JPH0124276B2 - - Google Patents

Description

[Detailed description of the invention] The present invention can reduce measurement errors.
This is related to FM-CW radar.

FM-CW radar is capable of changing the transmission frequency, for example, in the shape of a triangular wave, forming a beat signal from the transmission signal and the received signal of the reflected wave, and measuring relative distance and relative velocity based on the beat frequency. It is. Figure 1 shows the transmitted signal SS and received signal
It is an explanatory diagram of RS and beat frequency Fb, and the transmission signal SS is repeated with an up sweep period UP for changing the frequency from F to F + ΔF and a down sweep period DWN for changing the frequency from F + ΔF to F with one period 1/Fm. The received signal RS is obtained after a time T corresponding to the distance R (m) to a target such as a vehicle ahead, and the speed of light is C (3 × 10 ⁸ m/sec). Then, the time T becomes T=2・R/C(sec)...(1). and a part of the transmitted signal SS and a part of the received signal RS
By mixing the beat frequency Fb
beat signal is obtained.

The beat frequency Fb is expressed as Fb=2・Fm・ΔF・T=4・R・Fm・ΔF/C...(2) where the frequency deviation is ΔF (Hz) and the modulation frequency is Fm (Hz). Therefore, the distance R is as follows: R=Fb・C/4・Fm・ΔF (3) Therefore, the distance can be measured using the beat frequency Fb.

Also, the relative speed is determined by the difference between the beat frequency Fbu during the up sweep period UP and the beat frequency Fbd during the down sweep period DWN. Note that when the beat frequencies Fbu and Fbd of each period UP and DWN are different, the relative distance can be measured by setting (Fbu+Fbd)/2=Fb in (3).

The beat frequency Fb can be determined by counting the beat signal for a certain period within one period 1/Fm. In that case, since the count is an integer, it includes an error of one pulse per cycle. Here, by setting Fb/Fm=1 and substituting this relationship into equation 3, the distance error ΔR due to one pulse is obtained. That is, ΔR=C/4·ΔF(m) (4) If ΔF=15MHz, then ΔR=5m. That is, a counting error of one pulse causes an error of 5 m in the measured distance.

When the distance to the target is constant, the beat signal has the same waveform every period 1/Fm, so if you count the beat signal for a certain period based on the synchronization signal with the period 1/Fm. , the count content is always constant. That is, even if the average value is simply determined, the measurement accuracy cannot be improved.

Further, as a technique for solving such problems, there is a technique as disclosed in Japanese Patent Application Laid-Open No. 125873/1983. This technique involves counting the frequency of a repetitive signal having a predetermined period by shifting the timing for each period, and measuring the frequency from these counted values.

However, when this technology is applied to FM-CW radar, there are the following problems.

In other words, if the period of the beat signal matches the shift in frequency counting timing due to a change in the distance to the target, the counted value of the counter at each counting timing will be the same, improving the accuracy of the measured value. The problem is that it is not possible.

The present invention solves the above-mentioned problems, and it is possible to measure by changing the time length of the count period of the beat signal for each cycle and finding the average value of the count contents of the beat signal for each cycle. The purpose is to reduce errors. 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 transmitting antenna, 2 is a transmitting circuit, 3 is a modulation circuit, 4 is a synchronization signal generation circuit, 5 is a timing control circuit, and 6 is a one-shot multivibrator. 7 is a receiving antenna that receives reflected waves, 8 is a mixer that mixes a part of the transmitted signal and the received signal and outputs a beat signal, 9 is a bandpass filter, and 10 is a beat signal. 11 is a gate circuit, 12 is an arithmetic circuit including a counter, and 13 is a display circuit. Although this embodiment has separate antennas for transmitting and receiving, it is also possible to use a shared antenna for transmitting and receiving.

The synchronization signal generation circuit 4 uses the above-mentioned modulation frequency Fm
This circuit generates a synchronizing signal and applies the synchronizing signal to the modulating circuit 3 and the timing control circuit 5. The modulating circuit 3 changes the transmission frequency in synchronization with the synchronizing signal, and the timing control circuit 5 changes the transmitting frequency in synchronization with the synchronizing signal. The timing signal is sequentially shifted in phase with respect to the synchronization signal each time the synchronization signal is input. For example, it is possible to have a configuration in which synchronization signals are counted and the delay time is controlled according to the counted contents, and the same operation is repeated every N counts to output a timing signal.

The gate signal generation circuit 6 generates a gate signal of the OR logic of the signal generated from the time when the synchronization signal is generated to the time when the timing signal is generated and the fixed time length signal generated every time the timing signal is generated, and adds it to the gate circuit 11. Open circuit 11 and pulse forming circuit 1
A pulse from 0 is applied to an arithmetic circuit 12, a relative distance is calculated and added to a display circuit 13, and the relative distance is displayed in analog or digital form.

FIG. 3 is an explanatory diagram of the operation, and shows the case where the transmission frequency is changed in a sawtooth waveform, and a to g indicate signals a to g of each part in FIG. 2. The output signal of the modulation circuit 3 is as shown in FIG.
The frequency is changed from F to F+ΔF every cycle of 1/Fm, and the synchronizing signal b from the synchronizing signal generating circuit 4 is as shown in FIG. 3b. Transmission circuit 2
A mixer 8 mixes a part of the transmitted signal branched at , and a received signal whose reflected wave is received by the receiving antenna 7, and a beat signal c is obtained by removing unnecessary harmonic components by a bandpass filter 9, for example, as shown in FIG. In the pulse forming circuit 10, the pulse signal d shown in FIG.

Further, the timing control circuit 5 outputs a timing signal e as shown in FIG. 3e based on the synchronization signal b, and has an initial phase with a period of N/Fm. That is, when the initial phase for the synchronization signal b is zero, the timing signal e shown in FIG. The phase of the timing signal e is sequentially shifted as expressed by 1/2N)·i (1).

The gate signal generation circuit 6 generates the signal f2 generated between the synchronization signal b and the timing signal e, as shown in FIG. The gate circuit 1 outputs a gate signal f such as
Add to 1. The time TG' of this gate signal f is selected according to the relationship TG'(1/Fm)...(2).

Therefore, the pulse signal g applied to the arithmetic circuit 12
is as shown in FIG.
Count TG/N [TG+(N-1)T/2]
By multiplying, the average value within the period of N/Fm is obtained.

Figure 4 is for explaining the reduction in measurement error caused by sequentially increasing the time length of the gate signal f by TG/10 as described above. Assuming that the pulse signal shown in FIG. 4B is obtained and the gate signal shown in FIG. By sequentially shifting the phase of the timing signal from the timing control circuit 5, the time length of the gate signal is sequentially changed as shown in FIG. When 5, 6, 6, 6 pulse signals are applied to the arithmetic circuit 12 and the pulse signals for each gate signal are counted and the average value is determined, the number of pulse signals becomes "4.3".

Or, if the beat signal shown in Fig. 4 i is obtained, the pulse signal shown in Fig. 4 i is formed, and the pulse signal shown in Fig. 4 c is formed.
The number of pulse signals 4, 4, 4, 5, 5, 5 is added to the arithmetic circuit 12 by the gate signals shown in ~h, and the average value of the count contents is determined by the 6 gate signals. If it is, it will be "3.6". In this case, since the frequency of the beat signal shown in FIG. 4i is 0.8 times the beat signal frequency shown in FIG. 4a,
The average value of the count contents should be "3.2", but if only the gate signal with the time width shown in Figure 4 c is used, the average value of the count contents will be "4" and the error will be large. . However, as mentioned above, by changing the time width of the gate signal,
As the average value of the count approaches "3.2", the error in the distance measurement value becomes smaller.

If the beat signal shown in Fig. 4 k is obtained, the pulse signal shown in Fig. 4 is formed, and the beat signal shown in Fig. 4 k is obtained.
Since the frequency is 1.33 times the frequency of the beat signal shown in Figure a, the average value of the count contents should be "5.3" to be exact. The number of pulse signals is 6, 6,
7, 7, 8, 8, so the average value of the count is "5.6". However, if only the gate signal with the phase shown in FIG. 4c is used, the average value of the count contents will be "6" and the error will be large.

As explained above, the present invention provides means for converting a beat signal into a pulse signal and changing the time length of the gate signal f that controls the gate circuit 11, and The device is equipped with means such as an arithmetic circuit 12 that counts the pulse signals generated and calculates the average value to calculate the distance measurement value. The value error can be reduced to a small value of one pulse or less or zero. Note that it is of course possible to modulate the transmission signal with a triangular wave as shown in FIG. 1 other than the sawtooth wave shown in FIG. It would be good if it could be controlled. Furthermore, 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 transmitted signal, received signal, and beat signal, Fig. 2 is a block diagram of an embodiment of the present invention, Fig. 3 is an explanatory diagram of the operation, and Fig. 4 is a sequential change in the time width of the gate signal. FIG. 4 is an explanatory diagram of the operation when 1 is a transmission antenna, 2 is a transmission circuit, 3 is a modulation circuit, 4 is a synchronization signal generation circuit, 5 is a timing control circuit, 6 is a gate signal generation circuit, 7 is a reception antenna, 8 is a mixer, 9 is a bandpass filter, 10
1 is a pulse forming circuit, 11 is a gate circuit, 12 is an arithmetic circuit, and 13 is a display circuit.

Claims (1)

[Claims] 1. Mix a part of the transmitted signal and the received signal to form a beat signal, and measure the distance by pulsing and counting the beat signal.
In the FM-CW radar, a gate circuit that applies a pulse signal obtained by pulsing the beat signal, a gate signal generating means that changes the time length of a gate signal that opens the gate circuit, and a gate circuit that applies the pulse signal that is generated by pulsing the beat signal; 1. An FM-CW radar comprising: calculation means for calculating an average value of signal count contents and calculating a distance measurement value based on the average value.