JPH09243683A - Method and device for measurement of resistivity, electric conductivity and/or permittivity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and an apparatus by which a resistivity, an electric conductivity and/or a permittivity can be measured stably, at high-speed response and with high accuracy by a method wherein a circuit whose scale is small and which is fabricated easily is used and a feedbak loop is not required. SOLUTION: A voltage is applied to a voltage application pole PB at a measuring probe 1 from a D/A converter 2, and another voltage application pole PA is connected to a current amplifier 4. Arbitrary two out of respective electric- field detection poles P1 to Pn are selected by analog multiplexers 5a, 5b, and a voltage across them is detected by a differential amplifier 6. The output of the current amplifier and that of the differential amplifier are changed over by an analog switch 7, and any of the outputs is connected to an A/D converter 8. In a DSP(digital signal processor) 3, a voltage waveform which is input to the A/D converter and which is detected by an electric-field detection pole and a current waveform which flows to the voltage application pole are multiplied respectively by a plurality of sine waves which are output from the D/A converter, and a smoothing numerical value is then computed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for measuring resistivity, electric conductivity and / or permittivity in electrochemical measurement and electromagnetic property measurement.

[0002]

2. Description of the Related Art Heretofore, it has been common to construct a bridge circuit as a two-terminal circuit for measuring resistivity, electric conductivity and permittivity. However, this method has a problem that the balance of the bridge needs to be manually adjusted, and the contact resistance between the electrode and the object to be measured causes an error. For that reason,
A measurement method by a four-electrode method in which a voltage application electrode and an electric field detection electrode are separated has been proposed. The present inventor has also devised an electric conductivity measuring device applying this principle, and has already registered the utility model registration No. 207.
Registered as No. 7834 (Actual fair 6-4843
No. 6).

[0003]

The conventional four-electrode method requires a stable oscillator, an amplifier with a small phase error, and a high-precision analog multiplier, which makes the device complicated, large-scale, and expensive. It has the drawback of becoming This tendency is due to applications requiring a large number of electric field detection poles, for example, an electric field generated by a voltage applied to the skin is detected by two or more electric field detection poles, and electric conductivity or resistivity is calculated from the voltage-current relationship. However, it becomes extremely large in the measurement of the skin water content for estimating the water content. In the conventional method, the applied voltage is adjusted so that the voltage between the electric field detection electrodes is constant in the case of electrical conductivity and the current flowing through the voltage application electrode is constant in the case of resistivity. It is usual to perform feedback control. For this reason, there are drawbacks such that the stability of measurement is lowered and a long response time is required.

Therefore, an object of the present invention is to solve the above-mentioned drawbacks of the prior art, to use a circuit which is small-scale and easy to manufacture, does not require a feedback loop, and can respond stably and at high speed.
Moreover, it is an object of the present invention to provide a method and apparatus capable of measuring resistivity, electrical conductivity and / or permittivity with high accuracy.

[0005]

In order to achieve the above object, according to the present invention, the resistivity is measured for an object to be measured by using two voltage applying poles and two or more electric field detecting poles. In the method for measuring at least one selected from the group consisting of electrical conductivity and dielectric constant, a voltage is applied to the object to be measured through a voltage applying electrode, and the electric field generated thereby is detected by the electric field detecting electrode. There is provided a measuring method characterized by detecting, converting a detected voltage waveform and a detected current waveform flowing through a voltage application electrode into a digital variable, and calculating a resistivity, an electrical conductivity or a permittivity of an object to be measured. In a preferred mode, the voltage waveform detected by the electric field detection pole and the current waveform flowing through the voltage application pole are time-divided and converted into digital variables. Also,
The voltage applied to the voltage application pole is a sine wave, and the voltage waveform detected by the electric field detection pole and the current waveform flowing in the voltage application pole are each multiplied by a plurality of sine waves with the same period as the sine wave but with a constant phase difference. After that, it is smoothed.

Further, according to the present invention, there is also provided an apparatus which can be suitably used in the above method. That is, according to the present invention, at least one selected from the group consisting of resistivity, electric conductivity and dielectric constant is used for the object to be measured by using two voltage applying electrodes and two or more electric field detecting electrodes. In a device for measuring one, a D / A converter that generates a voltage waveform to be applied to a voltage application pole and an A / A converter that converts a voltage waveform detected by an electric field detection pole and a current waveform flowing through the voltage application pole into a digital variable There is provided a measuring device including a D converter and a numerical calculator that calculates a resistivity, an electric conductivity or a permittivity based on an output signal from the A / D converter.

In a preferred mode, the number of A / D converters used is less than the total number of the number of voltage waveforms detected by the electric field detection pole and the number of current waveforms flowing through the voltage application pole,
The signals are time-divided and input to the A / D converter. Also,
The voltage applied from the D / A converter to the voltage application pole is preferably a sine wave, and the numerical calculator flows into the voltage waveform detected by the electric field detection pole input to the A / D converter and the voltage application pole. The current waveform is multiplied by a plurality of sine waves having a constant phase difference in the same cycle as the sine wave output from the D / A converter, and then a numerical operation for smoothing is performed.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the method and apparatus of the present invention, the voltage waveform generated by the D / A converter is applied to the voltage application pole without using the analog oscillator. In addition, the voltage waveform detected by the electric field detection pole and the current waveform flowing in the voltage application pole are directly A / D
Numerical calculation is performed after conversion by the converter. A logic circuit, a high-speed CPU, or the like can be used for the numerical calculation, but using a DSP (digital signal processor) is preferable in terms of performance price ratio.

When there are a plurality of electric field detection poles, and when the voltage waveform detected by each electric field detection pole and the current waveform flowing through the voltage application pole are A / D converted, an A / D converter may be provided for each. But, switch with analog multiplexer, 1
It is also possible to use one A / D converter in a time division manner.

Although various AC waveforms can be used for the voltage applied to the voltage application pole via the D / A converter, it is preferable to use a sine wave because the calculation is simple and an error is unlikely to occur. . In this case, the phase lag between the numerical value input to the D / A converter and the voltage applied to the voltage application pole, and the voltage between the voltage detected by the electric field detection pole and the numerical value converted by the A / D converter The phase delay and the phase delay between the current waveform flowing through the voltage application pole and the numerical value converted by the A / D converter will not cause an error in the calculation if they do not change with time even if they exist independently. Can be corrected as follows.

Next, the operation of the present invention will be described with reference to the drawings. Fig. 1 shows the principle of impedance measurement with four electrodes. P _A and P _B are voltage application electrodes and P
Reference numerals ₁ and P ₂ denote electric field detection poles. In FIG. 1, Z _e1 , Z
_e2 , Z _e3 , and Z _e4 are contact impedances between the electrodes and the object to be measured, and Z represents the impedance to be measured.

The voltage detected by the electric field detection poles P ₁ and P ₂ is e
(T), the current flowing between the voltage application poles P _A and P _B is i
(T), assuming that the voltage applied to the voltage application pole is √2 Asin ωt, the relationship of each equation shown in the following Expression 1 is established.

[Equation 1] E in the formula (1) is an AC voltage in the complex number representation expressed by the notation, I in the formula (2) is also an AC current in the complex number representation, Z in the formula (3) is a complex impedance, and E in the formula (4) is Y
Is a complex admittance, and j is an imaginary unit. Further, t is time, ω is angular frequency, and A is an arbitrary constant. Equation (1)
In (2) and (2), n is the number of cycles for smoothing the data, which may be any natural number, but in the actual device, S /
From the N ratio, 10 or more is good. It is clear from the respective definitions that the real part of Z is proportional to the resistivity, the real part of Y is proportional to the electrical conductivity, and the imaginary part of Z is proportional to the dielectric constant.
Also, what is observed as e (t) does not flow a current between P ₁ and P ₂ , so that regardless of the values of Z _{e1 to} Z _e4 ,
Is the voltage across Z, i (t) is Z due to the continuity of the current
Z is calculated in this way because it is the current flowing through
Y is not affected by Z _{e1 to} Z _e4 .

Although the above equations (1) and (2) are shown in the continuous time system, even if they are replaced with the discrete time system, the sampling period Δt is the period of the sine wave Asin ωt applied to the application electrode, that is, ω. If it is 1/2 or less with respect to / 2π, it is theoretically equivalent, but in an actual device, 1/8 or less is preferable because the circuit can be simplified. If the DUT does not change over time,
Since E and I are constants, it is not always necessary to measure them simultaneously. Further, even if it changes with time, it is not necessary to perform simultaneous measurement if it is measured within a sufficiently short time compared to the rate of change. Accordingly, it is possible to measure a plurality of voltage waveforms detected by a plurality of electric field detection poles and a current waveform of a voltage application pole while sequentially switching with one A / D converter, and the circuit can be greatly simplified.

Further, in the above description, it is assumed that the voltage Asin ωt applied to the voltage application pole and the sine wave and the cosine wave used in the above equations (1) and (2) have the same phase. By adding the calculation described below, this precondition can be relaxed and correct measurement can be performed even if there is a constant phase difference. Although the following description will be made with reference to the drawings regarding an example having a large number of electric field detection electrodes, the same applies to the case where one set (two pieces) of electric field detection electrodes is used.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments shown in the accompanying drawings will be described below. In FIG. 2, reference numeral 1 denotes a measurement probe incorporating each electrode, of which P _A and P _B are voltage application electrodes and P
₁ , P ₂ ... P _{n denote} electric field detection electrodes. Voltage application pole P _B
Is applied with a voltage by the D / A converter 2, and the other voltage application pole P _A is connected to the current amplifier 4. The electric field detection poles P ₁ , P ₂ ... P _n are respectively connected to the analog multiplexers 5 a and 5 b, but for convenience of illustration, a bundle of wiring lines are shown by thick solid lines (between the electric field detection poles). Is not connected). Any two of the many electric field detection poles are analog multiplexers 5.
a, 5b, and the voltage between them is selected by the amplifier A ₁ ~
It is detected by the differential amplifier 6 composed of A ₃ . The outputs of the current amplifier 4 and the differential amplifier 6 are switched by the analog switch 7, and either output is connected to the A / D converter 8. DSP (Digital Signal Processor) 3
Operates the analog multiplexers 5a and 5b, the analog switch 7, the A / D converter 8 and the D / A converter 2 to perform the arithmetic processing described so far. Specifically, FIG.
The program corresponding to the flowchart shown in is executed.

First, prior to the measurement, the pure resistance network shown in FIG. 3 is connected to the device instead of the measurement probe 1. P _A and P _B are terminals for voltage application, P ₁ , P ₂ ... P
_n is connected to each of the electric field detection pole terminals and measured.
The values that should be measured should be R ₁ , R _2, ..., R _n-1 , but because of the phase difference between the applied voltage and the sine wave and cosine wave used in the calculation of equations (1) and (2) above. The imaginary part does not become 0,
Observed as complex impedance. This is Z ⁰ ₁ , Z
It is stored as ⁰ ₂ , ... Z ⁰ _n-1 . At the time of actual measurement, these values may be used to perform the calculation of Equation (5) below.

[Equation 2] In the equation (5), R _K represents the resistance values R ₁ , R ₂ ... R _n-1 of each element of the pure resistance network shown in FIG. 2, and Z ⁰ _K represents the above equation ( 1) to (3)
Is a calculation result value, Z ^m _K is a similar calculation result value by the object to be measured, and Z ^X _K is a corrected measurement value.

After that, the measuring probe 1 is connected to the apparatus, brought into contact with the object to be measured, and the program shown in FIG. 4 is executed to perform the measurement. The flowchart of the embodiment of the present invention shown in FIG. 4 will be described below. First, in step a, the task is activated by a periodic interrupt every 50 μs by a timer or the like. This is because it corresponds to a sampling period of 20 kHz. Next, in step b, A / D
From the converter 8, the conversion result of the voltage or current value from the channel that was previously set is taken out, and the variable "inp
Then, in step c, the content of the previously prepared variable “DAout” is sent to the D / A converter 2 and the output of the D / A converter 2 is updated.

Next, in step d, a sine wave function table (tsin previously created and stored in the memory).
[K] = sin (k * 2π / 20) and tcos [k] = co
From s (k * 2π / 20), k = 0, 1, ... 19 are calculated and stored in memory), and the sin and cos data (s and c) corresponding to the current phase are extracted. . In step e, this data (s, c)
The sum of products operation is performed by the input from the A / D converter stored in the variable "input". In preparation for the next cycle, in step f, the data (c) of cos that has just been used is held in DAout. Next, in step g, the phase holding counter variable "k" is incremented (incremented) and updated. If the counter variable “k” becomes 20 or more in step h, the measured frequency is 1 kHz.
Since one cycle of z is completed, the following processing is continued, but if not, the interrupt is completed. If one cycle is completed, the measurement cycle counter variable "n" is incremented and updated in step i. If, in step j, this counter variable "n" becomes 20 or more, the prescribed number of times of integration, 20 cycles, has been reached, so the following processing is continued. If not, the interruption ends.

When the specified number of times of integration is reached, step k
In step (1), the correction is performed according to the equation (5), and the result is output to the outside. It may communicate and send to another CPU,
It may be displayed on an LCD or the like, or may be recorded in a memory or an external storage device. Then, in step 1, the integration work variables “sumRe” and “sumIm” and the counter variable n are reset to 0. Then, in step m, the voltage between the next electric field detection poles or the current of the voltage application pole is measured by the A / D converter 8 for the measurement of the next another input channel.
To be input to the analog multiplexers 5a, 5b
And the analog switch 7 is switched to complete one interruption. Here, an example in which the measurement frequency is 1 kHz, the sampling frequency is 20 kHz, and the integration period is 20 cycles for smoothing the data is shown, but it goes without saying that it can be changed according to the situation.

[0020]

As described above, according to the present invention, a stable oscillator, an amplifier with a small phase error, and a high-precision analog multiplier as in the conventional four-electrode method are not required, and a large number of electric field detection poles are provided. The device can be configured with a small-scale circuit even when necessary. For this reason, a device that is easy to manufacture, inexpensive, and highly accurate can be realized in a small size. Further, according to the method and apparatus of the present invention, there is no feedback loop, and stable and fast response is possible. The D / A converter, the A / D converter, the differential amplifier, and the current amplifier required in the present invention do not cause an error even if an arbitrary delay is generated as long as the phase delay is constant in time. The advantages are that it is extremely easy and cheap to implement, and no adjustment is required. Therefore, the method and apparatus of the present invention, various conductors, measurement of resistivity of semiconductors, measurement of skin water content, water quality control of boiler water etc. and various solutions for concentration control of fertilizers in the agricultural field. It can be advantageously used for measurement of electric conductivity, measurement of dielectric constant of various dielectrics, and the like.

[Brief description of drawings]

FIG. 1 is a basic conceptual diagram showing the principle of impedance measurement with four electrodes.

FIG. 2 is a circuit diagram showing an embodiment of the device of the present invention.

FIG. 3 is a schematic configuration diagram showing a pure resistance network for testing.

FIG. 4 is a flow chart diagram of execution of the embodiment shown in FIG. 2 of the present invention.

[Explanation of symbols]

 1 Measuring Probe (Electrode) 2 D / A Converter 3 Digital Signal Processor 4 Current Amplifier 5a, 5b Analog Multiplexer 6 Differential Amplifier 7 Analog Switch 8 A / D Converter

Claims (7)

[Claims] 1. A voltage measuring electrode and two voltage applying electrodes are provided for an object to be measured.
In the method of measuring at least one selected from the group consisting of resistivity, electric conductivity and dielectric constant by using at least one electric field detection pole, a voltage is applied to an object to be measured through a voltage application pole. The applied electric field is detected by the electric field detection pole, the detected voltage waveform and the current waveform flowing in the voltage application pole are converted into digital variables, and the resistivity, electrical conductivity or permittivity of the DUT is calculated. A measuring method characterized by: 2. The method according to claim 1, wherein the voltage waveform detected by the electric field detection pole and the current waveform flowing through the voltage application pole are time-divided and converted into digital variables. 3. The voltage applied to the voltage application pole is a sine wave, and a plurality of voltage waveforms detected by the electric field detection pole and a current waveform flowing through the voltage application pole have a constant phase difference with the same period as the sine wave. Method according to claim 1 or 2, characterized in that each sine wave is multiplied and then smoothed. 4. A voltage measuring electrode and two voltage applying electrodes for the object to be measured.
D / A for generating a voltage waveform to be applied to a voltage applying pole in an apparatus for measuring at least one selected from the group consisting of resistivity, electric conductivity and dielectric constant using at least one electric field detecting pole A converter, an A / D converter for converting the voltage waveform detected by the electric field detection pole and the current waveform flowing through the voltage application pole into a digital variable, and the resistivity and electric power based on the output signal from the A / D converter. A measuring device comprising: a numerical calculator for calculating conductivity or permittivity. 5. The number of A / D converters used is smaller than the total number of the voltage waveforms detected by the electric field detection poles and the number of current waveforms flowing through the voltage application poles. The device according to claim 4, wherein the device is input to an A / D converter. 6. The method according to claim 4, further comprising an analog multiplexer connected to each electric field detection pole, and a differential amplifier that detects an output voltage of the analog multiplexer and outputs it to an A / D converter. 5. The device according to item 5. 7. A voltage waveform and a voltage application pole detected by the electric field detection pole input to the A / D converter by the numerical calculator, wherein the voltage applied from the D / A converter to the voltage application pole is a sine wave. The numerical calculation for smoothing is performed after multiplying each of the current waveforms flowing through the sine wave by a plurality of sine waves having a constant phase difference in the same cycle as the sine wave output from the D / A converter. 7. The device according to any one of 4 to 6.