KR100336307B1 - distance measuring apparatus and method using laser - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distance measuring apparatus and method using light, wherein light emitted from the light source 1 is passed through a beam splitter 3 to be divided into first and second split lights, and the first split light is predetermined. The second split light is reflected back to the moving reflector 5 and interferes with each other by being reflected by the inclined reflector 7 inclined at an angle and interfering with each other. It is characterized by measuring the moving distance of the reflector, and because of its excellent resolution and simple optical system, manufacturing cost is low and long distance can be measured precisely.

Description

Distance measuring apparatus and method using light

The present invention relates to a distance measuring apparatus and method using light, and more particularly to a distance measuring apparatus and method for measuring the moving distance of a moving object moved by a drive source.

Equipment such as precision inspection equipment, precision machine tools and three-dimensional measuring machines in various industries including the semiconductor industry will necessarily require a precision transfer device. The conveying apparatus is divided into three parts, and is composed of a motor as a power source, a distance measuring device for detecting a conveying amount, and a guide way for guiding a conveying motion.

The distance measuring device of the transfer device is the most important part that determines the precision of the transfer movement.

As one type of conventional distance measuring apparatus, an optical linear encoder is disclosed. This optical linear encoder is equipped with four light emitting elements and P / 4 intervals on both sides of a glass rod with a scale of 4 μm to 20 μm (Pitch = 'P') (called 'main grating'). When the main grid moves, the amount of light received by the light receiving element changes according to the position of the scale, and the four light receiving elements that generate an electrical signal proportional to the amount of light are the main grid. It generates a repetitive electrical signal with the same period as the scale pitch (P) of each output signal. Each output signal has a phase difference of 90 °, which is 1/4 of the main grid pitch of the UP / DOWN counter. The transfer amount of the main grid is determined by increasing and decreasing the count value at each cycle. That is, such a conventional distance measuring device has a resolution corresponding to 1/4 of the main grid pitch.

By the way, the conventional optical linear encoder as described above requires a regular interval scale as much as the measurement length, this scale has a direct influence on the accuracy of the distance measurement, so not only should be very precise, but also if the measurement length is longer Since the engraved main grid must also be long, there was a problem that the price of the measuring device increases proportionally with the measuring length. Due to this problem, the conventional optical linear encoder is mainly used for distance measurement of 0.2 m or less.

Accordingly, the present invention has been made to solve the above problems, and to provide a distance measuring apparatus and method that is excellent in resolution, inexpensive, and can accurately measure long distances.

1 is a schematic configuration diagram showing a distance measuring device according to an embodiment of the present invention;

2 is a schematic structural diagram showing a distance measuring device according to another embodiment of the present invention;

3 is a graph illustrating a process of forming an interference fringe between a first split light and a second split light and a relationship between a phase grating and a light receiver;

4 is a graph showing an output signal of a light receiving unit that changes when the moving reflector moves in the present invention;

5A is a block diagram showing an apparatus for generating a pulse signal from an interference fringe incident on a light receiving portion;

FIG. 5B is a graph showing a process of generating a pulse signal in FIG. 5A;

6 is an apparatus for testing the effect of the distance measuring device using light according to the present invention;

7 is a graph showing the output of the linear image sensor used to confirm the pitch of the interference fringe formed in the photodiode of FIG.

FIG. 8A is a graph showing the signal of the photodiode when the piezoelectric driver of FIG. 6 is increased by 0.125V and driven at 100 steps; FIG.

8B is a graph showing a signal of a gap sensor when the piezoelectric actuator of FIG. 6 is increased by 0.125V and driven at 100 steps;

FIG. 9 is a graph comparing the displacement measured by the gap sensor and the displacement obtained at the output of the counter when the piezoelectric actuator of FIG. 6 is driven at 100 steps in 0.125V increments.

<Description of the symbols for the main parts of the drawings>

1: light source 3: beam splitter

5: moving reflecting mirror 7: tilt reflecting mirror

9: light receiving unit 11: phase grating

13: beam enlarger

In the distance measuring device using light according to the present invention, the distance measuring device for measuring the moving distance of the moving object moved by the drive source, the light source for generating light, and the light from the light source is separated from the second separation light and the first separation A beam splitter provided at one side of the light source to separate the light, a moving reflector fixed to the moving body so that the second split light is reflected and returned, and the beam to be reflected back to the first split light at an angle A light receiving unit provided on the other side of the non-split splitter to detect an interference fringe generated by combining an inclined reflector coupled to one side of the splitter, a second split light reflected from the moving reflector, and a first split light reflected from the tilt reflector; And a phase grating disposed between the beam splitter and the light receiving unit so as to pass only a specific portion of the interference fringe. It characterized in that comprises.

A beam enlarger may be provided between the light source and the beam splitter to enlarge light emitted from the light source.

In the distance measuring method using light according to the present invention, light from a light source passes through a beam splitter, and is divided into first and second split lights, and the first split light is reflected and returned to the inclined winter which is inclined at a predetermined angle. At the same time, the second split light is reflected by the moving reflector and returned to interfere with the first split light and the second split light, and the interference pattern is detected by the light receiving unit to detect the moving distance of the moving reflector. It is characterized by measuring.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

As shown in Fig. 1 (in this drawing, the moving object and the driving source for driving the same) are not shown. On one side of the light source 1, the light emitted from the light source 1 is separated by the first split light O1 and the second split light. A beam splitter 3 is provided so as to be separated by (O2), and the movable reflector 5 is fixed to a moving body (not shown) so that the second split light (O2) is reflected and returned, and the first split light ( An inclined reflector 7 is coupled to one side of the beam splitter 3 so that O1 is inclined at a predetermined angle and returned, and the second split light O2 reflected by the moving reflector 5 and the second split light O2 are reflected. A light receiving portion 9 is provided on the other side of the beam splitter 3 so as to detect an interference pattern generated by the aggregation of the first split light O1 reflected from the inclined reflector 7, and passes only a specific portion of the interference pattern. The beam splitter 3 and the light receiving unit Is (9) it is a phase grating (11) (phase grating) installed between.

As shown in FIG. 2, a beam expander 13 may be provided between the light source 1 and the beam splitter 3 to enlarge the light emitted from the light source 1.

The light source 2 is made of helium-neon laser light, but any kind of laser beam can be used.

The beam splitter 3 is a known unpolarized beam splitter. In this case, the beam splitter may be a polarizing beam splitter. When using a polarizing beam splitter, a λ / 4 plate is provided between the polarizing beam splitter and the reflecting mirror so as to change the path of the light returned from the reflecting mirror by 90 degrees.

The moving reflector 5 has its surface perpendicular to the traveling direction of the second split light O2, and the inclined reflector 7 has its surface running in the first split light O1. It is installed to be inclined at a predetermined angle θ in a vertical plane with respect to the.

The light-receiving portion 9 is a photodiode known as a light-receiving element utilizing the photovoltaic effect, and is preferably a quadrant photodiode. As shown in FIG. 5A, the light receiving unit 9 includes a comparator 21 and 23, a direction discriminator 25, and an up / down counter 27. The light receiving circuit is combined.

The phase grating 11 is known as a component for reflecting and returning from the moving reflector 5 and the oblique reflector 7 to pass a portion of the interference fringe wavelength at predetermined intervals.

In the distance measuring device according to the present invention configured as described above, the light from the light source 1 is incident on the beam splitter 3. The light incident on the beam splitter 3 is divided into a first split light O1 and a second split light O2, and the second split light O1 is reflected by the moving reflector 5 to radiate the beam splitter 3. Returned to). On the other hand, since the inclined reflector 7 is inclined by θ, the first split light O1 reaches the beam splitter 3 in a state inclined by 2θ with respect to the optical axis when reflected by the inclined reflector 7. The first split light O1 and the second split light O2 gathered from the beam splitter 3 are interfered and enter the phase grating 11. The interference fringe formed at this time has a straight stripe shape and the interval of the stripe has a constant value according to the inclination angle of the inclined reflector 7. When one period of the linear fringe interference pattern is 360 °, the phase grating 11 arranged at intervals of 90 ° passes through the interference fringe of only the corresponding part and enters the photodiode 9. The photodiode 9 outputs an electrical signal proportional to the incident light energy.

At this time, the light emitted from the light source 1 may be enlarged while passing through the beam expander 13 shown in FIG. 2 and may be incident on the beam splitter 3. The structure passing through the beam expander 13 makes the measurement easier.

This process will be described in detail with reference to FIG. As shown in Fig. 3, since the reflective surface of the moving reflector is perpendicular to the optical axis, the second split light reflected here and incident on the phase grating maintains a wave front parallel to the phase grating plane. The wave front of the first split light reflected by the inclined reflector inclined by θ with respect to the optical axis is inclined by 2θ with the phase lattice plane. As a result, the light energy has the maximum value at the position where the second split light and the first split light are in the same phase, and the light energy is minimized at the 180 ° shifted phase. It shows the distribution of light energy. Here, the pitch of the interference fringes (Pitch = 'P') is determined by the wavelength of the light used (Wave Length = 'λ') and the tilt angle of the inclined reflector (Tilting Angle = 'θ'). .

When the moving reflector 5 moves by L in the same direction as the second splitting light, the moving distance of the second splitting light becomes 2L, so that the interference fringe is moved in the perpendicular direction of the stripe. Obtained as

The theoretical background of this equation is explained below.

The light reflected by the moving reflector 5 has the same phase over the front surface, and the wave front thereof is expressed as follows.

Where a is the reflected light amplitude of the moving reflector, d is the distance between the beam splitter 3 and the oblique reflector, and j and k are respectively And 2π / λ.

When the inclined reflector is inclined by θ, the reflector surface can be expressed by the following equation according to the position ｘ.

Where {z} _ {0} represents the distance from the same phase.

Therefore, the wavefront of the light reflected by the inclined reflector is expressed as the following equation (3) as a function of the position of the inclined reflector 7 surface.

Where b is the reflected light amplitude of the oblique reflector 7 and z (x) is a function of the position of the oblique reflector surface. The wavefronts of the two lights reflected by the moving reflector and the tilt reflector are combined in the beam splitter 3, and an interference fringe is generated by the phase difference between the wavefronts generated by the optical path difference of the two lights. Is represented by the following equation (4) from equations (1) and (3).

Here, I ₀ represents the mean intensity of the interference fringes, Where gamma is the fringe contrast And k represents a constant of 2π / λ.

The shape of the interference fringe I (x) generated by Equation (4) shows an image of a periodic shape according to the spatial position of the moving reflector with respect to the inclined reflector. Here, the pitch (Pitch, P) of the interference fringe is determined by the wavelength (Wave Length, λ) of the light used and the tilting angle (θ) of the tilting mirror, and has the following relationship.

When the moving reflector moves by L in the x-axis direction, the interference fringe moves in the direction perpendicular to the stripe. If this amount is l, it is obtained as follows.

This equation shows that when the moving reflector moves by λ / 2, the interference fringes move one pitch. Therefore, when a 0.6328 μm wavelength light source is used as the He-Ne laser which is widely used for measurement, a pitch having a pitch of 0.3164 μm may be easily implemented.

This can significantly improve the position detection resolution as compared to using a grating having a pitch of 4 μm to 20 μm in a conventional linear linear encoder.

In the optical energy distribution of the harmonic function type such as Equation (5) and Equation (6), when one cycle is 360 °, the phase grating 11 is illustrated to detect four light energy at 90 ° intervals. 2 and the output signal of each photodiode 9 is detected.

4 shows four photodiode 9 output signals A, A ', B, and B' as the moving reflector 5 moves, each of which has a 90 ° phase difference. It can be seen. Since this signal is the same as the output of a conventional optical linear encoder, by using a commercial four-channel UP / DOWN counter, it is possible to obtain a distance measuring resolution corresponding to λ / 8 in terms of the amount of movement of the moving reflector. You can get it.

FIG. 5B shows a process of generating four pulse signals for one period of harmonic function in a quadruple UP / DOWN counter, which is widely used in a conventional optical linear encoder. Omit.

Next, an experimental apparatus and results of experiments will be described to know the performance and effects of the present invention.

In this experiment, the signals of A, B, A ', and B', each of which are 90 degrees out of phase from the optical interference fringe, are used, and the quadruple technique commonly used in the conventional optical linear encoder is used.

6 shows the configuration of the experimental apparatus of this experiment.

As a light source, a He-Ne laser (5 mW) having a wavelength of 633 nm was used, a beam expander (not shown) used a beam expander that enlarges the laser light to a diameter of 20 mm, and a photodiode used a four-segment photodiode.

In addition, the linear image sensor was used to adjust the pitch of the interference pattern by adjusting the angle of the inclined mirror using a fine adjustment screw, and to match the pitch of the formed interference pattern with the pitch of the phase grating. The quad split photodiode used here is HAMAMATSU S5106, and the linear image sensor is OPA512T, OKI's MOS image sensor. And, in order to drive the moving mirror and measure the amount of displacement generated at this time, a precision feed system using a leaf spring structure as a motion guide surface was constructed.

In addition, TOKIN's piezoelectric actuator (AE0505D16) was used for micro-driving, ADE's capacitive gap sensor (ADE-2102) was used for microdisplacement measurement, and displacement sensor and amplifier were 1.0 V / μm. Adjusted to have an output of.

As shown in Fig. 7, the output signal of the linear image sensor used to check the pitch of the interference fringes formed in the four-segment photodiode by the experimental apparatus. The period of this output signal was adjusted using the fine adjustment screw to coincide with the pitch of 1.16 mm of the phase grating.

8A and 8B show a signal of a four-segment photodiode and a gap sensor signal when the piezoelectric actuator PZT of the precision transfer system is driven by 0.125V in driving 100 steps. As a result of constant driving of about 11.85 nm per step by the piezoelectric actuator, the signals A, B, A ', and B' of the four-segment photodiode are shown in the form of a cosine function having 90 degrees of phase difference.

The signals A and A 'and B and B' in Fig. 8A are converted into square waves by a comparator and input to UP / DOWN counters (LS7166, LSI Co., Ltd.). In the counter, because it is 632.8 nm of the used laser wavelength, it is quadrupled, and finally the counter value is increased every 79.1 nm, which is 1/8 of the wavelength. Fig. 9 compares the displacement measured at the gap sensor (denoted as 'G') and the displacement obtained at the output of the counter (79.1 nm x counter value, denoted as 'C') when the piezoelectric actuator is driven 100 steps. It is. According to these results, the error of the linear encoder developed in this study was 2σ = 56.54nm at the displacement of 1168nm.

As shown in the above experimental results, since the laser wavelength used in the experiment is 632.8nm, the output of the four-segment photodiode is quadrupled to obtain a resolution of 79.1nm, which is 1/8 of the wavelength. have. In addition, when comparing the displacement measurement with the gap sensor, the error of the linear encoder was 2σ = 56.54 nm. This can be expected to improve the resolution of about 12 times or more compared to the resolution obtained by four times the pitch of the main grating having a pitch of about 4 μm to 20 μm in the optical linear encoder that is generally used.

In addition, the laser has excellent coherence, so there is no additional cost to measure a long distance of about 20m.

According to the distance measuring device and method using the light according to the present invention configured as described above, since the resolution is excellent and the optical system is simple, the manufacturing cost is low and the long distance can be measured accurately.

Claims (3)

In the distance measuring device for measuring the moving distance of the moving object moved by the drive source, A light source for generating light, A beam splitter installed at one side of the light source to separate the light emitted from the light source into a second separation light and a first separation light; A moving reflector fixed to the moving body so that the second split light is reflected and returned; An inclined reflector installed at one side of the beam splitter inclined with respect to the optical axis such that the first split light is inclined at a predetermined angle and reflected back; A light receiving unit provided on the other side of the beam splitter to detect an interference fringe formed by the combination of the second split light reflected by the moving reflector and the first split light reflected by the tilt reflector; And a phase grating provided between the beam splitter and the light receiving portion so as to pass only a specific portion of the interference fringe. The method of claim 1, And a beam expander is provided between the light source and the beam splitter to enlarge the light emitted from the light source. The light from the light source is passed through a beam splitter and divided into a first separation light and a second separation light, The first split light is reflected back to the inclined reflector inclined at a predetermined angle, and the second split light is reflected back to the moving reflector to interfere with the first split light and the second split light. The distance measuring method using light which detects this interference pattern by a light receiving part, and measures the moving distance of the said moving reflector.