Laser doppler velocimeter

Abstract

To provide a laser Doppler velocimeter that has higher accuracy and measures the distance between a focal point that is an intersection of two light beams and a surface of a moving object to correct a speed error.SOLUTION: A differential laser Doppler velocimeter irradiates a moving object with two laser beams separated from a laser source and receives scattered reflected light from the object to measure the velocity of the moving object on the basis of the Doppler effect, and has: a first optical receiving system that receives regular reflected light and scattered reflected light from the moving object; a second optical receiving system that receives the scattered reflected light from the moving object and provides the velocity of the moving object; and a signal processing system that is connected to the first optical receiving system and the second optical receiving system. The signal processing system calculates the distance between the moving object and an intersection of the two beams and the velocity of the moving object from a received signal from the first optical receiving system and a received signal from the second optical receiving system.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a speedometer that uses the Doppler effect of laser light. [Background technology]

[0002] Laser Doppler velocimeters, which use laser light and the Doppler effect, can accurately measure the speed of an object without contact, and are therefore used in many fields today. One example of such applications is measuring the conveying speed during the production of high-performance films. High-performance films are used in fields such as electronics, automobiles, building materials, pharmaceuticals, and food packaging, and non-contact laser Doppler velocimeters are used to monitor and control the stretching speed during production to ensure uniform quality.

[0003] The operation of the laser Doppler velocimeter was verified in 1964. An example of this is shown in Non-Patent Document 1. Furthermore, as an advanced version of the laser Doppler velocimeter, the differential laser Doppler velocimeter shown in Non-Patent Document 2 has been developed.

[0004] A differential laser Doppler velocimeter is configured to irradiate a moving object with two beams of light separated from a laser light source. One of the two beams is irradiated from the front of the object, so the Doppler scattered light has an increasing optical frequency, while the other beam is irradiated from the rear of the object, so the Doppler scattered light has a decreasing optical frequency. When these two scattered lights are optically heterodyned by a photodetector, the frequency of the output electrical signal is the difference between the two scattered light components. The amount of deviation in the beat frequency of the output electrical signal is twice that of the system in Non-Patent Document 1, which used only one scattered light, which has the advantage of increasing the velocity detection sensitivity. Furthermore, because the optical system is a differential type, it is possible to eliminate the effects of noise components generated from unevenness on the surface of the object to be measured and noise components generated from the intensity noise of the light source itself, thereby improving the sensitivity of the laser Doppler velocimeter.

[0005] Furthermore, small-sized differential laser Doppler velocimeters have also been developed recently, one example of which is disclosed in Patent Document 1. FIG. 3 shows the configuration of the laser Doppler velocimeter disclosed in Patent Document 1. In Figure 3, in contrast to the conventional differential laser Doppler velocimeter, mirrors 6 and 10 are moved in the same direction and simultaneously to adjust the optical path lengths on the left and right to be equal, making it possible to detect velocity even when the semiconductor laser oscillates in multimode, thereby achieving stability and low cost as a laser Doppler velocimeter.

[0006] In Figure 3, laser light source 1 is a semiconductor laser with a wavelength of 660 nm operating in multimode. The laser beam emitted from laser light source 1 is collimated by collimator lens 2. This laser beam is incident on frequency shift element (AOM) 3, and a 40 MHz fm signal is applied to this AOM, resulting in a 40 MHz frequency shift of a portion of the incident laser beam, resulting in first-order diffracted light 8, which is shifted to S polarization, and zeroth-order diffracted light 12, which remains P polarization without frequency shifting. These beams are incident on polarizing beam splitter 4, where they are split into P-polarized and S-polarized beams. The P-polarized beam travels straight to the transmission side and is reflected by mirror 9 before being converted into a circularly polarized beam by λ / 2 wave plate 5 and irradiated onto moving object 0. The S-polarized beam from polarizing beam splitter 4 is reflected by mirror 6 and converted into a circularly polarized beam by λ / 2 wave plate 7, which irradiates moving object 0.

[0007] Light scattered from moving object 0 is collected by light-receiving lens 13 and incident on light-receiving element 15 via mirror 14, where it undergoes optical heterodyne detection through photoelectric conversion. The beat signal frequency of the electrical signal output shifts in the plus and minus directions around 40 MHz due to the Doppler effect, and the amount of frequency shift is proportional to the moving speed of object 0. The moving speed of object 0 is calculated by processing this electrical signal.

[0008] This laser Doppler velocimeter is highly sensitive because it employs a differential laser Doppler system, and because it uses frequency shift modulation, it is possible to measure velocities when the velocity is zero or in the reverse direction. Furthermore, it uses a semiconductor laser as the light source, making it possible to make it compact. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 6788928 [Non-patent literature]

[0010] [Non-Patent Document 1] Y.Yeh et al., "Localized Fluid Flow Measurements with an He-Ne Laser Spectrometer", Applied Physics Letters, vol.4, no.10, pp.176-178, May, 1964. [Non-patent document 2] Bruce E. Truax et al., "Laser Doppler velocimeter for velocity and length measurements of moving surfaces", Applied Optics, vol.23, Issue 1, pp.67-73, 1984. [Non-patent document 3] Eiji Okada and Haruyuki Minamitani, "Laser Doppler Measurement of Velocity and Angle on Solid Surfaces Using Specular Reflection," Transactions of the Society of Instrument and Control Engineers, Vol. 22, No. 10 (October 1986), pp. 1101-1106. Summary of the Invention [Problem to be solved by the invention]

[0011] Laser Doppler velocimeters require high measurement accuracy and a wide operating range. However, conventional differential laser Doppler velocimeters have the problem that velocity errors are easily generated when the distance to the object fluctuates. The fluctuation in the distance to the object refers to ΔZ in Figure 3, which is the fluctuation in the distance between the focal point, which is the intersection point of the two light beams, and the surface of the moving object 0.

[0012] Figure 4 shows the velocity error characteristics of a differential laser Doppler velocimeter versus the distance ΔZ between the focal point and the moving object. Ideally, the velocity error should have zero ΔZ dependence. However, in actual laser Doppler velocimeters, slight velocity errors occur when the distance ΔZ between the focal point and the moving object changes. Such velocity errors can lead to poor control of the stretching speed during the film manufacturing stretching process using a laser Doppler velocimeter, resulting in a deterioration in film quality. For example, if abnormal vibrations are applied to the laser Doppler velocimeter in the conveying system, the distance between the focal point and the moving object surface changes, resulting in an error in the measured stretching speed. Furthermore, pulley eccentricity in the conveying system or uneven rotation in the drive system can cause the film surface to move up and down, changing the distance between the focal point and the moving object surface, resulting in variations in the measured conveying speed. This results in poor control of the conveying speed, which can affect film thickness and quality.

[0013] To solve these problems, it is desirable to realize a laser Doppler velocimeter that does not generate velocity errors even when the distance ΔZ between the focal point and the surface of a moving object changes. A velocimeter with the velocity error characteristics shown in Figure 4 may not be applicable depending on the application. In such cases, to further reduce the velocity error, the optical components are replaced, and the velocimeter is remanufactured and readjusted, but this results in problems such as a decrease in yield and an increase in manufacturing man-hours, which increases the cost of the velocimeter.

[0014] Through optical analysis of a differential laser Doppler velocimeter, it was found that the above velocity error occurs due to slight optical distortion of the optical components used. The process by which the velocity error occurs is explained below.

[0015] As an example of optical analysis, Figure 5 shows the beam intersection diagram (A) and the beam wavefronts (B) and (C) of a differential Doppler velocimeter. The beam intersection diagram in Figure 5(A) shows how the zeroth-order diffracted light and first-order diffracted light of the differential Doppler velocimeter intersect at an angle 2α (set to 20° in Figure 5) when the distance between the focal point and the moving object surface is ΔZ=0. At ΔZ=0 mm, the center line of the light beam of the zeroth-order diffracted light and the center line of the light beam of the first-order diffracted light intersect, and this intersection point becomes the focal point of the differential laser Doppler velocimeter.

[0016] The wavelength of the light beam is 785 nm, and the semiconductor laser oscillates in a single mode by controlling the injection current and operating temperature of the semiconductor laser. The light beam is focused by a collimator lens to form a parallel beam with a slightly convergent wavefront. The cross-sectional shape of the light beam is elliptical, with a width (horizontal to the paper surface) of 2.5 mm and a height (vertical to the paper surface) of 0.3 mm, and the light beam intensity is close to that of a Gaussian beam. The width and height of the light beam are the distance between the points where the light intensity is 13.5% of its maximum value.

[0017] The wavefronts of the zeroth-order and first-order diffracted light were obtained by placing a Shack-Hartmann wavefront sensor at ΔZ = 0 mm and observing them. The solid lines in Figures 5(B) and 5(C) represent the wavefronts of the zeroth-order and first-order diffracted light on the XZ plane. The object whose velocity is being measured moves only in the X-axis direction in Figure 5(A), and the Y-axis wavefront component perpendicular to the paper is not involved in the velocity measurement, so the wavefront on the YZ plane is omitted. In the graphs of Figures 5(B) and (C), the horizontal axis represents the beam position in millimeters, and the vertical axis represents the wavefront aberration in micrometers. The wavefront characteristics shown by the solid lines in Figures 5(B) and (C) show that a 1 mm change in the horizontal axis results in an increase of approximately 0.2 μm, and the wavefront shapes of both light beams are gently converging, similar to a plane wave.

[0018] In Figure 5(A), the zeroth-order diffracted light and the first-order diffracted light intersect at an angle of 2α, and interference between the two light beams begins near ΔZ=-5mm, with the interference of the light beams reaching a maximum on the center line of the optical interference (dashed line) in Figure 5(A). At ΔZ=-5mm, the zeroth-order diffracted light wavefront in Figure 5(B) intersects with the center line of the optical interference at ΔW=+0.87mm, and the first-order diffracted light wavefront in Figure 5(C) intersects with the center line of the optical interference at ΔW=-0.87mm. Here, the relationship ΔW=ΔZ cosα tanα holds.

[0019] At ΔZ=0 mm, the ΔW=0 mm point of the 0th-order diffracted light wavefront intersects with the center line of optical interference, and the ΔW=0 mm point of the 1st-order diffracted light wavefront intersects with the center line of optical interference. The point where the ΔZ=0 mm line and the center line of optical interference intersect is the focal point of the differential laser Doppler velocimeter.

[0020] Since the wave vector points in the direction perpendicular to the wavefront, the wave vector K0 (solid arrow in the figure) of the zeroth-order diffracted light wavefront points vertically at the point ΔW = 0 mm in Figure 5(B), and the wave vector K1 (solid arrow in the figure) of the first-order diffracted light wavefront also points vertically at the point ΔW = 0 mm in Figure 5(C). The positional relationship of these vectors is shown in the center of Figure 5(A) (the focus of the differential laser Doppler velocimeter, which is the point ΔZ = 0 mm). The wave vectors K0 and K1 coincide with the traveling direction of each light beam and form a perfect isosceles triangle.

[0021] On the other hand, at the point ΔZ=+5 mm, the 0th-order diffracted light wavefront (B) intersects with the center line of the optical interference at a point ΔW=-0.87 mm, and the 1st-order diffracted light wavefront (C) intersects with the center line of the optical interference at a point ΔW=+0.87 mm. At ΔW = -0.87 mm on the 0th-order diffracted light wavefront in Figure 5(B), the wave vector K0 (dashed arrow in the figure) is slightly tilted toward the beam center due to the influence of unevenness on the wavefront, while at ΔW = +0.87 mm on the 1st-order diffracted light wavefront in Figure 5(C), the wave vector K1 (dotted arrow in the figure) is significantly tilted toward the beam center due to the influence of unevenness on the wavefront. The positional relationship of these vectors is shown on the right side of Figure 5(A) (at ΔZ = +5 mm). Wave vectors K0 and K1 are tilted toward the beam center from the direction of propagation of each light beam, and because the tilt angles of the two are different, the isosceles triangle collapses and the difference vector K0 - K1 is larger than the difference vector at ΔZ = 0 mm.

[0022] In the configuration of the differential laser Doppler velocimeter shown in FIG. 5, the Doppler frequency fd from a moving object 0 with a velocity V can be expressed by the following equation, as shown in Non-Patent Document 3. 2πfd=(K0-K1)·V (1) Here, K0 is the wave vector of the 0th-order diffracted light, K1 is the wave vector of the 1st-order diffracted light, V is the velocity vector of the moving object 0, and · is the dot product of each vector.

[0023] If the 0th-order diffracted light and the 1st-order diffracted light are perfect plane waves, the wave vectors K0 and K1 will coincide with the direction of travel of each light beam, forming an isosceles triangle as shown in the center of Figure 5(A), Since |K0|=|K1|=2π / λ, The Doppler frequency fd is fd=2|V| cosψ sinα / λ (2) Here, ψ is the angle between the difference vector (K0-K1) and the velocity vector V, and λ is the oscillation wavelength of the semiconductor laser.

[0024] Conventional differential laser Doppler velocimeters calculate the velocity V of a moving object 0 from the measurement of the Doppler frequency fd based on the above equation (2). However, in an actual differential laser Doppler velocimeter, the wavefronts of the 0th-order diffracted light and the 1st-order diffracted light are not perfect plane waves. Therefore, there are some places where velocity errors occur.

[0025] At the point ΔZ=+5 mm, the wavefronts of the 0th-order diffracted light and the 1st-order diffracted light are uneven, so their wave vectors K 0、 The direction of K1 changes. At the point ΔZ=+5mm in Figure 5(A), the wave vector K 0、 The angle formed by K1 is the intersection angle between the 0th-order diffracted light and the 1st-order diffracted light that is larger than 2α, so the absolute value of the difference vector (K0 - K1) becomes larger. Since the actual velocity V does not change, fd also becomes larger. As a result, even though the actual velocity V does not change, the increase in fd results in an increase in the velocity error. This velocity error corresponds to the rise in the error curve near ΔZ = 5 mm in Figure 4.

[0026] It is theoretically impossible to realize a light beam that always forms a plane wave along the light propagation direction. Therefore, the wavefronts of the zeroth-order and first-order diffracted lights are adjusted so that they gradually change from a convergent bowl shape to a wavefront that approaches a plane wave as they move from ΔZ = -5 mm to +5 mm. In other words, the wavefront curvature gradually decreases in the positive Z direction. With this changing wavefront state, the ideal wavefront shape that eliminates the ΔZ dependence of the velocity error is the parabola shown by the dotted lines in Figures 5(B) and (C). If the wavefront curvature becomes a parabola shown by the dotted lines in Figures 5(B) and (C) and the irregularities disappear from both wavefronts, the velocity error of the differential laser Doppler velocimeter can always be zero.

[0027] The wavefront shapes of the zeroth-order and first-order diffracted light can be made closer to an ideal parabola by adjusting the distance between the semiconductor laser and the collimator lens. However, it is extremely difficult to remove the irregularities that exist on the wavefront. Evaluation of the optical components revealed that the irregularities on the wavefront were caused by the optical characteristics of the lenses and mirrors. The wavefront irregularities common to Figures 5(B) and (C) are due to optical distortion of the collimator lens, while the inconsistent wavefront irregularities arise from the non-flatness of the mirrors (6, 9, and 10 in Figure 3) inherent to the beam. A semiconductor laser light source is a point source of light on the order of micrometers or less, and it is extremely difficult to expand this beam and achieve a smooth wavefront over a width of 2.5 mm. To achieve a wavefront with minimal irregularities, wavefront adjustment is performed by changing the combination of lenses and mirrors. This trial and error method is the only way to suppress velocity errors.

[0028] In addition, velocity errors that are not based on the unevenness of the wave surface (for example, those due to positional fluctuations of the speedometer) also need to be suppressed in a similar manner. [Means for solving the problem]

[0029] A differential laser Doppler velocimeter that irradiates two laser beams separated from a laser light source onto a moving object and receives scattered reflected light from the object to measure the speed of the moving object based on the Doppler effect has a first optical receiving system that receives specular reflected light and scattered reflected light from the moving object, a second optical receiving system that receives scattered reflected light from the moving object, and a signal processing system connected to the first optical receiving system and the second optical receiving system, and the signal processing system calculates the distance between the moving object and the intersection of the two beams and the speed of the moving object based on the received signals from the first optical receiving system and the second optical receiving system.

[0030] The first optical receiving system detects specularly reflected light and scattered reflected light from the moving object, and the signal processing system calculates the distance between the moving object and the intersection of the two beams.

[0031] The signal processing system also has a logic operation circuit and a lookup table, and the lookup table stores the dependency of the speed error relative to the moving speed measured by the second optical receiving system on the distance between the moving object and the intersection of the two beams, and corrects the measured moving speed according to the measurement value of the distance between the moving object and the intersection of the two beams.

[0032] The first optical receiving system has a two-dimensional image sensor and detects the position and shape of the laser beam irradiated onto the moving object, the signal processing system calculates the distance between the moving object and the intersection of the two beams, and calculates the pitch angle and roll angle which are the inclination of the laser beam, and the signal processing system corrects the measured moving speed using the dependency characteristic of the speed error stored in the lookup table on the distance between the moving object and the intersection of the two beams, the measured value of the distance between the moving object and the intersection of the two beams, and the calculated values of the pitch angle and the roll angle. [Effects of the Invention]

[0033] The effect of the present invention is that in a laser Doppler velocimeter that measures the speed of a moving object, the measurement accuracy of the laser Doppler velocimeter can be improved by also measuring the distance between the moving object and the differential laser Doppler velocimeter. [Brief explanation of the drawings]

[0034] [Figure 1] 1 is a configuration diagram of a laser Doppler velocimeter according to a first embodiment. [Figure 2] 1 is a configuration diagram of a laser Doppler velocimeter according to a first embodiment. [Figure 3] FIG. 1 is a diagram showing the configuration of a conventional laser Doppler velocimeter. [Figure 4] 1A and 1B are diagrams illustrating problems with a conventional laser Doppler velocimeter. [Figure 5] 1 is a diagram for explaining the operation of a conventional laser Doppler velocimeter. [Figure 6]FIG. 10 is a configuration diagram of a laser Doppler velocimeter according to a second embodiment. [Figure 7] FIG. 10 is a configuration diagram of a laser Doppler velocimeter according to a third embodiment. [Figure 8] FIG. 10 is a configuration diagram of a laser Doppler velocimeter according to a third embodiment. [Figure 9] FIG. 10 is a configuration diagram of a laser Doppler velocimeter according to a fourth embodiment. [Figure 10] 10A to 10C are diagrams illustrating the operation of the laser Doppler velocimeter according to the fourth embodiment. [Figure 11] FIG. 10 is a configuration diagram of a laser Doppler velocimeter according to a fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0035] In a differential laser Doppler velocimeter, when the distance between the focal point, where two light beams intersect, and the surface of a moving object changes, velocity errors occur due to slight optical distortions of the lenses, mirrors, etc. used in the differential laser Doppler velocimeter. Distorted areas in optical components cause irregularities in the wavefront of the light beam, and interference between the irregular wavefronts of the zeroth-order diffracted light and the first-order diffracted light causes the velocity error to depend on the distance between the focal point and the surface of the moving object. Since distortions in lenses, etc. appear as velocity errors, if the relationship between the velocity error and the distance between the focal point and the surface of the moving object, as shown in Figure 4, can be understood, the velocity error can be corrected by detecting the distance ΔZ between the focal point and the surface of the moving object, enabling more accurate detection of the velocity of the moving object.

[0036] A differential laser Doppler velocimeter illuminates a moving object with two light beams, and by capturing the specular and diffuse reflected light from these two light beams with a photodetector using a triangulation method or similar, the distance ΔZ between the focal point and the surface of the moving object can be detected. If the velocity of the moving object detected using conventional methods is corrected using this distance information between the focal point and the surface of the moving object, an accurate velocity can be detected without error.

[0037] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes an embodiment of the present invention. However, the present invention is not limited to the following embodiments, and any combination thereof may be used. In addition, although the same reference numerals are generally used for the same components in the drawings relating to each embodiment, in order to avoid complication of explanation due to an increase in the number of reference numerals, they may be used independently in each drawing. Therefore, even if common reference numerals are used in other drawings, they may not necessarily have the same configuration as in other drawings.

[0038] [First embodiment] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a diagram showing the configuration of a differential laser Doppler velocimeter according to a first embodiment. In Example 1, a first optical displacement sensor 19 and a second optical displacement sensor 20 are added to the differential laser Doppler velocimeter in comparison with the conventional configuration example shown in Fig. 3, and the distance between the focal point and the surface of a moving object is calculated from the outputs of these sensors, and this distance information is used to correct the velocity measurement value of the moving object.

[0039] In the first embodiment shown in Figure 1, laser light source 1 is a semiconductor laser with a wavelength of 785 nm. By controlling the current injected into the laser and the temperature of the laser element, the semiconductor laser operates in single-mode oscillation and maintains constant output power. The laser beam from laser light source 1 is collimated by collimator lens 2. This laser beam is incident on frequency shifter (AOM) 3, which applies a 40 MHz signal to a portion of the incident laser beam, resulting in the emission of first-order diffracted light 8, which is shifted to S polarization, and zero-order diffracted light 12, which remains P polarization without frequency shifting. These emitted beams are incident on polarizing beam splitter 4, where they are split into a P-polarized beam and an S-polarized beam. The P-polarized beam travels straight to the transmission side, reflects off mirror 9, and is converted into a circularly polarized beam by λ / 2 wave plate 5, which is then irradiated onto moving object 0. The S-polarized beam from the polarizing beam splitter 4 is reflected by a mirror 6, and then converted into a circularly polarized beam by a λ / 2 wave plate 7, which is then irradiated onto the moving object 0.

[0040] A moving object 0 generates specularly reflected light and diffusely reflected light. A portion of the diffusely reflected light is collected by a light-receiving lens 13 and optically heterodyned by a light-receiving element 15. The beat signal frequency of the electrical signal output has a Doppler frequency fd centered around 40 MHz, corresponding to the Doppler shift of the object. This electrical signal is amplified by an amplifier 16 and mixed with a 35 MHz signal by a mixer 17 in a processor 26 to downconvert it to a 5 MHz + fd signal. Further, high-frequency noise is removed by a low-pass filter 18, and the signal is converted into a digital signal by an A / D converter 21 and input to an FPGA 25. Within the FPGA 25, the input signal is converted into a frequency-domain signal by a fast Fourier transform (FFT), the Doppler frequency fd proportional to the velocity of the moving object 0 is read, and the velocity V is calculated according to equation (2).

[0041] On the other hand, a first optical displacement sensor 19 and a second optical displacement sensor 20 were used to measure the distance between the focal point and the surface of the moving object. Figure 2 shows how the sensors are attached to a differential laser Doppler velocimeter. Figure 2 shows a configuration in which the specular reflection component of light from a moving object 0 is used to measure the distance between the focal point and the surface of the moving object. As shown in the front view in the lower left of Figure 2, the first optical displacement sensor 19 is installed directly above the exit window for the first-order diffracted light 8, and the second optical displacement sensor 20 is installed directly above the exit window for the zeroth-order diffracted light 12. The optical path of the specular reflection light of the light beam incident on sensors 19 and 20 is shown in the upper right of Figure 2.

[0042] Sensor 19 receives the specularly reflected light component of zeroth-order diffracted light 12, and sensor 20 receives the specularly reflected light component of first-order diffracted light 8. Optical displacement sensors 19 and 20 consist of lenses 31 and 32 and horizontally elongated, one-dimensional CMOS image sensors 33 and 34, and operate on the detection principle of triangulation. When the positional relationship between object 0 and the focal point changes, the optical path of the specularly reflected light of the light beam also changes, causing the imaging position on the CMOS image sensor to change, and the sensor outputs an electrical signal corresponding to the imaging position. In the first embodiment, two sensors 19 and 20 are used to calculate the difference in imaging position. The advantage of using two sensors is that the measurement error of ΔZ is small even if the installation pitch angle ψ of the differential laser Doppler velocimeter deviates from 0°, and the measurement error is also small for changes in the surface angle during acceleration and deceleration of moving object 0.

[0043] The output difference d of the electrical signals from the sensor 19 and the sensor 20 has the following relationship with the distance ΔZ between the focal point and the surface of the moving object 0: ΔZ = ad + b (3) Here, a and b are constants whose accurate values can be obtained by measurement and calibration on an actual device. These a and b are used by the processor 26 as constant values for measuring ΔZ.

[0044] The bottom right of Figure 2 shows an installation diagram of the differential laser Doppler velocimeter viewed from the side. The differential laser Doppler velocimeter was installed with a yaw angle θ of 3° so that the specularly reflected light from object 0 would efficiently enter sensors 19 and 20. With the setup in Figure 2, even if the yaw angle of the differential laser Doppler velocimeter deviates from zero, there is no effect on the measured velocity. Note that the pitch angle ψ and roll angle φ affect the Doppler frequency fd and velocity V, and are related by the following equation: V = fd λ / (2 sinα cosψ cosφ) (4) By including the pitch angle ψ and roll angle φ in the velocity calculation, more accurate velocity measurements can be made.

[0045] In FIG. 1, the outputs of the sensors 19 and 20 are converted into digital signals by A / D converters 22 and 23 in a processor 26. The signal is digitized and input to the FPGA 25. Within the FPGA, the output difference d of the electrical signal is calculated from the difference between the outputs of the A / D converters 22 and 23, and the distance ΔZ between the focal point and the surface of the moving object 0 is calculated from equation (3) and the calibrated constants a and b.

[0046] Meanwhile, the dependency of the velocity error of the differential laser Doppler velocimeter on the amount of deviation from the focus is written into the look-up table (LUT) in the FPGA 25. The velocity error data has a granularity of 0.1 mm for ΔZ.

[0047] Meanwhile, velocity calculations via FFT in the FPGA are performed every 10 ms. To match this output cycle, the distance ΔZ between the focal point and the surface of the moving object is calculated every 10 ms, and velocity error data is read from the LUT based on the ΔZ value. A correction calculation is performed on the velocity calculation result using the velocity error data, and a corrected velocity closer to the true value is output.

[0048] It is also possible to input the pitch angle ψ and roll angle φ when the differential laser Doppler velocimeter is installed into FPGA 25 so that velocity correction can be performed using equation (4). Because the physical distance between the differential laser Doppler velocimeter and the surface of object 0 cannot always be set to ΔZ = 0, the deviation in the distance between the focus at the time of setting and the surface of the moving object is input as a zero-point correction value and calibrated within FPGA 25.

[0049] The FPGA 25 outputs the uncorrected speed, the distance traveled which is the time integral of the uncorrected speed, the corrected speed obtained by correcting the error from ΔZ, the corrected distance, ΔZ, and the rate of change of ΔZ. Using the above configuration, velocity measurements were performed using a glossy organic film. The organic film was made into a loop, and tension was applied between two pulleys to cause it to rotate. The surface of the moving organic film was placed horizontally on the floor, and the differential laser Doppler velocimeter of the first embodiment was placed above it, and set to project a light beam onto the film surface. The pitch angle ψ and roll angle φ were both set to 0°, and ΔZ was allowed to vary from -10 mm to +10 mm.

[0050] The moving speed of the organic film was set to exactly 6 meters per minute, and the light beam of the differential laser Doppler velocimeter was irradiated onto it. The total power of the two light beams was 36 mW, and with ΔZ = 0 mm, the diffuse reflected light power incident on the APD of the light receiving element 15 was 5 μW. This output provided a signal with a good signal-to-noise ratio, allowing the velocity V of the organic film to be measured. The velocity of the organic film was also measured while changing the distance ΔZ between the focal point and the surface of the moving object. Changing ΔZ caused the measured velocity V to fluctuate slightly, and the velocity error had the characteristics shown in Figure 4. The velocity error characteristics of Figure 4 were written in 0.1 mm increments into the lookup table (LUT) of the FPGA 25.

[0051] Sensors 19 and 20, which measure the distance ΔZ between the focal point and the surface of the moving object, receive specularly reflected light from moving object 0, and therefore each receive an optical input of 30 μW. This optical input is powerful enough to determine the position output of the CMOS image sensor, and also achieves a position resolution of 0.1 mm or less for ΔZ. In addition, the electrical output bandwidth of sensors 19 and 20 is 10 MHz or more, so there is sufficient response margin for readout every 10 ms and speed correction calculation processing within FPGA 25.

[0052] An evaluation was carried out using the distance measurement function between the focal points of sensors 19 and 20 and the surface of a moving object, as well as the LUT and speed correction function in FPGA 25. Speed measurements were performed at ΔZ = -5 mm, 0 mm, and +5 mm, with the speed correction function operating to measure the corrected speed. As a result, the speed error was within ±0.01% at each ΔZ.

[0053] The above measurements were evaluated with the distance between the focal point and the surface of the moving object fixed, but it is also possible to handle situations where the surface position of object 0 fluctuates over time. For example, it is also possible to handle a motion system in which the film surface moves up and down at high speed in a moving transport system with an eccentric pulley shaft. In the first embodiment, the speed calculation time for the Doppler frequency fd in the FPGA 25 was set to 10 ms, but this can be increased to 1 ms.

[0054] The response speed of sensors 19 and 20 is 100 ns or less, and the speed correction calculation is a simple calculation of reading the error value from the LUT and multiplying it by the calculated speed, so the speed correction calculation can be performed as fast as 1 ms. Furthermore, as long as the distance between the focal point and the surface of the moving object 0 can be found for ΔZ, it does not have to be the above equation (3).

[0055] In this example, two sensors were used to measure the distance between the focal point and the surface of the moving object, but it is also possible to use one sensor. In this case, it is desirable that the yaw angle deviation is clear, the surface of the moving object is perpendicular to the perpendicular line of the two laser beams, and the specular reflection light of the beams is incident on the sensor as set.

[0056] Furthermore, although horizontally long, one-dimensional CMOS image sensors 33 and 34 are used for the optical displacement sensors 19 and 20 in this embodiment, cheaper PSD elements (Position Sensitive Devices) or CCD image sensors may also be used. Any embodiment that can measure the distance between the focal point and the surface of a moving object can be included in the present invention.

[0057] In this embodiment, the frequency shift element 3 is used to give a frequency shift of 40 MHz to the first-order diffracted light, but the frequency shift element 3 may be omitted. By eliminating the frequency shift element 3, the speedometer output will be only the absolute value of the moving speed without any information on the moving direction, but the speedometer can be simplified.

[0058] [Second embodiment] Fig. 6 is a diagram showing the configuration of the optical part of a differential laser Doppler velocimeter of the second embodiment. Example 2 is a modified example of the optical part of Example 1 shown in Fig. 2, and has a mechanism in which the yaw angle does not need to be precisely set when installing the velocimeter, and the light beam of the velocimeter only needs to be set in the perpendicular direction to the moving object 0.

[0059] A first half mirror 27 and a second half mirror 28 were used between the first optical displacement sensor 19, the second optical orientation sensor 20, and the moving object 0. The transmittance of the half mirrors 27 and 28 was set to 50%. The downward light beams reflected by the half mirrors 27 and 28 were sufficiently attenuated by optical terminators to prevent light leakage to the sensors 19 and 20. The configuration of FIG. 6 makes it possible to install the ΔZ measurement sensors 19 and 20 in the space above the differential laser Doppler velocimeter, thereby reducing the number of manufacturing steps. The processing unit connected to the optical section of the differential laser Doppler velocimeter of the second embodiment shown in FIG. 6 has the same configuration as the processing unit 26 in the first embodiment shown in FIG. 1.

[0060] [Third embodiment] FIG. 7 is a diagram showing the optical configuration of a laser Doppler velocimeter according to a third embodiment. Example 3 utilizes scattered light components reflected from a moving object 0 to measure ΔZ. A single optical displacement sensor 19 is used, and as shown in FIG. 7, the sensor 19 is positioned above the differential laser Doppler velocimeter. The position of the beam projected onto a vertically elongated, one-dimensional CMOS image sensor 33 via a lens 31 is detected, and the distance ΔZ between the focal point and the surface of the moving object is calculated. An optical bandpass filter 37 with a center wavelength of 785 nm and a bandwidth of 10 nm is used within the sensor 19 to remove noise light. FIG. 8 shows the configuration of a laser Doppler velocimeter including a processor 26. Because a single sensor 19 is used, only a single AD converter 22 is required to convert the beam position information into a digital signal.

[0061] In this embodiment, a single sensor 19 is used to calculate the distance between the focal point and the surface of the moving object from the position where the scattered reflected light from object 0 is projected onto the CMOS image sensor 33, and the calculation formula is the same as formula (3) used in the first embodiment. The electrical signal output L indicating position information from the optical direction sensor 19 has the following relationship with the distance ΔZ between the focal point of object 0 and the surface of the moving object. ΔZ= a L + b (5) Here, a and b are constants whose accurate values can be found through measurements and calibration on an actual device. These constants are input into FPGA 25, and the distance ΔZ between the focal point of object 0 and the surface of the moving object is calculated from the electrical signal output L. Other processing within FPGA 25 is the same as in the first embodiment, and the dependency of the velocity error of the differential laser Doppler velocimeter on the amount of deviation from the focal point is written into the lookup table (LUT) within FPGA 25. The velocity error data has a granularity of ΔZ of 0.1 mm.

[0062] Velocity calculations were performed every 10 ms via FFT on the FPGA. In line with this output cycle, the distance ΔZ between the focal point and the surface of the moving object was calculated every 10 ms, and velocity error data was read from the LUT based on the ΔZ value. A supplementary calculation was performed on the velocity calculation results and velocity error data, outputting a corrected velocity closer to the true value.

[0063] The FPGA 25 also inputs the pitch angle ψ and roll angle φ when the differential laser Doppler velocimeter is installed, and performs velocity correction using equation (4). The deviation in the distance between the focal point and the surface of the moving object at the time of setting is also input as a zero-point correction value, and is calibrated within the FPGA 25. The output of FPGA25 is the uncorrected velocity, the distance traveled which is its time integral, the corrected velocity obtained by error correction from ΔZ, the corrected distance, ΔZ, and the velocity change of ΔZ.

[0064] Using the above configuration, velocity measurements were performed using a rubber belt with a high level of scattered reflected light. The rubber belt was rotated by applying tension between two pulleys. The surface of the rubber belt was placed horizontally to the floor, and the differential laser Doppler velocimeter of this example was placed above it, with a light beam projected onto the rubber belt surface. The pitch angle ψ and roll angle φ were both set to 0°, and the distance ΔZ between the focal point and the surface of the moving object was allowed to vary from -10 mm to +10 mm.

[0065] The rubber belt was set to a moving speed of precisely 10 meters per minute, and a 785 nm light beam from a differential laser Doppler velocimeter was irradiated onto it. The total power of the two light beams was 36 mW, and with ΔZ = 0, the diffusely reflected light power incident on the APD of the light-receiving element 15 was 20 μW. A signal with a good signal-to-noise ratio was obtained from this output. The optical input to the sensor 19 was 10 μW, which was sufficient power to output position information from the CMOS image sensor, and could achieve a position resolution of less than 0.1 mm in measuring the distance between the focal point and the surface of the moving object.

[0066] The distance ΔZ between the focal point of the optical displacement sensor 19 and the surface of the moving object was measured, and an evaluation was carried out using the LUT and speed correction function in the FPGA 25. Speed measurements were performed at ΔZ = -5 mm, 0 mm, and +5 mm, and the speed correction function was activated to measure the corrected speed. At each ΔZ, the speed error was within ±0.01%.

[0067] In addition, FPGA25 can also extract information on changes in the distance ΔZ between the focal point and the surface of the moving object, so the correlation between ΔZ and the speed of the rubber belt can be used to analyze malfunctions in the movement system using the rubber belt.

[0068] In this embodiment, a CMOS image sensor 33 is used as the sensor 19, but a PSD A position sensitive device or a CCD image sensor may also be used. Furthermore, the number of sensors 19 is not limited to one, and for example, two sensors may be used, one above the other, to sandwich the differential laser Doppler velocimeter, and the scattered reflected light may be received differentially to improve the accuracy of the distance between the focal point and the surface of a moving object. Also, although an optical bandpass filter 37 is used within sensor 19, the optical bandpass filter may be omitted if natural light does not leak into the sensor.

[0069] [Fourth embodiment] 9 is a configuration diagram of the optical part of a laser Doppler velocimeter according to the fourth embodiment. Example 4 is characterized in that the position and shape of the light beam of the differential laser Doppler velocimeter are observed by a camera 35 using a two-dimensional CMOS image sensor, the distance between the focal point and the surface of a moving object is measured from the beam position, and the roll angle φ and pitch angle ψ of the differential laser Doppler sensor in its installed state are measured from the beam shape and used to correct the measured velocity.

[0070] In Figure 9, camera 35 uses a 1280x1024 pixel high-speed CMOS image sensor and is equipped with a macro lens 36 to capture detailed images of the light beam reflected on nearby moving object 0. An optical bandpass filter 37 with a center wavelength of 785 nm and a bandwidth of 10 nm was installed in front of lens 36 to remove noise light. The pitch angle of camera 35 was set to 0° so that it was perpendicular to the surface of moving object 0, and the roll angle was set to 0° so that it was perpendicular to the direction of movement of moving object 0. The yaw angle and camera position were set so that the light beam was centered at the camera's center when the distance between the focal point and the surface of the moving object was ΔZ = 0 mm. Camera 35 was fixed with support 30 to prevent it from being affected by external vibrations. The differential laser Doppler velocimeter was fixed with support 29. A rubber belt was used as moving object 0, and ΔZ = 0 mm was set 100 mm ahead of the differential laser Doppler velocimeter.

[0071] Figure 10(A) is an image of a light beam on a rubber belt captured by camera 35. The light beams are overlaid when the differential laser Doppler velocity is set to ΔZ = 0 and -5 mm, showing the 13.5% light intensity positions when only the zeroth-order diffracted light 12 and the first-order diffracted light 8 are irradiated, respectively. When ΔZ = 0 mm, the two light beams coincide, and when ΔZ = -5 mm, the two beams do not overlap. The light beams are shaped using a collimating lens and a cylindrical lens, resulting in a flat beam with a width of 2.5 mm and a height of 0.3 mm. The differential laser Doppler velocimeter is set to a roll angle of -1° and a pitch angle of +1°, so the right side of the light beam is tilted upward, and the center position of each beam is shifted to the left.

[0072] Figures 10(B) and (C) are traces of the maximum light beam intensity mapped in the X-axis direction of the image sensor, with the vertical axis representing the intensity, with the maximum value when the zeroth-order diffracted light and the first-order beam are combined being 100%. Figure 10(B) shows the light beam when ΔZ = 0 mm, where the zeroth-order and first-order beams overlap and the combined light beam is single-peaked. Figure 10(C) shows the light beam when ΔZ = -5 mm, where the zeroth-order and first-order beams have different peak positions and the combined light beam is double-peaked. Although the light beam shapes in Figures 10(B) and (C) are different, by calculating the center of gravity G of these light beams and the end points L and R of the beam shapes, the pitch angle, roll angle, and ΔZ of the differential laser Doppler velocimeter installation can be calculated. The distance between the focal point and the surface of the moving object and the pitch angle can be calculated from the center of gravity G of the light beam intensity. The calculation method is shown below.

[0073] When the zeroth-order beam and the first-order beam are irradiated onto a rubber belt simultaneously, the information for each pixel obtained from the two-dimensional image sensor is Voutij = (Xi, Yj, Iij). Here, i and j are integers, Xi and Yj are the coordinate position of each pixel, and Iij is the intensity information of the light beam. If each pixel information from the image sensor is observed on the X-axis and used as the X-axis mapping component, and only the maximum light beam intensity IMAXi is updated and rewritten, the information becomes VXOUTi = (Xi, IMAXi).

[0074] Here, the center of gravity Xg on the X axis can be calculated by calculating Xg = Σ( Xi IMAXi ) / Σ( IMAXi ). Note that Σ is an abbreviation that represents the sum of elements from i=0 to i=1279. Furthermore, if the maximum light beam intensity IMAXj is updated on the Y axis to find VYOUTj = (Yj,IMAXj), the center of gravity Yg on the Y axis can be found from Yg = Σ(YjIMAXj) / Σ(IMAXj), where Σ means the sum of the elements from j=0 to j=1023. When the zeroth-order beam and the first-order beam are irradiated simultaneously, the center of gravity position G is expressed as coordinates (Xg, Yg). From this result, the distance ΔZ between the focal point and the surface of the moving object can be calculated from the following equation, which is a modification of equation (5). ΔZ= a Yg + b (6) Here, a and b are constants whose accurate values are determined by measurement and calibration on the actual equipment.

[0075] The pitch angle ψ can be calculated from the following equation (7) using the distance from the lens 13 to the object 0=100 mm+ΔZ and Xg of the center of gravity position G. ψ=Arctan( ( c Xp) / (100mm+ΔZ)) (7) Here, c is a constant related to the installation position and angle of the camera 35 and the camera lens focus, and an accurate value is obtained by measuring the actual device.

[0076] The roll angle is calculated from the end points L and R of the beam shape where the intensity of the light beam is 13.5% of the maximum value IMAX. If the point where Xi is minimum and the point where Xi is maximum are found from the image sensor output under the condition Iij = 0.135 IMAX, end point L has coordinates (Xmin, YL) and end point R has coordinates (Xmax, YR). From these coordinate positions, the roll angle φ can be calculated using the following equation (8). φ=Arctan(( YR - YL) / ( Xmax - Xmin )) (8)

[0077] Figure 11 shows the configuration of a laser Doppler velocimeter including a processing unit 26. A control interface CONT for controlling a camera 35 using a two-dimensional CMOS image sensor is configured within the FPGA 25. An interface for processing the digital output signal of each pixel from the camera 35 and a function for calculating the position and shape of the light beam are also configured within the FPGA 25. The function of calculating the distance between the focal point and the surface of a moving object from the light beam position and transmitting this to the LUT is the same as in the first embodiment, but the fourth embodiment is characterized by automatically detecting the pitch angle ψ and roll angle φ of the differential laser Doppler velocimeter using the angle detection function of the camera 35 and FPGA, and further adding angle-dependent velocity correction to the corrected velocity using the distance between the focal point and the surface of a moving object.

[0078] Using the above configuration, we performed velocity measurement and evaluation using a rubber belt with a high level of scattered and reflected light. The rubber belt was rotated between two pulleys under tension. The moving surface of the rubber belt was placed horizontally relative to the floor, and a camera 35 using a two-dimensional CMOS image sensor was installed above it, perpendicular to the surface of the rubber belt. The camera 35 was operated at an operating speed of 100 fps. The differential laser Doppler velocimeter was set to a pitch angle of +1° and a roll angle of -1°, a wavelength of 785 nm, and a distance ΔZ between the focal point and the surface of the moving object was set to 0 mm. The rubber belt was rotated at a speed of 5 meters / min, and the pitch angle ψ and roll angle φ detection functions of the differential laser Doppler velocimeter were evaluated.

[0079] The magnification of the macro lens 36 connected to the camera 35 was adjusted so that the light beam width occupied one-third of the X-axis of the image sensor, enabling detailed measurement of the position and shape of the light beam. Furthermore, since the settings of the pitch angle ψ and roll angle φ do not change over time, the calculation of these angles was performed using a moving average of 100 frames of camera output to reduce noise and improve measurement accuracy. The output results from the FPGA 25 were highly accurate, with a pitch angle ψ = +1.0° and a roll angle φ = -1.0°, and the resolution of each angle was 0.05° or less.

[0080] In addition, because the change in ΔZ, the distance between the focal point and the surface of a moving object, can be rapid, ΔZ measurement was performed using a moving average output every 10 frames, or 10 ms. The measurement error of ΔZ was 0.1 mm or less. Next, the surface position of the rubber belt was adjusted so that ΔZ = -5 mm, and the pitch and roll angles were measured. In this case, the light beam shape became bimodal, as shown in Figure 10(C), but there was no error in each angle measurement, and the angular resolution was within 0.05°. The ΔZ measurement error was also within 0.1 mm. The speed of the rubber belt was measured at ΔZ = -5 mm. As a result, the speed measurement result was smaller than the actual speed, but by applying the speed error correction value written to the LUT and speed correction for a pitch angle ψ = +1° and a roll angle φ = -1°, the corrected speed output was the same as the actual speed. These corrected speeds and the distance ΔZ between the focal point and the surface of the moving object were output from the FPGA 25 every 10 ms.

[0081] In this embodiment, the moving speed of the rubber belt can be accurately measured even if the distance between the rubber belt surface position and the focus of the light beam varies due to eccentricity of the pulleys or unevenness of the rubber belt. In addition, the camera 35 using a two-dimensional CMOS image sensor can accurately measure the pitch angle and roll angle of the differential laser Doppler velocimeter, so more accurate corrections that take into account this angle deviation information can be reflected in the speed measurement. [Industrial Applicability]

[0082] The present invention can measure the speed, length, and distance of an object more accurately and without contact, thereby enabling the conveyance speed to be controlled with high precision during the production of high-performance films, contributing to improved film quality. The present invention can also be applied to measuring the dimensions of steel billets in rolling processes in the steel industry and controlling the speed of rolling processes, measuring the accurate speed and travel distance of trains on railways, measuring the accurate speed and travel distance of inspection vehicles in road infrastructure, measuring the accurate speed and travel path of vehicles in the automotive industry, and measuring the accurate speed and length of manufactured products in industrial equipment in the chemical industry, construction industry, etc. Furthermore, the differential laser Doppler velocimeter outputs a pitch pulse corresponding to the accurately measured speed, which enables precise image capture of accelerating and decelerating vehicles, contributing to high-speed visual inspection of railway trains. [Explanation of symbols]

[0083] 1. Laser light source 2. Collimator lens 3. Frequency shift element 4. Polarizing beam splitter 5. Modulator driver 6. Mirror 7, λ / 2 wavelength plate 8. First-order diffracted light 9. Mirror 10. Mirror 11, λ / 2 wavelength plate 12. 0th-order diffracted light 13. Lens 14. Mirror 15. Photodetector 16. Amplifier 17. Mixer 18. Low-pass filter 19. Sensor 20. Sensor 21, 22, 23, AD converter 24. Oscillator 25. FPGA 26, processing unit 27, 28, Half mirror 29, 30, Support 31, 32, Lens 33, 34, CMOS image sensor 35. Camera 36. Macro lens 37. Optical bandpass filter 0, object

Claims

1. A differential laser Doppler velocimeter measures the velocity of a moving object based on the Doppler effect by irradiating two laser beams separated from a laser light source onto the object and receiving scattered reflected light from the object, a first optical receiving system that receives specularly reflected light and scattered reflected light from the moving object; a second optical receiving system that receives scattered reflected light from the moving object; a signal processing system connected to the first optical receiving system and the second optical receiving system; The signal processing system is characterized in that it calculates the distance between the moving object and the intersection of the two beams and the velocity of the moving object based on the received signal from the first optical receiving system and the received signal from the second optical receiving system.

2. In the differential laser Doppler velocimeter according to claim 1, the first optical receiving system detects specularly reflected light and scattered reflected light from the moving object, and the signal processing system calculates the distance between the moving object and the intersection of the two beams.

3. In the differential laser Doppler velocimeter according to claim 1, the signal processing system has a logic operation circuit and a lookup table, and the lookup table stores the dependency characteristic of the velocity error relative to the moving velocity measured by the second optical receiving system on the distance between the moving object and the intersection of the two beams, and corrects the measured moving velocity in accordance with the measurement value of the distance between the moving object and the intersection of the two beams.

4. In the differential laser Doppler velocimeter according to claim 3, the first optical receiving system has a two-dimensional image sensor and detects the position and shape of the laser beam irradiated onto the moving object, the signal processing system calculates the distance between the moving object and the intersection of the two beams, and calculates the pitch angle and roll angle which are the inclinations of the laser beam, and the signal processing system corrects the measured moving velocity using the dependency characteristic of the velocity error stored in the look-up table on the distance between the moving object and the intersection of the two beams, the measured value of the distance between the moving object and the intersection of the two beams, and the calculated values of the pitch angle and the roll angle.

Images