Communications: directive radio wave systems and devices (e.g. – Determining velocity
Reexamination Certificate
2002-08-02
2004-03-09
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Determining velocity
C342S111000, C342S115000, C342S116000, C342S117000, C342S192000, C342S194000, C342S196000
Reexamination Certificate
active
06703966
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to methods for measuring the absolute speed of a moving body relative to the ground, and to apparatuses for implementing said methods. It may be applicable to automobiles or trains.
Driving assistance and safety systems require knowledge not only of the speed of rotation of the wheels, but also of the absolute speed of the vehicle relative to the ground.
When a vehicle encounters a sheet of ice for example, and if the driver brakes suddenly, the wheels lock owing to the loss of friction between the tire tread and the road, in these conditions, a speed indicator entirely dependent on the speed of rotation of the wheels would indicate a speed of zero when the vehicle is skidding on a sheet of ice or would remain at the speed before braking, during aquaplaning.
Such a particularly dangerous situation can only be taken into account by the driving assistance systems in question if the information collected is truly representative of the actual situation.
Furthermore, conventional speed sensors commonly used today, which measure the number of rotations of the wheel of the vehicle made during a specific time period can lead to inaccurate speed measurements if there are variations in wheel diameter due for instance to poor tire inflation in the case of automobiles, or to wearing of the wheel in the case of trains, or if the vehicle is skidding on the ground.
Various methods have been suggested to overcome such a problem and to allow a measurement of absolute ground speed independent of the speed of wheel rotation to be made by using the Doppler effect.
In such methods a Doppler effect radar comprises an antenna which transmits an acoustic or electromagnetic wave towards the ground and uses the frequency shift between the frequency of said wave and that of the wave reflected by a ground element or a surface irregularity situated in the zone scanned by the radar beam, the element or surface irregularity hereafter defined as a reflecting obstacle.
This frequency shift, hereafter called the Doppler frequency and designated f
d
, which results from the relative movement between the antenna and the reflecting obstacle is proportional to the speed of the moving body relative to the ground and to the cosine of the angle &agr; defined as the angle between the direction of the wave at the reflecting obstacle, and the ground according to the equation:
f
d
=
2
v
cos
(
α
)
c
f
0
where f
0
is the transmitter frequency, v the speed of movement of the vehicle and c the speed of the wave, which leads to:
v
=
cf
d
2
cos
(
α
)
f
0
Because the accuracy on the speed depends directly on the accuracy of the angle &agr;, Doppler effect radars use highly directional antennae having a radar beam with a small aperture angle, so that the main part of the radiation transmitted and received by the antenna is centered along an angle &bgr; with the ground, the angles &agr; and &bgr; being equal in such situations.
A drawback of this solution arises from unexpected variations in angle &bgr; resulting from a change in tilt of the vehicle, due for example to a modification of the load.
In order to remedy this drawback, U.S. Pat. No. 4,107,680 suggests using at least two antennae, one pointed in the direction of travel and the other pointed in the opposite direction in order to compensate for the variation in tilt.
A second serious drawback of this prior art originates from the fact that when using a narrow radar beam, a reflected wave is only generated if a reflecting obstacle is encountered on the small surface of ground scanned by the beam, this condition not being necessarily fulfilled when the ground is smooth for example, as applies in the presence of ice.
In order to remedy this second drawback, French patent n° 2 722 301 suggests instead of using a highly directional antenna, using an antenna with a wide aperture angle to increase the probability of the presence of reflecting obstacles on the ground, in the beam.
Nevertheless in this case, the angle &agr; corresponding to the reflecting obstacle which reflects the wave towards the antenna constitutes an additional unknown variable.
In the said method, and in order to measure this angle and the speed, the sensor transmits simultaneously or non-simultaneously, two waves, one of fixed frequency and the other of varying frequency, the measurement of speed being made by identifying the Doppler frequencies for each of the above transmissions, produced by reflection on the same reflecting obstacle.
This method which brings a large improvement to apparatuses of this kind, requires however, sensitive instrumentation with large signal processing capacities.
The present invention which is particularly aimed at remedying these drawbacks, allows one to benefit from the advantages associated with the use of antennae with a wide aperture angle, even though it only requires simple instrumentation and simple methods of signal processing.
SUMMARY OF THE INVENTION
The present invention relates to the method of measuring the speed v of a moving object traveling in a direction parallel to the ground, the measurement being made by means of a Doppler radar with transmitter and receiver antennae fixed to the moving object at a certain height h above the ground and designed to transmit a radar beam towards the ground along a mean axis extending forwards or backwards relative to the direction of movement, said method including the following steps:
generating an electrical signal at a certain frequency by means of an oscillator,
from said signal and possibly after amplification, transmitting an incident wave towards the ground, by means of the transmitter antenna having a wide aperture angle in a vertical plane,
receiving a reflected wave, generated by reflection of the incident wave by a reflecting obstacle on the ground, by means of the receiver antenna with a wide aperture angle in the vertical plane,
mixing together part of the electrical signal supplied by the oscillator and the received signal, possibly after amplification thereby generating two signals, one signal at a frequency equal to the sum and the other at a frequency equal to the difference of the two signal frequencies entering the mixer,
filtering the signal from the mixer to generate a filtered signal proportional to the signal at a frequency equal to the frequency difference,
amplifying the filtered signal at a frequency equal to the frequency difference to generate a signal called Doppler signal,
looking for the different Doppler frequencies present in the Doppler signal, at close successive instants,
said method being essentially characterized in that it further includes the following steps
identifying in each Doppler signal obtained, Doppler frequencies at close successive instants corresponding to reflecting objects on the ground located in the zone scanned by the wave transmitted by the transmitter antenna, called identified Doppler frequencies,
calculating the theoretical evolution function representing the evolution as a function of time, of the Doppler frequency corresponding to a reflecting object, for a given speed, height of the transmitter and receiver antennae above the road and position of the reflecting obstacle,
selecting from the identified Doppler frequencies, those which correspond to the same reflecting obstacle at different successive instants using them to determine the speed of the moving body.
In preferred implementations of the method of the invention, use is also made of one or more of the following dispositions:
when the identification of the different Doppler frequencies is made using a Fourier transform method in order to determine the corresponding spectra, called Doppler spectra, the frequencies corresponding to the spectrum peaks in the Doppler spectra are searched for,
when the identification of the different Doppler frequencies is made using a Fourier transform method in order to determine the corresponding spectra, called Doppler spectra, the Doppler spectrum is decomposed in
Carreel Eric
Corbrion Céline
Ditchi Thierry
Lewiner Jacques
Marshall & Gerstein & Borun LLP
Sotomayor John B.
LandOfFree
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