|Publication number||USRE40551 E1|
|Application number||US 10/803,198|
|Publication date||Oct 28, 2008|
|Filing date||Mar 18, 2004|
|Priority date||Aug 21, 1998|
|Also published as||CA2341432A1, CA2341432C, DE69904338D1, DE69904338T2, EP1104543A1, EP1104543B1, US6359691, US20010006421, WO2000011431A1|
|Publication number||10803198, 803198, US RE40551 E1, US RE40551E1, US-E1-RE40551, USRE40551 E1, USRE40551E1|
|Inventors||Olivier M. Parriaux|
|Original Assignee||Parriaux Olivier M|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (12), Non-Patent Citations (2), Classifications (6), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a con of PCT/EP99/06057 Aug. 19, 1999.
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.
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.
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 to 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.
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.
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.
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.
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.
Another object of the invention is to provide a device of this type allowing a very flat arrangement which can easily be miniaturised.
A particular object of the invention is to provide a device of this type at least partially integrated in a silicon or semiconductor substrate.
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.
The features of this measuring device allows an easy miniaturisation and its integration by microelectronic and microsystem technologies.
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.
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.
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.
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.
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:
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.
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.
Gratings 6 and 20 have a spatial period Λ and gratings 12 and 14 have a period which is substantially two times smaller, i.e. substantially equal to Λ/2 and preferably equal to Λ/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 gratings 20. There is thus perfect superposition of the two beams interfering along said first direction of diffraction.
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
According to a particular feature of the present invention, beam FI incident upon first grating 6 has an angle of incidence α which is not zero. Consequently, in the plane of
Another consequence of non-zero incidence angle α is to prevent the spurious z-dependent modulation signal due to self-mixing when the source is a semiconductor laser.
The device according to
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.
By way of example, for a wavelength Λ=0.67 μm and an angle of incidence α=10°, diffraction grating 6 is formed in dielectric layer 36 of refractive index approximately n=2.2, in particular made of Ta2O5 or TiO2 deposited by a technique known to those skilled in the art, on glass substrate 4, the total thickness E1 of this layer being comprised between 0.4 and 0.5 μm. The depth P1 of the grooves situated between lines 30 of grating 6 is comprised between 0.30 and 0.35 μm. Transmission of approximately 80% of the total luminous energy of beams 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 P1 than in a lower index layer, or than directly in the transparent structure 4.
Those skilled in the art can also optimise the profile of the section of grating 6 along the transverse plane of
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 polarization, by using a grating structure comprising 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.
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 E2=0.34 μm and a depth P2=0.18 μm 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 λ and period Λ generating propagation of beams 8 and 10 to the right and left of the direction perpendicular to grating 6 respectively.
The arrangement of gratings 12 and 14 described in the example hereinbefore is provided for a situation in which the light received is not polarized. However, if the light is TE polarised (electric field vector parallel to the grating lines), thickness E2 of gratings 12 and 14 is approximately 0.1 μm, while the depth P2 is situated at around 0.08 μm and can even be equal to thickness E2. Substrates 24 and 26 are made for example of aluminum 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 dffracted 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 μm. 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
Dielectric layer 42 of grating 20 has a thickness E1 and a groove depth P1 substantially identical to those of grating 6 so as to assure reciprocity of the diffraction even 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.
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.
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 α 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.
It will be noted that substrate 50 is here staitonary 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.
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 α,λ and Λ given hereinbefore, grating 46 is formed of a dielectric layer 52 of refractive index n=2.2 approximately and having a thickness E1 comprised between 0.35 and 0.40 μm with a groove depth P1 equal to approximately 0.24 μm. 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.
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 E2 comprised between 0.25 and 0.30 μm with a groove depth P2=0.22 μm. As in
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 to 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 α. 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.
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 α 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 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.
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 Λ/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 into region R4. In these conditions those skilled in the art can calculate that the offset of Λ/8 provided in region R4 finally generates a phase shift of II/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 II/2 relative to the interference originating from region R2. By separately detecting the contributions from region R1 and R2, the detector receives two alternating luminous intensity signals phase shifted by II/2 in relation to each other. In a variant, it is possible to provide three gratings in parallel with an offset of Λ/6 to give three luminous intensity signals phase shifted by 120°. 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 Λ/16 with respect to region R1 in order to provide an optical intensity phaseshift of II/2, or by Λ/12 for a phaseshift of 120°. Grating 48 can also be divided into four regions similar to R1 and R2 with three regions having respectively offsets of Λ/16, Λ/18, 3Λ/16 relative to the last one in order to obtain the full set of four quadrature optical power signals.
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 Λ/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
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.
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.
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.
In conclusion, whatever the divergence or numerical aperture of beam FI, only partial beam FI* contributes to the displacement measurement and only regions RI, 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.
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.
With reference to
An incremental angle of rotation of wheel 60 corresponds to period Λ 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.
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
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.
According to the variant of
When the light beam sweeps gratings 118 of
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 Λ(m/4+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 II/2 relative to the alternating signal detected by detector 96′. However, in order to be free of any dilatation problem, it is preferable to provide two additional gratings phase shifted or offset by Λ/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.
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.
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
Another use according to the invention of the devices corresponding to
A further embodiment of the invention for velocity measurement corresponds to
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 Λ/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 Λ no longer has a fixed position relative to the source and to the detector, but translates at a constant and known velocity vr along X, vr 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 mesured by the detector is related to the velocities V and vr through f=4/Λ(V+vr). 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 slower end by integrating the velocity V over time t.
where t0 and t1 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.
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.
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.
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.
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|1||Interference Polarizers For The Ultraviolet Spectral Region, pp. 215-219, Use of Reflecting Diffraction Gratings in Interference Systems for Measuring Linear Shift, RASSUDOVA.|
|2||International Search Report by G. Vorropoulos, PCT/EP 99/06057 Nov. 29, 1999, 2 pages.|
|International Classification||G01B9/02, G01B11/02, G01D5/38|
|Aug 27, 2009||FPAY||Fee payment|
Year of fee payment: 8
|Aug 26, 2013||FPAY||Fee payment|
Year of fee payment: 12