DE3930272A1 - LIDAR - Google Patents

Abstract

The lidar (1) comprises a transmitter (3) for linear-polarized radiation (12), a receiver (5) having a first receiving device which measures the intensity of the back-scattered radiation (15) from the transmitted cone of rays (12) in the polarization plane thereof and perpendicular thereto, and a second receiving device which measures the intensity of the back-scattered radiation (17) from an annular spatial region outside the transmitted cone (12), and an evaluating device (7) for evaluating the measured signals in order to determine whether a wall of fog (14) or snow or rain or a solid obstacle to visibility is present at a distance (d) to be measured. The nature of the obstacle and its distance (d) from the lidar (1) are indicated in an indicator (43a, 43b, 43c, 43d, 44). The lidar (1) according to the invention can be incorporated as a distance monitor in a motor vehicle and, in conjunction with a data processing device and a tachometer measuring the speed of the vehicle, can be used to obtain an optimum speed of the vehicle by acting on the drive and/or brake system, depending on the environmental conditions determined from the measured data. <IMAGE>

Description

The invention relates to a lidar, according to the preamble of the patent claims 1 and 10 and a distance warning device for motor vehicles, according to the preamble of claim 8.

It is known to use a lidar to carry out transmission, extinction and backscatter measurements in the atmosphere in order to detect gases or particles present in it, to determine their concentration and distance from the position of the lidar. As an example, here are the publications by VE Derr, ′ ′ Estimation of the extinction coefficient of clouds from multiwavelength lidar backscatter measurements ′ ′, Appl. Opt., Vol. 19, No. 14, pp. 2310-2314 for the investigation on clouds, by JS Randhawa et al, "Lidar observations during dusty infrared Test-1", Appl. Opt., Vol. 19, No. 1/4, pp. 2291-2297, for backscatter and transmission measurements on explosion clouds from TNT using a lidar equipped with a CO ₂ or ruby laser and from DK Kreid, "Atmospheric visibility measurement by a modulated cw lidar", Appl. Opt., Vol. 15, No. 7, pp. Mentioned 1823-1831. With the measurement of the absorption and / or scatter of light in the atmosphere also deal z. B. DE-OS 23 51 972 and DE-OS 23 28 092.

In the publications mentioned a lidar is described, the sen laser beam is transmitted via a transmission optics. The sent out Laser radiation is absorbed and scattered in the atmosphere. An emp optics, the opening angle of which approximates the opening angle of the corresponds to the transmitted beam, measures the scattered light. From the intensity of the scattered light, the known scattered cross sections of different Gases and particles as well as the spectral absorption is because of the wavelength of the laser is known on the extinction coefficient of the atmosphere and thus inferred the density of admixed gases and particles.

Another type of lidar is also known, in which two La Ser rays of adjacent wavelength are used, the one Wavelength particularly strongly absorbed by the gas to be determined or is scattered, and the neighboring wavelength through this gas almost undergoes no absorption or scattering. The "unaffected" scattered back Laser radiation then serves as a reference for the absorption to be determined of the gas in the atmosphere.

Since the intensity of the backscattered light increases with the distance of the scattering volume from the lidar with a higher than square power decreases, electronic gate circuits are used, which only always evaluate a certain distance range. The lasers send in usually short radiation pulses in the order of a few tens of nano seconds off; however, modulated are also continuously worked ting laser used.

The known lidar designs are suitable for measuring the Ex Absorption coefficients of the atmosphere in general and for measuring the Ex tinktion of an admixed gas or particles. However, you are not suitable, different types of atmospheric turbidity from each other distinguish clearly and quickly, such as B. Snow from fog, rain or a fixed obstacle to visibility.

The invention has for its object to provide a lidar which clearly and quickly differentiates between different atmospa  rischer cloudiness such. B. snow, fog and rain from each other takes.

The solution to the problem is the subject of claims 1 and 10. With the subject of claim 8, the further task solved a distance warning device for motor vehicles with a lidar, which indicates the type of obstruction and its distance fen.

Preferred embodiments are in claims 2 to 7 of the lidar according to the invention and a preferred in claim 9 Embodiment of the distance warning device according to the invention described.

The following is an example of the lidar and of the distance warning device according to the invention based on drawings Illustrations explained in more detail. Show it:

Fig. 1 is a schematic representation of a lidar according to the invention,

Fig. 2 is an idealized distance-corrected lidar signal of a smoke screen,

Figure 3 is an idealized not corrected distance lidar signal belwand a Ne.,

Fig. 4 is an idealized distance-corrected lidar signal wall of a snow,

Fig. 5 is an idealized distance-corrected lidar signal of a rain wall,

Fig. 6 shows an idealized, non-distance-corrected lidar signal of a most visible obstacle, and

Fig. 7 is a block diagram of additional components for a speed control of a motor vehicle.

The lidar 1 shown in Fig. 1 has a transmitter 3, a Em pfänger 5 and an evaluation device. 7 The transmitter 3 is a laser 9 , the emitted pulsed and linearly polarized radiation in the infrared wavelength range is above 1.4 μm wavelength, so that its radiation is not harmful to the eyes. The electric field vector of the linearly polarized radiation of the laser 9 lies in the plane of the drawing. The radiation is expanded by means of a transmission optics 11 to a beam 12 with an opening angle of approximately 10 mrad.

In front of the lidar 1 there is a fog wall 14 at a distance d , into which the beam 12 penetrates and is partially reflected or scattered back against the incident droplet direction in a beam 15 . Another part of the incident radiation 12 is scattered several times and returns to the lidar 1 outside the radiation cone of the radiation 12 mainly in a radiation cone 17 of 10 to 30 mrad. For a clear representation, the angles of the beams 12 and 15 and of the radiation cone 17 are shown greatly enlarged in FIG. 1.

A first receiving device for the detection of the beam 15 has a Cassegrain mirror arrangement 19 (main mirror 20 pierced in the middle and the secondary mirror 22 is located in the optical axis above the borehole 21 ), a diaphragm 23 , an optical system 24 which detects the diaphragm 23 emerging rays 15 and 17 transformed into parallel rays and only lets radiation that comes from the spectrum of the laser 3 (narrow band filter 26 ), a Wollaston prism 25 , which ches the rays 15 and 17 in two rays 15 a / 17 a and 15 b / 17 b divides and a focusing lens 27 a and 27 b , which focuses the radiation 15 a and 15 b onto a photodiode 29 a and 29 b as a detector. The Wollaston prism 25 is arranged in such a way that linear polarized radiation of the beam 15 , the electric field vector of which is parallel to the electric field vector of the beam 12 , ie lies in the plane of the zei , leaves it as the beam 15 a . For this purpose, perpendicularly polarized radiation from the beam 15 leaves the Wollaston prism 25 as beam 15 b .

A second receiving device is used to detect the beam 17 . It includes, like the first receiving device, the Casse grain mirror arrangement 19 , the aperture 23 , the optical system 24 and the Wollaston prism 25 , which emits the radiation of the beam 17 into a linearly polarized beam 17 a , the electric field vector of which lies in the plane of the drawing , and a linearly polarized beam 17 b , the polarization plane of which is perpendicular to beam 17 a , splits, and additionally a focusing lens 31 a and 31 b , which beam 17 a and 17 b onto a photodiode 33 a and 33 b focused as a detector. In order not to overload Fig. 1, only the "beam limits" 12 , 15 , 15 a , 15 b , 17 , 17 a and 17 b are shown.

The electrical signals _generated by the photodiodes 29 a , 29 b , 33 a and 33 b depending on the radiation intensity I ₁₅ _∥ , I ₁₅ _⟂ , I ₁₇ _∥ and I ₁₇ _{⟂ are generated} with an amplifier 34 a , 34 b , 34 c and 34 d , the gain of which is adjustable, as described below, reinforced.

A small part of the beam 12 emitted by laser 9 is guided via an optical fiber 35 to a photodiode 36 , which is connected to a timing element 38 . An output 37 a of the timing element 37 is connected to a signal processing element 41 and each of four further outputs is connected to one of the four amplifiers 34 a , 34 b , 34 c and 34 d . Four displays 43 a to 43 d , which are labeled with the words fixed obstacle, fog, snow and rain, are electrically connected to the signal processing element 41 . A fifth display 44, which is also connected to the signal processing element 41 , is present to represent the distance d from the lidar 1 to the smoke screen 14 .

The laser 9 transmits pulses with a repetition frequency of a few hundred Hertz with a pulse width of a few tens of nanoseconds, which are widened with the transmitting optics 11 to the beam 12 with an opening angle of approximately 10 mrad. A fog wall 14 is located at a distance d from the lidar 1 in 200 meters. A part of each laser pulse passes through the light guide 35 to the photodiode 36 , which starts the timer 37 . The timer 37 is designed such that after the first laser pulse at its outputs 37 a to 37 e after 60 ns, digitally coded information "90.0" corresponding to the running time of 60 ns for a return trip of 180 m, ie a distance d of a possible visual obstacle of 90 m.

By means of the digitally coded information, the amplifiers 34 a to 34 d are set to a sensitivity for approximately 10 ns, which is an electrical signal from the relevant photodiode 29 a , 29 b , 33 a or 33 b corresponding to backscattered radiation to be expected at a distance of 90 m Would increase visibility obstacles sufficiently. After about a millisecond (corresponding to the set repetition frequency of the laser pulses), another laser pulse is emitted and the timer 37 now gives digitally coded information "91.5" after 70 ns corresponding to a possible obstruction in sight in 91.5 m from the lidar 1 to the amplifier 34 a to 34 d and the signal processing element 41 , etc.

Since the intensity of the backscattered radiation 15 and 17 decreases in dependence on the distance with a higher than quadratic power, an electronic "window" is used, in which a route is divided into small areas and after each laser pulse sent a different area is examined for backscattered radiation of the pulse just emitted. As a result, strong stray radiation from the areas close to lidar 1 can be suppressed and the amplification of the received signals can be adjusted in such a way that their signal level is corrected for distance. The distance of the scattered radiation from the room area is given by the switch-on time of the electronic "window". The geometric resolution of the distance results from the opening time of the "window" (10 ns∎1.5 m).

Since the fog wall 14 is only 200 m away, the above measurement does not provide a signal. Only after the timing element 37 transmits the digitally coded information “201” to the amplifiers 34 a to 34 d and to the signal processing element 41 after 1340 ns, does the photodiode 29 a , which contains only scattered and reflected radiation within the beam 15 , have their polarization plane parallel to the plane of polarization of the laser radiation in the beam 12 , and the photodiodes 33 a and 33 b receive radiation from the multiply scattered radiation of the beam 17 , a signal. With the following laser pulses, with each laser pulse backscattered radiation is received from a part of the room 1.5 m further away, grows, as in Fig. 2a (distance- _corrected electrical signal I ₁₅ _∥ the photodiode 29 a after the amplifier 34 a ) with logarithmic ordinate for the signal I ₁₅ _∥ , the processed electrical signal of the parallel polarized backscattered radiation of the beam 15 over the distance as the abscissa up to a maximum value I _max and then decreases again. The distance-corrected signals I ₁₇ _∥ and I ₁₇ _⟂ of the scattered light received by the photodiodes 33 a and 33 b of the beam 17 are plotted logarithmically over the linear distance d from the lidar 1 in FIGS . 2c and 2d. In Fig. 2b, the corrected scattered light I ₁₅ _⟂ with perpendicular polarization of the beam 15 is logarithmically plotted over the linear distance d . The signals I ₁₇ _∥ and I ₁₇ _{⟂ also} increase to a maximum value and then decrease again. The signal I ₁₅ _⟂ reaches no values above the noise.

In Fig. 3 the measured with the photodiode 29 a , but not distance-corrected signal is shown for better understanding up to a distance of 500 m, the 200 m thick smoke wall 14 being at a distance d of 200 m.

If these measurements are repeated with a wall of snow and rain wall so obtained for the snow 5a nit in FIGS. 4a to 4d and rain in FIGS. 5d shown idealized resulting. The assignment of the representations to the photodiodes 29 a , 29 b , 33 a and 33 b is selected analogously to the representations of FIGS. 2a to 2d. To differentiate the measurement _results , the measured distance-corrected idealized intensities I ₁₅ _∥ , I ₁₅ _⟂ , I ₁₇ _∥ and I ₁₇ _{⟂ receive} a further index N for the measurement on the fog wall, R for the rain wall and S for the snow wall.

Surprisingly, the measurement has now turned out accordingly from the process described above:

At the smoke _screen 14 , a distance- _corrected signal I ₁₅ _{∥ N} , I ₁₇ _{∥ N} and I ₁₇ _{⟂ N is} obtained; the signal I ₁₅ _{⟂ N} is not present;
with a snow wall (not shown), a distance-corrected signal I ₁₅ _{∥ S} , I ₁₅ _{⟂ S} , I ₁₇ _{∥ S} and I ₁₇ _{⟂ S} are obtained;
with a rain wall (not shown) a distance-corrected signal I ₁₅ _{∥ R is} obtained; the signals I ₁₅ _{⟂ R} , I ₁₇ _{∥ R} and I ₁₇ _{⟂ R} are negligibly small.

If there is a fixed obstacle to vision (not shown), whose non-distance-corrected signal from photodiode 29 a is shown in FIG. 6 , a signal I ₁₅ _{∥ H} with pointed, needle-shaped rise is detected compared to the above signals from photodiode 29 a . Depending on the type of surface (specular or diffuse), a portion I ₁₅ _{⟂ H} can also be.

From the above criteria, the signal processing element 41 determines an unambiguous assignment as to whether the measured obstruction is a wall of fog, snow or rain or a fixed obstruction. In a fog, snow or rain wall determines the signal processing element 41-founded from the data transmitted from the timer 37 digitalco values for each measurement a respective distance value d. From the position value d is approximately in the middle of the signal increase I ₁₅ _{∥ N} , I ₁₅ _{∥ S} or I ₁₅ _{∥ R.} In the case of a fixed obstacle to vision, the averaging is omitted due to the needle-shaped, sharp signal rise of the signal I ₁₅ _{∥ H.} The average distance d is shown in the display 44 as a number in meters. In addition, according to the above evaluation, the respective display 43 a , 43 b , 43 c or 43 d lights up according to a wall of fog, snow or rain or a fixed obstacle to visibility.

If there is a fixed visual obstacle within the wall of fog, snow or rain, the curves shown in FIGS . 2a, 2b, 4a, 4b, 5a, 5b are superimposed by a pointed, needle-shaped pulse which is generated by the signal processing element 41 as a fixed visual obstacle is recognized. In this case, in addition to the display 43 a , 43 b or 43 c for the fog, snow or rain wall, the display 43 d for the fixed sight obstacle. The distance indicator 44 shows the distance d of the fixed sight.

The above lidar 1 can be installed in a motor vehicle (not shown), the above measuring method being carried out in the direction of travel. The displays 43 a , 43 b , 43 c and 43 d of the wall of fog, snow or rain or a fixed obstacle to visibility are excellent driving aids for the driver. If the data of the signal processing element 41 , as shown in FIG. 7, is transmitted to a data processing device 47 , which determines the instantaneous speed of the vehicle via a tachometer 49 , the instantaneous driving speed of the vehicle can be optimally controlled in dependence on visual obstacles that occur data processing means 47 depending on the data transmitted from the tachometer 49 and the signal processing element 41 to the fuel supply 50 and thus to the drive system or the braking device 51 acts of the vehicle.

The display 43 c of a rain wall is also activated when water is whirled up from the road by a motor vehicle in front.

As has emerged from the above measurements, the results obtained with the photodiodes 33 a and 33 b differ only slightly. One of the photodiodes 33 a and 33 b with its amplifier 34 b and 34 d can therefore be omitted without reducing the accuracy of the values determined. The use of both diodes 33 a and 33 b allows under ge environmental conditions a more precise evaluation.

Depending on the distances to be measured, the start and end values can be set with the timer 37 , other repetition frequencies of the laser 9 and a resolution other than 10 ns corresponding to 1.5 m can also be selected.

Claims (11)

1. Lidar ( 1 ), characterized by a transmitter ( 3 ) for emitting a beam ( 12 ) with a predetermined opening angle, a receiver ( 5 ) with a first and a second receiving device for back-scattered radiation ( 15 , 17 ) of the emitted beam ( 12 ), the receiving optics of which are designed in such a way that their reception area areas do not essentially overlap, and an evaluation device ( 7 ) for evaluating the electrical signals of the first and second receiving device in accordance with the received backscattered radiation ( 15 , 17 ), to determine the type of turbidity in the atmosphere. 2. Lidar ( 1 ) according to claim 2, characterized in that the transmitter ( 3 ) is constructed such that it emits pulsed, linearly polarized radiation in the infrared spectrum, which is not harmful to the eyes. 3. Lidar ( 1 ) according to claim 1 or 2, characterized in that the receiving optics of the first receiving device is constructed such that essentially only the emitted beam ( 12 ) lies in its reception area, and the receiving optics of the second receiving device is constructed in such a way that the emitted beam ( 12 ) is not in its reception area. 4. Lidar ( 1 ) according to claim 3, characterized in that the receiving optics of the second receiving device is constructed such that its reception area is around the receiving area of the receiving optics of the first receiving device and the second receiving device a first detector ( 33 a ) Detection of multiple scattered radiation ( 17 ) of the beam ( 12 ). 5. Lidar ( 1 ) according to claim 4, characterized in that the second receiving device has a second detector ( 33 b ) and in front of both detectors ( 33 a , 33 b ) each has a polarization-analyzing element ( 25 ) to detect the beam ( 12 ) to determine multiple backscattered radiation intensity according to their polarization parallel and perpendicular to the polarization direction of the emitted beam ( 12 ). 6. Lidar ( 1 ) according to any one of claims 1 to 5, characterized in that the first receiving device has a polarization-analyzing element ( 25 ) in front of a first and second detector ( 29 a , 29 b ) to the beam ( 12 ) to determine backscattered radiation intensity according to its polarization parallel and perpendicular to the polarization direction of the emitted beam ( 12 ). 7. Lidar ( 1 ) according to claim 6, characterized in that the evaluation device ( 7 ) has a first display ( 43 d ) for a fixed visual obstacle, which indicates when the evaluation device ( 7 ) from the first and / or second detector ( 29 a , 29 b ) of the first receiving device receives a needle-shaped pointed pulse, has a second display ( 43 a ) for fog as a visual obstacle ( 14 ), which indicates when the evaluation device ( 7 ) from the second detector ( 29 b ) of the first Receiving device compared to its first detector ( 29 a ) negligible signal and from the detector ( 33 a ) or from the detectors ( 33 a . 33 b) of the second receiving device a slowly rising and falling distance corrected signal ( I ₁₇ _{∥ N} , I ₁₇ _{⟂ N} ), has a third display ( 43 c ) for rain or water whirled up from the ground as a visual obstacle, which indicates when the evaluation device ( 7 ) from the second detector ( 29 b ) of the first E mpfangsvorrichtung a ge compared to the signal ( I ₁₅ _{∥ R} ) of the first detector ( 29 a ) negligible signal ( I ₁₅ _{⟂ R} ) and from the detector ( 33 a ) or from the detectors ( 33 a , 33 b ) of the second receiving device only receives a negligibly small slowly rising and falling distance-corrected signal ( I ₁₇ _{∥ R} , I ₁₇ _{⟂ R} ), has a fourth display ( 43 b ) for snow as a visual obstacle, which indicates when the evaluation device ( 7 ) from the first and second detector ( 29 a , 29 b ) of the first receive device over the course of time an approximately equal signal ( I ₁₅ _{∥ S} , I ₁₅ _{⟂ S} ) and from the detector ( 33 a ) or from the detectors ( 33 a , 33 b ) of the second Receiving device _receives a slowly rising and falling distance-corrected signal ( I ₁₇ _{∥ S} , I ₁₇ _{⟂ S} ). 8. Distance warning device for motor vehicles, with a lidar ( 1 ) according to claim 7, characterized in that the evaluation device ( 7 ) has a fifth display ( 44 ) for the distance ( d ) of the visual obstacle from the vehicle equipped with the device, which from the running time of the lidar ( 1 ) to the obstacle to vision ( 14 ) and radiated back by this radiation pulses is determined by means of the electrical pulses of the first receiving device. 9. Distance warning device according to claim 8, characterized by an electronic data processing device ( 47 ) which determines a maximum speed from the distance ( d ) and the type of visual obstacle, taking into account the roadway state to be expected thereon, indicating and / or exceeding the same prevented by action on the drive and / or braking system ( 50 , 51 ) of the vehicle. 10. Lidar ( 1 ) to determine the causes of turbidity of gases, characterized in that the transmitter ( 3 ) for linearly polarized radiation ( 12 ) is designed, and that the receiver has a first receiving device with a first detector ( 29 a ) for radiation parallel to the direction of polarization of the transmitter ( 3 ), which is reflected in the direction of radiation reflected back ( 15 ), and a second detector ( 29 b ) for radiation perpendicular to the direction of polarization of the transmitter ( 3 ) in the direction of transmission radiation ( 15 ) and has an evaluation device ( 7 ) for determining the ratio of the electrical output signals of the two detectors ( 29 a , 29 b ). 11. Lidar ( 1 ) according to claim 10, characterized in that the receiver ger a second receiving device with a detector ( 33 a , 33 b ) for backscattered radiation ( 17 ) of the transmitter ( 3 ) from a beam ( 12 ) not including it Has space area, the evaluation device ( 7 ) additionally determines the ratio of the electrical output signal of the detector ( 33 a , 33 b ) of the second receiving device to that of the first ( 29 a) and / or second ( 29 b) detector of the first receiving device and the first and second detectors ( 29 a , 29 b ) are designed such that they receive approximately only the backscattered radiation ( 15 ) within the emitted beam ( 12 ).