JPS6235280A - Frequency tracer - Google Patents

Abstract

PURPOSE:To make sure a tracing operation and to expand the input frequency range by changing over frequency dividers of different dividing ratio on the output terminal of a VCO, and each frequency divider, LPF of LPFs of different band ranges on the output terminal of a mixer according to a range change-over command. CONSTITUTION:An input frequency and a local frequency generated by a VCO 9g are mixed in a mixer 9a, and the differential frequency is taken out from LPFs 9b, 9b2 having different band ranges, whose output are applied to a first range change-over switch 9n1. The output of the switch 9n1 is processed through a synchronous detector 9d, a lock detector 9j, an integrator 9e, and an adder 9f, depending on the frequency of a local oscillator 9h, and is applied to the VCO 9g. On the output side of the VCO 9g are provided frequency dividers 9m1, 9m having different dividing ratio, whose outpus are fed to a second range change-over switch 9n2. The switches 9n-1, 9n2 are changed over by an external changer-over command A, and the local frequency from the switch 9n2 is fed to the mixer 9a, resulting in an expanded region of the input frequency.

Description

[Detailed description of the invention] [Industrial application field] This invention utilizes the Doppler effect of light to produce steel.

This invention relates to a frequency tracker for a laser Doppler velocimeter that non-contactly measures the speed of a moving object on a manufacturing line for non-ferrous metals.

[Conventional technology]

Generally, a laser Doppler velocimeter having a configuration as shown in FIG. 2 has been used to measure the speed of a moving object using the Doppler effect of light. Figure 2 shows, for example, Mitsubishi Electric branch shaft Vol. 1.58- No. T 1984 page 34~
It is a configuration diagram of a conventional laser Doppler velocimeter shown in the optical fiber sensor laser interferometer described on page 38.
In the figure, (1) is a moving object, +21 is a laser, (
3) is a beam splitter that splits the laser beam into two, (4
a) and (4b) are optical fiber cables for guiding the laser beam split into two by the beam splitter (3) to the transmitting optical system (5a) + (5b); (40) U for receiving the receiving optical system described later; Fiber optic cable that transmits light (6
) is a receiving optical system for receiving the light scattered from the moving object +11, ((8) is a photodetector that converts the received light into an electrical signal, (8) is an amplifier, and (8) is a frequency tracker.

As shown in Figure 2, the laser beam emitted from the laser (2) to the moving object (1) is split into two by the beam splitter (31), and each beam is sent to the optical fiber couple (4a) l (4
b) and transmitting optical system (5a)p (5), so when the moving object (1) is irradiated crosswise from opposite directions, the wavelength of the scattered light of the moving object (1) corresponding to each transmitted light beam is , a so-called positive and negative Doppler shift occurs depending on the velocity V of the moving object fl). The scattered light that has undergone the two positive and negative Doppler shifts is received by a receiving optical system (6).

Guided to the photodetector (7) with an optical fiber cable (4C).

When converted into an electrical signal, some of this electrical signal.

There are a DC signal proportional to the intensity of received light and an AC signal (hereinafter referred to as a Doppler signal) having a Doppler frequency fd shown in equation (1).

fd-geki・ainju ・・・・・・・・・・・・・・・
・tl + here V: Speed of the moving object λ: Wavelength of the laser beam ψ: Intersection angle of the two transmitted beams The Doppler signal converted into an electrical signal by the photodetector (7) is weak, so it is converted to an electric signal by the amplifier (8). By measuring the Doppler frequency f,1 with the 9 frequency tracker (9) after being amplified,
The velocity V of the moving object f11 can be determined.

This is a known fact.

In the Doppler signal, the spread of the two light beams is 9° and the intersection angle ψ
Therefore, it has a spectrum with frequencies spread around the Doppler frequency fd. Figure 3 is.

This shows an example of the spectrum of a Doppler signal.

The horizontal axis shows frequency, and the vertical axis shows power spectrum density.

In FIG. 3, the spectrum of the Doppler signal shows a maximum value at the Doppler frequency fd, and the frequency spread width Δf1/! is 3 dB lower than the maximum value. is called the half width. The signal Q is the Doppler frequency fd divided by the half width △f 17°.
It is defined by the index. Further, the ratio between the maximum value of the spectrum and the noise level is called the S/N of a signal, and the Q and B/H of these signals are generally used as important factors for evaluating the properties of Doppler signals.

Q sw f Buddha f172 ・・・・・・・・・
(2) Normally, the Q of the Doppler signal of a laser Doppler speedometer is 10 to 30 degrees, and the signal level 4 is 10 to 30 degrees.
The signal quality is poor at about 30 dB, and the performance of the laser Doppler velocimeter depends on accurately measuring the Doppler frequency fd from this Doppler signal using a frequency tracker (9).

Figure 4 shows 2For example, the Bulletin of the Institute of Aeronautics, University of Tokyo, Vol. 2, No. 8 (
An example of a conventional frequency tracker (9) shown in the Layer Wavenumber Tracker for Airborne Dragler Radar (December 1961) is shown.

In the figure, (9a) is a mixer that mixes the Doppler signal input d and the output of a voltage controlled oscillator (hereinafter referred to as VCO) (9g), and (9b) is the Doppler signal input frequency of the output of the mixer (9a) and VC! (9c) is a detector that detects the output of the low-pass filter (9b), (9a)
is a synchronous detector, (9e) is an integrator, (5)r) is an adder, (9h) is a low frequency oscillator for providing VCO (9g) Ic IF M modulation, (91) is a level adjuster, (9,+) is a lock detector that detects the signal level of the detector (9C) to determine lock; (9/
) is a sweep switch which switches between the sweep generator (9k) and the frequency tracking loop in response to a lock determination signal from the lock detector (9j).

In the frequency tracker +91 with the above configuration, vco(
9g) is an oscillator (9g) that oscillates a square wave at a constant frequency.
h) The output frequency changes in proportion to the voltage obtained by adding the output voltage of the integrator (9θ) with the signal whose level is adjusted with the level adjuster (91) and the output voltage of the integrator (9θ) with the adder (9f). It is a vessel. That is, the output frequency of V(:!O(9g) is the oscillation frequency f of the two oscillators (9h) centered on the frequency fLo corresponding to the output voltage of the integrator (9θ).
.

It is an FM modulated signal, and at this time vc
The frequency fII of O(9g) is given by a linear equation.

Δfv fL−fL□+1−* sgn (ain (2πfm
t) ) ・−・−・・・+31 where Δfv: Modulation width of FM modulation sgn(sin(2πfmt)): Denotes the descriptive function. 1 when sin (2πfmt)>O, 0 sin when sin(2πfmt)11=0 (2πf
rnt) (-1 when 0.

The modulation width Δfv in equation (3) is determined by the level adjuster (91)
This is a constant determined by the amplitude of the modulation frequency fm adjusted by .

Now, when a Doppler signal having a Doppler frequency fd as shown in Fig. 3 is input, it is frequency-mixed with the frequency fL of voo (9g) and the mixer (9a).
) Output includes Doppler frequency fd and voo (91g)
(The sum and difference frequencies with the D frequency fL are mixed. The low-pass filter (9b) is configured to filter the frequency of the difference between the Doppler frequency fd and the frequency fl of the VCO (9g) among the output frequencies of the mixer (9a). Take out the chisel and use it.

The output of the low-pass filter (9b) is the modulation frequency f! of Woo (9g). At tl, the waveform becomes AM modulated. This AM modulated signal is VOo (9g),... If the Q center frequency fLo is equal to the Doppler frequency fd, the modulation degree is 0.

The degree of modulation increases in proportion to the size of fd-fL□ fd
>fb and fa<fL, the phase of the modulation frequency fm changes between positive and negative with respect to the output phase of the oscillator (9h). Therefore, when the output of the low-pass filter (9b) is subjected to AM detection by the detector (9c) and then synchronously detected by the output signal of the first oscillator (9h) and the synchronous detector (9d), as shown in Fig. 5, the VCO ( 9g) center frequency fLo is.

A positive or negative DC signal is obtained depending on whether it is higher or lower than the Doppler frequency fd. In other words, the output of the synchronous detector (9d) is the center frequency fLo of the VCO and the Doppler frequency fd.
The frequency difference between the two signals is used as a discrimination signal.

In addition, in FIG. 5, 8 is the linear zone of the output of the synchronous detector (9d), KD is the discretization gain in the linear zone S, and this discretization gain KD is 8/N of the input Doppler signal, half-width Δf j72゜vco (9g
) varies depending on the PM modulation width Δfy.

Now, when the input Doppler frequency f4 changes from the state of fLo-fd, the output voltage of the synchronous detector (9a) will be:

A positive or negative error voltage is generated depending on the difference between fd and fLo.

The integrator (9e) is charged and the synchronous detector (9d
) The center frequency fLo of vco (9g) changes in the direction in which the error voltage of
The center frequency fLo of CO (9g) will track the input Doppler frequency fd.

fLo■fd・・・・・・・・・・・・
...(4) That is, VC! The output center frequency fLo of O(9g) is.

It will be outputted tracking the Doppler frequency fd, which is equal to the input Doppler frequency fd.

In the above explanation, 2. Input Doppler frequency fd
In contrast, it is assumed that the center frequency fLo of vco (9g) is within the linear zone S of the output of the synchronous detector (9d) shown in FIG. In an actual system, the linear zone S is considerably narrower than the frequency range of VOO (9g), so when Doppler signal power of any frequency is input, it is not necessarily vC! The difference between the center frequency fLo of O(9g) and the Doppler frequency f6 does not necessarily mean that there are people within the linear sea 78, and the frequency tracking loop does not function.
Forcibly sweep the center frequency fLo of VCO (9g) over the entire frequency range by some method
You need to put it inside.

The lock detector (9j), the sweep switch (9υ), and the sweep generator (9k) are vao(s+g)o center station tll number fLOf, IJ of the output of the 151st period detector (9d) to put it in the near zone s This is a sweep circuit.The operation is to detect the output signal level of the wave detector (9C) with a lock detector (9j), generate a lock-on signal, and use the lock-on signal to switch the loop of the sweep switch (9j). In other words, if there is no lock-on signal, the signal from the sweep generator (9k)
The output frequency of (9g) is swept over the entire frequency range, and the output center frequency fLo of voo (9g) is
When it enters the linear zone S, the output signal is detected by the detector (9C), the lock detector (9j) detects lock-on, and the sweep switch (91) is switched to form a frequency tracking loop, and the VCO (9g) The center frequency fLo tracks the input Doppler frequency f4.

By the way, in a frequency tracker, it is possible to increase the loop gain of one frequency tracking loop to increase the responsiveness of the system, and to
9. Over a wide speed range of l+. That is, it is important to follow the input Doppler frequency fd over a wide frequency range.

For example, the speed range of a moving object (1) such as a steel line is
1 to 2000 m7 minutes, and a frequency tracker that tracks these entire speed ranges would require a dynamic range that varies by a factor of about 2000.

In the conventional frequency tracker shown in Fig. 4, the mixer (9a)
The output includes the difference frequency fd-fL between the Doppler frequency f4 and the frequency fL of VOO (9g), and the sum frequency f, 1
+fL are mixed, and if the band fc of the low-pass filter (9b) is sufficiently lower than the Doppler frequency fd, only the difference frequency f,1-tL is extracted by the low-pass filter (9b) and the frequency of the VCO (9g) is The center frequency fLo is 'LO-fd
The frequency tracker operates so that K. However, the Doppler frequency fli becomes lower, and the Doppler frequency fd and vco
(9g) Frequency Q, (Frequency of D sum f4 + f
When I, is within the band f0 of the low-pass filter (9b), the output of the low-pass filter (9b) is fd-fl, f4
+Q, mixed and influenced by the sum frequency fd+fl, the center frequency fLo of VOO (9g) does not track the two-input Doppler frequency fd (this results in a tracking error in the frequency tracker).

Generally, the lowest frequency f a m 1n ld of the Doppler frequency fd that can be tracked by the two-frequency tracker, and the band f of the low-pass filter (9b). It is about 5 times that of the previous year. On the other hand, the highest frequency Qmax is the band fc of the low-pass filter (9b).
is determined by the overall response speed of the frequency tracker, which depends on
Although the higher the Doppler frequency fd becomes, the response speed of the system becomes lower (the system no longer tracks even slight fluctuations in the input Doppler frequency fd).

Currently, the frequency variable range of the Woo (9g) cannot be wide enough to ensure stable operation.

Therefore, with a conventional frequency tracker, the frequency range that can be tracked is vfamax/fdmin''1'~2
It is impossible to track the Doppler frequency range, which varies by a factor of 0 to 2000 times.

[Problem that the invention seeks to solve]

The conventional frequency tracker described above has a problem in that the range of input Doppler frequencies fd that can be tracked is limited by the band fc of the low-pass filter (9b). Also, Wo
There was also the problem that the variable frequency of o(9g) could not be widened.

The present invention was made to solve such problems, and an object of the present invention is to obtain a frequency tracker that can track the input Doppler frequency fd over a wide frequency range.

[Means for solving problems]

The frequency tracker according to the present invention has a plurality of low-pass filters with different bands and a VCO (9g) at the output end of the mixer (9a).
) is provided with a range switcher for switching each of a plurality of frequency dividers, a plurality of low-pass filters, and a plurality of frequency dividers depending on the input Doppler frequency.

[Operation] In the present invention, a plurality of low-pass filters having bands preset according to the input Doppler frequency range and frequency dividers with different division ratios can be simultaneously switched from the outside according to the input Doppler frequency range. allows the frequency tracker to track over a wide frequency range.

〔Example〕

FIG. 1 is a block diagram of a frequency tracker showing one embodiment of the present invention, and will be described below with reference to the drawings.

In the figure, (9a), (9c) to (90) are the same as the above conventional ones. (9b1) and (9b2) are low-pass filters with different bands, respectively + (''N) y
(9m2) are frequency dividers with different division ratios + (”J
) is a low-pass filter (9b1) depending on the command from the outside,
(9b2) is the first range switcher that switches and outputs the output signal, (9n2) is the frequency divider (9ml) + (9mz
) is a second range switch that switches and outputs the output signal of the range switch according to command A.

In the frequency tracker with the above configuration, Woo (9
The output frequency of g) is determined by the frequency divider (9m1) t (9m2)
The 9 frequency change ratios are the same, but the frequencies are divided so that they partially overlap each other and have different frequency ranges. Low-pass filters (9b1) and (9b2) are frequency dividers (9m1)
, (9m2) each having a band of about 1A at the lower limit frequency of the output. Depending on the input Doppler frequency range, the first range switch (9
By switching the output of each of the low-pass filters (9b1) and (9b2) with n1) and at the same time switching the output of each of the frequency dividers (9m1) and (9m2) with the second range switch (9n2), a wide input can be achieved. This results in a frequency tracker that can track over a frequency range.

For example, if the output frequency change ratio of VOO (9g) is 20
The frequency division ratio of the frequency divider (9m1) is approximately doubled, and the frequency division ratio of the frequency divider (9m1) is
If the frequency division ratio of 9m2) is set to 1/1oo, it is possible to track a frequency range approximately 200 times larger.

〔Effect of the invention〕

As described above, according to this invention, voo (9g)
Multiple frequency dividers and mixers (9a) with different frequency division ratios are connected to the output terminal of
), the low-pass filters with different bands and the low-pass filters of each 2% frequency divider are switched by an external range switching command A. By providing the second range switch, a frequency tracker capable of tracking a wide input frequency range can be provided.

In the above embodiments, switching between two ranges has been described, but it is clear that switching between two or more ranges can also be realized and is included in the present invention.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a frequency tracker showing an embodiment of the present invention, Fig. 2 is a block diagram of a laser Doppler velocimeter, Fig. 3 is a diagram showing the characteristics of a Doppler signal, and Fig. 4 is a diagram of a conventional frequency tracker. FIG. 5 is a diagram showing the characteristics of the frequency tracker. In the figure, (9a) is a mixer, (9b), (9b+
), (9b2) is a low-pass filter, (9c) is a detector, (9d) U-synchronous detector, (9s) welfare integrator,
(qt) is an adder, (9g) Id Woo, (
9h) is an oscillator, (91) is a level adjuster, (
9j) is a lock detector. (9k) is a sweep generator, (9/) is a sweep switch. (9m1)e(9m2) is a frequency divider, (9n1), (9n
2) is the first. This is a second range switch. In addition, the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] A mixer that mixes the input frequency and the local frequency, multiple low-pass filters with different bands that extract the difference frequency between the input frequency and the local frequency, and the output signals of the multiple low-pass filters are switched according to the input frequency range. a first range switcher for outputting, a detector for detecting the output signal of the low-pass filter switched and outputted by the first range switcher, a synchronous detector for discriminating the difference frequency, and the synchronous detection for the above-mentioned. an oscillator that applies frequency modulation to the local frequency; a level adjuster that inputs the output signal of the oscillator and adjusts its frequency modulation width; and an output signal of the level adjuster. and a voltage-controlled oscillator that receives a signal obtained by adding the output signals of the integrator and outputs the local frequency that tracks the input frequency, and a plurality of dividers with different frequency division ratios provided at the output end of the voltage-controlled oscillator. A frequency tracker comprising: a frequency divider; and a second range switch that switches the output signals of the plurality of frequency dividers according to an input frequency range and outputs them to the mixer.