SU864157A1 - Ac voltage root-mean square digital voltmeter - Google Patents

Description

(54) DIGITAL VOLTMETER MEDIUM SQUADRATE VARIABLE VOLTAGE

one

The invention relates to electrical measuring equipment and is used to construct rms voltmeters of an alternating current voltage with a wide frequency range, including a range of low frequencies.

A digital voltmeter of alternating voltage RMS is known, containing two thermal-resonance converters, each of which is equipped with two heaters and auto-oscillators, a reference frequency generator, a frequency-current converter, two frequency subtraction devices, a counter, a code-current converter, a control device, an indicating device . In this case, the input of the voltmeter is connected to the first heater of the first thermal resonance converter, the code-current converter input is connected to the counter output, and its output is connected to the first heater of the second thermal resonance converter, the second heaters of the thermal-resonance converters are connected to the output of the frequency converter, which connects to the output of the first device frequency subtraction, the first input of the first frequency subtraction device is connected to the reference frequency generator, and the second input with the output of the oscillator the first thermal resonance converter, the outputs of both oscillators are connected to the inputs of the second frequency subtractor, the input of which is connected to the input of the counter, and the input of the display device is connected to the output of the counter 1.

A disadvantage of this device is the presence of an error due to the ripple of the output frequency of the first thermorezion converter at

15 measuring signals at low and infra-low frequencies, which reduces the accuracy of measuring low-frequency voltages and makes it impossible to measure voltages

20 infra-low frequencies.

The purpose of the invention is to extend the frequency range of a digital voltmeter towards infra-low frequencies and to increase measurement accuracy.

25 This goal is achieved by the fact that, in a well-known digital voltmeter of a mean-square value of alternating voltage, containing an indication device, two thermoreement 30 converter converters, outputs

which are connected to the first inputs of the autogenerators, the second inputs of which are connected to the common bus, while the input bus through a resistor is connected to the first heater of the first thermoresonant converter, the control device, the first output of which is connected to the first input of the first counter, code-current converter, the output of which connected to the first heater of the second thermo-resonant converter, the second heaters of the thermal-resonant converters are connected to the outputs of the frequency-current converter, whose input is one with the output of the first subtractor frequencies WMOs this first subtractor frequencies co. It is one with the reference frequency generator, and its second input is with the output of the oscillator of the first thermal resonance converter, the outputs of both oscillators are connected to the inputs of the second frequency subtraction device, additionally entered measurement | period timer, second counter, two one-shot, trigger, comparator, two keys, time interval converter - code and arithmetic unit, the first input of the period meter is connected to the input of the voltmeter, and its output is connected to the input of the second counter, the first input of the comparison device the inputs of both mono-vibrators, as well as the S-input of the trigger, the R-input of which is connected to the output of the second counter, the output of the first one-vibrator is connected to the second input of the comparator, the output of which is connected to the control the first key input, the first and second information inputs of this key are connected to the flip-flop and the second one-oscillator outputs, respectively, and the first switch output is connected to the first input of the converter; the time interval is the code and to the control input of the second key connected between the output of the second frequency subtractor and the first the input of the first counter, the output of which is unified} with the first input of the arithmetic device, the second input of the arithmetic device is connected to the output of the converter, the time interval - the code, the output a The device is connected to the inputs of the display device and the code-current converter, and the output of the control device is connected to the second inputs of the period meter, the first counter and the time interval converter code.

FIG. 1 shows a block diagram of the proposed device; in fig. 2 - time diagrams of the device at infra-low frequency | 1 st input signal.

The device contains two thermal resonance converters 1 and 2 with autogenerators 3 and 4, a generator 5 of the reference frequency, two frequency subtraction devices 8, a frequency-current converter 7, two keys 9 and 10, two counters 11 and 16, a control device 12, a meter period 13, two one-shot 14 and 15, the device 17 comparison, the trigger 18, the Converter 19 time interval code, the arithmetic unit 21, the code-current converter 20, the display device 22.

Thermal resonance converters 1 and 2 together with autogenerators 3 and

4, the frequency calculator 8, the key 10, the counter 11, the arithmetic unit 21 and the code-current converter 20 form the basic automatic balancing system, equalizing the temperatures of the thermistor converters 1 and 2. The reference frequency generator 5, the frequency subtractor 6 and the frequency converter the current 7 forms an auxiliary automatic control loop, by which the temperature of both teEMresonance transducers is maintained at a constant level set by the oscillator frequency 5 o pore frequency.

The above blocks form an automated comparator of simultaneous comparison on thermal resonance transducers 1 and 2, working in the isother of 1, cx 1 mode.

in steady state operation. when measuring signals at low and infra-low frequencies (fig. 2a), the difference frequency D f at the output of the frequency subtraction device 8 becomes pulsiruadei (fig 26) and, as a first approximation, has the form

if ufo Af, slnZUJ t (1)

where Afo is the average value of the frequency Af proportional to the actual value of the measured voltage; Af is the amplitude of the pulsations of the frequency Af, depending on the frequency of the measured voltage W) (and the parameters of the thermal-to-reso nance transducers. To reduce the conversion error caused by the second term in the formula, the count time of the output pulses of the frequency subtraction device 8 at low and infra low frequency input signals Signgshah is chosen equal to or a multiple of the period of the input signal

PT

x

where tr

Claims (1)

the counting time of the output pulses of the frequency subtraction device 8; T - the period of the input signal; n is a certain natural number. For this, when working in the field of low and infra-low frequencies, a device is passed to the counter 11 from the output. 8 pulses during the counting time interval T. The number recorded in the counter at the end of this interval will be equal to N jAiat –J (Afc ufrti9 n luJX) t + WTx since 5inQuuxLdt OH N becomes t proportional to the input signal period and the average value of the difference frequency ufjj, which is proportional to the effective value of the input voltage. To obtain a measurement result that does not depend on T Np (must be divided by a number equal to the counting time interval, the Device operates as follows. Input voltage) through a resistor R to convert the voltage to current IX (t) to the first heater thermo-resonant converter 1 and the input of the period meter 13. Upon a command, counters 11, 16 and the time interval transducer are combined with the control unit 12 - code 18, and the period meter 13 begins measuring the period of the input signal. The period meter 13 generates a pulse with a duration equal to TU, which is fed to the counter 16, one-shot 14 and 15, a comparison device 17 and a trigger 1B. The single-oscillators 14 and 15 produce pulses with fixed durations Tj; and T. The leading edges of both pulses coincide with the leading edge of pulse 1 formed by the period meter 13. The value of Tr (Fig. 2d) is chosen equal to the maximum period of such an input signal for which the error from the ripple of the output frequency of the frequency subtraction device 8. does not exceed the specified value . The trigger 18 is set to one state by a pulse from the output of the period 13 meter. Thus, at the output of the trigger 18, an impulse appears whose leading edge coincides with the leading edge of the pulse formed by the period meter 13 and the duration is equal to pT, de n the conversion factor of the counter 16, the pulse of overflow from the output of the higher bit of which the trigger returns to its original position. Comparison device 17 compares the durations T and T and produces a signal that controls the key 9. When measuring NI-INI signals (T T0), the torsion bar 9 is set to 1 position with the output signal of the comparator device 17, the duration of which is T. nT. When measuring signals in the center and high frequencies (T i Td), the signal from the output of the device of Equation 17, whose duration is equal to T T, where T., is an arbitrarily chosen value; the key 9 is set to position P. The control signal to the key 10 comes from the contact of the key 9. The duration of this signal increases the counting time interval T. and is equal to either Tx, or T, depending on which position the key 9.1 is in. Thus , at the end of the time interval for counting TC, the counter 11 records the number Nj proportional to iHoe Tp and the mean-square value of the input voltage. This number from counter 11 enters. The arithmetic unit 21, into which the number is proportional to the counting time interval T (- |, the time interval formed by the converter is count 19. The arithmetic unit 21 performs the operation of dividing the number NX by the number formed by the time interval - code 19. The result of the calculations, proportional to the mean-square value of the input voltage from the arithmetic unit 21, goes to the display device 22. Thus, when working in the field of averages High voltmeters in the voltmeter maintain high metrological characteristics, and when operating in the low and infralow frequencies, the voltmeter error does not increase with a decrease in the frequency of the input signal, which makes it possible to use it for measuring voltages in a wide frequency range, starting from infralow frequencies. The digital voltmeter of the mean square value of the alternating voltage containing the device. Indications, two thermal resonance converters, the outputs of which are connected to the first inputs 1 autogenerators, the second inputs of which are connected to the common bus, while the input bus through a resistor is connected to the first heater of the first temperature-resonant converter, the control device, the first output of which is connected to the first input of the first counter, code-current converter, the output of which is connected to the first heater the second thermal resonance converter, the second heaters of the thermal resonance converters are connected to the outputs of the frequency-current converter, the input of which is connected to the output of the first device frequency, the first input of which is connected to the reference frequency generator, and the second input is connected to the output of the oscillator of the first thermal resonance converter, the outputs of both oscillators are connected to the input of the second frequency subtraction device, which is different in order to expand the frequency a range in the direction of infra-low frequencies and an increase in measurement accuracy; a second counter, two one-vibrators, a comparator, a trigger, two keys, a time interval converter are added to the voltmeter; - a code and an arithmetic unit; the first input of the period meter is connected to the input of a voltmeter, and its output is connected to the input of the second counter, the first input of the device by comparison with the inputs of both single vibrators, and also to the S input of the trigger, the R input of which is connected to the output of the second counter is connected to the second input of the comparator, the output of which is connected to the control input of the first key; the first and second information inputs of this key are connected to the outputs of the trigger and the second one-vibrator with Respectively, the output of the first key is connected to the first input of the converter; the time interval is a code, and the control input of the second key connected between the output of the second frequency subtraction device and the first input of the first transmitter whose output is connected to the first input of the arithmetic unit; the second input of the arithmetic the device is connected to the output of the converter, the time interval is the code, the output of the arithmetic device is connected to the inputs of the display device and the code-current converter, and the output of the control device with it is uniform with the second inputs of the period meter, the first counter and the converter; the time interval is the code. Sources of information taken into account during the examination 1. Kudr sev E.A. Theoretical and experimental sequences of some possibilities of using piezoelectric crystals in measurement technology. Dissertations, l., LPI, 1974, p. 161 (prototype).