JP3558492B2 - Apparatus and method for measuring dry density of soil - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention detects the propagation speed of radio waves inside the embankment in order to know the degree of compaction of the embankment, and calculates the dry density of the embankment from the detected propagation speed and the separately measured or estimated water content of the soil. The present invention relates to a measuring device and a measuring method for measuring, and more particularly, to a measuring device and a measuring method of a dry density of soil which improve measurement accuracy by improving detection accuracy of a reception point of a received electric signal.
[0002]
[Prior art]
Conventionally, as disclosed in a patent application entitled "Soil compaction measuring device" previously filed by the present applicant (Japanese Patent Application Laid-Open No. 2-196960), the density of the embankment, and thus the compaction condition, is measured. Measurement devices are known. This measuring device is based on the principle that the relative permittivity of an embankment increases as its water content (moisture content) and density increase, and as a result, the propagation speed of radio waves decreases as the water content and density of the embankment increase. It was used.
[0003]
According to this measuring device, a pulse-like radio wave is propagated inside the embankment to be measured, and the propagation speed of the radio wave inside the embankment is measured from a time difference (required propagation time) between a transmission time point and a reception time point. The dry density and compaction of the embankment are measured from the propagation speed and the water content of the embankment measured or estimated separately.
[0004]
Transmitting and receiving pulsed electrical signals to the ground of the kind measuring device is characterized as being the following such as a radar apparatus to become fundamentally different are installed in a watercraft. First, the radio wave propagating in the ground is attenuated as much as 10 dB per meter, so that the measurement range cannot be too long and is limited to several meters at most. Second, radio waves propagating in the ground have different propagation paths and speeds depending on frequency components.
[0005]
The first characteristic is that the measurable range is narrow, so the time interval between transmission and reception of radio waves is extremely short. For example, if the relative dielectric constant in the ground is about 16, a propagation distance of 1 meter is required. The time is approximately 13 nsec. As described above, in this type of measuring apparatus, the time interval from transmission to reception is short, and in order to secure a required resolution, an isolated pulse to be transmitted needs to be extremely sharp with a half width of about 1 nsec. .
[0006]
The sharp isolated pulse having a half width of about 1 nsec as described above includes an extremely wide range of frequency components from DC to a maximum of about 2 GHz. By the way, according to a patent application (Japanese Patent Application Laid-Open No. 9-89807) entitled "Method and Apparatus for Measuring Degree of Compaction of Soil", which was filed earlier by the present applicant, as the frequency component becomes higher, A phenomenon similar to the skin effect in which the penetration depth of the skin is reduced is disclosed.
[0007]
That is, as illustrated in FIG. 3, the propagation path of the radio wave formed through the soil near the ground surface between the transmitting antenna TX and the receiving antenna RX arranged substantially parallel to the ground surface G has a high frequency component, a low The frequency components and the intermediate frequency components have different depths and path lengths such as fh, fl, and fm. As described above, since the depth of penetration of the propagation path into the soil, and therefore the length thereof, differs depending on the frequency component, the waveform (B) of the received signal becomes the waveform (A) of the transmitted signal as illustrated in FIG. Is synthesized under various delay times to form a ringing waveform in which large distortion has occurred.
[0008]
Further, the water having a large relative dielectric constant contained in the soil increases the dielectric constant of the soil. The relative permittivity of this water shows a large value of about 80 for the DC component, but in the GHz band, it approaches the dielectric relaxation frequency of water as the frequency increases, so that the relative permittivity decreases as the frequency increases. It is estimated that it will continue. This suggests that the propagation speed of radio waves in the ground increases with higher frequency components.
[0009]
According to a patent application entitled "Radar Antenna" (Japanese Patent Application Laid-Open No. Hei 9-116189) related to the earlier application of the present applicant, an underground radar for transmitting and receiving a sharp pulse-like electric signal having a half width of about 1 nsec is disclosed. It is also disclosed that the waveform of the reflected electric signal reflected and received by the buried object becomes a ringing waveform similar to the received waveform (B) in FIG. 2 due to the limitation of the frequency band of the antenna.
[0010]
In the above-described transmitting / receiving apparatus for electromagnetic waves such as laser beams that propagate in the air instead of underground, the waveform of the received reflected wave is slowed down, but still maintains a pulse shape similar to the transmitted waveform. A patent application entitled “Laser radar” according to the earlier application of the present applicant (Japanese Patent Application Laid-Open No. Hei 6-214025) discloses that the present point of time of the peak value of a received pulse amplified to be substantially constant is detected as the reception time Is disclosed. Another patent application entitled “Laser radar” related to the applicant's earlier application (Japanese Patent Laid-Open No. 6-235765) discloses that a peak value of a received pulse is detected and a certain ratio to the peak value is detected. A configuration is disclosed in which a value of 1/2 is dynamically set as a threshold, and a point in time when the amplitude of the received pulse exceeds the threshold is set as a reception point.
[0011]
[Problems to be solved by the invention]
As described above, in the measurement and the measurement method of the dry density of the embankment, the reception waveform of the transmitted wave and the reflected wave in the ground causes the formation of multipath due to a phenomenon similar to the skin effect, and the frequency dependence of the propagation speed and the like. Based on various factors such as band limitation of the antenna, a ringing waveform having a large distortion as illustrated in FIG. 2B is obtained. For this reason, the appearance of such a characteristic point on the waveform is determined as the time of reception, and the appearance of the characteristic point thus determined is detected accurately. Is a problem.
[0012]
[Means for Solving the Problems]
According to the apparatus and method for measuring the dry density of soil according to the present invention that solves the above-mentioned problems of the prior art, the receiving circuit samples and holds the received electric signal at a sampling period slightly longer than a predetermined transmission period. A sample-and-hold circuit that extends the time axis of the received electric signal, a variable gain amplifier circuit that amplifies the received electric signal with the extended time axis, and a digital waveform signal that converts the waveform of the amplified received electric signal into a digital waveform signal. An A / D conversion circuit for converting, and controlling the amplification gain of the variable gain amplifier circuit so as to keep a maximum value of the amplitude first appearing in the digital waveform signal at a predetermined value, and controlling the amplitude of the digital signal to A control / detection unit that detects a time point when a threshold value of a predetermined ratio with respect to a predetermined value is first reached as a reception time point of the electric signal.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
According to a preferred embodiment of the present invention, by dividing the distance between the center of the transmitting antenna and the center of the receiving antenna by the elapsed time from the transmission of the electric signal to the time of reception, the radio wave of the underground The radio wave speed is calculated. According to another preferred embodiment of the present invention, the predetermined threshold is set to a value of 3 to 4% of the predetermined value.
[0014]
【Example】
FIG. 1 is a functional block diagram showing a configuration of an embankment dry density measuring apparatus according to one embodiment of the present invention, wherein 1 is a digital signal processor (DPS), 2 is a timing control circuit, 3 is a transmission circuit, 4 is a balun transformer and 5 is a transmitting antenna. Further, 6 is a receiving antenna, 7 is a balun transformer, 8 is a high frequency amplifier circuit, 9 is a sample and hold circuit, 10 is a band pass filter circuit, 11 is a low frequency amplifier circuit, 12 is a variable gain amplifier circuit, and 13 is an A / A A D conversion circuit, 14 is a D / A conversion circuit, and 15 is a moisture content measuring unit using gamma rays, neutron rays, and the like.
[0015]
When the timing control circuit 2 is activated by the digital signal processor 1 and starts operating, the timing control circuit 2 supplies a transmission trigger pulse having a constant period T (20 μsec in this embodiment) to the transmission circuit 3 and the period of the transmission trigger pulse. A sampling pulse having a period (T + τ) larger than T by a small amount τ (τ≪T, in this example, 0.1 nsec) is delayed by (T + τ) from the first transmission trigger pulse to the sample-and-hold circuit 9. Supply.
[0016]
Upon receiving the transmission trigger pulse supplied from the timing control circuit 2, the transmission circuit 3 uses a sharp isolated pulse of high power (in this embodiment, an isolated pulse having a half width of about 1 nsec and a peak value of about 200 volt) as a transmission pulse. appear. The transmission pulse is supplied to a planar transmission antenna 5 called a bow-tie antenna or the like through a balun transformer 4 that converts two lines into four lines. Referring to FIG. 3, the transmission pulse is radiated from the transmission antenna 5 (TX), which is held substantially parallel to the ground, into the ground through a narrow gap between the transmission antenna 5 and the ground.
[0017]
Referring to FIG. 3, the transmission pulse radiated into the ground propagates on the surface of the ground almost in parallel with the ground, and then a narrow space between the receiving antenna 6 (RX) held substantially in parallel with the ground and the ground. The signal is received by the receiving antenna 6 (RX) via the gap. The reception signal received by the reception antenna 6 passes through a balun transformer 7 for converting four lines into two lines, and after being amplified by a high frequency amplifier circuit 8, is supplied to a sample and hold circuit 9, and is supplied to a timing control circuit 2. And is held in synchronization with the period (T + τ) sampling pulse supplied from.
[0018]
The received waveform signal is sampled and held, and its time axis is expanded by (T / τ) times (in this embodiment, 20 μsec / 0.1 nsec = 2 × 10 ⁵ = 200,000 times). It becomes a signal waveform. A predetermined number (2,000 in this embodiment) of the transmission pulses are continuously transmitted in the above-described fixed transmission cycle (20 μsec in this embodiment), so that one reception signal waveform expanded in the time axis is obtained. can get. Therefore, the time required to obtain one such reception waveform extended on the time axis is 40 msec, and the detection range of the reception signal is 200 nsec (= 0.1 nsec / piece × 2000 pieces) from the transmission of the transmission pulse. ) Range.
[0019]
By receiving 200,000 times longer axis extension, the received signal waveform that has been converted into a low frequency signal of the frequency band of several kHz, through the bandwidth frequency amplifier circuit 10, which is fixed in a low-frequency amplifier circuit 11 amplifies After receiving the low frequency amplification of the gain, it is supplied to a variable gain amplification (AGC) circuit 12 where it is subjected to variable gain amplification controlled by the digital signal processor 1. The digitally-received signal waveform expanded on the time axis which has been amplified by the variable gain is supplied to an A / D conversion circuit 13, where it is converted into a digital signal while being sampled at a period of 5 μsec. 13 is supplied.
[0020]
The digital signal processor 1 receives the received signal waveform supplied from the A / D conversion circuit 13 and expanded on the time axis and digitized. The received signal waveform is as illustrated in FIG. 2C, and the digital signal processor 1 detects the first maximum value Vpf appearing in such a received signal waveform. As a specific example, the digital signal processor 1 appears in this waveform as the maximum value within the interval from when the amplitude of the received waveform signal exceeds the positive threshold value Vth to when the amplitude of the received waveform signal falls below the threshold value Vth again. It is detected as the first maximum value Vpf.
[0021]
The digital signal processor 1 compares the first appearing local maximum value Vpf with a predetermined peak value Vpp (1 volt in this embodiment) being preset through the input circuit (IN) 1a. The digital signal processor 1 increases the gain to be newly set in the variable gain amplifier circuit 12 when the comparison result is Vpf <Vpp, and decreases the gain when the comparison result is Vpf> Vpp. Output a signal. This gain control signal is converted into an analog signal by the D / A conversion circuit 14 and set in the variable gain control circuit 12.
[0022]
The digital signal processor 1 changes the gain control signal when Vpf and Vpp become substantially equal by the variable gain control, specifically, when the absolute value of the difference between the two becomes smaller than a predetermined minute amount. Stop. In this state, the digital signal processor 1 sets the threshold Vth (30 mvolts to 40 mvolts) in which the amplitude of the rising portion of the digitized received signal waveform that has been extended on the time axis is set as 3 to 4% of the peak value Vpp. ) Is detected as the point of time tr of the reception pulse.
[0023]
As shown in FIG. 3, the digital signal processor 1 determines the center of the transmission antenna 5 (TX) and the reception antenna 6 ( By dividing the distance d from the center of RX), the propagation time of the transmitted pulse in the ground is calculated. The digital signal processor 1 calculates the dry density of the ground surface and the compaction degree from the calculated propagation speed and the water content of the ground measured by the water content measuring unit 15, and comprises a display device and the like. To the output circuit (OUT) 1b.
[0024]
The configuration in which the propagation path length of the radio wave is the distance between the centers of the transmission antenna and the reception antenna has been described above. However, a value obtained by multiplying this distance by an appropriate coefficient set in consideration of the depth of penetration into the soil may be used as the propagation path length.
[0025]
【The invention's effect】
As described in detail above, since the soil dry density measuring device according to the present invention is configured not based on the peak value of the received waveform signal but based on the maximum value of the amplitude that first appears. In addition, high measurement accuracy based on the high-frequency component appearing first in the received waveform can be realized.
[0026]
In other words, the lower-frequency component arrives at the receiving antenna with a slower propagation speed through a deeper and longer propagation path farther from the ground surface, and is formed by the synthesis of such a low-frequency component over time. The waveform of the received signal is complicated, and the peak value appearing later than the first rise of the signal waveform has a large fluctuation with respect to the point of occurrence and the amplitude.
[0027]
Therefore, instead of using the peak value accompanied by such a large fluctuation as the reference value, the detection accuracy at the time of reception can be improved by using the first maximum value appearing earlier as the reference value. And, focusing on the high-frequency component propagating in such a shallow part of the ground, preferably, the propagation distance of the radio wave is determined by the shortest distance between the center of the transmitting antenna and the center of the receiving antenna. Approximated.
[0028]
In addition, since the control of the variable gain amplifier circuit is performed by a control / detection unit realized by a digital signal processor or the like, the first local maximum value is not a peak value appearing in the received waveform signal but a substantially predetermined value. The complicated function of controlling the amplification gain so as to maintain the gain can be easily realized with a simple configuration.
[Brief description of the drawings]
FIG. 1 is a functional block diagram showing a configuration of an apparatus for measuring a dry density of soil according to one embodiment of the present invention.
FIG. 2 is a waveform diagram exemplifying a transmission pulse waveform transmitted from a transmission antenna to the ground and a reception waveform transmitted to the reception antenna after propagating through the ground.
FIG. 3 is a conceptual diagram for explaining that a penetration depth into the ground and a propagation path length are different depending on each frequency component included in a transmission pulse signal.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 digital signal processor 2 timing control circuit 3 transmission circuit 5 transmission antenna 6 reception antenna 9 sample and hold circuit 12 variable gain amplifier circuit 15 water content measurement unit

Claims (4)

A transmission circuit that generates and outputs a pulse-like electric signal of a predetermined transmission cycle, and a transmission antenna that is disposed substantially parallel to the ground and transmits the pulse-like electric signal output from the transmission circuit to the ground , A receiving antenna that is disposed substantially parallel to the ground and receives the electric signal propagated in the ground, a receiving circuit that detects a reception time point from a waveform of the received electric signal received by the receiving antenna, A soil drying apparatus comprising: a processor that calculates an underground propagation speed of an electric signal from an elapsed time from transmission to reception, and calculates a dry density of the soil from the calculated propagation speed and the underground moisture content. In the density measuring device,
The receiving circuit,
A sample and hold circuit that extends the time axis of the received radio wave by sampling and holding the received electric signal at a sampling period slightly longer than the predetermined transmission period,
A variable gain amplifier circuit for amplifying the received electric signal whose time axis has been extended,
An A / D conversion circuit for converting the waveform of the amplified received electric signal into a digital waveform signal;
The amplification gain of the variable gain amplifying circuit is controlled so that the maximum value of the amplitude first appearing in the digital waveform signal is kept substantially at a predetermined value, and the amplitude of the digital waveform signal is set at a predetermined ratio to the predetermined value. A control / detection unit for detecting a time point at which the threshold value is first reached as a reception time point of the electric signal, wherein the soil dry density measurement device is provided. In claim 1,
The processor calculates the propagation speed of the underground radio wave by dividing the distance between the center of the transmission antenna and the center of the reception antenna by the elapsed time from the transmission of the electric signal to the reception time. An apparatus for measuring dry density of soil. The said predetermined threshold value was set to the value of 3-4% of the said predetermined value, The measuring apparatus of the dry density of soil characterized by the above-mentioned. A pulse-like electric signal of a predetermined transmission cycle is transmitted into the ground from a transmitting antenna arranged substantially parallel to the ground, and an electric signal propagated in the ground is received by a receiving antenna arranged substantially parallel to the ground. The reception time is detected from the waveform of the received electric signal, and the propagation speed of the underground radio wave is calculated from the elapsed time from the transmission of the electric signal to the reception time, and the calculated propagation speed and underground water content are calculated. In the method of measuring the dry density of the soil to calculate the dry density of this soil from
When receiving the radio signal transmitted in the ground,
Extending the time axis of the received electric signal by sampling and holding the received electric signal at a sampling period slightly longer than the predetermined transmission period,
The received electric signal whose time axis is extended is amplified by a variable gain amplifier circuit,
The amplified waveform of the received electric signal is converted into a digital waveform signal,
The amplification gain of the variable gain amplifying circuit is controlled so that the maximum value of the amplitude first appearing in the digital waveform signal is kept substantially at a predetermined value, and the amplitude of the digital waveform signal is set at a predetermined ratio to the predetermined value. A method for measuring dry density of soil, wherein a time point when a threshold value is first reached is detected as the reception time point.

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