increases with the growth of the transmission inclination, and this growth trend gradually slows down when the transmission inclination is greater than 60° (Fig.3). Therefore, to enhance the signal-to-noise ratio of the Doppler signal, the transmission angle should be greater than 60°. However, increasing the launch angle will augment the velocity measurement error, especially when the launch angle is greater than 80°, the velocity measurement error will escalate rapidly with the increase of the launch angle (Fig.7). Based on the outcomes of the simulation analysis, the method of setting the launch angle or sampling frequency in segments is proposed for different measuring ranges and measuring accuracy requirements, and the steps of determining different measuring ranges are provided. Compared with unsegmented measurement, segmented measurement can significantly reduce the velocity measurement error of the system (Fig.9), and the equivalence of the segmented setting of the launch angle and the segmented setting of the sampling frequency is elucidated (Tab.1-Tab.2).
Conclusions To reduce the system bandwidth and enhance the signal-to-noise ratio of the Doppler signal, a large transmission angle should be selected. However, an excessively large launch angle will give rise to an increased velocity measurement error of the system. When choosing the actual launch angle, a balance should be struck among several factors, and the launch angle is preferably between 60° and 80°. When the range of velocity measurement is overly large, to concurrently meet the requirements of the signal bandwidth and velocity measurement error, the method of setting the transmission angle or sampling frequency in segments can be employed. It holds significant guiding significance for the structural design and practical application of LDV.
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