• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
  • 同族专利
  • 引用文献
  • 法律状态
  • 优先权
    • 专利摘要:
    The general field of the invention is that of Doppler lidars for measuring the speed of a target. The lidar according to the invention comprises: first means for modulating the optical frequency of the transmission signal, said frequency being the sum of a constant frequency and a variable frequency of determined amplitude modulated by a periodic time function; Second means (50 to 58) for calculating the spectrum of the measured heterodyne signal and for creating two measurement spectra obtained by shifting the spectrum of the heterodyne signal by a positive and negative frequency value, said resetting frequency equal to the difference between the instantaneous frequency of the transmission signal and the frequency of a signal transmitted at a time offset from the travel time between the lidar and the target; Third means (59) for comparing the two measurement spectra, the difference in amplitude between the two spectra at the Doppler frequency determining the direction of the speed of the target.
    公开号:FR3022349A1
    申请号:FR1401344
    申请日:2014-06-13
    公开日:2015-12-18
    发明作者:Philippe Rondeau；Jean Pierre Schlotterbeck；Xavier Lacondemine
    申请人:Thales SA；
    IPC主号:
    专利说明:

    [0001] BACKGROUND OF THE INVENTION The field of the invention is that of Doppler anemometry requiring to know either the direction of the wind speed, or that of the speed of the wearer of the anemometer when it is mounted on a vehicle. More specifically, the field of applications is aeronautics and more specifically, that of helicopters. Current helicopter airspeed systems, like those of airplanes, are based on total pressure measurements by pitot probes and static pressure. These systems, however, are not adapted to the need of helicopters because they do not cover their entire flight envelope. On the one hand, the air velocity measurement is unavailable at low speed, up to about 35 knots due to disturbances generated by the rotor flow. On the other hand, the anemobarometric systems of the helicopters do not provide the three components of the velocity vector but essentially its component along the longitudinal axis of the carrier. The "lidar" Doppler, LiDAR being the acronym for "Light Detection And Ranging", solves some of these disadvantages. It makes it possible to carry out a measurement of speed at a distance, outside the flow of the rotor of the helicopter, without having recourse to a perch of nose. The use of a plurality of laser beams or a beam scanning system provides access to the three components of the air velocity vector throughout the wearer's flight range. The signal from the simple homodyne Doppler lidar, which corresponds to the beat between a wave backscattered by the atmospheric particles and a copy of the transmitted wave, gives access only to the absolute value of the projection of the velocity vector along the axis of measurement and information of the speed sign is then lost. Indeed the heterodyne signal being real, its spectrum, obtained as the square of the module of its Fourier transform, is even in frequency and nothing makes it possible to determine if the measured Doppler shift is positive or negative.
    [0002] On aircraft, this poses no particular problem insofar as one can find configurations of the sighting axes for which the sign of the speed is always the same on the whole field of flight. On the other hand, considering the helicopter's evolution capabilities in all directions but also the need to measure the module and the orientation of the wind on the ground, it is imperative to have a measurement of signed speed. There are different solutions for determining the sign of speed. The use of an acousto-optical modulator or "MAO" makes it possible to shift the frequency of the emitted optical wave or equivalently of the local oscillator, so that at zero speed, the beat between the backscattered wave and the local oscillator is no longer at zero frequency but offset by a frequency fmAo For example, for a lidar operating in the near infrared at the wavelength of 1.55 pm, it is common to choose a MAO providing a 40 MHz offset. The frequency range covered by the frequencies between 0 Hz and 40 MHz corresponds then to the negative speeds of about -30 m / s to 0 m / s and the frequencies beyond the frequency fmAo correspond to the positive speeds. The use of an acousto-optic modulator nevertheless has several disadvantages: Reliability: the acousto-optic modulator is a fragile component, especially in a severe thermal and vibratory environment and is therefore not adapted to the aeronautical environment; Cost: the cost of the acousto-optic modulator is high compared to the cost of the overall optical architecture; Increase of the frequency domain. For a symmetric speed domain, the Doppler frequency domain to be analyzed is doubled, the processing power required at the level of the processing is increased accordingly. FIG. 1 represents an optical architecture of "CW" type, an acronym for "Continuous Wave" with acousto-optic modulator. A laser source 10 emits an optical wave of frequency VL or wavelength XL. This is shifted in frequency by means of the modulator 11, passes through the optical separator 12 and is then focused in the atmosphere by a reception emission telescope 13. The wave backscattered by the P particles naturally present in the The air is Doppler frequency shifted by a magnitude fp carrying the velocity information V along the axis of the laser beam. We have the classical relation: fp = 2.VaL is again: V = fp. X / 2 The beat between this backscattered wave and the local oscillator produced by the interferometer 14 is detected by the photodetector 15 and produces an electrical signal of frequency fmAo + fb. Spectral analysis by processing means 16 which may be, for example, an averaged periodogram, allows the signal to emerge from the noise and to extract the frequency information. A different device provides access to the sign of speed without using an acousto-optic modulator. It is represented in FIG. 2. The operating principle consists in modulating the frequency of a laser source 20 by means of a frequency ramp generator 21. This device makes it possible to measure the speed V = fD.X / 2 and the distance D separating the target from the anemometer.
    [0003] A and -a are the slopes of the frequency ramps of the ramp generator. The device comprises a transmission-reception channel comprising a separator 22, an amplifier 23, a circulator 24 and a transmission-reception telescope 25. The device also comprises a reference channel comprising a first delay line 26, a second separator 27 , a third separator 28, a second delay line 29, a first interferometer 30 and a first detector 31. Finally, the device comprises a measurement channel comprising a second interferometer 32 and a second detector 33. By treating separately the signals coming from ramps of slope frequency + a and -a, the frequencies ff = and f_ = + 2aD 2aD are respectively measured by means of the measuring channel. For example, if the slope a is equal to 6 MHz / ls, if the distance Da Da is 25 m, then 2. = 1 MHz. If the velocity is positive, then the Doppler shift is +5 MHz, the frequency f. is 4 MHz and the frequency f_ is 6 MHz. On the other hand, if the speed is negative, then the Doppler shift is -5 MHz, the frequency ff is 6 MHz and the frequency f is 4 MHz. Thus, we can find, by comparing the difference between the frequencies f, and f_ not only the value of the speed but its sign. The difference between the two frequencies is representative of the distance to the object.
    [0004] This type of device, however, has several disadvantages. Among other things, it requires the use of specific laser sources, with a well-controlled and controlled waveform by means of an additional detection channel, thus increasing the number of necessary components.
    [0005] The lidar according to the invention does not have these disadvantages. More precisely, the subject of the invention is a Doppler lidar for measuring the speed of a target, said lidar comprising at least one laser source emitting an optical signal, optical means for transmitting said optical signal and receiving a signal. an optical signal backscattered by said target illuminated by said optical signal, heterodyne detection means for causing the transmission optical signal and the backscattered optical signal to beat and for measuring the beat frequency of the beat heterodyne signal, said beat frequency comprising a peak at the so-called Doppler frequency representative of the absolute speed of the target relative to the lidar, characterized in that the lidar comprises: first means for modulating the optical frequency of the optical signal so that said frequency is the sum of a constant frequency and a variable frequency of determined amplitude modulated by a periodic time function ue; Second means for calculating the spectrum of the measured heterodyne signal and creating two measurement spectra, the first spectrum and the second spectrum being obtained by shifting the spectrum of the measured heterodyne signal by a positive and negative frequency value, said value resetting being equal to the difference in frequency between the instantaneous frequency of the transmission signal and the frequency of a transmission signal transmitted at a time offset from the travel time between the lidar and the target; Third means for comparing the two measurement spectra, the difference in amplitude between the two spectra at the Doppler frequency, making it possible to determine the direction of the speed of the target. Advantageously, the second means comprise: a first measurement stage of the reception heterodyne signal comprising the following means: an analog-digital converter A fixed-duration observation window limiting the duration of the digital heterodyne signal; Means for performing the Fourier transform of the digital heterodyne signal and calculating the square of its module to obtain its spectrum; A second digital spectrum processing stage originating from the first stage comprising the following means: two analysis channels arranged in parallel, each channel performing the functions of resetting the spectrum by a positive or negative value in frequency, of accumulation of a predetermined quantity of digital signals and of estimating the Doppler frequency and the peak amplitude corresponding to said Doppler frequency; Calculation of the value of the target speed and direction relative to the lidar. Advantageously, the second means comprise: an analog-digital converter; A fixed-duration observation window limiting the duration of the digital heterodyne signal; Means for multiplying the digital heterodyne signal by a complex exponential function whose exponent is proportional to the resetting value in order to obtain a complex digital signal; Means for performing the Fourier transform of the complex digital signal and for calculating the square of its module to obtain its spectrum; An analysis chain providing the functions of: accumulating a determined quantity of signed digital spectra and estimating the Doppler frequency, analyzing the amplitude and the width of the peak corresponding to said Doppler frequency; Calculating the value of the target speed and direction relative to the lidar as a function of said Doppler frequency and the amplitude and width of the corresponding peak. Advantageously, the duration of the observation window depends on the atmospheric coherence time.
    [0006] Advantageously, the emission frequency is located in the near infrared. Advantageously, the amplitude of the variable frequency of the transmission optical signal is between 10 MHz and 100 MHz and its modulation frequency is between 2 kHz and 20 kHz.
    [0007] Advantageously, the distance separating the lidar from the target is between 10 m and 100 m. The invention will be better understood and other advantages will become apparent on reading the following description given in a non-limiting manner and by virtue of the appended figures among which: FIG. 1 already commented represents a first optical architecture of "CW" type a Doppler lidar according to the prior art; Figure 2 already commented represents a second optical architecture without optical modulator Doppler lidar according to the prior art; Figures 3 and 4 show the block diagram of a lidar according to the invention; FIGS. 5, 6 and 7 show the measurement signals in three different cases, the first without frequency adjustment, the second by shifting the frequency by one positive value and the third by shifting the frequency by a negative value. . Figures 3 and 4 show the block diagram of a lidar according to the invention. Figure 3 shows the transmission and reception assemblies of the lidar. FIG. 4 represents an example of processing of the electrical signal 35 coming from the reception assembly.
    [0008] The transmission assembly comprises a laser source 40 modulated by a frequency modulation assembly 41 whose function is detailed below. The emission beam passes through an optical splitter 42. A first portion is amplified by the amplifier 43, passes through the circulator 44 and is then focused in the atmosphere by a reception emission telescope 45 at the distance D lidar. A second portion passes through a delay line 46 of optical length equal to twice the distance and is directed to the first port of an interferometer 47. The backscattered wave passes through the telescope 45, the circulator 44 and is directed to the second port of the interferometer 47. The beating of the two optical waves injected into the interferometer produces the electrical signal at the output of the photodetector 48. The operating principle of a lidar according to the invention is the following. At a given instant t, the emission laser 40 emits a light wave whose optical frequency viaser (t) is modulated by the modulator 41. The expression of this modulation is as follows: V laser (t) = V 0 ± A. sin (27-tft) With vo carrier frequency of the optical wave 20 A amplitude of the frequency modulation F modulation frequency As a non-limiting example, the emission laser source can emit in the near infrared. The frequency vo is then 1.94 x 1014 Hz and corresponds to a wavelength I of 1.551am. The amplitude A is between 10 MHz and 100 MHz and more precisely can be 50 MHz. The frequency f is between 2 kHz and 20 kHz and more precisely can be 10 kHz. When this transmitted wave reaches the target located at the distance D, the frequency Vrrody) of the backscattered wave is, by Doppler effect,: 2D 30 V retouch (t) = V laser t J Doppler c being the celerity of the light. D is usually a few tens of meters. In the following numerical examples, D is chosen equal to 25 m. The frequency fpoppier corresponds to the Doppler shift in / frequency which is: f Doppler = V being the speed of the target along the axis of the emission laser beam. With the previous wavelength of 1.55 lm, the Doppler shift frequency is 1.3 MHz for a speed of 1 m / s.
    [0009] The lidar according to the invention comprises an interferometer 47 capable of ensuring a heterodyne detection, that is to say to produce an electrical signal proportional to the beat between the optical transmission wave and the backscattered optical wave. The frequency of this beat signal fi,, therefore, is equal to, at the instant t: 8 fbaitemen, O = V retrodif t) V kmer (t1 _2D Let still be the beat (t) - A. sin 27-tf.t_ -sin (27-ift) fDoppler And replacing the difference of the two functions in sinus by their associated trigonometric expression: f beat (t) = 2A.sin f 2nf (t- + f Doppler is therefore the This term is that the Thus, the sum frequency of the modulated sinusoidal frequency with C of the modulation modulation beat signal (Doppler modulation frequency) is instantaneous and of the same emission and has a modified amplitude of a factor 2sin 2711D. This factor is small and, taking the previous numerical values, that is to say the frequency f equal to 10 kHz, D equal to 25 m and A equal to 50 MHz, we obtain: sin (21111) J 5 , 2 X 10-3 The total amplitude of the variations of the beat signal frequency is therefore 2.08 MHz with the previous numerical values. the beat frequency is conventionally done by spectral or Fourier analysis. In the absence of this term of modulation, the beat frequency is strictly equal to the absolute value of the Doppler frequency and it is impossible to go back to the sign of the speed of the target with respect to the lidar. By adding the modulation of the optical frequency of the laser source, it becomes possible to determine the sign of this speed. The principle is to do two calculations. In the first calculation, a recalibration of the instantaneous spectrum of the received signal is performed by adding the modulation term to the measured frequency. In the second calculation, we add the opposite of this same recalibration term. We note these terms of registration f -recalage + and frecalage-. Their expressions are as follows / f + 0 = 2A. sin 271fD cos 2, -tf. t - D recalibration-0 = 2A. sin (211ID) cosf2rifft - D jj c The two spectra obtained are therefore identical but offset with respect to each other in frequency. However, the frequency signal resulting from the beat is usually embedded in the noise for an individual spectrum. It is then necessary to make a non-coherent integration over several tens of milliseconds to make it emerge noise. This integration time being long compared to the modulation of the signal, this results in a spectral spread of the signal line on about 2 MHz. This spectral spread is to be compared to the Doppler shift of 1.3 MHz for a speed of 1 m / s with the same digital data. FIG. 4 represents a possible implementation of the whole of the heterodyne signal processing according to the invention. This treatment has two stages. A first measurement stage of the reception heterodyne signal 25 comprising the following means: An analog-digital converter 50 of the electronic signal coming from the photodetector 48; An observation window 51 of definite duration limiting the duration of the digital heterodyne signal; A means 52 for performing the complex Fourier transform of the heterodyne signal means 53, 54 and 55 for performing the calculation of the squares of the real and imaginary parts of the Fourier transform of the digital heterodyne signal, then of summon them. The size of the Fourier transform must be chosen long enough to obtain a frequency discretization step of about one tenth of the amplitude of the expected frequency modulation. For example, the Gaussian weighting window of width 1 / e2 is 1 ps and the Fourier transform has 4096 points with a sampling frequency of about 400 MHz. The step of frequency discretization is therefore of the order of 100 kHz.
    [0010] A second stage of processing spectra from the first stage comprising the following means: two analysis chains arranged in parallel, each chain performing the 56+ addition or subtraction functions 56_ of a spectrum frequency adjustment term, accumulating 57 a determined amount of digital spectra, detection and estimation 58 of the Doppler frequency and the amplitude of the peak corresponding to said Doppler frequency; Calculation 59 of the value of the speed of the target and its direction relative to the lidar. A synchronization signal 60 makes it possible to synchronize the processing chains with the frequency modulation unit 41.
    [0011] FIGS. 5, 6 and 7 represent the spectra of the measurement signals in three different cases, the first one represented in FIG. 5 without frequency matching, the second represented in FIG. 6 by shifting the frequency of a positive value and the third shown in Figure 7 by shifting the frequency of a negative value. The positive resetting value corresponds to the assumption of a positive velocity and the negative resetting value corresponds to the assumption of a negative velocity. The abscissa corresponds to the frequencies expressed in megahertz and the ordinate corresponds to the amplitudes of the spectra expressed in decibels. In these three examples, the speed of the target is approximately 23 m / s, which corresponds to a Doppler frequency of 30 MHz, the distance D to the target is 25 m, the frequency A is equal to 50 MHz and the modulation frequency f is 10 kHz. In all three cases, the spectra obtained have a significant line at the same 30 MHz Doppler frequency. However, the amplitude MAX of the lines is different. It is maximum when the registration frequency has the same sign as the Doppler frequency. This is the case of Figure 7. The speed of the target is negative, in this example. This provides a simple process for determining the direction and value of the target speed relative to the lidar.
    [0012] The calculation of the resetting can also be done in one step. Indeed, the heterodyne signal being real, its Fourier transform has a hermitian symmetry and the square of the module of this Fourier transform is even. The two readjustments can thus be carried out in a single step by shifting the signed frequency spectrum of the received signal by the previously calculated resetting term. This corresponds in fact to adding the term of registration for the positive frequencies and to subtract it in absolute value for the negative frequencies. This calculation method has a second advantage. Staggering the spectrum mathematically corresponds to convoluting it by a Dirac distribution at the desired frequency. Such convolution in the frequency domain corresponds to a multiplication by a complex exponential function in the time domain. It is therefore possible to perform the registration of the signed frequency spectrum for any value of the registration by multiplying the heterodyne signal measured by a numerical complex exponential function at said registration frequency before performing the Fourier transformation. The set of treatments is then performed on the signed spectra. In the cumulative spectra, the line corresponding to the incorrect sign of the velocity has a noticeable spectral spread which makes it possible to unambiguously choose the line corresponding to the correct velocity sign.
    权利要求:
    Claims (7)
    [0001]
    REVENDICATIONS1. Lidar Doppler for measuring the speed of a target, said lidar comprising at least one laser source (40) emitting an optical signal, optical transmission means (42, 43, 44, 45) of said optical and reception signal ( 44,45,46) of an optical signal backscattered by said target illuminated by said optical signal, heterodyne detection means (47,48) for causing the transmission optical signal and the backscattered optical signal to be pulsed and for measuring the beat frequency of the beat heterodyne signal, said beat frequency comprising a peak at the so-called Doppler frequency representative of the absolute speed of the target relative to the lidar, characterized in that the lidar comprises: first means (41) for modulating the optical frequency of the optical signal so that said frequency is the sum of a constant frequency and a variable frequency of a given amplitude modulated by a time function. periodic; Second means (50, 51, 52, 53, 54, 55, 56, 57, 58) for calculating the spectrum of the measured heterodyne signal and for creating two measurement spectra, the first spectrum and the second spectrum being obtained by shifting the spectrum of the heterodyne signal measured by a positive and a negative value in frequency, said resetting value being equal to the frequency difference between the instantaneous frequency of the transmission signal and the frequency of a transmission signal transmitted to a off-set time of round-trip time between the lidar and the target; Third means (59) for comparing the two measurement spectra, the difference in shape between the two spectra at the Doppler frequency making it possible to determine the direction of the speed of the target.
    [0002]
    2. Doppler Lidar according to claim 1, characterized in that the second means comprise: a first measurement stage of the reception heterodyne signal comprising the following means: an analog-digital converter (50); (51) of fixed duration limiting the duration of the digital heterodyne signal; Means (52, 53, 54, 55) for performing the Fourier transform of the digital heterodyne signal and calculating the square of its module to obtain its spectrum; A second digital spectrum processing stage coming from the first stage comprising the following means: two analysis channels (56) arranged in parallel, each chain performing the functions of resetting the spectrum by a positive or negative frequency value, accumulating a predetermined amount of digital signals and estimating the Doppler frequency and peak amplitude corresponding to said Doppler frequency; Calculation (59) of the value of the target speed and direction relative to the lidar.
    [0003]
    3. Doppler Lidar according to claim 1, characterized in that the second means comprise: an analog-digital converter; A fixed-duration observation window limiting the duration of the digital heterodyne signal; Means for multiplying the digital heterodyne signal by a complex exponential function whose exponent is proportional to the resetting value to obtain a complex digital signal; Means for performing the Fourier transform of the complex digital signal and calculating the square of its module to obtain its spectrum; An analysis chain providing the functions of: accumulating a determined quantity of signed digital spectra and estimating the Doppler frequency; analyzing amplitude and width of the peak corresponding to said Doppler frequency; - Calculation of the value of the target speed and direction relative to the lidar according to said Doppler frequency and the amplitude and width of the corresponding peak.
    [0004]
    4. Doppler Lidar according to one of the preceding claims, characterized in that the duration of the observation window depends on the atmospheric coherence time.
    [0005]
    5. Doppler Lidar according to one of the preceding claims, characterized in that the emission frequency is located in the near infrared.
    [0006]
    6. Doppler Lidar according to one of the preceding claims, characterized in that the amplitude of the variable frequency of the transmission optical signal is between 10 MHz and 100 MHz and its modulation frequency is between 2 kHz and 20 MHz. kHz.
    [0007]
    7. Doppler Lidar according to one of the preceding claims, characterized in that the distance separating the lidar from the target is between 10 m and 100 m.
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    同族专利:
    公开号 | 公开日
    EP2955542B1|2017-04-26|
    FR3022349B1|2016-09-23|
    EP2955542A1|2015-12-16|
    US20160170023A1|2016-06-16|
    US9778362B2|2017-10-03|
    引用文献:
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    US6608669B2|2000-09-22|2003-08-19|Virginia Tech Intellectual Properties|Quadrature processed LIDAR system|
    US6621561B2|2000-09-22|2003-09-16|Virginia Tech Intellectual Properties|Doppler rotational velocity sensor|
    US20050083513A1|2002-12-20|2005-04-21|Rogers Philip L.|Quadrature processed lidar system|
    FR2870004A1|2004-05-04|2005-11-11|Thales Sa|MEASURING DEVICE WITH LOW COST OF FREQUENCY SHIFTING BY DOPPLER EFFECT|
    FR2965064A1|2010-09-22|2012-03-23|Onera |TELEMETRIC MEASUREMENT USING A HETERODYNE DETECTION LIDAR TYPE DEVICE|US20160170023A1|2014-06-13|2016-06-16|Thales|Relative speed measuring doppler lidar|
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    FR3022349B1|2014-06-13|2016-09-23|Thales Sa|LIDAR DOPPLER WITH RELATIVE MEASUREMENT OF SPEED|CN107003411A|2014-12-12|2017-08-01|三菱电机株式会社|Laser radar apparatus|
    US20190018144A1|2016-01-27|2019-01-17|Mitsubishi Electric Corporation|Coherent lidar|
    DE102017106226A1|2017-03-22|2018-09-27|Metek Meteorologische Messtechnik Gmbh|LIDAR measuring device|
    CN108415031B|2018-01-15|2020-08-28|北京航空航天大学|Hyperspectral full-waveform laser radar system based on spectral splitting|
    US10901089B2|2018-03-26|2021-01-26|Huawei Technologies Co., Ltd.|Coherent LIDAR method and apparatus|
    US10838061B1|2019-07-16|2020-11-17|Blackmore Sensors & Analytics, LLC.|Method and system for enhanced velocity resolution and signal to noise ratio in optical phase-encoded range detection|
    法律状态:
    2015-06-08| PLFP| Fee payment|Year of fee payment: 2 |
    2015-12-18| PLSC| Search report ready|Effective date: 20151218 |
    2016-05-26| PLFP| Fee payment|Year of fee payment: 3 |
    2017-05-30| PLFP| Fee payment|Year of fee payment: 4 |
    优先权:
    申请号 | 申请日 | 专利标题
    FR1401344A|FR3022349B1|2014-06-13|2014-06-13|LIDAR DOPPLER WITH RELATIVE MEASUREMENT OF SPEED|FR1401344A| FR3022349B1|2014-06-13|2014-06-13|LIDAR DOPPLER WITH RELATIVE MEASUREMENT OF SPEED|
    EP15171034.0A| EP2955542B1|2014-06-13|2015-06-08|Doppler lidar with relative speed measurement|
    US14/734,989| US9778362B2|2014-06-13|2015-06-09|Relative speed measuring doppler LiDAR|
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