JPH0128914B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention measures positional distance and relative velocity.
This is related to FM-CW radar.

FM-CW radar transmits a frequency modulated wave with frequency changes in the form of a triangular wave, for example, and uses a beat signal from the transmitted wave and a received wave that receives a reflected wave from a target etc. to measure relative distance and relative speed. It can be applied to automobile collision prevention radar systems. In such a system, the positional relationship with the vehicle in front and the relationship with the road etc. change while driving, which causes multi-path from the road surface and the state of radio wave reflection from the vehicle in front to change, causing the reception level to increase. If the reception level becomes significantly low, errors will occur in the measured values of relative distance and relative velocity.

That is, when the reception level decreases to below a predetermined level, the beat signal component also decreases. Also,
It is known that the beat signal is waveform-shaped and counted in order to find the frequency of the beat signal.
As the beat signal component decreases, the S/N ratio deteriorates, causing errors in count values and errors in measured values of relative distance and speed.

Therefore, it has been proposed to correct the measured value by detecting a decrease in the reception level. When it is detected that the beat signal level has fallen below the threshold level, it is determined that there is no target and the relative distance is
This would interrupt speed control.

As a result, the control is interrupted before the level of the beat signal reaches a level below which the S/N ratio deteriorates, so there is no problem even if the level of the beat signal decreases.

However, in such a configuration, the smoothing process in the detection circuit causes a time delay in the change in the level of the beat signal, and if a sudden drop in level occurs, the output of the detection circuit will fall below the threshold level. At times, the actual level of the beat signal may be much lower than the level at which the S/N ratio is good. Therefore, an error occurs in the distance data that is the measured value during the delay time, and during that time, the relative distance including the error,
Since the control is continued based on speed, there is a problem in that the reliability of the control decreases.

Therefore, by detecting the received level of the beat signal with a detection circuit and then comparing the output of the detection circuit with a second threshold level that is higher than the aforementioned threshold level, the delay time of the detection circuit is It is possible to eliminate the delay in judgment due to

However, in such a configuration, even if the received level of the beat signal gradually decreases and is equal to or higher than the conventional threshold level, correction is performed at the same time as it falls below this second threshold level. The disadvantage is that even if the reception level is such that distance measurement is possible, no actual measurement is performed.

The present invention solves the problems of the prior art, and is applicable to an FM-CW radar that measures relative distance and velocity using a beat signal obtained by mixing a part of a transmitted wave and a received wave. In the
a first detection circuit that detects the beat signal; a low-pass filter that obtains an average level of the beat signal output from the first detection circuit; a first comparison circuit that detects a sudden drop; a second detection circuit that detects the beat signal; and a detection circuit that detects that the level of the beat signal output from the second detection circuit has fallen below a certain level. a second comparator circuit that
and a measuring section that outputs the immediately preceding measured values of relative distance and relative velocity as correction data when both of the detection outputs of the second comparison circuit occur. Examples will be described in detail below.

FIG. 1 is a block diagram of an embodiment of the present invention, where 1 is a radar transmitting/receiving section, 2 is a detection circuit, 3 is a high-speed detection circuit with a small time constant to enable high-speed detection, and 4 is a low-pass detection circuit. Filter, 5 is variable resistor,
6 and 7 are first and second comparison circuits, 8 is a reference voltage power supply, 9 is an AND circuit, 10 is a relative distance measuring section,
11 is a relative speed measuring section, 12 is an output terminal for applying the output signal of the detection circuit 2 to other processing circuits, such as a circuit for determining the presence or absence of a target, 13 is a distance data output terminal, and 14 is a speed data output terminal. .

The transmitting/receiving section 1 outputs a beat signal obtained by mixing a part of the transmitted wave and the received wave, and the detection circuit 2 is a normal detection circuit that detects the beat signal, and the detection circuit 2 is a normal detection circuit that detects the beat signal and passes through a low-pass filter 4 and a comparison circuit 6. Add to. The output terminal 12 is also added to a circuit that determines the presence or absence of a target by determining the reception level. Even if the beat signal level changes rapidly, the low-pass filter 4 outputs a signal at a level corresponding to the average level.The low-pass filter 4 outputs a signal at a level corresponding to the average level, and uses the level set by the variable resistor 5 as a threshold level to compare the detection output level with the comparison circuit. Compare by 6. Therefore, the output signal level of the low-pass filter 4 changes to follow a gradual change in the beat signal level, but the output signal level of the low-pass filter 4 cannot follow a sudden change in the beat signal level. When the beat signal level suddenly decreases, the output of the comparator circuit 6 becomes "1".

In addition, the high-speed detection circuit 3 has a time constant that is smaller than that of the detection circuit 2, and produces a detection output at a level that follows changes in the beat signal level. When compared with the reference voltage, when the detection output level becomes lower than the reference voltage, the output of the comparison circuit 7 becomes "1".

When the outputs of comparison circuits 6 and 7 both become "1",
The output of the AND circuit 9 becomes "1", and the relative distance measuring section 10 and the relative speed measuring section 11 perform correction processing. That is, in the relative distance measuring section 10 and the relative speed measuring section 11, the beat signal from the transmitter/receiver section 1 is waveform-shaped by comparing it with a reference level for waveform shaping into a pulse signal, and the pulse signal is generated at predetermined time intervals by a counter or the like. When the output of the AND circuit 9 becomes "1", the measured value obtained from the beat signal at that point is invalidated, and the immediately preceding measured data is retained and used as correction data. It is something to do.

FIG. 2 is an explanatory diagram of the operation. If curve A in the figure a is the detection output of the detection circuit 2, curve B is the detection output of the high-speed detection circuit 3. Due to the small time constant, fluctuations in the detection output level are , which follows the beat signal level from the transmitting/receiving section 1. Further, the threshold level of the output signal of the low-pass filter 4 set by the variable resistor 5 is shown in curve C, and the comparator circuit 6 outputs "1" when the level of curve A becomes lower than the level of curve C. Therefore, the output is as shown in FIG. 2b.

Further, assuming that the level of the reference voltage is curve D, the comparator circuit 7 outputs "1" when the level of curve B becomes lower than the level of curve D, so the output is as shown in FIG. 2c. That is, the measured value is corrected only when the detection output level by high-speed detection becomes less than the reference voltage and the detection output level is lower than the average level by the low-pass filter 4.

The relationship between the reception level and the beat signal level is
As shown in the figure, when the reception level is below L1, it corresponds to ground clutter, and when it is above L2, it becomes possible to shape the waveform of the beat signal, etc.
In this case, it becomes possible to determine the presence or absence of a target by processing the beat signal, and furthermore, it becomes possible to determine the presence or absence of a target by processing the beat signal.
If L2>L1) is the correction judgment level, it is possible to measure the relative distance and relative speed if the reception level is at least the correction judgment level L4, so correction processing is not performed even if there is a sudden change in the reception level. The correction processing is performed only when the reception level is lower than the correction determination level L4 and the drop is rapid.

According to the configuration of the present invention, when the reception level of the beat signal detected by the second detection circuit is below the threshold level and the reception level decreases gradually, the output of the AND circuit 9 is It remains at "0", and the relative distance measuring section 10 and relative speed measuring section 11 are not corrected by the output of the AND circuit 9. However, when the reception level decreases gradually, the output of the first detection circuit almost follows the beat reception level, so the presence or absence of the target is detected without time delay based on the output from the output terminal 12. Therefore, no malfunction will occur.

Furthermore, the low-pass filter 4 is preferably a Bethel active filter with little phase lag, and the threshold level applied to the comparator circuit 6 can be set to a level that follows the beat signal level.

FIG. 4 is a block diagram of main parts of another embodiment of the present invention, in which the same reference numerals as in FIG. 1 indicate the same parts,
The operation of the comparator circuit 6 is controlled by the output of the comparator circuit 7,
The output of the comparison circuit 6 is used as a correction control signal for the relative distance measuring section and the relative speed measuring section. That is, the output of the comparison circuit 7 is "0" when the high-speed detection output level of the high-speed detection circuit 3 is higher than the reference voltage from the reference voltage power supply 8, so the threshold level of the comparison circuit 6 is 0, and the output of the comparison circuit 6 is "0". The output of is "0". Also, when the high-speed detection output level of the high-speed detection circuit 3 falls below the reference voltage, the output of the comparison circuit 7 becomes "1", and the threshold level of the comparison circuit 6 is determined by dividing the output signal of the low-pass filter 4 with a resistor. level, and the detection circuit 2
When the detection output level of the detection output level falls below the threshold level, the output of the comparator circuit 6 becomes "1".

As explained above, the present invention forms a threshold level corresponding to the average level of the received signal using the low-pass filter 4, and compares it with the detection output level of the beat signal obtained from the first detection circuit. It is possible to detect a sudden drop in reception level, and even if there is a sudden drop in reception level, if it is above the correction judgment level, it is possible to measure the relative distance and relative speed, so correction processing can be performed. For example, if the car in front brakes suddenly while driving at high speed, even if the reception level suddenly drops, it is usually above the measurable reception level, so no correction processing is performed. , accurate relative distance and relative velocity at that point in time can be measured, and controls such as collision prevention can be performed. Note that the measurement units such as the relative distance measurement unit 10 and the relative velocity measurement unit 11 can also be configured by a microprocessor or the like.

[Brief explanation of drawings]

Fig. 1 is a block diagram of one embodiment of the present invention, Fig. 2 is an explanatory diagram of level comparison operation, Fig. 3 is an explanatory diagram of reception level and beat signal level, and Fig. 4 is an illustration of another embodiment of the present invention. FIG. 3 is a block diagram of the main parts of the embodiment. 1 is a transmitting/receiving section, 2 is a detection circuit, 3 is a high-speed detection circuit, 4 is a low-pass filter, 5 is a variable resistor, 6,
7 is a comparison circuit, 8 is a reference voltage power supply, 10 is a relative distance measuring section, and 11 is a relative speed measuring section.

Claims (1)

[Claims] 1. In an FM-CW radar that measures relative distance and relative velocity using a beat signal obtained by mixing a part of a transmitted wave and a received wave, a first detection circuit that detects the beat signal; A low-pass filter that obtains the average level of the beat signal output from the first detection circuit, and a first comparison circuit that uses the output of the low-pass filter as a threshold level to detect a sudden drop in the level of the beat signal. , a second detection circuit that detects the beat signal, a second comparison circuit that detects that the level of the beat signal output from the second detection circuit has decreased below a certain level, and the first and second detection circuits. An FM-CW radar, characterized in that it is equipped with a measurement section that outputs immediately preceding measured values of relative distance and relative velocity as correction data when both detection outputs of the comparison circuit occur.