System and method for on-axis measurement of surface movement based on the Doppler effect

Abstract

A system and method for optically measuring the axial divergence of a surface at an angle relative to the surface. The optical system includes a laser source and a detector. A laser beam of coherent light is emitted axially from the laser source of the optical system toward the surface while the surface is moving transversely relative to the emitted laser beam, either because the system is attached to a moving object such as a truck, or because the surface is moving, e.g., a conveyor belt. The laser beam is measured at 1 / e of the surface. 2 The beam radius is [Equation 1] JPEG2025527462000040.jpg1726 where λ is the wavelength of the emitted laser beam and L is the distance from the laser source to the surface. A detector is used to measure the optical signal of the reflected beam from the surface, and the Doppler frequency of the reflected beam is determined. Based on the Doppler frequency, the movement of the surface along the axis is determined.

Description

[Technical Field]

[0001] The present invention relates to the field of deflectometry and surface geometry divergence, in particular to non-contact deflectometry based on Doppler shift measurements. [Background technology]

[0002] Measuring the deflection of a surface under a given load is known to be useful in determining the properties of that surface, such as load-bearing capacity, durability, wear, and even the integrity of the underlying material.

[0003] One known method for determining the deflection of a surface moving relative to a deflectometer is to detect the Doppler shift of coherent light reflected from the deflected surface.

[0004] Doppler-based deflectometry offers several advantages. One such advantage is that the measurement is non-contact, i.e., based on optical reflection. Another advantage is that the measurement is performed while the deflectometer, the surface, or both are moving, eliminating the need to completely pause the operation of the system to take a measurement. For example, if the surface being measured is a road, the deflectometer can be installed in a vehicle, eliminating the need to block the road and stop traffic to take a measurement. Similarly, if the surface being measured is a conveyor belt, the conveyance does not need to be stopped while measurements are taken.

[0005] However, due to their fundamental principle of high sensitivity to small changes in the distance between the sensor and the surface, these Doppler-based deflectometers are also highly sensitive to speckle decorrelation on optically rough surfaces, as well as to surface contamination such as sand and gravel on roads or conveyor belt structures that increase friction. This speckle decorrelation causes signal fluctuations that indicate characteristics other than the deflection of the object. A proposed solution to this problem is receiver diversification, which allows the deflectometer system to maintain a constant distance to the surface being detected. Such compensation for speckle decorrelation is technically complex and expensive. Summary of the Invention

[0006] Numerous objects and advantages which will become apparent from the description of the invention are obtained, in accordance with a first aspect of the invention, by:

[0007] 1. A method for optically measuring the axial divergence of a surface at an angle relative to said surface, said method comprising: providing an optical system, the optical system including a laser source and a detector; emitting a laser beam of coherent light from the laser source of the optical system in the axial direction toward the surface while the surface is moving laterally relative to the emitted laser beam; The laser beam has a beam radius on the surface of

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[0008] The axial direction is understood to be a first direction intersecting the surface, in which the divergence between the surface and the underlying material is measured. Divergence refers to the dynamic change in the distance between the detector and the surface, for example, due to deflection or vibration under an applied force. The optical system is positioned at a distance L from the surface along the axial direction. Coherent light emitted from the laser light source is emitted in the axial direction. In some variations, the optical system may be positioned so that the axial direction is substantially perpendicular to the surface, e.g., 0 to 10 degrees from perpendicular, typically 0 to 2 degrees from perpendicular. In some variations, the optical system may be positioned so that the axial direction is at an angle between 30 degrees and 150 degrees relative to the surface. In some variations, the angle between the optical system and the surface may change during measurement. For example, if the change in the distance between the surface and the optical system is measured due to deflection of the surface, the load causing the deflection of the surface may also change the angle. Furthermore, depending on the relative positions during measurement, the angle may change due to surface irregularities, and therefore the angle may change depending on the relative movement of the optical system and the surface.

[0009] Because the axial velocity depends on how much the material below the surface is affected by the deflection force, the force causing the divergence in the surface and underlying material, e.g., the applied force, must move laterally relative to the surface for the axial velocity to occur. Alternatively, standing waves or vibrations through the material can generate the axial velocity. For example, the deflection means, i.e., the means for applying the deflection force, can be mounted with the optics such that the optics and the deflection force move together relative to the surface, thereby causing the axial velocity to change in response to a given deflection force as the material deflects to varying degrees depending on the structural strength of the material. In variations in which the means for applying the deflection force is fixedly mounted relative to the optics, the divergence-causing force and the optics move together laterally relative to the surface.

[0010] By knowing the distance of the relative lateral movement, the measured Doppler shift can be correlated to a location on the surface, for example, identifying locations where the material is weaker and the bending forces will result in a greater axial velocity than locations where the material is stronger.

[0011] Measuring the reflected beam axially, i.e., along the axis of the emitted laser beam, interference occurs between the emitted coherent laser beam and the Doppler-shifted reflected beam. This interference pattern can be measured with a detector, and from such measurements, the relative velocity between the optical system and the surface can be determined. While it is the reflection from the surface that is measured, and therefore the beam width at the intersection with the surface is of interest, it should be understood that the measurement relates to properties of the underlying structure, such as the integrity of the material and how much the material can deflect under an applied deflection force, rather than the surface structure.

[0012] Coaxial measurement of reflection, i.e., the coherent laser beam and the reflected beam are coaxial, is preferred because it simplifies the measurement. However, it should be understood that in other variations, the system may be configured to measure a reflected beam that is not coaxial with the emitted coherent laser beam, such as by collecting the reflected beam at an angle or directing it to the detector via a separate path. The benefits of an optimized beam width are not limited to coaxial measurements.

[0013] The optical system is adapted to provide a laser beam having a beam radius within a certain range, and the arrangement of the components of the optical system is such that, at a distance L, the beam radius of the Gaussian beam emitted from the laser source of the optical system is within a range where the light intensity is 1 / e 2 In other words, the beam radius of the present invention is determined according to the Gaussian beam radius criterion, and the intensity of the Gaussian beam is determined within a range of 1 / e from the maximum intensity at the central axis of the beam. 2 This applies to both circular and elliptical beam profiles, where at least one of the major or minor radii satisfies the beam radius requirement. For example, in the case of an elliptical beam, a larger radius may satisfy the requirement, but a smaller radius would be significantly smaller, providing the benefit of a wider beam according to the present invention while still benefiting from the higher power of the reflected laser beam due to its smaller radius. Such an arrangement may result from a combination of the selection of the laser source, the guidance means within the optical path, such as an optical fiber, and the selection of one or more lenses for shaping the laser beam and their relative distances. Those skilled in the art will understand that several combinations can be selected, all of which result in a beam radius according to the present invention, for example, by arranging multiple lenses in sequence.

[0014] Increasing the beam radius is beneficial to minimize the adverse effects of speckle decorrelation, while decreasing the beam radius is beneficial to increase the power of the reflected beam, improving measurement accuracy. The beam radius on the surface is

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[0015] According to a further embodiment of the first aspect of the present invention, the distance that the optical system has moved laterally relative to the surface and / or the rate of relative lateral movement is monitored.

[0016] In some variations, the optical system is mounted with a means for applying a deflection force that causes the surface to diverge, forming a composite system. In such variations, monitoring the distance the composite system moves laterally relative to the surface allows for correlation between the measured Doppler frequency and the surface position corresponding to that measurement. The distance moved may be measured, for example, by a wheel sensor that accurately determines distance based on rotation. In other variations, the rate of relative lateral movement is monitored and used to determine the distance traveled during the measurement time, allowing correlation between the point-to-point positions of the measured Doppler frequency and the surface position corresponding to the measurement.

[0017] According to a further embodiment of the first aspect of the present invention, the relative movement between the surface and the optical system is due to movement along the surface of a vehicle on which the optical system is mounted.

[0018] Mounting the optical system on a self-propelled vehicle has the advantage that the optical system is mobile and therefore easy to use on the move, for example for measuring roads, etc. In another variant, the optical system may be mounted separately from the vehicle, for example on a trailer that can be transported by another vehicle.

[0019] The vehicle or trailer on which the optical system is mounted may also include auxiliary devices, such as weights, that can be applied to the surface to induce deflections while the optical system is measuring the deflections. The loads may be adjustable or fixed.

[0020] According to a further embodiment of the first aspect of the present invention, the relative movement between the surface and the optical system depends on a translation and / or rotation of the surface while the optical system remains stationary.

[0021] Statically mounted optics are useful for surfaces that are positioned to move, such as conveyor bells or moving, e.g., rotating, parts of machinery. In such systems, the stability or wear of the moving part may be monitored with statically mounted optics. For example, if the measured Doppler shift exceeds a certain threshold, it may indicate that the part is worn or mounted at an angle.

[0022] According to a further embodiment of the first aspect of the present invention, the measurement results are transmitted to a data processing unit for storage and / or processing.

[0023] Sending the measurement results to an external data processing unit has several advantages. This potentially allows for a smaller optical unit due to the need for less on-board processing power. Furthermore, the external data processing unit may receive data from multiple optical systems, allowing for central mapping of the surface and / or calibration of one or more optical systems based on a reference optical system that also sends measurement data to the external data processing unit. The external data processing unit may also contain data from previous measurements of the surface, allowing for comparison and monitoring of changes over time, e.g., due to wear.

[0024] Another object of the present invention is to provide an optical system for use in the aforementioned method.

[0025] According to a second aspect of the present invention, the above objects and advantages are obtained by:

[0026] 1. An optical system for use in a method for measuring surface divergence according to the present invention, said optical system comprising: a laser source for emitting coherent light; a first optical path positioned to direct coherent light from the laser source for directing a coherent laser beam onto a surface, the first optical path being polarization-maintaining; a detector for detecting a reflected beam of light reflected from the surface; a second optical path for collecting and directing the reflected beam to the detector, the second optical path being polarization-maintaining; a polarizing beam splitter disposed between the first optical path and the second optical path for redirecting the on-axis reflected beam into the second optical path; The optical system is configured such that the beam radius at the intersection with the surface is

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[0027] A laser light source is understood to be any light source configured to emit coherent light, i.e. laser light, which may be any known laser light source, such as, but not limited to, a gas laser, a solid-state laser, a fiber laser, a dye laser or a laser diode.

[0028] In some variations, the laser source may emit coherent light at a wavelength in the range of 1000 nm to 3000 nm, for example in the range of 1400 nm to 2000 nm, such as 1500 nm to 1600 nm, with a wavelength of 1550 nm typically being a cost-effective solution due to the availability of suitable lasers.

[0029] The detector is adapted to perform a measurement correlated to the axial velocity between the optical system and the surface used for the measurement. In some variations, the detector is capable of measuring the intensity of the reflected beam of light reflected from the surface, thus determining an interference pattern and allowing the Doppler frequency to be measured.

[0030] In some preferred variations, the optical system further includes a bypass optical path arranged to direct a portion of the coherent light emitted from the laser light source directly to the detector.

[0031] Having a bypass optical path allows a wider range of detectors to be used in the optical system, as light directed directly from the laser source can interfere with reflected light in the detector.

[0032] In such a preferred variant, in which the optical system includes a bypass optical path, an acousto-optical modulator (AOM) is arranged in the bypass optical path, such an AOM being able to shift the frequency of the emitted laser light.

[0033] The first and second optical paths are understood to be any means for directing light to and from components of an optical system according to the present invention. For example, the optical paths may include optical fibers, free space paths directed by lens systems, and / or other waveguides. In some variations, all optical paths may be of the same type, e.g., all optical paths may be based on optical fibers, while in other variations, the first optical path, second optical path, and / or bypass optical path may be of one or more different types.

[0034] The first and second optical paths include optical fibers, allowing the laser source and detector to be positioned relative to each other in the optical system configuration. By making the first and second optical paths polarization-maintaining, a polarizing beam splitter can be used to split the emitted laser beam and the reflected beam, and the reflected light can be controlled to be directed to the detector. This allows on-axis collection of the reflected beam, i.e., the emitted beam and the reflected beam coincide between the surface and the polarizing beam splitter.

[0035] According to a further embodiment of the second aspect of the present invention, the beam radius at the intersection of the laser beam with the surface is

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[0036] The lens system is (λL / π) 1 / 2 From (λL / π) 1 / 2 (λL / π) 1 / 2 The beam radius may include one or more lenses arranged to provide a beam radius of at least 1 / e of a Gaussian beam emitted from a coherent laser source, which may have a circular or elliptical beam profile. 2 Note that this refers to the intensity beam radius.

[0037] It should be understood that the specific choices of number of lenses, lens types, and distances between lenses are interrelated and also depend on the choice of laser and fiber. Lens choices, such as concave and / or convex lenses, spherical or aspherical lenses, and how their curvature affects focal length and beam radius, are known to those skilled in the art.

[0038] In a preferred variant, the lens system is arranged between the beam splitter and the surface. In such a variant, the lens system may preferably include two lenses.

[0039] In other preferred variations, the lenses of the lens system may be arranged between the first optical path and the beam splitter, and between the second optical path and the beam splitter. In yet other preferred variations, the lenses of the lens system may be arranged both before and after the beam splitter, for example between the laser source and the beam splitter and / or between the detector and the beam splitter, as well as between the beam splitter and the surface.

[0040] In some variations, the lenses of the lens system are mounted so that their relative distance can be changed, thereby allowing the lens system to be adjusted so that the same optical system can be used in different situations where the distance L to the surface is changing.

[0041] In some variations, the optical system may be constructed without a lens system. The intended beam width can be obtained based on the beam width emitted from the laser source and the placement of the laser source relative to the surface to be measured.

[0042] According to a second aspect of the present invention, the above objects and advantages are obtained by:

[0043] 1. An optical system for use in a method for measuring surface divergence according to the method of the present invention, comprising: a laser light source for emitting coherent light; a detector for detecting a reflected beam of light reflected from the surface; a first optical path configured to direct coherent light from the laser light source and direct a coherent laser beam toward the surface; a second optical path for collecting and directing the reflected beam to the detector; a circulator disposed between the laser light source, the detector, and the surface to keep the emitted coherent light and the collected reflected beam separate; and The optical system is arranged to intersect with the surface

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[0044] It should be understood that the laser source and detector may be of the same type as in the optical system embodiments previously described.

[0045] In this embodiment, the presence of an optical circulator allows the use of an optical fiber that does not need to be polarization-maintaining but allows the emission and collection of the reflected beam on-axis. The circulator allows the transmission of the coherent laser light emitted by the laser source and the collection of the reflected beam through the same first optical fiber.

[0046] In some variations, the optical system may be constructed without a lens system. The intended beam width can be obtained based on the beam width emitted from the laser source and the placement of the laser source relative to the surface to be measured.

[0047] In some preferred variations, the optical system further includes a bypass optical path arranged to direct a portion of the coherent light emitted from the laser light source directly to the detector.

[0048] Having a bypass optical path allows a wider range of detectors to be used in the optical system, as light directed directly from the laser source can interfere with reflected light in the detector.

[0049] In such a preferred variant, in which the optical system includes a bypass optical path, an acousto-optical modulator (AOM) is arranged in the bypass optical path, such an AOM being able to shift the frequency of the emitted laser light.

[0050] According to a further embodiment of the second aspect of the present invention, the optical system is configured such that the beam radius at the intersection of the laser beam with the surface is

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[0051] In variations in which a lens system is present, the components are preferably arranged such that the laser source provides coherent light to a first port of the circulator, the lens system is positioned after a second port of the circulator, and the detector is positioned after a third port of the circulator. In some variations, a polarizer may be positioned between the third port and the detector.

[0052] According to a third aspect of the present invention, the above objects and advantages are obtained by:

[0053] A measurement system for use in a method for measuring a surface, the measurement system comprising a plurality of optical systems, the plurality of optical systems being arranged along a mounting bar.

[0054] By having a measurement system that includes multiple optical systems, it is possible to measure the axial velocity between each of the optical systems in the multiple optical systems relative to the surface. Therefore, by relating the lateral velocity of the relative movement between the optical system and the surface, it is possible to determine the dynamic topology of the surface at multiple locations. For example, multiple optical systems can be arranged parallel to the direction of relative lateral movement between the optical system and the surface, which allows for mapping a larger area of ​​the surface in a single pass. For example, this may allow for mapping of surface curvature due to applied bending forces or standing waves through the material below the surface. In another variation, instead of or in addition to a parallel arrangement, multiple optical systems may be arranged perpendicular to the direction of relative lateral movement between the optical system and the surface. This may enable detection across the width of a conveyor belt, for example. Alternatively or additionally, using multiple optical systems may also allow for validation by having multiple optical systems detect the same surface in close succession. Yet another use of multiple optical systems is to place one or more optical systems in areas where little wear or no applied forces are expected and therefore no bending is expected in those areas, allowing those optical systems to serve as a reference system for calibration.

[0055] The use of mounting bars to position multiple optical systems is advantageous because it allows the positions of the optical systems within the measurement system to be fixed relative to one another. For example, the optical systems can be mounted at the same distance from the surface of the object to be measured, but at different positions relative to the lateral movement of the surface. The same distance is understood to mean, for example, the same distance to the expected plane or average distance of the surface if the surface of the object to be measured is flat.

[0056] The choice of material for the mounting bar is important to ensure its robustness. This is important to ensure that any axial movement of the surface relative to the optics is due to surface divergence and not due to a change in the optics' mounting position over time. During measurement system operation, the surrounding environment can cause the mounting bar on which the optics are located to heat up. This is particularly problematic if the mounting bar is not heated uniformly—for example, if the side facing the object to which the mounting bar is attached becomes warmer than the side facing the surface being measured, or vice versa. Depending on the material, such uneven heating can cause the mounting bar to expand and / or bend, which can result in movement of the optics compared to the unheated bar, i.e., the bar's position before the measurement. Bending also causes a change in the relative angle of the optics' measurement axis, which will affect the measured axial velocity.

[0057] In some variations, the mounting bar is made of steel.

[0058] According to a further embodiment of the fourth aspect of the present invention, the mounting bar includes a passive thermal enclosure including a first insulating layer and a second insulating layer of insulating material, the first insulating layer and the second insulating layer of insulating material being arranged to form a cavity therebetween.

[0059] The presence of a passive thermal enclosure further contributes to thermal stabilization of the mounting bar, minimizing deformation over time.

[0060] In some variations, the insulating layer is a foam material. In some variations, both the first and second insulating layers have a thickness in the range of 10-50 mm, for example, 20 mm, because this thickness is a good compromise for providing sufficient insulation while maintaining the desired size of the mounting bar.

[0061] In the following, example embodiments according to the present invention are described. [Brief explanation of the drawings]

[0062] [Figure 1] FIG. 1 is a schematic diagram of the surface measurement. [Figure 2] Figure 2 shows a conceptual diagram of the relationship between optically rough surfaces and beam radius. [Figure 3] 3A and 3B show the components of the optical system in two different embodiments. [Figure 4] FIG. 4 is a schematic diagram of multiple optical systems mounted on a mounting bar.

[0063] Detailed Description of the Drawings The invention will now be explained in more detail, by way of example, with reference to the accompanying drawings, in which:

[0064] However, the present invention may be embodied in forms other than those set forth below and should not be construed as limited to the examples set forth herein. Rather, any examples are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. Accordingly, like elements will not be described in detail with respect to the description of each figure.

[0065] FIG. 1 shows schematically how axial velocity d (shown as a double arrow, it being understood that this movement can be in either direction) can be measured due to the divergence of a surface 1 under an axial force F (shown as an arrow) on an optical system 10.

[0066] Optical system 10 includes a laser source positioned to emit a coherent laser beam 22 axially so that the laser beam 22 coincides with surface 1. When coherent laser beam 22 strikes surface 1, it is reflected. The portion of the reflected light that is reflected back on-axis, i.e., along the emission axis, is referred to as reflected beam 32. Reflected beam 32 is detected by a detector in optical system 10.

[0067] The deflection force F, shown conceptually as an arrow in FIG. 1 , and the surface 1 move relative to each other in a direction transverse to the axial direction of the laser beam 22 and the reflected beam 32. For example, if the surface 1 diverges due to the applied deflection force F or due to vibration waves propagating through the material below the surface, the velocity d at which the surface 1 diverges will vary depending on the location on the surface 1 where the divergence occurs, because the amount of surface divergence depends on the structural integrity of the material at that location. In some exemplary preferred embodiments discussed below, a means for applying the deflection force F is mounted with the optical system 10, so that the optical system 10 can undergo a lateral movement v (shown by arrow v) relative to the surface 1 with the applied deflection force F to measure the axial velocity d. Due to the lateral movement v and the applied divergence force F, the axial velocity, and therefore the measured Doppler shift, will vary depending on the strength of the subsurface structure, depending on how large the surface divergence is at a particular lateral location. The lateral movement velocity v and the axial velocity d of the on-axis divergence cause the distance L traveled by the laser beam 22 along the axial direction to remain constant. In other embodiments not described in further examples, the optical system 10 may remain stationary relative to the surface 1, but only the deflection force F moves laterally.

[0068] Because the reflected beam 32 is defined as the portion of the reflected light that is on-axis, this light interferes with the coherent laser beam 22 due to the axial velocity of divergence d. This interference can be detected by the optical system 10, and the Doppler frequency can be determined from this measurement. It is understood that only the on-axis reflection is considered the reflected beam 32. Therefore, optically rough surfaces cause diffuse reflections, which in turn cause axial reflections, allowing the measurement of on-axis velocity fluctuations. Specular surfaces are not suitable for the optical system 10 unless it is guaranteed that the reflection will coincide with the detector position for a given lateral velocity v.

[0069] Coaxial measurement of reflection, i.e., coaxial between the coherent laser beam 22 and the reflected beam 32, is preferred because it simplifies the measurement. It should be understood that in other embodiments, the system may be configured to measure the reflected beam 32 that is not coaxial with the emitted coherent laser beam 22, for example, by collecting the reflected beam at an angle or directing it back to an adjacently located detector without a beam splitter. The benefits of an optimized beam width are not limited to coaxial measurements.

[0070] By knowing the starting position and rate of lateral movement of the optical system 10 relative to the surface 1, it is possible to correlate the magnitude of the Doppler frequency shift to a particular position on the surface 1.

[0071] The lateral movement v between the optical system 10 and the surface 1 is relative. Thus, in some embodiments, the optical system 10 may be stationary while the surface 1 is moving. An example of such an embodiment is a conveyor belt, which may continuously pass the detecting optical system 10 at one position while the surface 1 is changed by bending forces applied to the conveyor belt or vibrations propagating through the conveyor belt. In another embodiment, the moving surface may be part of a machine that rotates, vibrates, or translates at a particular frequency, and the stationary optical system 10 may be used in such an embodiment to determine unintended fluctuations or changes in the movement pattern due, for example, to vibration, wear, malfunction, or tilted installation.

[0072] In other embodiments, the lateral movement v between the optical system 10 and the surface 1 may be caused by lateral movement of the optical system 10 while the surface 1 is stationary. An example of such an embodiment is mounting the optical system 10 on a vehicle that can be driven along the property to be measured, such as a road or railroad surface. In such an embodiment, the vehicle may also have a load mounted thereon such that a known distortion affects the surface 1. In such a case, the optical system 10 may measure the deflection of the surface 1 caused by a load mounted on the same vehicle as the optical system 10. Such a load may be adjustable or fixed.

[0073] FIG. 2 shows schematically a surface 1 comprising different segments 5, 5′, 5″. The different segments 5 are shown merely as areas of irregular size, which may correspond to different segments of stone or gravel on a surface such as a road. However, these are only examples, and the different structures may follow a regular repeating pattern, for example the surface of a conveyor belt having a surface structure to adjust the coefficient of friction.

[0074] In FIG. 2, the laser spot 25 of the laser beam is 1 / e 2 It is shown for simplicity as a circle showing the intensity radius, the radius of which corresponds to the beam radius w according to the invention.

[0075] Two factors are relevant when determining the appropriate beam size. One factor to consider is speckle decorrelation, also known as speckle boiling. When surface 1 moves relative to the optical system a distance corresponding to laser spot 25 on surface 1, a new area on surface 1 becomes responsible for the reflection and therefore the speckle pattern in the reflected light. This is illustrated in Figure 2 by the shifted laser spot 25' next to laser spot 25. The shifted laser spot 25' is where the laser beam would impinge if surface 1 were moved twice the beam radius w.

[0076] In the marked laser spot 25, three different segments 5', 5", 5'" contribute to the speckle pattern scattered from the surface 1. In the shifted laser spot 25', only two segments 5, 5' contribute to the scattering of light from the surface, only one of which is the same as in the original laser spot 25. The difference in the speckle pattern of the reflected light due to the change in the structure of the surface 1 causes an undefined phase change in the Doppler signal, which causes speckle decorrelation.

[0077] Therefore, considering the speckle decorrelation, it is beneficial to increase the beam radius w.

[0078] Another factor to consider in selecting the beam radius w is that a smaller radius w will result in more light being reflected from the surface 1, thereby increasing the signal power, which may further improve the accuracy of the measurement and / or reduce the required power consumption of the laser light source.

[0079] Therefore, signal power considerations make it beneficial to reduce the beam radius w.

[0080] Therefore, the lower limit of the functionality of the optical system and method is governed by the phase noise due to speckle decorrelation, and the beam radius is at least

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[0081] It is further noted that this applies to the preservation of Gaussian beam profiles, which may be circular or elliptical: for elliptical beams, the equation must hold for either the semimajor axis or the semiminor axis, and may optionally hold for both.

[0082] The upper limit is governed by the signal power, which can be adjusted by reducing the beam radius or by providing more power from the laser source. Therefore, the upper radius limit is primarily governed by practical considerations of the setup, such as the available laser source, the cost of the power supply, and / or the maximum area of ​​the surface that can be illuminated, which may be limited, for example, by the distance between the wheels of the vehicle on which the optics are mounted or the width of the conveyor belt being inspected.

[0083] For the preferred practical use of the optical system and method, the upper limit is:

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[0084] This strikes a balance between two factors: speckle decorrelation and signal power.

[0085] As previously mentioned, those skilled in the art will know how to arrange the optical system components 10 to adjust the beam radius w within a preferred range. Once a laser light source is selected, the wavelength of the emitted coherent light is fixed, and one skilled in the art can then select one or more lenses and arrangements of the lens system, such as the distance between the laser and the beam-shaping lens, and the mutual distances between such lenses, if multiple lenses are present, to obtain a beam radius w within the desired range.

[0086] In one exemplary embodiment, the optics may be mounted such that the distance to the surface, L, is 1.6 m and the wavelength of the light emitted by the laser source is 1550 nm, resulting in a beam radius at the surface of at least 0.89 mm.

[0087] In another exemplary embodiment, the optics may be mounted such that the distance L to the surface is 1.8 m, while using the same wavelength of 1550 nm emitted by the laser source. In this case, the beam radius at the surface is instead at least 0.94 mm. However, due to the effects of the measurement environment, slight variations in this width may occur throughout the measurement procedure. Therefore, in some embodiments, it may be beneficial to shape the beam to be larger than the lower limit, so that slight variations do not cause the beam radius to fall below the lower limit.

[0088] 3A and 3B show schematic diagrams of components of optical system 10 in two different embodiments of the present invention.

[0089] FIG. 3A shows an embodiment of optical system 10 in which polarizing beam splitter 44 is positioned to split and direct emitted laser beam 22 and reflected beam 32 from on-axis propagation to detector 30.

[0090] In this embodiment, the laser source 20 emits coherent laser light. The emitted laser light passes through a first optical path 41, which in the illustrated embodiment is in the form of an optical fiber. In this embodiment, the first optical fiber 41 is polarization-maintaining, so that the light sent to the polarizing beam splitter 44 maintains its polarization. A lens system 47 is disposed between the polarizing beam splitter 44 and the surface 1 and is used to reduce the beam radius on the surface 1 to a minimum beam radius w by falling within the range of the beam radius according to the present invention. min The lens system 47 is configured to control the beam radius to be greater than . The lens system 47 can include a single lens or multiple lenses. The lens or lenses of the lens system 47 can be positioned to provide an intended beam radius based on their position relative to the emitted laser beam and their curvature.

[0091] It should be noted that, according to the present invention, several positions of lens system 47 relative to the other components of optical system 10 are possible: lens system 47 may be located in the optical path between polarizing beam splitter 44 and surface / target 1 as shown, or it may be located between optical path 41 and polarizing beam splitter 44, and between polarizing beam splitter 44 and second optical path 42, or a combination of the above.

[0092] The emitted coherent laser light passes through a polarizing beam splitter 44, which is positioned to direct the emitted laser beam 22 toward surface 1 so that it coincides with surface 1 (the arrows along the optical path indicate the direction of beam travel). The coherent laser light is reflected from surface 1, and the reflected beam 32 counter-propagates on-axis toward the polarizing beam splitter 44.

[0093] A polarizing beam splitter 44 directs the reflected beam 32 into a second optical path 43 in the form of an optical fiber, the second optical fiber 43 being polarization-maintaining. The second optical fiber 43 directs the reflected light towards the detector 30.

[0094] In some preferred embodiments, such as that shown in Figure 3A, the optical system further includes a bypass optical path 46, which in the illustrated embodiment is in the form of a bypass optical fiber. The bypass optical path 46 directs a portion of the emitted laser light from the laser source 20 directly to the detector 30. The light emitted directly from the laser source 20 and the reflected beam 32 interfere in the detector 30, allowing the Doppler frequency to be determined. Some detectors may not require such a bypass optical path.

[0095] In some preferred embodiments that include a bypass optical path 46, an AOM 50, such as a Bragg cell, may be placed within the bypass optical path 46 to allow for controlled frequency shifting of the emitted laser light before it enters the detector 30.

[0096] The detector 30 may include a processing unit for calculating the axial velocity of the divergence d based on the detected Doppler shift, and / or the detector may include a transmitter for transmitting the recorded signal to an external processing unit where the data can be stored and / or subsequent calculations can be performed. The processing unit, whether integrated into the optical system or external, may further perform calculations to correlate the measured Doppler frequency to a position on the surface 1.

[0097] While this embodiment is shown using optical fibers for all optical paths, it should be understood that in other embodiments having the same principles of operation, other waveguides or free space optics may be used to guide the light.

[0098] In some variations of the optical system 10, the lens system 47 may be mounted such that the optical system 10 can be calibrated by adjusting the position of one or more lenses of the lens system 47. For example, one or more lenses of the lens system 47 may be mounted on rails so that they can be translated along such rails, e.g., by a motor. In other embodiments of the optical system 10, the lens system 47 may be mounted such that lenses can be exchanged for other lenses to adjust the configuration for different use cases, e.g., when mounted on different equipment that changes the axial distance L. Another method of calibration may be achieved by adjusting the placement of the optical system 10 relative to the surface. Such calibration methods may be used in combination or separately. Additionally or alternatively, mathematical correction of the detected signal may be performed by an on-board or external processing unit based on reference measurements.

[0099] 3B schematically illustrates an alternative arrangement of the optical system 10 in which a circulator 45 is disposed between the surface 1 and the laser source 20 and detector 30, respectively. The circulator 45 allows the emitted coherent laser beam 22 and the reflected beam 32 to be directed along the same emission and reflection optical path 42, while still allowing for the discrimination of the reflected signals at the detector 30. The coherent laser light emitted from the laser source 20 is directed to a first port of the circulator 45 through a first optical path 41 and then directed towards the underlying material of the surface 1 via emission and reflection optical path 42 through a second port. The reflected beam 32 is also collected at a second port of the circulator 45 and then exits the circulator 45 through a third port, traversing a second optical path 43 to reach the detector 30.

[0100] In embodiments having a circulator 45, there is a lens system 47 having one or more lenses. The lens system 47 is positioned to ensure that the emitted laser beam is shaped so that the beam radius w on the surface 1 is within the scope of the present invention. The lens system 47 is positioned between the laser source 20 and the surface 1. In some embodiments, the lens system 47 may be positioned between the circulator 45 and the surface 1, for example, by being placed in the emitted and reflected light path 42 between the circulator 45 and the surface 1. As shown, the optical path of the optical system may consist of fiber, although in other embodiments, other waveguides may be used, as well as free-space optics.

[0101] As with the previous embodiment, the optical system including the circulator can have an adjustably positioned lens system 47 to allow for calibration. Also, as with the previous embodiment, the optical system may include an integrated processing unit and / or transmitter to allow for storage of measurements and / or calculations based on Doppler frequency.

[0102] In some preferred embodiments, such as that shown in Figure 3B, the optical system further includes a bypass optical path 46, which in the illustrated embodiment is in the form of a bypass optical fiber. The bypass optical path 46 directs a portion of the emitted laser light from the laser source 20 directly to the detector 30. The light emitted directly from the laser source 20 and the reflected beam 32 interfere in the detector 30, allowing the Doppler frequency to be determined. Some detectors may not require such a bypass optical path.

[0103] Although not shown, some preferred embodiments having a circulator and bypass path 46 further include an AOM 50, such as a Bragg cell, positioned in the bypass path 46 to allow for controlled shifting of the emitted laser light before it enters the detector 30.

[0104] 3A and 3B show configurations in which the coherent laser beam 22 and the reflected beam 32 propagate coaxially, it should be understood that in other embodiments in accordance with the present invention, the propagation need not be coaxial. The benefits of the present invention are also realized in situations in which the laser source 20 and detector 30 are positioned such that the reflected beam 32 is detected at an angle relative to the coherent laser beam 22.

[0105] In all the illustrated embodiments, the optical system 10 is shown as being gravity mounted above the surface 1 from which light is reflected. However, those skilled in the art will appreciate that different relative mountings may be used in different ways, for example to measure vertical surfaces, or that the optical system 10 may be positioned to measure a plate or material above which the surface 1 is gravity mounted.

[0106] 4 illustrates the concept of multiple optical systems 10 mounted on a mounting bar 70. Each optical system 10 secured to the mounting bar 70 may be of the same type, e.g., having a polarizing beam splitter and similarly arranged lens systems. In other variations, some or all of the optical systems 10 mounted on the same mounting bar 70 may be of different types.

[0107] The optics 10 may be mounted on the mounting bar 70 so as to be evenly distributed along the length of the mounting bar 70, some or all of the optics 10 may be clustered, or the optics 10 may be distributed non-uniformly based on an estimate of where it is most important to measure the divergence of a given surface, for example, based on the location of an applied deflection force F. In some embodiments, groups of optics 10 may be grouped together and one or more optics 10 may be mounted separate from the group, for example, so as to act as a reference when the mounting bar 70 is used in connection with a load-bearing vehicle and deflections are measured by the optics.

[0108] The mounting bar 70 itself is preferably configured to minimize the effects of heating from external sources in the environment in which the optical system is mounted and used.

[0109] The mounting bars are preferably made of metal, and in some embodiments, the mounting bars are made of steel.

[0110] The shape of the mounting bar can also contribute to the sturdiness and rigidity of the mounting bar 70 during use. In some embodiments, the mounting bar 70 may be configured as an H-beam. In other embodiments, the mounting bar 70 may be configured as a C-beam.

[0111] In some embodiments, the mounting bar may be configured with a passive thermal enclosure such that heating effects from the external environment are reduced. Such a passive thermal enclosure may include a hollow section having first and second insulating layers of insulating material.

[0112] The above embodiments are exemplary and other components can be added within the scope of the invention, for example to filter the signal, split the transmitted and reflected beams, change the polarization, or generally shape the laser beam.

[0113] Below is a list of reference numbers used in the detailed description of the present disclosure and in the drawings referenced in the detailed description of the present disclosure. [Explanation of symbols]

[0114] 1 surface 5, 5', 5" segments L Axial distance d Axial velocity F deflection force v Relative lateral velocity w beam radius 10 Optical system 20 Laser light source 22 Coherent laser beam 25 laser spots 30 detectors 32 Reflected Beam 41 First optical path 42 Emission and reflection paths 43 Second optical path 44 Polarizing Beam Splitter 45 Circulator 46 Bypass optical path 47 Lens System 50 AOM 70 Mounting bar

Claims

1. 1. A method for optically measuring the axial divergence of a surface at an angle relative to said surface, said method comprising: providing an optical system, the optical system including a laser source and a detector; emitting a laser beam of coherent light from the laser source of the optical system in the axial direction toward the surface while the surface moves laterally relative to the emitted laser beam; The laser beam has a beam radius on the surface of [Equation 1] where λ is the wavelength of the emitted laser beam, L is the distance from the laser source to the surface, and e.g., the beam radius on the surface is [Equation 2] from [Equation 3] and a step within 5 times of measuring an optical signal of the reflected beam reflected from the surface; determining the Doppler frequency of the reflected beam; determining the axial movement of the surface based on the Doppler frequency; A method comprising:

2. The method of claim 1 , wherein the distance and / or rate of relative lateral movement of the optical system relative to the surface is monitored.

3. 10. A method according to any one of the preceding claims, wherein the relative movement between the surface and the optical system is due to movement along the surface of a vehicle to which the optical system is mounted.

4. The method according to any one of claims 1 to 2, wherein the relative movement between the surface and the optical system is due to a translation and / or rotation of the surface while the optical system remains stationary.

5. 10. The method according to any one of the preceding claims, wherein the measurement results are transmitted to a data processing unit for storing and / or processing said data.

6. An optical system for use in a method for measuring surface divergence according to any one of claims 1 to 5, comprising: a laser source for emitting coherent light; a first optical path positioned to direct coherent light from the laser source for directing a coherent laser beam onto a surface, the first optical path being polarization-maintaining; a detector for detecting a reflected beam of light reflected from the surface; a second optical path for collecting and directing the reflected beam to the detector, the second optical path being polarization-maintaining; a polarizing beam splitter disposed between the first optical path and the second optical path for redirecting the on-axis reflected beam into the second optical path; The optical system is configured such that the beam radius at the intersection with the surface is [Equation 4] or more, for example, the beam radius is [Equation 5] from [Equation 6] an optical system for providing the laser beam within a range of 5 times.

7. the beam radius at the intersection of the laser beam with the surface is [Equation 7] For example, if the beam radius is [Equation 8] from [Equation 9] 7. The optical system of claim 6, including a lens system for controlling the focus of the emitted laser beam to be within five times of the reference wavelength.

8. An optical system for use in a method for measuring surface divergence according to any one of claims 1 to 5, comprising: a laser light source for emitting coherent light; a detector for detecting a reflected beam of light reflected from the surface; a first optical path arranged to direct coherent light from the laser light source for directing a coherent laser beam toward the surface; a second optical path for collecting and directing the reflected beam to the detector; a circulator disposed between the laser light source, the detector, and the surface for keeping the emitted coherent light and the collected reflected beam separate; The optical system is configured such that the beam radius at the intersection with the surface is [Equation 10] or more, for example, the beam radius is [0011] from [0012] an optical system for providing the laser beam within a range of 5 times.

9. the beam radius at the intersection of the laser beam with the surface is [0013] For example, if the beam radius is [0014] from [Equation 15] 9. The optical system of claim 8, including a lens system for controlling the focus of the emitted laser beam to be within five times of the focal point.

10. A measurement system for use in the method for measuring surface emanation according to any one of claims 1 to 5, said measurement system comprising: A plurality of optical systems according to any one or more of claims 6 to 9, wherein the plurality of optical systems are arranged along a mounting bar. A measurement system including:

11. 11. The measurement system of claim 10, wherein the mounting bar includes a passive thermal enclosure including a first insulating layer and a second insulating layer of insulating material, the first insulating layer and the second insulating layer of insulating material arranged to form a cavity therebetween.

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