RU2746186C1 - Scanning optical wave recorder - Google Patents

Abstract

FIELD: hydrometry; oceanography.

SUBSTANCE: invention relates to the field of hydrometry and oceanography, methods for measuring the height of the interface between liquid and air, methods for measuring the slopes of the interface, for example, measuring the wave height, wave spectrum, slope spectrum. Disclosed is a method for measuring the angles of inclination and wave height of the water surface relative to its equilibrium state. The laser beams are directed along the vertical Z axis. The scattered radiation is recorded using a digital video camera. The boundaries of the illumination of the laser beams on a digital matrix, taking into account the calibration, are converted into wave applicates. The laser beams are scanned over the surface with the video recording frequency, two beams are synchronously scanned in mutually perpendicular directions X and Y, or the third beam is added to the two perpendicular beams, which, synchronously with the first two, scans at a certain distance from the first two, and the instantaneous profile of the surface along the scan path of two or three beams is calculated taking into account the scanning speed and the speed of vertical movement of the surface height at each point of the trajectories.

EFFECT: multiple increase in the accuracy and reliability of measurements of heights (applicates) of the wave surface along the scanning lines, slopes at each point in selected directions and two-dimensional slopes averaged at various spatial scales (from millimeters to meters) due to which the spatial resolution increases and it is possible to register not only gravitational-capillary, but also capillary waves.

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Description

The invention relates to the field of hydrometry and oceanography, methods for measuring the height of the interface between liquid and air, methods for measuring the slopes of the interface, for example, measuring the wave height, wave spectrum, slope spectrum.

A known method for determining the height of the air-liquid interface and surface slopes using a string wave recorder [1]. It measures the electrical resistance of vertically positioned strings partially immersed in a liquid. When the level of the liquid changes, the resistance of the immersed part of the string decreases, and the height of the interface is determined by the total resistance of each string. The disadvantage of this method is the low accuracy of measurements due to a change in the resistance of the string when it is wetted, due to the deposition of salts and algae on the string and electrode, due to fluctuations in the resistance of the liquid, due to the capacitive properties of the electrical circuit. Another method is to measure the angular position of the sun's glare, reflected from the surface from the height of the aircraft [2]. But this method is not all-weather and is difficult to implement. It is possible to measure the slopes by the reflected laser radiation [3]. The disadvantage of this method is that the glare leaves the receiving system aperture at even small slopes, which does not allow measuring slopes even at moderate waves. In the method described in [4], measurements of the wave height of the water surface relative to its equilibrium state and measurement of surface slopes are carried out. In the method, three laser beams are directed vertically along the Z axis, the scattered radiation is recorded using a digital video camera, the boundaries of laser beams illumination on a digital matrix, taking into account spatial calibration, are converted into wave applicates. At the same time, the simultaneous measurement of the applicate at three points provides the possibility of registering slopes in two directions and obtaining a two-dimensional distribution of slopes. The disadvantage is the fixed scale of the position of the coordinates of three points, as well as the summation of the image during the exposure of one frame, which leads to blurring of the exposure boundary and deterioration of the measurement accuracy.

The closest analogue (prototype) is measuring the applicate of the wave surface using a laser beam or a laser knife or two orthogonal laser knives directed along the vertical axis, and registering a single or extended mark on the surface using a digital video camera [5]. In this case, the position of the marks on the video frame is recalculated into spatial coordinates with the further calculation of the applicate and slopes of the surface. The disadvantage of this method is the summation of the mark image during the exposure time of the frame, which leads to blurring of points in height. At a speed of vertical movement of the wave of 0.6 m / s (which occurs even with a wind of 10 m / s) and a shutter speed of 1/40 s, the blurring will be 15 mm, which leads to a low measurement accuracy of both the applicate at individual points of the surface and the slopes.

The technical result of the proposed method consists in a multiple increase in the accuracy and reliability of measurements of heights (applicate) of the wave surface along the scanning lines, slopes at each point in the selected directions and two-dimensional slopes averaged at various spatial scales (from units of millimeter to meters). This increases the spatial resolution and it is possible to register not only gravitational-capillary, but also capillary waves.

To achieve the technical result in the proposed method, the laser beams are directed along the vertical Z-axis, the scattered radiation is recorded using a digital video camera, the boundaries of the illumination of the laser beams on the digital matrix, taking into account the calibration, are converted into wave applicates.

The novelty in the method is that the laser beams are scanned over the surface with the video recording frequency, two beams are scanned synchronously in mutually perpendicular directions, in another version, a third beam is added, which, synchronously with the first two, scans at some distance from the first two, and the instantaneous profile of the surface along the scanning trajectories of two or three beams are calculated taking into account the scanning speed and the speed of vertical movement of the surface height at each point of the trajectories. The measurement accuracy is increased due to the fact that during scanning, the time of illumination of each point of the surface and the blurring of the image due to its movement is reduced tenfold. Measurement reliability is increased by increasing the signal-to-noise ratio in video frames. In addition, when three synchronous scanning beams are used, there are always three synchronous points on each double video frame at which an instant triangle can be constructed and its two-dimensional slope in space can be determined using the standard procedure. At the same time, on each video frame, you can find many such triangles of various scales (from mm to the maximum distance between the points of the rays). Taking into account the scanning speed of the rays and the speed of vertical displacement of the surface height at each point of the trajectories makes it possible to bring the surface profile to a single moment in time equal to the moment of the beginning of scanning, i.e. get an instant surface profile. Analysis of a series of video frames makes it possible to obtain the time dependence of heights and slopes, which makes it possible to calculate the spatial and temporal characteristics of waves. The use of interference filters when registering laser radiation with cameras will ensure selection of only useful laser radiation and reduce background radiation. This will allow measurements to be taken at any time of the day.

FIG. 1 shows a diagram of optical measurements. The laser system 1 scans the water surface 2 with beams 3. Two beams are synchronously scanned in mutually orthogonal directions X and Y, and the third, synchronously with the first two, scans at a certain distance from the first two beams at an angle of 45 ° to the X and Y axes. in the plane composed by the bisector between the X and Y axes and the vertical Z axis. At any given time, three points of the rays create a triangle, the dimensions of which change during the scan cycle.

FIG. 2. shows the scheme of surface scanning by two or three laser beams with the frequency of the video camera. The first ray along the X-axis (trajectory 0-1), the second along the Y-axis (trajectory 0-2), and the third along the trajectory 3-4. Video camera 4 is in a plane composed by a bisector between the X and Y axes and the vertical Z axis.

FIG. 3 shows a video frame of a single laser beam entering the sea water. The upper point is clearly recorded and corresponds to the water-air interface. When scanning at a frame rate, the top edge records the exact line of the instantaneous surface profile without averaging over the exposure time.

In the prototype method, the surface is continuously illuminated with a knife beam. Considering that the speed of vertical movement of surface points along the Z axis can reach 0.5-2 m / s, it turns out that during a holding time, for example 0.02 s, the surface illuminated by a laser knife will shift vertically by 10-40 mm, and the accuracy of registering the height in each point will have approximately the same scale. This will not allow obtaining slopes by registering the surface height in adjacent pixels, which are located at a distance of about 1-10 mm. Consequently, the prototype method will experience difficulties even when registering gravitational capillary waves. Unlike the prototype, the proposed method does not blur the surface boundary in video frames, since the illumination time of each surface point (matrix pixel) is more than 100 times less than the exposure time. For example, with a scan length of 600 mm, a camera resolution of 1 mm, and a beam diameter of 2 mm, the duration of illumination of one pixel will be 1/300 of the exposure duration. At 25 frames per second and shutter speed of 1/40 s, the pixel illumination time will be 1/12000 s. In this case, the displacement of the interface, even at a vertical surface velocity of 2 m / s, will not exceed 0.17 mm, and the resolution along the horizontal coordinates X, Y will be close to the pixel size, i.e. 1 mm. This will make it possible to register not only gravitational capillary waves, but also purely capillary components of waves.

In the proposed method, there is a gain not only in spatial resolution, but in the signal-to-noise ratio. Indeed, in the prototype the laser beam is knife-edge, so the illumination of the information pixels at the interface is proportional to the laser energy emitted during the exposure time W _rad divided by the number of illuminated pixels at the interface. With a shutter speed of 1/40 s and a typical vertical wave velocity of 0.6 m / s, the blurring of the border will be 15 mm. If the length of the knife beam is 600 mm, and the size of one pixel is 1 mm ² , then the light energy per pixel will be W _rad / (600 * 15). In the proposed solution, when scanning a surface with a beam of the same horizontal length in the vertical direction, only one pixel at the interface will be illuminated (the boundary displacement is 0.1 mm) and the light energy per information pixel at the surface boundary will be W _rad / (600 * 1), those. will increase 15 times. This increases the signal-to-noise ratio at the information pixels of the interface and increases the reliability of measurements, makes it possible to automate the processing process and conduct online measurements of wave parameters.

When the laser beam hits the surface, the upper point at the interface is clearly visualized, below which the illumination occurs due to the scattering of the beam that has penetrated into the water, as in the photograph in Fig. 3. When scanning with a laser beam along a certain straight line, a clear upper boundary of the surface with a blur not exceeding a fraction of a millimeter will be visualized, and below the lower half-space will be illuminated. When scanning a surface in two orthogonal directions, the upper bounds of beams will be recorded from the 0 point of their intersection along lines 0-1 and 0-2. Alternatively, when scanning with three beams, the upper bounds of beams 3-4 will also be recorded.

An important step is to take into account the displacement of the interface during the movement of the beam over the surface and to bring the height of the interface to a single instant of time equal to the instant of the beginning of the sweep. For this, the instantaneous profile of the surface along the scanning path of two or three beams is calculated taking into account the scanning speed and the speed of vertical movement of the surface height at each point of the path.

For better selectivity, the wavelength of the third beam can be selected different from the first two and recorded on another channel of the video camera, no less than the registration of the first two beams.

The use of interference filters will also significantly reduce the background radiation and allow measurements to be taken at any time of the day.

The use of the proposed method allows simultaneously, with a time accuracy of up to 1/1000 s, to obtain the values of the applicate at any two or three selected points on the rays of the surface, Fig. 2, with a spatial accuracy of 1 pixel (1 mm). From this value, according to the standard technique, the two-dimensional instantaneous slope of the surface is determined, averaged over the selected triangle, Fig. 2. Processing a video sequence from a sequence of frames allows one to obtain a two-dimensional distribution of slopes on the scale of the selected triangle. With synchronous scanning, the dimensions of the triangles smoothly change from tens of centimeters to a few millimeters, which makes it possible to obtain the spatio-temporal characteristics of waves with high accuracy and in a wide range of scales.

An example of the implementation of the method using two orthogonal beams is illustrated in FIG. 1. Laser system 1 is installed permanently on a remote console on an offshore platform at a height of 4 m above the water level and probes downward along the Z axis. In this case, two orthogonally scanning laser beams are formed, which are simultaneously and uniformly deflected in time in an angle so that the length of the illuminated segments 0-1 and 0-2 on the sea surface were 0.6 m each.Camera 4 is located in the plane made up by the bisector between the X and Y axes and the Z axis, is placed on a platform at a height of 4 m and is oriented to the illuminated areas at an average angle of 50 ° to vertical, fig. one.

и

и вычисляются значения высот поверхности (аппликаты) в каждой точке

и

The frequency of the video cameras is synchronized with the scanning frequency of the beams. In this case, on each i-th frame of camera 4, the upper boundaries of the surface will be recorded on trajectories 0-1 and 0-2, starting from point 0 of intersection of the rays to points 1 and 2, Fig. 2. The matrix of the video camera is calibrated according to the standard technique described, for example, in the prototype [5]. As a result, for each i-th frame and for each j-th discrete coordinate x _j , along the track of the boundary 0-1 on the video camera, you can assign coordinates X _j and Z _j in space, i.e. find the dependence Z _{i, j} (X _j ), and for each i-th frame and for each k-th discrete coordinate y _k along the track of the boundary 0-2, we can determine the coordinates Y _k and Z _k in space, i.e. find the dependence Z _{i, k} (Y _k ). Then, the zero level of the unperturbed surface is calculated at each point j and k as an average over a large number of frames N:

and

and the values of the surface heights (applicates) are calculated at each point

and

определяются аналогичным образом, они служат для расчета двумерных уклонов большого масштаба, фиг. 2.In an alternative solution, the third track is located perpendicular to the bisector of the X0Y angle at a distance of 1 m from the intersection of the orthogonal rays. Moreover, its length is, for example, 0.2 m.Applications on the third track

are defined in a similar way, they serve to calculate large scale 2D slopes, FIG. 2.

It should be taken into account that, during the scan, the surface has time to shift vertically. It turns out that the surface height (applicates ξ _i ) at different points of the scanning line are measured at different times. Therefore, the procedure is carried out to bring all heights in the i-th frame to a single moment in time, t _i equal to the beginning of scanning in the i-th frame. This requires taking into account the displacement of heights at each point during the movement of the ray along the trajectory. This is taken into account as follows. Let us consider this accounting for the example of the trajectory 0-1 along the X axis.

поддерживается постоянной, при этом скорость ν_x,i движения луча по поверхности на i-том кадре составит величину:• Angular scanning speed

is maintained constant, while the speed ν _{x, i} of the beam movement on the surface on the i-th frame will be:

- средняя высота поверхности на i-том кадре, М - число дискретных точек по оси X (на треке 0-1).where Z ₀ is the height of laser 1 above the unperturbed surface,

is the average surface height on the i-th frame, M is the number of discrete points along the X axis (on track 0-1).

• Time of movement of the beam along the X coordinate from the beginning of the scan X _{i, 1} to the point X _{i, j} (delay):

Δt _{back, i, j} = (X _{i, j} -X _{i, 1} ) / ν _{x, i}

• The rate of vertical displacement of each point X _{i, j is} calculated from comparing the applicate in two adjacent frames:

ν _{z, i, j} = (ξ _{i + 1, j} -ξ _{i, j} ) / τ _rep , where τ _rep is the frame repetition period.

- «мгновенный» профиль высоты поверхности вдоль оси X, приведенный к моменту начала развертки t_i, в котором учитывается вертикальное смещение точек за время развертки:• As a result, we get

- "instantaneous" profile of the surface height along the X axis, reduced to the time of the beginning of the sweep t _i , which takes into account the vertical displacement of points during the sweep:

Similarly, the instantaneous surface profile is calculated along the second orthogonal scanning path 0-2 and along the third scanning path 3-4.

One-dimensional surface slopes is the tangent of the slope of the normal to the surface at each point on the trajectory. For slopes along the X axis, they are found using the usual formula:

Similarly, one-dimensional surface slopes are determined in other scanning directions.

При альтернативном варианте с тремя лучами верхний масштаб треугольников существенно возрастает.Two-dimensional slopes of the surface are calculated according to the standard technique from the values of the instantaneous heights at any three points of the interface that do not lie on one straight line. When scanning in two orthogonal directions, three points of the triangle can be selected in sections 0-1 and 0-2, changing the scale from units of millimeters to sizes

In the alternative with three beams, the top scale of the triangles increases significantly.

Statistical processing of a series of video frames makes it possible to obtain time dependences of the heights and slopes of the water surface.

As a result of the application of the proposed method, it is possible with high temporal and spatial accuracy to obtain both temporal and spatial dependences for the heights of the water surface and surface slopes in the range of scales from millimeters to meters, which is necessary for a wide range of scientific and applied problems. At the same time, the method is remote, practically all-weather, does not lead to disturbance of the measured medium, is not subject to the influence of wind, waves and currents, allows for the automation of processing and registration of wave parameters in real time.

Literature

1. USSR author's certificate No. 513250, MKI G01C 13/00. Volnograf, A.K. Kuklin. -Stated 01/23/1975. Publ. 05/18/1976.

2. Sokh S., Munk W. Slopes of the sea surface deduced from photographs of sun glitter, J. Optical. Soc. America, 1954, Vol. 44, No. 11, pp. 838-850.

3. Zapevalov A.C. Statistical models of the sea surface in problems of scattering of acoustic and electromagnetic radiation // Diss. ... doct. physical and mathematical sciences. Sevastopol: Marine Hydrophysical Institute of the National Academy of Sciences of Ukraine, 2008.290 p.

4. A. Aleksandrov, E. Vayndruzh, V. Legeza "Light beam multichannel wave recorder", pp. 205-209 in collection. "Non-contact methods for measuring oceanographic parameters", Moscow: Gidrometeoizdat, 1983.

5. Karaev V.Yu., Meshkov E.M. Method for determining the angle of inclination and wave height of the water surface relative to its equilibrium state. RF patent No. 2 448 324 G01C 13/00.

Claims (3)

1. A method for measuring the angles of inclination and wave height of the water surface relative to its equilibrium state, in which the laser beams are directed along the vertical Z axis, the scattered radiation is recorded using a digital video camera, the boundaries of the illumination of laser beams on a digital matrix, taking into account the calibration, are converted into wave applicates, which differ by the fact that the laser beams are scanned over the surface with the video recording frequency, two beams are synchronously scanned in mutually perpendicular directions X and Y, or a third beam is added to the two perpendicular beams, which, synchronously with the first two, scans at some distance from the first two, and the instantaneous profile of the surface along the scanning trajectory of two or three beams is calculated taking into account the scanning speed and the speed of vertical movement of the surface height at each point of the trajectories. 2. The method according to claim 1, characterized in that the wavelengths of the laser beams are different. 3. The method according to claim 1, characterized in that when the scattered radiation is recorded, interference filters are used.

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