JP3075371B2 - Target object detection method using FM-CW radar device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting a target using an FM-CW radar apparatus, and more particularly to a method for detecting a small target in front of a target without being masked by a large target. And a method for detecting a target (target).

[0002]

2. Description of the Related Art Generally, in this type of FM-CW radar device, as shown in FIG. 3, the frequency of a voltage controlled oscillator (VCO) 21 is changed by a triangular wave modulation signal sent from a microcomputer 1 to a sensor 2 side. The transmission signal T _F that has been modulated and thus FM-modulated is radiated to the outside via the transmission antenna 31, and a part of the transmission signal is branched via the directional coupler 22 to become a local signal. ,
The local antenna is reflected by the target ahead and the receiving antenna 3
The received signal R _F, which is received via a 2 to obtain a beat signal by mixing by the mixer 23, the local signal component resulting amplitude variation the beat signal by a high pass filter (HPF) 24 (change of the modulation signal frequency In order to prevent the output of modulated frequency components from being detected by AM detection even when there is no received signal, to align the beat signal level of a short-range target with the beat signal of a long-range target, etc. Used) and low-pass filter (LPF)
25 (used to limit the detection range of the target to a certain distance, for example, 60 m, or to limit noise when the band is widened too much), and is sent to the microcomputer 1 to be pulsed. The microcomputer 1 counts the number of pulses of the pulse signal of the beat signal for each of the rising and falling slopes of the modulation signal (triangular wave), thereby setting the beat frequency for each of the slopes. Then, the distance to the target from which the received signal is reflected is measured based on the average value of these beat frequencies, and the relative speed with respect to the target is measured based on the frequency difference between these beat frequencies. You.

FIG. 8 shows a signal transmitted from the FM-CW radar device.
Transmitted signal T_FFrom the target object such as the preceding vehicle
Received signal R_FWith time t on the horizontal axis, and
The vertical axis indicates the frequency f. That is, the transmission signal
T_F(Shown by a solid line in FIG. 8)
Changes periodically (in the form of a triangular wave). Note that f ₀
Indicates the oscillation frequency of the voltage controlled oscillator 21.

On the other hand, a reception signal R _F (shown by a dotted line in FIG. 8)
Is delayed by the time Δt at which the transmitted signal is reflected back from the target, and thus the transmitted signal T _F is
(Here, the distance to the target where the transmission signal reflects is R,
(If the speed of the electromagnetic wave is C, ΔT = 2R / C). As a result, at a certain point in time, the transmission signal T _F and the reception signal R _F have a frequency difference of f _B as shown in FIG. 8, and the f _B becomes the frequency of the beat signal. And the frequency f _{B of the} beat signal
Increases as the distance R to the target increases (the Δt increases), and depends on the magnitude of the frequency of the beat signal (by pulse counting the frequency as described above). R can be determined.

[0005] The received signal R_FIs the radar
There is no relative velocity between the
(The distance does not change).
Therefore, in this case, the rising slope of the modulated signal (triangular wave)
The beat signal corresponding to the minute also
The frequency of the heartbeat signal is also f_BAnd the f _B
The distance to the target is calculated.

However, when there is a relative speed between the radar and the target (ie, when the target approaches or moves away from the radar), the received signal is affected by the Doppler shift. Moves upward (for example, when the target approaches) like R ′ _F (indicated by a dashed line in FIG. 8), or conversely moves below the received signal R _F (for example, When the target moves away).

Now, if the received signal is R '_Fof
Ascending, the rising of the modulated signal (triangular wave)
The frequency of the beat signal corresponding to the inclined portion is f '_BBecomes
F_BOn the other hand, on the other hand, in the downward slope portion of the triangular wave.
The frequency of the corresponding beat signal is f ″_BAnd the f_BYo
Also rises. And in this case, f '_BAnd f ″_BWhen
The distance to the target is determined from the average value of_B
And f ″_BThe relative speed of the target is determined from the difference between
In FIG. 8, the received signal is R ' _FOpposite to
Above R_FWhen moving down, the modulated signal
The frequency of the beat signal corresponding to the rising slope of (triangle wave)
The number is f_BOn the other hand, while the downward slope of the triangular wave
The frequency of the beat signal corresponding to the_BLower
You.

FIG. 7 shows two targets A and B in front.
When the relative speeds of the two targets A and B are both 0, the frequency spectrum of the beat signal corresponding to the rising slope of the modulated signal (FIG. 7) and the falling spectrum of the modulated signal correspond to the falling slope. The frequency spectrum of the beat signal (FIG. 7), the frequency f is plotted on the horizontal axis, and the level L is plotted on the vertical axis
(Received signal level). Note that A1 in FIG. 7 and A2 in FIG. 7 are the frequency spectra of the beat signal obtained from the received signal reflected from the target A, the frequencies of which are both f _A, and B1 in FIG. And B2 in FIG.
A frequency spectrum of the beat signal provided example by receiving signals reflected from the target B, the frequency is both a f _B.

As described above, when the relative speed of each of the targets A and B is 0, the frequency of the beat signal corresponding to the upward slope portion and the downward slope portion is changed with respect to each of the targets A and B, respectively. it is possible to fix the f _a and f _B, if there is a small target a (e.g. motorcycles) in front of the large target in the radar detection area as shown in Figure 7 B (e.g. tracks) Even in such a case, the beat signal (frequency f _B ) corresponding to the target B is cut off by setting the cut-off frequency of the low-pass filter (LPF) 25 between the frequencies f _A and f _B. to the distance to the small target a being in the front, it is possible to measure on the basis of the frequency f _a. In addition, among the arrows shown in FIG.
The white arrow indicates the moving direction of the beat signal frequency due to the influence of the Doppler shift when the target A or B approaches the host vehicle, and the hatched arrow indicates the target A or B. FIG. 7B shows the direction in which the beat signal frequency moves due to the influence of the Doppler shift when B moves away from the own vehicle.

FIG. 6 shows the frequency of the beat signal corresponding to the upward and downward slope portions of the modulated signal when the two targets A and B have a relative speed (approaching or moving away from the own vehicle). FIG. 6A shows a frequency spectrum (of FIG. 6A) of the beat signal corresponding to the upward slope portion when the target A approaches and the target B moves away. FIG. 6B shows the frequency spectrum (of FIG. 6A) of the beat signal corresponding to the downward slope portion, and FIG. 6B shows that the target A moves away from the target B
, The frequency spectrum of the beat signal corresponding to the rising slope portion (of FIG. 6 (2)) and the frequency spectrum of the beat signal corresponding to the falling slope portion (of FIG. 6 (2)). Is shown.

Considering the case of FIG. 6A, the frequency of the beat signal corresponding to the upwardly sloping portion is lower than the frequency f _A for the target A, while the frequency of the beat signal is lower for the target B. Rises above the frequency f _B (see FIG. 6 (1)), the beat signal corresponding to the target B is cut off by the low-pass filter (LPF) 25, and the small target A in front of it is cut off. Can be detected. However, the frequency of the beat signal corresponding to the downward slope portion is higher than the frequency f _A for the target A, and lower than the frequency f _B for the target B (FIG. 6 (1)). ), The beat signal corresponding to the target B is supplied to the low-pass filter (LPF) 25.
, The beat signal corresponding to the target A having a low level is masked (or, in some cases, the beat signal corresponding to the target A passes through the low-pass filter (LPF)). As a result, the frequency of the beat signal corresponding to the target A cannot be identified. As a result, the distance to the target A and the relative speed thereof cannot be correctly detected. .

Similarly, in the case of FIG.
As shown in FIG. 6B, the frequency of the beat signal corresponding to the downward slope portion is obtained by cutting off the beat signal corresponding to the target B by the low-pass filter (LPF). Although the frequency of the beat signal corresponding to A can be detected, the frequency of the beat signal corresponding to the upwardly sloping portion is, as shown in FIG. For similar reasons to
The frequency of the beat signal corresponding to the target A cannot be identified, so that the distance to the target A and the relative speed thereof cannot be correctly detected.

In particular, conventionally, in order to accurately detect the relative speed of a target, FM modulation was performed using a modulation signal of a somewhat lower frequency (the lower the modulation signal, the more the influence of the Doppler shift due to the relative speed (ie, the Doppler shift). However, when the modulation frequency is reduced in this manner, the change in the beat signal frequency according to the magnitude of the relative speed causes the above-described change in the beat signal frequency. The phenomenon described in FIG. 6 (2) with FIG. 6 (1) occurs, and the cut-off frequency of the low-pass filter (LPF) is conventionally fixed. Small target B (in the low-pass filter band) before large target A
If there is a relative speed between these and your vehicle,
When both the large target A and the small target B are within the band, the beat signal based on the target A having a large level becomes dominant when the beat signal is pulsed (that is, the beat signal). Target B with low level due to signal
There is a problem that it is difficult to identify the presence of the target B (therefore, its distance and relative speed).

[0014]

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem. When a target having a low reflection level is present in front of a target having a high reflection level, the targets have a relative velocity. However, even if the target has the low reflection level, the target having the low reflection level can be reliably detected without being masked by the target having the high reflection level.

[0015]

According to the present invention, there is provided a motor vehicle having an upward slope and a downward slope.
A local signal obtained by shunting a part of a transmission signal frequency-modulated by a modulation signal consisting of a triangular wave that repeats one cycle and a reception signal received through a reception antenna are mixed with each other, and the rising and falling slopes of the triangle wave are mixed. A target detection method for detecting a beat signal corresponding to a target in each part and measuring a distance to the target based on an average value of frequencies of the detected beat signals. The distance to the first target is calculated by using a modulated signal with a low repetition frequency, and then the influence of the Doppler shift component of the beat signal based on the relative speed of the first target is negligible. Modulation signal repetition
And increasing the frequency, further determines the cutoff frequency of the low-pass filter to cut off the beat signal component of the frequency corresponding to the distance to the target of the first to rise with increasing modulation signal repetition frequency Then, it is detected whether or not each of the beat signals corresponding to the second target exists in a frequency band passing through the determined low-pass filter, and the presence of each of the beat signals is detected. The present invention provides a target detection method using an FM-CW radar device, wherein a distance to the second target is calculated based on the frequency of each detected beat signal.

[0016]

According to the above arrangement, the distance to the first target having a large reflection level and the relative speed are calculated by using a relatively low frequency modulation signal, and then the frequency of the modulation signal is increased as described above. And determining the cutoff frequency of the low-pass filter, it is possible to determine the distance to the second target having a small reflection level in front of the first target.

[0017]

1 and 2 are flow charts showing the operation procedure of the method of the present invention. First, in steps 1 to 6 in FIG. 1, a relatively low modulation frequency is used to reach the first target. 2 shows a procedure for obtaining the distance and the relative speed. That is, the counting is started from n = 0, n is incremented by one in step 1, and the beat frequency corresponding to the rising slope portion of the triangular wave is calculated in step 2 by the pulse count (the result is, for example, F1 and F1). Then, in step 3, the beat signal frequency corresponding to the downward slope portion of the triangular wave is calculated by the pulse count (the result is, for example, F2). Then, based on these results, the distance to the first target is determined by (F1 + F2) · K / 2 in step 4 (where K is the distance per beat signal / hertz). The relative speed with respect to the first target is expressed by (F2
−F1) · L / 2 (where L is a relative speed per beat signal / hertz, and it can be determined whether the value is closer or farther depending on whether the value is positive or negative). Next, in step 5, it is determined whether or not the count value n of the counter has reached a predetermined value a. If the answer is yes, the process proceeds to step 6, where the number of times calculated in step 4 above (in this case, a times Based on the average of the distance and the relative speed (over the distance), the distance to the first target and the relative speed are determined.

Next, at step 7, the Doppler shift component of the beat signal based on the relative speed of the first target (changes in the up-slope portion and the down-slope portion according to the magnitude of the relative speed as described above) The frequency of the modulation signal is increased to such an extent that the influence of the change in the beat signal frequency is substantially negligible, and the frequency of the modulation signal is increased in accordance with the increase in the frequency of the modulation signal. A low-pass filter cuts off a beat signal frequency component (if the modulation signal frequency is increased by n times, the beat signal frequency corresponding to the distance to the first target is also n times the same distance). (LPF) Determine the cutoff frequency of 25. Here, as the low-pass filter, for example, S.P. C. F. (Switched capacitor filter) can be used. In this case, by changing the frequency of the clock signal supplied from the microcomputer 1 (ie, the clock for SCF shown in FIG. 3), the cutoff frequency can be varied. Can be switched.

After the cutoff frequency of the low-pass filter is determined in this way and the beat signal frequency component (including the Doppler shift) corresponding to the first target is cut off, the cutoff frequency is shown in FIG. In steps 8 to 13, it is detected whether the second target is present before the first target, and if it is present, the second target is detected in the same procedure as in steps 1 to 6. Find the distance to the target. In this step, the modulation frequency is increased as described above, and therefore the influence of the Doppler shift component based on the relative speed can be ignored, but the relative speed cannot be determined with high accuracy.
For the second target, first, only the distance to the target is obtained (as shown in FIG. 5 described later, depending on the distance to the detected second target, the modulation signal of the modulated signal is again obtained). While lowering the frequency, the cutoff frequency of the low pass filter may be determined again to detect the relative speed of the second target).

In the second step 8, the count number K of the counter is counted up, and in steps 9 and 10, the presence or absence of beat signals corresponding to the rising and falling slopes of the triangular wave is checked. Pulse count the frequency of the beat signal,
On the basis of the result, the distance to the second target is calculated in step 11 in the same manner as in step 4 described above, and if the count value of the counter reaches the predetermined value b in step 12, step 13 Then, the distance to the second target is determined based on the average value of the distance calculated for each number of times (in this case, b times) calculated in step 11. By repeating the above procedure, it is also possible to obtain the distances to the third, fourth,... Targets having a lower reflection level further in front of the second target.

FIG. 4 is a timing chart of various signals used in the third device. FIG. 4A shows a triangular wave modulation signal, and as described above, the distance to the first target and its relative position. After determining the speed, it shows that the modulation frequency is increased. (B) is a signal indicating the timing of calculating the beat signal frequency in the microcomputer, and is set to a high level in an upward slope portion of the modulation signal (triangular wave).
The low level is set in the downward slope portion. Further, (C) is the clock frequency switching signal for SCF, and as described above, the low-pass filter is used by the switching signal according to the change of the modulation signal frequency as described above. C. F. , The cutoff frequency is switched.

FIG. 5 shows each of the targets A and B (the target B having a high reflection level corresponds to the first target and the target having a low reflection level in front of the targets A and B when the modulation signal frequency is changed). A indicates a relationship between the frequency spectrum of the beat signal corresponding to the second target and the cutoff frequency f _C of the low-pass filter (LPF) 25.

FIG. 5 shows a state in which the distance to the first target B and the relative speed are calculated by lowering the modulation signal frequency (corresponding to steps 1 to 6 in FIG. 1). B1 and B2 represent the case where the frequency spectrum B of the beat signal corresponding to the first target B is Doppler-shifted according to the relative speed of the target B in the upward and downward slopes of the modulation signal. Similarly, A1 and A2 denote frequency spectra A of the beat signal corresponding to the second target A in the up-slope part and the down-slope part according to the relative speed of the target A. The frequency spectrum when shifted is shown. In this case, since the level of the beat signal of the first target A is low, the beat signal of the first target B is masked by the beat signal of the first target B. As a result, the beat signal of the first target A corresponds to the spectra B1 and B2 of the target B. The distance to the target B and the relative speed thereof are obtained as described above based on the frequency to be performed.

Next, FIG. 5 shows that the frequency of the modulation signal is increased and the cutoff frequency f _C of the low-pass filter is switched to change the distance to the second target A as described in step 7 of FIG. Is calculated (corresponding to steps 8 to 13 in FIG. 2). In this case, by increasing the modulation frequency (for example, n times) as described above, the beat frequency (the center frequency of the beat frequency corresponding to the distance to the target) corresponding to each of the targets A and B is also increased. (That is, n times)
Since the Doppler shift of the beat frequency based on the relative speed of each target does not change unless the relative speed changes, the effect of the Doppler shift can be reduced by increasing the modulation frequency. In this way, by increasing the modulation frequency to such an extent that the influence of the Doppler shift of the beat frequency can be almost ignored, the beat signal components (B1 to B1) of the target B as shown in FIG.
2) can be frequency-separated from the beat signal components (A1 to A2) of the target A, so that the low-pass filter cuts between them (ie, so as to cut the beat signal component of the target B). The off frequency f _C is determined, and the distance to the target A within the pass band of the low-pass filter is calculated based on the frequencies corresponding to the spectra A1 and A2. Note that the relative speed cannot be easily obtained with high accuracy because of the high modulation frequency, so that only the distance is obtained here as described above.

Further, as shown in FIG. 5, as a result of obtaining the distance to the target A, as shown in FIG. 5, as shown in FIG. In the case where A1 and A2) can be frequency-separated from the beat signal components (B1 and B2) of the target B (when the target A is far from the target B), the modulation is performed again. The signal frequency is reduced, and the cutoff frequency f _C of the low-pass filter is determined again so as to cut the beat signal component for the target B at that time. The target A within the pass band of the low-pass filter is determined again. Can be calculated based on the frequencies corresponding to the spectra A1 and A2 at that time.

In this way, it is possible to identify the presence of the target A having a low reflection level in front of the target B having a high reflection level, so that the distance to the target A (and, in some cases, the relative speed thereof) can be identified. ) Can be performed, for example, for controlling the distance between vehicles. As described above, by repeating the above-described procedure, it is possible to identify a target having a lower reflection level further in front of the target.

[0027]

According to the present invention, even when a target having a low reflection level is present in front of a target having a high reflection level, the target is not masked by the target having a high reflection level, but is positioned in front of the target. Since it is possible to identify the presence of a target having a low reflection level in the vehicle, it is possible to perform, for example, an inter-vehicle control based on the target having a low reflection level.

[Brief description of the drawings]

FIG. 1 is a flowchart showing the operation procedure of the method of the present invention.

FIG. 2 is a flowchart showing an operation procedure of the method of the present invention.

FIG. 3 is a diagram illustrating a configuration of an FM-CW radar device used in the present invention.

FIG. 4 is a timing chart of various signals used in the apparatus of FIG. 3;

FIG. 5 shows respective targets A and B when the modulation frequency is changed.
FIG. 5 is a diagram showing a relationship between the frequency spectrum of the beat signal and the cutoff frequency of the low-pass filter.

FIG. 6 is a diagram illustrating a frequency spectrum of a beat signal corresponding to an upward and downward slope portion of a modulation signal when targets A and B have a relative speed.

FIG. 7 is a diagram illustrating a frequency spectrum of a beat signal corresponding to an upward and downward slope portion of a modulation signal when the relative speeds of the targets A and B are both 0.

FIG. 8 is a diagram showing a relationship between a transmission signal and a reception signal in the FM-CW radar when there is a relative speed with respect to a target.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Microcomputer 2 ... Sensor 21 ... Voltage controlled oscillator 22 ... Directional coupler 23 ... Mixer 24 ... High-pass filter 25 ... Low-pass filter 31, 32 ... Transmission and reception antenna

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G01S ⁷ /00-7/42 G01S 13/00-13/95

Claims (2)

(57) [Claims] 1. One round formed by an upward slope and a downward slope
Mixing a local signal obtained by shunting a part of a transmission signal frequency-modulated with a modulation signal consisting of a triangular wave that repeats a period and a reception signal received through a reception antenna,
A target that detects a beat signal corresponding to the target at each of the rising slope and the falling slope of the triangular wave, and measures a distance to the target based on an average value of frequencies of the detected beat signals. Detection method, first relatively low
The distance to the first target is calculated by using the modulation signal of the repetition frequency, and then the modulation is performed to such an extent that the influence of the Doppler shift component of the beat signal based on the relative speed of the first target is negligible. increasing the repetition frequency <br/> number of signals, the low-pass to further cut off the beat signal component of the frequency corresponding to the distance to the target of the first to rise with increasing modulation signal repetition frequency Determine the cut-off frequency of the filter, within the frequency band passing through the determined low-pass filter,
It is detected whether or not each beat signal corresponding to the second target exists, and when the presence of each beat signal is detected, the second beat signal is detected based on the frequency of each detected beat signal. 2. A target detection method using an FM-CW radar apparatus, wherein a distance to a target is calculated. 2. The beat signal corresponding to the second target is converted to the first target even if the repetition frequency of the modulation signal is reduced as a result of calculating the distance to the second target. If it is determined that the frequency can be separated from each of the beat signals corresponding to the object, the modulated signal
The repetition frequency is reduced, and the cutoff frequency of the low-pass filter is reduced again so as to cut off a beat signal component having a frequency corresponding to the distance to the first target, which decreases as the modulation signal repetition frequency decreases. And calculating a relative speed with respect to the second target based on a frequency difference between the beat signals corresponding to the second target existing in a frequency band passing through the determined low-pass filter. 2. The target detection method according to claim 1, wherein the target is detected.