JPH0362203B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for detecting the direction of arrival of waves, and particularly to a method for separately detecting the directions of arrival of waves arriving from multiple directions in the ocean and having different wavelengths.

(Prior art) Conventionally, it has been common to rely on visual observation from an aircraft or ship to investigate the direction of arrival of ocean waves generated by typhoons, etc., but there are uncertainties in such observation methods, From the viewpoint of discontinuity, danger to observers, or cost, a buoy equipped with a wave arrival direction measuring device should be released and the measurement data should be continuously transmitted to a ground station via an appropriate relay station such as an artificial satellite. is being studied.

As a method for detecting the direction of wave arrival when employing the method described above, it has been proposed, for example, to obtain the direction of arrival of waves relative to the buoy by observing the elevation of the buoy due to the waves and its inclination.

However, although such a method is theoretical, in reality, the amount of inclination of a buoy has a strong correlation with the size of the wave, that is, the wavelength, wave height, and the size or displacement of the buoy. Not only is it impossible to accurately observe waves, but it is also impossible to distinguish between the direction of wave arrival and the direction of wave arrival that is deviated by 180 degrees from this direction. Therefore, it is impossible to accurately detect the direction of arrival of waves. The flaw was that it was impossible.

In addition, a stable platform is provided inside the buoy, on which acceleration sensors in horizontal (X-axis and Y-axis) directions that are orthogonal to each other are fixed, and the outputs of these sensors are subjected to FFT (fast Fourier transform). It has also been proposed to calculate the power spectrum for each wavelength of waves arriving from multiple directions and to detect the direction of arrival of waves of a specific wavelength from the ratio of the X direction and Y direction.

However, such a method is not applicable when the stable platform is a high-class and therefore expensive one, but when a simple and inexpensive stable platform such as a pendulum type is used, especially when the stable platform is a buoy. There was a problem in that the wave arrival direction detection accuracy was unreliable because it tilted following the oscillation and the calculated values of the power spectrum in the X and Y directions varied.

(Objective of the Invention) The present invention has been made to eliminate the deficiencies of the conventional wave arrival direction detection method as described above. Easy stable
The purpose of this invention is to provide a wave arrival direction detection method that can achieve high accuracy even when using a platform.

(Summary of the Invention) In order to achieve the above object, the wave arrival direction detection method according to the present invention uses the following configuration and method.

That is, a stable platform is placed inside a floating body, and horizontal (X, Y)
Directional and vertical (Z) direction acceleration sensors are provided, and their outputs are subjected to FFT (Fast Fourier Transform), and the cross-correlation of the X and Y direction outputs with respect to the Z direction output is used as a cross spectrum to calculate various waves arriving at the floating body. Detecting the arrival direction of a wave of a specific wavelength with respect to the X-axis or Y-axis by separately calculating each wavelength and calculating the arc tangent of the ratio of the imaginary parts (quadrature spectra) of both cross spectra. It is.

(Embodiments of the Invention) The present invention will be described in detail below based on its theory.

Before explaining the embodiments, in order to help understand the present invention, we will explain a method of detecting the arrival direction of waves using only horizontal direction (X-axis and Y-axis) acceleration sensors fixed on a stable platform. Considering this, when the stability platform is tilted by an angle δ, the output of these horizontal acceleration sensors will be observed to be smaller by g sin δ when the acceleration of gravity is g, so the output level will also be extremely small. As mentioned above, the conventional method of applying FFT to detecting the direction of wave arrival from the ratio of the power spectra of the two is unreliable because the detection accuracy varies greatly depending on the amount of inclination of the stable platform or its direction.

In order to solve this problem, in the present invention,
In addition to the acceleration sensors in both Y directions, a vertical direction (Z-axis) acceleration sensor is added to these.

The output of the Z-direction acceleration sensor is the true value Ζ¨ even if the stable platform on which it is mounted is tilted by an angle δ.
It can be considered that the error is negligible when δ is small since it is only Ζ¨cosδ for .

Therefore, we have come up with the idea that if we find the cross-correlation between the Z-direction acceleration sensor output and those in the X and Y directions, we can accurately determine the arrival direction of the wave.

The wave arrival direction detection method described above follows the following theoretical calculation.

That is, if the rise and fall of the sea level is η, it can generally be expressed as η=∫∫∫B(〓)exp{i(κ・〓−σt)}d〓 ₁・d〓 ₂・dσ (1).

Here, B(〓) is the Fourier transform of η(〓, t), and when time t and coordinate 〓 in physical space are transformed into frequency space, they are transformed into frequency σ and coordinate 〓, respectively.

However, since the velocity potential φ is gη = −∂φ/∂t …(2) at the reference plane, φ=−〓gηdt = (ig/σ)∫∫∫B(〓)exp{i (〓・〓 −σt)}d〓 ₁・d〓 ₂・dσ (3) Also, since K・X=k・xcosθ+k・ysinθ, the wave direction spectrum φ(σ, θ) is given as follows.

φ (σ, θ) = 1/2・() ^* ()/
dσdθ...(4) Here, let the accelerations in the directions of the X, Y, and Z axes be x, y, and z, respectively, and show the cross-correlation between Z-X and Z-Y from the covariance function of each of these elements. Since the cross spectra can be obtained, the real part (cospectrum) and imaginary part (quadrature spectrum) of each can be calculated.

To avoid the complexity of the explanation, we omit the intermediate calculations and use the cospectra Pm, n and quadratia spectrum.
^To _{find Qm} ^{, n} _{; _} ^{_ _ _} _{_} σ)=∫ ^2x ₀ g ² k ² sin ² θφ(σ, θ)dθ P _XY (σ)=∫ ^2x ₀ g ² k ² sinθsecθφ(σ, θ)dθ Q _ZX (σ)=∫ ^2x ₀ gkσ ² cosθφ(σ, θ)dθ Q _ZY (σ)=∫ ^2x ₀ gkσ ² sinθφ(σ, θ)dθ …(5).

Here, the cospectra P _ZZ , P _XX and P _YY are respectively X,
It is equivalent to the power spectrum that shows autocorrelation regarding the Y and Z direction acceleration sensor outputs, and _P
is a cospectrum of a cross spectrum showing the cross-correlation of the X-direction and Y-direction acceleration sensor outputs.

By the way, the wave direction spectrum φ(σ, θ)
The coefficients aν, bν (ν=0, 1,
2,...) Generally, it is given by a complex number aν+ibν, which is aν+ibν=1/π∫ ^2x _O φ(σ, θ)exp{iνθ}dθ...(6) Here, considering that exp{iνθ}=cosνθisinνθ Comparing equations (5) and (6), a ₀ = 1/πσ ⁴ P _ZZ (σ) a ₁ = 1/πgkσ ² Q _ZX (σ) a ₂ = 1/πg ² k ² P _XX ( σ)P _YY (σ) b ₁ = 1/πgkσ ² Q _ZY (σ) b ₂ = 2/πg ² k ² P _XY (σ) …(7).

By calculating these five coefficients, an approximate wave direction spectrum φ(σ, θ) can be developed as follows.

φ(σ,θ)≒1/2a ₀ +(a ₁ cosθ+b ₁ sinθ) + (a ₂ cos2θ+b ₂ sin2θ)+ …(8) By the way, θ under the condition where the energy is maximum in equation (8) Since the value of indicates the direction of arrival of the wave with frequency σ, the second and third terms of the Fourier coefficients in equation (8)
Determining the arrival directions θ ₂ (σ) and θ ₃ (σ) from the terms, ₂ (σ) = tan ^-1 (b ₁ /a ₁ ), ₃ (σ) = tan ^-1 (b ₂ /a ₂ ).

Therefore, the wave arrival direction ₂ (σ) is θ(σ)=tan ^- ₁ (b ₁ /a ₁ )=tan ^-1 {Q _ZY (σ)/Q _ZY (σ)}
…(9) Or ₃ (σ)=1/2tan ^-1 (a ₂ /b ₂ )=1/2・tan ^-1 [2P _XY (σ)/P _XX (σ)−P _YY (σ)} ] …It can be calculated using (10).

Here, the quadratia spectra Q _ZY and Q _ZX can take either positive or negative values depending on the direction of arrival of the wave, so if calculations are performed based on the above (9), the true direction of arrival of the wave and the direction deviated by 180° from this direction can be calculated. It is possible to uniquely determine the direction of arrival of the waves without making it impossible to distinguish between them.

On the other hand, as mentioned above, the above-mentioned equation (10) uses only the power spectrum, so the above-mentioned discrimination is difficult.

Finally, for reference, we will explain the actual processing process based on the above theory using a microprocessor equipped with a 3-axis acceleration sensor output mounted on a drifting buoy.
The flowchart shown in Figure. This uses the calculation according to the above equation (8) in order to detect not only the dominant wave but also all waves above a certain level (about the size of ripples) superimposed on the dominant wave.

Note that this calculation process describes the coordinate transformation of the direction using a magnetic compass, but in order to detect the absolute arrival direction of the wave by combining the magnetic compass, it is necessary to use the stable platform X or Y. The deviation angle between the axis and the finger ratio of the magnetic compass can be detected by using an appropriate bit-off, and this is extremely easy, so a detailed explanation thereof will be omitted.

(Effects of the Invention) Since the present invention detects the arrival direction of waves based on the method described above, it is possible to accurately detect waves without making any special changes in hardware compared to conventionally proposed methods. Since it is possible to detect the direction of arrival, especially when applied to a drifting buoy for observing ocean waves, it can be constructed at low cost and has a remarkable effect on improving the reliability of observation data.

In addition, since the method of the present invention can also detect wave height data, it is possible to obtain detailed data showing the characteristics of the ocean, which was previously unknown. It is also extremely effective in collecting basic materials for offshore development, such as the design of oil field development rigs.

[Brief explanation of drawings]

FIG. 1 is a flowchart of a calculation procedure showing an embodiment of the wave arrival direction detection method according to the present invention.

Claims (1)

[Claims] 1. Acceleration sensors in the horizontal (X-axis and Y-axis) and vertical (Z-axis) directions that are orthogonal to each other are mounted on a stable platform provided within the floating body. Detecting acceleration vectors generated based on waves of different wavelengths arriving at the floating body from multiple directions, and cross-correlating the detected force in the X-axis and Y-axis directions with the detected force in the Z-axis direction of each arriving wave. By separating and calculating each wavelength of each wave as a cross spectrum, and calculating the arc tangent of the ratio of the imaginary parts (quadrature spectrum) of these two cross spectra, the various wavelengths with respect to the X axis or Y axis can be calculated. A wave arrival direction detection method characterized in that the arrival directions of different waves are detected separately. 2. The wave according to claim 1, characterized in that the absolute direction of arrival of the various waves is detected by detecting the relationship between the absolute direction indicated by a magnetic compass and the X-axis or the Y-axis. Arrival direction detection method.