JPH0128913B2 - - Google Patents

Description

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

The FM-CW radar measures relative distance and relative speed using a beat signal made up of a transmitted wave that is frequency-modulated with a triangular frequency change and a received wave that is a reflected wave. Therefore, it can be applied to a radar system for automobile collision prevention. In this case, the reception level will vary greatly depending on the positional relationship with the vehicle ahead due to meandering, curves, etc. while the vehicle is traveling. If the reception level is extremely low, the S/N ratio will deteriorate and 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. Furthermore, it is known that the beat signal is waveform-shaped and counted in order to find the frequency of the beat signal, but when the reception level of the beat signal decreases, the S/N ratio deteriorates, resulting in an error in the count value of the counter. This results in large errors in relative distance and velocity measurements.

For example, when the distance and reception level change as time t passes, as shown in Figure 1 a and b,
Even if the actual distance changes linearly, the measured distance will also change as shown by the solid line as the reception level decreases. Therefore, it has been proposed that when the reception level falls below a threshold level, it is determined that there is no target and control regarding relative distance and speed is interrupted.

However, it is common to use an AM detection circuit to detect the received level, but when the detected received level changes rapidly, this AM
Since a response delay occurs between the actual reception level and the detection output due to the smoothing section of the detection circuit, in such a configuration, when the detection output falls below the threshold level, the actual reception level has already reached S/ In many cases, the N ratio deteriorates to the point where errors occur in the measured relative distance and speed, and control continues based on the incorrect measurement data during the time corresponding to the response delay. There is a problem in that the reliability of the device itself decreases.

The present invention improves the reliability of the control itself by immediately detecting a sudden drop in reception level that may cause an error in the measured relative distance or speed and correcting the measured value. The purpose is to Examples will be described in detail below.

FIG. 2 is a block diagram of an embodiment of the present invention, in which 1 is a radar transmitting/receiving section, 2 is a low-pass filter, 3 is a variable resistor, 4 is a comparator circuit, 5 is a relative distance measuring section, and 6 is a relative speed measuring section. 7 is a distance data output terminal, and 8 is a speed data output terminal. The detected beat signal is output from the transmitting/receiving section 1,
The level corresponds to the reception level.
This beat signal is applied to a low-pass filter 2, and the output of the low-pass filter 2 indicates the average level of the beat signal. As the low-pass filter 2, it is desirable to use a filter with a small phase delay, such as a Bessel active filter.

The output level of the low-pass filter 2 becomes a voltage-divided level by adjusting the variable resistor 3, and becomes one input of the comparator circuit 4 as a threshold level. Also, since the other input is a beat signal, the comparator circuit 4 compares the beat signal level with the threshold level, and when the beat signal level suddenly decreases, the threshold level does not follow the sudden change. , a signal indicating that the beat signal level has become low is applied to the relative distance measuring section 5 and the relative speed measuring section 6. As a result, the relative distance measuring section 5 and the relative speed measuring section 6
invalidates the measurement data, retains the previous measurement data, and outputs it as correction data.

Fig. 3 is an explanatory diagram of the operation, in which a shows the relative distance, b shows the beat signal level, and c shows the invalid signal, and the output signal of the low-pass filter 2 is varied in response to changes in the beat signal level. The level of the signal adjusted by the resistor 3 and input to one side of the comparator circuit 4,
That is, the threshold level becomes a low frequency component of the change in the beat signal level, as shown by the dotted line in FIG. The previous measured value is output as correction data. still,
When the received level of the beat signal suddenly decreases from a high level, it is often the case that the vehicle in front of you has disappeared due to a curve, etc. In such a case, the measured relative distance and speed will become impossible to measure. In reality, it is considered that the preceding vehicle is present at approximately the interval before the reception level drops, and there is no problem even if control is performed using correction data holding measured values of relative distance and speed.

In addition, if the received level of the beat signal suddenly decreases when the received level is low, as shown in Figure 1, the measurement value using the beat signal with a decreased S/N ratio will have a large error. , it is better to immediately adopt a correction value with a relatively small error and perform control when the level of the beat signal suddenly decreases as in the present invention, since control can continue based on such a measured value with a large error. The reliability of control is improved compared to

As explained above, the present invention includes a low-pass filter 2 and a comparison circuit 4, the low-pass filter 2 outputs the average value of the level of the beat signal as a threshold level, and the comparison circuit 4 adjusts the level of the beat signal. When the beat signal level is low compared to the threshold level, an invalid signal is output to invalidate the measurement values in the measurement units such as the relative distance measurement unit 5 and the relative velocity measurement unit 6, and the measurement values immediately before them are invalidated. The value is output as correction data, and the threshold level applied to the comparator circuit 4 corresponds to the average value of the beat signal level, so it follows changes in the reception level based on differences in relative distance. It is possible to immediately detect a sudden drop in the reception level.

In addition, since the beat signal is directly applied to the measuring section, there is no delay in measuring relative distance or relative velocity, and in the event of an erroneous measurement due to an extreme drop in the reception level, it can be invalidated by the invalid signal from the comparator circuit 4. Since the immediately preceding measurement value is used as correction data, control such as rear-end collision prevention can be performed without error. Incidentally, the measuring section can also be constituted by a microprocessor or the like, and the present invention is not limited to the above-described embodiments, but can be modified in various ways.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the conventional reception level and measurement distance, FIG. 2 is a block diagram of the embodiment of the present invention, and FIG. 3 is an explanatory diagram of the operation. 1 is a transmitter/receiver section, 2 is a low-pass filter, 3 is a variable resistor, 4 is a comparison circuit, 5 is a relative distance measurement section, 6
is the relative velocity measuring section.

Claims (1)

[Claims] 1. In an FM-CW radar that measures relative distance and relative velocity using beat signals from transmitted waves and received waves, a low-pass filter is added to the beat signal, and the output of the low-pass filter is used as a threshold level to A comparison circuit that compares the level of the beat signal with an invalid signal output from the comparison circuit when the level of the beat signal is below the threshold level, and a relative distance and relative velocity measured using the beat signal. 1. An FM-CW radar, comprising: a measurement unit that invalidates the FM-CW radar and outputs the immediately preceding relative distance and relative velocity as correction data.