JPH05297119A - Beat count type fm-cw distance measuring device - Google Patents

Abstract

PURPOSE:To provide an FM-CW distance measuring device of beat count type, which is equipped with an enhanced distance resolution and can even reduce the step error. CONSTITUTION:An FM-CW distance measuring device of beat count type comprises the first counter 15, another counter 8, a free counter 9, and a CPU 6. The first counter 15 measures the linear frequency modulation period while the counter 8 counts the zero-cross points per cyclic period of the beat signals obtained during the said linear frequency modulation period, and the free counter 9 measures the time from initial change of the beat signal counted value till its final change. The CPU 6 determines the distance on the basis of the counted values obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a beat count type frequency modulation continuous wave.
e, hereinafter abbreviated as FM-CW. ) In a distance measuring device, a beat count type FM-
The present invention relates to a CW distance measuring device.

[0002]

2. Description of the Related Art In FM-CW ranging with linear frequency modulation, the distance H is given by the following formula 1.

[0003]

[Equation 1] C: speed of light T: modulation period ΔF: frequency modulation width f _b : beat signal frequency By measuring the number n of zero crossings of the beat signal at the period T as shown in FIG. 2, the beat signal frequency can be approximately calculated by the following formula 2 expressed.

[0004]

[Equation 2] Therefore, by substituting the equation 2 into the above equation 1, the following equation 3 is obtained.

[0005]

[Equation 3] The distance can be obtained by measuring n according to the equation (3). This is called a beat counting device.

[0006]

However, according to the equation (3) described above, the beat count number n can take only an integer, so the resolution of the distance is inversely proportional to ΔF. or,
Even at beat frequencies that can be regarded as equivalent, a measurement error of a frequency corresponding to within ± 1 count occurred due to the initial phase, and this became a problem as a step error.

Therefore, conventionally, the resolution was improved and the step error was reduced by making ΔF as large as possible. However, in order to further reduce the step error, the hardware limit and the radio wave resource From a point of view, a different method was desired.

Therefore, in view of the above-mentioned drawbacks, a technical object of the present invention is to provide a beat count type FM-CW distance measuring device which can improve the distance resolution and reduce the step error.

[0009]

According to the present invention, a first counter for measuring a linear frequency modulation period and a second counter for counting the zero-cross points of the beat signal obtained during that period for each cycle. And a beat count type FM-, which has a third counter for measuring the time from the initial change to the final change of the count value of the beat signal, and a calculating means for obtaining a distance from the obtained count value. CW
A range finder can be obtained.

That is, according to the present invention, the distance resolution is improved and the step error is reduced by measuring the count number obtained by the beat count with a substantial decimal point.

[0011]

In the present invention, the zero-cross timing of the beat signal for counting is synchronized with the cycle of the beat signal, so that the timing C counted first is
_From the time T ′ from ₁ to the last counted C _n ,
The beat frequency is obtained from the following equation (4).

[0012]

[Equation 4] According to the equation (4), the measurement is always accurate based on the measurement accuracy of T'without counting error.

At this time, the frequency modulation width Δ corresponding to T '
Assuming the linear frequency modulation, the following Formula 5 holds for F ′.

[0014]

[Equation 5] Therefore, when substituting the equations 4 and 5 into the equation 1,
The following equation 6 is obtained.

[0015]

[Equation 6] That is, the count number n is (T /
Since T ') can be given as a real number, T, T'
The distance resolution is improved by measuring with high resolution.

[0016]

Embodiments of the present invention will now be described with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of the present invention. In the figure, 1 is a transmitting antenna, 2 is a directional coupler, 3 is a VCO, 4 is a sawtooth wave generator, 5 is a D / A converter, and 6
Is a CPU, 7 is a timing generator, 8 is a counter,
9 is a free counter, 10 is a clock generator, 11
Is a zero-cross comparator, 12 is an amplifier, 13 is a mixer, and 14 is a receiving antenna.

D / A converter 5 controlled by CPU 6
Through the sawtooth wave generator 4 to generate a modulated sawtooth wave corresponding to the period T and the frequency modulation width ΔF. This modulation wave causes the VCO3 to oscillate a frequency modulation wave,
It is radiated from the transmitting antenna through the directional coupler 2.

The radio wave reflected from the surface to be measured is delayed in proportion to the distance and is supplied to the mixer 13 as a frequency modulated wave through the receiving antenna 14. Here, a part of the transmission wave distributed from the directional coupler 2 and the frequency modulation wave are mixed to obtain a beat signal.

The weak signal is amplified to a predetermined level by the amplifier 12 and converted into a square wave zero-cross signal by the zero-cross comparator 11 having a hysteresis characteristic.

The counter 8 is reset outside the linear modulation period, and when it is enabled during the modulation period, it is counted up by the edge on one side of the zero cross signal (FIG. 2 (c)), and the count value (FIG. 2 (d)) is output.

On the other hand, the free counter 9 is continuously counted up every cycle by the frequency f _CLK of the clock generator 10, and the count value of the free counter 9 is latched in synchronization with the count up of the zero-cross signal. .. The latched value is stored in the memory of the CPU 6 as a time capture value (FIG. 2 (g)).

The counter 15 measures the linear frequency modulation period by the count value from the counter 8 and the frequency f _CLK of the clock generator 10 and outputs it to the CPU 6.

Here, as shown in FIG. 2 (FIG. 2 (g)), after the initial time capture value C ₁ , the previous capture value is sequentially updated, and finally the time capture value C at the end of the modulation cycle is obtained. Remember _n . From this binary value, the CPU 6 measures the cycle of an integral multiple of the beat signal by the following equation (7).

[0025]

[Equation 7] The value of T ′ measured by the equation (7), the modulation cycle T set by the timing generator 7, the beat count value n obtained by the counter 8, and the initially set Δ
Based on the value of F, the distance H is calculated by the above equation (6).

As is clear from the above equation 1, even if the distance changes, the term of T / ΔF is changed so that f
It is possible to keep _b constant.

From the obtained altitude data, the slope of the sawtooth wave for reaching a predetermined f _b is reset in the D / A converter 5, and at the same time, in order to keep ΔF constant, a cycle T is set in the timing generator 7. Reset.

By doing so, the band of the receiver can be narrowed, so that the S / N is improved and, as a result, the detected jitter component of the zero-cross comparator 11 is reduced and the variation of the measured value is suppressed.

[0029]

As described above, according to the present invention,
The range resolution is greatly improved, and as a result, step errors can be reduced. Therefore, the resolution comparable to the conventional one can be achieved with a smaller frequency modulation width, and the oscillator can be simplified and the radio resources can be effectively used. Further, since the analog beat signal is digitized by a simple circuit and the subsequent processing can be achieved only by a simple digital circuit, the circuit can be downsized and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a time chart of processed signals.

[Explanation of symbols]

 1 Transmitting Antenna 2 Directional Coupler 3 VCO 4 Sawtooth Wave Generator 5 D / A Converter 6 CPU 7 Timing Generator 8 Counter 9 Free Counter 10 Clock Generator 11 Zero Cross Comparator 12 Amplifier 13 Mixer 14 Reception Antenna

Claims (1)

[Claims] 1. A first counter for measuring a linear frequency modulation period, a second counter for counting a zero-cross point of a beat signal obtained in the linear frequency modulation period for each cycle, and a beat counter for the beat signal. Beat count type FM-CW range finder having a third counter for measuring the time from the initial change to the final change of the count value, and an arithmetic means for obtaining the distance from the respective obtained count values. ..