JPS61280577A - Digital type frequency detecting method - Google Patents

Abstract

PURPOSE:To detect frequency with high accuracy even when a DC component was superposed to an input signal by simple circuit constitution, by calculating the frequency on the basis of the generation time of the extreme value of an AC signal. CONSTITUTION:An AC signal subjected to A/D conversion receives processing by CPU21 in a digital operating part 2 and the generation time of the extreme value such as a max. value of said AC signal is determined on the basis of the difference between sampling values at every equal time intervals and, further, the frequency of the AC signal is calculated from the time difference with the next extreme value generation time to be displayed on a display part 4. By the method not based on the zero point generation time of the AC signal but based on the extreme value generation time, the frequency of the AC signal is detected with high frequency by simple circuit constitution even when a DC component is superposed thereon.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention samples a sine wave signal and performs a predetermined calculation based on the sampled value to numerically (digitally) calculate the frequency of the sine wave signal. This invention relates to a frequency detection method for detecting frequencies.

[Conventional technology]

Conventionally, as a method for detecting frequency using values obtained by sampling a sine wave AC signal, for example, the sign (±) of the signal is detected.
A zero point detection method is known that uses the point where the value is reversed, that is, the zero point.

FIG. 4 is a reference diagram for explaining such a zero point detection method. This is, for example, the zero point t2□ in the same figure.

Since the time difference '22-tz1 of tz, corresponds to 1/2 of the period T, the frequency f is determined as f-1/T-1/2 (t2□-t2.).

[Problem that the invention seeks to solve]

According to the zero point detection method as described above, if the input signal is a sine wave signal, the accuracy (frequency) can be determined, but if the DC component d is superimposed on the AC signal as shown in Figure 5, "Z2"Z1'←'zs because the difference in occurrence time between adjacent zero points does not correspond equally to the period of the sine wave.
't22'←T/2), there is a problem in that an error occurs in frequency detection.

Therefore, it is an object of the present invention to enable accurate frequency detection even when a DC component is superimposed on an input signal and the zero point drifts.

[Means to solve the problem]

A sampling means for sampling a sine wave input signal at a predetermined period, and a calculation means for performing a predetermined calculation based on the sampled values are provided.

[Effect]

In detecting the frequency (period), for example, the difference between the occurrence times of adjacent extreme values is used.

That is, as shown in Fig. 3, it includes a DC component and has a frequency f.
Even in the case of an alternating current signal v(t) that changes periodically at
2 (equivalent to half a cycle), or the difference T (1
It utilizes the principle that it can be detected correctly from the cycle (corresponding to the period: T-1/f).

[Example] FIG. 1 is a reference diagram for explaining the present invention in detail, and is for explaining a method for determining the time of occurrence of an extreme value (maximum value or minimum value).

First, the case of the local maximum value will be explained using FIG.

When sampling the AC signal v(t) at equal intervals every time Δt, time t1. t2. t5(t2 cod t1+Δ
t, It, Minority t2+Δ1. If the signal values at Δt3>O) are respectively expressed as V'1 + Y2 * Y5, then
vl(V2 If p V2 > V5 holds, then v(
The maximum value of t) is at time 'max between times t and t (t,<tmax x t,). Now, if we apply a quadratic function to these values, this time 'r
nax can be approximated by the following equation (1).

Similarly, for the minimum value, the six consecutive sampling times that sandwich the minimum value are 14.15 as shown in the same figure (b).
．． 16 (15 tan t4+ΔtsIt6baset, +Δ1s), and the value of the signal at each time is V4 # '1
5 tv 6 (v 4 ) v5* v s (v b
), the time tfT1i□ at which the minimum value occurs can be approximately determined by the following equation (2).

Using equations (1) and (2) above, we calculate the occurrence time 1 of one maximum value of the AC input signal and the adjacent minimum value. 1
−, the difference is “min −tmaxm
Since ax min corresponds to a half cycle of the AC signal, the frequency f can be calculated using the following equation (3).

When a DC component is superimposed on the input signal v (t), (1
) and (2), only the quadratic functions to be approximated are different, so no error occurs due to DC drift. Additionally, since sampling values near the extreme values where the absolute value of the AC signal is close to full scale are used for calculation, quantization errors are also improved.

FIG. 2 is a block diagram showing a digital device to which the present invention is applied. This digital device mainly consists of an analog input section 1, a digital calculation section 2, an input/output interface section 3, and a display section 4.
is an isolation transformer 11. A-1-Fl /r jsu n-A4
θ Jl-'/Go angle, -ni-n, ! The digital arithmetic unit 2 includes a processing unit (CPU) 21 such as a microcomputer, a ROM
(Read Only Memory) 22, RAM (Random Access Memory) 23, etc.

The AC signal input to the analog input section 1 is given to the analog filter 12 via the isolation transformer 11 that protects the calculation section 2 from surges, etc., where harmonics are removed.
The sample and hold circuit 15 converts the signal into a discrete value signal. If there are a plurality of AC inputs as shown in the figure, the signals are rearranged into serial data by the multiplexer 14, and then converted into digital signals by the analog/digital converter 15 and provided to the arithmetic unit 2.

The CPU 21 performs calculations as shown in equations (1) to (3) above based on the signal provided via the analog/digital converter 15. This calculation result is displayed numerically on the display section 4, and is also converted into a contact signal by the input/output interface section 3 and output to a communication device or the like.

Incidentally, the above-mentioned digital device is well-known as, for example, a sequencer or a digital operation type relay. That is, by adding a ROM or the like that executes calculations such as equations (1) to (6) above to an existing digital device such as a digital relay,
Another feature of this method is that it is possible to add a frequency detection function very easily.

In order to compare this method with the conventional zero point detection method, we set the following conditions and performed numerical seerations. The results are shown in the following table.

Input signal: An AC voltage signal with an effective value of 110 cm.

Assume that the frequency of two fundamental station wave numbers is 60 Hz, and consider a fluctuation of ±1L.

Quantization bits = 11 bits (2”-2048) DC drift: +5% of the effective value is superimposed on the sine wave.

Sampling Frequency Near 20Hz Table Note that the numbers in the table indicate the percentage of frequency detection error with respect to the frequency of the input signal. As is clear from this table, it can be seen that this method can reduce the error to a fraction of a fraction.

〔Effect of the invention〕

According to this invention, since the frequency is determined based on the occurrence time of the extreme value of the AC signal, it is not only possible to detect the frequency with high accuracy even when a DC component is superimposed on the input signal. This provides the advantage of easily adding frequency detection functionality to existing digital devices.

[Brief explanation of drawings]

Figure 1 is a reference diagram for explaining the invention in detail;
Figure 6 is a block diagram showing a digital device to which this invention is applied, Figure 6 is a waveform diagram for explaining the invention in detail, and Figure 4 is a diagram for explaining the relationship between the zero point and period of a sine wave signal. FIG. 5 is a waveform diagram for explaining the influence of DC drift on a sine wave signal. Symbol explanation 1...°/°analog input section, 2...digital operation section, 3...input/output interface section,
4... Display unit, 11... Disruptive transformer, 12... Analog filter, 13...
・Sample hold circuit, 14...multiplexer, 15...analog/digital converter,
21... Processing unit (CPU), 22...
・ROM (read only memory), 23...R
AM (-yy dumb access memory). Agent Patent Attorney Akio Namiki Agent Patent Attorney Matsuzaki Liquidation I Figure 2 Zumei 1 Eka

Claims (1)

[Claims] Sampling a sine wave input signal with a predetermined period,
calculating the occurrence times of at least two extreme values of the sine wave signal using a predetermined quadratic approximation formula from the sampled values;
A digital frequency detection method, characterized in that the frequency of the sine wave input signal is detected from the difference between the two generation times.