CN202886315U - Culture medium moisture capacitive sensor - Google Patents
Culture medium moisture capacitive sensor Download PDFInfo
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- CN202886315U CN202886315U CN 201220521545 CN201220521545U CN202886315U CN 202886315 U CN202886315 U CN 202886315U CN 201220521545 CN201220521545 CN 201220521545 CN 201220521545 U CN201220521545 U CN 201220521545U CN 202886315 U CN202886315 U CN 202886315U
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Abstract
The utility model discloses a culture medium moisture capacitive sensor. The culture medium moisture capacitive sensor comprises a strip capacitive probe for manufacturing a sensitive culture medium moisture PCB (Printed Circuit Board), a C/V (Current-Voltage) converting circuit main board for measuring the capacitance value of the probe, a sealed rubber shell, and an input-output interface cable. The capacitive probe consists of two strip printed circuit boards with the same length, and form an integrated structure with the C/V converting circuit main board. The C/V converting circuit main board consists of a power supply control module, a C/V converting module and a differential voltage converting module. The C/V converting module is further provided with one path of temperature output signals. The utility model provides the capacitive sensor for measuring the moisture of a culture medium, and the capacitive sensor has the advantages of simple structure, convenience in machining, small size, low cost and high precision, can be used for effectively reducing the influence of electric conductance, is conveniently inserted in the culture medium and has one path of the temperature output signals.
Description
Technical field
The utility model relates to a kind of cultivation matrix moisture capacitive transducer, relates in particular to a kind of based on measuring the moisture capacitive transducer that inserts probe capacitance parameter size in the cultivation matrix under the fixed frequency.
Background technology
China is the large agricultural country of a lack of water, and efficient water is subject to people and more and more pays close attention to.Rapid popularization along with China's soilless substyate culture technology, carrying out accurate irrigation control in cultivation matrix also comes into one's own gradually, the accurate measurement of the moisture of cultivation matrix then is the prerequisite that realizes moisture precision irrigation and control, and the low cost, low-power consumption, the high-precision sensor that is applicable to the cultivation matrix moisture measurement that therefore design a kind of suitable China's national situation have realistic meaning.
The scientific worker generally adopts the commercialization instrument of existing measurement soil moisture or sensor to carry out to the measurement of cultivation matrix moisture at present.According to measuring principle, can realize that the soil moisture method for real-time measurement mainly contains: TDR (time domain reflectometry), FDR(frequency domain reflectometry), SWR (standing wave ratio method) and capacitance method.But there is weak point separately in they:
The TDR method that measuring accuracy is higher is measured development by the liquid dielectric property and is formed, and it is by the decipher of high speed electromagnetic burst transmissions reflected signal being measured the dielectric property of soil.Because this technology relates to the high-speed electrical signals analysis, technology is very complicated, makes the instrument cost of this technology of employing also very high, and about 100,000 yuans, these have all greatly limited applying of the method.The FDR method is to measure to insert the specific inductive capacity that watered ground is determined in the admittance of watered ground probe under the certain frequency, and then estimation soil moisture.Belong to strictly speaking a kind of of capacitance-type sensor for moisture content.This measuring method precision is relatively low, and cost is higher, and about 10,000 yuans, performance can satisfy most application demand.The soil moisture measurement method of SWR principle also belongs to the measurement of Soil Moisture Dielectric constant, this measuring method that different from the TDR method is does not recycle the high speed lag line, and to measure incident-reflection interval poor, but measure its standing-wave ratio (SWR), test shows that the change of ternary mixture dielectric constant can cause the marked change of standing-wave ratio (SWR) on the transmission line.The sensor that utilizes the standing-wave ratio (SWR) principle to develop can drop to hundreds of yuan on cost, but agricultural extension is used, and particularly builds distributed moisture multimetering network system, and cost is still higher.In addition, all be to adopt stainless steel pin type structure based on the soil moisture sensor of TDR, FDR, SWR principle, it is large to measure time error in the soil of high conductance.
Capacitance method is to measure the equivalent capacity of inserting the electrode in the soil to measure soil moisture, and the relatively simple advantage that possesses skills is subject to very early people and extensively notes.Disclosing a kind of capacitance probe such as China Patent No. CN1504745A is to scribble one deck by the surface to consist of through the two metal sheets with insulation and non-absorbency ceramic glaze film of high temperature sintering.Patent CN101694475B and CN201034964 all disclose similarly a kind of with the two copper ring electrode sleeves that are provided with insulating isolation ring on the hollow insulation rod and will can be used for the condenser type moisture-sensitive device that moisture of soil profile is measured in its pvc pipe of packing into.The probe of above-mentioned 3 patents exists to make trouble or be not suitable for and inserts cultivation matrix and the deficiency such as good contact with it.Moreover the circuit design principle of these 3 patents all is to realize measuring capacitance size by the equivalent capacity take soil as medium as the part of resonant circuit.Because under different frequency, the cultivation matrix of certain moisture shows the equivalent capacity of different sizes, therefore adopts Dissipative Resistance by Harmonic Resonance Method cultivation matrix moisture to certainly exist systematic error.
Summary of the invention
The utility model is in order to overcome the shortcoming of existing soil moisture measurement sensor, different physicochemical property for the relative soil of cultivation matrix, provide a kind of simple in structure, easy to process, size is little, cost is low, precision is high, can effectively reduce electricity and lead impact, be convenient to insert in the cultivation matrix and measure with the output of one road temperature signal capacitance type sensor of cultivation matrix moisture.
To achieve these goals, the disclosed a kind of cultivation matrix moisture capacitive transducer of the utility model comprises capacitance probe, is used for C/V change-over circuit mainboard, caulking gum shell and the IO interface cable of measuring sonde capacitance size.Described C/V change-over circuit mainboard is located in the caulking gum shell, and is connected with the IO interface cable; Described capacitance probe and C/V change-over circuit mainboard are structure as a whole; Capacitance probe is made of two isometric rectangular printed circuit board (PCB)s.
The feature of described rectangular printed circuit board (PCB) is the dual platen that is made by the ordinary circuit board process technology, and the upper and lower surface of dual platen is covered the copper processing fully, and the copper that covers of the upper and lower bottom surface of the rectangular printed circuit board (PCB) of same passes through a via hole conducting.
For penetration depth and the signal intensity of taking into account electromagnetic field, the ratio of the width of described two isometric rectangular printed circuit board (PCB)s and the spacing between them is between 2-3.
Described rectangular printed circuit board (PCB) tip designs triangularity is convenient to insert cultivation matrix.
Described C/V change-over circuit mainboard is comprised of energy supply control module, C/V modular converter and differential voltage modular converter.
Described energy supply control module refers to adopt the voltage stabilizing chip to be used for the externally fed power supply is converted to the stabilized voltage supply of 5V, for C/V change-over circuit components and parts provide source of stable pressure.
The function of described C/V modular converter is take cultivation matrix as medium, and two rectangular double-sided printed-circuit boards are pole plate, and formed capacitor C mois parameter changes into differential voltage signal output.
Described C/V modular converter core devices is that the model that German AMG Co. produces is the chip of CAV424, and it is the C/V conversion integrated chip with signal acquisition process, differential voltage output function.
Described C/V conversion integrated chip CAV424 also carries one road 8mV/K temperature output signal.
The characteristics of described C/V conversion integrated chip CAV424 measuring junction are take cultivation matrix as medium, and two rectangular printed circuit board (PCB)s are pole plate, formed capacitor C mois one and reference capacitance electric capacity of a size in parallel.
Described its effect of differential voltage modular converter is the differential voltage of CAV424 chip to be exported be converted into single ended voltage output.
Wherein, the C/V change-over circuit mainboard of measuring sonde capacitance size scribbles successively marine glue and is sealed with rubbery outer cover, be after the electron device of whole cultivation matrix moisture transducer is all processed through waterproof and dampproof, insulation and the good circuit board waterproof glue embedding of heat conductivility, use again the rubbery outer cover insulated enclosure, carry out long-time surveying work in the cultivation matrix in order to sensor can be imbedded.
The intentionally effect of the utility model design: only used simple PCB process technology, C/V conversion CAV42 chip and some simple components and parts, do not use all complicated circuits of traditional soil capacitance-type sensor for moisture content, developed a kind of capacitive transducer of suitable cultivation matrix moisture measurement.It is low that it has a cost, stable performance, and the advantage such as IO interface is simple, and carry one road temperature output signal, being convenient to the secondary instrument development carries out temperature correction and compensation, can further improve measuring accuracy.
Description of drawings
Fig. 1 is the physical model of planar capacitance sensor.
Fig. 2 is plane capacitance Electric Field Distribution synoptic diagram.
Fig. 3 is sensor one-piece construction synoptic diagram.
Fig. 4 is the embodiment circuit theory diagrams.
Wherein, the 1st, capacitance probe, the 2nd, C/V change-over circuit mainboard, the 3rd, caulking gum shell, the 4th, IO interface cable.
Embodiment
Below in conjunction with concrete drawings and Examples technical scheme of the present invention is described in detail.
The measuring principle of present embodiment: the capacitance measurement water percentage mainly causes that to change in dielectric constant this rule of capacitance variation realizes according to observed matter.Cultivation matrix can be regarded the porous medium that air, butt matter and water form as.Under the electromagnetic frequency of 100MHz, the relative dielectric constant of water is about 80, and the relative dielectric constant of butt matter is about 2-6, and the relative dielectric constant of air is about 1.As seen, the content of the main Ying Youshui of specific inductive capacity of watered ground decides.Can reach the purpose of measuring volumetric soil water content by the relative dielectric constant of measuring soil.
Fig. 1 is the physical model of planar capacitance sensor.Wherein L is that pole plate is long, and b is plate width, and a is polar plate spacing, thinks that ideally pole plate is enough thin, and thickness is disregarded, and long enough is ignored edge effect.Fig. 2 is coplanar scattered field formula capacitive transducer Electric Field Distribution synoptic diagram.Adopt the element integral method to calculate the electric capacity of coplanar scattered field, the upper capacitance that forms of the small area increment of capacitance-sensitive element (Δ F=L Δ x), by electric field line shown in Figure 2, replace (also available approximate ellipsoidal line replaces) with the semicircular curved line, the electric capacity computing formula is
(1)
In the formula:
ε 0Be permittivity of vacuum;
ε rRelative dielectric constant for medium;
ABe polar plate area;
dBe polar plate spacing.
Approximate have
(2)
Total capacitance
(3)
Found out that by formula (3) in the certain situation of L, sensitivity depends mainly on the ratio b/a in plate width and gap, ratio is larger, and detection sensitivity is higher, but also will take into account investigation depth.Learnt that by theoretical analysis the field penetration degree of depth between pole plate is directly proportional with the centre distance of adjacent plate.When surpassing 2/3 pole plate centre distance a+b along the pole plate normal direction degree of depth, can think that effective electric-field intensity is zero.The centre distance of capacitor two-plate is larger, and its electric field radiation scope is larger, and depth measurement is larger, but the spacing increase, its sensitivity descends, and this just need at L one regularly select suitable b/a, obtains the investigation depth that meets the demands in the higher situation of detection sensitivity.In order to design the sensor probe with desirable depth measurement and signal intensity, on the basis that above-mentioned theory is analyzed, use the Maxwell modeling and simulating, simulation result shows that b/a effect between 2-3 is best.
As shown in Figure 3, be cultivation matrix moisture capacitive transducer one-piece construction synoptic diagram of the present invention.From structural representation, the present invention includes the capacitance probe 1 for responsive cultivation matrix moisture, C/V change-over circuit mainboard 2, caulking gum shell 3 and the IO interface cable 4 of measurement capacitance probe capacitance size.
Described capacitance probe 1 is comprised of two isometric rectangular printed circuit board (PCB)s.
Described two isometric rectangular printed circuit board (PCB)s and C/V change-over circuit mainboard 2 are structure as a whole.
The feature of described two isometric rectangular printed circuit board (PCB)s is the dual platens that are made by the ordinary circuit board process technology, and the upper and lower surface of dual platen is covered the copper processing fully, and bottom surface covers copper by a via hole conducting on the rectangular printed circuit board (PCB) of same.
For penetration depth and the signal intensity of taking into account electromagnetic field, the ratio of the width of described two rectangular printed circuit board (PCB)s and the spacing between them is between 2-3.
Described rectangular printed circuit board (PCB) 1 tip designs triangularity is convenient to insert cultivation matrix.
Described C/V change-over circuit mainboard 2 is comprised of energy supply control module, C/V modular converter and differential voltage modular converter.
Described energy supply control module refers to adopt the voltage stabilizing chip to be used for the externally fed power supply is converted to the stabilized voltage supply of 5V, for C/V change-over circuit components and parts provide source of stable pressure.
The function of described C/V modular converter is take cultivation matrix as medium, and two rectangular printed circuit board (PCB)s are pole plate, and formed capacitor C mois parameter changes into differential voltage signal output.
Described C/V modular converter core devices is that the model that German AMG Co. produces is the C/V conversion integrated chip of CAV424.CAV424 has larger application flexibility, is the complete translation interface integrated circuit of the various capacitance type sensor signals of multiduty processing, and it has simultaneously, and signals collecting (relatively electric capacitance change) is processed and the function of differential voltage output.CAV424 can measure the difference of a measured capacitance and reference capacitance.It can detect 10pF to the capacitance of 2nF (being 5% to 100% variation range of reference capacitance value).Its Voltage-output level is a differential signal that can reach Vmax=± 1.4V, can more easily carry out subsequent treatment.
The principle of work of CAV424 is: one drives two by the adjustable reference oscillator of capacitor C OSC frequency and constructs symmetrical integrator and make them synchronous on time and phase place.The amplitude of two controlled integrators is to be decided by capacitor C r and Cm, and Cr electric capacity for referencial use and Cm are as measuring electric capacity here.Because integrator has very high common-mode rejection ratio and resolution, so the signal that the difference of two amplitudes provides just reflects the difference of two capacitor C r and Cm.This voltage difference is filtered into dc power signal (rectifying effect) by the active filter of back, then delivers to adjustable amplifier, adjusts the value of Rl end outer meeting resistance, can obtain needed output voltage values.If two capacitor C r are identical with the Cm value, a d. c. voltage signal that obtains through overcommutation and filtering so is exactly zero.Changed Δ Cm if measure capacitor C m, the output voltage that obtains so is directly proportional with it.If two capacitor C r are not identical with the Cm value, so when Δ Cm=0, what obtain at output terminal is a bias, and it is superimposed upon on the d. c. voltage signal all the time.
The oscillation frequency of the reference oscillator that the frequency of CAV424 is adjustable is shown below:
Wherein oscillator electric capacity value should satisfy as far as possible: Cosc=1.6Cr, Δ Vosc are output voltage peak-to-valley value poor of reference oscillator, and it is by the built-in resistor definition and a fixed voltage value 2.1V is arranged.From following formula as seen, the oscillation frequency of CAV424 is to be determined by factors such as reference capacitance, external circuitses, is a steady state value in measuring process.Therefore, can solve the problem of frequency stabilization, guarantee the constancy of cultivation matrix specific inductive capacity in the measuring process.
Described C/V conversion integrated chip measuring junction is namely take cultivation matrix as medium, and two rectangular printed circuit board (PCB)s are pole plate, and formed capacitor C mois one and reference capacitance electric capacity of a size in parallel are so that Cm equals Cmois and Cr sum.
Described differential amplification output circuit refers to have integrated transporting discharging and the peripheral circuit of differential enlarging function, through differential amplifier circuit the CAV424 differential output is converted into single ended voltage output.
Described C/V conversion integrated chip CAV424 carries the temperature sensor device, and port 7 is one road 8mV/K temperature output signal, can be used for temperature correction.
The feature of described C/V conversion integrated chip CAV424 measuring junction is as medium take cultivation matrix, article two, rectangular printed circuit board (PCB) is pole plate, formed capacitor C mois in parallel one with reference capacitance electric capacity of a size so that the differential output voltage of CAV424 port 5 and port 6 is permanent in just.
Described its effect of differential voltage modular converter is the differential voltage of the port 5 of CAV424 chip and port 6 to be exported be converted into single ended voltage output.
Wherein, the C/V change-over circuit mainboard of measuring sonde capacitance size scribbles successively marine glue and is sealed with rubbery outer cover, be after the electron device of whole cultivation matrix moisture transducer is all processed through waterproof and dampproof, insulation and the good circuit board waterproof glue embedding of heat conductivility, use again the rubbery outer cover insulated enclosure, carry out surveying work in the cultivation matrix in order to sensor can be imbedded.
Connection cable 4 is the quad that mutually shields, and is respectively power input line, ground wire, C/V switching signal output line and temperature signal output line, is used to sensor that power input interface and measuring-signal output interface are provided.
Embodiment
Adopt protel99se Software for Design cultivation matrix moisture transducer circuit theory and pcb board structure.Embodiment circuit board size as shown in Figure 3.
Wherein rectangular printed circuit board (PCB) covers effectively that copper is long to be 90mm, and spacing is 5mm, and the ratio of the width of two rectangular printed circuit board (PCB)s and the spacing between them is about 2, and rectangular printed circuit board (PCB) tip designs becomes triangle between the isosceles.According to cultivation butt matter to saturated specific inductive capacity in the 3-80 scope, the capacitance size scope between the two rectangular printed circuit board (PCB)s of can estimating is between 3-65pF.
Fig. 4 is the embodiment circuit theory diagrams, also is described C/V change-over circuit.LM7805 is energy supply control module, for C/V change-over circuit components and parts provide source of stable pressure.The CAV424 chip is the core devices of C/V modular converter, and capacitor C mois is for take cultivation matrix as medium, and two rectangular printed circuit board (PCB)s are pole plate, formed electric capacity, the namely amount that will measure of this circuit; Cr is reference capacitor; Cfix is a fixing electric capacity, and its value equates with Cr, consists of with the Cmois sum and measures capacitor Cm, can go out by circuit measuring the variation capacitance of relative reference capacitance 5%-100%; Cosc is for determining the reference oscillator electric capacity of frequency; Cl1, Cl2 are the external electric capacity with the composition wave filter of chip; Rl1, Rl2 are the resistance of regulation output differential voltage gain; Rcxl, Rcx2, Rosc are respectively the resistance of regulating chip integrator electric current; Cvm is load capacitance, to stablize the fixedly output of 2.5V voltage; The fixing 2.5V Voltage-output of 5 pin output voltage V out and 6 pin consists of output, its difference size and the linear corresponding relation of measured capacitance amount Cmois.AD623 chip and peripheral component have consisted of the differential voltage modular converter.AD623 is a integrated form single supply, rail-to-rail, low-cost instrument amplifier, has the integrated transporting discharging of differential voltage enlarging function.R1, R2 are current-limiting resistance, and R3 is gain-adjusted resistance, and it is converted into suitable single ended voltage output with the output of CAV424 difference.
It should be noted that at last: above embodiment is unrestricted real invention technical scheme in order to explanation only, although with reference to above-described embodiment the present invention is had been described in detail, those of ordinary skill in the art is to be understood that: still can make amendment or be equal to replacement the present invention, and not breaking away from any modification or partial replacement of the spirit and scope of the present invention, it all should be encompassed in the middle of the claim scope of the present invention.
Claims (6)
1. cultivation matrix moisture capacitive transducer, comprise capacitance probe (1), be used for the C/V change-over circuit mainboard (2) of measuring sonde capacitance size, it is characterized in that described C/V change-over circuit mainboard is located in the caulking gum shell (3), and be connected with IO interface cable (4); Described capacitance probe (1) is structure as a whole with C/V change-over circuit mainboard (2); Capacitance probe (1) is made of two isometric rectangular printed circuit board (PCB)s.
2. cultivation matrix moisture capacitive transducer according to claim 1 is characterized in that described two isometric rectangular printed circuit board (PCB)s are dual platens that upper and lower surface is covered copper fully, and the copper that covers two-sided about the rectangular printed circuit board (PCB) of same passes through a via hole conducting; Article two, the ratio of the width of rectangular printed circuit board (PCB) and the spacing between them is between 2-3.
3. cultivation matrix moisture capacitive transducer according to claim 1 is characterized in that described C/V change-over circuit mainboard (2) is comprised of energy supply control module, C/V modular converter and differential voltage modular converter.
4. cultivation matrix moisture capacitive transducer according to claim 3 is characterized in that described C/V modular converter carries one road temperature output signal.
5. according to claim 3 or 4 described cultivation matrix moisture capacitive transducers, it is characterized in that described C/V modular converter is that handle is take cultivation matrix as medium, article two, rectangular double-sided printed-circuit board is pole plate, and formed capacitor C mois parameter changes into differential voltage signal output.
6. cultivation matrix moisture capacitive transducer according to claim 5 is characterized in that described differential voltage modular converter is for differential voltage output is converted into single ended voltage output.
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| CN 201220521545 CN202886315U (en) | 2012-10-12 | 2012-10-12 | Culture medium moisture capacitive sensor |
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| CN 201220521545 CN202886315U (en) | 2012-10-12 | 2012-10-12 | Culture medium moisture capacitive sensor |
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| CN202886315U true CN202886315U (en) | 2013-04-17 |
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102890104A (en) * | 2012-10-12 | 2013-01-23 | 江苏大学 | Culture substrate moisture capacitance transducer |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102890104A (en) * | 2012-10-12 | 2013-01-23 | 江苏大学 | Culture substrate moisture capacitance transducer |
| CN102890104B (en) * | 2012-10-12 | 2014-10-29 | 江苏大学 | Culture substrate moisture capacitance transducer |
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Granted publication date: 20130417 Termination date: 20141012 |
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