DE2525317A1 - METHOD AND DEVICE FOR MEASURING DISPLACEMENT BY USING A DIFFUSION GRID - Google Patents

Description

SOCIETE GENEVOISE D'INSTRUMENTS DE PHYSIQUE, Geneva (Switzerland)

Method and device for measuring displacement by means of a diffraction grating

The invention relates to an optical method and an optical device for displacement measurement by means of the interference of rays diffracted by a grating.

There are already FeS devices, the diffraction gratings use, with different beams z. E. with the order 0 and 1 or -1 and 0 in interference come. However, these devices have the disadvantage that they require double diffraction by the grating

56- · 5380 -HdSl

5098S1 / 0846

- 2 - 2 5 2 b 3 1 7

and can only work with monochromatic light and homogeneous light rays, which is extremely difficult to understand. real is.

There are also other devices for measuring position by diffraction. However, to get a good resolution to obtain, rays diffracted with each other with an order + n and -n have to iriterate, where η is relatively g .- 'oß, z. B. η> - 3, to a sufficient Frequency. Yet these high-order diffracted rays have little energy, so one a special grid geometry must be chosen in order to selectively the energy contained in the beams used to reinforce. This leads to the construction of very complicated grids with a relatively large grid constant.

Finally, the existing devices require the use of a monochromatic one Light source like a laser. In the case of high diffraction orders, the dispersion of the white light becomes too large.

It is therefore the object of the invention to provide a device for measuring position by diffraction, which uses a source of white light and is therefore simple, inexpensive and easy to manufacture Grid that does not require any special geometry and a corresponding measuring method.

A method for measuring relative displacements between two mechanical bodies, one of which carries a diffraction grating, is according to the invention there-

509851/0845

characterized in that on the diffraction grating from a light source carried by the other mechanical body two coherent incident rays are projected symmetrically to a plane perpendicular to the diffraction grating will; that symmetrically to this perpendicular plane at least one ray of the diffraction order + n of one of the incident rays with at least one ray of the diffraction order -n of the other incident Beam merged v / ground to let them interfere in pairs, and that the so obtained Modulation in at least one of these combined beams as a measure of the relative displacements of the two mechanical bodies is detected.

An apparatus for carrying out the method according to the invention with a diffraction grating, which is from a mechanical body is, according to the invention, characterized in that the diffraction grating has a constant geometry that a light source is supported by the other mechanical body, that means on the diffraction grating two coherent incident rays from the light source projected symmetrically to a plane perpendicular to the diffraction grating; that a device symmetrical to the vertical plane at least one ray of diffraction order + n of one of the two incident rays with at least one beam of the diffraction order -n of the other incident beam merges again in order to interfere with them in pairs and in that means the modulation thus obtained in at least one of the merged beams recorded.

509851/08

Lie invention is explained in more detail with reference to the drawing. Show it:

Fig. 1 schematically, as by means of a Kösters prism diffracted rays of the order + n and -n are brought together again:

Fig. 2 schematically shows the electro-optical arrangement of a AusfUhrungsbeispiels the inventors; external device;

3 shows the signals emitted by the device; and

4 schematically shows a modification of the method of operation of the device.

The inventor Ae measuring method differs from the state of the art in that there are two diffracted Rays of the order + n and -n, which originate from two different incident rays, interfere with each other leaves. These incident rays come from one and the same light source, are coherent with each other and symmetrical relative to a plane perpendicular to the plane of the diffraction grating. If wrong. rest taken care of becomes that the incident and the reflected rays subject to the same number of reflections, it is also possible to work with non-monochromatic light, z. With white light, which none of the previous devices allow.

In Fig. 1, which shows schematically an embodiment of the device according to the invention, two are incident To see rays 21 and 22 coming from the same light source 20, coherent with each other and

509851/0845

symmetrical to the normal on one: lattice ~ jl and converge on it. After diffraction, the two beams 21 and 22 produce beams 23 and 24 of order -1 and beams 25 and 26 of order +1. The higher-order diffracted rays are present, but not shown in FIG.

The aim of the invention is, in particular, to make the device work with white light and thus to provide a device. In order to interrelate the diffraction spectra of the rays of the order -1 and +1 to let interfere that the interference takes place for all V. 'wavelengths.

This problem is z. B. solved by using a Kösters-Prisnas 18. The two rays 24 and 25 or 23 and 26 fall on the Prisira 18, unite get on a semi-reflective flee 19 and leave the prism symmetrically in the form of combined rays 27 and 28 or 29 and> 0. So that the interference completely with respect to one of the united rays, z. 3. 27, takes place, must be the difference in the number of reflections of the incident beam 21 and the diffracted Ray 25 relative to the number of reflections of the incident ray 22 and the diffracted ray 24 be zero or even.

The beams 21 and 22 are made from a single beam 20 by decomposition by means of a Kösters prism generated so that they can be subjected to a number of reflections which differ by one. The Kb'sters prism is also used to make the diffracted rays interfere.

509851/0845

Big's device. 1 shows the interference of the diffracted beams of order +1 and -1, it understands however, that the device works in the same way with diffracted beams of higher order, if these jev / eils interfere in twos.

The interference obtained by rays 27-30 depends only on the position of the diffraction grating relative to the incident rays. Actually learns each diffracted ray has a phase shift of

= 2-7Γ -

η · -τ

with η = number of the order

χ = displacement of the diffraction grating d = grating constant.

If one lets a beam of the order +1 and a beam of the order -1 interfere, the result is relative phase shift:

from which it follows that the interference describes two complete periods if the diffraction grating is around one Lattice constant is shifted.

If a beam of order +2 and a beam of order -2 are allowed to interfere with each other, the relative phase shift ψ = 81T ^ shows that the interference describes four complete periods when the diffraction grating is shifted by one grating constant, and so on.

509851/0845

-% - 2 5 2 Fi 31 7

The device of Fig. 1 operates with white light, although the device can also be operated with monochromatic and coherent light, without its mode of operation to change.

A preferred embodiment of the device according to the invention is shown in FIG.

The image of a filament of a lamp 1 is focused through a lens 2 onto a diffraction grating 31, the Lines or engravings run perpendicular to the plane of the drawing. The incident beam 20 is through the semi-reflective surface 19 of the Kösters prism 18 divided into two beams 21 and 22 which are symmetrical to the axis of the caster-prisir.as 18 and to the normal the diffraction grating 31 run. The two rays 21 and 22 are diffracted at the diffraction grating 31, and the rays 24 are obtained, which is a diffracted ray of the order -1 of the ray is 22, and 25, which is a diffracted The +1 order beam of the beam 21 is to reunite them in the prism l8. Rays 24 and 25 interfere on the semi-reflective surface 19 and leave the Kösters prism 18 symmetrically, and in the form of combined rays 27 and 28, each of which is the sum of half the ray 24 and half the ray is. Depending on the relative phases of the two beams 24 and 25, beams 27 and 28 have one Maximum or a minimum of the light intensity. When the diffraction grating 31 is shifted, the rays 27 become and 28 light intensity modulated.

In order to detect the direction of displacement of the diffraction grating 31, a conventional procedure is used.

509851/08 A5

The beam 20 is linearly passed through a polarizer 3 polarized, the polarization direction of which runs parallel to the lines of the diffraction grating 31. The bowed one Ray 25 passes through a ¼ plate that looks like this is aligned that the linear polarization at the input is transformed into circular polarization at the output. The diffracted beam 24 passes through a compensation plate 5 », the thickness of which is so dimensioned that it is the Lengthening of the optical path of the beam 25 as a result of passing through the p (/ 4 plate is compensated Rays 27 and 28 which symmetrically form the Kösters prism Leaving 18, have two superimposed partial beams, one of which is linearly polarized and the other circularly polarized is. This results in two interference systems, which are polarized by 90 against each other and phase shifted by 90 ° that can be separated by analyzers.

The beam 27 is focused on a lens 6 and through the semi-reflective lamella 7 on photoelectric cells 10 and 11. The analyzers 8 and 9 are inclined at 45 ⁰ and symmetrically aligned to the polarization direction of the polarizer 3, the photoelectric cells 10 and 11 each receive one of the two interference systems as just explained. When the diffraction grating 3I is shifted, electric signals I 10 and I 11 are output from the photoelectric cells 10 and 11 as shown in FIG.

The beam 28 is through the lens 12 and through the semi-reflective lamella 13 on photoelectric Cells 16 and 17 focused. Analyzers 14 and 15 are 45 ° and symmetrical relative to the direction of polarization of the polarizer 3 aligned so that the photoelectric cells l6 and I7 each one

509851/0845

of the two aforementioned interference systems received.

To facilitate the electronic processing of the signals emitted by the photoelectric cells, is proceeded in a conventional manner, in that the rays 24 and 25 on the reflective surface 19 to the Interference brought v / ground, which consists of a -.- dielectric layer. Experienced during this reunion the rays 27 and 28 a mutual phase shift from l8o °.

When the diffraction grating 31 is shifted, the photoelectric cells Io and 17 give electrical signals I 16 and I 17 as shown in FIG.

The difference between the signals I 16 - I 10 and I 17 I 11 results in two signals I a and I b, which are 90 out of phase and be symmetrical to the zero point, regardless of the total light and the front contrast of the Interference.

The electronic counting of the number of zero crossings the signals I a and I b indicate the shift. The relative phase of the signals I a and I b, + 90 ° or -90 °, indicates the direction of shift.

In the case of monochromatic lighting and coherent light, a modification of the invention is shown in FIG.

Because of the conochromaticity of light, two diffracted rays of any order can interfere with one another. V- / enn with n, and n ~ denote the orders of the diffracted rays to be interfered,

509851/0845

results in the number of complete periods that can be described by the interference if the diffraction grating is shifted by a grating constant to:

if _r = 21Tn ₁ - 2Tn ₂ ,

4 T / for Ti ₁ = +1 and n ₂ = -1 or 2 7 / for n. = +1 and n _? = 0.

In the device shown in FIG. 4, an incident beam 1 perpendicular to the diffraction grating 2 is used. The beam 1 is diffracted into a beam 3 of the -1 order and a beam 4 of the +1 order, which are allowed to interfere on a semi-reflective lamella 6. The beam 4 reaches the lamella β by reflection at a ·! Mirror 5. The composite beams 7 and 8 see their light intensity modulated by the displacement of the diffraction grating 2 at the point of interference.

The entire description of FIG. 2 remains applicable here too, in particular as regards the properties of the beams 7 and 8, which symmetrically leave the lamella formed by a dielectric layer, and the obtaining of two modulations out of phase by 90.degree Unit of polarizer, } / h plate and analyzers.

In the first exemplary embodiment described wi d

a Kö'sters prism is used because this is a simple one

at
Construction allows where / it can be seen that the same result can also be achieved with sets of reflective and semi-transparent mirrors.

509851/0845

To work with white light, ν: is as beneficial, it is necessary, the covered ^V: eg and the number of reflections of the incident rays and the diffracted beams are identical and make the incident rays coherent.

The main advantages achieved by the device according to the invention are:

Use of significantly lower diffraction orders, preferably - 1, since it has a lot of energy contain,

identical geometric shape of the grid grooves, which is otherwise unimportant, which is not the case is, if one uses higher diffraction orders, which do not have enough energy, why then the grid grooves must have precisely defined geometries that are difficult to manufacture,

the neglect of inaccuracies in the lattice constants, since such inaccuracies are significant enter into less if the design order is low is,

the use of a very small lattice constant, e.g. B. 2 / um, v / as allows fine resolution, e.g. B. of 0.5 or 1 / um, without interpolation and without having to use higher diffraction orders, and

the use of any light source, in particular white light, which has hitherto been the case with higher diffraction orders was not possible because the dispersion became too large.

509851 / 08A5

Claims (5)

Expectations (JlJ method for measuring relative displacements or movements between two mechanical bodies, one of which carries a diffraction grating, characterized in that two coherent incident rays are projected symmetrically to a plane perpendicular to the diffraction grating from a light source carried by the other mechanical body ; that symmetrically to this perpendicular plane at least one beam of the diffraction order + n from one of the incident beams is recombined with at least one beam of the diffraction order -n of the other incident beam, so that they interfere in twos, with n <^ 3, preferably equal to 1; and that the modulation thus obtained is detected in at least one of these recombined beams as a measure of the relative displacements of the two mechanical bodies. 2. The method according to claim 1, characterized in that the light source is a source of white light, and that the difference in the number of reflections of an incident ray and the corresponding one diffracted beam versus the number of reflections of the other incident beam and the corresponding one diffracted beam is zero or an integer. 50985 1/0845 3. Apparatus for carrying out the method according to claim 1 or 2, with a diffraction grating carried by a mechanical body, characterized in that the diffraction grating (31) has a constant geometry; that a light source (20; 1) is carried by the other mechanical body; that means projecting onto the diffraction grating two coherent incident beams (21, 22) from the light source symmetrically to a plane (19) perpendicular to the diffraction grating (31); that a device symmetrically to the vertical plane unites at least one beam (25, 26) of the diffraction order + n of one (22) of the two incident beams with at least one beam (23 * 24) of the diffraction order -n of the other incident beam (21), in order to let these interfere in pairs, with η <T 3, preferably equal to 1; and that a device detects the modulation thus obtained in at least one of the recombined beams (27-30). 4. Apparatus according to claim 2, characterized by a Kösters prism (18), the semi-permeable separating surface (19) divides a beam (20) coming from a light source (1) into two incident beams (21, 22) and two diffracted rays (24, 25) of the order -n of one of the incident rays reunites the diffracted rays of order + n of the other incident ray. 5. Apparatus according to claim 3 or 4, characterized by at least one photoelectric cell '10, 11, 16, 17) to detect the modulation of a reunited diffracted beam (27, 28). 509851/0845