Abstract:
A bolt sensor assembly provides a body adapted for insertion into a hole in a component, the body having a first end and a second end, and a longitudinal axis extending therebetween. The first end includes an attachment means for attachment of the bolt sensor assembly to the component, and the second end mounts a sensor element, such as a strain gauge element. The sensor element is movable with respect to the body in the direction of the longitudinal axis of the body and the sensor element is constrained against rotation about the longitudinal axis with respect to the body.

Description:
CROSS-REFERENCE 
     This application is the U.S. National Stage of International Application No. PCT/EP2014/074737 filed on Nov. 17, 2014, which claims the benefit of priority from Great Britain Patent Application No. 1320282.5 filed on Nov. 18, 2013, the contents of which are both herein incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to mounting of sensor elements, and in particular to sensor elements mountable in a threaded hole. 
     BACKGROUND OF THE INVENTION 
     In certain machines it is desirable to be able to measure the load on internal components of the machine. For example, in a wind turbine it is desirable to be able to measure the load on certain bearings. 
     Many different load measurement devices are available, but often at least a certain amount of disassembly of the wind turbine is required in order to gain access to the bearing. 
     One possibility for mounting a sensor to measure a load on a bearing is to drill a hole through a part of the component in which the bearing is housed and mount a sensor in that hole. Alternatively, a hole that is present in the component may provide a suitable location to mount a sensor, for example a hole that receives a bolt which is used to remove the bearing from the component may be used. 
     Strain gauge sensors which can be mounted in holes are well known. 
     One such strain gauge is described in U.S. Pat. No. 2,873,341. This strain gauge comprises a bolt having a central bore in which is mounted an epoxy core with a resistance wire is embedded therein. 
     The bolt strain gauge described in U.S. Pat. No. 2,873,341 has the strain measuring element in the centre of the bolt, which is not ideal for measuring strain in a component situated at the end of the bolt. 
     A more suitable form of strain sensor is a friction strain sensor. Such strain sensors are used in many applications. Such friction strain gauges must be pre-loaded to a certain extent in order to function. Furthermore, friction strain gauges are very sensitive to damage, for example when subject to shear forces. Friction strain gauges are described in general terms in GB 2,367,628. 
     There seems to be room for improvement. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a bolt sensor assembly comprising a body adapted for insertion into a hole in a component. The body has a first end and a second end, and a longitudinal axis extending therebetween, wherein the first end includes an attachment means for attachment of the bolt sensor assembly to the component. The second end mounts a sensor element and the sensor element is movable with respect to the body in the direction of the longitudinal axis of the body. The sensor element is further constrained against rotation about the longitudinal axis with respect to the body. 
     The body is preferably rotatable about the longitudinal axis to engage and disengage the body from the hole. The body may include a threaded portion for engagement with a correspondingly threaded portion of the hole. 
     Suitably at least a part of the body comprises the second end and the hole are provided with cooperating elements which prevent relative rotation therebetween, and wherein the sensor element is biased to project from said second end of the body. The attachment means then may comprise a collar that is slidably mounted on the body or the attachment means may be mounted on the body for relative rotation therebetween. Suitably a biasing element may be comprised located between the body and the attachment means, the biasing element configured to urge the body and the attachment means in opposite directions along the longitudinal axis. The sensor element may also be comprised in a sensor assembly, which assembly is mounted at the second end of the body, the assembly comprising a base including an opening in which the sensor element is slidably mounted, a reaction member and a biasing means situated between the sensor element and the reaction member. The sensor assembly may further include a mount to which the sensor element is attached and wherein the biasing mean is situated between the mount and the reaction member. 
     The sensor element may be movable between a first position and a second position. In the first position the sensor element lying behind the second end of the body and in the second position at least a part of the sensor element lying proud of the second end of the body. The bolt sensor assembly may further comprise an actuator adapted to move the sensor element between the first and second positions. The body may include a bore and the actuator then acts on the sensor element through the bore. The sensor element may be mounted on a shaft, and the actuator then engages the shaft. The bolt sensor assembly may further comprise biasing means arranged to exert a force on the sensor element biasing it into the second position. The end of the body in which the sensor element mounted may be configured such that the sensor element is constrained against rotation with respect to the body about the longitudinal axis. The second end of the body may comprise a cap removably attachable to the remainder of the body, the sensor element is then mounted in the cap. 
     Suitably the sensor element is one or more of an acoustic emission sensor, a vibration sensor, a strain gauge sensor, a friction strain gauge sensor, and a temperature sensor. 
     According to a second aspect of the invention there is provided a friction strain gauge sensor comprising a body adapted for insertion into a hole in a component, the body having a first end and a second end, and a longitudinal axis extending therebetween, wherein the first end includes an attachment means for attachment of the sensor to the component, wherein the second end mounts a friction strain gauge element and wherein the friction strain gauge element is movable with respect to the body in the direction of the longitudinal axis of the body and wherein the friction strain gauge element is constrained against rotation about the longitudinal axis with respect to the body. 
     Preferably, at least a part of the body is rotatable about the longitudinal axis to engage and disengage the body from the hole. 
     The body may include a threaded portion for engagement with a correspondingly threaded portion of the hole. 
     The body may include a hollow portion, preferably extending from one of the first and second ends. 
     Advantageously, at least a part of the body comprising the second end and the hole are provided with cooperating elements which prevent relative rotation therebetween, such as a recess and a protrusion engaging the recess. Part of the body may include a rib and the hole may include a recess for receiving the rib. 
     Preferably the friction strain sensor element is biased to project from said second end of the body. 
     The attachment means may be slidably mounted on the body and may comprise a collar that is slidably mounted on the body. 
     The attachment means may be mounted on the body for relative rotation therebetween. The attachment means may comprise a threaded portion for engaging a correspondingly threaded portion of the hole, and the threaded portion may be part of the collar. 
     The friction strain gauge sensor may further comprise a biasing element located between the body and the attachment means, the biasing element configured to urge the body and the attachment means in opposite directions along the longitudinal axis. The biasing element may be a spring, such as a coil spring. 
     The body may include a shaft portion of reduced diameter, the biasing element and the attachment means being mounted on the shaft portion. 
     The body may include an attachment means retaining portion, the retaining portion limiting movement of the attachment means relative to the body. 
     The friction strain sensor element may be comprised in a friction strain sensor assembly, which assembly is mounted at the second end of the body, the assembly comprising a base including an opening in which the friction strain sensor element is slidably mounted, a reaction member and a biasing means situated between the friction strain sensor element and the reaction member. 
     The friction strain sensor assembly may further include arms configured for engagement with recesses formed in an inner wall of the body. The arms may be printed circuit boards. The arms may be attached to the base, for example by brackets. 
     Preferably, the friction strain sensor assembly further includes a mount to which the friction strain sensor element is attached and wherein the biasing means is situated between the mount and the reaction member. 
     Advantageously, the base is mounted on pins which are slidingly engaged with holes in reaction member. Biasing elements may be provided between the reaction member and the base to urge the base in the longitudinal direction of the body and away from the reaction member. The biasing means may be springs mounted on the pins. 
     The reaction member is preferably a bridge extending across a hollow portion of the body. 
     Advantageously, the friction strain sensor element is movable between a first position and a second position, in the first position the friction strain sensor element lying behind the second end of the body and in the second position at least a part of the friction strain sensor element lying proud of the second end of the body, the sensor further comprising an actuator adapted to move the friction strain sensor element between the first and second positions. 
     The body may include a bore and the actuator may act on the friction strain sensor element through the bore. 
     Preferably, the friction strain sensor element is mounted on a shaft, and the actuator engages the shaft. The shaft may comprise a collar, the collar configured to constrain the shaft against rotation. 
     The friction strain gauge may further comprise biasing means arranged to exert a force on the friction strain sensor element biasing it into the second position. 
     The end of the body in which the friction strain sensor element is mounted may be configured such that the friction strain sensor element is constrained against rotation with respect to the body about the longitudinal axis. 
     The second end of the body may comprise a cap removably attachable to the remainder of the body, the friction strain sensor element mounted in the cap. 
     The second end of the body is provided with a seal. The seal may be an O-ring seal. The purpose of the seal is to prevent ingress of contaminants to the area of the friction strain sensor element. 
     The friction strain gauge sensor may mount other sensors such as, but not limited to, acoustic emission sensors, vibration sensors, temperature sensors. 
     A second aspect of the invention provides a friction strain gauge sensor according to the first aspect of the invention mounted in a hole in a first component, and the friction strain gauge sensor element engaging a second component. 
     Other preferred features of the invention can be found in the detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the Drawings, which illustrate preferred embodiments of the bolt strain sensors according to the invention: 
         FIG. 1  is a schematic representation of a bolt sensor according to a first embodiment of the invention; 
         FIG. 2  is an exploded view of the bolt sensor illustrated in  FIG. 1 ; 
         FIG. 3  is a schematic representation of a component of the bolt sensor illustrated in  FIGS. 1 and 2 ; 
         FIG. 4  is a cross-sectional schematic representation of the bolt sensor illustrated in  FIG. 1 ; 
         FIG. 5  is a detail view of an end of the bolt sensor illustrated in  FIG. 1 ; 
         FIG. 6  is a schematic representation of the bolt sensor illustrated in  FIG. 1  showing the internal components; 
         FIG. 7  is an illustration of the bolt sensor shown in  FIG. 1  mounted in a housing; 
         FIG. 8  illustrates the hole in the housing in which the bolt sensor is mounted; 
         FIG. 9  is a schematic view of the end of a component of the bolt sensor illustrated in  FIG. 1 ; 
         FIG. 9 a    is a schematic cross-sectional top view of the component illustrated in  FIG. 9 ; 
         FIG. 10  is a schematic representation of a bolt sensor according to a second embodiment of the invention; 
         FIG. 11  is an exploded view of the bolt sensor illustrated in  FIG. 10 ; 
         FIG. 12  is a detail view of one end of the bolt sensor illustrated in  FIG. 10 ; and 
         FIGS. 13 a  and 13 b    show schematic end views of the bolt sensor illustrated in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIGS. 1 to 4 , there is shown a bolt sensor  1  comprising a bolt member  2 , and a sensor housing  10 . 
     The bolt member  2  includes a hexagonal head  3  and a threaded collar  4 . Referring in particular to  FIG. 4 , it can be seen that a bore  5  extends axially through the bolt member  2 , the bore  5  providing two bore sections  5   a  and  5   b , each of a different diameter. 
     The sensor housing  10  comprises a hollow portion  11 , a shaft portion  12  and a head  13 . The shaft portion  12  is of smaller diameter than the hollow portion  11  providing a radially extending rim where the hollow portion  11  meets the shaft portion  12 . The head  13  is attached to the end of the shaft portion  12  after the bolt member  2  has been mounted on the shaft portion  12 . 
     A spring  15  is also mounted on the shaft portion  12 , between the bolt member  2  and the hollow portion  11 . One end of the spring  15  abuts the rim  14 . The other end of the spring  15  is engaged by the end face  4   a  of the threaded collar  4 . 
     The shaft portion  12  passes through and is slidably mounted in the bore section  5   b , with the head  13  being located in the bore section  5   a . The spring  15  therefore urges the sensor housing away from the bolt member  2 , the extent of relative movement between the two components being limited by the head  13  situated in the first bore section  5   a.    
     By mounting the bolt member  2  and the spring  15  on the shaft portion  12 , as the bolt member is tightened the spring exerts a force on the housing  10 . The head  13  retains the housing  10  in the bolt member  2 . 
       FIGS. 2 and 3  illustrate a sensor assembly  20 , such as a friction strain gauge assembly, comprising a base  21 , which is circular in the example, and extending from the base  21  a pair of arms  22  connected together at their ends distal from the base by a connector element  23 . The arms  22  are printed circuit boards are connected to the base plate by brackets  40  each including a tab  41  which attaches to one of the arms. One of the brackets  40  may suitably mount a vibration and temperature sensor  43  and the other may mount an AE sensor  44 . 
     The base  21  includes an aperture  24 , which is rectangular in the illustrated example, and which is configured to receive the sensor element  30 , such as a friction strain gauge, strain gauge, vibration or other type of sensor, as will be described in greater detail below. Two small holes  25  are situated to either side of the aperture  24 . The holes  25  each receive a pin  26 . The pins  26  each comprise a pin head  26   a  and a pin shaft  26   b . The end of the pin shaft  26   b  distal from the pin head  26   a  sits in one of the holes  25 . In the present example, the holes  25  and the ends of the pin shafts  26   b  are provided with corresponding threads to secure the pins  26  to the base  21 . 
     A spring  27  is mounted on the pin shaft  26   b . The function of the spring will be described in greater detail below with reference to  FIGS. 4, 5, 9 and 9   a.    
     The friction strain sensor element  30  forms part of the friction strain sensor assembly  20 . The friction strain sensor assembly includes a plate  31  which mounts a pin  32 , the pin  32  fixed to the plate  31 , a spring  33  and a bridge  35  extending from one side of the hollow portion  11  to the other. The bridge  35  includes holes  38  through which the pin shafts  26   b  pass. The pin shafts  26   b  may slide in the holes  38 , the pin heads  26   a  providing an end stop to movement of the pins  26  and hence the base  21 . 
     The underside of the bridge  35  comprises a central spigot  36 , with recesses  37  to either side. One end of the spring  33  sits in the recesses  37 . The other end of the spring  33  engages with the plate  31 , sitting in an annular recess  31   a  and receiving the pin  32 . The end  32   a  of the pin  32  is attached to the friction strain sensor element  30 . 
     The friction strain sensor assembly  20  is inserted into the hollow portion  11  of the sensor housing  10  without the cross-member  23  in place. The arms  22  engage in slots  39  in the inner wall of hollow portion  11  and may slide therein. The cross-member  23  is attached to the arms  22 , by soldering from example, post insertion of the friction strain sensor assembly  20  into the hollow portion. Cross-member  23  in some embodiments may be omitted. The mounting of arms  22  in slots  39  assists in resisting rotational forces experienced by the base plate  21 . 
     The base  21  is slidable into and out of the hollow portion  11 , this securing the friction strain sensor assembly  20  in the hollow portion  11 , sliding of the base  21  out of the hollow portion  11  being constrained by the pin heads  26   a  coming into engagement with the surface of the bridge  35 . 
     The lower edge of the hollow portion  11  is provided with a rebate  11   a  in which a seal  50  sits. The hollow portion also includes on its outer surface a protrusion, which in the present example is a rib  11   b.    
     Referring to  FIGS. 3 and 6  in particular, it can be seen that the plate  31  includes curved recesses  31   b  into which the springs  27  extend. 
       FIGS. 7 and 8  illustrates a component  60  having a bolt sensor  1  mounted therein. The component  60  includes a bore  61  which has a recess  62  in the wall thereof. The bolt sensor  1  is aligned with the bore  61  such that the rib  11   b  is aligned with the recess  62 . 
     The friction strain sensor assembly functions as follows: 
     The springs  27  exert a force urging the base plate  21  out of the hollow portion  11 , this movement limited by abutment of the underside of pin heads  26   a  with the top surface of the bridge  35 . The spring  33  pushes the plate  31  away from the bridge  35 , which acts as a reaction member, and hence urges the friction strain sensor element  30  out of the aperture  24 . The rectangular shape of the aperture  24  and the friction strain sensor element  30  constrain the friction strain sensor element to reciprocal movement in the axial direction of the hollow portion  11 . 
     The spring  33  is specified such that the friction strain sensor element  30  is urged against a surface engaged by the lower surface of the base plate  21  with a pre-determined force on the friction strain sensor element  30  when the bolt sensor  1  is in place. 
     Hence, when a bolt sensor  1  is introduced into a bore  61 , as the bolt head  3  is rotated base plate  21  comes into abutment with a surface of a component adjacent the end of the bore  61 , and further rotation of the bolt head causes the base plate  21  to slide into the hollow portion  11 , compressing the springs  27  and the seal  50 . 
     As the base plate  21  slides into the hollow portion  21 , the friction strain sensor element  30  moves closer to the surface of the component adjacent the end of the bore  61  and is urged against the said surface by the spring  33 . 
     When the bolt sensor is released from the bore  61  the reverse occurs. 
     The seal  50  prevents ingress of lubricant or contaminants to the region immediately surrounding the friction strain sensor element  30 . 
     The base  21  therefore protects the sensitive sensor element  30 , as the sensor element  30  sits behind the surface of the base plate  21 , until the base plate  21  engages a surface that is to be instrumented by the sensor element  30 . 
     An antenna  52  is mounted in a collar  51  that sits in the head  3  of the bolt member  2 . The antenna  52  is powered by a power connector  53 , which includes a cap  54  and a socket  55 . The friction strain sensor  30 , the AE sensor and the vibration and temperature sensors may communicate with the antenna wirelessly, or may be connected by wires. The antenna  52  may relay information wirelessly. 
     A second embodiment of the invention is illustrated in  FIGS. 10 to 13 . In this embodiment, shear forces on the friction strain sensor element are avoided by positioning the friction strain sensor element so that it may be moved to engage the component to be monitored by moving the friction strain sensor element along the axis of the bolt, after the bolt has been positioned in the hole, when the bolt is stationary, thereby avoiding shear forces acting on the friction strain sensor element  30 . 
     The bolt sensor  100  comprises a bolt member  101  including a threaded collar  102  and a hexagonal head  103 . A elongate portion  104  extends from and is attached to the end of the collar  102 . The elongate portion  104  includes a central bore  105  and two threaded holes  106  situated to either side of the central bore  105  and extending into the free end of the elongate portion  104 . 
     An end cap  107  attaches to the free end of the elongate portion  104  by means of screws  108 . The end cap  107  includes screw head seats  111  and holes  112 , which are aligned with the holes  106 , with a shank  109  of one of the screws  108  passing through a hole  112  and engaging with a hole  106  until the screw&#39;s head  110  is seated in the screw head seat  111 . 
     The end of the cap  107  that abuts the end of the elongate portion  104  includes a channel  115  and a bore  116 , which aligns with the central bore  105 . 
     A friction strain sensor element assembly  130  includes a mount  131  mounting a friction strain sensor element  131   a , and a shaft  132 . The shaft  132  includes a collar  133  extending around the shaft  132  and fast with respect to the shaft  132 , and a spring  134 . The shaft  132  is attached to the friction strain sensor element mount  131  such that the axial movement of the shaft  132  causes axial movement of the mount  131 . 
     The shaft  132  sits in the bore  116 , with the collar  133  situated in the channel  115 , the flat sides of the collar engaging the parallel walls of the channel  115 , thereby preventing rotation of the shaft  132 . The bore  116  is smaller in diameter than the spring  134 , the spring  134  engaging the wall immediately around the bore  116 , thereby urging the mount  131  axially out of end cap  107 . 
     An actuating thumb screw  140  is mounted in the central bore  105 . One end  141  of the thumb screw  140  is configured to attach releasably to the shaft  132 . The end  141  of the thumb screw includes an internally threaded bore, and the shaft  132  is threaded externally between the free end of the shaft proximate the thumb screw  140  and the collar  133 . By rotating the thumb screw  140  the corresponding threads of the shaft  132  and thumb screw  140  cause the shaft  132  and hence the friction strain sensor element assembly  130  to move in the axial direction of the thumb screw  140 . Rotating the thumb screw  140  in one direction causes the shaft  132  and hence the mount  131  and frictions sensor element  131   a  to retract into the channel  135  compressing the spring  134 . The mount  131  is most retracted when the collar  133  engages the end  141  of the thumb screw  140 . 
     Rotating the thumb screw  140  in the other direction causes the mount  131  and friction sensor element  131   a  to move axially in a direction out of the channel  135  so that the friction sensor element  131   a  engages a component facing the end of the end cap  107 . 
     A hole  150  accommodates a cable that attaches to the friction strain sensor  131   a.    
     Before introducing the bolt sensor  100  into a hole, the thumb screw is rotated to draw the friction strain sensor mount  131  into the channel  135  such that the friction strain sensor element  131   a  lies behind the end face of the bolt cap  107 . The bolt sensor  100  is introduced into a hole and turned until the end of the end cap  107  is in abutment with the surface of a component to be monitored. The thumb screw  140  is then turned in the opposite direction which moves the mount  131  and friction sensor element  131   a  toward the component to be monitored. The force of the spring  134  pushes the friction strain sensor element  131   a  into engagement with the surface of the component to be monitored. 
     Both illustrated embodiments of the invention provide a bolt sensor utilising a friction strain sensor element, which is desirable, and prevent the friction strain sensor element from being subjected to the kind of forces which are likely to damage it. 
     In both embodiments, the springs urging the friction strain sensor element against the component being monitored ensure that the friction strain sensor element is subjected to the required load for operation of the friction strain sensor element. 
     Individual technical features of the illustrated embodiments are not limited to use in those embodiments, and may where suitable, be used with any embodiment falling within the scope of the claims.