FR2653552A1 - OPTICAL TRANSDUCER WITH SPECTRAL CODING OF REFLECTING SURFACE WITH MONOBLOC HEAD AND HOLOGRAPHIC COMPONENTS. - Google Patents

Abstract

Optical Transducer Provided is an optical displacement transducer having good efficiency and low vibration sensitivity. Wideband optical radiation is transmitted to an optical head (3) through a beam splitter (36) and an optical fiber (35). The head (3) has a glass block (30) with a holographic collimator (33) which directs radiation from the fiber (35) onto a holographic diffraction element (32) mounted on an inclined surface of the block. A holographic focusing element (37) reflects the diffracted radiation and focuses it into a spectrum (60) on a rotating disc (4) carrying reflecting tracks (51, 52). The reflected radiation returns by the same path to the beam splitter (36) which transmits it to a wavelength decoder.

Description

The present invention relates to an optical transducer comprising a

  optical source which emits radiation over a wavelength band, an optical head provided with a diffraction element arranged to disperse the radiation according to the wavelengths that compose it along a transverse zone with respect to the path radiation, so that the wavelength of the radiation delivered by the optical head at any point in this zone depends on the location of this point of the zone, a movable element capable of moving relative to said zone and transversely to the radiation path, a detector, and a radiation guide extending between the optical head and the detector, the moving element being optically encoded to reflect radiation of different wavelengths to the head according to the transverse position of the movable element, the diffraction element being arranged to diffract the reflected radiation, according to its wavelength, from

  to form an image on one end of the spoke guide.

  ment, and the detector being arranged to respond to radiation at different wavelengths emerging from the radiation guide, so as to output an output signal depending on the position of the

the moving element.

  Optical displacement transducers are well known and generally utilize a light source and a receiver, for example, optical cable ends, and means for varying the amount of radiation reaching the receiver as a function of displacement. The means for varying the radiation may use a moving mask provided with an opening of variable size, or a neutral density filter whose density varies in the direction of travel. These transducers can function satisfactorily provided that the intensity of the radiation reaching the receiver does not vary for any other reason. On the other hand, any modification of the radiation emitted by the light source, caused for example by variations in the power supply of the

  light source, produces a false reading.

  One possibility of reducing the effect of these variations in light intensity is to modulate the spectral content of the radiation as a function of the displacement. However, a problem inherent with known transducers of this kind lies in the difficulty of obtaining a linear output signal, since the devices producing the spectral dispersion are not linear and the wavelength decoders are not linear.

  do not usually present a linear response.

  In another arrangement, the light is dispersed in its spectral components and an encoded mask is moved within the spectrum to allow transmission of different parts of the spectrum depending on the position of the mask. Radiation passed through the mask is focused on one end of an optical cable and dispersed back to the other end of the cable. By measuring the intensity of the radiation in different parts of the spectrum, it is possible to determine the position of the mask. However, the problem with such an arrangement is that it is difficult to ensure that all the radiation transmitted through the mask is focused on the end of the receiving fiber, because the image formed will be spread on the image plane because of the scattered nature of the spectral image on the mask. There is also a problem that, under certain circumstances, it is not possible to use a transparent mask that requires both sides of the mask

be accessible.

  GB 2 188 144 A discloses an optical transducer which avoids these problems. This transducer comprises an optical head mounted above an optically encoded reflective disc. Broadband radiation is delivered to the head by means of an optical cable. The head is a glass block with a curved reflective surface that collimates and reflects radiation onto an ordinary diffraction grating. The radiation is dispersed by the grating and reflected towards the curved reflective surface. This has the effect of focusing the radiation on the disk in the form of a spectrum. The head combines the radiation reflected by the disc and focuses on the end of a return fiber. At the other end of this fiber, a wavelength decoder disperses the received radiation on a row of diode photos, which delivers an output signal representative of the position of the reflective portions of the disk, thereby determining the position of the disc. By measuring the spacing between two reflective tracks, concentricity errors can be eliminated. The displacement of a coded plate in two coordinate directions can be measured by measuring the spacing and absolute values of

reflected wavelengths.

  Although the above-mentioned transducer has considerable advantages over previous arrangements, it is relatively expensive to manufacture and its efficiency is lower than desired due to the use of an ordinary diffraction grating. Ordinary diffraction gratings have a relatively low efficiency, in particular because they depend on the polarization, so that the radiation in a plane of polarization is not used by the transducer. The high cost of this transducer of the prior art is due to the need to precisely shape the reflective faces having orientations.

  varied, which requires a realization in several different parts.

  An object of the present invention is to create an optical transducer that may be more efficient for a cost

relatively low.

  For this purpose, the invention relates to an optical transducer of the type indicated above, characterized in that the diffraction element is an element

holographic diffraction.

  Preferably, the optical head comprises a holographic element arranged to focus the radiation on the movable element and to focus the radiation reflected by the movable element. The holographic element arranged to focus the radiation can be separated from

  the holographic diffraction element.

  The optical head may comprise a holographic collimation element placed between the radiation guide and the holographic diffraction element. The optical head is preferably made of a one-piece block of transparent material, and the holographic diffraction element is mounted on a surface of this block. In an advantageous embodiment, said block has a substantially cubic general shape, the holographic diffraction element is mounted on an oblique surface formed on a first side of the block, a collimation element is mounted on an opposite side of the block, between the radiation guide and the diffractive element, and a reflective element is mounted on said opposite side so as to reflect the radiation coming from the element of

  diffraction to the movable element through the first side of the block.

  The radiation guide to the detector may include a beam splitter, and the radiation of the optical source may be transmitted to the optical head through the beam splitter and the radiation guide. At least a portion of the radiation guide can lead to a display device where the radiation reflected by the movable element is visible, so that the observation of the color of this

  radiation provides an indication of the position of the movable element.

  Preferably, the movable member has two elongated reflective tracks, and the mutual spacing of the two tracks is different at different points along their length. The movable member may be a rotating disk. An optical transducer according to the present invention will be described below by way of example, with reference to the accompanying drawings, in which FIG. 1 is a diagram of the transducer, FIG. 2 is a side view of the optical head of the transducer. Fig. 3 is a plan view of the moving encoder element used in the transducer, Fig. 4 is a plan view of another form of encoder element, and Figs. 4A through 4C show signals. of a detector in response to

  different positions of the encoder element of FIG.

  With reference to FIG. 1, the optical transducer comprises a source 1 of radiation in a wavelength band 1 to 2 'in the form of a light-emitting diode or of another source such as a tungsten lamp, and an electrical control 21. The radiation of the source 1 is delivered by a single-fiber optical cable 1 to an optical head 3 which will be described later in more detail. The optical head 3 disperses the radiation into spectral components and directs it onto an encoder disk 4 which rotates about its axis 5 as a function of the variations of an input variable. For example, the input variable could be given by a pressure or temperature sensor, rotating a tree 6

which carries the encoder disk 4.

  The encoder disc 4 reflects different parts of the spectrum towards the optical head 3, depending on the position of the disc. This reflected radiation is recombined in the optical head 3 and delivered on

  a first end of a monofibre optical return cable 7.

  The other end of the return cable 7 is connected to a wavelength decoder 8 which disperses the radiation coming from the cable into wavelength components. The decoder 8 may comprise any conventional form of dispersion device, for example a grating or a prism. The spectrum formed by the decoder 8 is focused on a linear array 9 of photodiodes or similar devices, delivering on lines 10 output signals which represent the intensity of the radiation in different parts of the spectrum. Since there will generally not be a single illuminated photodiode, but a multiple photodiode distribution, a signal processing device 11 is provided to identify the intensity peaks in the photodiode.

  spectrum. The output signals of the device 11 are transmitted to a device

  - 6 - display 12 which displays an indication of the position of the disk

  encoder 4, which can be calibrated to represent

  the variable to be measured, eg pressure or temperature.

  In another embodiment, the output signals may be delivered to other user means, for example to perform commands

according to the measured variable.

  If white light of sufficient intensity is used, a portion of the modified light may be derived and used to illuminate a point on a control panel. This means can give a simple and direct indication of the state of the variable. For example, as red is at one end of the visible spectrum, this color could indicate a dangerously high pressure, while green

would indicate a correct pressure.

  The optical head 3 will be described in more detail with reference to FIG.

  This head 3 is formed mainly of a single block of glass 30 having a generally cubic shape, except for an oblique surface 31 along its lower left edge according to the figure. On the surface 31 is mounted a volume holographic element 32 which serves as a grating or diffraction element. In the vertical direction of the holographic diffraction grating 32 is a holographic collimating element 33 which is mounted against a horizontal upper face 34 of the block 30. The collimating element 33 is placed directly under a fiber optic radiation guide 35 which is coupled to the input fiber 37 and the fiber of

  return 7 by means of a beam splitter 36.

  Also on the upper face 34 of the block 30, a holographic focusing element 37 is placed to receive the radiation of the diffraction grating 32. The focusing element is arranged to reflect this radiation vertically downwards and to focus it on the beam. Encoder disk 4. The bottom surface 38 of the block is treated so as to be non-reflective and, for this purpose, may have a multilayer dielectric coating or a surface treatment of the type

"fly eye".

  During operation, the radiation from the fiber 35 is transformed by the element 33 into a collimated vertical beam which is directed on the holographic diffraction grating 32.

  high efficiency, in particular because it does not depend on a polar

  sation, that is to say, it diffracts into wavelength components the incident radiation in any plane of polarization. The diffracted radiation arrives at the focusing element 37, which focuses the scattered radiation into a spectrum 60 along an area

  extending radially on the disc.

  The radiation reflected by the encoder disc 4 returns through the same path through the block 30 to the diffraction element 32. This reflected radiation is deflected at an angle that depends on the wavelength, so that the entire radiation reflected by the encoder disk

  forms an image at the same point on the end of the fiber 35.

  It should be noted that, thanks to the optical head 3, the radiation is dispersed before forming an image on the encoder disc 4 and that the wavelengths reflected by the reflective tracks of the disc are recombined by the diffraction element 32. In this way, the small-diameter optical object formed by the end of the input fiber 35 is scattered along the spectrum 60, on the encoder disk 4, and the reflected radiation is combined and returns focused in an image of

  small diameter on the end of the fiber.

  Thanks to the reflective coder, the same optical head is used to disperse and recombine the radiation, which avoids the need for a separate combination device, resulting in additional cost and space. In addition, by employing a reflective encoder, there is no need for free access behind the encoder. This allows you to use

  coding marks on an opaque element such as a steering wheel.

  Since the collimating element 33, the collimating element 32 and the focusing element 37 are all mounted on a compact block, the risk of relative movements by vibration between the optical surfaces

is reduced.

  In another arrangement, the holographic diffraction element 32 could be arranged to focus the radiation so that the holographic focusing element 37 can be replaced by a

simple reflective surface.

  An embodiment of the encoder disc 4 is shown in FIG. 3. This disc has a non-reflective background 50 and reflective marks in the form of two tracks 51 and 52. The outer track 51 is circular and centered on the axis 5 of the disc. The inner track 52 has a spiral shape, so that the distance between the two tracks 51 and 52 varies around the disc 4. The spectrum formed on the disc 4 is indicated by the zone 60 which extends radially across the two tracks. When the disc has rotated, other parts of the spectrum will be reflected towards the optical head 3 by the spiral track 52. The circular track 51 will reflect the radiation of the same part of the spectrum, assuming that the track is well concentric with the disk. However, by measuring the distance between the two tracks, as is the case here, we obtain a transducer which is insensitive to concentricity errors.

in the encoder disk.

  In another embodiment, the encoder disk could be encoded

  numerically, for example using the Gray coding system.

  The encoder is not necessarily a rotating disk as described

  above, but may be a movable plate in the direction of its length.

  An embodiment of such a plate 70 is illustrated in Figure 4 and has two reflective tracks 71 and 72 extending along the length of the plate. The tracks 71 and 72 have respective respective inclinations with respect to the longitudinal direction of the

  plate 70, so that their spacing varies on this length.

  With this form of plate, it is possible to measure orthogonal movements of the plate, that is to say movements along its length and at right angles along its width. FIG. 4A shows the output signal of photodiode array 9 when plate 70 has the position shown in FIG. 4, with respect to spectral image 60. If plate 70 moves to the left, FIG. The spacing between the portions of the tracks 71 and 72 on which the spectral image 60 falls becomes smaller, and the peaks detected by the detector approach, as shown in FIG. 4B. On the other hand, if the plate 70 moves in the direction of the width without moving in the direction of the length, the spacing between the peaks of the output signal remains unchanged, but the peaks move in absolute position, as shown Figure 4C. By measuring the absolute position of these peaks, it is therefore possible to measure the displacement of the plate in two coordinate directions

orthogonal.

  The present invention is not limited to the embodiments described above, but it extends to any modification or variant

obvious to a person skilled in the art.

- 10 -

Claims (10)

claims   An optical transducer comprising an optical source which emits radiation over a wavelength band, an optical head provided with a diffraction element arranged to disperse the radiation according to the wavelengths that compose it along a zone transverse to the path of the radiation, so that the wavelength of the radiation delivered by the optical head on any point of this zone depends on the location of this point of the zone, a movable element capable of moving relative to said area and transversely to the path of radiation, a detector, and a radiation guide extending between the optical head and the detector, the movable member being optically encoded to reflect radiation from different wavelengths to the optical head depending on the transverse position of the moving element, the diffraction element being arranged to diffract the radiation r bent along its wavelength to form an image on one end of the radiation guide, and the detector being arranged to respond to radiation at different wavelengths emerging from the radiation guide, so as to deliver a output signal as a function of the position of the moving element, characterized in that the diffraction element is a holographic diffraction element (32). 2. Transducer according to claim 1, characterized in that the optical head (3) comprises a holographic element (37) arranged to focus the radiation on the movable element (4, 70) and to focus the   radiation reflected by the movable element.   3. Transducer according to claim 2, characterized in that the holographic element (37) arranged to focus the radiation is separated from   the holographic diffraction element (32).   4. Transducer according to claim 1, characterized in that the optical head (3) comprises a holographic collimation element (33) placed between the radiation guide (35) and the diffraction element holographic (32). - 1i -   5. Transducer according to claim 1, characterized in that the optical head (3) is made of a block (30) in one piece of transparent material, and in that the holographic diffraction element (32) is   mounted on a surface (31) of this block.   6. Tranducer according to claim 5, characterized in that said block (30) has a substantially cubic general shape, in that the holographic diffraction element (32) is mounted on an oblique surface formed on a first side of the block, in that a collimating element (33) is mounted on an opposite side (34) of the block (30), between the radiation guide (35) and the diffraction element (32), and that reflective element (37) is mounted on said opposite side (34) to reflect radiation from the diffraction element (32) towards   the movable member (4, 70) through the first side of the block.   Transducer according to Claim 1, characterized in that the radiation guide (35, 7) to the detector (8, 9) has a beam splitter (36), and that the radiation of the optical source (1) ) is transmitted to the optical head (3) through the beam splitter (36) and the radiation guide (2, 35).   Transducer according to Claim 1, characterized in that at least a part of the radiation guide (35, 7) leads to a display device (12) where the radiation reflected by the mobile element (4, 70) is visible, so that the observation of the color of this radiation   provides an indication of the position of the movable member (4, 70).   9. Transducer according to claim 1, characterized in that the movable element (4, 70) comprises two reflective elongated tracks (51 and 52, 71 and 72), and in that the mutual spacing of the two tracks is different.   at different points of their length.   Transducer according to claim 1, characterized in that   the movable element (4) is a rotating disc.