SU1757095A1 - Method for comparation of alternating current from root-mean-square value - Google Patents

Abstract

The invention relates to a pulse technique and can be used to build variable voltage comparators. The purpose of the invention is to improve the accuracy by eliminating the constant component of the alternating voltage. For this purpose, the reference voltage is formed in a certain way so that it has the same constant component as the alternating comparable voltage. Upon subsequent generation of the difference between these voltages The wages are simply excluded, 3 Il.

Description

The invention relates to a measurement technique and can be used to compress, measure, maintain a given variable voltage of arbitrary shape.

There is a method of comparing the rms voltage, implemented in the device and based on the inversion of the measured voltage, the formation of the measured and inverted voltages, by alternately switching them to the input of the rms converter (PCPS) based on an electrothermal transducer ( ETP), the synthesized voltage, conversion using the PSCS based on the ETP of the synthesized voltage to a constant voltage, comparing this constant voltage with the set This voltage is obtained by means of a root-mean-square conversion of the reference constant voltage.

In the method, there is no error of the ESCP based on the ETP due to its asymmetry, but there is a measurement error due to the difference in the rms value of the inverted and measured voltages. . The error due to the non-ideal inversion increases with increasing frequency of the alternating voltage and at high frequencies (10 kHz), may exceed the error due to asymmetry, which is relatively small in the ESCR based on ECP (0.1-0.5%).

The closest to the present invention is a method of comparing alternating voltage according to RMS value, implemented in the device and including the formation of a reference two-pole voltage of the type meander, conversion using the PSCS based on ETP

variable voltage and reference two-voltage voltage in constant voltage, corresponding to their mean-hedratic values, fixation (measurement) of said voltage, compared with each other, according to the results of which the conclusion is obtained that the voltage and reference voltage are equal or predominance of one of them.

In the known method there is no need to invert the alternating voltage, but the error of comparing due to the asymmetry of the PSCS based on the ECD is absent only in the absence of a constant component in the input alternating voltage.

In the known method, when comparing a voltage of an arbitrary shape, the error of comparing due to asymmetry will be the greater, the larger the constant component in the RMS value of the voltage of an arbitrary shape. The maximum value equal to the asymmetry error of the PSKZ at a constant voltage, this component of the error will reach, when the variable component in the voltage of an arbitrary shape is close to zero, and the constant is maximum.

As is known, the error in the asymmetry of the PSKZ on the basis of the ETP depends on the type of PSKZ used. For example, in the PNTB type PSCS, the asymmetry error at a constant nominal voltage does not exceed 0.1% of the output voltage. For input voltages lower than the nominal voltage, the asymmetry error increases. For highly accurate measurements, however, it is necessary that the effect of asymmetry on the comparing error be less than 0.01%.

The purpose of the invention is to improve the accuracy of comparing.

The goal is achieved by the fact that in the method of comparing voltage by the root-mean-square value, including the formation of a rectangular two-pole reference pulse voltage with equal amplitudes of different-polarity pulses, the conversion of a variable voltage of the alternating voltage and the aforementioned reference voltage into constant voltages rms values, fixation (measurement) of the mentioned constant stresses, their comparison with each other, according to the results of which the derivation of the equation ve RMS kompariruemogo and reference voltages

or the predominance of one of them, prior to conversion to a constant voltage, the constant component Do

voltage, determine the ratio of the specified constant component Do to the amplitude A of the reference voltage and establish the ratio of the durations of the pulses of positive m + and negative m polarities in the reference voltage based on the expression

15

g + -7- uo

T And

where T is the period of the two-pole reference voltage,

FIG. 1 shows a block diagram of a voltage comparator implementing

the proposed method; in fig. 2 - stress diagrams for his work; Fig. 3 shows experimentally the dependences of the asymmetry error of the PSCP on the ratio of the constant component and the root mean square value of the input voltage.

A comparator (Fig. 1) that implements the method according to the invention contains a PSKZ 1 based on an ETP, indicator 1, connected to the input

PSKZ 1, switch 3, output connected to the PSKZ 1 input, and the first input - to the input bus, the source of 4 DC voltages of positive and negative polarity, switch 5,

through which the outputs of source 4 are connected to the second input of switch 3, filter b, the input connected to the input bus, the converter 7 voltage-time ratio, the first input connected to the output of filter b, and the second to one of the inputs, for example, positive, source 5 ,

The method is carried out as follows.

The compressed variable voltage is converted into a constant value corresponding to its rms value. For this purpose, the indicated voltage is supplied to the comparator PSKZ 1 through the switch 3, which at the initial moment of time is in the AB position. The obtained DC voltage value at the output of the PSKK 1 is captured (measured) with the aid of indicator 2 and stored (either by the operator or automatically in the display 2). Simultaneously with these operations, a two-pole reference voltage is formed, for which purpose, the constant component Uo (figure 26) is extracted from the compressed alternating voltage (Fig. 2a) before converting it to a constant voltage corresponding to the RMS value. . This is done with the help of low-pass filter 6 by applying a voltage from the input bus to it. At the same time, two constant voltage levels A of positive and negative polarity are formed using source 4. Find the relation Do to A, by

specified relationship

Uo

BUT

set

the ratio of the pulse durations of positive g + and negative T-polarities formed by switching the output voltage of source 4 with a switch 5. For this implementation of the method, after transmitting voltage 7 to filter converter inputs 6 and one of the outputs source 4 at the output of converter 7, having a characteristic described by formula 1, receive a unipolar pulse voltage with a follow-up period equal to T and a pulse duration equal to rm (Fig. 2d)

The said unipolar pulse voltage is used as a control for switch 5, thereby switching the outputs of source 4 and thus obtaining at the output A of switch 5 a two-pole reference voltage (2B) with a ratio of g + and T- defined as follows

G + -G- Up TA

The frequency of this following

On the one hand, it should not be greater than the minimum frequency of use of the PSKZ, but on the other hand, it should be small in order to avoid errors in the task

nor the reference stress.

The reference voltage generated by this method is converted into a constant voltage corresponding to its rms value, for which the switch 3 is switched to the AB position and the output voltage of the switch A of switch 5 is fed to the PSKZ 1. Then the indicator 2 is fixed (measured) the constant voltage U2 corresponding to the rms value of the reference voltage, after which the voltage value 1) 2 is compared with the previously recorded UL value. According to the results of the comparison, the operator makes the output nstve or prevalence of one of said values. If the rms value of the arbitrary waveform on the input bus is to be measured, then by changing the magnitude of the reference voltage, the readings Ui and LJ2 of indicator 2 are equal, and the voltage on the input bus is judged on the scale of source 4.

The reference voltage established in the proposed method (Fig. 2c) is described by the formula:

U A at 0 t t +; U -A at.

it has a rms value, equal to A regardless of the ratio of the durations of the pulses of positive and negative polarity, and a constant component U 0, described by the formula

U 0 A

T + -T A

2G + -T

0

five

0

five

0 5

It follows from this formula that by changing the ratio between g + and t - it is possible to change the value of U o from -A to A. the root mean square value A. At the same time, there is no change in the mean square value of the reference voltage. As a result, the PSKT error due to asymmetry in the conversion of both compared stresses, equivalent output values will be the same, and there will be no comparing error due to asymmetry.

A bipolar rectangular voltage with the same amplitudes of positive and negative polarity pulses was used as the input voltage for the PSKD (Fig. 2c). The constant component level at the input voltage was varied by changing. The ratio between the durations of the pulses of positive and negative polarities and was measured by a voltmeter after filtration. The rms input voltage level was 1 V. The input voltage following frequency was 200 Hz.

The asymmetry error b is defined as

6

E + -EE +

where E +, E --- are the PSKZ input voltages, respectively, when the PSKZ is directly and reversely connected to the input voltage source (the direct and reverse connections were achieved by changing the PSKZ input terminals when connected to the terminals of the operating voltage source), From the analysis the data shown in fig. 3 implies that in the absence of a constant component, there is no asymmetry in the input voltage of the PSCK; with an increase in the ratio of -r--.- asymmetry of the PSCP

Uc.K.3.

increases, the asymmetry of the PSKZ maximum

pgi, .. Up - when, .- I.

UC.K h.

In the proposed method, it is not necessary to very strictly establish the correspondence between the values of the constant components in the reference voltage and the voltage of an arbitrary shape, since the difference between the asymmetry errors at the input voltages of the same rms value, but with a small difference in the constant component , small and will not lead to a large error of comparison.

Thus, in the proposed method, there is no inversion of the voltage of an arbitrary shape, and consequently, the error associated with this operation,

The proposed method does not have an error due to the asymmetry of the PSKK with any arbitrary form of alternating voltage.

The formation of the reference voltage in the proposed method is associated with a change in the ratio of the pulse durations of the positive and negative polarities of the reference voltage. When using reference voltage with

The low frequency of the current element base allows this operation to be performed without changing the RMS value of the reference voltage, and, therefore, this operation will not cause an almost increase in the error of comparing.

Claims (1)

Claim Comparison Method voltage rms, including the formation of a reference two-pole pulse voltage of rectangular shape with equal amplitudes of different polarity pulses, converting the compressed alternating and said reference voltages into constant voltages corresponding to their rms values, measuring said direct voltages, comparing them to each other and by comparing the results of obtaining a conclusion about the equality of the rms values of the comparable and the reference stresses or about the predominance of one of them, characterized in that, in order to improve the accuracy, before the conversion to the constant voltage of the reference voltage they release a constant the component of the compressed alternating voltage, determine the ratio of the specified constant component to the amplitude of the pulses of the reference voltage and establish the ratio of the durations of the pulses in positive and negative polarities in the reference voltage based on the expression t + -t- U  Up where t + t-, T are the duration of the pulses of positive and negative polarities and the period of the reference voltage, respectively; U0 is the constant component of the variable voltage being compressed: A is the amplitude of the voltage reference pulses. Vh f Filter A source the permanent stress Converter Ratio Voltage - Time Input Voltage The output voltage of the filter 6 t 0.8 .1. Well-l.