Abstract:
An optical device for monitoring a rotatable shaft is disclosed. The optical device has an optical waveguide arranged on the rotatable shaft and an optical sensor. The optical device further has a transmitting unit transmit the light signal, a transfer device to transmit the light signal between the transmitting unit and the optical waveguide and an evaluation unit for determining a physical variable from a light signal coming from the optical sensor and transferred by the transfer device, the evaluation unit being associated with the transmitting unit. The transfer device has an optical ‘multimode’ waveguide associated with a coupling device, the waveguide being associated with the transmitting unit and a further optical ‘multimode’ waveguide, with an associated coupling device associated with the optical waveguide and arranged on the rotatable shaft eccentrically to its axis.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2007/058750 filed Aug. 23, 2007 and claims the benefit thereof. The International Application claims the benefits of German Patent application No. 10 2006 041 865.4 DE filed Sep. 6, 2006, both of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to an optical device for monitoring a rotatable shaft with an oriented axis. 
     BACKGROUND OF INVENTION 
     The measurement of temperatures and expansions on rotatable parts, such as drive shafts and generator shafts for example, is becoming increasingly important since, as a result of ever higher power demands, these parts will be operated right up the their load limits. As a rule conventional electrical temperature sensors, such as thermoelements for example, and electrical expansion sensors, such as piezoelectric sensors for example, are used for this purpose. In such cases the sensor signals must be conditioned to the shaft. This is normally done with special measurement amplifiers. Radio transmission or IR transmission is then used to send the measuring signals conditioned using this method from the shaft to a transceiver unit located in a fixed position in relation to the shaft. For this purpose auxiliary energy must be provided on the shaft in each case to enable the electronic components arranged there to be driven. This can be done for example through a battery or also through an inductive transmitter. Overall a large outlay is involved here. Since enormously high centrifugal forces can arise with shafts rotating at high speed, the corresponding electronic components must be adapted to these extreme conditions. The electronic components are usually encapsulated. 
     Instead of electrical sensors, the use of optical sensors based on optical waveguides, such as FBG (FBG: Fiber-Bragg-Grating) sensors, is known for these types of measurements on parts turning at high speeds. In such cases the actions of coupling and decoupling an optical transceiver unit arranged in a fixed location to and from the rotating shaft have proved especially difficult. For example optical transmitters arranged axially on a face side of the rotating shaft are especially suitable for this purpose, employing two collimators to transmit light signals between a transceiver unit arranged in a fixed position relative to the shaft and an optical waveguide arranged on the rotating shaft. In such cases the two collimators are arranged in a common housing which is embodied with one non-rotating and one rotating part. Such an optical transmitter is however not suitable for measurements on shafts for which there is no access to the end face side. 
     SUMMARY OF INVENTION 
     An object of the invention is to specify an optical device for monitoring a rotatable shaft which is robust and is able to have universal application all along the shaft. 
     This object is achieved in accordance with the invention by an optical device as claimed in the claims. 
     The inventive optical device involves an optical device for monitoring a rotatable shaft with an oriented axis, comprising
         at least one optical waveguide arranged on the rotatable shaft provided with at least one optical sensor, via which the at least one optical sensor is able to be interrogated by means of a light signal,   a transceiver unit mounted at a fixed location in relation to the rotatable shaft, with which the light signal is able to be emitted,   transmission means, with which the light signal is able to be transmitted between the fixed transceiver unit and the optical waveguide arranged on the rotatable shaft, and   an evaluation unit assigned to the transceiver unit for determining a physical variable from a light signal originating from at least one optical sensor and transmitted by the transmission means, with the transmission means having   at least one multimode optical waveguide with assigned coupling means arranged at a fixed location in relation to the rotatable shaft and assigned to the transceiver unit and   at least one further multimode optical waveguide with assigned coupling means arranged on the rotatable shaft eccentric to its axis connected to the at least one optical waveguide and   the light signals being able to be transmitted via the coupling element between the multimode optical waveguides.       

     The eccentric mounting of the further multimode waveguide and of the assigned coupling means on the rotatable shaft means that the transmission of the light signal between the coupling means arranged on the shaft and the coupling means arranged at a fixed location is only possible via a free beam coupling. In such cases the light signals can only be coupled over between the coupling means in a small time window during a rotation of the shaft. Since the transmission means has multimode optical waveguides there is a guarantee of hardly any coupling losses occurring on transmission of the light signal between the coupling means. The fact that the coupling path, i.e. the path between the two coupling means, is covered twice during a measurement would result in high losses for example, were single optical waveguides to be used instead of the multimode optical waveguides, making reliable measurement almost impossible. In addition a free beam coupling of “multimode optical waveguides by comparison with single optical waveguides demands less adjustment and is less sensitive to adjustment errors. The physical variable to be determined is especially the temperature and/or the expansion of the shaft. 
     Further advantageous embodiments of the optical device in accordance with the invention are claimed in the dependent claims. 
     It is thus advantageous for the at least one optical sensor to be at least an FBG sensor and for the at least one optical waveguide to be at least a single mode optical waveguide. An FBG sensor allows an almost punctate, i.e. a locally very narrowly restricted temperature and/or expansion measurement on the shaft. By contrast an optical sensor according to the Brillouin or Raman principle which can also be used normally has a certain local integrating effect, which can extend over several meters for example. A point measurement, i.e. especially a local restriction of the detection point to a few millimeters, can hardly be obtained with these types of optical sensor. It is easily possible with an FBG sensor however. In such cases a specific proportion of the supplied light signal is reflected back by the respective Bragg wavelength (focus wavelength) in the FBG sensor. The Bragg wavelength changes with the influencing variable predominating in the type of measurement, here especially the temperature and/or the expansion of the shaft at the location of the FBG sensor. This change in the wavelength content (or wavelength spectrum) of the respective (part) light signal reflected back can be used as a measure for the influencing variable (temperature and/or expansion) to be detected. To interrogate the FBG sensor by means of the light signal a wideband light source, such as an LED with a bandwidth of around 45 nm, an SLD with a bandwidth of around 20 nm or a tunable laser with a bandwidth of around 100 nm for example is used in particular. 
     Advantageously in such cases a number of FBG sensors are provided at different end points along the at least one single-mode optical waveguide. It is also useful for the at least one multimode optical waveguide arranged on the rotatable shaft to be connected to a number of single-mode optical waveguides. On the one hand this allows a locally-resolved temperature and/or expansion distribution and on the other with point-type events, such as for example a sudden locally-restricted increase in temperature and/or expansion, the precise pinpointing of the event location. The resolution in such cases will merely be determined by the spacing of the individual FBG sensors from each other. 
     It is of advantage for the FBG sensors to have different Bragg wavelengths from each other. If for example with the optical device the so called wavelength multiplex method is employed, as a rule up to 10 FBG sensors can be arranged one after another in a waveguide. The light signal injected by the transceiver unit into the multimode optical waveguide must have a wavelength range for this which covers all Bragg wavelengths. If on the other hand, as an alternative to the wavelength multiplex method the so-called time division multiplexing method (OTDR: Optical Frequency Domain Reflectometry) is used, an almost unlimited number of FBG sensors can be arranged in an optical waveguide. In such cases the sensors can also be spatially different for identical Bragg wavelengths. 
     Expediently the light signal wavelength emitted by the transceiver unit features wavelengths from the visible wavelength range, i.e. from 380 nm to 780 nm, and/or the near infrared wavelength range (NIR), i.e. from 780 nm to 2500 nm. 
     It is expedient for the light signal able to be emitted by the transceiver unit to be at least one light pulse. Advantageously the emission of the light signal is triggered with the rotation of the shaft. Thus light signals are only emitted and received again as required. The transceiver unit operates with greater energy efficiency in this case and simultaneously has a longer service life. 
     Advantageously the evaluation unit features an optical mass spectrometer with a least one CCD row. It is however also expedient for the evaluation unit to feature at least one complementary edge filter with at least one photodetector. This embodiment of such an evaluation unit is cheaper to implement compared to the first embodiment mentioned above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred, but in no way restrictive exemplary embodiments of the method and of the arrangement will now be explained in greater detail with reference to the drawing. For the purposes of illustration the drawing is not drawn to scale and certain features are depicted schematically. The individual diagrams show 
         FIG. 1  an optical device for monitoring a rotatable shaft by means of optical sensors in a single-mode optical waveguide, 
         FIG. 2  an optical device for monitoring the rotatable shaft by means of optical sensors in a number of single-mode optical waveguides, 
         FIG. 3  a cross-section through the contact surface of a multimode optical waveguide and a number of single-mode optical waveguides, 
         FIG. 4  a cross-section through the contact surface of the multimode optical waveguide and a number of single-mode optical waveguides with an increased external diameter compared to  FIG. 3 , 
         FIG. 5  an optical device for monitoring a rotatable shaft with radially oriented coupling path and a serpentine arrangement of single-mode optical waveguides, 
         FIG. 6  an optical device for monitoring a rotatable shaft with means arranged on the shaft for diverting the coupling path and 
         FIG. 7  an optical device for monitoring a rotatable shaft with a fixed means for diverting the coupling path. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Parts which correspond to one another are provided with the same reference symbols in  FIGS. 1 to 7 . 
       FIG. 1  shows an optical device for monitoring a shaft  10 , especially a shaft connected to a machine or a generator and supported to allow it to rotate. The shaft  10  is able to be rotated around its oriented axis  13  here. The end face side of the shaft  10  is labeled  12 . Arranged on and/or in the outer surface  11  of shaft  10  is an optical waveguide  20  with optical sensors  21  for detecting temperature and/or expansion of the shaft  10 . As shown in  FIG. 1 , the sensors  21  are FBG sensors incorporated into single-mode optical waveguides  20 . With a number of FBG sensors  21  each individual sensor  21  can feature a specific focal point wavelength the so-called Bragg wavelength which differs especially from those of the other sensors  21 . The FBG sensors  21  are interrogated by a light signal LS which is created by a wideband light source  41 , especially an SLD (Super Luminescence Diode). The light source  41  in this cases is part of a transceiver unit  40  arranged in a fixed location. Created by the fixed transceiver unit  40  the light signal LS will be injected via a transmission means into the single-mode optical waveguide provided with the FBG sensors  21 . In each FBG sensor  21  a proportion of the injected light signal LS is reflected back with the respective Bragg wavelength as a part reflex light signal. The remaining proportion of the light signal LS on the other hand passes the relevant FBG sensor  21  and if necessary hits the next FBG sensor. A light signal LS′ reflected from the FBG sensor  21  is then present at the transceiver unit  40  which is made up of the part reflex light signal of the individual FBG sensors  21 . 
     The light signal LS′ coming from the FBG sensors  21  injected back into the transceiver unit  40  is directed by an optical bus coupler  42  to an evaluation unit  43 . This especially includes an optoelectrical converter, an analog/digital converter and a digital signal processor (not shown in the figures). The optical converter advantageously features a spectrally-selective element for selection of the individual part-reflex signals, for example the shape of a spectrometer with at least one CCD row. As an alternative to this, the optoelectrical converter can feature at least one complementary edge filter with at least one photodetector. Downstream from the optoelectronic conversion an analog/digital conversion is undertaken in the analog/digital converter. The digitized output signal of the analog/digital converter is fed to the digital signal processor, by means of which measured values M 1 , M 2 , . . . are determined for the reference temperatures and/or expansions detected in the FBG sensors  21 . 
     The light source  41 , the optical bus coupler  42  and the evaluation unit  43  are grouped together in the transceiver unit  40 . These subunits or parts thereof can however also be constructionally separated from one another, i.e. not embodied as a common transceiver unit  40 . In addition a purely analog evaluation, for example by means of a hard-wired electronic circuit, is possible. No analog/digital converter would then be present and the evaluation unit  43  would be implemented in analog technology. 
     The transmission means is used for transmission of the light signals LS and LS′ emitted by the transceiver unit  40  and received back again between fixed transceiver unit  40  and single-mode optical waveguide  20  turning with the shaft  10 . The transmission means thus consists of a fixed part assigned to the transceiver unit  40  and a part arranged on the rotatable shaft  10 . The fixed part comprises a multi-mode optical waveguide  30  connected to the transceiver unit  40  with a first coupling means  30 K, especially a collimator. On the other hand the part of the transmitter arranged on the rotatable shaft  10  comprises a multimode optical waveguide  31  connected to the single-mode optical waveguide  20  with a second coupling means  31 K facing towards the first coupling means  30 K which is especially likewise a collimator. The connection between multimode optical waveguide  31  and single-mode optical waveguide  20  is made via facing sides of the two fibers  20  and  21 . 
     The light signal LS emitted by the transceiver unit  40  thus reaches the first coupling element  30 K via the first multimode fiber  30  which transmits the light signal LS preferably collimated to the second coupling means  31 K almost loss-free if the two coupling means  30 K,  31 K are facing each other. The distance covered between the two coupling means is labeled S. The light signal LS travels via the second coupling means  31 K through the second multimode optical waveguide  31  into single mode optical waveguide  20 , in which the FBG sensors  21  are able to be interrogated by means of the light signal LS. The light signal LS′ reflected back from the FBG sensors  21  then returns on the reverse path for evaluation in the transceiver unit  40 . 
     Advantageously the transceiver unit  40  and thereby also the light source  41  and if necessary the evaluation unit  43  are operated in pulsed mode, so that the light signal LS is emitted in the form of light pulses. The light pulses are advantageously emitted synchronously with the revolutions of the shaft, so that light pulses are only emitted in the time window, in which the two coupling means  30 K,  31 K are opposite each other in a position suitable for transmission. In such cases the transceiver unit  40  can be operated in a triggered fashion for example by a means for determining the speed of rotation of the shaft  10 . 
     The exemplary embodiment of the optical device shown in  FIG. 2  differs from that shown in  FIG. 1  merely in that a number (in this case three) single-mode fibers  20  are connected to the multimode fiber  31 . Thus a (surface) covering sensor arrangement is implemented so that the temperature and/or the expansion of the shaft  10  can be determined with local resolution. 
       FIG. 3  depicts the cross-section for the sectional line III shown in  FIG. 2 . The cross-section in this case runs through the face-side contact surface of the multimode optical waveguide  31  and single-mode optical waveguide  20 . The seven single-mode optical waveguides  20  are grouped into a bundle in this case such that their cross-sections are arranged into a tightest package. The outer area of the multimode optical waveguide  31  represents the waveguide shroud  312  surrounding the optical waveguide core  311 . Similarly the single-mode optical waveguides  20  are also embodied with a waveguide shroud  202  and a waveguide core  201 . It should be pointed out that the waveguide core  201  to waveguide shroud  202  ratio is significantly smaller for a single-mode optical waveguide  20  than the ratio of waveguide core  311  to waveguide shroud  312  for multimode optical waveguides. So that the light signal LS, LS′ can be transmitted between the multimode optical waveguide  31  and the single-mode optical waveguide  20 , the waveguide cores  201  of the single-mode optical waveguide  20  must lie within the waveguide core  311  of the multimode optical waveguide  31 . With the arrangement depicted in  FIG. 3  the starting point used for the example is a core diameter of the multimode optical waveguide  31  of 200 μm and an external diameter of the individual single-mode optical waveguides  20  of 80 μm. The core diameter of an individual single-mode optical waveguide  20  generally amounts to appr. 5 μm. 
     In  FIG. 4 , as in  FIG. 3  the cross-section for the section line IV shown in  FIG. 2  is depicted. Here the external diameter of the single-mode optical waveguide  20  is however greater than selected, so that in this example only three single-mode optical waveguides  20  can be connected to the multimode optical waveguide  31 . For example in this case the single-mode optical waveguides  20  respectively have an external diameter of 125 μm for a core diameter of the multimode optical waveguide  31  of 200 μm. 
     While in  FIG. 1  and in  FIG. 2  the coupling path S is always oriented axially i.e. in parallel to axis  13 ,  FIG. 5  shows an exemplary embodiment in which the coupling path S is oriented radially, i.e. perpendicular to the axis  13 . As in  FIG. 1 , only one single-mode optical waveguide  20  is connected to the multimode optical waveguide  31 . The single-mode optical waveguide  20  with its plurality of FBG sensors  21  is in this case arranged in a serpentine form on and/or in the outer surface  11  of the shaft  10 . Such an arrangement also makes a “full-coverage” monitoring of the shaft  10  possible. Naturally, like  FIG. 2 , a number of such single-mode optical waveguides  20  laid in a serpentine shape can be connected to the multimode optical waveguide  31 . 
       FIG. 6  and  FIG. 7  respectively each depict a coupling path S consisting of a part S 1  running radially and a part S 2  running axially. The light signals LS and LS′ in this case are diverted by a reflection means  32 , especially a diversion prism, preferably by 90°. This means that the respective reflection means  32  in both exemplary embodiments is a part of the transmission means. The respective reflection means  32  is attached for mounting to a holder  33 ,  33   a  provided for it. 
     In  FIG. 6  the holder  33  is attached to the shaft  10  close to the second coupling means  31 K. The holder  33   a  in accordance with  FIG. 7  is arranged instead at a fixed location relative to the rotatable shaft  10 . The holder  33   a  can also be provided for fixing the first coupling means  30 K, as depicted in  FIG. 7 . To avoid contact between the holder  33   a  and the shaft  10 , the second coupling means  31 K is attached to a base  32  for example.