JPH10318942A - Apparatus and method for measuring dry density of soil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable highly accurate measurements by extending a time base of a receiving signal by a sample hold circuit, and setting as a receiving time point when an amplitude of the signal after subjected to variable gain amplification and A/D conversion reaches a threshold value of a constant ratio first. SOLUTION: A receiving signal of a receiving antenna 6 is held with a slightly longer cycle than a cycle of transmission trigger pulses at a sample hold circuit 9 through a balance transformer 7 and a high frequency amplification circuit 8, and a time base of a signal waveform is extended. The receiving signal is sent to a digital signal processor 1 via a low bandpass filter circuit 10, a low frequency amplification circuit 11, a variable gain amplification circuit 12 and an A/D conversion circuit 13. A maximal value appearing first in the signal waveform is detected. The variable gain amplification circuit 12 is controlled to hold a constant peak value. Thereafter, a time point when an amplitude of a first rise part of the signal waveform reaches a threshold value set to be approximately 3-4% of the peak value is used as a detection point, and a propagation time and a propagation speed are obtained. A dry density and a degree of compaction of a ground surface are obtained from the propagation time, propagation speed and a moisture content.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for detecting the propagation speed of radio waves inside an embankment in order to know the degree of compaction of the embankment. The present invention relates to a measuring apparatus and a measuring method for measuring the dry density of this embankment, and in particular, a measuring apparatus and a measuring apparatus for the dry density of the soil which improve the measurement accuracy by improving the detection accuracy of the reception point of the received electric signal. It is about the method.

[0002]

2. Description of the Related Art Conventionally, a patent application entitled "Soil compaction measuring device" previously filed by the present applicant (Japanese Patent Laid-Open No. 2-196)
960), the density of the embankment,
Therefore, a measuring device for measuring the degree of compaction is known. This measuring device increases the relative dielectric constant of the embankment as its water content (water content) and density increase,
It uses the principle that the propagation speed of radio waves decreases as the water content and density of the embankment increase.

According to this measuring device, a pulse-shaped radio wave is propagated inside the embankment to be measured, and the propagation speed of the radio wave inside the embankment is determined from the time difference (transmission time) between the transmission time and the reception time. Is measured, and the dry density and compaction degree of the embankment are measured from the propagation speed and the water content of the embankment separately measured or estimated.

A measuring device for transmitting and receiving a pulsed electric signal under the ground has the following characteristics which are fundamentally different from a radar device installed on a ship or the like. First,
Since the radio waves propagating in the ground are attenuated by as much as 10 dB per meter, the measurement range cannot be so long, and is limited to a few meters at most. Second, radio waves propagating in the ground are considered to have different propagation paths and speeds depending on frequency components.

[0005] The first characteristic, that is, the measurable range is narrow, 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, 1 meter 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 necessary resolution, an isolated pulse to be transmitted needs to be extremely sharp with a half width of about 1 nsec. .

A sharp isolated pulse having a half width of about 1 nsec as described above contains an extremely wide range of frequency components from DC to a maximum of about 2 GHz. Meanwhile, a patent application entitled "Method and Apparatus for Measuring Degree of Compaction of Soil" previously filed by the present applicant (Japanese Patent Application Laid-Open No. 9-89807).
Discloses a phenomenon similar to the skin effect in which the penetration depth of the propagation path from the ground decreases as the frequency component increases.

That is, as illustrated in FIG.
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 almost in parallel to the high frequency component, the low frequency component, and the intermediate frequency component between them , Fh, fl, and fm, the depth and the path length are different. As described above, since the depth of penetration of the propagation path into the soil, and thus 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 a 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,
In the z-band, the relative permittivity is estimated to decrease as the frequency increases, because it approaches the dielectric relaxation frequency of water as the frequency increases. This suggests that the propagation speed of radio waves in the ground increases with higher frequency components.

A patent application entitled "Radar antenna" according to the earlier application of the present applicant (Japanese Patent Laid-Open No. Hei 9-116189)
According to the underground radar, which transmits and receives a sharp pulse-like electric signal having a half-value width of about 1 nsec, the waveform of the reflected electric signal reflected and received by the buried object due to the limitation of the frequency band of the antenna. It is also disclosed that the ringing waveform becomes similar to the reception waveform (B) of FIG.

In the above-described transmitting / receiving apparatus for electromagnetic waves such as laser beams propagating in the air instead of in the ground, the waveform of the received reflected wave is slowed down, but still maintains a pulse shape similar to the transmission waveform. Patent application entitled “Laser radar” related to the applicant's earlier application (JP-A-6-214025)
Discloses a configuration in which the present point of time of the peak value of the received pulse amplified to be substantially constant is detected as the reception point. Further, another patent application entitled "Laser radar" (Japanese Patent Laid-Open Publication No. Hei 6-235765) related to the earlier application of the present applicant detects a peak value of a received pulse and detects a certain ratio to the peak value, for example. 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 a received pulse exceeds the threshold is set as a reception point.

[0011]

As described above, in the measurement and the method for measuring 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. A ringing waveform having a large distortion as illustrated in FIG. 2B is obtained based on various factors such as the frequency dependence of the propagation speed and the band limitation of the antenna. Therefore, the appearance of such a characteristic point on the waveform is determined as the time of reception, and how the appearance of the characteristic point thus determined is accurately detected. Is a problem.

[0012]

According to the apparatus and method for measuring the dry density of soil according to the present invention which solves the above-mentioned problems of the prior art, the receiving circuit operates at a sampling period slightly longer than a predetermined transmission period. A sample and hold circuit for extending the time axis of the received electric signal by sampling and holding the received electric signal; a variable gain amplifier for amplifying the received electric signal having the time axis extended; A D / A conversion circuit for converting a waveform of an electric signal into a digital waveform signal, and controlling an amplification gain of the variable gain amplifier circuit so as to keep a maximum value of an amplitude first appearing in the digital waveform signal at a predetermined value. Control / detection for detecting a point in time when the amplitude of the digital signal first reaches a threshold of a predetermined ratio to the predetermined value as a reception point of the electric signal. It is equipped with a door.

[0013]

According to a preferred embodiment of the present invention, the distance between the center of the transmitting antenna and the center of the receiving antenna is divided by the elapsed time from the transmission of the electric signal to the time of reception. , The radio wave speed of the underground radio wave 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]

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. 1 is a digital signal processor (DPS), 2 is a timing control circuit, Is a transmission circuit, 4 is a balun transformer,
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-pass filter circuit, 12 is a variable gain amplifier circuit, and 13 is an A / D converter.
A conversion circuit, 14 is a D / A conversion circuit, and 15 is a moisture content measuring unit using gamma rays and neutron rays.

When the timing control circuit 2 is activated by the digital signal processor 1 and starts operation, it supplies a transmission trigger pulse of a fixed period T (20 μsec in this embodiment) to the transmission circuit 3 and the transmission trigger pulse. A sampling pulse having a period (T + τ) larger by a small amount τ (τ≪T, in this example, 0.1 nsec) than the pulse period T is set to (T +
τ) and then supplies it to the sample and hold circuit 9.

When the transmission circuit 3 receives the transmission trigger pulse supplied from the timing control circuit 2, the transmission circuit 3 generates a high power sharp isolated pulse (in this embodiment, an isolated pulse having a half width of about 1 nsec and a peak value of about 200 volt). Generated as a transmission pulse. 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 transmitting antenna 5 (TX), which is held substantially parallel to the ground, into the ground through a narrow gap between the transmitting antenna 5 and the ground.

Referring to FIG. 3, the transmission pulse radiated into the ground propagates on the surface of the ground substantially parallel to the ground, and then, is transmitted between the receiving antenna 6 (RX) held substantially parallel to the ground and the ground. Through the narrow gap between the receiving antenna 6
(RX). 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 held in synchronization with the period (T + τ) sampling pulse supplied from.

The received waveform signal is sampled and held so that its time axis is expanded by (T / τ) times (in this embodiment, 20 μsec / 0.1 nsec = 2 × 10 ⁵ = 200,000 times). The received signal waveform. Note that the transmission pulse has the above-mentioned constant transmission cycle (20 μsec in this embodiment).
, And a predetermined number (2000 in this embodiment) is continuously transmitted, whereby one received signal waveform that is extended in the time axis is obtained. 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).

By undergoing a 200,000-fold time axis extension,
The received signal waveform converted to a low-frequency signal in a frequency band of several kHz passes through a low-pass filter circuit 10 and receives low-frequency amplification of an amplification gain fixed by a low-frequency amplification circuit 11, and then a variable gain. It is supplied to an amplification (AGC) circuit 12, where it is subjected to variable gain amplification controlled by the digital signal processor 1. The time-domain expanded digital reception signal waveform that has undergone the variable gain amplification is represented by A
/ D conversion circuit 13, where it is converted into a digital signal while being sampled at a period of 5 μsec,
The signal is supplied to the digital signal processor 13.

The digital signal processor 1 comprises:
It receives the time-domain expanded and digitized reception signal waveform supplied from the D / A conversion circuit 13. The received signal waveform is as illustrated in FIG. 2C, and the digital signal processor 1 detects the first maximum value Vpf that appears in such a received signal waveform. As a specific example, the digital signal processor 1 calculates the maximum value in the interval from when the amplitude of the received waveform signal exceeds the positive threshold Vth to when the amplitude again falls below the threshold Vth.
It is detected as the first maximum value Vpf appearing in this waveform.

The digital signal processor 1 includes:
The first maximum value Vpf that appears first and the input circuit (IN) 1
A predetermined peak value Vpp (1 volt in this embodiment) that is being set in advance is compared via a. 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 D / A
The signal is converted into an analog signal by the conversion circuit 14 and set in the variable gain control circuit 12.

The digital signal processor 1 comprises:
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, the change of the gain control signal is stopped.
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 present time tr of the reception pulse.

The digital signal processor 1 comprises:
As shown in FIG. 3, the time difference between the transmission time point of the transmission pulse and the output time point tr of the reception pulse is between the predetermined center of the transmission antenna 5 (TX) and the predetermined center of the reception antenna 6 (RX). By dividing the distance d, the propagation time of the transmission pulse in the ground is calculated. The digital signal processor 1 calculates the dry density of the ground surface and the degree of compaction 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. Output circuit (OUT) 1b.

In the above, the configuration in which the propagation path length of the radio wave is the distance between the centers of the transmitting antenna and the receiving antenna has been exemplified. 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]

As described in detail above, the apparatus for measuring the dry density of soil according to the present invention uses the first maximum value of the amplitude instead of the peak value of the received waveform signal as a reference. With such a configuration, it is possible to realize high measurement accuracy based on a high-frequency component appearing first in a received waveform.

That is, the lower frequency components reach the receiving antenna with a slower propagation speed through the deeper and longer propagation path farther from the ground surface and at a lower propagation speed. The waveform of the received signal to be formed becomes complicated, and the peak value appearing later than the first rising edge of the signal waveform has a large fluctuation with respect to the present time and the amplitude.

Therefore, instead of using the peak value accompanied by such a large fluctuation as the reference value, the first local maximum value appearing earlier is used as the reference value, so that the detection accuracy at the time of reception can be improved. 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.

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 used instead of the peak value appearing in the received waveform signal. The complicated function of controlling the amplification gain so as to keep it at a substantially predetermined value 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 soil dry density according to an embodiment of the present invention.

FIG. 2 is a waveform diagram illustrating 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]

 1 Digital signal processor 2 Timing control circuit 3 Transmitter circuit 5 Transmit antenna 6 Receive antenna 9 Sample and hold circuit 12 Variable gain amplifier circuit 15 Moisture content measurement unit

Claims (5)

[Claims] A transmitting circuit for generating and outputting a pulsed electric signal having a predetermined transmission cycle; a transmitting antenna for transmitting the pulsed electric signal output from the transmitting circuit to the ground;
A receiving antenna that receives an electric signal that has propagated in the ground, a receiving circuit that detects a receiving time point from a waveform of the received electric signal received by the receiving antenna, and an electric circuit that detects an elapsed time from the transmission of the electric signal to the receiving time point. A processor for calculating the underground propagation speed of the signal and a processor for calculating the dry density of the soil from the calculated propagation speed and the underground moisture content, wherein the receiving circuit includes: A sample and hold circuit for extending 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 cycle; and a received electric signal having the time axis extended. A variable gain amplifier circuit for amplifying the digital signal; a D / A converter circuit for converting the waveform of the amplified received electric signal into a digital waveform signal; The amplification gain of the variable gain amplifier circuit is controlled so as to keep the maximum value of the amplitude first appearing in the signal at a substantially predetermined value, and the amplitude of the digital waveform signal is initially set to a threshold of a predetermined ratio to the predetermined value. Control to detect the point in time when the
An apparatus for measuring dry density of soil, comprising: a detection unit. 2. The processor according to claim 1, wherein the processor divides a distance between a center of the transmitting antenna and a center of the receiving antenna by an elapsed time from transmission of the electric signal to reception of the electric signal. An apparatus for measuring the dry density of soil, which calculates the propagation speed of radio waves in the ground. 3. The method according to claim 2, wherein the predetermined threshold is 3 to 4 of the predetermined value.
An apparatus for measuring dry density of soil, wherein the apparatus is set to a value of%. 4. A transmission circuit for generating and outputting a pulsed electric signal having a predetermined transmission cycle, a transmission antenna for transmitting the pulsed electric signal output from the transmission circuit to the ground,
A receiving antenna that receives an electric signal that has propagated in the ground, a receiving circuit that detects a reception time point from a waveform of the received electric signal received by the reception antenna, and a ground circuit based on an elapsed time from the transmission of the electric signal to the reception time point. In the apparatus for measuring the propagation speed of an underground radio wave for calculating the propagation speed of an underground radio wave, the receiving circuit samples and holds the received electric signal at a sampling period slightly longer than the 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 converts the waveform of the amplified received electric signal into a digital waveform signal A D / A conversion circuit, and a variable gain amplifying circuit that keeps the maximum value of the amplitude first appearing in the digital waveform signal at a substantially predetermined value. And a control / detection unit for detecting the time when the amplitude of the digital waveform signal first reaches a threshold of a predetermined ratio with respect to the predetermined value as the reception time. A device for measuring the propagation speed of radio waves underground. 5. A pulse-like electric signal having a predetermined transmission cycle is transmitted into the ground, an electric signal transmitted through the ground is received, and a reception time point is detected from a waveform of the received electric signal. Calculate the propagation speed of radio waves in the ground from the elapsed time from the transmission of electrical signals to the time of reception, and calculate the dry density of the soil from the calculated propagation speed and the moisture content in the ground. In the method, when receiving the radio signal propagated in the ground, the time axis of the received electric signal is sampled and held 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 expanded is amplified by a variable gain amplifier circuit, and the waveform of the amplified received electric signal is converted into a digital waveform signal, which is first output in this digital waveform signal. Controlling the amplification gain of the variable gain amplifying circuit so as to keep the maximum value of the amplitude to be approximately a predetermined value, and the point in time when the amplitude of the digital waveform signal first reaches a threshold of a predetermined ratio to the predetermined value. A method for measuring the dry density of soil, which is detected as a reception point.