JPH0736028B2 - Precision comparison method for thermoelectric AC / DC converters - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a precision comparison method of a thermoelectric type AC / DC converter used when performing precise measurement of AC voltage, AC current and the like.

(Prior Art) In direct current measurements, the voltage standard is provided by a Josephson voltage standard device, whereas in alternating current measurements there is no known device that produces a known alternating voltage (current). Therefore, for precise measurement of AC voltage (current), when the measurement voltage and its equivalent DC voltage (current) are alternately input, a device that produces equal output is used to determine the effective value of the measurement voltage (current). A method of comparing with a known DC voltage (current) and making a determination is adopted. The devices used in this method include an AC / DC converter for converting an AC amount into a DC amount and an AC / DC comparator for comparing with the DC amount.

Since it is possible to obtain a precise DC amount, the accuracy of this method is mainly determined by the error of the AC / DC conversion and the AC / DC comparator. These errors are sometimes called AC / DC difference,
Determining the AC-DC difference is used in much the same sense as establishing an AC voltage (current) standard.

Regarding low-frequency AC current and voltage standards, thermoelectric AC / DC converters that utilize thermoelectric phenomena and electrostatic AC / DC comparators that use electrostatic torque balance meters have been tried in the past. In comparison, the thermoelectric AC / DC converter has a simple structure and is easy to handle, and thus has a high practical value as a low-frequency AC (hereinafter abbreviated as AC) current / voltage standard.

This thermoelectric AC / DC converter is one in which a current is passed through a resistance wire (heater) and the temperature rise is detected by a thermocouple.

The AC / DC difference δ of the thermoelectric type AC / DC converter is δ =, assuming that when the AC current of effective value I _a and the DC current of I _d are alternately passed through the heater of the converter, the same magnitude of output is obtained. It is defined by (I _a −I _d ) / I _d (1).

A precise comparison of the two transducers is done by measuring the difference in this alternating difference.

Now, assuming that the AC / DC difference of the converter (TCC1) is δ ₁ and the AC / DC difference of the converter (TCC2) is δ ₂ , and the DC input to TCC1.TCC2 is I _d , the AC / DC difference (δ ₁ −δ ₂ ) Is approximately given by the following formula according to the principle of the two-voltmeter method.

δ ₁ −δ ₂ = {(E _a2 −E _d2 ) / ΔE _d2 − (E _a1 −E _d1 ) /
ΔE _d1 } ・ ΔI _d I _d (2) Here, E _a1 and E _a2 are AC currents with effective value I _a TCC1.TCC2
Thermoelectromotive force generated in TCC1 and TCC2,
E _d1 and E _d2 are the thermoelectromotive force generated in TCC1 and TCC2 when a direct current I _d is applied to TCC1 and TCC2, ΔE _d1 and ΔE _d2 are the small changes in DC current ΔI _d in TCC1 and TCC2. The change of thermoelectromotive force is shown.

The voltage comparison can be expressed by a similar equation.

FIG. 2 shows a circuit configuration for measuring the difference between the AC / DC difference by the two-voltmeter method by current measurement (series), and an AC current is made to flow from the precision AC power supply V _a to the converters TCC1 and TCC2 by the switch S ₁ . Further, a direct current is made to flow from the direct current U _d by the switches S ₂ and S ₃ .

Further, the setting of the direct current I _d is performed as follows. Apply a rated AC current of a certain frequency to a converter, for example TCC1, measure the thermoelectromotive force E _a with a precision digital voltmeter (DVM1), then change the connection to a DC power supply, and change the DC current forward and backward. Add. Each of the thermal electromotive force Ed ^+, Ed ^- Read. Then, the average value of the thermoelectric power when given this direct current ^{(Ed + + Ed -) /} 2
The output of the precision DC power supply V _d is adjusted so that is equal to the AC thermoelectromotive force E _a .

Furthermore, E _a1 , E _a2 , E _d1 , and E _d2 are measured as follows.
That is, alternating current, direct current, forward direction, reverse direction and alternating current are sequentially applied by switches S ₁ , S ₂ and S ₃ , and this cycle is repeated several times, and two precision digital voltmeters DV are supplied each time.
Measure the thermoelectromotive force of TTC1 and TCC2 with M1 and DVM2. As a result, average E _a1 , E _a2 , E _d1 , and E _d2 are obtained.

After that, the switches S ₄ measure the gradients ΔE _d1 / ΔI _d and ΔE _d2 / ΔI _d of the input current thermoelectromotive force characteristics of the converters TCC1 and TCC2. From the above measurement, the difference (δ ₁ −δ
₂ ) is calculated.

(Problems to be Solved by the Invention) However, it is very difficult to equalize the thermoelectromotive forces when an alternating current and a direct current are passed as described above in the above conventional comparison device.

By the way, short-term stability of precision AC power supply is 0.007% in 10 minutes.
It is a degree.

In addition, the thermal time constant of the converter is several seconds, and the thermoelectromotive force is measured after 30 seconds or more until the current (voltage) flows through the converter and the thermoelectromotive force stabilizes. The measurement of
Since it is performed in the order of ppm (parts per millidn), it is extremely difficult to match the thermoelectromotive force to the same level as that.

Therefore, since there is fluctuation in the precision digital voltmeter in practice, the sensitivity of the precision digital voltmeter is reduced to make them almost equal.

(Means for Solving Problems) In order to solve the above problems, in the present invention, when comparing the AC / DC difference of two thermoelectric type AC / DC converters connected in series or in parallel, Measure the thermoelectromotive force when an AC rated current or voltage is applied to the thermoelectric AC / DC converter, and then measure the average value of the thermoelectromotive force when a DC current or voltage is applied in the forward or reverse direction. From these, the deviations from zero of the value obtained by normalizing the difference between the thermoelectromotive force at the time of applying the AC and the average value of the thermoelectromotive force at the time of applying the DC to two or more points are obtained. Further, the difference between the AC / DC difference of the two thermoelectric AC / DC converters corresponding to these is measured, and the relational expression between the deviation degree and the AC / DC difference obtained as described above is obtained, and these relational expressions are further calculated. The difference in AC / DC difference when the average value of the thermoelectromotive force when AC is applied is equal to the average value of the thermoelectromotive force when DC is applied. Provided is a precise comparison method for thermoelectric AC / DC converters.

In addition, the relational expression between the deviation degree and the difference between the straightness and straightness is represented as a linear function or a higher-order function by the least-squares method by graphically showing the difference between the deviation degree set at two places and the difference between the straightness and difference corresponding to this. Be done.

It should be noted that the graphical representation of the deviation degree and the difference of the straight-line difference is performed, for example, by taking the deviation degree on the horizontal axis and the difference of the straight-line difference on the vertical axis.

There are no particular restrictions on the number of points where the degree of deviation is set, but it is preferable that the average value of DC electromotive force is approximately equal to AC thermoelectromotive force, that is approximately several tens of ppm, and that where it is several tens of ppm. Set each to a small area and measure.

(Operation) That is, in the present invention, in the adjustment of the thermoelectromotive force before the comparative measurement as in the conventional case, the thermoelectromotive force is not equal to the AC thermoelectromotive force of the thermoelectric AC / DC converter, and the DC thermoelectromotive force is not set. The degree of deviation between the thermoelectromotive force when AC is applied and the average value of the thermoelectromotive force when DC is applied is arbitrarily set at two or more places, and the difference between this deviation and the corresponding AC / DC difference between the two thermoelectric converters is set. The relational expression between the degree of deviation and the difference between the AC and DC is calculated graphically, and from this relation, the difference between the AC and DC difference when the average value of the thermoelectromotive force when AC is applied and when the average value of the thermoelectromotive force when DC is applied is calculated. Therefore, the operation of equalizing the thermoelectromotive forces when an alternating current and a direct current are made to flow can be performed extremely easily and accurately in a diagram.

Therefore, according to the present invention, it is possible to perform precision measurement by removing the drift of the precision power source and the fluctuation of the precision digital voltmeter.

(Example) As described with reference to FIG. 2, the switches S ₁ , S ₂ , S ₃
AC current, DC current forward, same direction and AC current are sequentially applied, and this cycle is repeated several times, and thermoelectric power of TCC1 and TCC2 is measured by two precision digital voltmeters DVM1 and DVM2 each time. . This gives the average E _a1 , E _a2 , E
_{Find d1} and E _d2 .

After that, the gradients ΔE _d1 / ΔI _d and ΔE _d2 / ΔI _d of the input current thermoelectromotive force characteristics of the converters TCC1 and TCC2 are measured by the switch S ₄ , and the difference between the AC / DC difference (δ _1- δ
₂ ) is calculated.

Here, Fig. 1 shows the thermoelectromotive force E _a of TCC1 when an AC rated current (voltage) is applied to the converter, and the thermoelectromotive force Ea is applied when a DC current (voltage) is applied in the forward and reverse directions. when the, degree of deviation ^- Ed ⁺ and Ed of ^{_{{E a - (Ed + +}} Ed -) / 2)} / (Ed + + E
d ^-) / 2 The scale on the horizontal axis, also the difference of the AC-DC difference and the vertical axis ([delta] ₁
-Δ ₂ ) is a scaled representation, and both axes have units.
ppm.

At the point on the right side of the vertical axis in the figure, the groups are those in which both thermoelectromotive forces are made substantially equal {E _a to (Ed ⁺ + Ed ⁻ )}.

The group of points on the left side of the vertical axis is (Ed ⁺ ＋ Ed ⁻ ) / 2 from E _a about 10 pp
This is when m is set large. Rightmost ones than E _a ^- those of Lowering by / 2 about 40ppm (Ed ^{+ +} Ed).

For the measurement, an alternating current, a direct current in the forward direction, a reverse direction in the same direction, and an alternating current (this is called one cycle) are applied continuously,
The difference in the AC / DC difference is calculated by measuring the thermoelectromotive force at each time, and one black circle is the average of 3 cycles. The horizontal axis is a scale of the deviation degree in the first cycle.

Next, referring to FIG.
The empirical formula is obtained as a linear function or a higher-order function by the method of least squares.

In this example, the empirical equation is a linear equation y = 0.017x + 1.4
2. In the quadratic equation, y = -4 × 10 ^-4 x ² + 0.014x + 1.50.

In addition, setting the thermoelectromotive force to E _a = (Ed ⁺ + Ed ⁻ ) / 2 means obtaining an intersection, that is, an intercept, between the vertical axis and the empirical formula in the figure. The intercepts of the equation and the quadratic equation agree within the repeatability of measurement.

There are various empirical formulas such as a straight line, an upward convex, and a downward convex, but in order to express the empirical formula as a linear function, the above deviation degree may be set at two positions sandwiching the vertical axis. To express the empirical formula with a higher-order function, the deviation degree may be set at three or more times.

(Effects of the Invention) In summary, according to the present invention, the operation of equalizing the thermoelectromotive forces when an alternating current and a direct current are made to flow is extremely easy,
And it can be done accurately, drift of precision power supply,
Precise measurement is possible without the fluctuation of precision digital voltmeter.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing the deviation of the thermoelectromotive force at the time of applying an alternating current and the average value of the thermoelectromotive force at the time of applying a direct current, and the difference in the AC / DC difference between two thermoelectric converters, which is illustrated by the present invention. FIG. 2 is a circuit diagram for precise comparison of two thermoelectric converters.

Claims (1)

[Claims] 1. When comparing the AC / DC difference of two thermoelectric type AC / DC converters connected in series or in parallel, when an AC rated current or voltage is applied to the two thermoelectric type AC / DC converters. Measure thermoelectromotive force, then forward DC current or voltage,
Measure the average value of thermoelectromotive force when flowing in the reverse direction, and from these, calculate the difference between the thermoelectromotive force when AC is applied and the average value of thermoelectromotive force when DC is applied as the average value of thermoelectromotive force when DC is applied. Degree of deviation from zero of the standardized value is obtained at two or more places, and the difference in the AC / DC difference between the two thermoelectric type AC / DC converters corresponding to these is measured, and the degree of deviation obtained as described above is obtained. The relational expression with the difference of the AC / DC difference is obtained, and the difference between the AC / DC difference when the average value of the thermoelectromotive force when AC is applied and when the average value of the thermoelectromotive force when DC is applied is equalized is calculated from these relational expressions. Precision comparison method for thermoelectric AC / DC converters.