Conductivity detector

Abstract

The invention provides a conductivity detector. The conductivity detector is provided with a waveform output unit that supplies an AC signal to a measurement object, and a control device that controls the waveform output unit. The control device performs processing of multiplying a detection wave obtained by passing an AC signal through a measurement object by a reference wave. The control device causes the waveform output unit to generate a waveform in which the dummy peak is superimposed on the input wave, and performs a process for detecting the phase difference. The control device causes the waveform output unit to generate a waveform in which the virtual peak is not superimposed on the input wave, performs a process for correcting the phase difference with respect to the reference wave, and calculates the conductivity on the basis of the result of the multiplication process.

Description

Technical Field

[0001] The present disclosure relates to a conductivity detector. Background Art

[0002] The conductivity detector is used, for example, as a detector for detecting the components (ions) of a sample separated by a column in an ion chromatograph. The conductivity detector applies a voltage between a pair of electrodes arranged to sandwich the flow path of the eluent (sample solution) from the ion chromatograph to measure the current flowing between the two electrodes. The conductivity of the eluent flowing between the two electrodes is thereby calculated, and the conductivity detector detects the components separated in the ion chromatograph based on the conductivity. Here, if the voltage applied between the two electrodes is set to direct current, polarization occurs between the electrodes and accurate conductivity cannot be measured, so an alternating current (e.g., a sine wave) with a constant amplitude and frequency is used for the voltage.

[0003] International Publication No. 2017 / 208561 discloses a measuring device as an example of such a conductivity detector. The measuring device includes: a sensor unit that outputs an AC signal; a reference signal output unit that outputs a reference signal having the same frequency and phase as the AC signal; a multiplication unit that multiplies the AC signal by the reference signal; an A / D conversion unit that is arranged before or after the multiplication unit; and an accumulation unit that accumulates a digital signal that is an output of the multiplication unit. Summary of the invention

[0004] Among conductivity detectors, there is a conductivity detector that performs a lock-in measurement. When performing a lock-in measurement, a process of multiplying a detection wave by a reference wave (=same frequency and same phase as the drive wave) is performed. Depending on the measurement object, a phase difference sometimes occurs between the detection wave and the reference wave, which causes an error in the multiplication result. Therefore, the phase difference between the drive wave (reference wave) and the detection wave is calculated, and the error is eliminated by making the phase of the reference wave consistent with the phase of the detection wave. However, there is a case where a noise component is superimposed on the drive wave and the phase difference cannot be accurately detected, and the detection accuracy of the conductivity decreases.

[0005] The present disclosure is a conductivity detector that solves such a problem, and an object of the present disclosure is to provide a conductivity detector that is not easily affected by noise.

[0006] The first mode of the present disclosure relates to a conductivity detector. The conductivity detector includes a waveform output unit that provides an AC signal to a measurement object and a control device that controls the waveform output unit. The control device is configured to perform a multiplication process in which a detection wave obtained by passing the AC signal through the measurement object is multiplied by a reference wave for detecting conductivity. The control device is configured to cause the waveform output unit to generate a waveform with a virtual peak superimposed on an input wave as an AC signal, and to perform a phase difference detection process for detecting a phase difference between the correspondingly obtained detection wave and the AC signal. The control device is configured to cause the waveform output unit to generate a waveform with no virtual peak superimposed on an input wave as an AC signal, and to perform a process for correcting the phase difference with respect to the reference wave, and to perform a conductivity calculation process for calculating conductivity based on the result of the multiplication process.

[0007] The above-mentioned objects, features, aspects and advantages and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 : is a block diagram showing the structure of the conductivity detector according to the present embodiment.

[0009] Figure 2 This is a diagram for explaining locking measurement when there is a phase error.

[0010] Figure 3 This is a diagram for explaining locking measurement when there is no phase error.

[0011] Figure 4 It is a diagram for explaining the phase difference.

[0012] Figure 5 This is a diagram for explaining the influence of spike noise on the peak position.

[0013] Figure 6 This is a diagram showing an example of the shape of a virtual peak.

[0014] Figure 7 is a diagram showing the relationship between spike noise and virtual peaks.

[0015] Figure 8 It is a flowchart for explaining the details of the phase difference detection process.

[0016] Fig. 9 : is a flowchart for explaining the details of the conductivity calculation process. DETAILED DESCRIPTION

[0017] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. In addition, the same reference numerals are attached to the same or corresponding parts in the drawings, and their description will not be repeated.

[0018] Figure 1 : is a block diagram showing the structure of the conductivity detector according to the present embodiment. Figure 1 The conductivity detector 1 shown in the figure includes a D / A converter 11, an analysis chamber 12, an amplifier 13, an A / D converter 14, and a control device 100 for controlling a waveform output unit (D / A converter 11). The D / A converter 11 functions as a waveform output unit for providing an AC signal to a measurement object. The analysis chamber 12 accommodates a sample solution as a measurement object. The amplifier 13 amplifies the detection wave. The A / D converter converts the output of the amplifier 13 into a digital signal.

[0019] The control device 100 is composed of, for example, at least one CPU (Central Processing Unit), at least one ASIC (Application Specific Integrated Circuit), at least one FPGA (Field Programmable Gate Array), or a combination thereof.

[0020] The control device 100 includes a storage device. The storage device includes, for example, a ROM (Read Only Memory), a RAM (Random Access Memory), a non-volatile memory, an SSD (Solid State Drive) or a hard disk. The storage device stores programs for implementing various processes. The control device 100 implements the following various processes by executing the programs stored in the storage device.

[0021] The control device 100 is configured to execute a multiplication process in which a detection wave obtained by passing an AC signal through a measurement target (a sample solution in the analysis chamber 12 ) is multiplied by a reference wave for detecting conductivity. The multiplication process is executed by a multiplier 105 .

[0022] The control device 100 is configured to cause the waveform output unit to generate a waveform with a virtual peak superimposed on an input wave as an AC signal, and to perform a phase difference detection process for detecting a phase difference between the correspondingly obtained detection wave and the AC signal. The phase difference detection process is performed by the phase difference detection unit 102 .

[0023] The control device 100 is configured to cause the waveform output unit to generate a waveform without a virtual peak superimposed on the input wave as an AC signal and to execute a process of correcting the phase difference with respect to the reference wave. The process of correcting the phase difference is executed by the phase correction unit 103 .

[0024] The control device 100 is configured to execute conductivity calculation processing to calculate conductivity based on the result of multiplication processing. After the output of the multiplier 105 is integrated by the integrating unit 106 and the value is smoothed by the filter unit 107, the conductivity calculation processing is executed by the conductivity calculation unit 108.

[0025] According to the conductivity detector 1 , the accuracy of the correction for making the phase of the reference wave coincide with the phase of the detection wave is improved by the phase correction unit 103 , so that the conductivity can be detected with high accuracy.

[0026] The phase correction will be described in detail below. The conductivity detector generally performs lock-in measurement. Figure 2 This is a diagram for explaining locking measurement when there is a phase error. Figure 3 This is a diagram for explaining locking measurement when there is no phase error.

[0027] like Figure 2 , Figure 3 As shown in Figure 1, the driving wave mostly uses a fixed frequency sine wave. Figure 1 In the illustrated configuration, a D / A converter 11 is used when generating a sine wave, and an A / D converter 14 is used when reading a waveform.

[0028] In the case of a phase detection, the detection wave is multiplied by the reference wave (=same frequency and same phase as the drive wave) by the multiplier 105. Figure 2 As shown, depending on the measurement object, a phase difference occurs between the detection wave and the reference wave, and an error occurs in the calculation result.

[0029] Figure 4 is a diagram for explaining the phase difference. Figure 4 As shown, peak detection processing is usually performed before measurement to calculate the phase difference between the peak of the drive wave (or reference wave) and the peak of the detection wave, and the time corresponding to the phase difference is corrected to make the phase of the reference wave consistent with the phase of the detection wave, thereby eliminating the above-mentioned error.

[0030] like Figure 2 As shown in FIG. 1 , when there is a phase difference between the driving wave (= reference wave) and the detection wave, the correct calculation result cannot be obtained. The reference wave and the detection wave have different signs according to the amount of phase deviation, and the correct area cannot be obtained even if the multiplication result is accumulated. On the other hand, if Figure 3 As shown, a correct calculation result is obtained when there is no phase difference between the driving wave (=reference wave) and the detection wave.

[0031] Therefore, usually, the phase difference is detected by peak detection processing before measurement, and the detected phase difference is fed back to the reference wave to obtain the phase difference. Figure 2 The status becomes Figure 3state, thereby obtaining the correct calculation result.

[0032] However, when using an inexpensive D / A converter, spike noise may enter the output voltage. Figure 5 is a diagram for explaining the effect of spike noise on the peak position. Figure 5 In FIG. 1 , a driving wave containing spike noise (solid line) and a driving wave not containing spike noise (dashed line) are shown superimposed. Figure 5 As shown, the peak position varies depending on whether there is spike noise, so an error occurs in the peak detection of the sine wave due to the entry of the spike noise.

[0033] Therefore, in Figure 4 The peaks shown in the figure have errors when they are detected. This error is a phase error, which directly translates into an error in conductivity.

[0034] Since spike noise is averaged and becomes a wide peak within the sampling time of the A / D converter, it is often difficult to process based on ordinary threshold judgment when the amplitude of the detection wave is small. On the other hand, if a glitch-free D / A converter that does not produce spike noise is used, the error can be improved, but such a D / A converter is expensive.

[0035] In addition, although spike noise can be reduced by inserting an analog filter in the output section of the D / A converter, it is necessary to consider the delay caused by the filter and the attenuation of the detection signal. In addition, the cost increases due to the additional components.

[0036] Therefore, the conductivity detector of the present embodiment uses a virtual peak when detecting a phase difference. Figure 6 This is a diagram showing an example of the shape of a virtual peak. Figure 7 is a diagram showing the relationship between spike noise and virtual peaks.

[0037] In this embodiment, when performing the peak detection process, the sine wave data output unit 101 generates data by adding a virtual peak having an amplitude larger than that of the spike noise to the output signal of the D / A converter 11. Figure 7 As shown in FIG. 1 , since the amplitude of the virtual peak is larger than the spike noise, the peak position is no longer shifted by the spike noise when the peak is detected. On the other hand, since the sine wave data without the virtual peak is usually used during measurement, it does not affect the analysis result.

[0038] The amplitude of the virtual peak can be larger than the amplitude of the assumed spike noise and smaller than the amplitude of the AC signal as the input wave. For example, the amplitude of the virtual peak can be set to be greater than 5% and less than 15% of the amplitude of the AC signal. In a further limited way, the amplitude of the virtual peak can be set to be 10% of the amplitude of the AC signal.

[0039] Figure 8 It is a flowchart for explaining the details of the phase difference detection process.

[0040] First, in step S1 , the control device 100 outputs the sine wave data having the virtual peak to the D / A converter 11 , and in step S2 , the control device 100 acquires the detection wave data from the A / D converter 14 .

[0041] Next, in step S3, the control device 100 performs peak detection of the detection wave. In the peak detection, the control device 100 detects the time when the data value of the waveform converted into a numerical value by the A / D converter 14 becomes the maximum. Next, in step S4, the control device 100 calculates the phase difference (or delay time) based on the difference between the time of the peak of the input wave and the time of the peak of the detection wave, and sets the phase difference as the phase correction data in step S5.

[0042] Fig. 9 : is a flowchart for explaining the details of the conductivity calculation process.

[0043] First, in step S11, the control device 100 outputs the sine wave data without the virtual peak to the D / A converter, and obtains the data of the detection wave that has passed through the analysis chamber 12 from the A / D converter 14. Then, in step S12, the control device 100 outputs the rectangular wave data that has been phase-corrected, and in step S13, the detection wave data is multiplied by the rectangular wave data. Then, the multiplication result is integrated over a half cycle or a cycle of the sine wave to calculate the area.

[0044] At this time, if the rectangular wave data is not corrected, Figure 2 In this way, the rectangular wave data and the detection wave data have different signs, so the multiplication result is a negative value, and the area cannot be correctly accumulated. In this embodiment, the phase of the rectangular wave data is corrected when the rectangular wave data and the detection wave data are multiplied. Figure 3 As shown, the sign of the detection wave data and the sign of the rectangular wave data match over the entire range, and the area can be accurately calculated by integrating the results of multiplying the detection wave data and the rectangular wave data.

[0045] Thereafter, filtering processing is performed in step S15 , and in step S16 , the result of conversion is output as electrical conductivity by multiplying by a coefficient corresponding to the size of the analysis chamber or the like as necessary.

[0046] As described above, according to the conductivity detector of this embodiment, the following effects are obtained.

[0047] Since there is no need to use a D / A converter without interference in which spike noise does not occur and there is no need to install a filter circuit for removing spike noise, the conductivity detector can be manufactured at low cost.

[0048] The calculation cost hardly changes even if only the input wave data is changed, so there is no need to change the hardware (CPU, PLD, etc.), and it is easy to introduce processing that improves the accuracy of phase correction.

[0049] [Way]

[0050] It will be understood by those skilled in the art that the above-described exemplary embodiments are specific examples of the following aspects.

[0051] (Item 1) A first aspect of the present disclosure relates to a conductivity detector. The conductivity detector includes a waveform output unit that provides an AC signal to a measurement object and a control device that controls the waveform output unit. The control device is configured to perform a multiplication process in which a detection wave obtained by passing the AC signal through the measurement object is multiplied by a reference wave for detecting conductivity. The control device is configured to cause the waveform output unit to generate a waveform in which a virtual peak is superimposed on an input wave as an AC signal, and to perform a phase difference detection process for detecting a phase difference between the correspondingly obtained detection wave and the AC signal. The control device is configured to cause the waveform output unit to generate a waveform in which a virtual peak is not superimposed on an input wave as an AC signal, and to perform a process for correcting the phase difference with respect to the reference wave, and to perform a conductivity calculation process for calculating conductivity based on a result of the multiplication process.

[0052] According to the first aspect of the conductivity detector, since the accuracy of the correction for making the phase of the reference wave coincide with the phase of the detection wave is improved, the conductivity can be detected with high accuracy.

[0053] (Item 2) In the conductivity detector described in Item 1, the input wave is a sine wave and the reference wave is a rectangular wave.

[0054] According to the second conductivity detector, since the accuracy of correction for making the phase of the rectangular wave as the reference wave coincide with the phase of the sine wave as the detection wave is improved, the conductivity can be detected with high accuracy.

[0055] (Item 3) In the conductivity detector described in Item 2, the waveform output unit includes a D / A converter. The control device is configured to send the sine wave data to the D / A converter in the conductivity calculation process. The control device is configured to send data obtained by adding virtual peak data to the sine wave data to the D / A converter in the phase difference detection process.

[0056] According to the conductivity detector of the third item, since the waveform is generated by the D / A converter, it is possible to easily switch whether to superimpose a virtual peak on the input wave.

[0057] (Item 4) In the conductivity detector described in Item 3, the control device is configured to detect a peak value of the detection wave in the phase difference detection process and detect the phase difference based on a delay amount from a virtual peak of the AC signal to the peak value of the detection wave.

[0058] According to the fourth conductivity detector, since virtual peaks are superimposed, the time difference between peaks is less susceptible to noise, so the accuracy of correction for matching the phase of the reference wave with the phase of the detection wave is improved, and conductivity can be detected with high accuracy.

[0059] (Item 5) In the conductivity detector described in Item 2, the control device is configured to calculate the area of ​​the superposition portion of the reference wave and the sine wave by integrating the results of the multiplication process, and convert the calculated area into conductivity.

[0060] According to the conductivity detector of the fifth aspect, when the input signal is an AC signal, the accuracy of correction for making the phase of the reference wave coincide with the phase of the detection wave is improved, so that the conductivity can be detected with high accuracy.

[0061] (Item 6) In the conductivity detector described in Item 1, the height of the virtual peak is 5% or more and 15% or less of the amplitude of the AC signal.

[0062] According to the conductivity detector of item 6, since the virtual peak is larger than the noise and is in a range that does not affect the AC signal, the time difference between the peaks is not easily affected by the noise, and the detection accuracy of the conductivity can be optimized.

[0063] (Item 7) In the conductivity detector described in Item 6, the height of the virtual peak is 10% of the amplitude of the AC signal.

[0064] According to the seventh conductivity detector, since the virtual peak is larger than the noise and is in a range that does not affect the AC signal, the time difference between the peaks is not easily affected by the noise, and the detection accuracy of the conductivity can be optimized.

[0065] (Item 8) In the conductivity detector described in Item 1, a width of the virtual peak is shorter than half a period of the AC signal.

[0066] According to the conductivity detector of the eighth item, since the virtual peak is in a range that does not exert an influence on the AC signal, the time difference between the peaks is not easily affected by noise, and the detection accuracy of the conductivity can be optimized.

[0067] (Item 9) In the conductivity detector described in Item 1, the measurement object is a sample solution of ion chromatography. The conductivity detector further includes an analysis chamber for accommodating the sample solution. The analysis chamber includes a first electrode and a second electrode, wherein the first electrode is immersed in the sample solution, an AC signal is provided to the first electrode from a waveform output unit, and the second electrode is immersed in the sample solution to output a detection wave.

[0068] According to the conductivity detector of the ninth aspect, it is possible to optimize the detection accuracy of the conductivity used in ion chromatography.

[0069] Although the embodiments of the present invention have been described, the embodiments disclosed this time should be considered to be illustrative in all aspects and not restrictive. The scope of the present invention is indicated by the claims, and it is intended to include all modifications within the meaning and scope equivalent to the claims.

Claims

1. A conductivity detector, comprising: a waveform output unit that provides an AC signal to a measurement object; and a control device for controlling the waveform output portion, in, The control device is configured to perform a multiplication process in which a detection wave obtained by passing the AC signal through the measurement object is multiplied by a reference wave for detecting conductivity. The control device is configured to cause the waveform output unit to generate a waveform in which a virtual peak is superimposed on an input wave as the AC signal, and to perform a phase difference detection process for detecting a phase difference between the correspondingly obtained detection wave and the AC signal, The control device is configured to cause the waveform output unit to generate a waveform without superimposing the virtual peak on the input wave as the AC signal, execute a process of correcting the phase difference with respect to the reference wave, and execute a conductivity calculation process of calculating the conductivity based on a result of the multiplication process.

2. The conductivity detector according to claim 1, wherein: The input wave is a sine wave, and the reference wave is a rectangular wave.

3. The conductivity detector according to claim 2, wherein: The waveform output unit includes a D / A converter, The control device is configured to send sine wave data to the D / A converter in the conductivity calculation process. The control device is configured to send data obtained by adding the data of the virtual peak to the sine wave data to the D / A converter in the phase difference detection process.

4. The conductivity detector according to claim 3, wherein: The control device is configured to detect a peak value of the detection wave in the phase difference detection process, and detect the phase difference based on a delay amount from the virtual peak of the AC signal to the peak value of the detection wave.

5. The conductivity detector according to claim 2, wherein: The control device is configured to calculate an area of ​​a superimposed portion of the reference wave and the sine wave by integrating the result of the multiplication process, and convert the calculated area into the electrical conductivity.

6. The conductivity detector according to claim 1, wherein: The height of the virtual peak is greater than or equal to 5% and less than or equal to 15% of the amplitude of the AC signal.

7. The conductivity detector according to claim 6, wherein: The height of the virtual peak is 10% of the amplitude of the AC signal.

8. The conductivity detector according to claim 1, wherein: The width of the virtual peak is shorter than half of a period of the AC signal.

9. The conductivity detector according to claim 1, wherein: The measurement object is a sample solution of ion chromatography, The conductivity detector further includes an analysis chamber for accommodating the sample solution. The analysis chamber includes a first electrode and a second electrode, wherein the first electrode is immersed in the sample liquid and the AC signal is supplied to the first electrode from the waveform output unit, and the second electrode is immersed in the sample liquid and outputs the detection wave.

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