JP3122687B2 - Angle detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an angle detecting device capable of detecting an angle with high accuracy.

[0002]

2. Description of the Related Art Angle measurement is generally performed by using a mechanical standard such as a scale disk or a radial grating. Since it is necessary to manufacture the reference device itself such as a scale disk and a radial grating with extremely high accuracy,
It becomes difficult to manufacture a reference device. In addition, if comparison between the reference device and the object to be measured is not performed accurately, the measurement accuracy is reduced, and the apparatus for this is also large and complicated. On the other hand, as another measurement method, a method using a rotating magnetic field without using a mechanical reference device is known.
There is a problem because it is difficult to create an ideal rotating magnetic field, and the device is large and complicated.

In addition, these methods have a problem that the measurement accuracy is at most about one minute, and it is difficult to obtain a high accuracy of one second or more.

The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to adjust the angle with extremely high accuracy despite its relatively simple structure. An object of the present invention is to provide an angle detection device that can detect the angle.

[0005]

To achieve the above object, according to the Invention The angle detection apparatus according to the present invention comprises a light source means for generating two circularly polarized light or elliptically polarized light rotated in and opposite directions different frequencies, this Optical axis of incident light from light source means
The rotatably provided at the center, and polarizing splitting means for splitting the light emitted from the light source means into two linearly polarized light components perpendicular to each other, the two polarization components split by the polarization splitting means respectively received to and beat signal detection means for detecting a beat signal representing the difference in frequency of the respective polarization components, and the phase difference detecting means for respectively generating a signal representative of the phase difference between the separate set-digit reference signal and the beat signal mentioned above, Adding means for generating a signal representing the sum of the two phase difference signals obtained by the phase difference detecting means; and detecting the rotation angle of the polarization splitting means based on the sum signal.

[0006]

When the polarization splitting means rotates about the incident optical axis, the phase of the beat signal detected by the beat signal detecting means changes according to the rotation. Therefore, by detecting this phase change, it is possible to know that the polarization splitting means has rotated.

When the light emitted from the light source is elliptically polarized light, when this light is decomposed into two linearly polarized light components, there is an amplitude difference and a phase difference between the respective polarized light components. The parameter included in the phase difference between the beat signal and the reference signal detected when the polarization splitting means is rotated increases, and the phase difference signal is detected for only one of the linearly polarized light components obtained by the polarization splitting means. However, the rotation angle of the polarization splitting means cannot be known. Therefore, in the present invention, a phase change is detected for each of the two linearly polarized light components split by the polarization splitting means, unnecessary parameters are eliminated by adding the results, and light emitted from the light source means is elliptically polarized light. In this case, the rotation angle of the polarization splitting means can be accurately detected.

[0008]

FIG. 1 shows a first embodiment of the present invention. In the figure, 1 is a light source means, 2 is a semi-transparent mirror, 3 is an analyzer, 4 is a photodetector, 5 is a semi-transparent mirror, 6 is a polarizing prism, and 7 and 8 are photodetectors. The light source means 1 is, specifically, an internal mirror type Zeeman laser to which a magnetic field of an appropriate magnitude is applied in the direction of the optical axis. In this type of laser, two circularly polarized light beams having different frequencies rotate in opposite directions. Is obtained, and the beat of the light observed after passing through the analyzer is detected, and the wavelength is stabilized by controlling the frequency to be constant.

Next, the operation will be described. A part of the light beam emitted from the light source means 1 is reflected by the semi-transparent mirror 2, passes through an analyzer 3 that passes linearly polarized light oscillating in the plane of the paper (x direction), and is converted into an electric signal by a photodetector 4. Is done. In this case, assuming that the frequencies of the circularly polarized lights are ω ₁ and ω ₂ , respectively, an optical beat signal having a frequency (ω ₁ −ω ₂ ) is obtained from the photodetector 4 and is used as a reference signal. The light transmitted through the semi-transparent mirror 2 has the same characteristics as the semi-transparent mirror 2 and the direction of the incident surface with respect to the semi-transparent mirror 2 is rotated by 90 ° around the optical axis. The direction is the y direction)
And enters the polarizing prism 6. In this case, the semi-transparent mirror 5 has the function of restoring the polarization state disturbed by the semi-transparent mirror 2. The polarizing prism 6 transmits almost 100% of the p-polarized light component oscillating in the plane of incidence, and reflects almost 100% of the s-polarized light component oscillating in the plane perpendicular to the plane. Thus, the light transmitted through the polarizing prism 6 is detected by the photodetector 7 integrated therewith, and the light reflected by the polarizing prism 6 is detected by the photodetector 8 integrated therewith, thereby obtaining an optical beat signal. .

Now, assuming that the polarizing prism 6 is tilted by θ with respect to the paper about the incident optical axis as shown in FIG. 2, the amplitude of the counterclockwise circularly polarized light is a, the frequency is ω ₁ , The amplitude of clockwise circularly polarized light is also a, and the frequency is ω
_Assuming that ₂ , the synthesized amplitude A _x in the x-axis direction is A _x = a (cos ω ₁ t + cos ω ₂ t) (1) The amplitude A _y in the _y -axis direction is A _y = a (sin ω ₁ t) −sin ω ₂ t) (2) Amplitude of p-polarized light transmitted through the polarizing prism 6 and incident on the photodetector 7 Optical beat detected by photodetector 7 Next, whereas the optical beat I _x detected by the reference signal or light detector 4 in the formula (4), at the _{θ = 0, I X = 2a} 2 cos {(ω 1 -ω 2) t} (5 ). As can be seen by comparing equations (4) and (5),
There is a 2θ phase difference between the two beats. Therefore, by measuring the phase difference of the optical beat, the tilt angle (rotation angle) of the polarizing prism 6 can be known.

In the above description, the amplitudes of the two circularly polarized lights are assumed to be equal. However, the amplitudes are generally not exactly equal due to an error. The light emitted from the Zeeman laser is not actually circularly polarized light, but is slightly elliptically polarized light. Even in this case, the tilt angle can be measured without error, as described below. Now, x-axis counterclockwise elliptically polarized light, respectively the y-axis direction component s A _1x, When _{A 1y, A 1x = a 1x} cos ω 1 t (6) A 1y = a 1y sin (ω 1 t + Δ 1 ) (7). Here, a _1x and a _1y are x
The amplitudes in the axis and y-axis directions are assumed to be substantially equal. Delta ₁ is a phase error, a value close to zero. Similarly, for clockwise elliptically polarized light, A _2x = a _2x cos (ω ₂ t + φ) (8) A _2y = −a _2y sin (ω ₂ t + φ + Δ ₂ ) (9) φ is the initial phase difference between the two elliptically polarized lights.

The amplitude of the p-polarized light detected by the photodetector 7 However, For counterclockwise elliptically polarized light, using equations (6) and (7), ^{_{However, a 1 2 = (a 1x}} cos θ + a 1y sin Δ 1 sin θ) 2 + (a 1y cos Δ 1 sin θ) 2 (14) Given by From this, if the second or higher order error of a _1x −a _1y , Δ ₁ is omitted, Similarly, for clockwise elliptically polarized light, equations (8),
Using (9), ^{_{However, a 2 2 = (a 2x}} cos θ-a 2y sin Δ 2 sin θ) 2 + (a 2y cos Δ 2 sin θ) 2 (18) Given by From this, if errors of second order or more of a _2x −a _2y , Δ ₂ are omitted, Becomes Substituting equations (13) and (17) into equation (10) gives here, It is. Therefore, the beat signal of the light detected by the photodetector 7 From equations (16) and (20), It is. The s-polarized light component is detected by the photodetector 8, and the optical beat signal Becomes Accordingly, the inclination angle θ is obtained from the following equation from the measured values of the phase angles of the optical beat signals of the p-polarized component and the s-polarized component, that is, the values on the left side of equations (25) and (26). Here, since the last term of the equation (27) is a constant value, it does not affect the angle measurement. Thus, the first-order error due to the deviation from the circularly polarized light is completely removed.

FIG. 3 shows the first equation for obtaining the result of equation (27).
Although an example of a signal processing unit used in the embodiment is shown, 9, 10, and 11 are bandpass filters, 12 and 13 are phase difference detection circuits as phase difference detection means, and 14 is an adder as addition means. is there. The signal processing unit operates as follows. That is, the optical beat signals from the respective photodetectors 4, 7 and 8 obtained as described above are respectively subjected to bandpass filters 9, 10 and 11 configured to pass their fundamental wave and N-th harmonic. After passing through, the phase difference detection circuit 12
In, the difference between the phase of the N-th harmonic from the photodetector 7 and the phase of the N-th harmonic of the reference signal from the photodetector 4 is detected. Similarly, in the phase difference detection circuit 13, the photodetector 8
The difference between the phase of the N-th harmonic and the phase of the N-th harmonic of the reference signal is detected. The outputs from the phase difference detection circuits 12 and 13 thus obtained are added in an adder 14 to obtain an output θ from which an error has been removed.

Since the above-described phase difference detection circuit is known in both analog and digital systems, further detailed description is omitted, but the digital system can easily obtain higher accuracy. Is advantageous. The reason for using N times higher harmonics is to make the sensitivity N times higher. or,
In order to easily detect the phase difference, the phase difference detection circuit 12,
It goes without saying that the input frequency of the thirteenth input signal may be lowered by frequency conversion such as heterodyne detection.

FIG. 4 shows a second embodiment of the present invention. In this embodiment, a Wollaston prism 6 'is used instead of the polarizing prism 6 in the first embodiment. The light incident on the Wollaston prism 6 ′ is separated into linearly polarized lights orthogonal to each other, and optical beat signals are obtained in the photodetectors 7 and 8. In this embodiment, the other configurations and operations are the same as those in the first embodiment, and therefore, detailed description thereof will be omitted.

In the above embodiments, the light source means 1
Although a laser that emits two circularly polarized lights having different frequencies is used, a laser that emits two linearly polarized lights having different frequencies may be used instead. That is, by passing through a quarter-wave plate whose direction is set so that the fast axis (or the slow axis) becomes 45 ° with respect to the polarization direction, the linearly polarized light is turned in two directions opposite to each other and having different frequencies. It can be converted to circularly polarized light. 5 used a laser 1 'which emits only the frequency omega ₁ of the linearly polarized light, shows a third embodiment of the present invention. In the drawing, 15 and 18 are polarizing prisms, 16 and 17 are reflection prisms, 19 is an optical frequency converter, 20 is a quarter-wave plate, and other components that are the same as those in the above-described embodiment have the same reference numerals. Is attached. In this case, the oscillation direction of the linearly polarized light from the laser 1 'is set at 45 degrees with respect to the paper surface.

Next, the operation will be described. Of the light emitted from the laser 1 ′, the p-polarized light component oscillating in the paper surface passes through the polarizing prism 15 and is reflected by the reflecting prism 16,
The light passes through another polarizing prism 18. On the other hand, the s-polarized light component oscillating perpendicular to the plane of the paper is reflected by the polarizing prism 15 and further reflected by the reflecting prism 17, then converted into light having a frequency ω ₂ by the optical frequency converter 19, and The light is reflected, passes through the same optical path as the p-polarized light component that has passed through another optical path, and exits from the polarizing prism 18. The fast axis (or slow axis) is 4 with respect to the polarization direction
The light is converted into circularly polarized light of opposite directions by the quarter-wave plate 20 placed in the 5 ° direction. The optical frequency converter 19 can be realized, for example, by using a known ultrasonic cell and exciting it at a frequency of ω ₂ −ω ₁ . The signal processing after the counterclockwise circularly polarized light thus obtained is split into two by the polarizing prism 6 and detected by the photodetectors 7 and 8 is essentially the same as in the case of the above-described embodiment. However, the excitation signal of the ultrasonic cell used as the optical frequency converter 19 is used instead of the signal detected by the photodetector 4 as a reference signal.

FIG. 6 is a schematic perspective view of an apparatus in the case where the present invention is applied to the measurement of a prism surface angle. In the figure, reference numeral 21 denotes a fixed base, which houses the system from the laser 1 to the semi-transparent mirror 5 shown in FIG. 1, that is, the light source means and the reference signal generating means.
22 is a turntable rotatably mounted on the fixed base 21;
Reference numeral 23 denotes an autocollimator fixedly attached to the fixed base 21, and reference numeral 24 denotes a prism as an object to be measured mounted on the rotary base 22. The rotating base 22 has polarizing prisms 6 and 6.
A system of the photodetectors 7 and 8, that is, a polarization splitting means, a beat signal detecting means, a phase difference detecting means, and an adding means are mounted. In use, the turntable 22 is turned so that one of two adjacent surfaces of the prism 24 is perpendicular to the optical axis of the autocollimator 23 and the other surface is at the position of the autocollimator 2.
If the rotation angle of the turntable 22 with respect to the position perpendicular to the optical axis 3 is detected by the method according to the present invention, the surface angle can be obtained with high accuracy. In the same manner, the calibration of the scale disk can be performed with high accuracy.

[0019]

As described above, according to the present invention, it is possible to provide an angle detecting device capable of measuring an angle with extremely high accuracy despite its relatively simple structure.

[Brief description of the drawings]

FIG. 1 is a main part configuration diagram of a first embodiment of an angle detection device according to the present invention.

FIG. 2 is a diagram for explaining the principle of the present invention.

FIG. 3 is a configuration diagram of a signal processing unit used in the first embodiment shown in FIG. 1;

FIG. 4 is a configuration diagram of a main part of a second embodiment of the device of the present invention.

FIG. 5 is an overall configuration diagram of a third embodiment of the device of the present invention.

FIG. 6 is a schematic overall perspective view showing an example of the device of the present invention.

[Explanation of symbols]

 1,1 'laser 2,5 semi-transparent mirror 3 analyzer 4,7,8 photodetector 6,15,18 polarizing prism 6' wollaston prism 9,10,11 bandpass filter 12,13 phase difference detection circuit 14 addition Apparatus 16, 17 Reflecting prism 19 Optical frequency converter 20 Quarter-wave plate 21 Fixed base 22 Turntable 23 Autocollimator 24 DUT (prism)

Claims (1)

(57) [Claims] 1. A light source means for generating two circularly polarized lights or elliptically polarized lights having different frequencies and rotating in mutually opposite directions, and rotatable about an optical axis of incident light from the light source means.
A polarization splitting means for splitting light emitted from the light source means into two linearly polarized light components orthogonal to each other; and receiving the two polarized light components split by the polarized light splitting means, respectively, and receiving a difference between the frequencies of the respective polarized light components. Beat signal detection means for detecting a beat signal representing the following; phase difference detection means for respectively generating a signal representing a phase difference between each of the beat signals and a reference signal provided separately; and the phase difference detection means An angle detecting device comprising: an adding unit that generates a signal representing a sum of the two phase difference signals; and detecting a rotation angle of the polarization splitting unit based on the sum signal.