Abstract:
A sensor unit for a bearing including at least one inner ring and at least one outer ring, wherein the sensor unit is configured to detect at least one state variable of the bearing. The sensor unit is embedded in a recess of one of the inner ring or the outer ring.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a United States National Stage Application claiming the benefit of International Application Number PCT/EP2013/056473 filed on 27 Mar. 2013 (27.03.2013), which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a sensor unit for use with bearings, a bearing including such a sensor unit and a control unit for controlling or monitoring an apparatus including such a bearing. 
     BACKGROUND OF THE INVENTION 
     It is known to provide bearings with sensor units, which may be mounted in a housing or on the outer or inner ring of the bearing. Sensors which may be used are e.g. temperature sensors and rotation sensors. 
     Usually, sensor units are mounted on a surface of a bearing, e.g. on an axial side face of a bearing outer ring, wherein the sensors are arranged in a housing which is fixed to the surface of the bearing by adhesion or with screws. 
     This way of mounting the sensors has drawbacks, in particular in terms of robustness and precision of measurement. The position of the sensor housing on the surface is susceptible for damages due to unintended collisions with other parts or tools used during the assembly of the device provided with the bearing and the sensor may be fairly remote from the rolling elements such as rollers or balls the properties of which are measured. 
     SUMMARY OF THE INVENTION 
     The invention tries to overcome the above drawbacks of the prior art by providing a sensor unit, a bearing and a pertinent control unit. 
     The invention starts from a sensor unit for a bearing including at least one inner ring and at least one outer ring, wherein the sensor unit in configured to detect at least one state variable of the bearing. The state variables may include amplitudes of vibrations, acoustic emissions, temperature or strain values. 
     It is proposed that the sensor unit is configured to be embedded in a recess of the inner ring or of the outer ring of the bearing. 
     Due to the embedding, the sensors can be brought closer to the rolling elements such as rollers, tapered rollers, needles or balls of the bearing while at the same time protecting the sensor from collisions with tools or other parts of the device in which the bearing with the sensor unit is mounted. 
     Presently, the invention is mainly applicable to large-size bearings with diameters of several tenths of centimeters such as bearings for wind turbines, generators, ship propellers or the like. However, it is to be expected that the progress in miniaturization will make the invention applicable to smaller bearings as well. Special advantages of the invention may be achieved in the context of devices requiring a detailed remote monitoring, in particular wind turbines. 
     Preferably, the sensor unit includes multiple sensors in one single sensor housing. This may help to reduce costs of multiple housings and multiple recesses to be provided in the rings of the bearing and facilitates the assembly. 
     Further, it is proposed that the sensor unit is provided with a wireless transmitter for transmitting signals obtained by the sensor unit. 
     The machining of a suitable recess mating with the sensor unit is particularly simple and cost-saving when the sensor unit has a housing with a circular cross-section axially fitted into the recess. 
     Preferably, the housing of the sensor unit comprises at least one deeper portion and one shallower portion, wherein bolts for fixing the sensor unit or larger circuit boards or circuit elements may be provided in the shallower portion in a well-protected way and only the parts of the sensor which need to be as close as possible to the rolling elements of the bearing may be provided in the bottom portion of the deeper portion. The machining of a recess mating with a housing of this shape is particularly simple and cost-saving if the deeper portion and the shallower portion are formed in essentially cylindrical shapes of different diameters. 
     In a preferred embodiment, a bottom face of the sensor unit including at least one contact plate is configured to abut with a bottom surface of the deeper portion of the recess. This helps to bring the contact plate as close as possible to the rolling elements of the bearing. 
     Preferably, the sensor unit includes at least one strain sensor. It is in particular the strain measurement which is greatly improved in precision by bringing the sensor close to the rolling elements. 
     Further, the sensor unit may include at least one acoustic emission and/or vibration sensor. 
     In a preferred embodiment of the invention, it is proposed that the sensor unit includes a circlip to hold the sensor in the recess. The use of a circlip may help to avoid the use of screws and bolts, which affect the strain measurement in a detrimental way. The circlip may in particular engage with a circumferential notch provided in the inner surface of the recess. 
     Screws for closing the housing may be avoided by using a housing which is sealed with an adhesive bond, in particular with ultra-sonic welding. 
     Further, the invention proposes a bearing including an inner ring, an outer ring and a sensor unit as described above. A recess is provided in the outer ring or in the inner ring and the sensor unit is embedded in the recess. The recess may in particular comprise a deeper portion and a shallower portion mating with the deeper portion and the shallower portion of the sensor unit. 
     In embodiments using a circlip for fixing the sensor unit exclusively or in addition to screws or bolts, the recess may be provided with a circumferential notch for receiving the circlip for retaining the sensor unit in the recess. Alternatively, the circlip could be replaced with other elastical elements engaging with notches or holes in the side walls of the recess. 
     A further aspect of the invention relates to a control apparatus configured to receive data from the sensor unit and to generate status information or warning signals based on the received data. 
     The above embodiments of the invention as well as the appended claims and figures show multiple characterizing features of the invention in specific combinations. The skilled person will easily be able to consider further combinations or sub-combinations of these features in order to adapt the invention as defined in the claims to his specific needs. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic cross-sectional view of a bearing including a sensor unit according to the invention; 
         FIG. 2  is a view of the sensor unit as mounted in a recess in the inner ring of the bearing; 
         FIG. 3  illustrates the sensor unit according to the invention outside the recess; 
         FIG. 4  is an exploded view of the sensor unit according to  FIGS. 1-3 ; 
         FIG. 5  is a view of a sensor arrangement arranged in a deeper portion of the recess; 
         FIG. 6  is a sectional view of a sensor unit according to a second embodiment of the invention including a circlip; 
         FIG. 7  shows the sensor unit of  FIG. 6  outside from the recess; and 
         FIG. 8  shows an exploded view of the sensor unit according to  FIGS. 6 and 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows a cross-sectional view of a sensor unit  14  embedded in an inner ring  10  of a taper roller bearing including an inner ring and an outer ring  12 . 
     The sensor unit  14  is embedded in a recess  16  machined in the axially outer surface of the inner ring  10 . The recess  16  comprises a deeper portion  16   a  and a shallower portion  16   b , each being formed as concentrical bores with different diameters such that the shallower portion  16   b  forms a step. 
     The sensor unit  14  comprises a housing  18  formed with a T-shaped cross-section mating with the shape of the recess  16  and the housing  18  of the sensor element is fitted into the recess  16 . A top-surface of the sensor unit  14  is flush with the axial side surface of the inner ring  10  and the sensor unit  14  is fixed in the recess  16  by means of screws  28  engaging with threaded bores (not shown) in the shallower portion  16   b  of the recess  16 . 
     The sensor unit  14  comprises a circuit board  30  with a diameter larger than the diameter of the deeper portion  16   a  and is arranged in the transversal bar of the T-shape of the housing  18 . 
     A wireless transmitter  20  including an antenna is arranged on the top side of the housing  18 . 
       FIG. 2  illustrates a perspective view of the sensor unit  14  as mounted in the inner ring  10 . A connector  32  with a wire is guided out of the housing  18  and enables to read out data or update firmware of a processor provided in the sensor unit  14  in a wired manner. It is to be noted that the wireless transmitter  20  or the connector  32  may be provided alternatively or in combination. 
       FIG. 3  is a perspective view of the sensor unit  14  isolated from the recess  16 . A plastic cover  34  of the housing  18  of the sensor unit  14  is fixed to a bottom part of the housing  18  by means of screws  36 . 
       FIG. 4  shows an exploded view of the sensor unit  14  according to the invention. The circuit board  30  is provided with a processor and a wireless transmitter units  20  and connected via flexible elements with a bottom part of the sensor unit  14  including the actual sensors and contact plates  38 ,  40  fitted into through holes (not shown) in the bottom surface of the lower part of the housing  18 . An O-ring  42  is provided and engages with a circumferential notch  44  in the circumferential outer wall of the bottom part of the housing  18  and protects the contact plates  38 ,  40  from being contaminated by penetrating oil or other liquids. The contact plates  38 ,  40  are configured to be directly pressed against a bottom surface of the recess  16  by means of springs  46 . 
       FIG. 5  shows the bottom part of the sensor unit  14  including the contact plates  38 ,  40  in more detail. The first contact plate  38  is a metallic contact plate equipped with a vibration sensor  48 , an acoustic emission sensor  50  and a temperature sensor  52 . The second contact plate  40  is a contact plate of a strain sensor  54  formed as a friction strain gage. 
     The sensors  48 - 54  are mounted on flexible circuit boards  53  adhered to the actual contact plates  38 ,  40 . 
       FIGS. 6 to 8  show a second embodiment of the invention. In the following description, the same reference numbers are used for features with similar functions. The description is limited to differences to the embodiment of  FIGS. 1 to 5  and it is referred to the above description of  FIGS. 1 to 5  for details of features which are unchanged. 
     The sensor unit  14  of the second embodiment is provided with a circlip  56  engaging with a notch  58  in the circumferential side surface of the shallower portion  16   b  of the recess  16 . The notch  58  is deep enough to securely fix the sensor unit  14  in combination with the circlip  56  and its depth is chosen such that the outer diameter of the notch  58  is slightly smaller than the outer diameter of the circlip  56  in an unloaded configuration. The width of the notch  58  corresponds to the thickness of the circlip  56  such that the circlip  56  may be easily fitted into the notch while axial play of the circlip  56  is avoided. It is to be noted that the pressure of the springs  46  pushes the sensor unit  14  in the direction out of the housing  18  and this force is retained by the circlip  56 . 
       FIG. 7  shows the sensor unit  14  in a perspective view. It should be noted that no screws are provided for closing the cover plate  34  of the housing  18  because the cover plate  34  is adhered to the housing  18  using ultra-sonic welding. 
     As illustrated in the exploded view of  FIG. 8 , no screws or bolts are provided for fixing the cover  34  to the base portion of the housing  18  or for fixing the sensor unit  14  in the recess  16 . The functions of the screws  28 ,  36  are fulfilled by the adhesive bonding and the circlip  56 , respectively.