JPH032520A - Position detector - Google Patents

Abstract

PURPOSE:To offer a small-sized, inexpensive position detector with high accuracy by composing the position detector of a semiconductor laser, a diffraction grating, an interference means which makes diffracted light beams interfere with each other, and a photodetecting means. CONSTITUTION:The light from a light source 13 composed of a laser diode is passed through a collimator lens 2 and made incident vertically on the diffraction grating 10. The primary diffracted light is reflected by Porro prisms R1 and R2, made incident on one point X2 on the grating 10, and diffracted again. Polarizing plates P1 and P2 are provided, so while diffracted light of 0th order is cut off at the point X2, both the diffracted light beams are made incident on a beam splitter 11. Both linear polar ized light beams which are transmitted pass through a polarizing plate P3 become mutually coherent light beams, which are made incident on a photodetecting element D1. Both linear polarized light beams which are reflected pass through a polarizing plate P4 and are made incident on a photodetecting element D2. Signals corresponding to the amplitude center levels of sine signals S1 and S2 obtained from the elements D1 and D2 are compared to generate a pulse signal. This pulse is generated at intervals of the movement quantity of the diffraction grating 10 and counted to know the movement quantity.

Description

[Detailed description of the invention] [Field of application of the invention] The present invention relates to a position detector using a diffraction grating.

[Background of the Invention] Conventionally, the so-called More I scale, in which two transmission diffraction gratings are superimposed, has been widely used as a means of measuring linear displacement in units of microns (p, m). It is being Known examples disclosing such techniques include, for example, Journal Otuphysinox I12 Scientific Instruments 1 1972 No. 5
Volume No. 193 (, Iourna1, ofphys)
cs E: 5 scientific d+
s1. rumCnt1972 Vol, 5. p193
). An example of this is shown in Figure 1. The light from the light source is made into parallel light by the collimator lens 2, and the period is 81.
The main scale 3, on which grating grooves of about t m are formed, is irradiated.

An Intex scale 4 having grating grooves with the same period as the main scale 3 is arranged to face the main scale 3, and the irradiated light can pass through or , may be shielded. The transmitted light is condensed by a condenser lens 5, enters a photoelectric conversion element 6, and is converted into an electric signal corresponding to the light intensity. When using a scale with a grating groove period of approximately 8 μm, as the main scale 3 moves, a sinusoidal signal with one period of 8 μm is generated, and by counting the number of peaks in this signal, the amount of movement of the main scale can be calculated. It can be measured.

FIG. 2 shows an example of a case where a reflection type diffraction grating is used instead of the transmission type scale 3. The light from the light source 1 is made into parallel light by the collimator lens 2, and the light from the reflective scale 7
incident at an angle to the As a result, the light that hits the reflective part of the lattice groove of the reflective snail lure is reflected, but the light that hits the transparent part is either transmitted or absorbed, so the cross section of the reflected light beam is a stripe of light and dark. The situation is as follows. This stripe moves as the reflection scale 7 moves.

This striped reflected light travels back along the optical path it came from by combining the condensing lens 5 and the reflecting mirror 8. This backward striped reflected light moves with the movement of the reflection scale 7,
The direction of movement is reversed by the reflecting mirror 8. This light is reflected again by the reflection groove part of the reflection scale 7, and is reflected by the lens 2.
, enters the photoelectric element 6 via the half mirror 9. The intensity of light incident on the photoelectric element 6 changes in brightness and darkness in a sinusoidal manner as the reflection scale 7 moves, but as mentioned above, the light reflected by the reflection mirror 8 moves in the opposite direction to the reflection scale 7. , the period is ]/2 of the grating groove pitch. That is, the displacement sensitivity is twice that of the one shown in FIG.

In the two types of moiré scales mentioned above, the grating groove scale is simply used as a row of bright and dark slits 21, and the diffraction phenomenon of the grating is not utilized. Therefore, while it is possible to use simple white light as the light source 1, on the other hand, if the displacement sensitivity is increased and the grating focus of the J-scale is made smaller, the diffraction phenomenon of light cannot be ignored, and the grating can only reach up to about 5 μm. There is a drawback that the pinch cannot be reduced.

Unlike the above-mentioned type of moiré scale, one that utilizes the interference of diffracted light from a grating groove scale has been considered. For example, Optics and Spec] - Rothkopi 1st
Volume 3, page 295 (0ptics & 5pectrosc
opyVo1.13, p 295). FIG. 3 shows an example.

The light from the monochromatic point light source is made into parallel light by the collimator lens 2, and then obliquely enters the beam splitter 1, where it is split into two parts: the transmitted light a and the reflected light. . Each light is reflected by a reflection type diffraction grating 1o
The beam is incident obliquely on the beam and is diffracted. The direction of the reflected diffracted light is determined by the grating constant d of the diffraction grating 10, the incident angle, and the incident wavelength λ, and by appropriately selecting these, it is possible to diffract the reflected diffracted light in the direction of incidence. In this case, the diffracted light has a negative order.

Now, two lights a that are diffracted and follow the optical path of the incident light in the opposite direction,
b' is again divided into two by the beam splitter 1j. The lights a' and b' transmitted through the beam splitter II enter the photoelectric element 6 via the lens 2 and the half mirror 9. Since these two lights are coherent, they interfere, producing brightness and darkness in the light intensity. When the grating groove of the diffraction grating 10 is displaced by h'ti miscellaneous d/2, the electric signal generates a one-period sine wave. This type uses diffraction phenomenon, so the light source must be coherent, but the lattice constant can be made small, making it possible to obtain a highly sensitive position detector. It is.

However, there are the following drawbacks in implementing the type shown in FIG. In other words, (]) If you want to have a relatively large working distance (the distance between the diffraction grating 10 and one beam splitter, which is defined by the distance between the diffraction grating 10 and one beam splitter, which is the distance between the diffraction grating 10 and one beam splitter), it is necessary to The distance between the two beams of light irradiated onto the diffraction grating becomes larger, and a longer diffraction grating is required to detect a certain moving distance.

(2) The incident light from the light source is split by a beam splitter, and the transmitted light is not affected by the tilt angle of the beam splitter, but the reflected light depends on the tilt angle, so the reflected diffracted light from the diffraction grating 10 It is necessary to strictly align the optical axes of the beam splitter 1, the light source 1, the lens 2, etc. so that they will interfere well, and conversely, this diffraction grating 10
If the lens moves, the optical system is likely to go awry. and,
(3) The light that is caused to interfere with each other is the light that has been diffracted by two different parts of the diffraction grating.In order to obtain good interference, a diffraction grating that has particularly uniform grating groove performance is required. Also, the lattice is susceptible to dust and scratches, etc.

Furthermore, when a well-known gas laser is used as a light source, there are problems such as fluctuations in the angle of incidence on the diffraction grating.

[Object of the Invention] The present invention provides a position detector that eliminates the above-mentioned conventional drawbacks.

[Embodiments of the Invention] First, the basic principle of the present invention will be explained in detail with reference to FIG.

Light from a monochromatic point light source J is made into parallel light by a collimator lens 2, and is made perpendicular to the grating groove plane of the reflection type diffraction grating 10. Then, the diffracted lights of the positive and negative orders of R1 and R2 are reflected in directions determined by the incident wavelength and the lattice constant d. In this case, there are diffracted lights of many orders, but here we will focus only on the set of lights of the same order. Now, one diffracted light R1, R2 is reflected by mutually parallel reflecting surfaces Ml, M
The transmitted diffracted light R and the reflected diffracted light R2 travel in the same direction, interfere with each other, and enter the photoelectric element 6. The interference is between the bending point on the diffraction grating]O and the beam splitter 11
A sinusoidal signal is obtained with a period of d/2 (d is the grating constant) by which the diffraction grating is moved.

As explained above, one feature of the present invention is that the lights of the same positive and negative orders of the light vertically incident on the diffraction grating are made to interfere with each other, and as a result, it is possible to eliminate the two consecutive drawbacks of the conventional method. Become. That is, since the diffraction phenomenon is utilized, the lattice constant d is extremely small, for example, d20.
87jm can also be used, allowing highly sensitive position detection. Furthermore, since the light is incident perpendicularly to the diffraction grating, alignment of the optical system is easy, and by using a diffraction grating in which orthogonal grating grooves are formed, XY two-dimensional position detection becomes possible. This will be described in detail later. In FIG. 4, the basic structure of the present invention is explained using an example of a reflection type diffraction grating, but it is of course possible to use this as a transmission type diffraction grating.

FIG. 5 is a diagram illustrating an embodiment of the present invention. The light source 13 uses a laser diode 1~ with a wavelength of 830 nm,
The light is made into a parallel beam by the collimator lens 2 and is perpendicularly incident on the reflection type diffraction grating]0. The lattice constant d is 1
．． In the case of 6 μm, the positive and negative first-order diffracted lights are approximately 40°
It is reflected and diffracted at an angle of , and enters the polyprisms R1 and R2. Both diffracted lights are directed in the original direction by the respective polyprisms, and are again incident on one point X2 on the diffraction grating and diffracted again. Here, since polarizing plates P1°R2 having polarization axes perpendicular to each other are provided in each optical path, the O-th diffracted light at the point The two diffracted lights are passed through the beam splitter 1J with polarization planes perpendicular to each other.
incident on . Both linearly polarized lights transmitted through this beam splitter further pass through a polarizing plate P3 having a polarization plane at an angle of 45 degrees to the respective polarization planes, become mutually coherent light, and enter the light receiving element D1. On the other hand, the linearly polarized light reflected from the beam splitter 11 is further 90
'The light passes through the polarizing plate P4 having a rotated plane of polarization and enters the light receiving element D2.

obtained from each light receiving element Di, D2 (No. 3 Sl
, S2 are sinusoidal signals whose period is the movement amount d/4 (=0.4 μm) of the diffraction grating and whose phases are shifted by 90° from each other, as shown in FIG. 6a. After comparing the magnitude of this sine signal with respect to the level IT at the amplitude center and digitizing the signal as shown in Figure 6, a pulse signal as shown in Figure 6C is generated at the 3'l rising and falling edges of the signal. to occur. These signal processes can be performed using a known method θ.

Through the above processing, the pulse is moved by the amount of movement of the diffraction grating d/16
(=O, 1 μm), and by counting this, the amount of movement can be determined. In this example, the polyprism 1 is used to cause diffraction twice, but
This is true even if the movement of the diffraction grating 10 causes the diffraction grating to be slightly tilted.

This is to always match the traveling directions of the two diffracted lights and obtain good interference. Also, since it is diffracted twice, the sensitivity is 4th
It is twice as large as the one in the figure.

This sensitivity can be increased by further interfering multiple methods as diffracted light. It is also obvious that the same effect can be obtained by using a cube corner or the like instead of a polyprism.

FIG. 7 is a diagram showing an embodiment in which two-dimensional position detection is performed according to the present invention. As the diffraction grating 10', X, Y
A diffraction grating having grooves perpendicular to the direction (a right-angle grating) is used, and a laser beam is incident perpendicularly to the diffraction grating ]0', as in FIG. As a result, the light is reflected and diffracted in four directions at right angles to the grating grooves, and polyprolith 71%
+ l Rx −+ Ry + .

It is incident on Ry- and reflected. Polyprism R Tsuyato Ry
-The light reflected by - gathers at one point X2 of the diffraction grating I-,
It is diffracted vertically again. On the other hand, the lights in the Y-axis direction coincide at point Y2.

With such an optical system, X・1"111.Y
The movement of the diffraction grating in the axial direction can be obtained as interference of each independent diffracted light, and two-dimensional position detection can be performed. Nasida, 1 figure, 2', 31'13', S,
,D,′,S,′,one 2′1P3P4′ are 2, 11, 13°S, ,D,,S2 ., shown in FIG. 5, respectively. D2
．． Shows something similar to PJ, P, and PX+1PX-
indicates a polarizing plate arranged in the X direction, and Py++Py- indicates a polarizing plate arranged in the Y direction.

Next, the effects of the embodiment when a semiconductor laser is used as the light source will be described.

If a parallel beam is incident on the diffraction grating 10 using a T-(e-Ne gas 1 noser or the like as a light source), a phenomenon occurs in which the emission direction of the laser beam temporally fluctuates by several μrad to several μrad. In Figure 8, if the laser beam that was initially incident perpendicularly on the diffraction grating 1o (the optical path in this case is shown as a solid line) is incident at an angle of 80° as shown by the dotted line, then The position where it is turned back and enters the diffraction grating again moves from point A to point B. Then, the optical path length of the + co-order diffracted light increases by ΔL from the initial solid line, and conversely, the optical path length of the 1st-order diffracted light becomes It becomes shorter by ΔL. Here, Δ■ is approximately given by equation (1).

However, Δβ is the angle between the first-order diffracted light shown by the solid line toward C1 and the first-order diffracted light toward C] shown by the dotted line, and the point P is
This is the intersection of the dotted line from C1 to B and the perpendicular line drawn from A to that line.

ΔL=2 T--tanβ×Δβ sj-nβ=□ For example, L==25mm, β=36°, m=1. λ=84
,0 nm,'d=1. , 4-4 μm
, 80=1. ．． If 41Jp rad,
From equation (2), ΔL=0.4.4 μm.

The optical path length of +1st-order diffracted light increases by 0.44μIn, and conversely -11
Since the optical path length of the next diffracted light is shrunk by 0.44 μm, the interference optical path length ultimately changes by 0.88 μm. It is detected by a change in the optical path of the wavelength λ, causing a detection error. The size of this error cannot be ignored compared to the detection accuracy targeted by this detector, and is the biggest drawback when using a gas laser as a light source.

On the other hand, in the case of a semiconductor laser, unlike a gas laser, an external reflecting mirror is not used, so there is no fluctuation in the laser emission direction, and the above-mentioned detection error does not occur.

In this way, when a semiconductor laser is used as a light source for a single detector, there is a special effect.

[Effects of the Invention] As is clear from the above description, according to the present invention, position detection in units of 01-μm is possible even when the distance from the moving grating is about 10 mm, and there is no need for focusing. In addition, since two-dimensional position detection is easy, it has various advantages such as being able to be combined with an X and Y moving table, etc., and applied to measuring instruments that detect coordinates and various manufacturing machines that require precision positioning. The effect is great in practical use.

Furthermore, it is possible to provide an ultra-small, inexpensive, high-precision position detector that uses a semiconductor laser as a light source and is low-voltage and solid-state. It goes without saying that the present invention is not limited to the specific numerical values used in the second embodiment, but can be set as appropriate.

[Brief explanation of drawings]

Figures 1 to 3 are diagrams showing conventional position detectors, Figure 4 is a diagram explaining the basic principle of the present invention, and Figure 5 is a diagram showing the basic principle of the present invention.
6 is a signal waveform diagram showing an example of signal processing, FIG. 7 is a diagram showing two-dimensional position detection according to the invention, and FIG. 8 is a diagram showing an embodiment of the present invention. 2 is a diagram illustrating the error of the detector due to the difference between a semiconductor laser and a gas laser as a light source. In the figure, 1. point light source, 2. collimating lens, 6.
...Photoelectric element, 10. Diffraction grating, 11. Beam splitter. 1st diagram Ka 3 diagram muscle 4 diagram punishment Z diagram

Claims (1)

[Scope of Claims] 1. A semiconductor laser, a diffraction grating that diffracts a laser beam from the semiconductor laser, an interference means for causing the diffracted lights of the same positive and negative orders diffracted from the diffraction grating to interfere with each other, and interference by the interference means A position detector comprising a light receiving means for receiving light. 2. A semiconductor laser, a diffraction grating for obtaining first and second diffraction of the laser beam from the semiconductor laser, and making the diffracted lights of the same positive and negative orders among the diffracted lights diffracted by the diffraction grating enter the diffraction grating again. and a light re-entering means, and the diffracted light from the first diffraction passes through the light re-entering means, and the diffracted light of the same order of positive and negative is subjected to a second diffraction by the diffraction grating, and of the second diffraction, the diffracted light of the same order of positive and negative A position detector comprising a light receiving means for receiving interference light caused by interference between diffracted lights.