JP2009198272A - Radar system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radar system capable of finding the distance of a target to be measured, with high accuracy up to a long distance from point-blank distance. <P>SOLUTION: A radar system includes an oscillator for outputting a transmission wave, a control circuit for controlling the frequency of the oscillator, a transmitting antenna for transmitting the transmission wave into space from the oscillator, a receiving antenna for receiving a reflected wave of the transmission wave reflected by an object in the space, a mixer for mixing the transmission wave and the reflected wave to output a beat signal, a signal processing circuit for analyzing the beat signal and detecting at least one out of the distance from the radar system to the object, its azimuth, and speed, and a frequency changing section for changing the inclination of the frequency according to the distance between the object and the radar system. It does not matter that the frequency changing section changes the inclination of the frequency so that the frequency inclination becomes large when the distance between the object and the radar system becomes smaller than a predetermined value, or so that the frequency inclination becomes small when the distance between the object and the radar system becomes larger than a predetermined value. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an in-vehicle radar device that detects an obstacle using continuous radio waves.

  A radar device using millimeter waves is widely used as a device for measuring the distance to an obstacle or a forward vehicle when traveling in an automobile. The radar device emits radio waves and receives reflected waves from an object such as a vehicle. Then, the intensity of the received reflected wave, the Doppler shift of the frequency, the propagation time from the emission of the radio wave to the reception of the reflected wave, and the like are detected, and the distance and relative velocity to the object are measured from the result. Typical examples of such radar include an FMCW (Frequency Modulated Continuous Wave) method and a two-frequency CW (Continuous Wave) method.

  In the FMCW system, there is a problem that the distance and relative velocity cannot be measured correctly for an object that moves at a close speed at a large speed. Therefore, there is a technique that uses a transmission wave in which the absolute value of the frequency change rate is changed (patent) Reference 1).

  In the two-frequency CW method, a target with a relative speed of 0 cannot be detected because a beat signal is not detected because no Doppler shift occurs. In addition, when there are a plurality of targets having the same relative speed, their reflected signals are measured as the same beat frequency and cannot be detected separately. Therefore, there is a technique for transmitting a frequency pattern in which two transmission waves have a frequency gradient (see Patent Document 2).

Japanese Patent Laid-Open No. 10-253753 JP 2004-151022 A

  According to Patent Document 1, the beat signal measured in the section where the frequency of the transmission wave is increasing and the beat signal measured in the section where the frequency is decreasing are based on the reflected signal from the same target. It must first be determined whether or not. However, when there are a plurality of objects, this determination is difficult, and if the determination is incorrect, there is a problem in that there is a risk of outputting erroneous data indicating that an object exists at a position that does not actually exist.

  According to Patent Document 2, when a radio wave is transmitted from a radar mounted on a host vehicle traveling at a speed Vi and reflected back to a preceding vehicle, the beat frequency of the reflected wave of the radio wave is detected by the radar. . However, when the preceding vehicle has a relative speed of 0 with the host vehicle and is at a close distance, the beat frequency corresponding to the distance to the object is measured, but it becomes a very small beat frequency. There is a problem that the signal becomes the same signal as the DC component and an accurate distance value cannot be measured. By making the modulation slope steep, the beat frequency can be increased even at the same short distance as described above. However, since the frequency of the oscillator is increased, the linearity of the frequency with respect to time deteriorates. FIG. 6 is a diagram illustrating the relationship between the distance and the received signal level due to the influence of the linearity of the modulation gradient. It is known that when the linearity deteriorates, the received signal level decreases as the distance increases. Therefore, by making the modulation slope steep, it is possible to solve the distance measurement to an object existing at a close distance. However, since the measurement accuracy of a target existing at a long distance deteriorates, the performance of the radar as a whole is improved. Remains a challenge.

  Accordingly, an object of the present invention is to provide a radar apparatus that can accurately obtain the distance of a target to be measured from a close distance to a long distance.

  In order to solve the above problems, one of the desirable embodiments of the present invention is as follows.

  The radar device receives an oscillator that outputs a transmission wave, a control circuit that controls the frequency of the oscillator, a transmission antenna that transmits the transmission wave from the oscillator to the space, and a reflected wave of the transmission wave reflected by the object in the space. Receiving antenna, a mixer that outputs a beat signal by mixing a transmission wave and a reflected wave, and a signal processing circuit that analyzes the beat signal and detects at least one of the distance, direction, and speed from the radar device to the object And a frequency changing unit that changes the slope of the frequency according to the distance between the object and the radar apparatus.

  ADVANTAGE OF THE INVENTION According to this invention, the radar apparatus which can obtain | require accurately the distance of the target which should be measured from a short distance to a long distance can be provided.

  Hereinafter, embodiments will be described with reference to the drawings.

  FIG. 1 is a diagram illustrating a configuration of a radar apparatus.

  The oscillator 100 receives the modulation signal from the modulation signal generator 107 and modulates the frequency. The signal is amplified by the power amplifier 101 and transmitted from the transmission antenna 102. The signal reflected from the object (in this paper, the preceding vehicle) is received from the receiving antenna 103. The signal is mixed with the signal of the oscillator 100 by the mixer 104, and the beat frequency is taken into the AD converter 105 through the preamplifier. The digitized signal is subjected to FFT processing by the signal processing unit 106, and a distance value is calculated from the measured frequency and phase information.

  FIG. 2 is a diagram illustrating frequency modulation of a radar transmission wave.

  The radar apparatus transmits while switching between two frequencies having a frequency difference Δf with a period of time ΔT. Further, there is a section having a negative frequency gradient at a time interval T larger than the time interval ΔT, and the frequency gradient is changed according to the distance between the preceding vehicle and the radar apparatus.

  FIG. 3 is a diagram illustrating a relationship between the modulation inclination and the minimum detection distance of a stationary object.

  It can be seen that increasing the modulation gradient is effective for improving the minimum detection distance. Therefore, when the distance between the radar device and the preceding vehicle is smaller than a predetermined value, the frequency modulation is increased in accordance with the distance from the target (hereinafter referred to as logic A). Thereby, the minimum detection distance can be improved.

  FIG. 4 is a diagram for explaining a method of selecting a scene to which frequency modulation is applied.

  In Logic A, the minimum detection distance is improved, but since the frequency of the oscillator is increased, the linearity between the frequencies is deteriorated, and the reception signal level of the preceding vehicle at a long distance is lowered. In other words, by making the modulation slope steep, the distance measurement to the preceding vehicle existing at a close distance can be solved, but the distance measurement to an object existing at a long distance deteriorates, so the performance of the radar device as a whole is deteriorated. Issues remain as improvements.

  The purpose of improving the minimum detection distance is to detect the preceding vehicle when the radar device stops following the preceding vehicle. Therefore, the logic A is applied only when the distance between the radar apparatus and the preceding vehicle is, for example, a predetermined distance (hereinafter referred to as R1) or less and the own vehicle speed is equal to or less than a predetermined speed (hereinafter referred to as V1). As a result, there is no need to change the modulation gradient in an unnecessary scene. In other words, when you want to detect a preceding vehicle at a long distance while you are traveling, you can prevent problems such as logic A operating by an object different from the preceding vehicle, increasing the modulation slope, and reducing the received signal level. Is possible.

  A specific example of how to determine R1 is shown.

  In the case of the Doppler radar, if the relative speed is measured in the full range up to 160 km / h, 23 KHz is measured as the Doppler frequency fd as follows.

fd = 2fc / C * V0 = 23 kHz (1)
fc: Radar center frequency 76.5 GHz
C: Speed of light 3 * 10 ^ 8m / s
V0: Relative speed 160km / h
Assuming that the number of points when FFT is performed as signal processing is 1000, for example, the measurement frequency fd_s per LSB is expressed by the following equation.

fd_s = 23 kHz / 1000 = 23 Hz (2)
In time 1/23 = 44 ms.

  The frequency change frf generated by the frequency gradient (Fr / Ts) in FIG. 2 when the radio wave travels the distance R can be expressed by the following equation.

frf = t * (Fr / Ts) (3)
t = 2R / C
The time when frf = fd_s is the minimum distance resolution Rsmin.

Rsmin = c / 2 * Ts / Fr * fd_s = 1.52 m
Ts = 44ms
Fr = 100MHz
Therefore, for example, if the predetermined distance of R1 is set to be 5 times the distance Rsmin, the logic A can be operated before reaching the Rsmin when the preceding vehicle approaches.

  In addition, when the preceding vehicle is separated, if the set value of R1 is set to a value different from that at the time of approach, for example, a distance 10 times as large as Rsmin, the logic A repeatedly repeats operation / non-operation before and after the threshold value. There is no smooth transition of logic. The distance 5 times or 10 times is an example, and this value may be changed depending on the vehicle speed.

  FIG. 5 is a diagram illustrating frequency modulation of a radar transmission wave.

  The difference from the scene where the logic A is applied is that the frequency modulation has a predetermined monotonically increasing period and a monotonically decreasing period of time T, and there is no frequency modulation corresponding to ΔT in the first embodiment. Also in this modulation method, the same effect as described above can be obtained by changing the frequency gradient according to the distance between the radar device and the preceding vehicle.

  According to this article, when the target exists at a close distance, the frequency difference of the beat signal can be increased by increasing the modulation gradient, and the DC component can be separated even if FFT processing is performed. On the other hand, when the target exists at a long distance, it is not necessary to increase the frequency of the oscillator by reducing the modulation gradient, and the linearity of the frequency with respect to time can be ensured. As a result, it is possible to prevent the signal strength from being deteriorated due to the linearity when the distance is long, so that the performance of the radar apparatus can be improved from a close range to a long range.

The figure which shows the structure of a radar apparatus. The figure which shows the frequency modulation of the transmission wave of a radar system. The figure which shows the relationship between the said modulation | alteration inclination and the minimum detection distance of a stationary target. The figure for demonstrating the method to select the scene which applies frequency modulation. The figure which shows the frequency modulation of the transmission wave of a radar system. The figure which shows the relationship between the distance by the influence of the linearity of a modulation inclination, and a received signal level.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Radar device 2 Vehicle equipped with radar 3 Vehicle traveling in front of radar-equipped vehicle 100 Oscillator 101 Power amplifier 102 Transmitting antenna 103 Receiving antenna 104 Mixer 105 AD converter 106 Signal processor 107 Modulation signal generator

Claims (5)

An oscillator that outputs a transmission wave;
A control circuit for controlling the frequency of the oscillator;
A transmission antenna for transmitting a transmission wave from the oscillator to space;
A receiving antenna for receiving a reflected wave of the transmitted wave reflected by an object in the space;
A mixer that mixes the transmitted wave and the reflected wave to output a beat signal;
A signal processing circuit that analyzes the beat signal and detects at least one of a distance, a direction, and a speed from a radar device to the object;
A radar apparatus, comprising: a frequency changing unit that changes an inclination of the frequency according to a distance between the object and the radar apparatus.   The radar apparatus according to claim 1, wherein the frequency changing unit changes a slope of the frequency according to a speed of the radar apparatus.   The radar apparatus according to claim 1, wherein the frequency changing unit changes the slope of the frequency so as to increase when a distance between the object and the radar apparatus becomes smaller than a predetermined value.   The radar apparatus according to claim 1, wherein the frequency changing unit changes the slope of the frequency to be small when a distance between the object and the radar apparatus is greater than a predetermined value.   The radar apparatus according to claim 1, wherein the frequency has a predetermined monotonically increasing period and a monotonically decreasing period.

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