U.S. patent application number 09/788444 was filed with the patent office on 2001-07-05 for device for measuring translation, rotation or velocity via light beam interference.
Invention is credited to Parriaux, Olivier M..
Application Number | 20010006421 09/788444 |
Document ID | / |
Family ID | 8232496 |
Filed Date | 2001-07-05 |
United States Patent
Application |
20010006421 |
Kind Code |
A1 |
Parriaux, Olivier M. |
July 5, 2001 |
Device for measuring translation, rotation or velocity via light
beam interference
Abstract
The device for measuring translation, rotation or velocity
includes at least a light source, a light detector, a first grating
and a second grating, the first grating being mobile relative to
the second grating. A incident beam reaches the first grating where
it is diffracted in two beams whose directions are interchanged by
the second grating, the resulting beams being then again diffracted
by the first grating in an output diffraction direction where they
interfere together. Both gratings are used in reflexion.
Inventors: |
Parriaux, Olivier M.;
(Saint-Etienne, FR) |
Correspondence
Address: |
B. Franklin Griffin, Jr.
Griffin & Szipl, P.C.
Suite PH-1
2300 Ninth Street, South
Arlington
VA
22204-2320
US
|
Family ID: |
8232496 |
Appl. No.: |
09/788444 |
Filed: |
February 21, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09788444 |
Feb 21, 2001 |
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PCT/EP99/06057 |
Aug 19, 1999 |
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Current U.S.
Class: |
356/499 |
Current CPC
Class: |
G01D 5/38 20130101 |
Class at
Publication: |
356/499 |
International
Class: |
G01B 009/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 21, 1998 |
EP |
98115810.8 |
Claims
What is claimed is:
1. A device for measuring translation, rotation or velocity via
light diffraction including a light source, at least one light
detector, a first grating or a first grating and a fourth grating
of the same spatial period and located substantially in a same
first plane, and a second grating or a second grating and a third
grating of the same spatial period and located substantially in a
same second plane; the second and, where appropriate, third
gratings being mobile along a given direction of displacement
relative to the first and, where appropriate, fourth gratings, this
device being arranged so that a first light beam generated by said
source defined a beam incident upon said first grating where this
incident beam is diffracted into at least a second beam and a third
beam; so that these second and third beams then reach at least
partially said second grating or said second and third gratings
respectively, where they are respectively diffracted into at least
fourth and fifth beams whose propagating directions are
interchanged respectively with the propagating directions of said
second and third beams; so that these fourth and fifth beams then
reach at least partially said first grating or, when appropriate,
said fourth grating where they are respectively diffracted in a
same output diffraction direction so that they interfere at least
partially, said light detector being arranged to detect at least
partially light resulting from said interference; wherein said
first and second gratings and, where appropriate, said third and/or
fourth gratings are used in reflexion.
2. The device of claim 1, wherein said first and, where
appropriate, fourth gratings belong to a portion of the device
which is mobile relative to said incident beam, said second and,
where appropriate, third gratings being fixed relative to this
incident beam.
3. The device of claim 2, wherein said second grating and, where
appropriate, said third grating are arranged between said source
and said detector.
4. The device of claim 3, wherein said second and, where
appropriate, third gratings form together with said source and said
detector a measuring head of this device, said first grating
defining a scale of said device.
5. The device of claim 4, wherein said detector is integrated in a
region of a semiconductor substrate bearing said second grating
and, where appropriate, said third grating.
6. The device of claim 4 or 5, wherein said light source is
integrated or arranged in a region of a semiconductor substrate
bearing said second and, where appropriate, said third grating.
7. The device of claim 1, wherein the second and, where
appropriate, third gratings have a spatial period which is twice as
small as that of the first and, where appropriate, fourth gratings,
said second and third beams being diffracted respectively into the
<<+1>> and <<-1>> orders, said fourth and
fifth beams being diffracted respectively into the
<<-1>> and <<+1>> orders, and these fourth
and fifth beams being respectively diffracted into the
<<+1>> and <<1>> orders in said same output
diffraction direction by said first or, where appropriate, fourth
grating.
8. The device of claim 2, wherein the second and, where
appropriate, third gratings have a spatial period which is twice as
small as that of the first and, where appropriate, fourth gratings,
said second and third beams being diffracted respectively into the
<<+1>> and <<-1>> orders, said fourth and
fifth beams being diffracted respectively into the
<<-1>> and <<+1>> orders, and these fourth
and fifth beams being respectively diffracted into the
<<+1>> and <<-1>> orders in said same
output diffraction direction by said first or, where appropriate,
fourth grating.
9. The device of claim 7, wherein said output diffraction direction
defines an angle, in a plane perpendicular to lines forming the
gratings, which has a value substantially equal to the angle of
incidence of the incident beam multiplied by <<-1>>
relatively to an axis perpendicular to said gratings, only light
interfering along this output diffraction direction being used for
measuring a relative displacement.
10. The device of claim 8, wherein said output diffraction
direction defines an angle, in a plane perpendicular to lines
forming the gratings, which has a value substantially equal to the
angle of incidence of the incident beam multiplied by
<<-1>> relatively to an axis perpendicular to said
gratings, only light interfering along this output diffraction
direction being used for measuring a relative displacement.
11. The device of claim 9 or 10, wherein the light from said
incident beam forming said second, third, fourth and fifth beams
and finally detected by the detector reaches said first grating at
an angle of incidence which is not zero in a plane perpendicular to
lines forming the gratings, this angle of incidence being
sufficient so that the source providing said light and the
detection region of the detector receiving said light are spatially
separated from each other in projection in a plane perpendicular to
said lines.
12. The device of claim 7 or 8, wherein a diffraction region of
said first or fourth grating, from which originates said light
resulting from said interference and detected by the light
detector, is arranged so that other interference, along different
diffraction directions to said first direction and originating from
different diffraction orders of said fourth and fifth beams than
respectively <<+1>> and <<-1>>, have at
least one of the two contributions of these fourth and fifth beams
whose amplitude is considerably less than the amplitudes of the
fourth and fifth beams diffracted along said first diffraction
direction in said diffraction region.
13. The device of claim 7 or 8, wherein said first grating is
arranged, in a region of said first grating receiving the light
from said first beam finally detected by said detector, so that the
<<0>> diffraction order is relatively low, said first
beam being diffracted in this region mostly into said
<<+1>> and <<-1>> orders.
14. The device of claim 1 or 7, wherein said first grating and,
where appropriate, said fourth grating are formed in a dielectric
layer of index n greater than 1.8, so as to achieve a larger
diffraction efficiency with shallower grating grooves.
15. The device of claim 1 or 7, wherein said second grating and,
where appropriate, said third grating are formed in a dielectric
layer on top of a reflective substrate, so as to achieve a large
diffraction efficiency for the TE polarization.
16. The device of claim 1 or 7, wherein said first and second
gratings, where appropriate said third and/or fourth gratings are
each formed of several longitudinal secondary gratings of close but
different frequencies allowing an absolute displacement measurement
over at least one range of measurement.
17. The device of claim 1 or 7, wherein it further includes at
least one diffraction grating of increasing and/or decreasing
period, arranged beside at least one of said first and second
gratings, where appropriate said third and fourth gratings so as to
define at least one reference position for said detector or for
another detector provided for this purpose.
18. The device of claim 1 or 7, wherein it further includes at
least one diffraction grating having at least one offset or phase
jump in the arrangement of its lines so as to define at least one
reference position for said detector or for another detector
provided for this purpose.
19. The device of claim 1 or 7, wherein it is arranged for
measuring the relative velocity between said first and second
gratings, the sole measurement of the frequency of the detected
luminous intensity modulation providing said relative velocity.
20. The device of claim 1 or 7, wherein at least one grating among
said first and second gratings, and where appropriate said third
and fourth gratings has a region where its lines are offset or
phase shifted relative to the rest of this grating or is formed of
at least two secondary gratings of the same period and of phase
shifted or offset lines between these secondary gratings, this
phase shift or offset being provided so that said light resulting
from said interference has two partial beams or two distinct beams
whose alternating luminous intensity signals, which varies as a
function of the relative position between a first portion attached
to said source and a second mobile portion relative to said first
portion, are phase shifted, in particular by .PI./2, to allow
detection of the relative displacement direction between these
first and second portions and interpolation in an electric period
of the luminous intensity signals.
21. The device of claim 4, wherein said light source is formed of
an electroluminescent diode.
22. The device of claim 21, wherein it includes an optical
collimation element arranged between said source and said first
grating.
23. The device of claim 1 or 7, wherein said source emits light
forming a first partial beam incident upon said first grating at a
positive angle of incidence and another partial beam incident upon
said first grating at a negative angle of incidence, said first and
second gratings, and where appropriate, said third and fourth
gratings, being provided on either side of the two regions of
incidence of said first and second partial beams incident upon said
first grating so as to form on either side said first to fourth
beams and to generate on either side said interference between said
fourth and fifth diffracted beams, the light resulting from this
interference being detected on either side by at least two
detectors also arranged on either said of said regions of
incidence.
24. The device of claims 7 or 8, wherein said source emits light
forming a first partial beam incident upon said first grating at a
positive angle of incidence and another partial beam incident upon
said first grating at a negative angle of incidence, said first and
second gratings, and where appropriate, said third and fourth
gratings, being provided on either side of the two regions of
incidence of said first and second partial beams incident upon said
first grating so as to form on either side said first to fourth
beams and to generate on either side said interference between said
fourth and fifth diffracted beams, the light resulting from this
interference being detected on either side by at least two
detectors also arranged on either said of said regions of
incidence.
25. The device of claim 23, wherein said source is attached to said
second and, where appropriate, third gratings of which useful
portions situated on either side of said source are offset or phase
shifted relative to each other so that the alternating light
signals resulting from said interference and detected respectively
by the two detectors are phase shifted, in particular by .PI./2, in
relation to each other.
26. The device of claim 23, wherein it further includes a fifth
diffraction grating arranged between said source and said first
grating, this fifth grating diffracting mostly into the
<<+1>> and <<-1>> orders respectively on
either side of a direction perpendicular to said first grating.
27. The device of claim 26, wherein said source provides a
substantially collimated beam propagating along a direction
substantially perpendicular to said first grating.
28. The device of claim 1 or 7, wherein at least said first or
second grating defines a bi-directional diffraction grating of the
same spatial period along said two orthogonal axes.
29. The device of claim 1, wherein it includes at least first and
second reflective surfaces, the first reflective surface being
arranged to deviate said first beam, originating from said source
and propagating substantially along said displacement direction, in
the direction of said first grating in order to provide said
incident beam, said second reflective surface being arranged to
reflect said light interfering along said output diffraction
direction in a direction substantially parallel to said
displacement direction before being received by said detector.
30. The device of claim 29, wherein said source and said detector
are attached to said first and, where appropriate, fourth gratings
and said first and second reflective surfaces being formed on a rod
supporting said second and, where appropriate, third gratings.
Description
[0001] The present invention concerns a device for measuring
translation, rotation or velocity via interference of light beams
diffracted by diffraction gratings which are substantially parallel
to each other.
[0002] European application 0 672 891 discloses a device for
measuring relative displacements between a head unit and a scale.
This device is of the type where all diffraction gratings have the
same spatial period or pitch P. The head unit has a light-emitting
element (source), a cylindrical lens to condense the light beam
provided by the source and a first diffraction grating used in
transmission for splitting the light beam. The resulting diffracted
beams fall onto a second grating arranged on the scale where they
are diffracted in reflexion. The head unit further comprises a
third grating used in transmission for mixing the diffracted beams
coming back from the scale and a light-receiving element
(photodetector). In all embodiments, the source and the
photodetector are spatially separated respectively from the first
and third gratings so that the head unit has relatively large
dimensions. The distance between the mixing grating and the
photodetector is actually needed because there is a plurality of
interfering beams coming out of this mixing grating. Further, it is
to be noted that for each diffraction event, at least one
diffracted beam is not used. The unused diffracted beams represent
a loss of light power, generate noise, and may lead to spurious
interferences. The efficiency of such a measuring device is thus
relatively low.
[0003] U.S. Pat. No. 5,424,833 discloses a measuring device of
another type wherein the first and third gratings are replaced by
an unique index grating used in transmission with a pitch twice as
large as the pitch of the scale grating. Thus, the scale grating,
which is longer than the index grating, has a pitch or spatial
period smaller than that of this index grating. Further, all
embodiments in this document are arranged so that the incident beam
falling on the index grating has a main propagating direction
comprised in a plane perpendicular to the moving direction of the
scale grating and thus parallel to the lines of both gratings. In
order to spatially separate the light source and the photodetector,
this document proposes, in a first embodiment, to have said main
propagating direction oblique relative to the direction
perpendicular to the index grating in said perpendicular plane. In
a second embodiment, the incident beam falls perpendicularly onto
the index grating and a beam splitter is used which deflects the
interference beam coming back normally from the index grating into
a direction different from the light source. The first embodiment
needs an extended space in a direction perpendicular to the moving
direction (measurement direction) and to the direction
perpendicular to the gratings. The second embodiment has the
following drawbacks: it needs an extended space between the source
and the index grating, it is less efficient, and it involves more
parts.
[0004] European application 0 603 905 discloses a measuring device
wherein two gratings are formed on the scale, a first one for
splitting the light beam coming from the source and a second one
with a pitch twice smaller for interchanging the directions of the
two used beams diffracted by the first grating. The mixing grating
used in transmission is attached to the photodetector. This
arrangement is not very efficient because its resolution is twice
as small as the resolution of the device of U.S. Pat. No. 5,424,833
for gratings having pitches identical to those of the latter.
Further, the scale is transparent and either its two main surfaces
are arranged for diffracting and/or reflecting light beams, or an
additional mirror is needed. The scale is thus relatively difficult
to manufacture.
[0005] An object of the invention is to provide an optical device
for measuring relative movements which has great measuring accuracy
while remaining of relatively simple construction.
[0006] Another object of the invention is to provide such a
measuring device the arrangement of whose various parts, in
particular the scale or longer grating, can be made within
relatively large manufacturing tolerances without adversely
affecting the accuracy of measurements.
[0007] Another object of the invention is to provide a measuring
device of this type wherein the variation in wavelength of the
source and of its angular spectrum have no influence on the
accuracy of measurements.
[0008] Another object of the invention is to provide a device of
this type allowing a very flat arrangement which can easily be
miniaturised.
[0009] A particular object of the invention is to provide a device
of this type at least partially integrated in a silicon or
semiconductor substrate.
[0010] The invention therefore concerns a device for measuring
translation, rotation or velocity via light diffraction including a
light source, at least one light detector, a first grating or first
and fourth gratings of the same spatial period and located
substantially in a same first plane, and a second grating or second
and third gratings of the same spatial period and located
substantially in a same second plane; the first and, where
appropriate, fourth gratings being mobile along a given direction
of displacement relative to the second and, where appropriate,
third gratings, this device being arranged so that a first light
beam generated by said source defines a beam incident upon said
first grating where this incident beam is diffracted into at least
a second beam and a third beam; so that these second and third
beams then reach at least partially said second grating or, where
appropriate, said second and third gratings respectively, where
they are respectively diffracted into at least fourth and fifth
beams whose propagating directions are interchanged respectively
with the propagating directions of said second and third beams; so
that these fourth and fifth beams then reach at least partially
said first grating or, where appropriate, said fourth grating where
they are respectively diffracted in a same output diffraction
direction so that they interfere, said light detector being
arranged to detect at least partially light resulting from said
interference; the first, second and, where appropriate, third
and/or fourth gratings being used in reflexion.
[0011] The features of this measuring device allows an easy
miniaturisation and its integration by microelectronic and
microsystem technologies.
[0012] According to a preferred embodiment, said first and, where
appropriate, fourth gratings belong to a portion of the device
which is mobile relative to said incident beam, said second and,
where appropriate, third gratings being fixed relative to this
incident beam.
[0013] According to a particular embodiment, the first and, where
appropriate, fourth gratings have a pitch or spatial period which
is twice as large as that of the second and, where appropriate,
third gratings, said second and third beams being diffracted
respectively into the <<+1>> and <<-1>>
orders, said fourth and fifth beams being diffracted respectively
into the <<-1>> and <<+1>> orders and these
fourth and fifth beams being respectively diffracted into the
<<+1>> and <<-1>> orders in said same
output diffraction direction by said first or, where appropriate,
fourth grating.
[0014] According to a preferred feature of the measuring device
according to the invention, the light from said incident beam
forming said second, third, fourth and fifth beams and finally
detected by the detector reaches said first grating at an angle of
incidence which is not zero in a plane perpendicular to lines
forming the gratings, this angle of incidence being sufficient so
that the light source providing said light and the detection region
of the detector receiving said light are spatially separated from
each other in projection in a plane perpendicular to said
lines.
[0015] According to a particular feature, said output diffraction
direction defines an angle, in said plane perpendicular to lines
forming the gratings, which has a value substantially equal to the
angle of incidence of the incident beam multiplied by
<<-1>> relatively to an axis perpendicular to said
gratings, only light interfering along this output diffraction
direction being used for measuring a displacement. Thus, the
optical arrangement is fully symmetrical and so reciprocal.
[0016] Other objects, particular features and advantages of the
present invention will appear more clearly upon reading the
following detailed description, made with reference to the annexed
drawings, which are given by way of non-limiting example, in
which:
[0017] FIG. 1 shows schematically an optical device for measuring a
relative displacement,
[0018] FIGS. 2 and 3 show schematically a first embodiment of a
measuring device, according to the invention;
[0019] FIG. 4 shows schematically the spatial distribution of the
light beams used for the displacement measurement of a second
embodiment;
[0020] FIGS. 5 and 6 show schematically a third embodiment of a
measuring device, according to the invention;
[0021] FIGS. 7, 8 and 9 show schematically three other embodiments
of the invention;
[0022] FIG. 10 shows schematically and partially an embodiment
allowing an absolute measurement of the relative position between a
mobile scale and the fixed portion of the displacement measuring
device;
[0023] FIGS. 11 to 15 show schematically various alternatives for
defining a reference position of the mobile scale of the
displacement measuring device;
[0024] FIGS. 16 and 17 show schematically two other embodiments of
the invention;
[0025] FIGS. 18 and 19 show schematically an embodiment allowing
measurement of displacement along two orthogonal directions;
[0026] FIG. 20 shows schematically another embodiment of the
invention in which the beam emitted by the light source and the
interfered beam propagate parallel to the measured displacement
direction.
[0027] FIG. 1 shows a translation measuring device including a
light source 2 which supplies a first beam FI, which reaches a
first transparent structure 4 on one surface of which is arranged a
first grating 6 of period .LAMBDA.. Beam FI is diffracted into the
<<+1>> and <<-1>> orders and generates two
beams 8 and 10. Beams 8 and 10 reach respectively second and third
gratings 12 and 14 where they are reflected and diffracted
respectively into the <<-1>> and <<+1>>
orders. Beams 16 and 18 resulting from these two diffractions
propagate symmetrically to beams 10 and 8 and are joined together
as they reach a fourth grating 20 where they are diffracted,
respectively into the <<+1>> and <<-1>>
orders, along a same first direction of diffraction offset
angularly by angle .alpha. relative to an axis perpendicular to
grating 20, this angle .alpha. being identical in absolute value to
angle of incidence .alpha. of beam FI incident upon first grating
6.
[0028] The two beams generated by the diffraction of beams 16 and
18 in grating 20, along the aforementioned first direction,
interfere and together form a beam FR which again passes through
transparent structure 4 and is then directed towards light detector
22 arranged for measuring the variation in the luminous intensity
of beam FR resulting from said interference. The first and fourth
gratings are situated in a same first general plane and arranged on
a same face of transparent structure 4. Likewise, second and third
gratings 12 and 14 are arranged in a same second general plane of
the device. Grating 14 is arranged at the surface of a reflective
support 24 which is fixed relative to structure 4, while grating 12
is arranged at a surface of a mobile reflective support 26 moving
along a direction X parallel to the aforementioned first and second
general planes. In this embodiment, mobile portion 28, formed of
support 26 and grating 12 remains in a fixed position along axis Z
during measured displacements.
[0029] The path travelled by beams 8 and 16, on the one hand, and
beams 10 and 18 on the other hand, are identical. Consequently, the
phase shift between the two beams 16 and 18 incident upon grating
20 depends solely upon the displacement of mobile portion 28. Those
skilled in the art know how to calculate the phase shift generated
by a displacement along axis X of this mobile portion 28 for beam
16 generated by the diffraction of beam 8 in grating 12, this phase
shift increasing proportionally with the displacement of moving
portion 28 and the luminous intensity of beam FR detected by
detector 22 varying periodically. Measurement of this periodic
variation in the luminous intensity of beam FR allows the
displacement of mobile portion 28 to be determined with great
accuracy.
[0030] Gratings 6 and 20 have a spatial period .LAMBDA. and
gratings 12 and 14 have a period which is substantially two times
smaller, i.e. substantially equal to .LAMBDA./2 and preferably
equal to .LAMBDA./2. This ratio between the spatial periods of the
different gratings allows two reciprocal optical paths to be
obtained defining a symmetry relative to axis Z. Indeed, due to the
particular arrangement of the aforementioned different spatial
periods an incident beam FI at point A of grating 6 generates two
diffracted beams 8 and 10 which are diffracted respectively at
points B1 and B2 along two directions which are symmetrical to the
directions of beams 8 and 10 relative to axis Z. Consequently,
beams 16 and 18 meet at point C situated on grating 20. There is
thus perfect superposition of the two beams interfering along said
first direction of diffraction.
[0031] It will be noted however that the four gratings can be
situated in different general planes if required as long as the
relative displacements are effected in displacement planes parallel
to these general planes. However, such an arrangement loses certain
of the advantages of the device of FIG. 1, in particular its
independence relative to the wavelength .lambda. of beam FI and its
angle of incidence .alpha.. This is why, although such a solution
is not excluded, an arrangement in accordance with FIG. 1 is
preferred. Those skilled in the art can demonstrate mathematically
that the intensity of beam FR resulting from the interference is
independent of angle .alpha. and the wavelength of beam FI when
gratings 6 and 20 are situated in a first general plane and
gratings 12 and 14 are situated in a second general plane of the
device. This feature is particularly advantageous for light sources
emitting with a certain divergence or numerical aperture in a
spectral band of a certain width, i.e. non monochromatic.
[0032] According to a particular feature of the present invention,
beam FI incident upon first grating 6 has an angle of incidence
.alpha. which is not zero. Consequently, in the plane of FIG. 1
which is parallel to the direction of displacement of mobile
portion 28 and perpendicular to lines 30, 31, 32 and 33 of gratings
6, 20, 12 and 14, the point of incidence A on grating 6 and the
point of interference C on grating 20 are separated spatially so
that source 2 and detector 22 are separated spatially in projection
in this plane and can thus be arranged so as to be globally aligned
along a direction parallel to direction of displacement X. This
allows very flat measuring devices to be obtained given that the
source and the detector can both be arranged in a plane parallel to
the measured displacement direction.
[0033] Another consequence of non-zero incidence angle a is to
prevent the spurious z-dependent modulation signal due to
self-mixing when the source is a semiconductor laser.
[0034] The device according to FIG. 1 is favourable for measuring a
relative displacement between two bodies situated in a same general
plane.
[0035] Given that only diffraction orders <<+1>> and
<<-1>> of grating 6 are useful, this grating 6 is
arranged so that the majority of the luminous intensity of beam FI
is diffracted into these two diffraction orders to form
respectively beams 8 and 10. In particular, the light emitted into
diffraction order <<0>> is minimised. Likewise, in the
event that the second diffraction order may intervene, grating 6 is
arranged so that the light diffracted into this second order is
relatively weak.
[0036] By way of example, for a wavelength .LAMBDA.=0.67 .mu.m and
an angle of incidence .alpha.=10.degree., diffraction grating 6 is
formed in dielectric layer 36 of refractive index approximately
n=2.2, in particular made of Ta.sub.2O.sub.5 or TiO.sub.2 deposited
by a technique known to those skilled in the art, on glass
substrate 4, the total thickness E.sub.1 of this layer being
comprised between 0.4 and 0.5 .mu.m. The depth P.sub.1 of the
grooves situated between lines 30 of grating 6 is comprised between
0.30 and 0.35 .mu.m. Transmission of approximately 80% of the total
luminous energy of beam FI is thus obtained in diffracted beams 8
and 10. Defining the grating 6 in layer 36 composed of a high index
dielectric material is particularly advantageous since it allows a
large diffraction efficiency of the <<+1>> and
<<-1>> orders to be obtained with a shallower groove
depth P.sub.1 than in a lower index layer, or than directly in the
transparent structure 4.
[0037] Those skilled in the art can also optimise the profile of
the section of grating 6 along the transverse plane of FIG. 1 to
further increase this selective transmission of the luminous energy
or define other grating profilers in layers of different
transparent materials such as SiO.sub.2 or polymers or solgels. It
will be noted that, given that the diffraction events at point C
form a reciprocal situation with the diffractions at point A, a
difference in the percentage transmitted into the
<<+1>> and <<-1>> orders at point A is
re-established during diffraction at point C at angle .alpha. so
that the contributions of beams 16 and 18 along the direction of
diffraction selected are identical, which leads to maximum contrast
for the interference. It will also be noted that the diffraction
efficiency in the aforementioned example is substantially
independent of the polarisation of the incident light. The light
diffracted into <<0>> order is practically zero. With a
period .LAMBDA.=1 .mu.m, diffraction orders greater than 1 do not
exist.
[0038] Those skilled in the art will choose for reflection gratings
12 and 14 a corrugated metal surface. It is known that such metal
gratings exhibit high diffraction efficiency for beams 8 and 10 of
TM polarization only. High diffraction efficiency for the TE
polarization requires a large groove depth which is very difficult
to obtain in practice when the period is of the order of the
wavelength. Furthermore, it is practically very difficult to obtain
such metal grating exhibiting comparable large diffraction
efficiency for both TE and TM polarizations of beams 8 and 10 as is
requested in case the light source is unpolarized. An object of the
invention is to provide high diffraction efficiency for the TE
polarization, and for both TE and TM polarizations, by using a
grating structure comprizing a flat mirror substrate 26 or 24, a
dielectric layer 38 and 40, the grating 12 or 14 being realized in
the dielectric layer 38 or 40. Such structure associates the
diffraction of grating 12 or 14 with the reflection of the
reflective substrate 26 or 24 in order to give rise to constructive
interference effects in the direction of beam 16 or 18.
[0039] In a particular example, gratings 12 and 14 are both formed
of a dielectric layer respectively 38, 40 also having a refractive
index n=2.2. With a total thickness E.sub.2=0.34 .mu.m and a depth
P.sub.2=0.18 .mu.m for the grooves situated between lines 32 and
33, the luminous intensity diffracted into the <<-1>>
order for grating 12 and the <<+1>> order for grating
14 is approximately 50%, the remainder being essentially diffracted
into the <<0>> order. Given that beam 8 is diffracted
to the right of the direction perpendicular to grating 6, the light
diffracted into the <<0>> order by grating 12 does not
disturb the measurement in any way since it is not received by
detector 22. Likewise, the light diffracted at B2 into the
<<0>> order reaches grating 20 at a distance from point
C comparable to the distance separating point C from point A. It is
thus easy to arrange detector 22 so that the light diffracted at
point B2 into the <<0>> order is not detected. This
fact favours in particular a ratio between wavelength .lambda. and
period .LAMBDA. generating propagation of beams 8 and 10 to the
right and left of the direction perpendicular to grating 6
respectively.
[0040] The arrangement of gratings 12 and 14 described in the
example hereinbefore is provided for a situation in which the light
received is not polarised. However, if the light is TE polarised
(electric field vector parallel to the grating lines), thickness
E.sub.2 of gratings 12 and 14 is approximately 0.1 .mu.m, while the
depth P.sub.2 is situated at around 0.08 .mu.m and can even be
equal to thickness E.sub.2. Substrates 24 and 26 are made for
example of aluminium or coated with an aluminium film or another
suitable metal. Under these conditions, approximately 80% of the
luminous intensity of beams 8 and 10 is diffracted respectively in
beams 16 and 18. For a TM polarisation (electric field vector
perpendicular to the grating lines), one can omit the dielectric
layer and the aluminium substrate is micro-machined with a groove
depth of approximately 0.12 .mu.m. In a variant, substrate of any
type is micro-machined, then coated with a metal film. Thus, the
luminous intensity diffracted in beams 16 and 18 is approximately
70%. Again, the profiles of gratings 12 and 14 in the plane of FIG.
1 can be optimised by those skilled in the art so as to increase
the transmission of luminous energy in the respective useful
directions, in proportions substantially equal but not necessarily
equal at points B1 and B2. Other layer materials like other oxides,
fluorides, polymers, solgels can be chosen and deposited or coated
by different techniques like vacuum deposition, spinning, dipping,
in which the grating can be achieve by dry or wet etching,
lift-off, photo inscription or moulding techniques.
[0041] Dielectric layer 42 of grating 20 has a thickness E.sub.1
and a groove depth P.sub.1 substantially identical to those of
grating 6 so as to assure reciprocity of the diffraction event at C
relative to the diffractive event at A. The diffraction
efficiencies at C correspond to those given hereinbefore for the
diffractions occurring at A.
[0042] Finally, in a variant, transparent structure 4 is in two
portions which are mobile in relation to each other and carry
respectively the first and fourth gratings 6 and 20, while the
second and third gratings 12 and 14 are both attached to one of
these two portions.
[0043] FIGS. 2 and 3 show a first embodiment of the invention. Beam
FI generated by a source which is not shown passes through
transparent structure 44 and reaches grating 46, at an angle of
incidence .alpha., where it is diffracted into the
<<+1>> and <<-1>> orders to form beams 8
and 10, as in the first embodiment. However, this second embodiment
differs from the first in that beam 8 is diffracted to the left of
the direction perpendicular to grating 46. By way of example, the
light wavelength .lambda.=0.67 .mu.m, angle of incidence
.alpha.=20.degree. and period .LAMBDA.=2 .mu.m.
[0044] Beams 8 and 10 reach grating 48 arranged at the surface of
reflective substrate 50. Beams 8 and 10 are respectively diffracted
by grating 48 into diffraction orders <<-1>> and
<<+1>> to form respectively beams 16 and 18 which are
joined as they reach again grating 46 where they are diffracted
along a same diffraction direction, at an angle .alpha. relative to
the direction perpendicular to grating 46. Beam FR resulting from
this interference again passes through transparent structure 4
prior to being detected at least partially by a detector which is
not shown.
[0045] It will be noted that substrate 50 is here stationary
relative to the source and the detector, while structure 44 is
mobile along direction X. The luminous intensity of beam FR varies
periodically as a function of the displacement of structure 44
relative to substrate 50. This detected luminous intensity and the
periodic variation therein allows the relative displacement between
structure 44 and substrate 50 to be accurately determined.
[0046] In order to optimise the transmission of the luminous energy
of beam FI in diffracted beams 8 and 10 and also in order to
optimise the transmission of the luminous energy of these beams 16
and 18 in beam FR, for .alpha.,.lambda. and .LAMBDA. given
hereinbefore, grating 46 is formed of a dielectric layer 52 of
refractive index n=2.2 approximately and having a thickness E.sub.1
comprised between 0.35 and 0.40 .mu.m with a groove depth P.sub.1
equal to approximately 0.24 .mu.m. It will be noted that this
grating structure and these values are given by way of non-limiting
example and have been determined for a transparent structure 44
with an index of approximately n=1.5. Under these conditions,
approximately 60% of the luminous energy of beam FI is transmitted
in diffracted beams 8 and 10 in substantially equal parts,
independently of the polarisation of the light. The luminous
intensity transmitted into the <<0>> order is low. It
is approximately zero for TE polarisation while it reaches
approximately 5% for TM polarisation.
[0047] In the event that the light is not polarised, second grating
48 is formed by a dielectric layer 54 of refractive index n=2.2
having a total thickness E.sub.2 comprised between 0.25 and 0.30
.mu.m with a groove depth P.sub.2=0.22 .mu.m. As in FIG. 1, a high
efficiency grating comprising a dielectric layer 54 and a
reflective substrate 50 is provided, the grating 48 being made in
said dielectric layer. Approximately 55% of the luminous intensity
of beams 8 and 10 is diffracted respectively in beams 16 and 18.
Preferably, the refractive index of the dielectric layers mentioned
is greater than 1.8. For the sole TE polarised light, the luminous
intensity diffracted into the useful orders at grating 48 can be
increased to approximately 70% with a thickness E.sub.2 slightly
greater than 0.30 .mu.m. Under these conditions, it is possible to
obtain 70% of the energy transmitted in beams 16 and 18 while the
luminous energy diffracted into the <<0>> order is very
low; which is not the case for TE polarisation when thickness
E.sub.2 is less than 0.30 .mu.m.
[0048] The numerical example given here thus allows the luminous
energy transmitted into diffraction order <<0>> in
grating 46 to be reduced to the maximum and also, although to a
lesser extent, in grating 48. Then, the light transmitted into the
second diffraction order is relatively small. Consequently, the
only significant interference is that generated by the diffraction
of beams 16 and 18 in grating 46 respectively into the
<<+1>> and <<-1>> orders, at angle of
diffraction .alpha.. This favourable situation results essentially
from the fact that the transmission of beams 16 and 18 into the
<<0>> order of diffraction and the orders greater than
the first order of diffraction at point C is relatively low, or
even zero. Thus, a detector situated in proximity to point C
essentially receives beam FR as a light signal varying alternately
as a function of the displacement of substrate 44. The other
contributions received by this detector generate a substantially
constant signal independent of the relative displacement between
substrate 50 and structure 44.
[0049] In the example given here, the light is essentially
transmitted in the useful orders and the low intensity of the light
transmitted into the <<0>> order of diffraction at
points A and B1 allow any light generating a constant signal to be
reduced to the maximum for the luminous intensity received by the
detector. It will also be noted that given that the diffraction at
point C into the <<0>> order is relatively low, any
interference with a diffraction into the second order can generate
only a small luminous variation and thus a minor disturbance for
the measurement signal propagating at angle .alpha. and formed by
beam FR. In the examples given hereinbefore, most of the luminous
intensity of beams 16 and 18 is diffracted respectively into the
<<+1>> and <<-1>> orders, the amplitudes of
the diffracted beams into other orders being small or zero. It is
to be noted that no particular care must be taken of the luminous
intensity in the zero and second orders when the light source is a
broadband source like a Light Emitting Diode (LED) since their
contribution in the detected signal only amounts to a DC component
because of the short coherence length of a LED.
[0050] In order to be able to determine the direction of relative
displacement between structure 44 and substrate 50, grating 48 has
been divided into two regions R1 and R2 along the direction
perpendicular to direction of displacement X (FIG. 3). In region
R2, grating 48 is also divided into two distinct regions R3 and R4.
In region R3, lines 58 of grating 48 are in phase over the two
regions R1 and R2. However, in region R4, lines 58 have a
discontinuity given that the part of these lines situated in region
R2 is offset by .LAMBDA./8 relative to the part of these lines
situated in region R1. Grating 48 is arranged relative to the light
source so that beam 8 reaches grating 48 in region R3 while beam 10
reaches in region R4. In these conditions those skilled in the art
can calculate that the offset of .LAMBDA./8 provided in region R4
finally generates a phase shift of .PI./4 between beams 16 and 18
incident upon grating 46 at point C. Consequently, the luminous
intensity resulting from the interference originating from region
R1 has a phase shift of .PI./2 relative to the interference
originating from region R2. By separately detecting the
contributions from regions R1 and R2, the detector receives two
alternating luminous intensity signals phase shifted by rV2 in
relation to each other. In a variant, it is possible to provide
three gratings in parallel with an offset of .LAMBDA./6 to give
three luminous intensity signals phase shifted by 120.degree.. If
beams 8 and 10 are not spatially separated when they reach grating
48, region R2 does not have to be separated into regions R3 and R4.
Region R2 as a whole is offset by .LAMBDA./16 with respect to
region R1 in order to provide an optical intensity phaseshift of
.PI./2, or by .LAMBDA./12 for a phaseshift of 120.degree.. Grating
48 can also be devided into four regions similar to R1 and R2 with
three regions having respectively offsets of .LAMBDA./16,
.LAMBDA./18, 3.LAMBDA./16 relative to the last one in order to
obtain the full set of four quadrature optical power signals.
[0051] Thus, on the basis of these two, or three or four separately
detected signals, the electronic system of the measuring device can
determine the direction of relative displacement between structure
44 and substrate 50 and interpolate finely within the electric
period .LAMBDA./4 of the luminous intensity resulting from said
interference to further increase the accuracy of the measurement.
It will be noted that, in the case of the device of FIG. 1, this
electric period is .LAMBDA./2.
[0052] It will be noted that a variation in the spacing between
this structure 44 and substrate 50, i.e. a variation in the
distance separating gratings 46 and 48 has no influence on the
measurement of the displacement along axis X, the two optical paths
between points A and C remaining identical and the phase shift
between the two contributions forming beam FR and originating
respectively from beams 16 and 18 remaining dependent solely on the
relative displacement along axis X.
[0053] Finally, it will be noted that the phase shift for a given
displacement is twice as large in this second embodiment than in
the first embodiment of FIG. 1.
[0054] FIG. 4 shows schematically a second embodiment in which
transparent structure 44 is stationary relative to source 2 and
detector 22, reflective substrate 50 being mobile. Gratings 46 and
48 are the same as those described with reference to FIG. 2. FIG. 4
is given to allow the light useful for the displacement measurement
provided by source 2 to be visualised. This source 2 generates a
beam FI which has a divergence or numerical aperture and which
reaches grating 46 at an angle of incidence varying continuously
within a range of given values. It will be noted that this range of
values can include the value .alpha.=0, i.e. an incidence
perpendicular to grating 46. This beam FI generates beams 8, 10,
16, 18 and FR as described hereinbefore. The numerical aperture of
beam FI generates a divergence of these diffraction beams.
[0055] Since detector 22 is arranged relative to source 2 so that
their projections in a plane perpendicular to the lines of gratings
46 and 48 are not superposed, although they are globally aligned
along a substantially parallel direction to the direction of
displacement, only the light which is comprised in a partial beam
FI* and illuminates region RA of grating 46 (comprised between the
two arrows in the drawing) forms the partial beam useful for the
displacement measurement. According to the invention, the totality
of light FI* incident upon region RA has an angle of incidence
which is not zero, but sufficiently large for the light finally
incident upon detection element 80 to be spatially separated from
the light forming beam FI*, in projection in a plane perpendicular
to the lines of gratings 46 and 48 corresponding to the plane of
the drawing of FIG. 4. When detection element 80 is situated in
direct proximity to region RC where partial beams 16* and 18*
arrive which generate partial beam FI* detected by detector 22,
this condition corresponds to a spatial separation of regions RA
and RC of grating 46. Beam FI* which is useful for the displacement
measurement thus generates partial beams 8* and 10*, which reach
grating 48 respectively in regions RB1 and RB2. From there they are
diffracted to form partial beams 16* and 18* and are joined in
region RC of grating 46 where they are diffracted along a same
direction to form partial beam FR* of beam FR.
[0056] In conclusion, whatever the divergence or numerical aperture
of beam FI, only partial beam FI* contributes to the displacement
measurement and only regions R1, FB1, RB2 and RC define the active
regions of gratings 46 and 48 in which the optimising conditions
for maximum diffraction efficiency and maximum contrast of the
detected interference signal must be fulfilled. It will also be
noted that the light forming beam FI* can have a wide spectrum.
[0057] Hereinafter, the numerical references already described will
not be described again in detail, since they were only given as an
example. It is indeed an object of the invention that the gratings
can be manufactured with large tolerances without affecting the
measurement accuracy.
[0058] With reference to FIGS. 5 and 6 a third embodiment of the
invention will be described hereinafter, wherein an angular
displacement of a wheel 60 is measured, said wheel having at its
periphery a grating 62 formed of lines 64 parallel to the axis of
rotation of wheel 60. Grating 62 defines a scale of period
.LAMBDA.. Facing grating 62 there is provided a measuring head 66
formed of a transparent structure 68 having on its face opposite
grating 62 a diffraction grating 70 having a period .LAMBDA./2. The
ratio of the period of grating 70 to the period of grating 62 is
substantially 1/2when the angle between the normals to grating 62
at points A et C is close to zero. This ratio is smaller than
1/2when the radius of wheel 60 is small and when the spacing
between gratings is large. On the other face of structure 68 are
arranged a light source 72 and a detector 74. Beam FI generated by
source 72 passes through structure 68 and reaches grating 62 where
it is diffracted in reflection essentially into the two orders of
diffraction <<+1>> and <<-1>>. Beam FR,
resulting from the interference of beams 16 and 18 diffracted in
reflection at angle .alpha. at point C, again passes through
structure 68 prior to being detected by detector 74. Grating 70 is
formed in a reflective substrate 76 deposited at the surface of
transparent structure 68.
[0059] An incremental angle of rotation of wheel 60 corresponds to
period .LAMBDA. of grating 62. Thus, for every displacement of
grating 62 relative to measuring head 66 there is a corresponding
angle at centre of wheel 60. Consequently, the processing of the
alternating luminous signal detected by detector 74 allows an angle
of rotation of wheel 60 to be accurately determined.
[0060] As in the second embodiment, the direction of rotation of
wheel 60 can be detected. In order to do this, grating 70 shown in
plane in FIG. 6 has two regions R1 and R2 in which the lines 78 of
grating 70 are offset by .LAMBDA./16. This offset provided at
points B1 and B2 finally generates an optical intensity phase shift
of .PI./2 in beam FR between the two contributions originating from
regions R1 and R2.
[0061] FIG. 7 shows a fourth miniaturised embodiment which is
partially integrated in a semiconductor substrate 82. This
substrate 82 has an aperture 84 wherein is arranged a collimation
ball for the light emitted by electroluminescent diode 88 arranged
at or close to the surface of ball 86. Diode 88 is arranged so that
the central axis of beam FI leaving ball 86 has an angle of
incidence which is not zero when reaching grating 90 of period
.LAMBDA.. On the face of substrate 82 situated facing grating 90
arranged on reflective substrate 112 there is provided a reflection
grating 92 of period .LAMBDA./2. This grating 92 can be either
micro-machined directly in substrate 82, in particular in silicon,
or be obtained by deposition of one or more layers by deposition
techniques known to those skilled in the art. In particular, it is
possible to deposit a metal layer followed by a dielectric layer.
The lines of grating 92 can be obtained either by micro-machining
the dielectric layer or by a two phase deposition, the deposition
effected in the second phase forming the lines of grating 92. The
resulting beam FR originating from diffraction of beams 16 and 18
in grating 90 is finally detected by detector 98 integrated in
substrate 82. Such detectors are known to those skilled in the art,
as is the electronic circuit used for processing the light signals
received by said detector 98.
[0062] It will be noted that the light detector can be formed by a
unit which is materially distinct from substrate 82, in particular
by a detection unit preceded by a focusing element. In such case,
this detection assembly is arranged either in another aperture, or
in a recess provided on the face of this structure 82 situated
opposite grating 90.
[0063] FIG. 8 shows a fifth miniaturised and partially integrated
embodiment. Semiconductor substrate 82 comprising integrated
detector 98 has a recess 100 in which is arranged the source formed
of electroluminescent diode 88 and transparent ball 86. The bottom
of recess 100 is closed by a transparent layer 102, in particular
made of SiO.sub.2 or Si.sub.3N.sub.4, arranged on one face of
substrate 82 on the side of detector 98. At the surface of this
layer 102 is arranged a dielectric layer defining grating 104 of
period .LAMBDA.. Facing grating 104 is arranged reflection grating
106 of period .LAMBDA./2 at the surface of a mobile reflective
scale 108.
[0064] FIG. 9 shows a sixth entirely integrated embodiment. The
displacement measuring head is formed by semiconductor substrate 82
in which are integrated detector 98 and light source 110.
Preferably, source 110 is directly integrated in substrate 82. In a
variant, source 110 can be manufactured separately and arranged at
the surface of substrate 82 or in a recess provided for the source.
Although source 110 emits with a large numerical aperture in
several directions, only a portion of the beam generated defines
beam FI diffracted by gratings 90 and 92 is finally detected by
integrated detector 98. The optical paths of the two end beams FIA
and FIB of partial beam FI have been shown so as to visualise the
spatial distribution of the different diffracted beams useful for
the relative displacement measurement between substrates 82 and
112. The two end rays of each beam are referenced respectively by
the letters <<A>> and <<B>> after the
previously used numerical reference. This sixth embodiment allows
an ultimate miniaturisation of the measuring device according to
the invention and the integration thereof in mechanical and
micromechanical devices.
[0065] FIG. 10 shows schematically a seventh embodiment of the
invention which differs from the sixth in that, in place of a
single grating 90, three gratings 90A, 90B and 90C are provided,
arranged next to each other and having respectively three
different, although relatively close, spatial periods .LAMBDA.1,
.LAMBDA.2 and .LAMBDA.3. Grating 92 is also replaced by three
gratings (not shown) situated facing the three gratings 90A, 90B
and 90C, and each having a spatial period which is two times
smaller than the spatial period of the grating which it faces. For
each of the pairs of gratings, the application of the optical
principle disclosed in the present invention is identical. By
selecting appropriate values for .LAMBDA.1, .LAMBDA.2 and
.LAMBDA.3, the light intensities, received by a detector having
three distinct detection zones for the three pairs of gratings,
define a signal corresponding to a single relative position between
substrate 82 and substrate 112. Such a device thus enables the
absolute position of the mobile portion to be defined relative to
the fixed portion of the device. This constitutes an application of
the Vernier principle. The device can contain N paths of different
periods to assure univocal coding of each measured relative
position between substrates 82 and 112.
[0066] FIGS. 11 to 14 show schematically four alternative
embodiments of the mobile portion relative to the light source and
the detector each able to be arranged in any of the embodiments
described hereinbefore to define at least one reference position
between the fixed portion and the mobile portion of the
displacement measuring device.
[0067] According to the variant of FIG. 11, in addition to base
grating 116 of constant period .LAMBDA. or .LAMBDA./2, there is
provided beside this latter another grating 118 of variable spatial
period and decreasing to substantially an identical period to that
of grating 116, able to perform identically to grating 116 on a
certain number of lines, to increase again. The reference position
REF is defined by the symmetrical axis of grating 118. The variant
of FIG. 12 differs from that of FIG. 11 in that a grating 120 is
provided beside grating 116 whose period varies by increasing or
decreasing passing from a value higher than the value of the period
of grating 116 to a lower value than the latter. Reference position
REF corresponds to the middle position of the place of coincidence
between the periods of gratings 116 and 120 able to extend over a
certain number of lines.
[0068] When the light beam sweeps grating 118 of FIG. 11 or grating
120 of FIG. 12, an interference signal is generated on passing
across the reference region allowing the displacement measuring
detector or another detector to determine reference position REF.
This originates from the fact that grating 118 or 120 has only in
the reference region a period having a ratio 1/2 or 2/1 with the
grating situated opposite on the fixed portion of the displacement
measuring device. In other words, there is coding of an absolute or
reference position by mutual spatial coherence of the two
gratings.
[0069] FIG. 13 shows another variant wherein there is provided
beside grating 116 a grating 122 of decreasing then increasing
variable period passing from a period higher than that of grating
116 to a lower period. Grating 122 has symmetry relative to
reference position REF situated between two interference signals
occurring at two reference positions REF1 and REF2 where the period
is identical to that of grating 116. Grating 122 thus allows two
reference positions REF1 and REF2 to be determined, which allows
the detected signal processing means to define with great accuracy
the central reference position REF.
[0070] In FIGS. 11 to 13 it will be noted that in the event that
grating 116 has a period .LAMBDA./2, the mutual coherence at the
reference location must be verified at least partially for the
diffraction events at the diffraction points or regions of incident
beams 8 and 10. Consequently, the variant of FIG. 13 can only
define one reference position with a spacing between these two
points or regions substantially equal to the distance between REF1
and REF2.
[0071] FIG. 14 shows another alternative embodiment wherein the
mobile portion relative to the light source includes grating 126 of
period .LAMBDA./2. A second grating 128 is provided beside grating
126, these two gratings 126 and 128 being arranged facing the
grating of constant period .LAMBDA.. Grating 128 is formed of lines
130 defining a period .LAMBDA./2 with two discontinuities defining
an phase shift or offsetting of one portion of grating 128 relative
to the corresponding lines 132 of grating 126. Grating 128 thus has
a first offset of .LAMBDA./4 increasing a space between two lines
130 to 3.LAMBDA./4. At a certain distance from this offset a second
offset of .LAMBDA./4 is provided decreasing from period .LAMBDA./2,
generating a space .LAMBDA./4 between two other lines 130.
[0072] FIG. 15 shows the variation in the luminous intensity
detected by a detector as a function of the displacement of grating
128 when the light beam passes through the region including the two
offsets of opposite directions described hereinbefore. First, the
component AC of the intensity I of beam FR defined hereinbefore
decreases given that one increasing portion of this beam includes
an interference product having a phase difference of .PI.. When
more than half of the first offset of grating 128 has been passed
through or the second phase jump is reached, the component AC of
intensity I again increases to the maximum before again decreasing
and then increasing towards the initial mean value. Graph 134 of
FIG. 15 thus defines three reference points F1, F2 and F3 allowing
three reference positions to be defined or, using a processing
unit, central reference position F2 to be accurately defined. It
will be noted here that it is possible in another variant to
provide a single phase jump of .LAMBDA./4 thus generating a single
minimum in the AC component of intensity I.
[0073] FIG. 16 shows another embodiment of the invention which is
particularly advantageous and able to be miniaturised. The device
includes on the one hand a substrate 82 on one face of which is
arranged a light source, in particular an electroluminescent diode
or a light source integrated in a semiconductor region of substrate
82 and known to those skilled in the art. As in the embodiment of
FIG. 9, this source 110 can be a porous silicon light emitting
zone, an electroluminescent polymeric emitter in a recess zone, or
a LED chip bonded onto substrate 82. This embodiment differs
essentially from the sixth embodiment in that a partial beam
propagating to the right of light source 110 and another partial
beam propagating to the left of said source are used for the
displacement measurement. Thus, to the left and right of source 110
are provided two gratings 92 and 92' of period .LAMBDA./2. On
either side of these two reflection gratings are arranged two light
detectors 98 and 98' integrated in regions of semiconductor
substrate 82. The optical paths of the beams diffracted to the left
and right of source 110 and the two partial beams used for the
displacement measurement are substantially symmetrical. Facing the
face of substrate 82 having gratings 92 and 92' is arranged a
grating 90 of period .LAMBDA. on a reflective substrate 112.
[0074] In order to determine the direction of displacement of
grating 90 and to interpolate in a period of the detected luminous
intensity signal, a variant provides an offset of
.LAMBDA.(m/4+{fraction (1/16)}) between gratings 92 and 92' where m
is an integer number. Consequently, the alternating signal detected
by detector 98 is phase shifted by .PI./2 relative to the
alternating signal detected by detector 98'. However, in order to
be free of any dilatation problem, it is preferable to provide two
additional gratings phase shifted or offset by .LAMBDA./16 on each
side of source 110. The mention of possible expansion leads us to
mention here an application of the device according to the
invention to temperature measurements by expansion of the substrate
formed of materials determined for such application. This is
important in rotating or translating mechanical systems where the
temperature of the moving parts has to be monitored as a criterion
for the system's safety or lifetime.
[0075] FIG. 17 shows another particularly advantageous embodiment
which differs to that described in FIG. 16 in that an opening 100
is provided in the silicon substrate 82 in which a collimation ball
86 is arranged and a diode 88 arranged at the surface or at a
distance of said ball 86 so that the direction defined by the
centre of diode 88 and the centre of ball 86 is substantially
perpendicular to a diffraction grating 140 arranged so as to close
opening 100 on the side of the surface of substrate 82 having
diffraction gratings 92 and 92'. The light supplied by diode 88 is
collimated by ball 86 so that most of the light reaches grating 140
with a substantially perpendicular direction. Grating 140 has a
spatial period and a profile determined so that most of the
luminous intensity incident upon grating 140 is diffracted
substantially in equal parts into the <<+1>> and
<<-1>> diffraction orders. The angle of diffraction in
the air with respect to the direction perpendicular to grating 140
is for example comprised between 20.degree. and 50.degree.. Thus,
most of the luminous intensity provided by diode 88 is transmitted
in useful beams FI and FI'. Grating 140 can be formed in a
SiO.sub.2 or Si.sub.3N.sub.4 layer or in a multi-layered structure
including in particular a superficial dielectric layer of index n
greater than 2.0. Gratings 92 and 92' are formed at the surface of
substrate 82 by deposition of a metal layer 142 followed by
deposition of a dielectric layer 144, for example SiO.sub.2 or
Si.sub.3N.sub.4. Alternatively, the grating can be first etched
into substrate 82 followed by metal deposition.
[0076] In a variant, it is possible to provide a polarisation
element between ball 86 and grating 140. In another variant, it is
possible to provide a transparent layer formed in substrate 82 and
defining the bottom of recess 100. On this transparent layer is
deposited a dielectric layer in which are formed grating 140 and
gratings 92, 92'. It will be noted that any light source may be
provided in this embodiment, fixed to substrate 82 or at a distance
from the latter. Preferably, the incident light over grating 140 is
substantially collimated. However, even for a diverging source,
grating 140 allows transmission into the <<0>>
diffraction order to be limited and thus the luminous intensity to
be concentrated along directions defining a non zero angle of
incidence on grating 90.
[0077] FIGS. 18 and 19 show another embodiment of the invention
allowing a displacement along two orthogonal axes of displacement X
and Y to be measured. The arrangement along axis X, Y respectively
corresponds to the embodiment described hereinbefore in FIG. 17. A
bi-directional grating 150 diffracting along directions X and Y is
arranged on reflective substrate 112. This bi-directional grating
150 is formed of a set of studs 152 defining grating lines along
axes X and Y respectively. It may also be formed by a set of
recesses or square hollows, regularly distributed along axes X and
Y. Bi-directional grating 150 shown in FIG. 18 is mobile relative
to the portion forming the measuring head shown in FIG. 19 and
corresponding to the portion associated with the source. The
measuring head includes on one of its faces arranged facing grating
150, a bi-directional grating 140A having the same function as
grating 140 along the two directions X and Y. Grating 140A
diffracts a light of normal incidence essentially into the first
diffraction order in directions X and Y. Dotted line 154 represents
an opening in the measuring head while the light source supplying a
substantially collimated beam is represented by dotted line 156.
Grating 140A is formed of studs or square hollows 158 aligned along
the two directions X and Y. The measuring head further includes
four gratings 92, 92', 92A and 92A' of period .LAMBDA./2 and at
least four detectors 98, 98', 98A and 98A' arranged so as to allow
optical paths along the two directions X and Y as shown in the
embodiment of FIG. 17 for a unidirectional displacement along axis
X.
[0078] It will be noted that, in a less perfected variant, it is
possible to use a diverging source, in particular the source 110
shown in FIG. 16, and to omit diffraction grating 140A. It will
also be noted that the embodiments shown in FIGS. 1 to 8 can each
also be arranged in a bi-directional displacement device. In order
to do this, the light source in particular is arranged so as to
emit light along the two directions X and Y in a direction of
propagation which is not perpendicular to the diffraction grating
of period A similar to bi-directional grating 150 shown in FIG. 18.
In the case of a collimated beam, in particular a laser beam, this
beam will be oriented in a non-perpendicular way with respect to
the measuring device grating and will have a direction, in
projection in the plane X-Y, median to axes X and Y.
[0079] Another use according to the invention of the devices
corresponding to FIGS. 2, 4, 5, 7, 8, 9, 16, 17, 18 or 19 is the
measurement of the relative velocity V along direction X between
two gratings, by measuring the instantaneous frequency f of the
modulated signal detected in the direction of beam FR by at least
one detector. The relationship between f and V is given by
V=.LAMBDA.f/4. It allows a direct measurement of the velocity
without resorting to phase measurement and period counting.
[0080] A further embodiment of the invention for velocity
measurement corresponds to FIGS. 2, 4 or 8 whereby grating 48 or
106 is the rough surface of the moving substrate 50 or 108 whose
Fourier component along coordinate X corresponding to the spatial
frequency of period .LAMBDA./2 has non-zero amplitude. Substrate 50
or 108 can be a moving band or wire. Among all the beams scattered
in all directions at points B1 and B2 illuminated by beams 8 and
10, only those diffracted in directions 16 and 18 by the spatial
frequency corresponding to the spatial period .LAMBDA./2 will
interfere after recombination along beam FR by grating 46 or 104.
Two conditions may preferably be fulfilled for a high constructive
interference to take place along the beam FR. The first condition
is that the rough surface of substrate 50 or 108 is placed at a
distance from grating 46 where beams 8 and 10 have a non-zero
spatial overlap on said surface. The second condition is that the
length difference AB2-AB1 (FIGS. 1 and 2) between beams 10 and 8 is
smaller than the coherence length of source 2. This interference
appears as a peak of frequency f in the temporal frequency spectrum
of the optical power detected by at least one detector, f being
related to the instantaneous velocity V of substrate 50 or 108 by
V=.LAMBDA.f/4. Those familiar with the art will easily locate f in
the frequency spectrum by resorting to spectral analysis
instruments dedicated to Doppler velocimetry. The advantages of the
device according to the invention for velocity measurement are the
miniaturization, the possible small spacing between the readout
head, comprising the light source, the detector and the grating of
period .LAMBDA., and the moving substrate. Another advantage is the
possibility of using a Light Emitting Diode.
[0081] A further embodiment of the invention for velocity
measurement relates to the previous embodiment where grating 48 is
the surface, exhibiting a non-zero spatial component at period
.LAMBDA./2, of a substrate 50 moving at velocity V. The distinct
characteristics with respect to the previous embodiment is that the
transparent grating 46 of period .LAMBDA. no longer has a fixed
position relative to the source and to the detector, but translates
at a constant and known velocity v.sub.r along X, v.sub.r being
larger than the maximum which V can have. In one variant, grating
46 is a radial grating made at the surface of a large radius disk
rotating in a plane parallel to the displacement direction X and
normal to the plane of incidence of beam F1. In a second variant,
grating 46 is a closed grating band rotating on two drums having
their rotation axis normal to the incidence plane, the movement of
grating 46 between the source/detector assembly and the substrate
50 being rectilinear and in the X direction. Grating 46 is for
instance made by embossing in a polymeric foil. The frequency f of
the modulated optical power signal measured by the detector is
related to the velocities V and v.sub.r through
f=4/.LAMBDA.(V+v.sub.r). This embodiments allows the accurate and
fast measurement of the velocity V even in case V is close to zero.
As a consequence, this embodiment allows an accurate determination
of the length of a finite displacement L inclusive of its slow
beginning and of its slow end by integrating the velocity V over
time t. 1 L = t 0 t1 V t = 4 t 0 t 1 f t - v r ( t 1 - t 0 )
[0082] where t.sub.0 and t.sub.1 are the starting and stop times of
the displacement. The device according to the invention can
therefore be advantageously used to measure the length of long
strands of wire, bands, ribbons or sheets of different
materials.
[0083] FIG. 20 shows an embodiment of a measuring device with a
mobile scale 160 allowing a maximum measurement range for a given
grating length and having in addition the advantage that the whole
set of the gratings, source, detector(s) and optical paths used for
the measurement is entirely contained in a closed case (a tube for
example), without the mobile grating associated with scale 160
exiting the case, while the displacement range of this scale (a
metal rod for example) can reach a value only slightly smaller than
the length of the inner cavity 164 of case 162, and without the
scale 160 supporting the source and the detector. In order to do
this, a light source 166 emits a beam FI along a direction
essentially parallel to direction of displacement X. Rod 160 has in
its upper portion a plane 168 inclined at an angle greater than
45.degree. relative to axis X. This inclined plane 168 defines a
mirror for beam FI, which is reflected in the direction of a fixed
grating 170 of period .LAMBDA. arranged on a wall of cavity 164.
Beam FI thus reaches grating 170 at an angle of incidence which is
not zero according to the invention. Scale or rod 160 also includes
a reflective surface 162 defining a grating 174 of period
.LAMBDA./2. Following grating 174 is arranged an inclined plane 176
defining a mirror. This inclined plane 176 defines an angle,
relative to a direction perpendicular to gratings 170 and 174,
equal to the angle defined between inclined plane 168 and direction
X. Thus, the resulting beam FR is reflected along a direction
parallel to axis X and is directed towards detector 178.
[0084] Those skilled in the art will understand that it is possible
to invert the arrangement of source 166 and detector 178, the
optical paths remaining the same and the light propagating in a
reverse direction to that shown in FIG. 20. In order to assure a
stable displacement along axis X, two bearings 180 and 182 are
provided at the opposite end to that where the source and the
detector are arranged. It will be noted that any other guide means,
in particular a slide can be provided as an alternative
arrangement.
[0085] Other variants using mirrors to deviate and orient incident
beam FI and resulting beam FR can be designed by those skilled in
the art while remaining within the scope of the present invention
and, in particular, of the embodiment described with reference to
FIG. 20.
[0086] Finally, it will be noted that the gratings can be formed in
various ways by various methods known to those skilled in the art,
in particular by a periodic variation in the refractive index at
the surface of a plane dielectric layer. Moulding and embossing
techniques may also be envisaged. The profiles of the transverse
sections of the diffraction gratings can be optimised for each
particular device in order to increase the efficiency of the
displacement measurement according to the principle of the
invention.
* * * * *