CN201327478Y - Soil moisture measuring transducer - Google Patents

Abstract

The utility model relates to a soil moisture measuring transducer. The soil moisture transducer sends high-frequency signals through a high-frequency signal source, a detection circuit converts the change of dielectric constants to DC voltage and outputs the DC voltage, the determined mathematical relation between the output DC voltage and soil moisture content realizes accurate measurement of soil volumetric moisture content, and the accuracy of measurement is plus or minus two percent. Because the probe of the transducer of the utility model adopts a pin type, the transducer can be buried on a soil profile for continuous measurement, or matched with a special measuring instrument for movable itinerant measurement. The transducer has the advantages of relatively simple circuit, low cost and convenient debugging; and meanwhile, the price of the transducer is greatly lower than that of a soil moisture test instrument and, and the consistency of transducers in batch production is ensured.

Description

Soil moisture measurement sensor

Technical field

The utility model relates to a kind of soil moisture measurement sensor.

Background technology

The soil moisture measurement technology is the important techniques guarantee that saving water, resisting drought is implemented.Soil moisture sensor and surveying instrument then are the important technical that realizes variable irrigation and soil moisture content (damage caused by a drought) monitoring.

Utilize the dielectric property of soil come the measured soil water cut be a kind of effective, fast, easy, reliable method.That at first the dielectric property of soil is made systematic study was The former Russian scholar Chernyak, and he has published the academic masterpiece " wet soil Study on dielectric property method " that causes World Focusing in 1964.Based on this, the dielectric property of soil is applied to rapidly in the measuring technique of soil moisture content, and the specific implementation method varies.Wherein, high frequency capacitance probe measurement soil moisture content, very high frequency(VHF) transistor line oscillator measured soil water cut, time domain reflectometry (TDR), frequency domain reflectometry measuring methods such as (FDR) all belong to the soil water-containing measuring method based on the soil dielectric property.

The dielectric measuring method of soil moisture mainly contains time domain reflectometry (TDR) and frequency domain reflectometry at present.Time domain reflectometry (TDR) is by measuring the transmitted wave of frequency electromagnetic waves in soil and the mistiming measured soil water cut between reflection wave.The TDR technical sophistication, time-domain reflectomer costs an arm and a leg, and this technology is developed country's monopolizations such as America and Europe at present, still is blank in China.

Apparent dielectric constant changes this principle mensuration soil moisture content with soil moisture content in the frequency domain method application measured medium.The frequency domain method instrument is generally worked in 20～150MHz scope, the variation of specific inductive capacity can be converted to DC voltage or other output forms by multiple circuit, and the DC voltage of output is directly related with soil moisture content in the working range of broadness.The soil moisture curve of output and the TDR of frequency domain method are similar, by polynomial expression, should just can obtain good corresponding measuring accuracy with cubic polynomial.

Because the wide soil types in Chinese region is many, external soil moisture sensor can not given the calibration characteristic in all kinds of soil of China, and often measuring accuracy can not get guaranteeing, add that price is higher, technical protection is untimely etc., should not be in Chinese wide popularization and application.

The utility model content

The utility model has overcome above-mentioned shortcoming, provide a kind of theory fully, advanced technology, circuit is simple, accuracy of detection is high soil moisture measurement sensor.

The technical scheme in the invention for solving the technical problem is: a kind of soil moisture measurement sensor, comprise high-frequency signal source, transmission line circuit of equivalent network, detecting circuit, amplifying circuit, sensor probe, one end of described transmission line circuit of equivalent network connects high-frequency signal source, the other end connects described sensor probe, described two detecting circuits are connected to the two ends of transmission line circuit of equivalent network, and described amplifying circuit links to each other with the output terminal of described detecting circuit.

Described amplifying circuit is a differential amplifier circuit, and the two paths of signals input end of described differential amplifier circuit connects described detecting circuit output terminal respectively.

Described sensor probe is made up of four isometric stainless steel probes, wherein one is the center probe, be connected an end of transmission line circuit of equivalent network, its excess-three root is to pop one's head in the periphery of described center probe equidistant placement, be connected the earth terminal of transmission line circuit of equivalent network, described detecting circuit is connected to the two ends of transmission line circuit of equivalent network.

The utility model utilizes the measured medium medium dielectric constant microwave medium to change this principle with soil moisture content and measures soil moisture content.Soil moisture sensor sends high-frequency signal by described high-frequency signal source, by detecting circuit the variation of specific inductive capacity is converted to DC voltage output, the DC voltage of output (0-100%Vol) in the working range of broadness is fixed a number really with soil moisture content and is learned relation, realize the accurate measurement of volumetric water content of soil, measuring accuracy is ± 2%.Because the utility model sensor probe adopts probe-type, can be embedded in the soil profile continuous coverage, also can cooperate and do to move touring the measurement with the special measurement instrument, and circuit is simple relatively, therefore cost is low, price is significantly less than the TDR soil moisture tester, and debugging is convenient, has guaranteed the consistance when sensor is produced in batches.

Description of drawings

Fig. 1 is a schematic block circuit diagram of the present utility model;

Fig. 2 is a transmission line circuit of equivalent network of the present utility model;

Fig. 3 is a soil moisture sensor equivalent circuit diagram of the present utility model.

Embodiment

Below in conjunction with embodiment the utility model content is elaborated.

Schematic block circuit diagram of the present utility model comprises high-frequency signal source 1, transmission line circuit of equivalent network 2, two detecting circuits 3, differential amplifier circuit 4, sensor probe 5 as shown in fig. 1.Wherein high-frequency signal source 1 produces the high frequency sinusoidal signal of 100MHZ, this signal is leaded up to transmission line circuit of equivalent network 2 and is transferred to sensor probe 5, described sensor probe 5 is by four long 60mm, the stainless steel probe of diameter 3mm is formed, wherein one is the center probe, be connected an end of transmission line circuit of equivalent network 2, other three is to be the periphery probe that equilateral triangle is arranged, be connected the earth terminal of transmission line circuit of equivalent network 2, the high frequency sinusoidal signal is launched in soil by the center probe, generate an electromagnetic field in the soil media between center probe and peripheral probe, when sensor probe 5 inserts in the soil, sine wave signal is propagated in the soil media around it by probe, because transmission line circuit of equivalent network and probe impedance do not match, some electromagnetic wave reflects along the transmission line circuit of equivalent network at probe and transmission line circuit of equivalent network junction, incident wave and reflection wave stack forming standing wave on the circuit of equivalent network of transmission line, the utility model come measured soil moisture by measuring standing wave ratio on the transmission line circuit of equivalent network just.Can adopt a fairly large number of probe in actual applications, for example bury 3-4 probe underground at same soil profile and use simultaneously, consistance of the present utility model and exchange performance satisfy the needs of production application.

Referring to Fig. 1, incident wave during standing wave and reflection wave produce direct current signal through the detecting circuit 3 (detecting circuit adopts external state-of-the-art detuner integrated circuit (IC) chip) that is connected transmission line circuit of equivalent network 2 two ends respectively, input to differential amplifier circuit 4, after the zeroing and amplification through described differential amplifier circuit 4, produce the differential DC voltages signal output of 0-2.5V, the difference direct current signal of the utility model utilization output has been set up the cubic polynomial relation between soil volumetric(al) moisture content and the sensor output voltage through the demarcation of soil moisture sensor in soil.

Fig. 2 is the transmission line circuit of equivalent network.An available inductance capacitance network equivalent on the transmission line circuit of equivalent network principle.L, C are the equivalent transmission line parameter, C ₁Be the input capacitance of transmission line, C ₂Be transmission line load.The selection of component values is on working frequency points, LC series resonance, LC ₁Parallel resonance.Can determine the L value according to this principle, the value of C is determined by test according to the external structure of type of sensor and probe.

Transmission line circuit of equivalent network 2 can equivalence be one section uniform coaxial transmission line.Any one section uniform coaxial transmission line is divided into many differential section dz, for uniform transmission line, because its distribution parameter is equally distributed along the line, and because the length of line element dz is extremely short, so can will regard a lumped circuit as, and next equivalent with a Γ type network, its equivalent electrical circuit is as shown in Figure 3

When the length of described equivalent coaxial transmission line equals four of wavelength X/for the moment, the crest of standing wave and trough are just at the two ends of equivalent coaxial transmission line, equivalence coaxial transmission line length determines that by test must assurance produce standing wave thereon, the potential difference (PD) at two ends is:

${\hat{U}}_j-{\hat{U}}_0=2\mathrm{A\ρ}=2A\frac{Z_L-Z_C}{Z_L+Z_C}$

In the following formula, Z _CBe the characteristic impedance of transmission line, Z _LThe characteristic impedance of soil moisture sensor probe, the numerical value of A depends on the amplitude of high frequency oscillator, is steady state value.So the potential difference (PD) at transmission two ends is proportional to reflection coefficient ρ under the constant situation of A, and the available standing wave ratio of ρ is expressed as in transmission line theory:

$\mathrm{\Γ}=\frac{1-\mathrm{\ρ}}{1+\mathrm{\ρ}}$

So the soil moisture measurement method that the utility model will be realized just can realize based on the measuring method of standing wave ratio principle.

The isometric soil moisture sensor probe of four pins belongs to non-regular transmission line, and the utility model studies show that the isometric soil moisture sensor probe of four pins characteristic impedance Z _LAs shown in the formula:

$Z_L=\frac{K\·60}{\sqrt{\mathrm{\ϵ}}}\ln \frac{R}{r}$

Because the probe of sensor is concentric four needle constructions, satisfy inequality by its characteristic of geometric configuration analysis:

Z _S＞Z _L＞Z _C

Z _CBe the characteristic impedance of concentric cable, $Z_C=\frac{60}{\sqrt{\mathrm{\ϵ}}}\ln \frac{R}{r},$

Z _SBe the characteristic impedance of biaxial cable, $Z_S=\frac{120}{\sqrt{\mathrm{\ϵ}}}\ln \frac{R}{r}$

Wherein R is the outer conductor radius, and r is the inner wire radius, and ε is the specific inductive capacity of megohmite insulant in the cable.

Force relation principle according to folder, the span that can determine K is 2＞K＞1.

If the specific inductive capacity in the concentric cable is ε _{γ 0}, the specific inductive capacity of medium is ε in the probe, promptly obtains:

$\mathrm{\Δ}\hat{U}={\hat{U}}_j-{\hat{U}}_0=2A\frac{\sqrt{{\mathrm{\ϵ}}_{\mathrm{\γ}0}}-K\sqrt{\mathrm{\ϵ}}}{\sqrt{{\mathrm{\ϵ}}_{\mathrm{\γ}0}}+K\sqrt{\mathrm{\ϵ}}}$

Because of selected probe is solid stainless steel probe material, ε _{γ 0}=1, then

$\mathrm{\Δ}\hat{U}=2A\frac{1-K\sqrt{\mathrm{\ϵ}}}{1+K\sqrt{\mathrm{\ϵ}}}$

Wherein ε is the relative dielectric constant of soil, water and air potpourri.

Therefore pass through the voltage difference at sense transmission line circuit of equivalent network two ends

Can measure the DIELECTRIC CONSTANT of soil, determine volumetric water content of soil according to the funtcional relationship of volumetric water content of soil and soil dielectric constant more at last.

More than soil moisture measurement sensor provided by the utility model is described in detail, used specific case herein principle of the present utility model and embodiment are set forth, the explanation of above embodiment just is used for helping to understand method of the present utility model and core concept thereof; Simultaneously, for one of ordinary skill in the art, according to thought of the present utility model, the part that all can change in specific embodiments and applications, in sum, this description should not be construed as restriction of the present utility model.

Claims (4)

1. soil moisture measurement sensor, it is characterized in that: comprise high-frequency signal source, transmission line circuit of equivalent network, detecting circuit, amplifying circuit, sensor probe, one end of described transmission line circuit of equivalent network connects high-frequency signal source, the other end connects described sensor probe, described two detecting circuits are connected to the two ends of transmission line circuit of equivalent network, and described amplifying circuit links to each other with the output terminal of described detecting circuit.

2. soil moisture measurement sensor according to claim 1, it is characterized in that: described transmission line circuit of equivalent network comprises first capacitor, second capacitor and the 3rd capacitor in parallel in turn, the described first capacitor input end is electrically connected with described high-frequency signal source output terminal, the output terminal of described the 3rd capacitor is electrically connected with the end of sensor probe, wherein is connected with the earth with the second capacitor tie point at first capacitor.

3. soil moisture measurement sensor according to claim 1 is characterized in that: described amplifying circuit is a differential amplifier circuit, and the two paths of signals input end of described differential amplifier circuit connects described detecting circuit output terminal respectively.

4. soil moisture measurement sensor according to claim 1, it is characterized in that: described sensor probe is made up of four isometric stainless steel probes, wherein one is the center probe, be connected an end of transmission line circuit of equivalent network, its excess-three root is to pop one's head in the periphery of described center probe equidistant placement, be connected the earth terminal of transmission line circuit of equivalent network, described detecting circuit is connected to the two ends of transmission line circuit of equivalent network.

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