JP2009058335A - Radar device and relative distance detection method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve relative distance detection accuracy at close range by an FM-CW radar device. <P>SOLUTION: In this radar device for transmitting a radar wave as a transmission signal, and receiving the transmission signal reflected by a target as a reception signal, a relative distance detection means for performing the first frequency modulation so that the frequency of the transmission signal is raised and lowered repeatedly alternately, and performing the first relative distance detection processing for detecting the first relative distance to the target based on a frequency difference between the first transmission signal and the first reception signal when the first frequency modulation is performed, performs the second frequency modulation for switching the frequency of the transmission signal at close range, counts the wavenumber of a standing wave generated by the transmission/reception signal, and thereby detects the second relative distance to the target based on the count value and a frequency difference between a frequency before switching by the second frequency modulation and a frequency after switching, to thereby enable accurate detection of the relative distance. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a radar apparatus that transmits a radar wave as a transmission signal and receives the transmission signal reflected by a target as a reception signal, and a method for detecting a relative distance of a target using the radar apparatus, and in particular, the frequency of the transmission signal is increased. And a radar apparatus having a relative distance detecting means for detecting a relative distance from the target based on a frequency difference between the transmission signal and the received signal, and a relative distance of the target by the frequency modulation It relates to a detection method.

  An FM-CW (Frequency Modulated-Continuous Wave) radar device that detects a relative distance from a target by transmitting and receiving a frequency-modulated continuous wave is known, and an example thereof is described in Patent Document 1. . FIG. 1 is a diagram for explaining the principle by which an FM-CW radar apparatus detects a relative distance from a target. FIG. 1A shows a change in frequency (vertical axis) with respect to time (horizontal axis) of a transmission signal and a reception signal of an FM-CW radar apparatus that uses electromagnetic waves as radar waves.

  First, the transmission signal W1 represented by a solid line is frequency-modulated according to a triangular wave having a frequency fm and an amplitude ΔF. That is, the frequency of the transmission signal W1 gradually increases with the frequency modulation width ΔF during the frequency increase period UP corresponding to the rising period of the triangular wave, and gradually decreases with the frequency modulation width ΔF within the frequency falling period DN corresponding to the falling period of the triangular wave. . This is repeated with 1 / fm as one modulation period.

  When the transmission signal W1 is reflected by the target, the reception signal W2 is shifted by a time delay ΔT1 corresponding to the relative distance and a Doppler frequency ΔD corresponding to the relative speed, as indicated by a dotted line. . When the transmission signal W1 and the reception signal W2 are mixed, a beat signal B1 whose frequency changes as shown in FIG. 1B is obtained in accordance with the frequency difference between the two. At this time, the frequency of the beat signal B1 in the frequency increase period UP is the upbeat frequency fu, and the frequency of the frequency decrease period DN is the downbeat frequency fd.

  Here, since the time delay ΔT1 of the received signal W2 is the time for the radar wave to make one round trip relative to the target, the relative distance R is expressed by the following equation A1 when C is the speed of light. .

R = C · ΔT1 / 2 (Formula A1)
Further, since the frequency modulation period 1 / fm of the transmission signal W1 and the amplitude ΔF of the triangular wave are given, the time difference ΔT1 is expressed by the following expression A2 from these, the upbeat frequency fu, and the downbeat frequency fd. ΔT1 = (fu + fd) / (2 · ΔF · fm) (Formula A2)
Then, from the expressions A1 and A2, the relative distance R is expressed by the following expression A3.

R = C · (fu + fd) / (8 · ΔF · fm) (Formula A3)
Therefore, the FM-CW radar apparatus performs FFT (Fast Fourier Transform) processing on the beat signal B1 to detect the upbeat frequency fu and the downbeat frequency fd, thereby providing a frequency modulation period of the given transmission signal. Using 1 / fm and the frequency modulation width ΔF, the relative distance R to the target is calculated according to the above equation A3.

  FIG. 2 shows a usage situation when the FM-CW radar apparatus is used as a radar apparatus for rearward monitoring of a vehicle. As shown in FIG. 2A, a radar device 10 that is an FM-CW radar device is mounted on the rear portion of the vehicle 1. The radar apparatus 10 uses an antenna that reciprocates and swings within a predetermined angular range α centered on the rear front B of the vehicle 1, and transmits a transmission signal that has been frequency-modulated as described above. The rear of the vehicle 1 is scanned. At this time, the radar apparatus 10 synchronizes the modulation period of the transmission signal and the operation of the antenna so that a pair of frequency increase period and decrease period correspond to each predetermined azimuth angle θ.

At this time, since it is unknown from which azimuth angle the received signal is obtained, the above FFT processing is performed for each frequency modulation period of the transmission signal corresponding to the azimuth angle θ. By doing so, when the reception signal is obtained from the azimuth angle θ2 with respect to the rear front B where the target, that is, the following vehicle 2, exists, the relative distance R from the up / down beat frequency to the target at the azimuth angle θ2. Is calculated.
JP-A-8-170985

  By the way, in recent years, the development of a back sensor that detects a relative distance from an obstacle behind and notifies the occupant when the vehicle is moved backward and enters the garage has been developed. And it has been proposed to use the FM-CW radar device as a back sensor in combination with the limitation of the mounting space of equipment in the vehicle and the demand for cost reduction.

  FIG. 2B shows a usage situation when the radar apparatus 10 is used as a back sensor. Generally, when a driver enters a vehicle, the stop position is adjusted while moving the vehicle back and forth in small increments so as not to collide with obstacles such as the wall surface 3 of the garage. Therefore, the back sensor is required to detect the relative distance R from the target, that is, the wall surface, with an accuracy of centimeters, more preferably millimeters.

  However, in the FM-CW radar apparatus, when the relative distance to the target is short, as shown in FIG. 1C, the time difference ΔT2 from when the transmission signal is transmitted until the reception signal is received is It becomes smaller than the case of 1 (A). At this time, as shown in FIG. 1D, the beat signal B2 has a lower frequency in the frequency increase period UP and the decrease period DN than in the case of FIG. 1B. That is, the wavelength of the beat signal B2 becomes longer.

  Here, the FFT processing is performed on a signal having a length of one wavelength or longer on the assumption that the wave of the signal is infinitely continuous. Then, in the FM-CW radar device, for example, when using a radar wave obtained by modulating a millimeter wave band having a center frequency of 76 GHz with a modulation width of 100 MHz according to a triangular wave having a frequency of 1 KHz, at a close range of 10 meters to several meters or less, When the beat signal is FFT processed within one modulation period of the transmission signal, there is a problem that one wavelength of the beat signal cannot be detected within that time, and the relative distance cannot be detected accurately.

  Accordingly, an object of the present invention is to provide an FM-CW radar apparatus capable of detecting a relative distance with high precision even at a close distance, and a relative distance detection method thereof.

  In order to achieve the above object, according to a first aspect of the present invention, there is provided a radar device that transmits a radar wave as a transmission signal and receives the transmission signal reflected by a target as a reception signal. Based on the frequency difference between the first transmission signal and the first reception signal when the first frequency modulation is performed so as to alternately repeat the increase and decrease of the frequency. Relative distance detection means for performing a first relative distance detection process for detecting a first relative distance to the target, wherein the relative distance detection means is based on the first relative distance detected in the past. When the distance is greater than or equal to the distance, the first frequency modulation and the first relative distance detection process are repeated, and when the first relative distance is less than the reference distance, a second frequency for switching the frequency of the transmission signal is switched. Frequency modulation of the above, When a Doppler signal is detected from a second received signal received when frequency modulation of 2 is performed, and the level of the Doppler signal is less than a reference level, the number of times the level of the second received signal is increased or decreased. And counting the second relative distance from the target based on the count value and the frequency difference between the frequency of the transmission signal before switching by the second frequency modulation and the frequency of the transmission signal after switching. A second relative distance detection process for detection is performed.

  In a preferred aspect of the above aspect, the relative distance detection means, after the second relative distance detection processing, when the level of the Doppler signal becomes equal to or higher than the reference level, the wave number of the Doppler signal, and the first The distance traveled by the radar device is detected based on the wavelength of the second transmission signal transmitted when frequency modulation of 2 is performed, and the object is detected based on the second relative distance and the travel distance. A third relative distance detection process for detecting a third relative distance from the target is performed.

  When the radar device is used as a back sensor, it is desirable to accurately detect the relative distance to a stationary object in a situation where the vehicle repeats a stopped state and a moving state. According to the above aspect, it is determined that the relative distance to be detected this time is the closest distance from the relative distances detected in the past. When the distance is close, the relative distance detection means performs second frequency modulation for switching the frequency of the transmission signal, and performs Doppler from the second reception signal received when the second frequency modulation is performed. When the signal is detected and the level of the Doppler signal is lower than the reference level, that is, when the vehicle is in a stopped state, the number of times of increase or decrease of the level of the second reception signal is counted, and the count value, Second relative distance detection processing for detecting a second relative distance from the target based on the frequency difference between the transmission signal before switching by the second frequency modulation and the frequency difference between the transmission signals after switching. I do. That is, the relative distance in the close range is detected by using a standing wave generated from the transmission signal and the reception signal between the stationary object and the radar apparatus when the transmission signal has a plurality of different frequencies.

  Here, at a short distance of 10 to several meters or less, the relative distance detection accuracy by the FM-CW method is lowered due to the beat signal having a low frequency. However, since the standing wave has the same frequency as the transmission signal, the frequency is higher than the beat signal corresponding to the frequency difference between the transmission and reception signals, and this problem does not occur. Therefore, good relative distance detection accuracy can be obtained even at a close distance.

  According to a preferred aspect of the above aspect, the relative distance detection means, based on the wave number of the Doppler signal, when the level of the Doppler signal becomes equal to or higher than the reference level after the second relative distance detection processing. The third relative distance detection process for detecting the movement distance moved by the radar device and detecting the third relative distance to the target based on the second relative distance and the movement distance is performed. In terms of distance, the relative distance to the stationary object can be detected not only when the vehicle is stopped but also when the vehicle is moving.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to these embodiments, but extends to the matters described in the claims and equivalents thereof.

  FIG. 3 is a configuration diagram of the radar apparatus 10 according to the present embodiment. First, as shown in FIG. 2A, the radar apparatus 10 is mounted on the rear part of the vehicle and is used as an on-vehicle radar apparatus for backward monitoring that detects a relative distance from a target such as a subsequent vehicle. At this time, the radar apparatus 10 functions as an FM-CW radar apparatus.

  When the radar apparatus 10 functions as an FM-CW radar apparatus, the modulation signal generator 12 generates an FM-CW frequency modulation signal in response to an instruction from the signal processing unit 24. The oscillator 14 generates a radar wave that is frequency-modulated according to the frequency modulation signal. Here, in FIG. 4, the modulation pattern of the radar wave is shown with the horizontal axis representing time and the vertical axis representing frequency. Then, the modulation pattern of the radar wave generated by the FM-CW method at this time has a frequency modulation width ΔF (for example, a triangular wave having a frequency fm (for example, 1 kHz) as a baseband as shown in FIG. 1 MHz), the frequency modulation pattern P1 is obtained by alternately repeating the gradual increase and decrease of the frequency.

  The generated radar wave is amplified by the amplifier 15 and then output from the circulator 16 to the antenna 17. Then, the antenna 17 transmits this radar wave as a transmission signal. At this time, the antenna drive unit 18 reciprocates the predetermined angle range α by swinging the antenna 17. Thereby, the angle range α is repeatedly scanned by the radar wave. Further, the swinging operation of the antenna 17 by the antenna driving unit 18 is synchronized so that a pair of frequency increase periods UP and a decrease period DN correspond to each other for each predetermined azimuth angle unit θ. The antenna 17 receives the transmission signal reflected by the target as a reception signal. Then, the received signal is input from the circulator 16 to the mixer 20 via the amplifier 19.

  The mixer 20 mixes a part of the transmission signal and the reception signal, and generates a beat signal corresponding to the frequency difference between the two. The beat signal is input to the signal processing unit 24 after high frequency noise is removed by the band pass filter 22.

  As an example, the signal processing unit 24 includes a microcomputer having a CPU, a ROM, a RAM, and the like. In the signal processing unit 24, an analog input signal is AD-converted, and the CPU executes various arithmetic processes based on the digital signal while using the RAM as a work area in accordance with various processing programs stored in advance in the ROM.

  Specifically, the signal processing unit 24 first detects the up / down beat frequency by performing FFT on the beat signal, and detects the signal level of each frequency. This process is performed for each azimuth angle θ at which the antenna swings, that is, for each modulation period (1 / fm) of the transmission signal including the frequency increase period UP and the frequency decrease period DN. And the signal processing part 24 detects the relative distance with a target using these. Therefore, the signal processing unit 24 that executes such processing corresponds to the “relative distance detecting unit” 24a. In addition to the relative distance, the signal processing unit 24 detects a relative speed, an azimuth angle, and the like, and these detection results are output to an electronic control device (not shown) that performs vehicle collision response control.

  Further, the signal processing unit 24 controls the antenna driving unit 18 to increase or decrease the swing range of the antenna. By doing so, the angle range scanned by the transmission signal is changed. The signal processing unit 24 that performs such processing corresponds to "transmission / reception angle range control means" 24b.

  In the present embodiment, the radar apparatus 10 is also used as a vehicle back sensor, as shown in FIG. Here, when the radar apparatus 10 is used as a back sensor, it is assumed that the relative distance to the target is a close distance of 10 to several meters or less. In this case, as described with reference to FIG. 1, when the relative distance is detected by the FM-CW method, the beat frequency becomes low and one wavelength is not detected during the FFT processing time within one modulation period. Then, the relative distance detection accuracy is lowered.

  Further, when the radar apparatus 10 is used as a back sensor, it is assumed that there is a high probability that the stop state of the vehicle and the forward / rearward movement are repeated alternately. Further, at this time, the target to be detected is assumed to be a stationary object such as a garage wall. Therefore, the radar device 10 detects the stop state of the vehicle at a close distance, and detects a relative distance from a stationary object using a method different from the FM-CW method, that is, using a standing wave when the vehicle stops. To do. Therefore, the radar apparatus 10 further has the following configuration.

  First, the signal processing unit 24 instructs the frequency modulator 12 to change the frequency modulation pattern of the transmission signal. Specifically, the FM-CW modulation pattern P1 shown in FIG. 4A is instructed to be changed to the modulation pattern P2 shown in FIG. 4B. In the modulation pattern P2, after a transmission signal having a constant frequency f1 is generated, the frequency of the transmission signal is switched to a frequency f2 higher than the frequency f1, and vice versa. In this modulation pattern P2, switching from the frequency f1 to f2 and vice versa are performed within the same time as one modulation period of the modulation pattern P1.

  When the frequency of the transmission signal is fixed at f1, a standing wave is generated in the space between the radar apparatus 10 and the stationary object due to interference between transmission / reception signals of the same frequency. This standing wave is received by the antenna 17, and the amplified received signal is input to the mixer 20. Then, the mixer 20 mixes the standing wave received by the antenna 17 with the transmission signal and down-converts it to an intermediate frequency signal.

  The signal processing unit 24 performs FFT processing on the intermediate frequency signal to detect the Doppler frequency, and detects its level, that is, the level of the Doppler signal. When the level of the Doppler signal is zero, that is, when the relative speed is zero, it can be determined that the vehicle is stopped with respect to the target of the stationary object. Therefore, in this case, the signal processing unit 24 detects the level of the standing wave received by the antenna 17, and how the level changes when the frequency of the transmission signal is increased from f1 to f2. By observing, the relative distance to a stationary object at a close distance is detected.

  Therefore, the signal processing unit 24 that executes such processing corresponds to the “relative distance detecting unit” 24a. At this time, the radar device 10 functions as a back sensor by functioning as a standing wave radar.

  Here, a relative distance detection method using a standing wave will be described with reference to FIG. As shown in FIG. 5A, when the wavelength λ1 (= C / f1, C is the speed of light) of the standing wave SW1 generated when the frequency of the transmission signal is f1, the relative distance R to the stationary object is used. The wave number N1 of the standing wave SW1 generated in is expressed by the following equation B1.

N1 = R / λ1 (B1)
Next, as shown in FIG. 5B, when the signal processing unit 24 increases the frequency of the transmission signal from f1 to f2 by the frequency modulation pattern P2, the wave number increment ΔN is set to the level of the standing wave. Detection is performed by counting the amplitude. Then, as shown in FIG. 5C, using the wavelength λ2 (= C / f2) of the standing wave SW2 generated when the frequency of the transmission signal is f2, it is generated at a relative distance R from the stationary object. The wave number N2 of the standing wave SW1 is expressed by the following equation B2.

N2 = R / λ2 (B2)
Here, since the detected ΔN is ΔN = N2−N1, the following formula B3 is established from the above formulas B1 and B2.

ΔN = N2-N1 = R (1 / λ1-1 / λ2) (B3)
Since λ1 = C / f1 and λ2 = C / f2, the above-described equation B3 is as follows.

ΔN = R · (f1−f2) / C (B4)
Then, in the above equation B4, ΔN and frequencies f1 and f2 are known. Therefore, the signal processing unit 24 calculates the relative distance R by the following formula B5.

R = ΔN · C / (f1−f2) (B5)
With such a configuration, the radar apparatus 10 according to the present embodiment functions as an FM-CW radar apparatus at a relatively long distance, thereby functioning as an in-vehicle radar that monitors other vehicles behind, and is fixed at a close distance. By functioning as a standing wave radar, it functions as a back sensor that detects a relative distance from a stationary object behind the vehicle. That is, according to the present embodiment, the radar apparatus 10 can improve the detection accuracy of a close range while utilizing the hardware configuration as an FM-CW radar apparatus, and can also have a function as a back sensor.

  FIG. 6 is a flowchart for explaining the operation procedure of the radar apparatus 10 according to the present embodiment. The procedure shown in FIG. 6 corresponds to the operation procedure of the signal processing unit 24 that is executed every time the antenna 17 scans the angle range α once by the radar wave.

  First, the signal processing unit 24 instructs the modulation signal generator 12 to perform frequency modulation with the frequency modulation pattern P1 (S10). Then, a beat signal is generated by the mixer 20 at the azimuth angle where the target exists. Next, the signal processing unit 24 performs FFT processing on the beat signal, and the frequency corresponding to the frequency difference between the transmission wave and the reception wave in the frequency increase period and the decrease period of the transmission wave, that is, the up / down beat frequency, The level of the beat signal at the frequency is detected (S12). Then, the signal processing unit 24 detects, as the azimuth angle of the target, the azimuth angle θ having the largest beat signal level among neighboring azimuth angles (S16).

  Further, the signal processing unit 24 calculates a relative distance from the target at the azimuth angle (S18). At this time, as will be described in detail later, the signal processing unit 24 compares the relative distance detected in the previous scan with the reference distance, and the relative distance to be detected this time is a relatively long distance greater than or equal to the reference distance. In some cases, the relative distance is calculated using the FM-CW method, that is, the relative distance from the up / down beat frequency. If the distance is close to the reference distance, the relative distance from the target is calculated using a standing wave method. To do.

  Further, when the relative distance is detected by the FM-CW method, the signal processing unit 24 also calculates the relative speed of the target from the up / down beat frequency (S19). In addition, the relative speed V is calculated | required by following Formula A4. In the following expression A4, f0 represents the center frequency in the frequency modulation width ΔF of the transmission signal W1 shown in FIG.

V = C · (fd−fu) / (4 · f0) (formula A4)
Then, the signal processing unit 24 determines the reliability of the detection result based on the detection result in the past scan and the presence or absence of the continuity of the detection result in the current scan (S20), and determines that the detection result has reliability. The output result is output to the vehicle control device (S22).

  FIG. 7 is a flowchart showing a procedure corresponding to the subroutine of the relative distance detection procedure S18 shown in FIG. First, the signal processing unit 24 determines whether the relative distance detected in the previous scan exceeds the reference distance (S32). Here, the reference distance is set to a lower limit of a distance range in which accurate relative distance detection is ensured by the FM-CW method, for example, about several meters to 10 meters.

  When the determination result is “YES”, it is possible to accurately detect the relative distance by the FM-CW method. Therefore, the signal processing unit 24 detects the relative distance to the target by the FM-CW method (S44). . That is, the signal processing unit 24 calculates a relative distance from the up / down beat frequency obtained by the FFT processing based on the above-described equation A3. Then, the procedure of FIG. 7 is terminated, and the process proceeds to step S19 shown in FIG.

  On the other hand, when the determination result is “NO”, that is, when the accuracy of the relative distance from the target detected by the current scan is not secured in the FM-CW method, the process proceeds to step S34.

  Then, the signal processing unit 24 controls the antenna driving unit 18 to narrow the scan range by reducing the angle range in which the antenna swings (S34). By doing so, the required time per scan can be shortened, and the entire relative distance detection process with respect to a stationary object at a close distance can be executed faster. In addition, by doing so, the number of targets to be detected can be reduced when detecting the relative distance using a standing wave as will be described later, so that the influence of the multi-target can be reduced.

  In that case, the signal processing unit 24 switches the transmission signal modulation pattern from P1 to P2 (S35). Then, the transmission signal fixed at the frequency f1 interferes with the reflected signal reflected by the stationary object, and a standing wave is generated. Further, the mixer 20 generates a Doppler signal corresponding to the frequency difference between the transmission signal having the frequency f1 or f2 and the reception signal.

  First, the signal processing unit 24 detects the stop state of the vehicle from the level of the Doppler signal detected from the received signal. That is, when the level of the Doppler signal is 0 (NO in S36), the vehicle is determined to be in a stopped state because the relative speed with respect to the stationary object is zero.

  If the level of the Doppler signal is not greater than 0, that is, when the relative speed between the vehicle and the target is not generated and the vehicle is stopped (NO in S38), the signal processing unit 24 The relative distance from the stationary object is detected from the level change of the standing wave (S41).

  Furthermore, in this embodiment, when the vehicle moves again at a close distance, the relative distance of the moving vehicle at the close distance from the stationary object is detected by using a Doppler signal. First, when the level of the Doppler signal is greater than 0, the signal processing unit 24 detects that the relative speed between the vehicle and the target has occurred, that is, the start of movement of the vehicle (YES in S38).

  Here, according to the Doppler signal, even if the movement distance can be calculated, the movement direction cannot be calculated. Therefore, in that case, the signal processing unit 24 uses the standing wave and determines whether the moving direction is away from the stationary object, that is, in the far direction, that is, in the approaching direction, that is, in the following procedure (S40). ).

  FIG. 8 is a diagram illustrating a method for determining the far direction and the near direction. FIG. 8 shows the standing waves SW1 and SW2 generated between the target at the position L on the horizontal axis and the radar apparatus 10 at the position L1 or L2. The vertical axis indicates the standing wave reception level in the radar apparatus 10.

  8A and 8B show a standing wave SW1 (shown by a solid line) when the frequency of the transmission signal is f1, and a standing wave SW2 (shown by a dotted line) when the frequency of the transmission signal is f2. Here, it is assumed that the frequency of the transmission signal is f1 and the standing wave SW1 is generated.

  First, the position of the radar device 10 is either the position L1 on the left side of the position Lp where the peak of the standing wave SW1 is detected (FIG. 8A) or the position L2 on the right side (FIG. 8A). Is determined. Therefore, when the radar apparatus 10 increases the frequency of the transmission signal from f1 to f2, the number of standing waves formed between the radar apparatus 10 and the target increases. At this time, as shown in FIG. 8A, when the radar apparatus 10 is located at the position L1 on the left side of the position Lp where the peak of the standing wave SW1 is formed, the reception level V1 of the standing wave SW1. Decreases to the reception level V2 of the standing wave SW2. On the other hand, as shown in FIG. 8B, when the radar apparatus 10 is located at the position L2 on the right side of the position Lp where the peak of the standing wave SW1 is formed, the reception level V3 of the standing wave SW1 is This rises to the reception level V4 of the standing wave SW2.

  On the other hand, when the radar apparatus 10 lowers the frequency of the transmission signal from f2 to f1, the number of standing waves formed between the radar apparatus 10 and the target decreases. Then, in this case, from FIGS. 8A and 8B, a relationship opposite to the increase / decrease relationship of the standing wave reception level is established.

  In this way, by observing the reception level of the standing wave immediately after the frequency of the transmission signal is increased from f1 to f2 or decreased from f2 to f1, the position of the radar apparatus 10 has a peak of the standing wave. It can be determined whether the position L1 on the left side of the formed position Lp or the position L2 on the right side.

  Next, the far / near direction of the moving direction of the vehicle is determined based on the position L1 or L2. First, as shown in FIG. 8C, when the position of the radar apparatus 10 is a position L1 on the left side of the position Lp where the peak of the standing wave SW1 is formed, the frequency of the transmission signal is fixed to, for example, f1, When the radar apparatus 10 moves to the position Lf1, that is, far away from the target, the reception level V4 of the standing wave SW1 decreases to V5. On the other hand, when the radar apparatus 10 moves toward the position Ln1, that is, from the target, the reception level V4 of the standing wave SW1 rises to V6. On the other hand, as shown in FIG. 8D, when the position of the radar device 10 is a position L2 on the right side of the position Lp where the peak of the standing wave SW1 is formed, the frequency of the transmission signal is fixed at, for example, f1. When the radar apparatus 10 moves far from the position Lf2, that is, from the target, the reception level V7 of the standing wave SW1 increases to V8, and when it moves from the position Ln2, that is, from the target, to the reception level V9. .

  The correspondence as described above is shown in the table of FIG. Such a table is stored in advance in the ROM of the signal processing unit 24. And the signal processing part 24 determines a far direction or a near direction by referring to this table.

  Then, the signal processing unit 24 executes steps S32 to S36 again, and determines whether the vehicle is moving from the level of the Doppler signal in step S36. At this time, since it is after the start of movement, a relative speed has occurred and the level of the Doppler signal is greater than zero. Therefore, since the determination result is “YES”, the process proceeds to step S42.

  Then, the travel distance of the vehicle is calculated from the Doppler frequency. Here, if the Doppler frequency is fp and the frequency of the transmission signal is f1, the frequency of the reception signal is f1 + fp. When the relative speed between the vehicle and the target, that is, the moving speed of the vehicle is V and the speed of light is C, the following expression C1 is established.

f1 + fp = ((C + V) / C) · f1, that is, fp = (V / C) · f1 (Formula C1)
Here, if the Doppler signal is generated in N waves while the vehicle is moving at the speed V (m / sec) and the moving distance S (m) for T seconds, the relationship among N, T, V, S, and fp is It becomes as follows.

S = V · T (m) (Formula C2)
fp = N / T (Hz) (Formula C3)
Then, the distance S0 that the vehicle moves while one wavelength of the Doppler signal is detected is expressed by the following equation C4.

S0 = S / (2.N) (Formula C4)
Then, from equation C3, N = T · fp. When this and equation C2 are substituted into equation C4,
S0 = V · T / (2 · T · fp) = V / 2 · fp (Expression C4).

And substituting equation C1 into equation C4,
S0 = V / (2 · (V / C) · f1) = (1/2) · (C / f1) = λ / 2 (Formula C5)
(Λ is one wavelength of the transmission signal of frequency f1, ie, λ = C / f1).

  Here, since the movement distance calculated as described above is an absolute value of the movement distance, the movement direction is not included. Therefore, the relative distance R after the movement is determined based on the relative distance R0 at the stop time obtained in step S41 using the far or near direction determination result previously used in step S40. In the case of the near direction, the following formula C7 is used.

R = R0 + N · λ / 2 (Formula C6)
R = R0−N · λ / 2 (Formula C7)
The above procedure is executed as procedure S18. By doing so, the radar apparatus 10 can accurately detect the relative distance to the target at a close distance, and can function as a back sensor.

  In the above-described procedure, in step S42, the frequency may be fixed to a fixed frequency, for example, f1 in order to detect the Doppler signal instead of the frequency modulation pattern P2.

  In the above description, the case where the vehicle rear monitoring radar device also serves as a back sensor has been described as an example. However, as long as the radar apparatus detects the relative distance to the target at a close distance while having the configuration of the FM-CW radar apparatus, the present embodiment is not limited to the rear monitoring radar of the vehicle. .

  As described above, according to the present embodiment, when detecting a relative distance from a relatively far target, the distance is detected by the FM-CW method, and at a close distance, a relative distance is used using a standing wave. Is detected. At this time, since the standing wave has the same frequency as the transmission signal, it is higher in frequency than the beat signal corresponding to the frequency difference between the transmission and reception signals, and the beat signal has a low frequency, so that the relative distance detection by the FM-CW method is performed. It is possible to solve the problem that the accuracy is lowered, and to obtain a good relative distance detection accuracy.

It is a figure explaining the principle by which FM-CW radar apparatus detects the relative distance with a target. It is a figure which shows the use condition in case FM-CW radar apparatus is used as a radar apparatus for back monitoring of a vehicle. 1 is a configuration diagram of a radar apparatus 10 in the present embodiment. It is a figure explaining the modulation pattern of the radar wave in this embodiment. It is a figure explaining the relative distance detection method using a standing wave. It is a flowchart figure explaining the operation | movement procedure of the radar apparatus 10 in this embodiment. It is a flowchart figure which shows the procedure corresponding to the subroutine of relative distance detection procedure S18 shown in FIG. It is a figure explaining the determination method of the far direction and the near direction of the moving direction of a vehicle.

Explanation of symbols

10: radar device, 24: signal processing unit, 24a: relative distance detection means, 24b: transmission / reception angle range control means

Claims (5)

In a radar device that transmits a radar wave as a transmission signal and receives the transmission signal reflected by a target as a reception signal,
The first frequency modulation is performed so that the frequency of the transmission signal alternately repeats an increase and a decrease, and the frequency of the first transmission signal and the first reception signal when the first frequency modulation is performed. Relative distance detection means for performing a first relative distance detection process for detecting a first relative distance to the target based on the difference,
The relative distance detection means repeats the first frequency modulation and the first relative distance detection process when the first relative distance detected in the past is equal to or larger than a reference distance, When the relative distance is less than the reference distance, the second frequency modulation for switching the frequency of the transmission signal is performed, and the Doppler signal is obtained from the second reception signal received when the second frequency modulation is performed. When the level of the Doppler signal is detected and less than the reference level, the number of times the level of the second received signal is increased or decreased is counted, and the count value and the transmission before switching by the second frequency modulation are counted. 2. A radar apparatus, comprising: a second relative distance detection process for detecting a second relative distance from the target based on a signal frequency and a frequency difference between the transmission signal frequencies after switching. In claim 1,
After the second relative distance detection process, the relative distance detection means performs the wave number of the Doppler signal and the second frequency modulation when the level of the Doppler signal becomes equal to or higher than the reference level. And a third distance relative to the target is detected based on the second relative distance and the movement distance based on the second relative distance and the movement distance. A radar apparatus that performs a third relative distance detection process for detecting a distance. In claim 2,
The relative distance detection means detects a direction in which the radar apparatus has moved based on an increase or decrease in the level of the second received signal when the level of the Doppler signal is equal to or higher than the reference level, And a third relative distance detection process based on the third relative distance detection process. In claim 1,
When the first relative distance is greater than or equal to the reference distance, the first transmission signal has a first angle range. When the first relative distance is less than the reference distance, the first transmission signal has a second angle range that is smaller than the first angle range. A radar apparatus, further comprising transmission / reception angle range control means for performing transmission and reception of the first reception signal. In a method for detecting a relative distance from the target by a radar device that transmits a radar wave as a transmission signal and receives the transmission signal reflected by the target as a reception signal,
A first frequency modulation is performed so that the frequency of the transmission signal repeats rising and falling alternately;
A first relative distance detection process for detecting a first relative distance to the target based on a frequency difference between the first transmission signal and the first reception signal when the first frequency modulation is performed. And
When the first relative distance detected in the past is greater than or equal to a reference distance, the first frequency modulation and the first relative distance detection process are repeated, and the first relative distance is less than the reference distance. In this case, the second frequency modulation for switching the frequency of the transmission signal is performed, and the level of the Doppler signal detected from the second reception signal received when the second frequency modulation is performed is less than the reference level. In this case, the number of times of increase / decrease of the level of the second received signal is counted, the value of the count, the frequency of the transmission signal before switching by the second frequency modulation, and the frequency of the transmission signal after switching And a second relative distance detection process for detecting a second relative distance from the target based on the frequency difference between the target and the target.

Images