JPS58158506A - Optical measuring method of displacement - Google Patents

Abstract

PURPOSE:To make it possible to perform highly accurate measurement, by changing the length of a light path of reflected light; when the light is irradiated on a reference material and a material to be measured, respectively, the interference light obtained by synthesizing the reflected light beams is utilized, and the displacement is measured. CONSTITUTION:The laser beam emitted from a laser oscillator 1 is reflected by a beam splitter 2 and inputted into a 1/4 wavelength plate which is the reference material. A part of the laser beam is reflected by the surface of the 1/4 wavelength plate, and the other part is reflected by the surface of the material to be measured 13. The respective reflected light beams are inputted into a polarization beam splitter 3 through a beam splitter cube 2. The light beam reflected by the 1/4 wavelength plate passes a 1/4 wavelength plate 6, and is reflected by a mirror surface 10 and also by the polarization beam splitter 3, and then the beam is inputted into a polarizing plate 11. The light beam reflected by the material to be measured 13 passes a 1/4 wavelength plate 5, and is reflected by a mirror surface 9, and then the beam is inputted into the polarizing plate 11. The mirror 9 is controlled so that the interference light which has passed the polarizing plate 11 becomes the maximum or minimum.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical displacement measuring method used for measuring the displacement of machines that require accurate positioning, such as NC machine tools and coordinate measuring instruments.

A method for optically measuring minute gaps and displacements of the object to be measured is to treat the displacement of the object as a change in the optical path difference, and to feedback control the optical path length so that the optical path difference is always constant. A method has been proposed in which the amount of change is measured from the controlled amount. This f'L has already been proposed by Ganto Honda in Japanese Patent Application No. 55-4591.

However, in the above control method, since the measurement range is determined by the operating range of the control element, measurement becomes difficult when the displacement of the object to be measured is large.

In view of the above points, an object of the present invention is to provide an optical method for measuring displacement, which makes it possible to measure displacement at a high degree of accuracy even when the displacement of an object to be measured is large.

Hereinafter, a simple embodiment of the present invention will be explained with reference to FIG. 1 is a laser oscillator that oscillates a linearly polarized laser beam with wavelength λ, 2 is a beam splitter cube that transmits or reflects the laser beam regardless of the vibration plane of the incident V-beam, and 3 is the vibration plane of the incident laser beam. 4.5.6 is a l/4 wavelength plate, 7.8 is a mirror surface 9.10' (i = displacement An adjuster that varies the optical path length by adjusting the length of the optical path, for example, an electric flexure element.The electric flexure element 7 displaces the mirror surface 9 according to the brightness of the detected interference light, and the electric flexure element 8 displaces the mirror surface 10 with a high frequency wave as described below. It has a function of determining the sign of the optical path difference by displacing it with a high frequency voltage from an oscillator. 11 is a polarizing plate; 12
13 is a photoelectric conversion element, 13 is an object to be measured, 4 is a high-frequency oscillator, 5 is an AC amplifier, 17 is a synchronous detection circuit, 18
is a low-pass filter, 19.20 is a voltage comparison circuit, 2
1 is an up/down counter, 22 is an OR circuit, 23 is an analog switch, and 24 is a DC amplifier.The laser beam oscillated from the laser oscillator I is incident on the laser beam. A portion of this laser beam is reflected by the surface of the quarter-wave plate 4, and another portion is reflected by the surface of the object to be measured 13. Each of the reflected lights enters the polarizing heam splitter 3 via the beam splitter cube 2. Here, one of the lights reflected on the surface of the quarter-wave plate 4 is transmitted through the polarizing beam splitter 3, and the light reflected from the object to be measured 13 is reflected by the polarizing beam splitter 3. The light that has passed through the polarizing beam splitter 3 passes through the 1/4 wavelength plate 6, is reflected by the mirror surface 10, passes through the 1/4 wavelength plate 6 again, is reflected by the polarizing beam splitter 3, and enters the polarizing plate 11. do. At this time, the incident light makes one round trip through the 1/4 wavelength plate 6, so the vibration plane is 90°.
゜Rotate. The other light reflected by the object to be measured 13 is reflected by the polarizing beam splitter 3, transmitted through the 1/4 wavelength plate 5, reflected by the mirror surface 9, and transmitted through the 1/4 wavelength plate 5 again. , passes through the polarizing beam splitter 3 and enters the polarizing plate 11. Here, it is reflected on the surface of the 1/4 wavelength plate 40,
Since the vibration planes of the light that reached the polarizing plate 11 and the light that was reflected by the object to be measured 13 and reached the polarizing plate 11 are different from each other by 90 degrees,
Each other's ex doesn't interfere. For example, if the transmission axis of the polarizing plate 11 is set to Fi relative to the vibration plane of the light reflected from the object to be measured 13, the light reflected from the object to be measured 13 and 1
Each n component of the light reflected from the /4 wavelength plate 4 in the transmission axis direction of the polarizing plate 11 is transmitted through the polarizing plate 11'jk and causes interference.

The brightness of this interference light is converted into an electric signal by an electric conversion element.

This optical path difference is caused by the difference between the optical path length until reaching the photoelectric conversion element 12 and the optical path length until reaching the photoelectric conversion element 12 after being reflected by the next surface of the object to be measured 13. Next, this optical path difference will be explained.

In FIG. 1, t+ = t2.1/4 wavelength plate 4
Letting X be the optical path length from the surface of the object 13 to the front surface of the object to be measured 13, the optical path difference rl 2 X is obtained. Here, the brightness of the laser beam that is reflected from the surface of the quarter-wave plate 4 and reaches the photoelectric conversion element 12, and the brightness of the laser beam that is reflected from the surface of the object to be measured 13 and reaches the optical conversion element 12. are both equal to I, then the brightness IT of the interference light is expressed by equation (1).

IT=21 (1-cO89') -
(1) However, the phase difference ψ is expressed by equation (2).

4π, -X... (2) λ Figure 2 (a) shows the relationship between the optical path difference and the brightness of the interference light.

Returning to the front, when a high frequency sinusoidal voltage is applied to the electric flexure element 8 from the high frequency oscillator 14, the mirror surface 10 is slightly displaced. The displacement f at this time is ±Δ! It will be 7/2. Now, assume that the optical path difference 2X is at point a before the mirror surface 10 is displaced. When the mirror surface 10 is vibrated at high frequency in this state, the brightness of the interference light changes as shown by the dotted line in FIG. 2(o).

Further, in FIG. 2(o), the caustic line indicates the voltage applied to the gage element 8. The change in brightness of the interference light is converted into an electrical signal by the conversion element 12, amplified by the AC amplifier 16, and then detected by the synchronous detection circuit 17. This detected signal is further amplified by the DC amplifier 24 via the low-pass filter 18 and is applied to the electric flexure element 7.
is slightly displaced, and the optical path difference changes. on the other hand,
Assuming that the optical path difference is almost at point b in FIG. 2(A), when the mirror surface 10 is vibrated at high frequency in this state, the brightness of the interference light will be as shown by the point resistance in FIG. 2(C). Change. In FIG. 2 Pi, the voltage applied to the actual If#rj and the cage rope 8 is shown. When the brightness of the interference light is converted into an electrical signal by the photoelectric conversion element 12, synchronous detection is performed in the same manner, and the signal is passed through a low-pass filter, the output voltage becomes O.

That is, in FIG. 2(a), if the optical path difference is initially at point a, the brightness of the interference light at this time is converted into an electrical signal, and the synchronous detection circuit 17 and the low-pass filter 1
8. When an electric current is applied to the wire 7 through the DC amplifier 24, the mirror 9 is slightly displaced and the optical path difference reaches point b, resulting in balance. Thereafter, when the object to be measured 13 is displaced in the same way, the mirror 9 is also displaced accordingly, so that the optical path difference is always the second.
In the figure (a), it moves to point b. At this time, if the relationship between the applied voltage and the displacement of the electric flexure element 7 is calibrated in advance, the displacement of the object to be measured 13 can be measured by measuring the voltage applied to the electric flexure element 7. can. Measurement range 1ffil-j at this time: Determined by the operating range of the electric cable 7. In addition, since the measurement angle f″i is determined by the relationship between the applied voltage and the displacement of the wavy wire, high n-degree measurements can be performed.

Next, a case where the object to be measured 13 changes significantly and fc will be described.

FIG. 3(A) shows the relationship between the displacement X of the object to be measured 13 and the brightness of the interference light. Further, FIG. 3(b) shows the relationship between the displacement and the voltage applied to the electric wire 7. Position iX of the origin of the object to be measured 13. Suppose that the optical path difference is at point a at this time. When the device in the first section operates in this state, a voltage of -E0 is applied to the electric flexure element 7 as shown by the solid line in Figure 3 (B), and as shown in Figure 3 (A).
Balance at point b. From this state, if the displacement X of the object to be measured 13 increases and reaches Xl, then the electric wire 7
As shown in FIG. 3 (b), the voltage applied to the awn changes depending on the amount of displacement. At this time, if the soft voltage of the voltage comparison circuit 19 is set to 1, the output voltage of the voltage comparison circuit 19 is always 0, but becomes 1 when the output voltage of the low-pass filter 18 reaches E. , OR circuit 2
An electronic switch 23 is operated via 2. As a result, the output voltage of the DC amplifier 24 becomes 0, and 0 as shown in FIG.
balance on points. When the displacement X further increases, balance is achieved at point d in the same way, and when the displacement becomes X, a voltage of E is applied to the electric skew element 7, resulting in balance at point d.

Therefore, the origin position X of the object to be measured 13.

The displacement X from to X can be measured as follows. The output voltage of the voltage comparator circuit 9 is the object to be measured 13
Since it changes from 0 to 1 every time the needle is displaced by λ/2, this is counted by the up/down cuff/ter 21, and the result of the number of stitches is multiplied by λ/2. A case where the displacement X changes in a decreasing direction from the position fC will be described.

(9) In this case, a voltage of -Eo is applied to the balance element 7 in advance, and the voltage is balanced at point b. From this state, the displacement
Assuming that the object to be measured 13 is displaced in the direction in which
A negative voltage is applied to the voltage distortion element 7 as shown in FIG. 3(b). If the specific soft voltage ' of the voltage comparator circuit 2o is set to -E, the output voltage of the voltage comparator circuit 20 always changes from 0 to 1 and is balanced at point b'. Therefore, the amount of displacement of the output voltage of the voltage comparison circuit 20 to the up/down column XI' can be determined.

According to the measurement of the present invention, it is possible to measure not only small displacements but also large displacements. *It becomes possible to measure degrees.

[Brief explanation of the drawing]

FIG. 1 is a principle diagram showing the optical displacement measurement method of the present invention;
Figure 2 (a), (b), C → and Figure 3 (a), (0
) is a diagram for explaining the operating principle of the method of the present invention. 1...Nuza oscillator, 3...Polarizing beam splitter, 7.8...Military beam splitter, 12...Light 1 conversion element,
13 (10) ... Object to be measured, 21 ... Up-down counter,
23...Electronic switch. Thatch 2 fig.

Claims (1)

[Claims] In a method of measuring the displacement of an object to be measured by irradiating the reference object and the object to be measured with light and combining the respective reflected lights, the displacement of the object to be measured is measured using interference. A regulator that makes the optical path length fully variable is installed in the middle of the optical path, and the brightness of the interference light generated due to the optical path difference is detected.
The detected signal controls the adjuster so that the brightness of the interference light is always at the maximum or minimum, and when the control amount of the adjuster exceeds a certain value, the adjuster is returned to its original state. A displacement method characterized in that the number of times the amount exceeds a certain value is counted, and the controlled amount within the certain value is measured.≠4 The amount of displacement of the object to be measured is measured from the composite value of both. optical measurement method.