CN204789768U - Liquid conductivity measurement system based on complementary digital electric bridge - Google Patents

Abstract

The utility model relates to a liquid conductivity measurement system based on a complementary electric bridge. This system changes the structure of the traditional balanced bridge. The four bridge arms of the balanced bridge are the resistance to be measured R _C , the standard resistance R _S and two equivalent complementary resistances R ₁ and R ₂ , and the complementary resistance R ₁ and R2 adopt N _- level R-2R resistor network structure. However, this system uses a complementary digital bridge, which reduces the influence of the accuracy of the branch resistance of the lower bridge arm on the measurement; the accuracy of the complementary bridge only depends on R _S and is not affected by R ₁ and R ₂ , which reduces the measurement accuracy. Uncertainty. The main control unit of the system adopts the complementary resistance optimization algorithm to adjust the access structure of the R-2R resistance network and the balance bridge; uses the interpolation algorithm to accurately calculate the balance point of the balance bridge. The system has high measurement accuracy and meets the measurement requirements of high-precision liquid conductivity.

Description

Liquid Conductivity Measurement System Based on Complementary Digital Bridge

technical field

The utility model belongs to the technical field of instrument measurement and relates to a liquid conductivity measurement system.

Background technique

Liquid conductivity measurement technology is a very important engineering technology. Its main principle is to measure the conductivity characteristics of the liquid, which is convenient for the analysis of the liquid components. It is mainly used for the measurement and detection of industrial production water, human domestic water, seawater characteristics, battery electrolyte, etc.

The balanced bridge method is the main method for high-precision measurement of conductivity, suitable for use in advanced laboratories. The basic idea is to use the conductivity sensor as one arm of the bridge, and the other three resistors use high-precision resistors. The high-precision resistors have better consistency and similar temperature drift characteristics to ensure high precision.

Usually the conductivity sensor is composed of a conductivity cell. Different conductivity cells have different conductivity cell constants. In order to overcome the influence of the conductivity cell constant on the measurement results, both upper bridge arms use conductivity cells, and a standard solution is placed in one of the conductivity cells. , put the solution to be tested in another conductivity cell. The system operates according to a certain process to calculate the conductivity cell constant and make compensation.

Figure 1 is a schematic diagram of the existing balanced bridge method for measuring liquid conductivity. According to the principle of the balanced bridge, the calculation formula of the measured resistance R _C when the bridge is balanced is as follows:

${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}$

i.e. conductivity

${\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}$

Among them, R _S is the resistance of the standard solution, R ₁ and R ₂ are the resistances to be measured, R _C is the resistance of the solution to be tested, ρ _C is the conductivity of the solution to be tested, and ρ _S is the conductivity of the standard solution.

When using this bridge for measurement, the following problems will affect the measurement accuracy:

(1) Since R _S is a standard solution, it can be seen from the above formula that the measurement accuracy of ρ _C is affected by the accuracy of R ₁ , R ₂ and R _S ;

(2) In order to achieve digitization, R ₁ usually uses a digital potentiometer. At present, the number of taps of the digital potentiometer is not enough, and the resolution is low, so it is difficult to meet the needs of high-precision measurement;

(3) In order to prevent the polarization of the solution from affecting the measurement of resistivity, an AC bridge is usually used, but the waveform and frequency of the AC bridge are generally fixed;

(4) Due to the non-pure resistivity of the solution, when the AC bridge is used for measurement, there is waveform distortion, which seriously affects the measurement accuracy;

(5) The excitation source, galvanometer and electric bridge are not completely isolated electrically, and circuit interference may affect the measurement accuracy.

Utility model content

The purpose of the utility model is to propose a new type of high-precision liquid conductivity measurement system based on a complementary bridge type in view of the shortcomings of the existing bridge-type conductivity measurement method.

The technical scheme of the utility model is: a liquid conductivity measurement system based on a complementary digital bridge, including an excitation source, a balance bridge connected with the excitation source, and a digital galvanometer, as well as a main control circuit, an input transformer and an output transformer; The main control circuit is respectively connected with the excitation source and the digital galvanometer;

The four bridge arms of the balanced bridge are the resistance to be measured R _C , the standard resistance R _S and two equivalent complementary resistances R ₁ and R ₂ , wherein the second end of the resistance to be measured R _C is connected to the equivalent complementary resistance R ₁ The first end of the standard resistance R S forms a branch, the second end of the standard resistance R _S is connected with the first end of the equivalent complementary resistance R ₂ to form a branch; the first end of the resistance R _C on the two branches is connected with The first end of the standard resistance R _S is connected; the two ends of the secondary coil of the input transformer are respectively connected to the first end of R _C and the second end of the equivalent complementary resistance R ₁ and R ₂ , and the complementary resistance R ₁ and R ₂ The following complementary conditions are met:

$\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; k</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}<span\; class="notranslate">\; ;</span>$

$\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; k</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}<span\; class="notranslate">\; ;</span>$

Among them, k is the adjustable coefficient, and R _ref is the reference resistance of R ₁ and R ₂ ;

When k is given, according to the principle of bridge balance, the resistance to be measured Therefore, the k value and R _S value determine the precision of the resistance to be measured.

Complementary resistors R ₁ and R ₂ adopt an N-level R-2R resistor network structure, and the resistors on their branches are 2R _ref and R _ref respectively, the first 2R _ref branch of the R-2R resistor network is connected to the second of the standard resistor R _S The other 2R _ref branches are connected with the second end of the resistance R _C to be measured and the second end of the standard resistance R _S through a bidirectional selector switch;

The two ends of the primary coil of the output transformer are respectively connected to the second end of the standard resistance R _S and the second end of the resistance R _C to be measured, and the two ends of the secondary coil are connected to the digital galvanometer;

The main control circuit includes a signal acquisition unit, a complementary resistance calculation unit and an excitation waveform generation unit; the signal acquisition unit receives the feedback signal of the digital galvanometer, performs synchronous sampling and filtering, the excitation waveform generation unit generates an excitation signal, and transmits the excitation signal to the excitation source, the complementary resistance calculation unit calculates the resistance values _of the equivalent resistances R1 and R2 according to the feedback signal, so as to adjust the connection structure of the R _- 2R resistance network structure and the balance bridge.

Preferably: the digital galvanometer includes a signal conditioning circuit and an analog-to-digital conversion circuit, which is connected to the main control circuit through the analog-to-digital conversion circuit; the primary coil of the output transformer is connected to the second end of the resistance R _C to be measured and the standard resistance R _S The second end of the second end; the secondary coil is connected to the signal conditioning circuit, the signal of the transformer is amplified by the signal conditioning circuit and transmitted to the analog-to-digital conversion circuit for analog-to-digital conversion, and the digital quantity is transmitted to the main control circuit.

Preferably, the excitation source includes a digital-to-analog conversion unit, a signal bipolar conversion unit, and a power drive unit, and the excitation signal is transmitted to the digital-to-analog conversion unit of the excitation source, and after being converted into an analog signal, it is transmitted to the signal bipolar conversion unit; The signal bipolar conversion unit converts the unipolar signal into a bipolar signal and then transmits it to the power drive unit. The power drive unit is connected to the input end of the balanced bridge through the input transformer. passed to the balance bridge.

The beneficial effects of the utility model are:

(1) Compared with the traditional bridge-type liquid conductivity measurement system, the accuracy of the measurement system and method provided by the utility model is greatly improved. The measurement accuracy of the traditional balanced bridge depends on R ₁ , R ₂ and R _S , but this system uses a complementary digital bridge, which reduces the influence of the branch resistance accuracy of the lower bridge arm on the measurement; when k is accurately given, The accuracy of the complementary bridge only depends on R _S and is not affected by R ₁ and R ₂ , which reduces the measurement uncertainty and improves the accuracy.

(2) The complementary digital bridge adopts the R-2R resistor network structure, increasing the order N of the R-2R resistor network structure can effectively improve the measurement accuracy of the system.

(3) The excitation waveform can be programmed, and the excitation waveform parameters can be set according to different solution characteristics to reduce the influence of polarization on the accuracy.

(4) Using synchronous sampling and filtering technology, the accuracy of the system and the stability of the measurement are improved.

(5) The excitation source, the galvanometer and the electric bridge are completely isolated electrically, which reduces the interference of the circuit.

(6) The interpolation algorithm is used to predict the parameters of the balance when the bridge reaches quasi-balance, so as to further improve the accuracy.

Description of drawings

Figure 1 is a schematic diagram of the balanced bridge method for measuring liquid conductivity.

Fig. 2 is a structural schematic diagram of the utility model.

Fig. 3 is a schematic diagram of the simplified structure of the balance bridge of the present invention.

Fig. 4 is a schematic structural diagram of a balanced electric bridge based on the R-2R resistance network of the utility model.

Detailed ways

As shown in Figure 2, the liquid conductivity measurement system based on a complementary digital bridge includes an excitation source, a balance bridge connected to the excitation source, and a digital galvanometer, as well as a main control circuit, an input transformer and an output transformer; the main control The circuit is respectively connected with the excitation source and the digital galvanometer.

The four bridge arms of the balanced bridge are the resistance to be measured R _C , the standard resistance R _S and two equivalent complementary resistances R ₁ and R ₂ , wherein the second end of the resistance to be measured R _C is connected to the equivalent complementary resistance R ₁ The first end of the standard resistance R S forms a branch, the second end of the standard resistance R _S is connected with the first end of the equivalent complementary resistance R ₂ to form a branch; the first end of the resistance R _C on the two branches is connected with The first end of the standard resistor R _S is connected; the two ends of the secondary coil of the input transformer are respectively connected to the first end of R _C and R _S and the second end of the equivalent complementary resistance R ₁ and R ₂ . _The resistance _values of the equivalent complementary resistors R1 and R2 satisfy the following relationship:

$\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; k</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}<span\; class="notranslate">\; ;</span>$

$\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; k</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}<span\; class="notranslate">\; ;</span>$

Among them, _k is the adjustable coefficient, and _Rref is the reference resistance _of R1 and R2.

As shown in Figure 4, the complementary resistors R ₁ and R ₂ adopt an N-level R-2R resistor network structure, and the resistors on their branches are 2R _ref and R _ref respectively, and the first 2R _ref branch of the R-2R resistor network is the same as the standard The second end of the resistance R _S is connected; the other 2R _ref branches are connected with the second end of the resistance R _C to be measured and the second end of the standard resistance R _S through a bidirectional selector switch.

Its detection principle is:

Assuming that an N-level R-2R resistor network is used, D _i represents the state of the i-th switch, "0" indicates that the switch is connected to the "A" terminal, and "1" indicates that the switch is connected to the "B" terminal, then define:

$\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; no</span>}}{{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; N</span>}}}$

in,

Indicates the number of combined switches connected.

Further, the measured resistance can be obtained as

${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; no</span>}}{{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}$

Resistivity:

${\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; N</span>}}}{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}$

That is to say, the conductivity of the solution to be tested is determined by the conductivity ρ _s of the standard solution, the order N of the R-2R network, and the number of switch combinations n when the bridge is balanced.

The digital galvanometer includes a signal conditioning circuit and an analog-to-digital conversion circuit, which is connected to the main control circuit through the analog-to-digital conversion circuit; the primary coil of the output transformer is connected to the second end of the resistance R _C to be measured and the second end of the standard resistance R _S ; The secondary coil is connected with the signal conditioning circuit, and the signal of the transformer is amplified by the signal conditioning circuit and transmitted to the analog-to-digital conversion circuit for analog-to-digital conversion, and the digital quantity is transmitted to the main control circuit.

The excitation source includes a digital-to-analog conversion unit, a signal bipolar conversion unit, and a power drive unit. The excitation signal is transmitted to the digital-to-analog conversion unit of the excitation source, and after being converted into an analog signal, it is transmitted to the signal bipolar conversion unit; the signal bipolar The conversion unit converts the unipolar signal into a bipolar signal and then transmits it to the power drive unit. The power drive unit is connected to the input terminal of the balance bridge through the input transformer. The power drive unit drives the transformer to generate an excitation signal, which is transmitted to the balance circuit through the transformer bridge.

The main control circuit includes a signal acquisition unit, a complementary resistance calculation unit and an excitation waveform generation unit; the signal acquisition unit receives the feedback signal of the digital galvanometer, the excitation waveform generation unit generates an excitation signal, and transmits the excitation signal to the excitation source, and the complementary resistance calculation unit Calculate the resistance values _of the equivalent resistors R1 and R2 according to the feedback signal in order to adjust the R _- 2R resistor network structure.

The excitation waveform generation unit of the main control system can generate DC-2kHz excitation waveforms. Typical types of waveforms include square waves, trapezoidal waves, and sine waves. Users can define arbitrary waveforms according to their needs. The amplitude, frequency and phase of the waveform can be realized by programming.

The excitation waveform generated by the excitation waveform generating unit is transmitted to the balance bridge after passing through the digital-to-analog conversion unit, the signal bipolar conversion unit and the power drive unit. The balanced bridge works under the action of the excitation signal.

The digital galvanometer collects the output of the second end of the resistance R _C of the balance bridge to be tested and the second end of the standard resistance R _S. According to this signal, it can be judged whether the balance bridge has reached a balanced state. The signal collected by the digital galvanometer is amplified by the signal conditioning circuit, converted by the analog-to-digital conversion unit, and then transmitted to the signal acquisition unit of the main control circuit.

Define s as the bridge output.

When judging that the output s of the balanced bridge is positive, it means that the resistance R _C to be measured is smaller than the high-precision standard resistance R _S . To adjust the bridge to achieve balance, the complementary resistance R ₁ of the same branch as the resistance R _C to be measured must be reduced , at this time, add the 2R _ref branch of the R-2R resistor network connected to the standard resistance R _S branch.

On the contrary, when it is judged that the output s of the balance bridge is negative, add the 2R _ref branch of the R-2R resistance network connected to the resistance R _C branch to be tested.

Adjust the access structure of the R-2R resistor network until the output s of the balanced bridge is 0.

Therefore, when it is judged that the number of combined switches is n, the bridge output is 0. According to the formula:

${\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}}<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; N</span>}}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}$

Calculate the conductivity of the solution to be tested.

Claims (3)

1. The liquid conductivity measurement system based on complementary digital electric bridge, comprises excitation source, the balance electric bridge connected with excitation source and digital galvanometer, is characterized in that: also comprises main control circuit, input transformer and output transformer; Main control The circuit is respectively connected with the excitation source and the digital galvanometer; The four bridge arms of the balanced bridge are respectively the resistance R _C to be measured, the standard resistance R _S and two equivalent complementary resistances R ₁ and R ₂ , wherein the second end of the resistance R _C to be measured is connected to the equivalent complementary resistance The first end of R ₁ forms a branch, and the second end of the standard resistance R _S is connected to the first end of the equivalent complementary resistance R ₂ to form a branch; the first end of the resistance R _C to be measured on the two branches The end is connected to the first end of the standard resistance R _S ; the two ends of the secondary coil of the input transformer are respectively connected to the first end of R _C and R _S and the second end of the equivalent complementary resistance R ₁ and R ₂ ; The complementary resistors R ₁ and R ₂ adopt an N-level R-2R resistor network structure, and the resistors on the branches are 2R _ref and R _ref respectively, and the first 2R _ref branch of the R-2R resistor network is connected to the standard resistor R _S The second end is connected; the other 2R _ref branches are connected with the second end of the resistance R _C to be measured and the second end of the standard resistance R _S through a bidirectional selector switch; The two ends of the primary coil of the output transformer are respectively connected to the second end of the standard resistance R _S and the second end of the resistance R _C to be measured, and the two ends of the secondary coil are connected to the digital galvanometer; The main control circuit includes a signal acquisition unit, a complementary resistance calculation unit and an excitation waveform generation unit; the signal acquisition unit receives the feedback signal of the digital galvanometer, the excitation waveform generation unit generates an excitation signal, and transmits the excitation signal to the excitation source, and the complementary resistance _The calculation unit calculates the resistance _values of the equivalent resistors R1 and R2 according to the feedback signal. 2. The liquid conductivity measuring system based on complementary digital electric bridge as claimed in claim 1, is characterized in that: said digital galvanometer comprises signal conditioning circuit and analog-to-digital conversion circuit, through analog-to-digital conversion circuit and main control circuit Connected; the primary coil of the output transformer is connected to the second end of the resistance R _C to be tested and the second end of the standard resistance R _S ; the secondary coil is connected to the signal conditioning circuit, and the signal of the transformer is amplified by the signal conditioning circuit and then transmitted to the analog digital The conversion circuit performs analog-to-digital conversion, and the digital quantity is transmitted to the main control circuit. 3. The liquid conductivity measurement system based on complementary digital electric bridge as claimed in claim 1, is characterized in that: described excitation source comprises digital-to-analog conversion unit, signal bipolar conversion unit and power drive unit, and excitation signal is delivered to The digital-to-analog conversion unit of the excitation source is converted into an analog signal and transmitted to the signal bipolar conversion unit; the signal bipolar conversion unit converts the unipolar signal into a bipolar signal and then transmits it to the power drive unit. The driving unit is connected to the input terminal of the balanced electric bridge through the input transformer, and the power driving unit drives the transformer to generate an excitation signal, which is transmitted to the balanced electric bridge through the transformer.