CN102809687B - Digital measurement method for alternating-current frequency - Google Patents

Abstract

The invention discloses a digital measurement method for alternating current frequency. For continuous digital sampling signals, a sampling point P1 is selected after the signal passes the positive and negative peak values, and two consecutive points P2 and P3, P2 are selected in subsequent sampling points. The selection condition of P3 and P3 is that the negative value of P1 sampling value is in the interval [P2 sampling value, P3 sampling value]. Then, a point P4 can be calculated on the line segment composed of P2 and P3 by interpolation method, so that the value of P4 is equal to the negative value of the sampling value of P1, and the virtual occurrence time of point P4 can be obtained at the same time. The zero point of the electrical signal is calculated from the sampling occurrence time of P1 and the virtual occurrence time of P4, and the frequency or period of the electrical signal is calculated from a series of zero-crossing points of the electrical signal. The invention does not need to convert the low-frequency signal into a square wave, and is especially suitable for the frequency measurement of the low-frequency sinusoidal signal. The system is convenient for detection, and the hardware cost is small; the system has strong anti-interference ability; the implementation is simple and reasonable, and the data is accurate and reliable.

Description

A Digital Measuring Method of Alternating Current Frequency

technical field

The invention relates to a digital measurement method for alternating current frequency, which can also be used for frequency measurement of low-frequency sinusoidal signals, triangular wave signals, etc. with fixed frequency or little frequency change.

Background technique

In daily life and production process, it is often necessary to monitor the frequency of the signal. The more accurate the measured frequency, the better the application effect. In the existing frequency measurement methods for low-frequency signals, the low-frequency signal is first formed into a square wave by a shaping circuit, and the time interval T between two adjacent rising or falling edges of the square wave is detected, and the frequency is obtained by calculating the reciprocal. In addition, there are some methods for measuring frequency by software, such as: zero-crossing method, analytical method, error minimization principle algorithm, DFT algorithm, quadrature demodulation method, etc. However, some of these algorithms are easy to implement but have low precision, and some algorithms require too much computation.

Existing technology 1: Chinese patent application No. 02107167 discloses a method and device for measuring the frequency of three-phase alternating current. By sampling voltage or current signals, the angular velocity of the voltage or current rotation vector is obtained, and then converted by the angular velocity of the rotation vector. frequency. Although the measuring device has strong anti-interference ability, its disadvantage is that it needs a lot of complicated calculations when using the rotational vector angular velocity to convert the frequency, and the measuring device is dedicated to the frequency measurement of three-phase AC power, and cannot be used for the frequency measurement of civil single-phase AC power, let alone It is used for frequency measurement of ordinary non-three-phase low-frequency signals.

Prior Art 2: Chinese Patent Application No. 201010150157 discloses a frequency measurement method for low-frequency signals, which is essentially a zero-crossing detection method, but this method may have a large amount of computation.

Contents of the invention

The technical problem to be solved by the present invention is to propose a digital measurement method for AC frequency, which can also be used for fixed Frequency measurement of low frequency sinusoidal signals whose frequency or frequency does not vary much.

The present invention adopts the following technical solutions for solving the problems of the technologies described above:

A digital measurement method for alternating current frequency, comprising the following steps:

Step 1), sampling the electrical signal under test, obtaining the sampling value of the electrical signal under test, and then generating a sampling signal;

Step 2), in the sampling signal of the electrical signal under test, when the cycle of the electrical signal under test does not reach the zero point after passing through any peak value, select a sampling point as the first sampling point P1 _i , and set the first sampling point P1 _i The sampling time of is T1 _i , and the sampling value is S1 _i ;

Step 3), when the sampling signal does not reach the next peak value after crossing the zero point, select two consecutive sampling points in the sampling signal, that is, the second sampling point P2 _i and the third sampling point P3 _i , set these two The sampling times of the points are T2 _i and T3 _i respectively, and the sampling values are S2 _i and S3 _i respectively, where the sampling values S2 _i and S3 _i need to satisfy |S2 _i |≤|S1 _i |≤|S3 _i |;

Step 4), take the point corresponding to the opposite value of the sampling value of the first sampling point P1 _i as the virtual sampling point P4 _i , and calculate the virtual occurrence time of the virtual sampling point P4 _i ${T4}_i={T2}_i+\frac{({T3}_i-{T2}_i)\×({S1}_i+{S2}_i)}{{S2}_i-{S3}_i};$

Step 5), calculate the zero-crossing time:

Step 6), repeat step 2) to step 5) to obtain several zero-crossing times, T _z1 , T _z2 , T _z3 ... T _zi , T _zi+1 , T _zi+2 ... T _zn , where, i=1, 2,...n, n is a positive integer; then the period of the measured electrical signal: T _pi =T _zi+2 -T _zi or T _pi =2×(T _zi+1 -T _zi ), the measured The frequency of the electrical signal: $f_i=\frac{1}{T_p}=\frac{1}{T_{\mathrm{the\; zi}+2}-T_{\mathrm{the\; zi}}}$ or: $f_i=\frac{1}{T_p}=\frac{1}{2\×(T_{\mathrm{the\; zi}+1}-T_{\mathrm{the\; zi}})}.$

Furthermore, in the digital measurement method of AC frequency of the present invention, the sampling of the electrical signal under test in step 1) is sampling at equal time intervals or sampling at unequal time intervals.

Furthermore, in the digital measurement method of AC frequency of the present invention, the sampling in step 1) is to sample the entire cycle of the electrical signal under test.

Further, a digital measurement method of AC frequency in the present invention also includes selecting sampling points with different sampling values as the first sampling point P1 _i according to the principle of step 2), and then repeating steps 3) to 5) for several Take the average value after zero-crossing calculations to get the zero-crossing time.

Further, a digital measurement method of AC frequency in the present invention also includes the following steps: repeating step 1) to step 6), measuring several periods or frequency values of the electrical signal under test, and then analyzing the obtained several periods Or the frequency values are averaged to calculate the final frequency.

Furthermore, in the digital measurement method of AC frequency of the present invention, in the step 3), when the sampling values of the selected first to third sampling points meet |S2 _i |=|S1 _i |=|S3 _i |Condition, take any one of the following two processing schemes:

a), reselect the second sampling point P2 _i and the third sampling point P3 _i , so that the sampling value S2 _i of the second sampling point and the sampling value S3 _i of the third sampling point must satisfy |S2 _i |<|S3 _i | , then proceed to step 4) to step 6);

b) Select any point on the sampling time interval [T2 _i , T3 _i ] between the second sampling point P2 _i and the third sampling point P3 _i as the virtual sampling time T4 _i corresponding to the virtual sampling point P4 _i , and then proceed to step 5 ) to step 6).

Further, in the method for digitally measuring AC frequency of the present invention, in the scheme b), take the sampling time interval [T2 _i , T3 _i ] between the second sampling point P2 _i and the third sampling point P3 _i Midpoint, calculate the virtual occurrence time of the virtual sampling point P4 _i ${T4}_i=\frac{{T3}_i+{T2}_i}{2}.$

Compared with the prior art, the present invention adopts the above technical scheme and has the following technical effects:

1) The traditional zero-crossing method uses two consecutive points with opposite signs to determine the zero-crossing point. Although the physical concept of the algorithm is clear, it is easily disturbed by harmonics, measurement errors, etc., and the measurement accuracy is low. From the selected 3 sampling points, the zero-crossing point can be obtained through calculation. After determining the zero crossing point of the signal, the frequency and period of the AC signal can be calculated. Compared with the traditional zero-crossing method, the calculation amount is roughly the same, but the anti-interference ability of the system is improved.

2) The calculation amount of the present invention is small, and it is suitable for use in an embedded system.

Description of drawings

Figure 1 is a detailed schematic diagram of calculating and obtaining a zero-crossing point near a certain zero point of an AC signal;

Fig. 2 is a schematic diagram of performing zero-crossing calculation near each zero point after sampling the alternating current signal and further calculating the frequency.

Detailed ways

Below in conjunction with accompanying drawing, technical scheme of the present invention is described in further detail:

The essence of the present invention is to select a sampling point P1 after the signal crosses the positive and negative peak values for continuous digital sampling signals, and select two consecutive points P2 and P3 in the subsequent sampling points. The selection conditions of P2 and P3 are: P1 The negative value of the sampled value is in the interval [P2 sampled value, P3 sampled value]. Then, a point P4 can be calculated on the line segment composed of P2 and P3 by interpolation method, so that the value of P4 is equal to the negative value of the sampling value of P1, and the virtual occurrence time of point P4 can be obtained at the same time. The zero point of the electrical signal is calculated from the sampling occurrence time of P1 and the virtual occurrence time of point P4, and the frequency or period of the electrical signal is calculated from a series of zero points of the electrical signal. The selected P1 should not be too close to the zero point, if it is too close to the zero point, the measurement accuracy may not be high enough due to noise. Step 2) and step 3) in the claims of the present invention adopt a stricter method than the above method in selecting P1, P2 and P3, and this method has stronger anti-interference characteristics.

The principles of the invention are illustrated by measuring the frequency of two cycles of an AC signal.

Concrete implementation process of the present invention is as follows:

1. Sampling the electrical signal under test, the sampling mentioned here is the sampling of the whole cycle. It can be sampling at equal time intervals or sampling at unequal time intervals.

2. Select a sampling point P1 _i after the electrical signal under test passes the positive and negative peak values, set the sampling time of this sampling point as T1 _i , and the sampling value is S1 _i ;

3. When the sampling signal crosses the zero point, select two consecutive points P2 _i and P3 _i in the subsequent sampling values, and set the sampling time of these two points as T2 _i and T3 _i respectively, and the sampling values of the two points are respectively are S2 _i and S3 _i , and these two points have the following characteristics: if S1 _i is a positive number, then S2 _i and S3 _i are both negative numbers; if S1 _i is a negative number, then S2 _i and S3 _i are both positive numbers, and It satisfies |S2 _i |≤|S1 _i |≤|S3 _i | and |S2 _i |<|S3 _i |.

4. Calculate the virtual occurrence time T4 _i of point P4 _i , since the value of P4 _i is equal to the negative value of the sampling value of P1 _i , then: T4 _i can be expressed as ${T4}_i={T2}_i+\frac{({T3}_i-{T2}_i)\&times;({S1}_i+{S2}_i)}{{S2}_i-{S3}_i};$

5. When the sampling value of the selected sampling point meets the condition of |S2 _i |=|S1 _i |=|S3 _i |, two processing methods can be adopted: one method is to reselect P2 _i and P3 _i and recalculate; Another method is that T4 _i takes any point on the interval [T2 _i , T3 _i ], for the convenience of calculation, take the midpoint of this interval, then

Considering the up-and-down symmetry of the AC signal, the zero-crossing time is expressed as:

Refer to Figure 1 for specific operations. The hollow circle points P1 _i , P2 _i , and P3 _i in Figure 1 are three points selected according to the requirements, the hollow triangle point P4 _i is a virtual point obtained after interpolation, T1 _i is the sampling time of P1 _i , and T4 _i is The virtual occurrence time of P4 _i , the hollow circle point T _zi on the horizontal axis is the zero-crossing point obtained after calculation.

6. Repeat the above steps to get several zero-crossing times, T _z1 , T _z2 , T _z3 ... T _zi , T _zi+1 , T _zi+2 ... T _zn , then the measured AC signal Period: T _pi =T _zi+2 -T _zi or T _pi =2×(T _zi+1 -T _zi ), the frequency of the measured AC signal: $f_i=\frac{1}{T_p}=\frac{1}{T_{\mathrm{the\; zi}+2}-T_{\mathrm{the\; zi}}}$ or: $f_i=\frac{1}{T_p}=\frac{1}{2\&times;(T_{\mathrm{the\; zi}+1}-T_{\mathrm{the\; zi}})}.$

In one signal period, different P1 _i can be selected near a certain zero-crossing point for multiple zero-crossing point measurements, and the final zero-crossing point time can be obtained after multiple zero-crossing point times are averaged.

After sampling the electrical signal, calculate and measure the frequency near each zero point as shown in Figure 2. The AC signal in Figure 2 has four zero-crossing points, so four calculations were performed after the signal peaked. The hollow circles on the horizontal axis in the figure are calculated zero-crossing points, and the zero-crossing points are: T _zi-1 , T _zi , T _zi+1 , T _zi+2 in sequence. Then the period of the signal can be expressed as T _p =T _zi+1 -T _zi-1 or T _p =T _zi+2 -T _zi or T _p =2×(T _zi -T _zi-1 ) or T _p = 2×(T _zi+1 −T _zi ) or T _p =2×(T _zi+2 −T _zi+1 ). The frequency F _p is the reciprocal of T _p .

After measuring several period or frequency values of the electrical signal under test, the final frequency can be calculated by averaging the obtained several period or frequency values.

The preferred solution is that the above-mentioned sampling is sampling at equal time intervals, and the selected P1 _i should not be too close to zero or too close to the peak value.

By adopting the method, multiple frequency measurements can be carried out within one signal period.

To sum up, the digital measurement method of a kind of AC frequency involved in the present invention does not need to convert the AC signal into a square wave, and is especially suitable for the frequency measurement of low-frequency sinusoidal signals, the system detection is convenient, and the hardware overhead is small; the system Strong anti-interference ability; simple and reasonable to implement, accurate and reliable data; broad application prospects.

Claims (7)

1. A digital measuring method of alternating current frequency, is characterized in that comprising the following steps: Step 1), sampling the electrical signal under test to obtain a sampling value of the electrical signal under test, and then generating a sampling signal; Step 2), in the sampling signal of the electrical signal under test, when the cycle of the electrical signal under test does not reach the zero point after passing through any peak value, select a sampling point as the first sampling point P1 _i , set the first sampling point P1 _i The sampling time of is T1 _i , and the sampling value is S1 _i ; Step 3), when the sampling signal does not reach the next peak value after crossing the zero point, select two consecutive sampling points in the sampling signal that does not reach the next peak value after crossing the zero point, that is, the second sampling point P2 _i and the second sampling point Three sampling points P3 _i , assuming that the sampling time of these two points are T2 _i and T3 _i respectively, and the sampling values are respectively S2 _i and S3 _i , these two sampling points have the following characteristics: if S1 _i is a positive number, then S2 Both _i and S3 _i are negative; if S1 _i is negative, then S2 _i and S3 _i are both positive, and satisfy |S2 _i |≤|S1 _i |≤|S3 _i | and |S2 _i |<|S3 _i |; Step 4), use the point corresponding to the opposite value of the sampling value of the first sampling point _P1i as the virtual sampling point _P4i , and calculate the virtual occurrence time of the virtual sampling point _P4i ${T4}_i={T2}_i+\frac{({T3}_i-{T2}_i)\&times;({S1}_i+{S2}_i)}{{S2}_i-{S3}_i};$ Step 5), calculate the zero-crossing time: Step 6), repeat step 2) to step 5) to obtain several zero-crossing times: T _z1 , T _z2 , T _z3 ... T _zi , T _zi+1 , T _zi+2 ... T _zn , wherein, i=1, 2,...n, n is a positive integer; then the period of the electrical signal under test: T _pi =T _zi+2 -T _zi or T _pi =2×(T _zi+1 -T _zi ), the measured The frequency of the electrical signal: $f_i=\frac{1}{T_p}=\frac{1}{T_{\mathrm{the\; zi}+2}-T_{\mathrm{the\; zi}}}$ or: $f_i=\frac{1}{T_p}=\frac{1}{2\&times;(T_{\mathrm{the\; zi}+1}-T_{\mathrm{the\; zi}})}.$ 2. A digital measurement method for AC frequency according to claim 1, characterized in that: step 1) said sampling the electrical signal under test is sampling at equal time intervals or sampling at unequal time intervals. 3. The digital measuring method of a kind of alternating current frequency according to claim 1, characterized in that: the sampling described in step 1) is sampling the whole cycle of the electrical signal under test. 4. The digitized measuring method of a kind of alternating current frequency according to claim 1, is characterized in that: also comprises according to the principle of step 2) respectively selects the different sampling points of sampling value as the first sampling point P1 _i , then repeats step 3 ) to step 5) and take the average value after performing several zero-crossing calculations to obtain the zero-crossing time. 5. according to the digitized measuring method of a kind of alternating current frequency described in any one in claim 1 to 3, it is characterized in that: also comprise the following steps: repeat step 1) to step 6), measure some of measured electric signal cycle or frequency value, and then use the average method to calculate the final frequency of several cycle or frequency values obtained. 6. The digital measuring method of a kind of alternating current frequency according to claim 1, characterized in that: in said step 3), when the sampled values of the selected first to third sampling points meet |S2 _i |=| When S1 _i |=|S3 _i | condition, take any one of the following two processing schemes: a), reselect the second sampling point P2 _i and the third sampling point P3 _i , so that the sampling value S2 _i of the second sampling point and the sampling value S3 _i of the third sampling point must satisfy |S2 _i |<|S3 _i | , then continue step 4) to step 6); b) Select any point on the sampling time interval [T2 _i , T3 _i ] between the second sampling point P2 _i and the third sampling point P3 _i as the virtual sampling time T4 _i corresponding to the virtual sampling point P4 _i , and then proceed to step 5 ) to step 6). 7. the digitized measurement method of a kind of alternating current frequency according to claim 6, is characterized in that: in described scheme b), get the sampling time interval [T2 of the second sampling point P2 _i and the 3rd sampling point P3 _{i i} , the midpoint on T3 _i ], calculate the virtual occurrence time of the virtual sampling point P4 _i