U.S. patent application number 09/778287 was filed with the patent office on 2001-12-20 for radar system for determining optical visual range.
Invention is credited to Hahlweg, Cornelius.
Application Number | 20010052872 09/778287 |
Document ID | / |
Family ID | 7630152 |
Filed Date | 2001-12-20 |
United States Patent
Application |
20010052872 |
Kind Code |
A1 |
Hahlweg, Cornelius |
December 20, 2001 |
Radar system for determining optical visual range
Abstract
A radar system which is used for determining optical range
includes a frequency analyzer for the analysis of differential
frequency signals, which originate from a mixture of the
transmitted signal with the received signals, for example reflected
from a fog bank. The frequency spectrum images the distance profile
of the fog bank.
Inventors: |
Hahlweg, Cornelius;
(Hildesheim, DE) |
Correspondence
Address: |
Richard L. Mayer, Esq.
KENYON & KENYON
One Broadway
New York
NY
10004
US
|
Family ID: |
7630152 |
Appl. No.: |
09/778287 |
Filed: |
February 7, 2001 |
Current U.S.
Class: |
342/128 ;
356/5.09 |
Current CPC
Class: |
G01S 17/88 20130101;
Y02A 90/10 20180101; Y02A 90/19 20180101; G01S 17/95 20130101; G01S
17/34 20200101; G01S 17/931 20200101 |
Class at
Publication: |
342/128 ;
356/5.09 |
International
Class: |
G01C 003/08; G01S
013/08 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 8, 2000 |
DE |
1 00 05 421.8 |
Claims
What is claimed is:
1. A radar system comprising: means for generating a
frequency-modulated transmitted signal, a power of the transmitted
signal being frequency-modulatable with linear frequency variation
in time intervals; means for receiving signals reflected from an
object; means for multiplying the transmitted signal by the
reflected signals; a frequency analyzer for calculating a frequency
spectrum corresponding to a distance profile as a function of
differential frequency signals; a low-pass filter; and means,
coupled downstream from the low-pass filter upstream from the
frequency analyzer, for automatically compensating for a signal
strength of the reflected signals, the signal strength depending on
a distance of the reflecting object.
2. The radar system according to claim 1, wherein the radar system
is an FMCW radar system for a motor vehicle.
3. The radar system according to claim 1, wherein the frequency
analyzer includes a fast Fourier transformer.
4. The radar system according to claim 1, wherein the means for
compensating includes a fourth-order differentiating element.
5. The radar system according to claim 4, wherein the
differentiating element is formed by a corresponding edge of at
least one of a high-pass filter and a band pass filter.
6. The radar system according to claim 1, wherein the transmitted
signal is a light signal.
7. The radar system according to claim 6, wherein the means for
generating includes a laser diode for providing the light
signal.
8. The radar system according to claim 6, wherein the means for
generating includes a high-power light diode for providing the
light signal.
9. The radar system according to claim 1, wherein the transmitted
signal is divergently emissible.
10. The radar system according to claim 1, wherein the transmitted
signal is rectangularly modulatable.
Description
BACKGROUND INFORMATION
[0001] A radar system is described in German Patent Application No.
196 32 889, in which, however, the range finding is limited to the
measurement of distances to solid bodies.
SUMMARY OF THE INVENTION
[0002] In contrast to this, the radar system according to the
present invention has the advantage of being capable of creating
distance profiles to diffuse objects, in particular fog banks.
Thus, for example, quantitative information concerning visual range
can be obtained in a motor vehicle independently of the driver.
This functionality can be integrated here advantageously via simple
additional technical measures in a device which is already used for
distance sensing, for example for parking or for automatic vehicle
control systems. Also advantageous is the simple feasibility of
digital signal processing as a result of the relatively
low-frequency difference signals to be used, for which inexpensive
digital signal processors are available. Particularly advantageous
is the provision of automatically operating compensation means,
which permit realistic imaging of diffuse objects, in particular of
fog banks. In particular, the driver can detect the actual extent
and the actual spatial density distribution of such areas and thus
realistically evaluate the traffic situation/obstacle to
visibility.
[0003] If light which is frequency-modulated in its intensity is
used as a transmitted signal, a real image of areas of possible
obstacles to visibility is provided by the type of the measurement
itself, since the transmitted signal is of the same physical nature
as the light beams which are detected by the human eye and produce
the optical image in the eye. Moreover, in the case of using light
as an information carrier, as opposed to measurements in the radio
frequency range, it is not necessary to pay attention to spectral
purity or to a band limitation, so that frequency modulation taking
account of harmonics can also be performed in a rectangular form
without impairing the measurement result. In the case of systems in
the radio frequency range, on the other hand, strict adherence to
the assigned frequency bands (for the most part, moreover, very
narrow-band ranges, cf. Industrial Scientific Medical bands) is
necessary (key words: wireless service, electromagnetic
compatibility, etc.); therefore a radar signal used in the radio
frequency range cannot be simply a rectangular sequence, since the
harmonic waves transmitted in this case would cause a
"contamination" far from the assigned band. In addition,
high-performance laser diode drivers, which are offered for optical
communication and are designed for pulse modulation, may be used
for the process of visual range determination.
[0004] Furthermore, the transmitted signal is emitted divergently
in an advantageous way, in order to obtain a good averaging over
the density of any existing fog layers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1a shows a first embodiment.
[0006] FIG. 1b shows a detail of a second embodiment.
[0007] FIG. 2a shows a fog bank.
[0008] FIG. 2b shows a first distance profile corresponding to the
fog bank.
[0009] FIG. 2c shows a second distance profile corresponding to the
fog bank.
DETAILED DESCRIPTION
[0010] FIG 1a shows a radar system with a control unit 10. Control
unit 10 has an analog multiplexer 20, which is supplied with a
positive direct voltage V+and a negative direct voltage V-. The
multiplexer is connected with an integrator 30. Control unit 10
further has a digital signal processor (DSP) 60, which controls
multiplexer 20. Signal processor 60 contains a fast Fourier
transformer 50 (abbreviated "FFT"), and an analog-digital converter
40. DSP 60 is connected to a display unit 70. Integrator 30
controls a frequency-modulatable oscillator (VCO) 80, which in turn
operates a laser diode 95 using a control voltage. Laser diode 95
is part of transmitting unit 90, which further includes an emitting
element 96 in the form of a dispersion lens. In addition, the radar
system possesses a receiving element 106 (lens) for collecting
optical radiation. A photo diode 105 is located behind the
receiving element. The output of the photo diode is connected to a
mixer 110, which mixes the photo diode signal with the signal of
oscillator 80. The mixed signal passes through a low-pass filter
120 and then a compensator 130. The compensator in turn is
connected to the input of analog-digital converter 40 of DSP
60.
[0011] DSP 60 controls analog multiplexer 20 for generating a
rectangular signal, which oscillates between the voltage values
V+and V-. Integrator 30 generates from this rectangular signal a
triangular signal, which is used for controlling oscillator 80.
Oscillator 80 produces a high-frequency oscillation in the gigaherz
range, modulated with the triangular signal. The laser diode is
controlled with this high-frequency oscillation and thus emits
intensity-modulated light, the frequency of the modulation varying
linearly according to the triangular signal. Thus frequency
modulation of the light intensity takes place in stages
corresponding to a slope (that is, linear increase and decrease of
the modulation frequency). Light scattered back from solid or
diffuse objects is collected by receiving element 106 and converted
into an electrical signal by photo diode 105. Mixer 110 generates
sum and differential frequency signals. Low-pass filter 120 is
adjusted so that it lets through only differential frequency
components of the mixed signal of interest. This occurs, for
example, in the case of a limiting frequency of 500 MHz of the
filter, when the maximum frequency deviation of oscillator 80 is of
this order of magnitude.
[0012] Compensator 130 is given by the edge of a high-pass filter,
or band pass filter, acting as a fourth-order differentiating
element. This filter produces an intensity correction, by passing
components of higher differential frequencies better. In this way
the intensity decrease of the optical radiation is compensated in
the case of a far-away scattering object. The differentiating
element here is of the fourth order, since for objects in the
remote field of the transmitter the received intensity is damped
both by the path of the transmitted signal as well as by the path
of the scattered signal, so that the inverse quadratic
proportionality of the intensity to the distance is to be taken
into account two times, and the differential frequency signal of
each scattering object, passed by the low-pass filter, has a power
which is inversely proportional to the fourth power of the
differential frequency. This is exactly what is compensated by
fourth-order differentiating element 130.
[0013] Analog-digital converter 40 digitizes the mixed signal of
the differential frequency components obtained and corrected in
their intensity; FFT 50 provides a spectral analysis of the mixed
signal capable of being evaluated by the DSP. The relationship
between spectral intensity and differential frequency here is a
direct reflection, for example, of the spatial density distribution
of a fog bank representing the back-scattering object. The DSP
transmits the spectral analysis directly to the display unit, so
that the driver of the vehicle learns the distance at which a fog
bank is located, and how far the latter extends in the direction of
travel. Alternatively, in the case of recognizably short visual
range (for example when it is less than a specific distance to the
fog bank), to warn the driver accordingly via the display unit or
via separate lights or warning sounds, or to switch on the fog
lights automatically.
[0014] As is known in this case from German Patent Application No.
196 32 889, a differential frequency fdiff corresponds to a
specific distance L, at which the scattering object (either a solid
body or a particle-containing volume of air, for example a volume
element of a fog bank) is located. In this case L is given by
L=fdiff.times.c/(2.times.df/dt),
[0015] c representing the speed of light and df/dt the constant
rise, respectively fall, of the modulation frequency in the
frequency slopes induced by the triangular signal. For example, if
a frequency deviation of 500 MHz corresponding to the maximum
frequency deviation of the VCO is chosen, and a measuring time of 1
second is set for this (which then corresponds to a frequency of
the triangular signal of 0.5 Hz), df/dt=500 MHz/1s. A frequency
resolution of the FFT of 1 Hz results because of the measuring time
of 1 second. If the FFT has 512 interpolation points for the
analysis of the mixed signal, the highest resolvable differential
frequency amounts to 256 Hz. Because of the above-mentioned
relationship between L and fdiff, this corresponds to a distance of
76.8 m, and 1 Hz corresponds to 30 cm. Thus, in this example
(diffuse) objects can be detected up to a distance of 76.8 meters
with a local resolution of 30 cm. The measuring times can be
shortened in the case of a given frequency deviation within the
framework of the steerability (input time constants of the VCO), in
order to reduce the undesirable averaging influence of the motion
of travel. The differential frequency components of the signal
mixture generated by mixer 110 are accordingly of higher
frequency.
[0016] FIG. 1b shows an alternative transmitting unit 91, which may
be used instead of transmitting unit 90. Transmitting unit 91
includes a threshold switch 97, whose output signal controls laser
diode 95, which emits its light via emitting element 96 in the way
already described.
[0017] Threshold switch 97 converts the sinusoidal oscillation of
oscillator 80 into a rectangular signal, so that it is possible to
use laser diodes of higher power than laser diode 95, available for
optical communication, which are designed for pulse modulation. The
rectangular signal still includes the original sine wave as the
fundamental wave. Furthermore, mixing in the multiplier takes place
with the sine signal. The harmonic waves of the rectangular
oscillation remain without influence on the signal mixture to be
evaluated, as long as the limiting frequency of the low-pass filter
is chosen suitably. The arrangement composed of mixer and low-pass
filter in principle represents a correlator (multiplication of two
signals and integration of the product), in the case of which a
correlate that is variable in time is obtained because of the short
time integration (low-pass filter), that is, actually a "slowly"
variable continuous component is evaluated. Since the harmonic
waves of the rectangle are orthogonal to the fundamental wave, the
system also functions as described, namely with a rectangle and the
sine of the fundamental wave. The limit frequency of the low-pass
filter in this case must lie below the lowest used instantaneous
frequency of the VCO. This specific embodiment makes use of the
fact that if light is used as a carrier, it is not necessary to pay
attention to spectral purity or band limitation, since presently
there are no specifications for modulated light with respect to the
frequencies usable for modulation. At the present time this is also
not necessary, since light is transported for communication
purposes anyway, except that it is transported enclosed, preferably
in glass fibers.
[0018] FIG. 2a shows a schematic representation of a fog bank 210,
which is located at a distance range of 30 to 70 meters in front of
a vehicle, the x coordinate of which is at the origin. Area 220,
covered by emitting elements 90 and 91, is marked with dotted
lines. The receiving area of receiving element 106 is marked with
solid lines and with reference symbol 230.
[0019] The radar system provides display unit 70 with first curve
240 shown in FIG. 2b, for example, starting from the object
position shown in FIG. 2a. It represents the uncompensated (that
is, obtained without the use of compensator 130) Fourier spectrum I
of the differential frequency signal mixture, the differential
frequencies corresponding to the spatial extent of the fog bank in
the x-direction detecting a range between 100 and 233 Hz. If,
however, the radar system is equipped with compensator 130, the
normalized Fourier spectrum N (second curve 250), which represents
an image of the relative spatial distribution of the fog density,
is shown. Comparative values, which were determined from comparison
measurements on a test fog area of variable density (particles per
cubic meter) in transmitted light and with which the display is
calibrated, may be stored for representing the absolute fog
density. If individual "solid" or larger elongated objects are
found in the area investigated, the detected spectrum will contain
discrete "surges" or pronounced peaks. The latter may be detected
and masked out with an appropriate filter mask (not described
further here).
* * * * *