CN100403037C - Method and device for measuring AC electricity - Google Patents
Method and device for measuring AC electricity Download PDFInfo
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Abstract
The present invention relates to a measuring technique, particularly to a method and a device for measuring alternate current, which are used for measuring the frequency and the effective value of single-phase alternating current. The method comprises steps: an AC signal which is measured is rectified into a half wave signal; the half wave signal is rectified into a square signal with the same frequency; the pulse number of the square wave signal is counted, and the peak value of the half wave signal is sampled; the frequency and the effective value of the AC signal which is measured are determined according to the counted value and the sampled value. The measuring device comprises a half wave rectifying circuit which is used for rectifying the input AC signal into the half wave signal, a schmitt trigger circuit which is used for outputting the square wave signal, and a measuring circuit which is used for sampling the peak value of the half wave signal, counting the pulse number of the square wave signal, and determining the frequency and the effective value of the AC signal according to the sampled value and the counted value. The method and the device of the present invention can be used for safely and conveniently measuring the frequency and the effective value of the single-phase alternating current in a high precision way.
Description
Technical Field
The invention relates to a testing technology in electronics or communication, in particular to a method and a device for measuring alternating current, which can test the frequency and the effective value of single-phase alternating current.
Background
In the field of electronic or communication, it is necessary to use ac as an energy source for a system or a device to ensure the system or the device to work normally, so as to realize its function, the general ac is sinusoidal ac, and the ac voltage can be expressed as u (t) ═ a × sin (2 pi f × t), where: a is the peak value of the AC voltage, and the effective value of the AC voltage is
Average value of voltage of
The alternating voltage has a frequency f. The frequency and effective value of the sine alternating voltage or current and other parameters are detected, and the method can be used for power supply monitoring and component protection.
The first prior art is as follows: chinese patent No. 02289437 discloses a power information measuring card for measuring the magnitude of three-phase ac voltage and current, which converts 220V commercial power into a dc voltage signal of about 0 to 8V by using a current and voltage sensor, then measures the voltage and current values of the dc signal, and then calculates the effective values of the ac voltage and current by using the measurement results. The measuring circuit utilizes the resistor and the capacitor to complete conversion from an alternating current signal to a direct current signal, and meanwhile, the voltage division and the filtering effect are achieved. The disadvantages of this measuring circuit are: when an alternating current overvoltage or overcurrent is caused by accidental conditions such as lightning stroke, the measuring circuit is easily damaged and fails. Moreover, the voltage of 220V is directly introduced into the measuring circuit, and the personal safety of an operator can be damaged once the measuring circuit is operated improperly.
The second prior art is: chinese patent application No. 02107167 discloses a method and apparatus for measuring three-phase ac frequency, which obtains the angular velocity of voltage or current rotation vector by sampling voltage or current signal, and then converts the frequency by using the angular velocity of the rotation vector. Although the measuring device has strong anti-interference capability, the measuring device has the disadvantages of complex circuit structure, a large amount of complex floating point operation is needed when the angular velocity of the rotation vector is used for converting the frequency, the performance requirement on an ADC (analog-to-digital converter) is relatively high, and the measuring device is specially used for measuring the frequency of three-phase alternating current and cannot be used for measuring the civil single-phase 220V alternating current commercial power.
Disclosure of Invention
The invention provides a method and a device for measuring alternating current, which are used for effectively measuring the frequency and the effective value of single-phase alternating current, and the measuring method is realized by the following steps:
a: rectifying the tested alternating current signal into a half-wave signal;
b: shaping the half-wave signal into a square wave signal with the same frequency;
c: counting the pulse number of the square wave signal, and sampling the peak value of the half-wave signal;
d: determining the frequency and effective value of the tested alternating current signal according to the pulse number of the square wave signal and the peak value of the half wave signal in the step C; the frequency of the alternating current voltage signal to be measured is the same as that of the square wave signal, and the effective value of the alternating current to be measured is in direct proportion to the peak value of the sampled half-wave signal.
Before sampling the peak value of the half-wave signal in the step C, the method further comprises: integrating the half-wave signal.
Before the step A, the method further comprises the following steps: and (4) the tested alternating current signal is subjected to voltage reduction and isolation.
The pulse number of the square wave signal is the average value of multiple statistical values, and the peak value is the average value of multiple half-wave sampling values; the frequency of the tested alternating current signal is equal to that of the square wave signal, and the effective value is in direct proportion to the peak value of the half wave signal.
An apparatus for measuring alternating current, comprising:
a half-wave rectifier circuit: rectifying an input alternating current signal into a half-wave signal;
schmitt trigger circuit: outputting a square wave signal under the trigger of the half-wave signal;
a measurement circuit: sampling the peak value of the half-wave signal, counting the pulse number of the square wave signal, and determining the frequency and effective value of the alternating current signal according to the pulse number of the square wave signal and the peak value of the half-wave signal.
The measuring circuit also comprises an integrating circuit for integrating the half-wave signal, and the integrating circuit is connected between the output end of the half-wave rectifying circuit and the input end of the measuring circuit.
The isolation transformer is used for reducing the voltage of the alternating current signal, a primary side coil of the isolation transformer is connected with the alternating current signal after being connected with a first resistor in series, a secondary side coil of the isolation transformer is connected with a second resistor in series, one end of the second resistor is grounded, and a non-grounded end of the second resistor is connected with the input end of the half-wave rectification circuit.
The half-wave rectifying circuit comprises a first operational amplifier, the inverting input end of the first operational amplifier is connected with the non-grounding end of the second resistor through a third resistor, the non-inverting input end of the first operational amplifier is grounded through a fifth resistor, and the output end of the first operational amplifier is connected with the inverting input end through a fourth resistor; or,
the half-wave rectification circuit is a bridge consisting of four diodes.
The Schmitt trigger circuit comprises a second operational amplifier, the inverting input end of the second operational amplifier is connected with the output end of the half-wave rectification circuit, the non-inverting input end of the second operational amplifier is respectively connected with the positive power supply through a sixth resistor, is grounded through a seventh resistor and is connected with the output end through an eighth resistor, and the output end outputs the square wave signal; or,
the Schmitt trigger circuit is an integrated Schmitt trigger, the input end of the Schmitt trigger circuit is connected with the output end of the half-wave rectifying circuit, and the output end of the Schmitt trigger circuit outputs the square wave signal.
The measuring circuit comprises an analog-to-digital converter for sampling the half-wave signal; and the singlechip is used for counting the pulse number of the square wave signal and determining the frequency and effective value of the alternating current signal according to the sampling value and the counting value.
The alternating current measuring method and the alternating current measuring device can conveniently and safely measure the frequency and the effective value of an alternating voltage signal, particularly provide a measuring device with higher precision for civil single-phase sinusoidal alternating current, can also carry out accurate measurement by adding an integrating circuit when the measured signal has distortion, can improve the safety of a measuring circuit, reduce low-frequency distortion, inhibit high-frequency harmonic waves and improve the accuracy of a measuring result by using an isolating circuit, and can realize voltage reduction measurement by a transformer at the same time, thereby ensuring the personal safety of operators.
Drawings
FIG. 1 is a block diagram of an embodiment of an apparatus for measuring alternating current according to the present invention;
FIG. 2 is a schematic diagram of the waveform of the AC voltage to be measured, the waveform of a half-wave signal and a square-wave signal;
FIG. 3 is a circuit diagram of an apparatus for measuring AC power according to an embodiment of the present invention;
fig. 4 is a circuit structure diagram of an apparatus for measuring alternating current according to an embodiment of the present invention.
Detailed Description
As shown in fig. 1, fig. 1 is a circuit structure block diagram of the present embodiment, including an isolation circuit for stepping down a detected ac voltage signal; a half-wave rectifier circuit: rectifying an input alternating current signal into a half-wave signal; schmitt trigger circuit: outputting a square wave signal under the trigger of the half-wave signal; a measurement circuit: sampling the peak value of the half-wave signal, counting the pulse number of the square wave signal, determining the frequency and effective value of the alternating current signal according to the sampling value and the counting value, reducing the voltage of the alternating current voltage signal to be measured through an isolation circuit, rectifying the alternating current voltage signal through a half-wave rectifying circuit to form a half-wave signal of which the amplitude is in direct proportion to the amplitude, triggering a Schmitt trigger to output a square wave signal with the same frequency by the half-wave signal, wherein the waveform relationship is shown in figure 2 and is described in detail in the following specific embodiment.
As shown in fig. 3, the first embodiment specifically includes:
1. an isolation circuit:
the isolation circuit generally adopts a low-power isolation transformer, such as: the primary side coil of the isolation transformer is connected with the first resistor in series and then is connected with the alternating current signal, the secondary side coil of the isolation transformer is connected with the second resistor in series, one end of the second resistor is grounded, a non-grounded end of the second resistor is connected with the input end of the half-wave rectification circuit, the primary side of the transformer is connected with the detected alternating current signal, the primary side coil of the isolation transformer is connected with the attenuation resistor of 300K omega in series for voltage reduction, the secondary side coil of the isolation transformer is connected with the resistor of 1K omega in series for ensuring that the transformer enters a linear working area, and the ratio of the number of.
If the tested alternating current signal is introduced through the alternating current commercial power transformer, the alternating current commercial power transformer can be directly used as an isolation circuit, the purpose of isolating a tested signal loop and a measuring loop can be met, and an isolation transformer does not need to be added independently.
2. A half-wave rectifier circuit:
the half-wave rectifying circuit comprises a first operational amplifier, wherein the inverting input end of the first operational amplifier is connected with the non-grounding end of a second resistor through a third resistor, the non-grounding end of the second resistor is connected with the ground through a fifth resistor, the inverting input end of the first operational amplifier is connected with the inverting input end through a fourth resistor, an operational amplifier U1 with rail-to-rail (signals can reach positive and negative power supply voltages) output characteristics is selected during practical application and is matched with R3, R4 and R5 to realize proportional amplification, R3 and R4 form reverse amplification, R5 is used for balancing bias current of the operational amplifier, and when the operational amplifier only applies a forward working power supply, the output end outputs a half-wave signal in direct proportion to. U1 (including U2 below) may select a low speed rail-to-rail operational amplifier such as LMC6482 from NS corporation.
The half-wave rectification can also be realized by a bridge (also called a bridge circuit) consisting of four diodes, but compared with the half-wave rectification circuit realized by an operational amplifier, the precision of the bridge circuit is poorer, and the half-wave rectification circuit is suitable for measurement with low precision requirement.
3. Schmitt trigger circuit:
the Schmitt trigger circuit comprises a second operational amplifier, the inverting input end of the second operational amplifier is connected with the output end of the half-wave rectification circuit, the non-inverting input end of the second operational amplifier is respectively connected with the positive power supply through a sixth resistor, is grounded through a seventh resistor and is connected with the output end through an eighth resistor, and the output end outputs the square wave signal; or,
the Schmitt trigger circuit is an integrated Schmitt trigger, the input end of the Schmitt trigger circuit is connected with the output end of the half-wave rectifying circuit, and the output end of the Schmitt trigger circuit outputs the square wave signal.
The method specifically comprises the following steps: the operational amplifier U2 is matched with R6, R7 and R8 to realize Schmitt trigger, and the trigger level calculation formula is as follows:
when the output goes from 0 to 1, the trigger voltage: <math><mrow>
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when the output goes from 1 to 0, the trigger voltage: <math><mrow>
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if the power supply voltage Vcc is 5V, when R6, R7, and R8 are set to 4.7K Ω, 1K Ω, and 20K Ω, V1 and V2 are calculated:
it can be seen that the trigger voltage V1 is less than V2, which can effectively reduce glitch interference and improve accuracy. An integrated Schmitt trigger (such as 74LV123) can be used for replacing an operational amplifier structure, the input end of the Schmitt trigger is connected with the output end of the U1, and the output end of the Schmitt trigger is connected with/INT 0 of the C51 singlechip.
4. A measurement circuit:
the measuring circuit comprises an analog-to-digital converter for sampling the half-wave signal; and the singlechip is used for counting the pulse number of the square wave signal and determining the frequency and effective value of the alternating current signal according to the sampling value and the counting value. When the device is applied specifically, the measuring circuit can adopt a CPU (single chip microcomputer) of an integrated ADC (analog to digital converter), and can also be composed of a discrete ADC circuit and the single chip microcomputer, wherein the ADC circuit part can be a general low-speed ADC, such as ADC0809, and the CPU can be a C51 series single chip microcomputer. The sampling frequency of the low-speed ADC is about 10KHz, which is far higher than the frequency of the 50Hz alternating current commercial power signal, and the requirement of measurement accuracy can be met. If a higher frequency signal is measured, an appropriate ADC is selected for multiple measurements to improve accuracy.
The CPU is internally provided with an interrupt service program, firstly a measurement period is set, when the square wave output by the U2 is turned from Vcc to 0, the CPU is started to start to execute measurement (the mechanism is called as external interrupt of the CPU), in one measurement period, the CPU counts square wave pulses output by Schmidt trigger (the program is called as the interrupt service program), meanwhile, the ADC is started to sample half-wave signals output by half-wave rectification, and the maximum value of sampling is the peak value of the half-wave signal voltage. After the CPU finishes counting the square wave pulse and sampling the half-wave signal peak value, the frequency and the effective value of the tested alternating current signal are calculated according to the following methods:
1. calculating the frequency:
the frequency of the measured AC signal is equal to that of the square wave signal, the statistical pulse number is m, the measurement period is n, and the frequency of the measured AC signal The measuring period can be set according to specific conditions, generally set to be 1 second, so that the number of square wave pulses counted by the CPU is the frequency of the measured alternating current signal, multiple times of counting can be carried out for improving the precision, and finally the average value of the multiple times of counting results is taken as a final result.
2. Calculating an effective value:
if the scale factor of the measurement circuit is B and the peak value of the sampled half-wave signal is M, the effective value of the measured AC signal is
The scaling factor B, the transformation ratio of the transformer, the attenuation resistor R1 connected in series at the primary side of the transformer, and the operationThe gains of the operational amplifiers U1 and U2 are related and are either accounted for or directly measured according to the specific circuit element values. According to different precision requirements, the CPU can control the ADC to sample the peak value of each or several half-wave signals passing through in one measurement period, and then take an average value to calculate an effective value to improve the measurement precision, or sample the peak value of only one half-wave to calculate the effective value.
In a second embodiment, as shown in fig. 4, when the waveform distortion of the ac voltage signal is large, an RC integrating circuit may be connected between the output end of the half-wave rectifying circuit and the input end of the measuring circuit, the RC integrating circuit is composed of a resistor R9 and a capacitor C1, the peak value sampled by the ADC is the average value N of the half-wave signal, and the effective value of the measured ac signal is the average value N of the half-wave signal
To sum up, the following steps are required to complete the measurement of the frequency and the effective value of the ac mains signal:
s1: the isolation of the tested AC mains supply signal loop and the measuring circuit is realized by using an isolation transformer;
the purpose of adopting the isolation transformer to access the tested signal is as follows: firstly, the measuring circuit is prevented from being damaged and failing when overvoltage or overcurrent occurs in the circuit; and the second mode ensures the personal safety of operators. The isolation circuit can be implemented with a 1: 1 low power transformer. The secondary side of the isolation transformer outputs an alternating current signal proportional to the signal under test.
S2: rectifying an alternating voltage signal output by an isolation transformer into a half-wave signal;
the half-wave rectifier circuit may be implemented with an operational amplifier or a diode bridge having a rail-to-rail characteristic. Through the two steps, a sine half-wave signal with an effective value in direct proportion to the effective value of the tested alternating current signal is obtained.
S3: shaping the half-wave signal output by rectification into a square wave signal through a Schmidt trigger circuit;
as shown in fig. 2, in this step, a schmitt trigger is triggered by using the half-wave signal obtained by rectification, so as to obtain a square wave signal having the same frequency as the ac signal to be measured.
S4: counting the pulse number of the square wave signal in the set period, starting the analog-digital converter to synchronously sample the half-wave signal when the falling delay of the square wave signal reaches, and sampling the peak value of at least one half-wave signal.
S5: calculating the frequency and effective value of the measured signal according to the statistical combined sampling result;
the frequency of the square wave signal is equal to: the number of pulses is divided by the statistical period, and the calculation result is the frequency of the measured alternating current voltage signal. The effective value of the measured alternating current is in direct proportion to the peak value of the half-wave signal sampled in the previous step.
The method can also improve the measurement precision by two measures:
1. if the distortion of the alternating current signal is large, the half-wave signal can be filtered by an RC integrating circuit, the average value of the half-wave signal after integration is sampled, and then the effective value of the alternating current signal to be measured is calculated.
2. In step S4, the square wave pulses of multiple periods are counted, the average value is taken as the final result, the peak values of multiple half waves are sampled, and the average value is taken to calculate the effective value of the measured ac signal.
The alternating current measuring method and the alternating current measuring device can conveniently and safely measure the frequency and the effective value of an alternating voltage signal, particularly provide a measuring device with higher precision for civil single-phase sinusoidal alternating current, can also carry out accurate measurement by adding an integrating circuit when the measured signal has distortion, can improve the safety of a measuring circuit, reduce low-frequency distortion, inhibit high-frequency harmonic waves and improve the accuracy of a measuring result by using an isolating circuit, and can realize voltage reduction measurement by a transformer at the same time, thereby ensuring the personal safety of operators.
Claims (11)
1. A method of measuring alternating current, comprising: comprises the following steps:
a: rectifying the tested alternating current signal into a half-wave signal;
b: shaping the half-wave signal into a square wave signal with the same frequency;
c: counting the pulse number of the square wave signal, and sampling the peak value of the half-wave signal;
d: determining the frequency and effective value of the tested alternating current signal according to the pulse number of the square wave signal and the peak value of the half wave signal in the step C; the frequency of the alternating current voltage signal to be measured is the same as that of the square wave signal, and the effective value of the alternating current to be measured is in direct proportion to the peak value of the sampled half-wave signal.
2. A method of measuring alternating current according to claim 1, wherein: before sampling the peak value of the half-wave signal in the step C, the method further comprises: integrating the half-wave signal.
3. A method of measuring alternating current according to claim 1 or 2, characterized by: before the step A, the method further comprises the following steps: and (4) the tested alternating current signal is subjected to voltage reduction and isolation.
4. A method of measuring alternating current according to claim 1 or 2, characterized by: the pulse number of the square wave signal is the average value of multiple statistical values, and the peak value is the average value of multiple half-wave sampling values.
5. A method of measuring alternating current according to claim 1 or 2, characterized by: the frequency of the tested alternating current signal is equal to that of the square wave signal, and the effective value is in direct proportion to the peak value of the half wave signal.
6. An apparatus for measuring alternating current, comprising: the method comprises the following steps:
a half-wave rectifier circuit: rectifying an input alternating current signal into a half-wave signal;
schmitt trigger circuit: outputting a square wave signal under the trigger of the half-wave signal;
a measurement circuit: sampling the peak value of the half-wave signal, counting the pulse number of the square wave signal, and determining the frequency and effective value of the alternating current signal according to the pulse number of the square wave signal and the peak value of the half-wave signal.
7. An apparatus for measuring alternating current according to claim 6, wherein: the measuring circuit also comprises an integrating circuit for integrating the half-wave signal, and the integrating circuit is connected between the output end of the half-wave rectifying circuit and the input end of the measuring circuit.
8. An apparatus for measuring alternating current according to claim 6 or 7, wherein: the isolation transformer is used for reducing the voltage of the alternating current signal, a primary side coil of the isolation transformer is connected with the alternating current signal after being connected with a first resistor in series, a secondary side coil of the isolation transformer is connected with a second resistor in series, one end of the second resistor is grounded, and a non-grounded end of the second resistor is connected with the input end of the half-wave rectification circuit.
9. An apparatus for measuring alternating current according to claim 6 or 7, wherein: the half-wave rectifying circuit comprises a first operational amplifier, the inverting input end of the first operational amplifier is connected with the non-grounding end of the second resistor through a third resistor, the non-inverting input end of the first operational amplifier is grounded through a fifth resistor, and the output end of the first operational amplifier is connected with the inverting input end through a fourth resistor; or,
the half-wave rectification circuit is a bridge consisting of four diodes.
10. An apparatus for measuring alternating current according to claim 6 or 7, wherein: the Schmitt trigger circuit comprises a second operational amplifier, the inverting input end of the second operational amplifier is connected with the output end of the half-wave rectification circuit, the non-inverting input end of the second operational amplifier is respectively connected with the positive power supply through a sixth resistor, is grounded through a seventh resistor and is connected with the output end through an eighth resistor, and the output end outputs the square wave signal; or,
the Schmitt trigger circuit is an integrated Schmitt trigger, the input end of the Schmitt trigger circuit is connected with the output end of the half-wave rectifying circuit, and the output end of the Schmitt trigger circuit outputs the square wave signal.
11. An apparatus for measuring alternating current according to claim 6 or 7, wherein: the measuring circuit comprises an analog-to-digital converter for sampling the half-wave signal; and the singlechip is used for counting the pulse number of the square wave signal and determining the frequency and effective value of the alternating current signal according to the sampling value and the counting value.
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP3382406A1 (en) | 2017-03-30 | 2018-10-03 | NorthQ ApS | A system and a method for estimation of current transported in a cable |
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|---|---|
| CN1782719A (en) | 2006-06-07 |
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