FR2544875A1 - OPTICAL DISPLACEMENT SENSOR - Google Patents

Abstract

AN OPTICAL DISPLACEMENT SENSOR INCLUDES A PAIR OF 10 NETWORKS, EACH NETWORK INCLUDING AN ORDERED ROW OF FIRST AND SECOND SETS OF 12, 13 RAIES OF WHICH THE RESPECTIVE RAIES ARE ALTERNATIVELY ARRANGED. THE RAYS 13 OF THE FIRST SET SHOW AN OPTICAL CHARACTERISTIC ACCORDING TO WHICH THE TRANSMITTANCEREFLECTANCE OF THE RAYS IS SENSITIVELY INDEPENDENT OF THE WAVELENGTH OF THE INCIDENT RADIATION. THE 12 RAYS OF THE SECOND SET SHOW AN OPTICAL CHARACTERISTIC WHICH DEPENDS ON THE WAVELENGTH OF THE INCIDENT RADIATION. FOR A FIRST BAND, THE 12 RAYS OF THE SECOND SET HAVE THE SAME TRANSMITTANCREFLECTANCE AS THE 13 RAYS OF THE FIRST SET, WHILE, FOR A SECOND BAND, 12 RAYS OF THE SECOND SET HAVE A DIFFERENT TRANSMITTANCREFLECTANCE FROM THAT OF THE 13 RAYS OF THE FIRST SET.

Description

Optical displacement sensor.

  The present invention relates to

  displacement and optical network transducers.

  A known form of optical network includes a-

  optically transparent substrate, such as glass, on which is located a set of opaque lines arranged parallel to each other and separated from each other

  others from a predetermined distance These networks con-

  have various applications and, in particular,

  that they are used in pairs and illuminated by a light beam, they act as displacement sensors, because a relative movement of the two networks causes

  intensity modulation of the transmitted light beam.

  The degree of this modulation, as measured by a detector

  part of a transducer, provides a measurement

  However, any variation in the characteristics

  ticks of networks or of the illuminating light beam, or of the detector, such as that which can be brought

  by variations in the voltage of the electrical supply

  of the detector and the light source, or due to the aging of the components, reveals errors

as far as displacement.

  An object of the present invention is to provide an improved form of displacement sensor which

  overcomes or mitigates the above drawbacks.

  To this end, the present invention relates to a displacement sensor comprising first and second

  optical networks, the first network being mobile by comparison

  port to the second network for modulating the intensity of an interrogating radiation beam, each network comprising an ordered row of first and

  second sets of lines, the respective lines of the pre-

  mier and second sets being alternately arranged in the row, the lines of the first and second networks being parallel, and it is characterized in that one of the first and second optical networks comprises lines

  of the first set which have an op-

  tick that transmittance (or trans-

  mission) or the reflectance (or power of reflection) of the lines is substantially independent of the wavelength of the incident radiation, and the lines of the second set have a corresponding optical characteristic which depends on the wavelength of the incident radiation so that, for a band of a first wavelength,

  the lines of the second set have the same transmit-

  tance or reflectance as the lines of the first set, and that for a band of a second wavelength, the lines of the second set have a transmittance or a reflectance different from that of the lines of the first

together.

  In a particular form of the first network,

  the first and second sets in the row are supported

  ted by a transparent substrate and the second set is formed by a coating adhering to the substrate, the coating

  being a material forming a filter eliminating interference

  rences, and the first set consisting of parts

not coated with the substrate.

  In another form of the first network, is pre-

  seen a substrate made of a material forming an interference eliminating filter, the first set being

  consisting of a coating adhering to the substrate and defined-

  sant first highly reflective lines, the second set consisting of uncoated parts

of the substrate.

  In yet another form of the first network, there is provided a substrate made of a material forming a

  absorption filter adhering to a highly reflective layer

  chissant, the first assembly being formed by a coating adhering to the substrate and defining first highly reflective lines, the second assembly being

  made of uncoated parts of the substrate.

  The detectors according to the present invention find

  wind their application in the 'displacement transducers

  ment comprising means for emitting a light beam

  neux arranged to deliver to the sensor a first light beam in the band of the first wavelength and to deliver to the sensor a second light beam in

  the second wavelength band, means of re-

  ception of light beams arranged to collect the first and second beams from the sensor, and detection means at the output of the reception means, intended to measure the ratio between the intensities of the

first and second beams.

  According to an embodiment of an op-

  tick according to the present invention used in the displacement sensor, the sensor can operate either in a transmission mode or in a reflection mode In all cases, it will be noted that the sensors perform a

  modulation of the intensity of one of the two light beams

  due to the displacement of one of the two re-

  sensor buckets, while the other of the two beams

  light is in no way subject to a modulation of

  density, or is subject to it only to a small extent, whatever the displacements of the network, this beam thus taking the place of reference light beam In the case where the reference beam is subjected during displacements of the network to a modulation of intensity in

  a small measurement, this can be deleted by cali-

  electronic bration Consequently, any variation in

  characteristics of the transducer components is detected

  ted by the detection means and, since the output thereof has the form of an intensity ratio,

  displacement measurement is made independent of the

  characteristics of the components used in the transducer

tor.

  The light beam emission means can

  wind comprise a single light source emitting light in a wide band, a first part of this band constituting the first light beam and a second part of this band constituting the second light beam, as determined by the transition wavelength of the second set of lines of the network according to the present invention According to a variant, the light beam emission means may comprise a

  first light source emitting only the first beam

  light beam and a second light source emitting

than the second light beam.

  Note that, if you use only one source

  bright broadband, the wavelength of transi-

  tion must be precise enough to eliminate any ambiguity between the first and second beams, while, if two separate light sources are used, the accuracy of the transition wavelength is not necessarily critical However, the lengths

  of the first and second beams should preferably

  rence be close enough to avoid or half

  minimize divergence due to effects such as

Raleigh persion.

  The light beam receiving means may comprise a beam splitter intended to supply the first and second light beams to the detector means separately. In this case, the detection means comprise separate detection elements respectively intended to detect the intensities of the first and second light beams According to a variant,

  only one detection element can be used, the trans-

  conductor being arranged to supply the first and second beams at separate times In this arrangement, the detection means can comprise a data storage unit, this solution being advantageous in

  any deterioration or variation in the characteristics

  that the single sensing element will not affect the

displacement measurement.

  The detection means may include any known means for providing the desired report, such as a

  voltage divider The light source (s) mentioned-

  previously born may have the form of laser diodes, in which case feedback systems may be used to keep the output intensity stable over time. According to a variant, the light source may have the form of a photo-emissive diode. ,

  in which case a measurement of the output intensity is direct

  delivered to the detection means and the rate of

  pulses of the light-emitting diode is arranged so that the detection means receive separately the signals coming from the sensor and those which come directly

  of the light-emitting diode, the ratio between the intensities

  tees of these signals eliminating the influence of variations

  of the intensity of the light emitting diode.

  Preferably, the light beam emitting means and the light beam receiving means each comprise optical fibers, in order to make the

  transducer suitable for use in hostile environments.

  Certain embodiments of the present invention are described below by way of example, with reference to the accompanying drawings, in which:

  Figure 1 illustrates a form of op-

  tick intended to be used according to the present invention FIGS. 2 to 5 illustrate respective embodiments of displacement sensors according to the present invention, and FIGS. 6 to 8 illustrate respective embodiments of displacement transducers

according to the present invention.

  The network 10 illustrated in FIG. 1 comprises a glass substrate 11 comprising an anterior surface

  l 1 A on which a set of lines 12 is located

  tending parallel to each other, the lines ad-

  adjacent lines being separated by lines 13 The lines 13 consist of regions not coated with the substrate 11, and therefore have the transmission characteristics of the substrate 11, which are independent of the length

  wave of incident radiation Lines 12 are

  each formed of a coating adhering to the surface

  11 A and made of a material which transmits

  selective activity, as a function of the wavelength band of the incident radiation. For example, the

  lines 12 can consist of a covering

  carrying one or more layers and acting as an interference filter, which transmits below a predetermined wavelength (the transmission wavelength) and which does not transmit for wavelengths situated

  above the predetermined wavelength.

  FIG. 2 illustrates a sensor 50 formed of a pair of networks 10 as individually described with reference to FIG. 1 This sensor 50 operates in the

  transmission, so that the incident light is trans-

  put through the sensor 50 while being modulated in intensity

  or not, depending on whether the wavelength band is

  above or below the transition wavelength

or predetermined from lines 12.

  In the sensor 50, the lines 12 of the networks

  have the characteristic of transmitting long

  wavelengths located below the predetermined wavelength and not to transmit the lengths

  wavelengths above the predetermined wavelength

  However, it will be noted that the reverse arrangement can be used, in which case the lines 12 will not transmit below the predetermined wavelength and will transmit above the predetermined wavelength In all cases, - the incident light beam which

  has a wavelength for which the lines 12 trans-

  act as a reference beam and thus the

  precise transmission characteristic of lines 12 as well

  if that of the part of the substrate 11 on which are so-

  killed lines 12 must be equal to that of lines 13.

  In addition, the incident light beam which has a wavelength for which the lines 12 do not transmit

  acts as a signal beam and is intensively modulated

  sited by the displacement of a network 10 with respect to the

be network 10 of sensor 50.

  FIG. 3 illustrates a sensor 54 which comprises an array 55 comprising a glass substrate 56 on which are located lines 57 which do not transmit, which can be either mirror lines, or opaque lines, and which never transmit, whatever the wavelength of the incident radiation (that is to say that the network 55 is a known conventional network) The sensor 54 also includes a network 10 as described with reference to FIG. 1 This sensor operates in mode of transmission due to the fact that both the signal beam and

  the reference beam are transmitted through the cap-

  tor 54, but in this case the reference beam is subjected to an intensity attenuation (of the order of 50%)

  caused by the presence of lines 57.

  FIG. 4 illustrates a sensor 60 which comprises a conventional or known network 61 comprising opaque lines 63 on a glass substrate 62 The second network 66 of the sensor 60 is formed by mirror lines 65 mounted on a substrate 64 formed of a material forming an interference filter Thus, this sensor 60 operates in reflection mode, and the reference beam due to the

  presence of lines 63 is subject to attenuation d-'in-

  density when the substrate 64 does not transmit (and is non-reflective) Similarly, the signal beam, in addition to being

  modulated, is attenuated in intensity by the presence of the

trat 64 when this transmits.

  FIG. 5 illustrates a sensor 70 which comprises a conventional or known array 71 comprising opaque lines 72 on a glass substrate 73 The second array 76 consists of mirror lines 75 with high reflection on a substrate 74 formed of a material of absorption filtering 77 adhering to a mirror coating 78 with high reflection Thus, the sensor 70 operates in reflection mode and the reference beam, although subject

  attenuation of intensity, is obtained when the material

  riau 77 is in its integral transmission mode, the entire network 76 thus being highly reflective and acting simply as a mirror Of course, the signal beam is formed by reflection on the lines when the filter material 77 is in its mode

absorption or opaque.

  Various embodiments of displacement transducers are described below by way of example using the sensor 50 of FIG. 2, the transducer thus being in transmission mode, but it will be understood that the other sensors 54, 60, 70 previously described

  can replace sensor 50, an appropriate adaptation

  required being made in the reception of the light beams from the sensor when the latter operates in reflection mode. In the transducer 20 illustrated in FIG. 6, the sensor 5 r 0 is interrogated by light emitted along an optical fiber 22, and the transmitted light is collected by an optical fiber 23, the fibers 22 and 23

  thereby forming a bidirectional fiber optic cable.

  Light is introduced into the fiber 22 by a self-converging lens 24 from a broadband source 25

  which in this case consists of a photo diode

  emissive with a central wavelength of 820 nm

  lines 12 of each network 10 of the sensor 50 have a long

  830nm cutoff wavelength, so that the source

  simultaneously provides the first light beam

  with wavelengths less than 830 nm and the

  cond light beam consisting of longer wavelengths

  fibers at 830 nm Fiber 23 delivers the light beams

  neux collected at a self-converging lens 26 whose output is incident on a beam splitter 27, the frequencies constituting the first light beam being transmitted through the beam splitter 27 to a first photodiode 28 and the frequencies constituting the second light beam being reflected by the divider

  of beam 27 on a second photodiode 29 Each photo-

  diode 28, 29 measures the intensity of the light beam it receives and the electrical outputs of the photodiodes 28, 29 are supplied to a voltage divider 30 which

  provides at its output 31 the intensity ratio On no-

  tera that the characteristics of the beam splitter 27 must be chosen at the outset to be comparable to those of lines 12 of networks 10, insofar as the

  beam splitter 27 transmits the first beam beam

  losslessly and reflects the second beam

so losslessly.

  In the second transducer illustrated in figure

  gure 7, the light source 25 comprises a first laser diode 35 having a narrow band at 840 nm, a second laser diode 36 having a narrow band at 904 nm, and a beam dissector 37 arranged to transmit the light beam coming from the diode laser 35 and for reflecting the beam coming from the laser diode 36 The beam dissector 37 is arranged to reflect a small percentage of the light coming from the diode 35

  and to transmit the same small percentage of the light-

  re coming from the laser diode 36, these being detected by a photodiode 38 the output of which is delivered as a feedback signal to the pulse circuit and to

  power supply 39 which alternately control the di-

  35 and 36 so that their output beams

are intertwined in time.

  The optical fiber 23 delivers the two light beams interleaved in time to a single photodiode 41, the electrical output of which is supplied to a

  processing 42 which includes a data storage unit

  data and a data division unit.

  In the second transducer, the wavelength

  cutoff of lines 12 of networks 10 is 870 nm.

  It will be noted that the two laser diodes 35, 36 of the embodiment of FIG. 7 could be replaced respectively by photo-emissive diodes associated with narrow band filters, for example, by

  a photo-emissive diode having a central wavelength

  the of 820 nm in combination with a narrow band filter Ealing having a central wavelength of 820 nm on the one hand, and by a photoemissive diode having a central wavelength of 900 nm in combination with a band filter narrow Ealing having a central wavelength

900 nm on the other hand.

  It will also be noted that the beam splitter 37, which acts as a device intended to combine the two light beams, could be replaced by a

four-point optical coupler.

  In the transducer illustrated in Figure 8, the fluctuations in the intensity of the light beams

  emitted by the two light sources 35, 36 are controlled

  by sampling the light output from them i L and by coupling the sampled light to the photodiode

  41 by means of beam splitters 45, 46.

  By way of example, the networks 10 of FIG. 1

  can be produced by photolithic techniques

  first, the glass substrate is coated with the material from which the lines are to be formed, then the material is covered with a layer of resin

  photosensitive which is exposed to ultra-vio-

  lette through a model diffraction grating Resin

  photosensitive is then developed and engraved in an ap-

  propriée using a silicon dioxide etchant. Although each of the transducers described forms a single transducer system, it can be used in a passive multiplexing system using a set

  pairs of source wavelengths.

  It will also be noted that, to provide simultaneous-

  ment to the detector a signal beam and a

  ference, two separate light beam sources may

  can be used, each beam being modulated in

  quence and being separated in the detector by demodulation.

  This solution eliminates the need to memorize data.

  born, as previously described.

Claims (7)

  1 displacement sensor (50) comprising first (10) and second (10) optical networks, the first   network being mobile with respect to the second network for   read a modulation of the intensity of a beam of interrogating beam 4 F, each network (10) comprising an ordered row of first and sec nd sets of lines   (12, 13), the respective lines of the first and second en-   sembles being alternately arranged in the row, * 10 the lines of the first Pt second network being parallel, characterized in that one of the first and second optical networks (10) comprises lines (13) of the first set which have an optical characteristic that   the transmittance or reflectance of the lines (13) is felt   so independent of the wavelength of the ray   incident, and in that the lines (12) of the second   seems to have a corresponding optical characteristic   which depends on the wavelength of the incident radiation, so that, for a band of a first wavelength, the lines (12) of the second set have the same transmittance or reflectance as the lines (13) of the first set and that, for a band of a second wavelength, the lines (12) of the second set have a transmittance or a reflectance different from that   lines (13) of the first set.   2 sensor according to claim 1, characterized   in that, in the first network (10), the first and second   cond sets of the row are supported nar a transparent substrate (11) and the second set (12) is formed of a coating adhering to the substrate (11), the coating (12)   being a material forming a filter eliminating interference   rences, and the first set (13) consisting of   uncoated parts of the substrate (11).   3 sensor according to claim 1, characterized in that the first network (66) comprises a substrate (64)   made of a material forming a filter eliminating   tererences, and the first set (65) is formed of a   coating adhering to the substrate and defining pre-   highly reflective stripes, the second set consisting of parts not coated with the substrate (64). 4 sensor according to claim 1, characterized in that the first network (76) comprises a substrate (77) made of a material forming an absorption filter and adhering to a highly reflective layer (78) and the first assembly (7 ;) is made of an adherent coating   to the substrate (77) and defining first high lines   reflective (75), the second set being cons-   made up of parts not coated with the substrate (77).   Sensor according to claim 2, characterized   in that the other (55) of the first and second networks comprises   takes a transmitting substrate (56) supporting lines (57) having an optical characteristic that   the transmittance or reflectance of the lines is independent   the wavelength of the incident radiation.   6 Sensor according to any one of the claims.   tions 3 and 4, characterized in that the other (61) of the pre-   mier and second networks includes a substrate which transmits (62) supporting lines (63) which do not transmit and which do not reflect, whatever the   wavelength of the incident radiation.   7 Displacement transducer including a   displacement sensor according to any one of the res-   dications l to 6, characterized by the combination with   light beam emission means (25, 24, 22) arranged   cés to deliver to the sensor (50) a first beam beam   miners in the band of the first wavelength and for delivering to the sensor (50) a second light beam in the band of the second wavelength, light beam receiving means (23, 26, 27) arranged to collecting the first and second beams from the sensor (50), and detection means (28, 29, 30 at the   output of the receiving means (23, 26), intended for measurement   rer the relationship between the intensities of the first and second beams. 8 transducer according to claim 7, characterized in that the light beam emission means   (25, 24, 22) and the light beam receiving means   neux (23, 26) each include optical filaments, to make the transducer suitable for use in a hostile environment.   9 Transducer according to any one of the re-   vendications 7 and 8, characterized in that the detection means (28, 29, 30) comprise first and second detection elements (28, 29) and the light beam receiving means comprise a beam splitter (27) for to separately deliver the first and second light beams to the respective detection elements   (28, 29) detection means (28, 29, 30).   A transducer according to any one of the re-   vendications 7 and 8, characterized in that the detection means comprise a single detection element (41) and the light beam emission means (23, 26) are arranged to provide the first and second beams in separate moments.   11 transducer according to claim 10, ca-   characterized in that the detection means (41) comprise   further provide a data storage unit (42) for   to store beam intensity data   until the intensity of the other beam is   tected, in order to allow the measurement of said information ratio tensities.