APPARATUS FOR ASCERTAINING A VELOCITY COMPONENT OF AN OBJECT

Abstract

The invention relates to an apparatus for ascertaining a velocity component of an object that moves relative to the apparatus in a detection region at a measurement distance and that reflects light originating from a light source, the object generating reflected light that emanates from the detection region. To this end, the apparatus comprises an objective, a modulator, a reception optical unit and a light detector, wherein the objective detects the reflected light generated by the object in the detection region and elimages said light on the modulator, wherein the modulator modulates the reflected light into a sequence of light signals, wherein the reception optical unit images the sequence of light signals generated by the modulator on the light detector and wherein the light detector converts the sequence of light signals into a sequence of electronic signals. Further, the apparatus comprises an interface which transmits the sequence of electronic signals generated by the light detector to an evaluation unit.

Claims

1. An apparatus for ascertaining a velocity component of an object that moves in a detection region at a measurement distance relative to the apparatus and reflects light originating from a light source, wherein the object generates reflected light that emanates from the detection region, a) comprising a lens, a modulator, a receiving optical unit and a light detector, b) wherein the lens captures the reflected light generated by the object in the detection region and images it onto the modulator, c) wherein the modulator modulates the reflected light into a sequence of light signals, d) wherein the receiving optical unit images the sequence of light signals generated by the modulator onto the light detector, and e) wherein the light detector converts the sequence of light signals into a sequence of electronic signals, and f) comprising an interface, which forwards the sequence of electronic signals generated by the light detector to an evaluation unit.

2. The apparatus as claimed in claim 1, characterized in that the lens, the modulator, the receiving optical unit and the light detector are arranged on an optical receiver path, wherein the receiver path is embodied within the apparatus.

3. The apparatus as claimed in claim 1, characterized in that the lens images the detection region with a defined depth of field on the modulator.

4. The apparatus as claimed in claim 1, characterized in that the lens images the light reflected from the object onto a defined region of the modulator.

5. The apparatus as claimed in claim 1, characterized in that the modulator is provided with a pattern of alternating opaque and non-opaque lines.

6. The apparatus as claimed in claim 5, characterized in that the opaque and non-opaque lines are embodied in the form of Archimedean spirals.

7. The apparatus as claimed in claim 1, characterized in that the modulator is fixed within the apparatus or is mounted rotatably about an axis.

8. The apparatus as claimed in, characterized in that the light detector is arranged in a focal plane of the receiving optical unit.

9. The apparatus as claimed in claim 1, characterized in that a beam splitter is arranged between the lens and the modulator and splits the light reflected from the object into two partial beams, wherein the lens images the partial beams generated by the beam splitter into a first region and into a second region of the modulator, wherein the regions do not overlap.

10. The apparatus as claimed in claim 9, characterized in that the modulator modules the partial beams in a first sequence of light signals and a second sequence of light signals, wherein the receiving optical unit images the first sequence of the first light signals generated by the first region of the modulator onto the light detector, and in that a second receiving optical unit images the second sequence of the second light signals generated by the second region of the modulator onto a second light detector.

11. The apparatus as claimed in claim 10, characterized in that the second receiving optical unit and the second light detector are arranged on a second optical receiver path, wherein the receiver path and the second receiver path are arranged parallel.

12. The apparatus as claimed in claim 1, characterized in that the light source for the light to be reflected is a laser or an incoherent light source.

13. The apparatus as claimed in claim 1, characterized in that a mirror optical unit is provided, which directs the light emanating from the light source in the direction of the detection region and into the optical axis of the lens.

14. The apparatus as claimed in claim 13, characterized in that the light source, the imaging optical unit and the mirror optical unit lie on an optical transmission path, wherein the light source, the imaging optical unit and/or the mirror optical unit are/is arranged within the apparatus.

15. The apparatus as claimed in any of claim 1, characterized in that an evaluation unit calculates the velocity components of the object from the sequences of the electronic signals.

Description

DESCRIPTION OF THE DRAWINGS

[0094] In the exemplary embodiment below, an advantageous way of embodying the invention is explained with the aid of drawings, in which:

[0095] FIG. 1 shows a schematic illustration of an apparatus according to the invention for ascertaining a velocity component of an object;

[0096] FIG. 2 shows a schematic illustration of a modulator for the apparatus according to the invention for ascertaining a velocity component of an object; and

[0097] FIG. 3 shows a schematic illustration of another embodiment of an apparatus according to the invention for ascertaining a two-dimensional velocity vector of an object.

[0098] The apparatus designated generally by 10 in FIG. 1 is embodied as a LiDAR system and serves for wind measurement and thus for ascertaining a horizontal and/or transverse velocity component v.sub.x and/or v.sub.y of an object O, namely of an aerosol particle, that is moving in a detection region D at a measurement distance g relative to the apparatus 10. The object reflects light L emanating from a light source 20 and in the process generates reflected light RL that emanates from the detection region D.

[0099] The wind measurement is preferably effected in a ground-based manner, i.e. the apparatus 10 is fixed on the ground and the measurement is carried out as vertical remote measurement, wherein the horizontal components v.sub.x and v.sub.y of the wind velocity, i.e.

[0100] transverse components thereof relative to the incidence of light in the apparatus, are ascertained.

[0101] The apparatus 10 comprises the light source 20, a lens 30, a modulator 40, a receiving optical unit 50 and a light detector 60 in a housing 12. Furthermore, the apparatus 10 has an interface 70 between the light detector 60 and an evaluation unit (not illustrated). The lens 30, the modulator 40, the receiving optical unit 50 and the light detector 60 are arranged on an optical receiver path 80.

[0102] Besides the light source 20, the apparatus 10 also comprises a mirror optical unit 90 and an imaging optical unit (not shown) with a system of lens elements (likewise not illustrated). The light source 20, preferably a laser 22, the imaging optical unit and the mirror optical unit 90 lie on an optical transmission path 190.

[0103] It is evident in FIG. 1 that the laser 22, the mirror optical unit 90 and the imaging optical unit are arranged in the housing 12 of the apparatus 10 in such a way that the transmission path 190 and the receiver path 80 are combined at the output of the apparatus 10 and thus at the output of the LiDAR system upstream of the lens 30 on an optical axis. For this purpose, the mirror optical unit 90 has on the transmission path 190 a first deflection mirror 92 arranged in front of the laser 22, and also a second deflection mirror 94 positioned on the optical axis of the receiver path 80. The aperture of the lens 30 is distinctly larger than the dimensioning of the second deflection mirror 94, likewise lying on the receiver path 80.

[0104] The system of lens elements of the imaging optical unit comprises two converging lens elements, for example, and serves for expanding the laser beam L emitted by the laser 22 to a defined diameter and for subsequently collimating this beam. The collimated beam is emitted along the optical axis of the receiver path 80 in the direction of the detection region D with the aid of the deflection mirrors 92, 94. At a measurement distance of g=300 m, its diameter d that determines the horizontal spatial resolution according to equation (6) is 0.25 m, for example. If, for example, the overlap function O(g) is set to a value of O(g)=0.5 (for identical proportions of opaque and non-opaque regions 43, 44), then at the measurement distance g the diameter d of the laser beam L is equal to the diameter dab, of the detection region captured by the lens and is equal to the horizontal spatial resolution.

[0105] If the ratio between the proportions of opaque and non-opaque regions 43, 44 is chosen differently (deviating from a ratio of 1:1), the value of O(g) can also be greater or less than 0.5.

[0106] The laser 22 used as the light source 20 is a monochromatic pulsed laser having a wavelength of λ=532 nm, for example. Such a laser 22 is cost-effective and enables particularly simple adjustment of the LiDAR system 10. The further laser parameters should in each case be coordinated with the desired measurement distance g and the object to be measured, here aerosol particles.

[0107] The lens, for example a catadioptric lens, captures the reflected light RL generated by the object O in the detection region D and images it onto the modulator 40, wherein the reflected light RL is focused onto a selected first region 46 of the modulator 40.

[0108] If a lens 30 having an aperture D of 35.6 cm (14 inches), a focal length f of 3910 mm and an f-number k=f/D=11, then the values indicated in Table 2 result for the depth of field h. The parameters of the lens and the laser wavelength used are summarized in the right-hand column of Table 2.

TABLE-US-00002 TABLE 2 Depth of field depending on the object distance = measurement distance g Object Depth distance g [m] of field h [m] Focal length f [m] 50 0.05 3.91 100 0.2 Hyperfocal distance d.sub.h [m] 150 0.45 97338.38 200 0.8 Circle of least confusion Z [mm] 250 1.26 0.01 300 1.83 f-number k 11 Wavelength λ [nm] 532

[0109] The modulator 40 has on its surface a pattern 42 of opaque and non-opaque lines 43, 44 forming a grating. The lines 43, 44 are preferably spiral lines 41, particularly preferably in the form of Archimedean spirals. As a result, the light RL reflected by the object O, upon passing through the detection region D, is modulated into a sequence of light signals (LS) by the modulator 40.

[0110] Said sequence of light signals LS is imaged onto the light detector 60 by the receiving optical unit 50, said light detector correspondingly generating a sequence of electronic signals ES that are forwarded to the evaluation unit via the interface 70.

[0111] If the modulator 40 is stationary, i.e. if it is fixed in the housing 12, a velocity component v.sub.x of the detected object can be calculated from the detected electronic signals ES.

[0112] Preferably, however, the modulator 40, and thus the pattern 42, is arranged in a rotating manner in the housing 12, i.e. the pattern 42 of the modulator 40 is permanently rotated about an axis that is parallel to the receiver path 80. This gives rise to a defined line movement in a radial direction, which can be evaluated quantitatively. Two horizontal components v.sub.x, v.sub.y of the velocity of the object O and thus a two-dimensional velocity vector can be ascertained in this way.

[0113] In order to be able likewise to calculate two horizontal velocity components of a two-dimensional velocity vector—in another embodiment of the apparatus according to the invention—the light RL reflected from the object O is split into two partial beams RL1 and RL2 by a beam splitter 130 and is imaged onto the modulator 40 by the lens 30. In this case—as is furthermore shown in FIG. 3—each partial beam RL1, RL2 is imaged onto a dedicated region 46, 48 on the modulator 40, which do not overlap. The modulator 40 is thus traversed by the two partial beams RL1, RL2 on two parallel, spatially separated beam paths. Here, too, the modulator 40 can be arranged in a fixedly mounted manner or can rotate uniformly about its center axis.

[0114] The receiving optical unit 50 and the light detector 60 are arranged in the first beam path. A second receiving optical unit 150 and a second light detector 160 are arranged in the second beam path.

[0115] A filter unit 155, an iris stop (not shown) and a shutter (likewise not illustrated) can also be arranged between the receiving optical units 50, 150, which preferably have a system of lens elements. The shutter can be closed as soon as the incident radiation exceeds a predefined intensity threshold value. These components are not illustrated, for reasons of clarity.

[0116] The modulator 40 shown in FIG. 2 is additionally mounted rotatably about an axis A and driven by a motor (not illustrated).

[0117] It is evident in FIG. 2 that the modulator 40 is embodied as a circular, quasi-radially symmetrical grating mask (referred to as mask hereinafter), which rotates at a rotational frequency n, which can be set in a defined manner, about an axis of rotation running perpendicularly through its center point.

[0118] The mask has a pattern 42 of Ns.sub.p=64 identical spiral lines 41 in the form of Archimedean spirals, which run from the inner region of the mask to the outer side thereof. The lines 41 are embodied as opaque regions 43. Situated between two adjacent lines 43 in each case is a non-opaque region 44 having the same width as the opaque regions 43. This gives rise to a grating pattern formed by the lines 43, 44. The basic grating constant of said grating is given by the distance between the centers of two adjacent spiral lines 43. It is G.sub.0=0.25 mm, for example. For visualization purposes, in FIG. 2 an arbitrarily chosen Archimedean spiral 41 is highlighted by having been drawn with greater line thickness. By way of example, the external diameter of the pattern 42 is 5 cm, and the internal diameter is 1 cm.

[0119] In order that the conditions (15) and (16) are always reliably met, the following procedure is suitable:

[0120] A maximum wind velocity v.sub.max is defined which is intended to be measurable unambiguously by the LiDAR system 10. For the zero frequency f.sub.0 this then yields the condition

f.sub.0>v.sub.max/G   (19)

[0121] In the present case, this results in a suitable zero frequency f.sub.0=8000 Hz, which is realized by virtue of the modulator 40, which has 64 spiral lines, rotating at a rotational frequency n=125 Hz.

[0122] The modulator 40 rotates in the counterclockwise direction, as indicated by the arrow at the top left in FIG. 2, such that the spiral lines 41 move from the outer region inward. The light RL reflected from the object O is directed by the lens 30 through that region of the mask pattern 42 which is designated by 46, which region is illustrated again in an enlarged manner outside the mask. In the region 46, the spiral lines 41 move counter to the x-axis, wherein the reflected light RL is modulated with the zero frequency f.sub.0=8000 Hz.

[0123] The modulated light RL, as is shown in FIG. 1, is focused as a sequence of light signals LS onto the light detector 60 via the receiving optical unit 50, which light detector can be protected against excessively intense exposure by the shutter (not shown). The light detector 60 converts the light signals LS into electrical signals ES, amplifies them as necessary and forwards them to the evaluation unit synchronized with the laser.

[0124] The evaluation unit ascertains the frequency f.sub.x of the signal component that is caused by backscattering objects O, e.g. aerosol particles, in the detection region D at the distance g=300 m and calculates the transverse velocity component v.sub.x therefrom in accordance with equation (9).

[0125] In order also to be able to measure the transverse velocity component v.sub.y of the wind velocity, the reflected light RL is split by the beam splitter 130 and the second partial beam RL2 of the reflected light RL is directed through a region 48 offset by 90° relative to the region 46 on the mask, where it is modulated by the spiral lines 41, which here move counter to the y-direction.

[0126] The signal modulated in this way is then focused onto the second light detector 160 by the second receiving optical unit 150 and is registered and analyzed in the evaluation unit in the same way as was described for the x-component.

[0127] An overview of the expected frequencies for a velocity of 2 m/s and different measurement distances g is given in Table 3.

TABLE-US-00003 TABLE 3 Relationship between measurement distance, magnification, grating constant and frequency Measurement Magnification Grating Frequency distance g [m] V constant G [mm] for 2 m/s 15 2.8 0.71 10820.56 30 6.7 1.67 9198.93 45 10.5 2.63 8761.26 60 14.3 3.59 8557.68 75 18.2 4.55 8440.01 90 22.0 5.50 8363.34 105 25.9 6.46 8309.43 120 29.7 7.42 8269.45 135 33.5 8.38 8238.61 150 37.4 9.34 8214.11 165 41.2 10.30 8194.18 180 45.0 11.26 8177.64 195 48.9 12.22 8163.69 210 52.7 13.18 8151.78 225 56.5 14.14 8141.48 240 60.4 15.10 8132.49 255 64.2 16.05 8124.58 270 68.1 17.01 8117.55 285 71.9 17.97 8111.28 300 75.7 18.93 8105.64

[0128] The receiving optical unit 50 always ensures that the backscattered light modulated by the modulator 40 is focused onto the light detector 60. It is preferably embodied as a system of lens elements.

[0129] Table 4 summarizes again those parameters of the above-explained measurement which determine the backscattering signal RL. Table 4 shows that the backscattering signal consists only of a few photons. The scanning region, given by the region of the depth of field of 1.83 m at the measurement distance g=300 m, i.e. the height range 300 m±0.92 m, is traversed by the laser signal within 6.1 ns and excites an aerosol particle O present in this region to backscattering. The same time period of 6.1 ns should be taken into account for the backscattering, with the result that the backscattering signal should be detected in a time interval with a length of 12.2 ns. In this time period to be assigned to the region of the depth of field (1.83 m), on average 58.1 photons are registered, which corresponds to 3.2 photons/ns. This very weak backscattering signal is converted into an electrical signal by the light detector 60, for example a photomultiplier, is amplified and is fed to the evaluation unit.

TABLE-US-00004 TABLE 4 Values of the parameters of the backscattering signal Parameter Value Unit Transmission losses η.sub.t 0.8 Reception losses η.sub.x 0.7 Pulse energy E.sub.x 30 μJ Pulse repetition frequency PRF 50 kHz Average laser power P.sub.average 1.5 W Overlap function O(g) 0.5 Measurement distance g 300 m Atmospheric transmission losses T 95.1 % Backscattering coefficient β 10.sup.−6 m.sup.−1sr.sup.−1 Aperture D 35 cm Area of the aperture A.sub.R 962.1 cm.sup.2 Power P.sub.Sgathered via the aperture 1217.63 pW Wavelength λ 532 nm Photon flux 3.2 Photons/ns Scanning region 1.83 m Photons in the scanning region 39.54 Photons/1.83 m

[0130] By way of example, within the apparatus 10 it is possible to use a light source 20 in the form of a monochromatic pulsed laser 22 having a wavelength of λ=532 nm, which allows particularly simple adjustment of the LiDAR system.

[0131] The pulsed laser 22 additionally has the following parameters coordinated with a desired maximum measurement distance of g=300 m and with a chosen lens: [0132] Pulse repetition frequency (repetition rate) of the laser: PRF=50 kHz, [0133] Pulse duration of the laser pulses: ΘP=800 ps, [0134] Average laser power: Paverage=1.5 W, [0135] Energy per laser pulse: Ex=30 μJ

[0136] Between the pulse duration ΘP and the vertical spatial resolution s there is the relationship s=½.Math.c.Math.ΘP. The factor ½ arises because the measurement distance g is traversed twice, by the laser pulse and by the far weaker backscattering pulse that is registered.

[0137] The chosen pulse duration ΘP=800 ps ensures a vertical spatial resolution s of 12 cm, such that the predefined vertical spatial resolution of 15 cm is attained. The setting to the measurement distance g by way of the pulse propagation time tP: g=½.Math.c.Math.tP is effected in a corresponding manner.

[0138] With the apparatus 10 it is thus possible for example to measure wind velocities of up to 20 m/s in a plurality of detection regions situated at measurement distances (heights) g of between 50 m and 300 m.

[0139] It is evident that the apparatus 10 according to the invention has distinct advantages over the systems and apparatuses from the prior art: [0140] The apparatus 10 has a distinctly lower energy loss of the light beam compared to EP 2 062 058 B1. [0141] The construction of the transmission path 190 and that of the receiver path 80 are distinctly simplified. [0142] The emitted light beam L does not have to be shaped and adapted by means of a plurality of successive lens elements. This results in a distinctly more robust system. [0143] Interference generated by the mask 40 does not arise. [0144] The mask signal can be modulated on any received light reflection. [0145] The focal length of the lens 30 is adaptable, whereby the magnification of the grating can be controlled. [0146] The detection is simplified by virtue of the higher radiation power. [0147] The system 10 is easier to reproduce because the calibration of the receiving optical unit 50 is associated with significantly less complexity in comparison with the transmitting optical unit. As a result, the production of the apparatus is more economic in comparison with the conventional apparatuses. [0148] By virtue of the greater option for parameterization, the system 10 can be designed rapidly and simply for planned measurement distances. [0149] The size of the detection region D is defined by the optics and can therefore be given very small dimensioning even at a large distance.

[0150] It is furthermore evident that the invention relates to an apparatus 10 for ascertaining a velocity component v.sub.x, v.sub.y of an object O that moves in a detection region D at a measurement distance g relative to the apparatus 10 and reflects light L originating from a light source 20, wherein the object O generates reflected light RL that emanates from the detection region D. For this purpose, the apparatus 10 has a lens 30, a modulator 40, a receiving optical unit 50 and a light detector 60, wherein the lens 30 captures the reflected light RL generated by the object O in the detection region D and images it onto the modulator 40, wherein the modulator 40 modulates the reflected light RL into a sequence of light signals LS, wherein the receiving optical unit 50 images the sequence of light signals LS generated by the modulator 40 onto the light detector 60, and wherein the light detector 60 converts the sequence of light signals LS into a sequence of electronic signals ES. The apparatus furthermore has an interface 70, which forwards the sequence of electronic signals ES generated by the light detector 60 to an evaluation unit. With this apparatus 10, which forms a LiDAR system for the remote measurement of at least one transverse velocity component of an object O in a three-dimensional space, the direction of the measurement is freely selectable. In particular, the apparatus 10 allows the measurement of the velocity of drifting objects concomitantly moved in fluids, thereby enabling a ground-based measurement of the wind velocity at different heights with high spatial and temporal resolution and also a non-contact measurement of the flow velocity in liquids. The velocity of objects 0 that move independently of fluids can likewise be measured.

[0151] The invention is not restricted to any of the embodiments mentioned above. By way of example, the modulator 40 is preferably provided with a pattern 42 of Archimedean spirals 41. However, it is also possible to use other patterns and structures, e.g. other types of spirals or grating structures.

[0152] Symbols

[0153] A.sub.R Area of the aperture D of the lens: A.sub.R=(¼)πD.sup.2

[0154] b Opening diameter of the iris stop

[0155] b.sub.nop Width of the non-opaque lines of the modulator

[0156] b.sub.op Width of the opaque lines of the modulator

[0157] c Velocity of light in the respective medium

[0158] d Diameter of the laser beam (as a function of the measurement distance g)

[0159] D Aperture of the lens

[0160] d.sub.f Depth of field: Distance with respect to the far point

[0161] d.sub.n Depth of field: Distance with respect to the near point

[0162] d.sub.0 Diameter of the emergent laser beam after shaping

[0163] d.sub.Obj Diameter of the detection region captured by the lens

[0164] E.sub.x Pulse energy

[0165] f.sub.0 Frequency at which the grating lines of the modulator move (zero frequency)

[0166] f Focal length of the lens

[0167] f.sub.x Measured frequency of the backscattering signal in the x-direction

[0168] f.sub.y Measured frequency of the backscattering signal in the y-direction

[0169] Δf.sub.x Frequency shift of f.sub.x relative to the zero frequency

[0170] Δf.sub.y Frequency shift of f.sub.y relative to the zero frequency

[0171] g Measurement distance from the lens to the detection region

[0172] G Grating constant of the image of the modulator projected back into the detection region

[0173] G.sub.0 Basic grating constant of the modulator

[0174] h Depth of field in the detection region

[0175] n Rotational frequency of the modulator

[0176] N.sub.Sp Number of spirals of the modulator

[0177] O(g) Overlap function

[0178] P.sub.average Average power of the laser

[0179] P.sub.S Gathered power of the backscattering signal via the aperture d of the lens

[0180] PRF Pulse repetition frequency (repetition rate) of the laser

[0181] s Vertical spatial resolution

[0182] t.sub.p Pulse propagation time

[0183] T Atmospheric transmission losses

[0184] v Velocity vector of an object in two- or three-dimensional space

[0185] |v| Magnitude of the velocity v of an object

[0186] v.sub.x, v.sub.y Velocity components of an object transversely with respect to the measurement direction (transverse velocity components)

[0187] v.sub.z Velocity component of an object in the measurement direction (longitudinal velocity component)

[0188] v.sub.max Maximum value of the velocity to be measured

[0189] V Magnification factor G/G.sub.0

[0190] α Direction of the velocity v of an object

[0191] β Backscattering coefficient

[0192] η.sub.t Transmission losses

[0193] η.sub.x Reception losses

[0194] λ Wavelength of the light source

[0195] φ Divergence angle of the shaped laser beam

[0196] Θ.sub.P Pulse duration of the laser pulses (when a pulsed laser is used)

REFERENCE SIGNS

[0197]

TABLE-US-00005 10 Apparatus 12 Housing 20 Light source 22 Laser 30 Lens 34 Focal plane 40 Modulator 41 Spiral line 42 Pattern 43 Opaque line/region 44 Non-opaque line/region 46 First region of the modulator 48 Second region of the modulator 50 Receiving optical unit 54 Focal plane 60 Light detector 70 Interface 80 Optical receiver path 90 Mirror optical unit 92 First deflection mirror 94 Second deflection mirror 130 Beam splitter 150 Second receiving optical unit 155 Filter unit 160 Second light detector 180 Second optical receiver path 190 Optical transmission path A Axis D Detection region ES Electronic signals L Light LS Light signals LS1 Light signals LS2 Light signals O Object RL Reflected light RL1 Partial beam RL2 Partial beam