CN104777196A - Device for real-time measurement of fluid conductivity by use of electromagnetic method - Google Patents

Abstract

The invention belongs to the technical field of electromagnetic induction, in particular relates to a device for real-time measurement of fluid conductivity by use of electromagnetic method, and aims at the conditions of low fluid conductivity, and little effect on the amplitude of electromagnetic mutual inductance signals; the device uses a method of excitation frequency adjusting and phase component extracting to replace the amplitude for extraction of useful information to the maximum in order to obtain the most accurate measurement results; aiming at the condition that the measurement of fluid conductivity by use of the electromagnetic method is susceptible to the interference of the surrounding electromagnetic environment, the device uses a method of real-time calibration and probe shielding to prevent measurement result drift caused by electromagnetic interference; and aiming at the problem that the high frequency signal transmission is susceptible to noise interference when the fluid conductivity is low and the excitation signal frequency is high, the device uses a method of coaxial cable shielding for signal transmission to ensure the signal stability and reliability.

Description

A kind of device utilizing electromagnetic method to measure fluid conductivity in real time

Technical field

The invention belongs to electromagnetic induction technical field, be specifically related to a kind of device utilizing electromagnetic method to measure fluid conductivity in real time.

Background technology

Conductivity is one of key property of fluid, is widely used in the middle of water quality monitoring, medical and health, scientific research and industrial processes.The metal material higher with conductivity is compared, and due to the impact by factors such as self CONCENTRATION DISTRIBUTION, unstable form, temperature, accurate, the Quick Measurement that realize fluid conductivity exist certain difficulty.The method the most generally used is at present electrode method and electromagnetic method.

Electrode method is also called electrochemical process, the conductivity measurement instrument major part of current use is based on this kind of principle, and electrode method also also exists some singularity and limitation in actual measurement, the selection of such as excitation power supply, direct supply can make surrounding them produce polarization phenomena, and the stray capacitance of electrode or contact conductor can produce capacitance current when using AC power, in addition, electrode is owned by France in contact type measurement, can damage detected fluid to a certain extent, and there is the problems such as the long-term and liquid comes into contact of electrode can be corroded, these all can have a strong impact on accuracy and the practical of measurement result.Another comparatively novel method is inductive electromagnetic method, the advantage that this method is maximum be non-contact measurement, overcome some main shortcomings of contact type measurement (conductance method), have measurement range wide, measure accurately fast, structure is simple, the features such as cost is lower, therefore investigation and application is widely obtained, such as to the flaw detection of metal material, film thickness measuring, the parameter detecting of steel manufacture process and the liquid level of fluid and conductivity measurement etc., but research further of still needing in the measurement of low-conductivity fluid, simultaneously some researchs in the past mostly utilize the amplitude of induced signal to realize the measurement of conductance, because electrolyte flow bulk conductivity is general all lower, the decay of the magnetic line of force after fluid in amplitude is very little, be mainly manifested in the change in mutual inductance signal phase place.Make thus to measure accurate deficiency, measurement range is also large not.

Summary of the invention

(1) technical matters that will solve

The technical problem to be solved in the present invention is: how to provide a kind of device utilizing the undesirable mutual electromagnetic inductance information between coaxial coil to measure fluid conductivity fast and accurately.

(2) technical scheme

For solving the problems of the technologies described above, the invention provides a kind of device utilizing electromagnetic method to measure fluid conductivity in real time, this device comprises: sensor, electromagnetic detection module, host computer;

Described sensor is made up of probe and front-end circuit, and wherein, probe is made up of jointly coil and inserting tube, is specially a pair associate coil and is wrapped in inserting tube surface with the parallel mode of a determining deviation, surrounding parcel shielding material; In described a pair associate coil one as drive coil, another is as receiving coil; Two coils of dead in line centered by described a pair coaxial coil, are entwined by the coat of paint copper wire of surface insulation, and the radius of coil is certain, and the axial distance between coil is certain; The material of described inserting tube is insulating material, and inserting tube diameter dimension is grade;

Being inserted by probe during measurement treats in fluid measured, keeps the coaxial coil on probe to be all in below fluid level to be measured in measuring process; This probe has screen layer, and material surface is megohmite insulant;

Described front-end circuit is for realizing the preprocessing function of the shutoff of pumping signal, power amplification, mutual inductance signal;

After the process of electromagnetic detection module digitalized current, host computer is transferred to by USB by the mutual inductance signal of sensor senses;

Described electromagnetic detection module is for generation of encouraging AC signal, carrying out analog to digital conversion, performing phase demodulation and carry out instruction interaction with host computer;

Described host computer is used for arranging excitation frequency, on-line proving, in real time display measurement result;

Specifically, in the course of work, in drive coil, inject AC signal, the electromagnetic eddy electric field equation of constraint that surrounding produces is:

$\▿\×\▿\×E-{k_b}^{2}E=\mathrm{j\ω\μJ}+[k^2\left(r\right)-{k_b}^2]E---\left(1\right)$

Wherein, operator represent the curl of field amount, E is electric field, and J is exciting current, and k (r) is the wave number in air, k _bfor the wave number in conductive fluid, ω is exciting signal frequency, and μ is magnetic permeability;

Be similar to by Berne, namely suppose frequency and object conductivity all very low, then think that scattered field is very faint, when ignore displacement current and even fluid distribution, above formula (1) through derive obtain scattered field formula:

E _s(r，r′)＝-jωσμ ₁∫dr″G(r，r″)E ₀(r″，r′) (2)

Wherein, ∫ dr " G (r, r ") E ₀(r ", r ') regards a scale-up factor relevant with coil line size as, and σ is dielectric conductance rate, reflects the electric conductivity of material; And the intensity of scattered field is directly proportional to the induced voltage of receiving coil, testee is mainly manifested in imaginary part to the impact that induced signal produces as can be seen from the above equation; Because its magnetic permeability of fluid change of different conductivity is little, therefore the change of its phase place is proportional with the conductivity of detected fluid, its scale-up factor α, i.e. phase angle change value/fluid conductivity, both can by formulae discovery out, also can be obtained by demarcation in actual measurement, scale-up factor α is the function of coil radius R, coaxial coil space D, exciting signal frequency ω, α=f (R, D, ω) can be expressed as;

In addition, described electromagnetic detection is realized by FPGA, and it comprises: DDS module, MCU, USB interface; Described former-section circuit comprises: power amplifier, signal preprocessor; Detailed process is: first by DDS module, i.e. Direct Digital Frequency Synthesizers, produce the interchange reference signal for encouraging, carry out injecting drive coil after power amplification through power amplifier, then the induced signal that receiving coil collects generates discrete sample signals through the laggard digitized sampling of signal preprocessor, under MCU controls, utilize phase demodulation method to calculate the phase value of mutual inductance signal according to discrete sample signals, be then transferred to host computer by USB;

So-called phase demodulation method is the one of numerous demodulation method, can extract being polluted by white Gaussian noise the useful signal even flooded, purposes widely, also referred to as phase lock-in detecting, lock-in amplify, coherent detection and correlation demodulation method;

Discrete sample signals M obtained above [n] is expressed as herein:

$M\left[n\right]=A\cos (\frac{2\mathrm{\π}{f}_{m}n}{f_s}+\mathrm{\φ})0\≤n\≤N_s-1,$ Wherein, A is amplitude, and φ is phase place, f _mfor signal frequency, f _sfor sample frequency, N _sfor sampling number;

Whole phase demodulation method is in two steps:

The first step, is multiplied discrete sample signals M [n] with the Digital AC reference signal produced by DDS, obtains:

$I\left[n\right]=M\left[n\right]\×\cos \left(\frac{2\mathrm{\π}{f}_{m}n}{f_s}\right)=\frac{1}{2}A\cos \left(\mathrm{\φ}\right)+\frac{1}{2}A\cos (\frac{4\mathrm{\π}{f}_{m}n}{f_s}+\mathrm{\φ})0\≤n\≤N_s-1$

$Q\left[n\right]=M\left[n\right]\×\sin \left(\frac{2\mathrm{\π}{f}_{m}n}{f_s}\right)=\frac{1}{2}A\sin \left(\mathrm{\φ}\right)+\frac{1}{2}A\sin (\frac{4\mathrm{\π}{f}_{m}n}{f_s}+\mathrm{\φ})0\≤n\≤N_s-1$

Wherein, I [n] is mutual inductance signal real component, and Q [n] is imaginary;

Second step, by signal obtained in the previous step by a low-pass filter, by harmonic component filtering, just obtains the real imaginary part information representing demodulated signal, cuts calculate phase place further by negating;

Described electromagnetic detection is also set to receive the switching that excitation frequency is carried out in host computer instruction, adjustment conductivity measurement scope;

Described host computer carries out data interaction by USB and electromagnetic detection, obtains phase information rear weight scale-up factor and just can obtain conductivity value and display in real time; Operating personnel can also arrange correlation parameter by UI interface, as excitation frequency and calibrated reference, ensure measuring accuracy to greatest extent; Calibrated reference comprises the known electrolyte solution of air, pure water or conductivity;

In implementation and operation, whole measuring process is as follows: first in host computer, arrange excitation frequency, selects calibrated reference, probe is inserted in calibrated reference, require that coaxial coil keeps static under being all in liquid level, gather calibration result, according to calibration result real-time update scale-up factor a; Then probe is inserted in detected fluid, obtains measurement result.

(3) beneficial effect

The invention provides a kind of fluid conductivity measurement scheme based on electromagnetic induction principle, particularly for the conductivity measuring apparatus of low conductivity fluid.

Lower for fluid conductivity, affect little situation to undesirable mutual electromagnetic inductance signal amplitude, described device passes through the method that excitation frequency is adjustable, extract phase component replacement amplitude, extracts useful information to greatest extent, to obtaining measurement result the most accurately;

Measure for electromagnetic method the situation that fluid conductivity is subject to the interference of periphery electromagnetic environment, the method that described device is shielded by real-time calibration and probe, the measurement result preventing electromagnetic interference (EMI) from bringing is drifted about;

For lower at fluid conductivity, when exciting signal frequency is higher, high-frequency signal transmission is easily subject to the problem of noise, and described device carries out Signal transmissions by adopting the method for coaxial cable shield, ensures stability and the reliability of signal;

The present invention can realize quick, Measurement accuracy to detected fluid conductivity, probe design had both met measurement requirement, the problems such as the sensor that traditional electrode method can be avoided again to run into is corroded, by taking the measures such as the switching of real-time calibration, excitation frequency, signal shielding, make whole measuring process convenient, flexible, reliable, have obvious effect and advantage compared with traditional measurement scheme.

Accompanying drawing explanation

Fig. 1 is sensor schematic diagram.

Fig. 2 (a) to Fig. 2 (e) is emulation schematic diagram.

Fig. 3 is the functional block diagram of technical solution of the present invention device.

Fig. 4 is the interface schematic diagram of host computer embedded software.

Embodiment

For making object of the present invention, content and advantage clearly, below in conjunction with drawings and Examples, the specific embodiment of the present invention is described in further detail.

For solving the problem of prior art, the invention provides a kind of device utilizing electromagnetic method to measure fluid conductivity in real time, this device comprises: sensor, electromagnetic detection module, host computer;

As shown in Figure 1, described sensor is made up of probe and front-end circuit, and wherein, probe is made up of jointly coil and inserting tube, is specially a pair associate coil and is wrapped in inserting tube surface with the parallel mode of a determining deviation, surrounding parcel shielding material; In described a pair associate coil one as drive coil, another is as receiving coil; Two coils of dead in line centered by described a pair coaxial coil, are entwined by the coat of paint copper wire of surface insulation, and the radius of coil is certain, and the axial distance between coil is certain; The material of described inserting tube is insulating material, generally selects organic glass, and inserting tube diameter is unsuitable excessive, is generally grade;

Being inserted by probe during measurement treats in fluid measured, needs to keep the coaxial coil on probe to be all in below fluid level to be measured in measuring process; This probe has screen layer, and material surface is megohmite insulant, both can meet measurement demand, the direct contact between survey sensor and tested electrolyte can be avoided again, the sensor etching problem preventing Long contact time from causing, increases the service life, and ensures measuring accuracy;

In order to increase measurement range, the frequency domain of pumping signal is wider, because broadband signal is easily subject to noise in transmitting procedure, when the transmission of middle and long distance, relaxation phenomenon is also serious than low frequency signal, the present invention devises front-end circuit and solves this problem, and described front-end circuit is for realizing the function such as pre-service of the shutoff of pumping signal, power amplification, mutual inductance signal; Front-end circuit forms sensor together with probe.

After the process of electromagnetic detection module digitalized current, host computer is transferred to by USB by the mutual inductance signal of sensor senses;

Described electromagnetic detection module is for generation of encouraging AC signal, carrying out analog to digital conversion, performing phase demodulation and carry out instruction interaction with host computer;

Described host computer is used for arranging excitation frequency, on-line proving, in real time display measurement result;

Lower for fluid conductivity, affect little situation to undesirable mutual electromagnetic inductance signal amplitude, described device passes through the method that excitation frequency is adjustable, extract phase component replacement amplitude, extracts useful information to greatest extent, to obtaining measurement result the most accurately;

Measure for electromagnetic method the situation that fluid conductivity is subject to the interference of periphery electromagnetic environment, the method that described device is shielded by real-time calibration and probe, the measurement result preventing electromagnetic interference (EMI) from bringing is drifted about;

For lower at fluid conductivity, when exciting signal frequency is higher, high-frequency signal transmission is easily subject to the problem of noise, and described device carries out Signal transmissions by adopting the method for coaxial cable shield, ensures stability and the reliability of signal;

Specifically, in the course of work, in drive coil, inject AC signal, the electromagnetic eddy electric field equation of constraint that surrounding produces is:

$\▿\×\▿\×E-{k_b}^{2}E=\mathrm{j\ω\μJ}+[k^2\left(r\right)-{k_b}^2]E---\left(1\right)$

Wherein, operator represent the curl of field amount, E is electric field, and J is exciting current, and k (r) is the wave number in air, k _bfor the wave number in conductive fluid, ω is exciting signal frequency, and μ is magnetic permeability;

Be similar to by Berne, namely suppose frequency and object conductivity all very low, then can think that scattered field is very faint, when ignore displacement current and even fluid distribution, above formula (1) through derive obtain scattered field formula:

E _s(r，r′)＝-jωσμ ₁∫dr″G(r，r″)E ₀(r″，r′) (2)

Wherein, ∫ dr " G (r, r ") E ₀(r ", r ') the relevant scale-up factor such as and coil line size can be regarded as, σ is dielectric conductance rate, reflects the electric conductivity of material; And the intensity of scattered field is directly proportional to the induced voltage of receiving coil, testee is mainly manifested in imaginary part to the impact that induced signal produces as can be seen from the above equation; Because its magnetic permeability of fluid change of different conductivity is little, therefore the change of its phase place is proportional with the conductivity of detected fluid, in order to verify this proportionate relationship, we utilize Maxwell Ansoft V11.1 software creation three-dimensional model and carry out finite element simulation, as shown in Fig. 2 (a).Two coil parallel nested are outside pipeline, and pipe radius is 10mm, is full of fluid media (medium) in pipeline.One of them is as drive coil, adopts ac-excited source.Affect the factor of mutual inductance signal, as coil-span, excitation frequency and detected fluid conductivity etc. are set individually.Fig. 2 (b)-Fig. 2 (e) gives coil-span when being respectively 20mm, 40mm, 60mm and 80mm, under 3MHz, 6MHz and 12MHz three kinds of excitation frequencies, the relation of mutual inductance signal phase change value between detected fluid conductivity and coil.As can be seen from Fig. 2 (b)-Fig. 2 (e), there is obvious approximate ratio relation in both, its scale-up factor α, i.e. phase angle change value/fluid conductivity, both can by formulae discovery out, also can be obtained by demarcation in actual measurement, scale-up factor α is the function of coil radius R, coaxial coil space D, exciting signal frequency ω, α=f (R, D, ω) can be expressed as;

In addition, as shown in Figure 3, for the present invention utilizes electromagnetic method to measure the functional block diagram of the device of fluid conductivity in real time, described electromagnetic detection is realized by FPGA, and it comprises: DDS module, MCU, USB interface; Described former-section circuit comprises: power amplifier, signal preprocessor; Detailed process is: first by DDS module, i.e. Direct Digital Frequency Synthesizers, produce the interchange reference signal for encouraging, carry out injecting drive coil after power amplification through power amplifier, then the induced signal that receiving coil collects generates discrete sample signals through the laggard digitized sampling of signal preprocessor, under MCU controls, utilize phase demodulation method to calculate the phase value of mutual inductance signal according to discrete sample signals, be then transferred to host computer by USB;

So-called phase demodulation method is the one of numerous demodulation method, can extract being polluted by white Gaussian noise the useful signal even flooded, purposes widely, also referred to as phase lock-in detecting, lock-in amplify, coherent detection and correlation demodulation method etc.;

Discrete sample signals M obtained above [n] is expressed as herein:

$M\left[n\right]=A\cos (\frac{2\mathrm{\π}{f}_{m}n}{f_s}+\mathrm{\φ})0\≤n\≤N_s-1,$ Wherein, A is amplitude, and φ is phase place, f _mfor signal frequency, f _sfor sample frequency, N _sfor sampling number;

Whole phase demodulation method is in two steps:

It is multiplied with the Digital AC reference signal produced by DDS by the first step, obtains:

$I\left[n\right]=M\left[n\right]\×\cos \left(\frac{2\mathrm{\π}{f}_{m}n}{f_s}\right)=\frac{1}{2}A\cos \left(\mathrm{\φ}\right)+\frac{1}{2}A\cos (\frac{4\mathrm{\π}{f}_{m}n}{f_s}+\mathrm{\φ})0\≤n\≤N_s-1$

$Q\left[n\right]=M\left[n\right]\×\sin \left(\frac{2\mathrm{\π}{f}_{m}n}{f_s}\right)=\frac{1}{2}A\sin \left(\mathrm{\φ}\right)+\frac{1}{2}A\sin (\frac{4\mathrm{\π}{f}_{m}n}{f_s}+\mathrm{\φ})0\≤n\≤N_s-1$

Wherein, I [n] is mutual inductance signal real component, and Q [n] is imaginary;

Signal obtained in the previous step by a low-pass filter, by harmonic component filtering, is just obtained the real imaginary part information representing demodulated signal, cuts calculate phase place further by negating by second step;

Described electromagnetic detection is also set to receive the switching (f that excitation frequency is carried out in host computer instruction _m), adjustment conductivity measurement scope automatically;

Fig. 4 is host computer embedded software operation interface schematic diagram; Host computer carries out data interaction by USB and electromagnetic detection, obtains phase information rear weight scale-up factor and just can obtain conductivity value and display in real time; Operating personnel can also arrange correlation parameter by UI interface, as excitation frequency (the current excitation frequency upper limit can reach 10MHz) and calibrated reference, ensure measuring accuracy to greatest extent; Calibrated reference comprises the known electrolyte solution of air, pure water or conductivity;

Whole measuring process is as follows, first in host computer, arranges excitation frequency, selects calibrated reference, probe is inserted in calibrated reference, require that coaxial coil keeps static under being all in liquid level, click and demarcate button collection calibration result, according to calibration result real-time update scale-up factor a; Then probe is inserted in detected fluid, clicks and measure button, obtain measurement result.For ensureing measurement effect, the preheating time of about 5 minutes after device power, should be had.

The above is only the preferred embodiment of the present invention; it should be pointed out that for those skilled in the art, under the prerequisite not departing from the technology of the present invention principle; can also make some improvement and distortion, these improve and distortion also should be considered as protection scope of the present invention.

Claims (1)

1. utilize electromagnetic method to measure a device for fluid conductivity in real time, it is characterized in that, this device comprises: sensor, electromagnetic detection module, host computer;

Described sensor is made up of probe and front-end circuit, and wherein, probe is made up of jointly coil and inserting tube, is specially a pair associate coil and is wrapped in inserting tube surface with the parallel mode of a determining deviation, surrounding parcel shielding material; In described a pair associate coil one as drive coil, another is as receiving coil; Two coils of dead in line centered by described a pair coaxial coil, are entwined by the coat of paint copper wire of surface insulation, and the radius of coil is certain, and the axial distance between coil is certain; The material of described inserting tube is insulating material, and inserting tube diameter dimension is grade;

Being inserted by probe during measurement treats in fluid measured, keeps the coaxial coil on probe to be all in below fluid level to be measured in measuring process; This probe has screen layer, and material surface is megohmite insulant;

Described front-end circuit is for realizing the preprocessing function of the shutoff of pumping signal, power amplification, mutual inductance signal;

After the process of electromagnetic detection module digitalized current, host computer is transferred to by USB by the mutual inductance signal of sensor senses;

Described electromagnetic detection module is for generation of encouraging AC signal, carrying out analog to digital conversion, performing phase demodulation and carry out instruction interaction with host computer;

Described host computer is used for arranging excitation frequency, on-line proving, in real time display measurement result;

Specifically, in the course of work, in drive coil, inject AC signal, the electromagnetic eddy electric field equation of constraint that surrounding produces is:

Wherein, operator × representing the curl that field is measured, E is electric field, and J is exciting current, and k (r) is the wave number in air, k _bfor the wave number in conductive fluid, ω is exciting signal frequency, and μ is magnetic permeability;

Be similar to by Berne, namely suppose frequency and object conductivity all very low, then think that scattered field is very faint, when ignore displacement current and even fluid distribution, above formula (1) through derive obtain scattered field formula:

E _s(r，r′)＝-jωσμ ₁∫dr″G(r，r″)E ₀(r″，r′) (2)

Wherein, ∫ dr " G (r, r ") E ₀(r ", r ') regards a scale-up factor relevant with coil line size as, and σ is dielectric conductance rate, reflects the electric conductivity of material; And the intensity of scattered field is directly proportional to the induced voltage of receiving coil, testee is mainly manifested in imaginary part to the impact that induced signal produces as can be seen from the above equation; Because its magnetic permeability of fluid change of different conductivity is little, therefore the change of its phase place is proportional with the conductivity of detected fluid, its scale-up factor α, i.e. phase angle change value/fluid conductivity, both can by formulae discovery out, also can be obtained by demarcation in actual measurement, scale-up factor α is the function of coil radius R, coaxial coil space D, exciting signal frequency ω, α=f (R, D, ω) can be expressed as;

In addition, described electromagnetic detection is realized by FPGA, and it comprises: DDS module, MCU, USB interface; Described former-section circuit comprises: power amplifier, signal preprocessor; Detailed process is: first by DDS module, i.e. Direct Digital Frequency Synthesizers, produce the interchange reference signal for encouraging, carry out injecting drive coil after power amplification through power amplifier, then the induced signal that receiving coil collects generates discrete sample signals through the laggard digitized sampling of signal preprocessor, under MCU controls, utilize phase demodulation method to calculate the phase value of mutual inductance signal according to discrete sample signals, be then transferred to host computer by USB;

So-called phase demodulation method is the one of numerous demodulation method, can extract being polluted by white Gaussian noise the useful signal even flooded, purposes widely, also referred to as phase lock-in detecting, lock-in amplify, coherent detection and correlation demodulation method;

Discrete sample signals M obtained above [n] is expressed as herein:

wherein, A is amplitude, and φ is phase place, f _mfor signal frequency, f _sfor sample frequency, N _sfor sampling number;

Whole phase demodulation method is in two steps:

The first step, is multiplied discrete sample signals M [n] with the Digital AC reference signal produced by DDS, obtains:

Wherein, I [n] is mutual inductance signal real component, and Q [n] is imaginary;

Second step, by signal obtained in the previous step by a low-pass filter, by harmonic component filtering, just obtains the real imaginary part information representing demodulated signal, cuts calculate phase place further by negating;

Described electromagnetic detection is also set to receive the switching that excitation frequency is carried out in host computer instruction, adjustment conductivity measurement scope;

Described host computer carries out data interaction by USB and electromagnetic detection, obtains phase information rear weight scale-up factor and just can obtain conductivity value and display in real time; Operating personnel can also arrange correlation parameter by UI interface, as excitation frequency and calibrated reference, ensure measuring accuracy to greatest extent; Calibrated reference comprises the known electrolyte solution of air, pure water or conductivity;

In implementation and operation, whole measuring process is as follows: first in host computer, arrange excitation frequency, selects calibrated reference, probe is inserted in calibrated reference, require that coaxial coil keeps static under being all in liquid level, gather calibration result, according to calibration result real-time update scale-up factor a; Then probe is inserted in detected fluid, obtains measurement result.