RU2210782C2 - Converter of root-mean-square value of voltage - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: proposed converter is intended to measure level of AC voltage and is incorporated in AC voltmeters ( or universal voltmeters ). It can also be utilized in facilities measuring and stabilizing root-mean-square value of voltage or current, for instance, in calibrators. Its employment is advantageous in measuring facilities of medium ( 0.1-0.5% ) and high ( under 0.1% ) accuracy when achievement of better metrological characteristics is required. EFFECT: increased accuracy of converter. 1 cl, 2 dwg

Description

 The invention relates to electronics. The main field of application is measurement technology. It is used to measure the level of AC signals.

где U - среднеквадратическое значение напряжения, u(t) - мгновенное значение измеряемого сигнала и Т - период измерения (усреднения).Known converters rms values of alternating voltage [1], [2]. Their principle of operation is based on the equation:

where U is the rms voltage value, u (t) is the instantaneous value of the measured signal and T is the measurement period (averaging).

 The first known device [1] contains a quadrator, a low-pass filter (averaging device) and a square root calculator.

 The second known device [2], which is the closest technical solution to this invention, contains a quadrator, an averaging device, an analog-to-digital converter with a DC voltage source and a digital device for calculating the square root. The quadrator input is connected to the input terminal, and the output to the input of the averaging device, which is connected to the input of an analog-to-digital converter, whose output code value, which expresses the ratio of the input voltage to the reference voltage, is fed to the input of a computing device that outputs the measured rms value in the form of a digital code.

 A disadvantage of the known devices is the low accuracy, determined primarily by the parameters of the quadrator. The averaging device usually does not introduce errors, and the square root calculation errors in the known device [2] are eliminated by selecting an analog-to-digital converter and a digital computing device with sufficient resolution and accuracy.

 The aim of the invention is to improve the accuracy of measurement (conversion) of alternating voltage.

 This goal is achieved by adding a digital-to-analog converter, the input of which is connected to the output of the computing device, the first analog switch at the input of the quadrator, connecting its input to the input terminal or to the output of the digital-to-analog converter, and the second analog switch switching the filter capacitor, changing the constant the time of the averaging device, the third analog switch at the input of the analog-to-digital converter, connecting its input to the output of the averaging device, or Exit-analog converter, the analog switch control device and a computing device.

 The converter circuit is shown in the drawing (Fig. 1).

The converter contains a series-connected quadrator 1, an averaging device 2, an analog-to-digital converter (ADC) 3 with a reference voltage source 4, and a computing device 5. The quad input is connected to the output of the analog switch 6, through which the measured voltage Ux or output voltage Uc is applied to it digital-to-analog converter (DAC) 9. The analog key 7 provides a change in the time constant of the averaging device. Through an analog switch 8, the output voltage U ^{2 of the} averaging device or the output voltage of the digital-to-analog converter, which are compared with the reference voltage Ur, is supplied to the input of the analog-to-digital converter. The control device 10 generates signals that set the analog keys and the operating mode of the computing device to the required state.

 The converter operates as follows.

 The control circuit provides the formation of 2N + 1 states (phases) of the converter operation (hereinafter we will call them the 0th, 1a, 1b, 2a ... Na and Nb phases).

Вычисленное значение пропорционально величине входного напряжения, отличаясь на коэффициент передачи квадратора, и содержит составляющие погрешности, свойственные реальному квадратору, например, за счет нелинейности.In the 0th phase, the measured voltage is applied to the input of the quad, the averaging device is turned on with such a time constant to provide the necessary suppression of ripple of the output voltage of the quad at the lowest frequency of the measured signal. The output voltage of the averaging device is supplied to (ADC). The computing device, using the measured ADC value U0, determines the value of the measured signal Ux as:

The calculated value is proportional to the value of the input voltage, differing by the transfer coefficient of the quadrator, and contains the error components inherent in the real quadrator, for example, due to non-linearity.

 The subsequent phases of the converter operation provide the determination of the conversion coefficient and the correction of other errors. Moreover, the calibration of the converter quadrator (and this is the calibration operation) is performed at a level approximately equal to the measured voltage acting at the input at this moment, which significantly reduces the measurement error.

 To carry out the calibration, the input of the quadrator 1 is connected to the output of the DAC 9 by the analog switch 6. The analog switch 7 is opened, decreasing the time constant of the averaging device so that it is possible to measure the DC voltage supplied to the input of the square 1 at maximum speed. This state of the circuit remains unchanged in the progress of all calibration phases from 1a to Nb. The state of the analog switch 8 at this time changes, providing the ADC 3 the ability to measure the voltage at the output of the DAC 9 (phase with index a) and at the output of the averaging device (phase with index b). The change in the state of the converter circuit occurs under the influence of the signals of the control device 10.

From the output of the computing device, N code values are supplied to the input of the DAC, providing the reproduction of N calibration levels at its output. Each calibration level Uc operates during two working phases (with indices a and b) and is calculated as:
Uс _{1 ... N} = Ux • M _{1 ... N} ,
where M ₁ , M ₂ ... M _N - weighting coefficients that describe the expected law of distribution of the measured signal values (the shape of the measured signal). For example, for a sinusoidal signal at N = 8, it is convenient to use the following coefficient values: -1.4166, -1.0833, -0.75, -0.5, 0.5, 0.75, 1.0833, 1.4166 . They can be determined by other criteria that provide the best correction for the nonlinearity of the quadrator (or the entire converter). The choice of the weight coefficient also occurs according to the signal of the control device 10. Moreover, the order of application of the coefficients does not matter.

Коэффициент калибровки квадратора (преобразователя) К определяется как среднее значение масштабных коэффициентов K₁ ... K_N, полученных для каждого калибровочного уровня:

По завершении всего измерительного цикла производится вычисление результата с учетом коэффициента калибровки:
U=U_x•К
Результат в цифровом виде подается на выход преобразователя. Далее управляющее устройство приводит схему в состояние, соответствующее 0-й фазе, и снова повторяет весь измерительный цикл.In each of the calibration phases, voltage values supplied to the input of quadrator 1 (Ula ... UNa) and received at the output of the averaging device 2 (Ulb ... UNb) are supplied to the computing device from the ADC 3. Thus, it is possible to determine the transfer coefficient of the quadrator (transducer) K ₁ ... K _N for each calibration level:

The calibration factor of the quadrator (transducer) K is determined as the average value of the scale factors K ₁ ... K _N obtained for each calibration level:

Upon completion of the entire measurement cycle, the result is calculated taking into account the calibration coefficient:
U = U _x • K
The result is digitally fed to the output of the converter. Next, the control device brings the circuit to the state corresponding to the 0th phase, and again repeats the entire measuring cycle.

 It should be noted the special role of the reference source 4, which, only at first glance, does not carry any additional function, besides ensuring the operability of the ADC 3 circuit.

 It lies in the fact that the dimension of the constant voltage reproduced by him is transferred to the dimension of the measured alternating voltage. The transmission medium is the ADC, which acts as a measure of the ratio, and the calibration of the converter (conversion scale) is reduced to one simple operation - setting the level of the reference source 4.

 The DAC 9 can also play the role of an equivalent reference source of constant voltage. However, the practical implementation of such a solution is impractical - the DAC circuit is complicated, its calibration is complicated (it becomes a multi-valued voltage measure that needs to be calibrated at many points). At the same time, very high requirements for the parameters of the ADC remain. In the proposed version of the converter, the DAC should provide only short-term stability of the output voltage, sufficient for two measurements - at the input and output of the quad.

 Thus, in this device, the accuracy of measuring AC voltage is provided only by the parameters of the reference source 4 (accuracy and stability of the output voltage) and ADC 3 (linearity, dynamic range and resolution). The most critical unit of the converter - quadrator 1, as a means of converting AC voltage to DC, is excluded from the transmission circuit of the dimension of voltage.

 Compared with the prototype [2], in which the parameters of the quadrator directly affect the conversion coefficient, the proposed transducer scheme can significantly improve the measurement accuracy. When implementing the converter, the error is reduced by an order of magnitude on average when using quadrators of the same type. The only significant factor limiting the possibility of a complete correction of the error of the quadrator is its random (noise) component of the error.

In all cases of application, the proposed scheme makes it possible to abandon the careful selection of quadrator elements and reduce the requirements for them (initial displacement, asymmetry, temperature stability, and temporary drift). It is most effective:
- to build precision converters when the parametric accuracy of the existing quadrators is not enough:
- in devices operating in a wide temperature range, making it possible to compensate even the strong dependence of the parameters of the quadrator on the ambient temperature;
- in high-frequency measuring devices, when there is no possibility of using high-quality quadrators due to unsatisfactory frequency properties.

When using this converter in measuring instruments of high accuracy, for example voltmeters, it is possible:
- reduce the volume of calibration operations (reduce the complexity) due to the fact that the Converter, in fact, is calibrated automatically;
- Replace complex AC calibration with a more accurate DC calibration and achieve additional accuracy improvements.

It should be noted that the fees for improving accuracy are:
- additional hardware costs for improving the converter compared to the analogue [2], but they are insignificant - the cost increases by about a quarter or may not increase if the requirements for the element base are accordingly reduced;
- a decrease in speed by an average of half, however, it can be restored by turning off the calibration of the quad, and then the circuit goes into the same mode of operation as the known device [2]. Another possibility to restore the measurement speed may be, for example, the use of two parallel channels with alternating calibration;
- complication of the computing and control devices, but this is only the cost of programming a microprocessor device, since otherwise (hardware) it is difficult to implement the described algorithm for the operation of this converter.

 In the second embodiment, the converter described above contains the third additional input of the third analog switch, which allows you to connect the ADC input to the second additional input terminal. This addition makes it possible to measure DC voltage with higher accuracy, bypassing the quad. However, the purpose of this supplement is not to increase the accuracy of measuring DC voltage, which is obvious, but to calibrate the reference ADC source. At the same time, by applying a reference level of constant voltage to the converter input, by recalculating the ADC readings, you can calculate the actual value of the reference source, that is, calibrate it, than to provide a more accurate measurement of the alternating voltage in the future. Secondly, such a converter can operate in a mode of comparing (comparing) an alternating voltage with a constant voltage. In this case, the measurement error will be formed only due to the error (non-linearity) of the ADC, and the error of the reference source of the ADC is excluded. Thus, if the invention in the first paragraph is an RMS converter for a voltmeter, then the invention in the second paragraph is already an RMS converter for a voltmeter-voltage comparator. The converter circuit according to the second paragraph is shown in the drawing (Fig. 2).

 The presence of a second terminal for comparing is optional. The compared voltages can be applied to the combined input alternately.

The converter operates as a voltage comparator by alternating measurement cycles:
- AC voltage at the first input terminal in the same way as in the first version of the circuit;
- DC voltage at the second input (or combined) terminal in the third position of the key 8.

где Us - значение эталонного постоянного напряжения, с которым сравнивается переменное.According to the results of two measuring cycles in which the values of the alternating Uac and constant Udc voltages are determined relative to the internal reference source, the actual value of the alternating voltage applied to the input of the converter is calculated:

where Us is the value of the reference constant voltage with which the variable is compared.

Literature
1. W. Titze, L. Schenk. Semiconductor circuitry. M .: Mir, 1982, p. 473.

 2. AC VOLTMETER B3-71, B3-71 / 1. Technical description and instruction manual. Part 1, 1998, p.10 (Fig. 5.1) and p.13 (Fig. 5.2).

Claims (2)

 1. The RMS voltage converter, comprising a quadrator, an averaging device at its output, an analog-to-digital converter with a DC voltage reference source, and a computing device that determines the square root value, the input of which is connected to the output of the analog-to-digital converter, and the output to the output a converter, characterized in that a digital-to-analog converter is introduced, the input of which is connected to the output of the computing device, the first is introduced at the quad input an analog key connecting it to the input terminal or output of the entered digital-to-analog converter, a second analog key is entered to change the time constant of the averaging device, a third analog key is entered at the input of the analog-to-digital converter, connecting it to the output of the averaging device or the output of the digital-to-analog converter, a control device is introduced the outputs of which are connected to the control inputs of analog keys and a computing device.  2. The Converter according to claim 1, characterized in that an additional input of the third analog switch is introduced, which allows you to connect the input of the analog-to-digital converter with a second input terminal or a combined terminal.

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