JPH04155260A - Rotational speed measuring apparatus - Google Patents

Abstract

PURPOSE:To achieve higher stability and accuracy in measurement by turning reflection right and left circularly polarized lights from a 1/4 wavelength plate mounted on a rotor to linearly polarized lights with a fixed 1/4 wavelength plate to obtain a rotational speed thereof from a beat frequency generated through an analyzer via a half mirror. CONSTITUTION:Linearly polarized lights orthogonal to each other are impinged into and passes through a fixed 1/4 wavelength plate 2a so arranged that an advance phase axis of a crystal is 45 deg. in each linearly polarized light to be turned to right and left circularly polarized lights. A luminous flux thus obtained is impinged into a rotary 1/4 wavelength plate 3 mounted on a rotor 5 rotating within a polarization plane, subsequently, reflected with a mirror 4 disposed on the rear thereof and passes through the plates 3 and 2a again to be an orthogonal linearly polarized light. A speed of the light is inverted with a half mirror 1 to cause an interference with the passage through a polarizer 6 arranged at 45 deg. of an axial bearing and when the current intensity of the light is detected with a photoelectric detector 7, a light beat signal is detected corresponding to a rotational speed of the rotor 5. This enables stable and highly accurate measurement of the rotational speed free from effect of disturbance and vibration.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a rotational speed measuring device. For example, it is applied to measuring the rotation speed and speed unevenness of a transfer drum of a copying machine, and measuring wow and flutter of a drive motor of an optical disk, a polygon mirror, etc.

As a method of measuring the angular velocity of a rotary body with one vertebrae, it is known to use a laser interferometer or a ring laser. However, in either method, the optical systems must be arranged and combined in a complicated manner, resulting in a complicated and large-sized structure, making it difficult to apply to a tJs-type rotating body. In order to solve this problem, for example, an "angular velocity measurement method" has been proposed in Japanese Patent Application Laid-Open No. 63-222267. In this publication, a circularly polarized light source with a frequency of ω is installed on a rotating body, and a local circularly polarized light source (with a frequency of ω2) is placed externally. The frequency shift amount of ω□ is determined, and the angular velocity is determined from this. However, with this method, it is difficult to miniaturize the light source attached to the rotating body, including the power supply, and furthermore, it is difficult to miniaturize the light source attached to the rotating body. The disadvantage is that light sources are generally very expensive.

Also, “Fundamentals of Optical Applied Measurement (Society of Instrument and Control Engineers, 1)
983, p. 201-206) describes that velocity measurement methods using light can be broadly classified into correlation method, grating method, and laser Doppler method.

The laser Doppler method utilizes the fact that laser light scattered by moving particles undergoes Tosoplar displacement, and the two-incidence method (differential type) is the most commonly used method. However, it has the disadvantage that it is susceptible to speckle noise because it measures the velocity of a rough surface object.

Furthermore, the previously proposed ``rotational speed meter'' measures the rotational speed from a beat signal generated by attaching a small polarizer to a rotating body and making left and right polarized light incident thereon. However, the beat signal detected by this method is
This corresponds to twice the rotation frequency. The drawback was that it could not meet demands for higher sensitivity.

Conventionally, the main methods for measuring the rotational speed are a method using a rotary encoder and a method using the Doppler effect of light. The method using a rotary encoder calculates the speed from temporal changes in the rotational position.

The disadvantage of this method is that the encoder itself has some weight, so it is not suitable for small objects or for objects whose rotation is affected by the weight of the encoder.

In addition, although the optical Doppler velocimeter is non-contact, the rotating body must be a diffused object, and speckle noise occurs and is affected by this.

FIG. 7 is a diagram showing a conventional differential optical Doppler velocimeter, in which 21 is a laser light source, 22 is a collimator lens, 23 is a beam splitter, 24a and 24b are mirrors,
25 is a photodiode, 26 is a test object, and 27 is scattered light.

When a moving object is irradiated with laser light, the frequency of the scattered light shifts from the original frequency of the incident light due to the Doppler effect. This Doppler frequency fD is the velocity vector of the moving object ■, the wave number vectors Ko and Ks of the incident light and scattered light.
is given by 2π. Therefore, by determining fD, the speed ■ can be determined.6 The laser beam split into two by the beam splitter 23 is emitted toward the object to be measured at the intersection angle φ, and the scattered light is Heterodyne detection is performed by a photodetector via an optical lens system. At this time, the Doppler frequency fD of the scattered light with respect to the two incident irradiation lights.

fD2 is given by each. The angle φ, 80 represents the angle formed by the two incident irradiation lights, and the relative setting error angle between the optical system and the speed direction.

The beat frequency fD obtained by combining two types of scattered lights with different frequencies shown by the above two equations using a photodetector is as follows. The speed V can be determined by measuring fD.

In the conventional differential optical Doppler velocimeter shown in FIG. 7, the test object must be located at the intersection of two beams of light, which is susceptible to vibrational displacement in out-of-plane directions, and has the disadvantage that experimental setup is difficult.

Purpose The present invention was made in view of the above-mentioned circumstances.
Easy to place for measurement and resistant to external disturbances.

The purpose of this invention is to present a device that can measure up to the rotational speed of a small object, and to provide a rotational speed measuring device that improves the measurement sensitivity in rotational speed measurement by installing a quarter-wave plate on a rotating body. It has been done.

1-In order to achieve the above object, the present invention (1) injects orthogonal linearly polarized light, and aligns the fast axis direction of the crystal with the linearly polarized light.
A fixed 1/4 wavelength plate installed in the direction of 45 degrees and converts the linearly polarized light into left and right circularly polarized light, and a rotating 1/4 wavelength plate attached to a rotating body receives the left and right circularly polarized light polarized by the fixed 174 wavelength plate. /4 wavelength plate, a half mirror in which the left and right circularly polarized light from the rotating quarter wavelength plate is linearly polarized by the fixed quarter wavelength plate, and the linearly polarized light reflected by the half mirror is incident. , an analyzer installed at ±45° in the axial direction, and the rotational speed of the rotating body is measured from the beat frequency generated by passing the analyzer, or (2) orthogonal linearly polarized light is incident. , a fixed quarter-wave plate whose fast axis direction of the crystal is set at ±45° with respect to the linearly polarized light, and which converts the linearly polarized light into left-right circularly polarized light; and left-right circularly polarized light polarized by the fixed 174-wave plate. a rotating quarter-wave plate attached to a rotating body, and a half mirror in which left and right circularly polarized light from the rotating quarter-wave plate is linearly polarized by the fixed quarter-wave plate, and The rotational speed is made up of an analyzer installed at 45" above the axis, and the rotational speed of the rotating body is measured from the beat frequency generated by passing the linearly polarized light reflected by the half mirror through the analyzer. In the measuring device, an optical system is provided in which the light beam that has passed through the rotating quarter-wave plate twice is incident, reflected by a reflecting object provided outside and reflected by the reflecting object, and then enters the rotating quarter-wave plate again. further, (3) the rotating body and the reflective object installed outside are corner cube prisms, and the reflecting object installed on the rotating body 1/4
(4) The corner cube prism is movable in a direction perpendicular to the optical axis, and the number of reciprocations with the rotating body can be adjusted arbitrarily depending on its position. The feature is that it can be adjusted. Hereinafter, the present invention will be explained based on examples.

FIG. 1 is a configuration diagram for explaining one embodiment of the rotational speed measuring device according to the present invention, and in the figure, 1 indicates a half mirror 12.
a, 2b are fixed 1/4 wavelength plates, 3 is a rotating 174 wavelength plate,
4 is a mirror, 5 is a rotating body, 6 is an analyzer, and 7 is a photodetector.

Linearly polarized lights perpendicular to each other are made incident on a fixed 1/4 wavelength 2a. The fixed 1/4 wavelength plate 2a is arranged so that the fast axis of the crystal forms 45° for each linearly polarized light.
The linearly polarized light that has passed through the wavelength plate 2a becomes left and right circularly polarized light. This light flux enters the rotating quarter-wave plate 3 attached to the rotating body 5 that is rotating within the polarization plane, and is then reflected by the mirror 4 placed on the back surface of the rotating quarter-wave plate 3. The light then passes through the fixed 1/4 wavelength plate 2a and becomes orthogonally linearly polarized light. This orthogonally linearly polarized light beam is reflected by the half mirror 1 and interferes by passing through the analyzer 6 installed at an axial direction of 45 degrees, and the light intensity at that time is detected by the photoelectric detector 7. At this time, when the rotating body 5 is rotating at an angular frequency ω, the left and right circularly polarized lights each undergo a frequency shift of ±2ω, and as a result, an optical beat signal of 4ω is detected.

This can be explained using Jones Matrix as follows.

Let El be the right-handed circularly polarized light and EL be the left-handed circularly polarized light, (EO is the amplitude, ω is the angular frequency of light) ゛ A quarter-wave plate rotating at the angular frequency ω has an orientation of 45''
1/4 wavelength plate Q (45°) and analyzer A (45°) placed at
5°) can be written as . Therefore, left and right circularly polarized light E4. E
When L undergoes a series of polarization effects, E,'=A(45)・Q
(45”)・Q(ωrt)・Q(ω,1)・E.

EL'=A(45)・Q(45°)・Q(ωrt)・Q
(ωrt)・EL.

Therefore, the light intensity detected by the photoelectric detector 7 is I =
By calculating l El+EL l ”, I =
ll1 (1-5in4 ω, t), L= I
E, becomes +2. In other words, the interference light is intensity-modulated at a frequency four times the angular frequency of the rotating body, and by detecting this beat frequency, the rotational speed of the object can be determined. 1/4 wavelength plate 2 shown in Figure 1
A similar effect can be obtained by replacing a with position 2b. In other words, orthogonal linearly polarized light may be made incident on the rotating quarter-wave plate 3, and the reflected light may be passed through the quarter-wave plate 2b and the analyzer 6.

As a light source having orthogonal linearly polarized light, a transverse Zeeman laser is already commercially available. At this time, since the orthogonal linearly polarized lights have a frequency difference Δω, the generated beat frequencies are 4ω and 10Δω. Therefore, the rotational speed can be determined by determining the frequency difference from Δω. Not limited to transverse Zeeman lasers, by giving a frequency difference to two polarized lights that interfere with each other, it is possible to eliminate S due to thermal noise, shot noise, etc. when the rotation speed is around O in beat frequency detection.
/N can be suppressed and highly accurate measurement is possible, and the rotation direction can also be easily determined from the sign of the difference from Δω.

FIG. 2 is a diagram showing another embodiment of the rotational speed measuring device according to the present invention, in which 8 is a half-wave plate, 9 is a prism mirror, 10 is a polarizing beam splitter, 11 is a beam splitter, Other parts having the same functions as those in FIG. 1 are given the same reference numerals.

Since transverse Zeeman lasers are generally expensive, even if an inexpensive laser that emits only a single linearly polarized light is used, the same effect can be obtained by using the configuration shown in FIG. 2.

The half-wave plate 8 used in FIG. 2 is used to rotate the vibration direction of the linearly polarized light coming from the light source in order to equalize the ratio of light amounts separated by the polarizing beam splinter 10. be. By making the scene ratios equal, the S/N ratio of the generated beat signal can be increased. Moreover, even if a birefringent polarizing element such as a Savart plate or a Wollaston prism is used in place of the polarizing beam splitter 10, mutually orthogonal linearly polarized lights can be obtained.

FIG. 3 is a diagram showing still another embodiment of the rotational speed measuring device according to the present invention. In FIG. 1, numeral 12 is a corner cube prism, and other parts having the same functions as those in FIG. 1 have the same reference numerals. It is attached.

By using a corner cube prism 12 in place of the mirror 4 used in FIG. 1, the optical system can be easily set up.

As a light source other than a transverse Zeeman laser that provides orthogonal linearly polarized light, there is an external cavity type semiconductor laser (LD). By using such a semiconductor laser (LD), the overall structure can be made smaller;
In D), since the wavelength fluctuation is large, retardance occurs in the polarizing element, and errors are likely to occur in the measurement results. In such cases, errors due to wavelength fluctuations can be reduced by using an achromatic quarter-wave plate.

The rotational speed measurement device described above installs a polarizer or a quarter-wave plate on a rotating body, makes left-right polarized light or orthogonal linearly polarized light incident on it, and detects the frequency shift that occurs at this time as an optical beat signal. Then, the rotation speed is calculated from this.

However, the detection sensitivity is twice or four times as high as the rotational speed, and measurements requiring even higher precision will be described below.

FIG. 4 is a diagram showing a further improved embodiment of the present invention. In the figure, 13 is a fixed 174 wavelength plate, 14 is a mirror (reflection object), and other parts having the same function as in FIG. 1 are the same. Reference numbers are attached.

Linearly polarized light beams orthogonal to each other are made incident on the fixed 174-wavelength plate 2a. The fixed 1/4 wavelength plate is arranged so that the fast axis of the crystal makes an angle of 45° with each linearly polarized light.
The linearly polarized light that has passed through the wave plate becomes left and right circularly polarized light. This light flux enters a rotating quarter-wave plate 3 attached to a rotating body 5 rotating within the polarization plane, and is then reflected by a corner cube prism (CCP) 12 placed on the back surface.
The light passes through the rotating quarter-wave plate 3 again and becomes left-right circularly polarized light.

However, the direction of rotation of the polarized light at this time is reversed from that at the time of incidence. This left-right circularly polarized light beam passes twice through a quarter-wave plate 13 installed at an azimuth of 45 degrees by a mirror 14 fixed outside, thereby becoming a left-right circularly polarized light beam with the rotation direction of the polarized light reversed and emitted. . Subsequently, the light passes through the 1/4 wavelength plate 3 attached to the rotating body 5 twice, passes through the fixed 1/4 wavelength plate 2a, is reflected by the half mirror 1, and is set at an axial direction of 45°. When the light passes through the analyzed analyzer 6, the light interferes, and the light intensity at that time is detected by the photoelectric detector 7. At this time, if the rotating body 5 is rotating at an angular frequency ω, the left and right circularly polarized lights each undergo a frequency shift of ±4ω, and as a result, an 8ωf optical bit signal is detected.

This can be explained using Jones Matrix as follows.

Let El be the right-handed circularly polarized light and E be the left-handed circularly polarized light, (EO is the amplitude and ω is the angular frequency of light).A quarter-wave plate rotating at the angular frequency ω is placed at an azimuth of 45°. 174 wavelength plate Q (45°) and analyzer A (45°
) can be written as . Therefore, left and right circularly polarized light E, ,E
When L acts on the rotating quarter-wave plate (formula (8)) twice in each round trip, the luminous flux is E l' = Q (ω11)・Q (ω+1)・E1E
L'=Q(ω, 1)·Q(ωrt)·EL.

As can be seen from equations (11) and (12), right-handed circularly polarized light is converted to left-handed circularly polarized light, right-handed circularly polarized light is converted to right-handed circularly polarized light, and each ±2ω
, t undergoes a frequency shift. Furthermore, these two luminous fluxes are 1
/4 wavelength plate (formula (9)) twice, E,"=Q(45")・Q(45°)・El'EL"
=Q(45")・Q(45°)・E,,', and it can be seen that the polarization direction is reversed left and right again. Therefore, when this light beam is made incident on the rotating 174-wavelength plate (formula (8)),
Under the same effect as shown in equations (11) and (12),
As a result, a frequency shift of ±4ωrt occurs in each case.

Therefore, by repeating this process n times, the frequency shift of left and right circularly polarized light can be set to ±2nω, 1. Fourth
The figure corresponds to the case where n=2.

In order to cause left and right circularly polarized light to interfere and generate a beat signal, a quarter wavelength plate (formula (9)) and an analyzer (formula (10)) are required.
All you have to do is let it pass.

Expressing these together as a formula, ElL"'=A(45")・Q(45°)・Q(ω,
1)・Q(ω,1)・E1″E,″=A(45°)・Q
(45°)・Q(ω,1)・Q(ω,1)・EL″′・
... (15) The light intensity detected by the photoelectric detector 7 is calculated as I=1. (1-sin8ω, t) However, l0=
It becomes lE012. In other words, in the case of FIG. 4, it is shown that the interference light is intensity-modulated at a frequency eight times the angular frequency of the rotating body, and by detecting this beat frequency, the rotational speed of the object can be determined.

The same effect can be obtained even if the quarter-wave plate 2a shown in FIG. 4 is replaced in the position of 2b.

That is, it is sufficient to make orthogonal linearly polarized light incident on the rotating 174-wave plate 3, and finally pass the reflected light through the 174-wave plate 2b and the analyzer 6.

Figure 5 shows a rotating 1/4 wavelength plate 3 and a fixed 174 wavelength plate 13.
This shows a method of increasing the frequency shift by repeating reflections between the two times. Corner cube prism (CCP) with fixed 1/4 wavelength plate 13 attached
By moving 14 up and down in a direction perpendicular to the optical axis,
The number of repetitions can be adjusted.

Note that if the light beam that has gone back and forth through a quarter-wave plate that rotates once is returned directly to the rotating quarter-wave plate without passing through the fixed quarter-wave plate, the direction of polarization will be reversed, so it will be reversed. As a result, the one frequency shift disappears.

In addition, we will explain the case where a transverse Zeeman laser is used as a light source with orthogonal linearly polarized light, and we will also explain the case where a transverse Zeeman laser is used as a light source with orthogonal linearly polarized light.
The explanation when using rDiode:LD) is the same as the case of FIG.

FIG. 6 is a diagram showing still another embodiment of the present invention, in which reference numeral 15 is a 172 wavelength plate, and other parts having the same functions as in FIGS. 4 and 5 are given the same reference numerals. It is.

'Fixed 1/4 wavelength plate 13 used in Fig. 4
Instead, a 172 wavelength plate 15 is installed on the return path of the round trip optical path. Passing through the 174 wavelength plate twice is equivalent to passing through the 1/2 wavelength plate once, so
By inserting a 1/2 wavelength plate in either the outgoing path or the incoming path, results equivalent to those shown in FIG. 4 can be obtained.

As is clear from the above description, the present invention has the following effects.

(1) Effect corresponding to claim 1: Since the interference optical system is a completely common path, there is no disturbance or out-of-plane displacement of the rotating object.
It is completely unaffected by vibrations, allowing for stable and highly accurate measurements. Furthermore, since the 174 wavelength plate and the reflective object attached to the rotating body only need to be large enough to reflect the laser beam once, they can be configured sufficiently small, and can be used even for small rotating bodies. Measurements can be made without any influence.

In addition, compared to the method of measuring rotational speed by attaching a polarizer to a single rotating object (beats occur at a frequency twice the rotational speed), the two-rotation method generates beats at a frequency four times the rotational speed, making it less sensitive. is twice as large, making it possible to measure rotational speed with high resolution.

(2) Effect corresponding to claim 2: Since the interference optical system is completely common path, there is no disturbance or out-of-plane displacement of the rotating object.
It is completely unaffected by vibrations and can perform stable, high-precision measurements.

In addition, compared to the method of measuring the rotational speed by attaching a quarter-wave plate to the rotating body (beat generation occurs at a frequency 4 times the rotational speed), this method can be used to measure the rotational speed by 4n times the rotational speed (n is 4n times the rotational speed). It is now possible to generate beats at a frequency equal to the number of reciprocations, and the rotational speed can be measured with a resolution more than twice that of conventional methods.

(3) Effect corresponding to claim 3: By using a corner cube prism, it becomes easier to set up the optical system, and the number of times the rotating quarter-wave plate is reciprocated is two or more times (two times in the case of a mirror). ), making it possible to perform measurements with higher resolution.

(4) Effect corresponding to claim 4: By moving the corner cube prism, the number of times the rotating quarter-wave plate is reciprocated can be arbitrarily set, so that an arbitrary measurement sensitivity can be set.

[Brief explanation of the drawing]

FIG. 1 is a block diagram for explaining one embodiment of the rotational speed measuring device according to the present invention, FIGS. 2 and 3 are diagrams showing another embodiment of the present invention, and FIG. FIG. 5 is a diagram showing a case where a corner cube prism is used as the reflecting object in FIG. 4, and FIG. FIG. 7 is a diagram showing still another embodiment of the conventional differential optical Doppler velocimeter. half mirror, 2a, 2b = fixed 174 wavelength plate, 3
・Rotating 1/4 wavelength plate, 4 Mirror, 5...Rotating body, 6...
・Analyzer, 7...Photoelectric detector. Figure 1 Atsushi Figure 2 Figure 6 Figure 4

Claims (1)

[Claims] 1. Fixed 1/4 wavelength in which orthogonal linearly polarized light is incident, the fast axis direction of the crystal is set in a direction of ±45° with respect to the linearly polarized light, and the linearly polarized light is made left and right circularly polarized light. plate, and the left and right circularly polarized light polarized by the fixed 1/4 wavelength plate are made incident, and the left and right circularly polarized light from the rotating 1/4 wavelength plate and the rotating 1/4 wavelength plate is incident on the fixed 1/4 wavelength plate. It consists of a half mirror, into which the linearly polarized light is made incident by a quarter-wave plate, and an analyzer, which receives the linearly polarized light reflected by the half mirror and is installed at ±45° in the axial direction, and passes through the analyzer. A rotational speed measurement device that measures the rotational speed of a rotating body from the beat frequency generated by the rotation. 2. A fixed 1/4 wavelength plate into which orthogonal linearly polarized light is incident, the fast axis direction of the crystal is set in a direction of ±45° with respect to the linearly polarized light, and the linearly polarized light is made left and right circularly polarized light; The left and right circularly polarized light polarized by the /4 wavelength plate is incident, and the left and right circularly polarized light from the rotating quarter wavelength plate attached to the rotating body is transmitted by the fixed quarter wavelength plate. It consists of a half mirror into which the linearly polarized light enters, and an analyzer which receives the linearly polarized light reflected by the half mirror and is installed at ±45° in the axial direction, and the beat frequency generated by passing through the analyzer. In a rotational speed measuring device that measures the rotational speed of a rotating body from 1 to 2, a light beam that has passed through the rotating 1/4 wavelength plate twice is incident, is reflected by a reflecting object provided outside and the reflecting object, and is reflected again by the rotating 1/4 wavelength plate. 1. A rotational speed measuring device comprising a quarter-wave plate and another quarter-wave plate or a half-wave plate provided in an optical system that makes light incident on the quarter-wave plate. 3. Claim characterized in that the rotating body and the reflective object installed outside are corner cube prisms, and are capable of passing through a quarter-wave plate installed on the rotating body multiple times. 2. The rotational speed measuring device according to 2. 4. The rotation speed measuring device according to claim 3, wherein the corner cube prism is movable in a direction perpendicular to the optical axis, and the number of reciprocations with the rotating body can be arbitrarily adjusted depending on its position.