JPH0355798B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a laser Doppler speedometer that uses the Doppler effect of light to measure the speed of moving objects on manufacturing lines for steel, non-ferrous metals, etc. in a non-contact manner. be.

[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. Second
The figure is, for example, Mitsubishi Electric Technical Report Vol. 58, No. 7, 1984 No. 34.
This is a configuration diagram of a conventional laser Doppler velocimeter shown in the optical fiber sensor laser interferometer described on pages 1 to 38, in which 1 is a moving object, 2 is a laser, and 3 is a beam beam that divides the laser beam into two. The splitters 4a and 4b are optical fiber cables for guiding the laser beam split into two by the beam splitter 3 to the transmitting optical systems 5a and 5b, and 4c is an optical fiber cable for transmitting the received light of the receiving optical system, which will be described later. ,6
1 is a receiving optical system for receiving light scattered from the moving object 1, 7 is a photodetector that converts the received light into an electrical signal, 8 is an amplifier, 9 is a frequency tracker, and 10 is a velocity calculator.

As shown in FIG. 2, a laser beam emitted from a laser 2 to a moving object 1 is split into two by a beam splitter 3, and each beam is sent to an optical fiber cable 4a, 4b.
When the transmitting optical systems 5a and 5b cross-irradiate the moving object 1 from opposite directions, the wavelength of the scattered light of the moving object 1 corresponding to each transmitted light beam will depend on the speed v of the moving object 1. This causes a so-called positive and negative Doppler shift. The scattered light that has undergone the two positive and negative Doppler shifts is received by the receiving optical system 6, guided to the photodetector 7 by the optical fiber cable 4c, and converted into an electrical signal. There are a DC signal proportional to , and an AC signal with a Doppler frequency f _d (hereinafter referred to as a Doppler signal) shown in equation (1).

f _d = 2v/λ・sinφ/2 ...(1) where v: Speed of the moving object λ: Wavelength of the laser beam φ: Intersection angle of the two transmitted light beams Converted into an electrical signal by the photodetector 7 Since the Doppler signal is weak, it is amplified by the amplifier 8, and then the Doppler frequency f _d is measured by the frequency tracker 9, and the velocity calculation unit 10 calculates the velocity using equation (1) to calculate the velocity v of the moving object 1. You can ask for it. This is a known fact.

FIG. 3 is a diagram showing the intersection of two beams from the transmission optical systems 5a and 5b, in which D is the beam diameter, Δθ is the beam spread, and points A, B, and C are 2
This indicates the intersection of the beams in the row, and indicates the intersections of the lower, middle, and upper limits of the beam, respectively.

l is the measured distance from the transmission optical system 5a, 5b to the moving object 1, 2.Δl is the distance between the intersections A and C of the two beams, φ is the beam intersection angle at point B,
φ ₁ is the beam intersection angle at point A, and φ ₂ is the beam intersection angle at point C.

In the laser Doppler velocimeter, the moving object 1 must be located at the beam intersection (between points A and C) in FIG. 3 in order to obtain a Doppler signal by receiving Doppler-shifted scattered light from each of the two beams.

Normally, a traveling line P (hereinafter referred to as a path line) of a moving object 1 passes through an intersection B of two beams 2
The transmission optical systems 5a and 5b are arranged perpendicular to the center line where the striped beams intersect, the measured distance lo at this time is used as the reference distance, and the speed measurement value v' of the speed calculator 10 is used as the speed of the moving object 1. Calibrate it to be equal to v.

In this case, even if the path line P of the moving object 1 moves in parallel, as long as it does not exceed points A and C, a Doppler signal is obtained and the velocity v of the moving object 1 can be measured. In other words, the length of the beam intersection 2・Δl is
The permissible path line P fluctuation range in which the speed of the moving object 1 can be measured is shown.

When the path line P of moving object 1 passes through point B, the beam intersection angle is φ, so the Doppler frequency
f _d is given by equation (1) for velocity v. However, when the path line P changes and passes through point A, the beam intersection angle φ ₁ becomes φ ₁ due to the beam spread Δθ.
=φ+2·Δθ, so the Doppler frequency f _d1 is given by the following equation.

f _d1 = 2v/λ・sin (φ ₁ /2) = 2v/λ・sin (φ/2 + Δθ) ...(2) Similarly, when the path line P changes and passes through point C, the beam intersection angle φ ₂ Since φ ₂ =φ−2Δθ, the Doppler frequency f _d2 is given by the following equation.

f _d2 = 2v/λ・sin (φ ₂ /2) = 2v/λ・sin (φ/2−Δθ) ...(3) As is clear from equations (1) to (3), the movement object
(1) causes a change in the path line P, the Doppler frequency obtained for the velocity v of the moving object (1) becomes a different value, and the velocity calculator 10 that calculates the velocity v of the moving object 1 from the Doppler frequency The speed measurement value v′ will include an error.

Now, the measurement error rate ε when the path line P changes from point A to point C is calculated from equations (1) to (3) by the Doppler frequency f _d when the path line passes through point B.
is given by the following formula based on

ε=f _d2 −f _d1 /f _d =2V/λ・sin(/2−Δθ
)−2V/λ・sin(/2+Δθ)/2V/λ・sin(/
2)=-2·sin(Δθ)/tan(/2) (4) The length of the beam intersection 2·Δl can be approximately approximated by the following equation from the beam diameter D and the beam intersection angle φ.

2・Δl=D/sin(/2) (mm)...(5) Therefore, the measurement error δ per unit distance change of the path line P is calculated by the following equation from equations (4) and (5). Given.

δ=ε/2・Δl=−2・sin(Δθ)・cos(/2
)/D (1/mm) ...(6) Usually, the beam spread Δθ is designed to be several mrad or less, and the beam intersection angle φ is designed to be 10° or less, so sinΔθ≒
Δθ, cos(φ/2)≈1, and equation (6) is approximated by the following equation.

δ=-2・Δθ/D (1/mm)……(7) For example, if the beam spread Δθ is 3 ^mrad and the beam diameter D
The measurement error δ when is set to 4 ^mm is δ=-2×3× ^10-3 rad/4 ^mm =-1.5× ^10-3 (1/mm)...(8), and the pass line P variation is 1 mm. Approximately -0.15 (%) per hit
This results in a measurement error of . Here, the sign of equation (8) is (-)
indicates that a negative measurement error is given when the path line P changes in the direction of increasing the measurement distance l, and gives a measurement error that is approximately proportional to the measurement distance variation.

If the path line P passing through point B in FIG. 3 is a reference and the amount of path line variation from the measured distance lo at this time is Δlp, the speed measurement value v' of the speed calculator 10 is
It is given by the following formula.

v'=v(1+δ·Δlp) (9) For the pass line variation amount Δlp, the direction in which the measured distance l becomes longer than the reference distance lo is defined as (+), and the direction in which it becomes shorter than the reference distance lo is defined as (−).

In general, the path line P of a moving object 1 such as a steel line may vary from several millimeters to several millimeters due to the fluttering of the plate during threading.
It fluctuates by 10mm. Therefore, the measurement distance between the transmission optical systems 5a, 5b and the moving object 1 changes,
This causes an error in the speed measurement value.

[Problem that the invention seeks to solve]

The above-mentioned conventional laser topper speedometer uses a reference distance for measurement distance due to path line fluctuations of the moving object 1.
This has the problem of producing a speed measurement error that is approximately proportional to the amount of variation Δlp from lo.

The present invention was made to solve these problems, and an object of the present invention is to obtain a laser Doppler speedometer that reduces speed measurement errors caused by changes in measurement distance.

[Means for solving problems]

The laser Doppler velocimeter according to the present invention has a displacement meter that measures the amount of variation Δlp of the measurement distance between the transmission optical systems 5a, 5b and the moving object 1 from the reference distance lo, and an output of the displacement meter that measures the measurement distance l. A servo mechanism is provided to keep the distance lo equal to the reference distance lo.

[Effect]

In this invention, a displacement meter that measures the amount of variation Δlp of the measurement distance between the transmission optical systems 5a, 5b and the moving object 1 from the reference distance lo, and
b. A probe in which the receiving optical system 6 and a displacement meter are integrated, and a servo mechanism that controls the position of the probe using the output of the displacement meter are provided to detect the moving object 1.
By controlling the measured distance l to always be equal to the reference distance lo in accordance with the path line variation of the speed measurement error caused by the variation of the measured distance.

〔Example〕

FIG. 1 is a block diagram of a laser Doppler velocimeter showing one embodiment of the present invention, which will be described below with reference to the drawings.

In the figure, numerals 1 to 10 are the same as those of the above conventional example.
11 is a non-contact displacement meter represented by an optical displacement meter using the principle of triangulation; 12 is a transmission optical system 5a;
5b, a probe that integrates the receiving optical system 6 and the displacement meter 11; 13, the output of the displacement meter 11 and the reference value A;
14 is a servo motor, 15 is a ball screw screw connected to the rotor of the servo motor 14 to rotate, 16 is a ball screw nut fixed to the probe 12, 1
7 is a case in which the stator of the servo motor 14 is fixed.

In the laser Doppler speedometer with the above configuration,
When the measured distance l varies from the reference distance lo due to a change in the path line of the moving object 1, the servo amplifier 13 outputs a voltage proportional to the difference between the measured distance l and the reference distance lo, and drives the servo motor 14. Servo motor 1
The rotor 4 rotates forward or reverse depending on the positive or negative output of the servo amplifier, and rotates a ball screw screw 15 directly connected to the rotor of the servo motor 14.
Ball screw nut 1 fixed to probe 12
5, the probe 12 is moved linearly in the direction of the moving object 1. The probe 12 is constrained in its rotational direction via a linear bearing, but is configured to freely slide in the direction of the moving object 1.

Now, if the measured distance l becomes larger than the reference distance lo, in the direction in which the probe 12 approaches the moving object 1,
On the other hand, if l<lo, the probe 12 is moving object 1
By determining the rotation direction of the servo motor 14 so that the servo motor 14 slides in the direction away from the moving object 1,
Even if the path line fluctuates, the measured distance l can always be controlled to be equal to the reference distance lo, resulting in a laser Doppler velocimeter that reduces speed measurement errors caused by fluctuations in the measured distance.

〔Effect of the invention〕

As described above, according to the present invention, there is provided a displacement meter that measures the amount of variation Δlp of the measured distance l between the transmitting optical systems 5a, 5b and the moving object 1 from the reference distance lo, and the transmitting optical systems 5a, 5b, the receiving A probe in which an optical system 6 and a displacement meter are integrated, and a servo mechanism that controls the position of the probe using the output of the displacement meter are provided, so that the measurement distance l is always the reference according to the path line fluctuation of the moving object 1. By controlling the distance so that it is equal to the distance lo, it is possible to provide a laser Doppler speedometer that reduces speed measurement errors caused by fluctuations in the measurement distance.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a laser Doppler velocimeter showing an embodiment of the present invention, FIG. 2 is a block diagram of a conventional laser Doppler velocimeter, and FIG. 3 is a diagram showing a beam intersection of a transmitting optical system. In the figure, 11 is a displacement meter, 12 is a probe, 13 is a servo amplifier, 14 is a servo motor, 15 is a ball screw screw, 16 is a ball screw nut, and 17 is a case. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1 a laser that outputs a specific wavelength, a beam splitter that splits the output beam of the laser into two,
An optical fiber cable that transmits the two divided laser beams, two transmission optical systems that receive the transmitted laser beam and irradiate it onto a moving object by crossing it from opposite directions, and each of the two irradiation beams. a receiving optical system that simultaneously receives the scattered light that has undergone a Doppler shift in accordance with the speed of the moving object; an optical fiber cable that transmits the scattered light that includes the Doppler signal received by the receiving optical system;
a photodetector that electrically converts the transmitted scattered light; an amplifier that amplifies the output of the photodetector; a frequency tracker that detects a Doppler frequency from the amplified signal; and an output signal of the frequency tracker. In a laser Doppler speedometer equipped with a speed calculator that calculates the speed of a moving object from a Doppler frequency, the distance is always kept constant in order to reduce speed measurement errors due to distance fluctuations between the moving object and the transmitting optical system. A laser Doppler speedometer characterized by being provided with a servo mechanism that can control the position of the transmitting optical system so that the position of the transmitting optical system is controlled.