Patent Publication Number: US-2023139352-A1

Title: Nut and/or bolt rotation indicator

Description:
TECHNICAL FIELD 
     The invention relates to systems for monitoring rotation of a nut or bolt. More specifically, the invention relates to (but need not be limited to) systems configured to detect loosening of a nut or bolt. 
     BACKGROUND 
     The humble nut and bolt has been used for decades in order to fasten components together in a wide range of technical fields. Nuts or bolts (hereafter referred to simply as a nut) can however, progressively loosen over time, especially when exposed to environmental effects such as heat expansion and contraction, and vibration, or alternatively as a result of improper torqueing on installation. 
     The consequences of nuts loosening may be catastrophic. Typically, a visual inspection to check for loosening may be conducted. This can however be time consuming, especially where large numbers of nuts are utilised and/or nuts to be monitored are spread over a large area, for example in railways or the aviation industry. Monitoring nuts may also be expensive and/or logistically challenging, for example, when checking the nuts of offshore wind turbines. Additionally, relying on visual inspections leaves room for human error. 
     SUMMARY 
     According to the invention in a first aspect, there is provided a system for monitoring rotation of a nut or bolt, the system comprising an indicator for mounting to the nut or bolt and configured to rotate therewith, a detector configured to detect a rotational position of the indicator, and a transmitter configured to transmit data indicative of the detected rotational position of the indicator. 
     Optionally, the transmitter is configured to transmit an alarm signal indicating a loose nut condition if the detected rotational position of the indicator exceeds a threshold. 
     Optionally, the detector is configured to detect a change in rotational position of the indicator, and wherein the transmitter is configured to transmit an alarm signal indicating a loose nut condition if the detected change in rotational position of the indicator exceeds a threshold. 
     Optionally, the system further comprises an actuator, wherein the detector is configured to detect a change in relative rotational position between the actuator and the detector. 
     Optionally, the detector comprises a sensor configured to detect a property of the actuator indicative of a relative direction and/or range of the actuator from the detector. 
     Optionally, the property comprises a strength and/or a pattern of a magnetic field. 
     Optionally, the actuator generates the magnetic field and optionally comprises a magnet. 
     Optionally, the indicator comprises one of the actuator and the detector. 
     Optionally, the other of the actuator and the detector is for mounting in a location adjacent to the nut or bolt and is configured not to rotate therewith. 
     Optionally, the other of the actuator and the detector is fixed relative to the nut or bolt. 
     Optionally, the indicator comprises the actuator. 
     Optionally, the detector comprises the transmitter. 
     Optionally, the indicator and the detector comprise alignment features configured to indicate positions of the actuator and the sensor. 
     Optionally, the alarm signal comprises identification data configured to identify the indicator and/or the detector. 
     Optionally, the alarm signal comprises location data indicative of a geographic location of the indicator and/or the detector. 
     Optionally, the system further comprises an auto-calibrator configured to determine the threshold based on an initial property of the actuator sensed by the sensor at the time of fitting the system. 
     Optionally, the system further comprises a locator configured to cooperate with the detector and/or the indicator to facilitate positioning and mounting of the detector in a predetermined orientation and/or range with respect to the indicator. 
     Optionally, the locator is configured to couple to the detector and/or indicator to facilitate positioning and mounting of the detector and decouple from the detector and/or indicator after positioning and mounting of the detector. 
     Optionally, the locator comprises a coupling feature configured to couple the detector thereto. 
     Optionally, the locator comprises a locating feature configured to engage at least a portion of the indicator, such that when the detector is coupled to the locator and the locating feature is engaged with the at least a portion of the indicator, the detector is positioned in the predetermined orientation and/or range. 
     Optionally, the locator comprises a release mechanism configured to decouple the detector and/or indicator from the locator after positioning and mounting of the detector. 
     According to the invention in a further aspect, there is provided a method for monitoring rotation of a nut or bolt, the method comprising mounting an indicator to the nut or bolt such that the indicator rotates therewith; detecting, by a detector, a rotational position of the indicator; and transmitting, by a transmitter, data indicative of the rotational position of the indicator. 
     Optionally, the method further comprises transmitting, by the transmitter, an alarm signal indicating a loose nut condition if the detected rotational position of the indicator exceeds a threshold. 
     Optionally, the detector is configured to detect a change in rotational position of the indicator, and the method further comprises transmitting, by the transmitter, an alarm signal indicating a loose nut condition if the detected change in rotational position of the indicator exceeds a threshold. 
     Optionally, detecting the rotational position of the indicator comprises sensing, by a sensor, a property of an actuator indicative of relative direction and/or range of the actuator from the detector. 
     Optionally, the property of the actuator comprises a strength and/or a pattern of a magnetic field of the actuator. 
     Optionally, the indicator comprises one of the actuator and the detector, and the method further comprises mounting the other of the indicator and the detector in a location adjacent to the nut or bolt such that it does not rotate therewith. 
     Optionally, the method further comprises determining the threshold based on an initial property of the actuator sensed by the sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows an exemplary system for monitoring rotation of a nut and/or bolt; 
         FIG.  2    shows a schematic representation of an exemplary detector; 
         FIG.  3    shows a flow diagram of a method of monitoring rotation of a nut and/or bolt; and 
         FIG.  4    shows an exemplary system for monitoring rotation of a nut and/or bolt. 
     
    
    
     DETAILED DESCRIPTION 
     Generally disclosed herein are exemplary systems for detecting rotation and/or loosening of a nut. The exemplary systems may detect rotation of the nut and be configured to transmit a signal, and/or data, indicative of a rotational position of the nut. Alternatively, or in addition, exemplary systems may be configured to transmit an alarm signal if the rotational position of the nut is indicative of a loose nut condition. For example, exemplary systems may be configured to transmit an alarm signal if a change in rotational position of the nut exceeds a threshold. The inventors have realised that providing systems that transmit an alarm signal when a loose nut condition is detected eliminates the need for time consuming, expensive and difficult visual inspections, which may also be prone to human error. Instead, it is only necessary to respond to the alarm signals. In exemplary systems, the signals transmitted may comprise identification data and/or location data to allow the location of the loose nut to be determined. This allows the loose nut to be directly located to be tightened/replaced. 
       FIG.  1    shows an exemplary system  100  for monitoring rotation of a nut. The exemplary monitoring system  100  of  FIG.  1    comprises an indicator  102  and a detector  104 . 
     The indicator  102  is configured to be mounted to a nut  106 . The indicator  102  may be configured to be mounted to the nut  106  such that the indicator  102  rotates therewith. 
     In the exemplary arrangement shown in  FIG.  1   , the indicator  102  comprises a collar comprising an aperture  108  configured to receive the nut  106 . The aperture  108  may be dimensioned to provide an interference fit with the nut  106 , such that the indicator  102  does not fall away from the nut  106  and may not be removed from the nut  106  without a force being applied. The skilled person will appreciate that alternative indicators may comprise a cap, cover or housing and may be configured to substantially enclose the nut  106 . The skilled person will further appreciate that there are alternative ways of mounting the indicator  102  to the nut  106  such that the indicator  102  rotates therewith. For example, the indicator may be configured to be mounted to a surface of the nut, such as a sidewall or upper surface of the nut, such that the indicator rotates therewith. The skilled person will be able to envisage further arrangements. 
     The exemplary indicator  102  further comprises an indicator element  112 . The indicator element  112  may be configured to rotate with the indicator  102 . In exemplary arrangements, the indicator element  112  is fixed relative to the indicator  102 . 
     In the exemplary arrangement of  FIG.  1   , the indicator element  112  is received within an aperture  114  of the indicator  102 . In alternative arrangements, the indicator element  112  may be mounted to a surface of the indicator  102 . For example, the indicator element  112  may be mounted to a sidewall, base or upper surface of the indicator  102 . 
     The detector  104  may be configured to detect a property of the indicator element  112 . The property of the indicator element  112  may be indicative of a direction of the indicator element  112  relative to the detector  104 . Alternatively, or additionally, the property of the indicator element  112  may be indicative of a distance and/or range of the indicator element  112  from the detector  104 . As will be described in more detail below, the detected property of the indicator element  112  may be used to determine whether the nut  106  has loosened (i.e. to determine whether there is a loose nut condition). 
     In Exemplary systems  100 , the indicator element  112  may be configured to generate a magnetic field and may comprise a magnet. In exemplary arrangements, the indicator element  112  may comprise a permanent magnet. In such exemplary arrangements, the detector  104  may be configured to detect a strength and/or pattern of a magnetic field of the indicator element  112 , as will be described in more detail below. In exemplary arrangements, the indicator element  112  may comprise a flat disc shape. 
     The skilled person will appreciate that in alternative arrangements, alternative indicator elements  112  may be used. For example, in alternative arrangements, the indicator element  112  may be configured to emit an electromagnetic field or, specifically, a radiofrequency (RF) field, and may comprise an electromagnetic field emitter or an RF emitter. In such arrangements, the detector may be configured to detect a strength of the electromagnetic or RF field of the indicator element. In further alternative arrangements, the indicator element may be configured to generate and/or reflect electromagnetic radiation, such as visible light. In such arrangements, the indicator element may comprise a surface configured to reflect the visible light or an electromagnetic radiation emitter, for example, one of an LED, a photodiode, a photodetector, and a light sensor. The detector may be configured to detect a strength of the electromagnetic radiation generated and/or reflected by the indicator element. The skilled person will be able to envisage further arrangements in which the indicator element emits and/or generates a wave, field strength or field pattern, or else comprises a property, which may be detected by the detector. 
     The exemplary indicator  102  of  FIG.  1    further comprises an alignment feature  116 . The alignment feature  116  may indicate a location of the indicator element  112 . In the arrangement shown in  FIG.  1   , the alignment feature  116  comprises a radially extending protrusion. The alignment features  116  further comprises an indicia, which in the exemplary arrangement of  FIG.  1    is located on a top surface of the indicator  102 . The skilled person will appreciate that in alternative arrangements, alternative ways of providing an indication of the location of the indicator element may be used. For example, alternative arrangements may comprise an indicia located on substantially any surface of the indicator. In further alternative arrangements, the indicator element may be visible when the indicator is mounted to the nut, for example, the indicator may comprise a transparent portion through which the indicator element is visible. The skilled person will be able to envisage further arrangements. 
     In exemplary arrangements, the indicator  102  may be adjustable to accommodate a range of different sizes of nuts and bolts. In exemplary arrangements, the indicator  102  may be adjustable to vary the size of the aperture  108  configured to receive the nut  106 . For example, the indicator  102  may comprise a clamp comprising opposed ends joined by a threaded bolt. The threaded bolt may be configured to alter the distance between the opposed ends and therefore alter the diameter of the aperture. Such an arrangement may be similar to an adjustable pipe clamp. The skilled person will be able to envisage further arrangements to achieve an adjustable indicator. Advantageously, this allows the indicator  102  to be used in different applications, which may use different nut/bolt sizes, without the need to custom make the indicator for each application. 
     The detector  104  may comprise a sensor  216  (not visible in  FIG.  1   ) configured to sense a property of the indicator element  112 , as described above. In exemplary systems, the sensor  216  may comprise a magnetic field sensor, such as a hall effect sensor configured to sense a magnetic field strength and/or pattern of a magnetic field generated by the indicator element  112 . In alternative arrangements, the sensor  216  may comprise an electromagnetic field sensor or an RF sensor configured to sense a field strength of the electromagnetic or RF field generated by the indicator element  112 . In further arrangements, the sensor  216  may comprise a photosensor configured to sense an amount of visible light reflected or generated by the indicator element  112 . 
     As described in more detail below, the detector  104  may comprise further detector electronics. The detector electronics may be housed in a water-tight housing  120 . 
     The detector  104  may further comprise an alignment feature  118 . The alignment feature  118  may be configured to indicate a location of the sensor  216 . The alignment feature  118  of the exemplary detector  104  of  FIG.  1    comprises an indicia. The indicia of  FIG.  1    is located on an upper surface of the detector  104 , however the skilled person will appreciate that in alternative arrangements, the indicia may be located on substantially any surface of the detector. The skilled person will further appreciate that in alternative arrangements, alternative alignment features may be used, for example as mentioned above in respect of the alignment feature  116  of the indicator  102 . 
     In exemplary arrangements, the detector  104  is configured for mounting in a location adjacent to the nut  106 . For example, the detector  104  may be configured for mounting to a surface adjacent to the nut  106 . In exemplary arrangements, the detector  104  may be configured for mounting adjacent to the nut  106  such that the detector does not rotate therewith. 
     In the exemplary arrangement shown in  FIG.  1   , the detector  104  may be mounted to a location adjacent to the nut  106  using an adhesive. The adhesive may comprise a double-sided tape. In alternative arrangements, the detector  104  may be mounted using alternative mounting means, comprising, for example, mechanical fasteners. In further alternative arrangements, the system  100  may further comprise a bracket configured to mount the detector  104  adjacent to the nut  106 . The bracket may comprise a mounting plate configured to receive the detector and the mounting plate may be configured to be fastened to a surface adjacent to the nut to secure the detector  104 . 
     In exemplary arrangements, the detector  104  and/or indicator  102  may further comprise a visual indicator (not shown in  FIG.  1   ) configured to provide a visual indication if a loose nut condition is detected. The visual indicator may comprise an LED configured to illuminate if a property of the indicator element  112  sensed by the sensor  216  is indicative of a loose nut condition. In alternative arrangements, the detector  104  and/or indicator  102  may alternatively or additionally comprise an audible indicator configured to emit an audible alert if a loose nut condition is detected. The skilled person will appreciate that alternative indicators may be used to provide a short range indication that a loose nut condition has been detected. This may aid the on-site identification of the nut  106  to be tightened/replaced. 
       FIG.  2    shows a schematic representation of a detector  200 , which may be the detector  104  shown in  FIG.  1   . The detector  200  comprises a transmitter  202  and may optionally comprise a receiver  204 . In exemplary arrangement, the detector  200  may comprise an antenna, which may function as at least one of the transmitter and receiver. The transmitter  202  and/or receiver  204  may be in data communication with other entities, such as user equipment, servers and/or functions in a telecommunications network and are configured to transmit and receive data accordingly. 
     The detector  200  may further comprise a memory  206  and a processor  208 . The memory  206  may comprise a non-volatile memory and/or a volatile memory. The memory  206  may have a computer program  210  stored therein. The computer program  210  may be configured to undertake the methods disclosed herein. The computer program  210  may be loaded in the memory  206  from a non-transitory computer readable medium  212 , on which the computer program is stored. The processor  208  may be configured to undertake one or more of the functions of an alarm generator  214  and auto-calibrator  218 , as set out below. The detector  200  also comprises a sensor  216  and the processor may be configured to control this feature. 
     Each of the transmitter  202 , receiver  204 , memory  206 , processor  208 , alarm generator  214 , sensor  216  and auto-calibrator  218  may be in data communication with the other features  202 ,  204   206 ,  208 ,  210 ,  214 ,  216 ,  218  of the detector  200 . The detector  200  can be implemented as a combination of computer hardware and software. In particular, the alarm generator  214  may be implemented as software configured to run on the processor  208 . The memory  206  may store the various programs/executable files that are implemented by a processor  208 , and also provides a storage unit for any required data. The programs/executable files stored in the memory  206 , and implemented by the processor  208 , can include the alarm generator  214  and auto-calibrator  218 , but are not limited to such. The transmitter may use transmission frequency of, for example, one of 315 MHz, 433 MHz and 868 MHz. 
     Although not shown in  FIGS.  1  or  2   , exemplary detectors  104  may further comprise one or more of: a temperature sensor and vibration sensor. The temperature sensor may be configured to sense a temperature indicative of the temperature of an environment surrounding the nut  106 . The vibration sensor may be configured to sense vibrations indicative of vibrations that the nut  106  is exposed to. As mentioned above, environmental effects such as heat expansion and contraction and vibration can lead to nuts loosening. In exemplary arrangements, the transmitter may be configured to transmit an alarm signal if a temperature condition is sensed by the temperature sensor indicative of adverse temperature conditions for the nut (e.g. a temperature that suggests loosening of the nut is likely). This may be a threshold temperature or a threshold rate of change of temperature. Similarly, the transmitter may be configured to transmit an alarm signal if a vibration condition is sensed by the vibration sensor indicative of adverse conditions for the nut (e.g. a vibration level or cycle that suggests loosening of the nut is likely). This may be a threshold vibration level or a threshold rate of change of vibration levels (e.g. a threshold number of vibration cycles). 
     The skilled person will appreciate that indicator  102  may comprise one or more features of the detector  104 . For example, in exemplary arrangements, the indicator  102  may comprise the detector  104  and the indicator element  112  may be configured for mounting on a surface adjacent to the nut  106 . 
       FIG.  3    shows a flow chart for a method for monitoring rotation of a nut. 
     In exemplary methods, the indicator  102  is mounted  300  to the nut  106 . In exemplary arrangements, the nut  106  may be tightened to a desired torque before mounting the indicator  102  thereto. In the arrangement shown in  FIG.  1   , mounting the indicator  102  to the nut  106  comprises locating the indicator  102  over the nut  106  and applying a force to the indicator  102  such that the nut  106  is received in the aperture  108 . 
     The indicator  102  may be mounted to the nut  106  such that relative rotation therebetween is prevented. As described above, relative rotation between the indicator  102  and the nut  106  may be prevented due to an interference fit between the aperture  108  of the indicator and the nut  106 . In alternative arrangements, alternative means of preventing relative rotation between the indicator  102  and the nut  106  may be used. For example, an adhesive may be used to secure the indicator  102  to the nut  106 . 
     The detector  104  may be mounted  302  in a location adjacent to the nut  106 . In the arrangement shown in  FIG.  1   , the detector  104  is mounted such that it does not rotate with the nut  106 . In exemplary arrangements, the detector  104  may mounted such that it is fixed relative to the nut  106 . The skilled person will appreciate however that in alternative arrangements, the detector  104  may be mounted such that it is moveable relative to the nut  106 . For example, the detector  104  may be mounted to a further nut, such that the detector  104  rotates with the further nut, wherein the further nut is adjacent to the nut  106  to which the indicator  102  is mounted. In such arrangements, rotation of either the nut  106  or the further nut will cause a change in proximity of the indicator element  112  relative to the detector  104 . 
     In the exemplary arrangement of  FIG.  1   , the detector  104  may be mounted to a surface adjacent to the nut  106 . The skilled person will appreciate that there are many ways of mounting the detector  104  to a surface adjacent to the nut  106 , as described above. For example, the detector  104  may be secured to the surface adjacent to the nut  106  using an adhesive, fasteners or a mounting plate, for example. 
     The detector  104  may be mounted such that the sensor  216  is located adjacent to the indicator element  112 . This may comprise mounting the detector  104  such that the alignment feature  118  of the detector  104  aligns with the alignment feature  116  of the indicator  102 . In alternative arrangements, the detector  104  and the indicator  102  may be mounted in substantially any orientation, that is, in alternative arrangements, the alignment features  116 ,  118  may not align on mounting the detector  104  and the indicator  102 . 
     The skilled person will appreciate that in alternative methods the detector  104  may be mounted at the location adjacent to the nut  106  before the indicator  102  is mounted to the nut  106 . In such arrangements, the indicator  102  may be mounted to the nut  106  such that the alignment feature  116  of the indicator  102  aligns with the alignment feature  118  of the detector  104 . 
     In exemplary arrangements, the detector  104  is configured to detect a rotational position of the indicator  102 . The detector  104  may be configured to detect a rotational position of the indicator  102  by sensing  304  a property of the indicator element  112 . Since the indicator  102  rotates with the nut  106 , by detecting a rotational position of the indicator  102 , rotation (and therefore loosening) of the nut  106  may be detected. 
     In the exemplary arrangement of  FIG.  1   , the detector  104  may be configured to sense a strength and/or a pattern of a magnetic field of the indicator element  112 . The strength and/or pattern of the sensed magnetic field may be indicative of a distance between the detector  104  and the indicator  102 . Specifically, the sensed strength and/or pattern of the magnetic field may be indicative of a distance (or a direction and/or range) between the indicator element  112  and the sensor  216 . For example, as the nut rotates, the distance between the indicator element  112  and the sensor  216  may change. As such, the strength of the magnetic field sensed by the sensor  216  will also change, increasing as the distance between the sensor  216  and the indicator element  112  reduces, and decreasing as the distance between the sensor  216  and the indicator element  112  increases. 
     The sensor  216  may be configured to sense a property of the indicator element  112  at intervals. For example, the sensor  216  may be configured to sense the property of the indicator element  112  at intervals of 1 minute, 10 minutes, 1 hour, 24 hours and 48 hours. The skilled person will appreciate that the interval length may be varied based on application. 
     In alternative arrangements, in which the indicator element  112  may generate a signal for detection by the detector  104 , the indicator element  112  may be configured to generate a signal at intervals. For example, the indicator element may comprise an RF emitter configured to emit a RF signal at intervals. The indicator element may be configured to generate a signal for detection by the detector at intervals of 1 minute, 10 minutes, 1 hour, 24 hours and 48 hours as above. 
     In exemplary arrangements, the sensor  216  may be configured to sense the property of the indicator element  112  at an increased sensing rate (that is, the interval length may be reduced) if adverse conditions for the nut are detected. For example, as mentioned above, exemplary arrangements may comprise a temperature sensor and/or a vibration sensor. In such arrangements, the sensing rate, and therefore the length of the interval, may be changed based on the temperature and/or vibration level detected by the temperature sensor and the vibration sensor. For example, the sensor  216  may be configured to sense the property of the indicator element  112  at an increased sensing rate (that is, the interval length may be reduced) if the vibration sensor detects adverse conditions for the nut (e.g. a vibration level or cycle that suggests loosening of the nut is likely). Similarly, the sensor  216  may be configured to sense the property of the indicator element  112  at an increased sensing rate (that is, the interval length may be reduced) if the temperature sensor detects adverse temperature conditions for the nut (e.g. a temperature that suggests loosening of the nut is likely). 
     In arrangements in which the indicator element  112  generates a signal for detection, the indicator element  112  may be configured to generate the signals for detection at an increased rate if adverse conditions for the nut are detected, as described above. 
     In exemplary arrangements, the transmitter  202  may be configured to transmit  308  data indicative of the sensed rotational position of the indicator  102 . The data indicative of the sensed rotational position of the indicator  102  may comprise an absolute value of the property sensed by the sensor  216 . As discussed above, the value of the property of the indicator element  112  sensed by the sensor  216  may be indicative of the distance between the indicator  102  and the detector  104 . 
     The transmitter  202  may be further configured to transmit identification data configured to identify the indicator  102  and/or detector  104 . Alternatively, or additionally, the transmitter  202  may be configured to transmit location data indicative of a geographic location of the indicator  102  and/or detector  104 . This data may allow identification of the nut  106  to which the rotational position data relates. 
     In exemplary arrangements the transmitter  202  may be configured to transmit an alarm signal indicative of a loose nut condition. In such arrangements, the alarm signal may or may not comprise an absolute value sensed by the sensor  216 . 
     In exemplary arrangements, the alarm generator  214  of the detector  104  may be configured to compare  306  a property of the indicator element  112  sensed by the sensor  216  with a threshold and determine whether an alarm signal should be generated based on the comparison. The threshold may be indicative of a loose nut condition. For example, the threshold may comprise a value of the property of the indicator element  112  that if sensed indicates that the rotational position of the indicator  102  suggests that the nut has loosened. If the sensed property is not outside of the threshold, then no alarm signal is generated and the property of the indicator element  112  is sensed again by the sensor  216  after the next interval period. If the alarm generator  214  determines that the sensed property is outside of the threshold then the alarm generator  214  generates an alarm signal indicating a loose nut condition and the transmitter  202  transmits the alarm signal. 
     In alternative arrangements, the alarm generator  214  may be configured to compare a property of the indicator element  112  sensed by the sensor  216  with a property of the indicator element previously sensed by the sensor  216 . The alarm generator may be configured to generate an alarm signal for transmission if the property of the indicator element sensed by the detector differs from the previously sensed property by a threshold amount. 
     There are a number of ways to implement such alarms that will be known to the skilled person. 
     The alarm signal may comprise identification data and/or location data configured to identify the indicator  102  and/or the detector  104 . As such, the nut  106  associated with the indicator  102  and/or the detector  104  may be identified by the alarm signal. This allows the nut  106  to be easily located and tightened/replaced. 
     In exemplary arrangements, the transmitter  202  may be configured to transmit the alarm signal to an apparatus  120 . The apparatus  120  may be a user equipment, such as a mobile phone or other device. In exemplary arrangements, the user equipment may be configured to upload data indicating that an alarm signal has been received to a server. This may be a cloud-based server configured to transmit the indication that an alarm signal has been received to a further apparatus. For example, the further apparatus may be a further user equipment located in a control/maintenance centre. 
     The apparatus  120  may receive the alarm signal and indicate to a user that a loose nut condition has been detected. Alternatively, the apparatus  120  may be configured to transmit an indication that an alarm signal has been received to a further apparatus. 
     The alarm signal and the identification data and/or location data allow the nut  106  to be identified. As such, maintenance can travel directly to the location of the nut  106  and tighten/replace the nut  106  without the need to perform a visual check on every nut used in an assembly (e.g. ever nut in a section of railway track, or every nut on a wind turbine). 
     In exemplary arrangements, the apparatus  120  that received the alarm signal and/or data from the system  100  may be one of a plurality of ground receivers. The plurality of ground receivers may form a network. At least one of the ground receivers may be located near to the system  100 . The data transmitted by the system  100  (and in exemplary arrangements, the transmitter  202  of the detector  104 ) may be transmitted between the network of ground receivers to boost the signal over large distances. In exemplary arrangements, the location of the system  100  may be determined based on the number of signals transmitted by the system  100  and received by a ground receiver directly. That is, the number of signals received by the ground receiver from the system  100 , and not from a further ground receiver. This allows the ground receiver nearest to the relevant nut to be determined, and therefore the location of the relevant nut to be determined. In exemplary arrangements, at least one of the ground receivers may be battery and/or solar powered or mains powered. 
     As will be appreciated by the skilled person, in arrangements comprising the alignment features  116  and  118 , as the nut  106  loosens and rotates, the alignment features  116  and  118  will be brought out of alignment. This allows a visual check to additionally be performed. This may aid the identification of the nut to be tightened/replaced in response to the alarm signal. 
     Once the nut has been tightened/replaced, the indicator  102  may be mounted to the tightened/new nut. 
     In exemplary arrangements, the detector  104  may comprise an auto-calibrator  218 . The auto-calibrator  218  may be configured to calibrate the sensor  216 . Calibrating the sensor  216  may comprise determining at least one threshold indicative of a loose nut condition. The at least one threshold may be determined based on an initially sensed property of the indicator element  112 . For example, a property of the indicator element  112  sensed on fitting of the system  100 . 
     The auto-calibrator  218  may be configured to calibrate the sensor  216  on installation of the system  100 . In exemplary arrangements auto-calibration may occur when the detector  104  and the indicator  102  are brought within range of one another. For example, auto-calibration may occur when the indicator  102  and the detector  104  are mounted to the nut and a location adjacent to the nut. This may “wake up” the detector  104  which may initiate auto-calibration. 
     In exemplary arrangements, the threshold may be determined as a percentage of the initially sensed property of the indicator element  112 , which may be the value of the property sensed on fitting the system  100 . The threshold may be determined to be 25%, 50%, 100% or more of the initial sensed property. For example, in the exemplary arrangement of  FIG.  1   , the threshold may be determined as a percentage of the strength of the magnetic field sensed on fitting the system. Alternatively, the threshold may be calculated by adding or subtracting a predetermined absolute amount from the initial property sensed. The threshold may then be stored in the memory  206  of the detector for comparison with subsequently sensed properties of the indicator element  112 . Advantageously, calibrating the sensor in such a way reduces the need for accurate alignment of the sensor and actuator on installation. 
     In alternative arrangements, the detector may not undergo a calibration process, and instead a manufacturer set threshold may be stored in the memory  206  of the detector  104 . 
     The inventors have appreciated that installation of systems for monitoring rotation of a nut, such as the system described above, may be further simplified by facilitating accurate location of the indicator with respect to the detector on installation. In particular, installation may be further simplified by facilitating accurate location of the indicator element of the indicator with respect to the sensor of the detector. Exemplary systems may comprise a locator configured to interact with the detector and indicator to facilitate installation of the detector in a predetermined position with respect to the indicator. In exemplary systems the locator may also act as an applicator, allowing the user to pick up the detector and secure the detector to a surface in the predetermined position with respect to the indicator. 
       FIG.  4    shows a further exemplary system  400  for monitoring rotation of a nut. The exemplary monitoring system  400  of  FIG.  4    comprises an indicator  402  and a detector  404 . Many of the features of the indicator  402  and the detector  404  are similar to those described above in respect of the indicator  102  of  FIG.  1    and the detector  104 ,  200  of  FIGS.  1  and  2   . As such, a description of these features is not given again here and corresponding reference numerals are used to identify them in  FIG.  4   . Thus,  412  is the indicator element (not visible in  FIG.  4   ) and  408  is the aperture of the indicator  402 , and  416  is the sensor of the detector  404 . 
     The exemplary system  400  further comprises a locator  422 . The locator  422  may be configured to cooperate with the detector  404  and/or indicator  402  to facilitate positioning and mounting of the detector  404  with respect to the indicator  402 . The locator  422  may be configured to cooperate with the indicator  402  and/or the detector  404  to locate the detector  404  in a predetermined orientation and/or range of the indicator  402 . In exemplary arrangements, the locator  422  may be configured to cooperate with the indicator  402  and/or the detector  404  to locate the sensor  416  of the detector  404  in a predetermined orientation and/or range of the indicator element  412  of the indicator  402 . As will be described in more detail below, the locator  422  may be configured to engage with the detector to allow installation of the detector on a surface to which the detector is to be mounted in the predetermined orientation and/or range of the indicator  402 . 
     In the exemplary arrangement of  FIG.  4   , the locator  422  comprises a locating feature  424 . The locator  422  may comprise a coupling feature  428  configured to couple the detector  404  to the indicator  402 . 
     The locating feature  424  of the locator  422  may be configured to cooperate with the indicator  402  to locate the detector  404  in a predetermined orientation and/or range of the indicator  402 . In exemplary arrangements, the locating feature  424  of the locator  422  may be configured to cooperate with the indicator  402  to locate the coupling feature  428  (and therefore the detector  404 , when the detector  404  is engaged with the coupling feature  428 ) in a predetermined orientation and/or range of the indicator  402   
     The locating feature  424  of the locator  422  may comprise a mechanical feature, such as a recess or a protrusion. In the exemplary arrangement shown in  FIG.  4   , the locating feature  424  comprises a keyed feature. The keyed feature of the locator  422  shown in  FIG.  4    is configured to receive at least a portion of the indicator  402  therein. The skilled person will appreciate that in alternative arrangements, the indicator  402  may comprise the keyed feature and at least a portion of the locator  422  may be received therein. 
     In the exemplary arrangement of  FIG.  4   , the locating feature  424  of the locator  422  is configured to cooperate with a corresponding locating feature  426  of the indicator  402 . The locating feature  426  of the indicator  402  may be indicative of a position of the indicator element  412  of the indicator  402 . In exemplary arrangements, engagement of the locating feature  424  of the locator  422  and the locating feature  426  of the indicator  402  may align the indicator element  412  and the sensor  416  when the detector  404  is coupled to the locator  422 . 
     The corresponding locating feature  426  of the indicator  402  may be received within the locating feature  424  of the locator  422 . In the exemplary arrangement of  FIG.  4   , the corresponding locating feature  426  of the indicator  402  comprises a protrusion. The protrusion may be configured to be received within the keyed feature of the locator  422 . In the exemplary arrangement of  FIG.  4   , the shape of the keyed feature of the locator  422  corresponds to the shape of the protrusion of the indicator  402 . 
     The locator  422  may be configured to couple to the detector  404 . In exemplary arrangements, the locator  422  may couple to the detector  404  such that movement of the locator  422  causes corresponding movement of the detector  404  (and vice versa). 
     The locator  422  may comprise a coupling feature  428  configured to couple the detector  404  to the locator  422 . In exemplary arrangements the detector  404  may be positioned in the predetermined orientation and/or range of the indicator  402  when the locating features  424 ,  426  of the locator  422  and the indicator  402  respectively cooperate, and when the detector  404  is coupled to the locator  422 . 
     In the exemplary arrangement shown in  FIG.  4   , the coupling feature  428  comprises a recess configured to receive at least a portion of the detector  404  therein. The recess may be sized and shaped such that when the detector  404  is received therein, it is retained within the recess due to a friction fit. The detector  402  may be configured to provide a poka-yoke fit within the recess of the locator  422 . The poka-yoke fit may cause the detector  404  to be coupled to the locator  422  such that the sensor  416  of the detector  404  is positioned in a predetermined orientation with respect to the locating feature  424  of the locator  422 . In turn, this causes the sensor  416  to be positioned in a predetermined position with respect to the indicator element  412  of the indicator  402  when the locator  422  is used to position and mount the detector  404 . 
     The skilled person will appreciate that in alternative arrangements, alternative coupling features may be used. For example, the coupling feature of the locator  422  may comprise an adhesive configured to adhere to a surface of the detector  404 . In further alternative arrangements, the coupling feature  428  may comprise a recess or protrusion configured to engage a corresponding recess or protrusion on the detector  404  to couple the detector thereto. In further alternative arrangements, the coupling feature may comprise a locating feature, similar to the locating feature  424  described above, but configured to cooperate with at least a portion of the detector  404 . In further alternative arrangements, the coupling feature may comprise an abutment surface and the detector  404  may be configured to contact the abutment surface when the locator  422  is engaged with the indicator  402  to position the detector  404  correctly with respect to the indicator  402 . The skilled person will be able to envisage other arrangements for coupling the detector  404  and the locator  422 . 
     The locator  422  may be configured to releasably couple to the detector  404 . The exemplary locator  422  shown in  FIG.  4    comprises a release mechanism  430 . The release mechanism  430  may be configured to decouple the detector  404  from the locator  422 . In exemplary arrangements, the release mechanism  430  may be configured to decouple the detector  404  from the coupling feature  428  of the locator  422 . The release mechanism  430  may act to decouple the detector  404  from the locator  422  and apply the detector  404  to the surface to which it is to be mounted. 
     The release mechanism  430  of the exemplary locator  422  shown in  FIG.  4    comprises a first portion  434  and a second portion  436 . The first and second portions  434 ,  436  may be axially moveable with respect to each other. In the exemplary arrangement shown in  FIG.  4   , the first portion comprises the locating feature  424  and the coupling feature  428 . The second portion may be telescopically received within the first portion  434 . 
     The second portion  436  may be configured to interact with the detector  404  when the detector  404  is coupled to the locator  422 , such that relative axial movement between the first portion  434  and the second portion  436  decouples the detector  404  from the locator  422 . In the exemplary arrangement shown in  FIG.  4   , an abutment surface of the second portion  436  may be configured to abut a surface of the detector  404  such that relative axial movement between the first portion  434  and the second portion  436  disengages the detector  404  from the recess of the locator  422 . 
     Installation of the detector  404  using the locator  422  is described below with reference to  FIG.  4   . 
     The indicator  402  is mounted to the nut  406 , similarly to as described above in respect of  FIG.  3   . As described above, the locating feature  426  of the indicator  402  may correspond to the position of the indicator element  412  of the indicator  402 . 
     The locator  422  may be coupled to the detector  404 . In exemplary methods, the locator  422  may be coupled to the detector  404  by engaging the coupling feature  428  with at least a portion of the detector  404 . In the exemplary arrangement shown in  FIG.  4   , engaging the coupling feature  428  with the detector  404  comprises receiving the detector  404  within the recess of the locator  404 . The user may push the locator  422  over the detector  404  such the detector  404  is received within the recess. 
     Once the locator is coupled to the detector  404 , movement of the locator  422  causes corresponding movement of the locator  422 . The locator  422  and the detector  404  may therefore be moved towards the installation site adjacent to the nut  406 . 
     The locating feature  424  of the locator  422  may be engaged with the corresponding locating feature  426  of the indicator  402 . This may comprise sliding the keyed feature of the locator  422  over the protrusion of the indicator  402  until the detector  404  contacts the surface to which it is to be mounted. In this position, the detector  404  is positioned in the predetermined orientation and/or range of the indicator  402 . Specifically, the sensor  416  of the detector  404  is positioned in the predetermined orientation and/or range of the indicator element  412 . 
     In exemplary arrangements, the detector  404  may be secured to the surface to which it is to be mounted using an adhesive. For example, the detector  404  may comprise a double sided tape. The pressure applied to the detector  404  via the locator  422  when engaging the locator  422  and the indicator  402  may adhere the detector  404  to the surface. 
     The locator  422  may be decoupled from the detector  404  and disengaged from the indicator  402  once the detector  404  is positioned (and in some arrangements, once the detector  402  has been secured to the surface). 
     Decoupling the detector  404  and indicator  402  from the locator  422  may comprise applying a force to the locator  422  to overcome the friction fit between the locator  422  and the detector  404 . 
     In the exemplary arrangement of  FIG.  4   , the detector  404  may be decoupled from the locator  422  and secured to the surface to which it is to be mounted using the release mechanism  430 . In the exemplary arrangement of  FIG.  4   , the abutment surface of the second portion  436  is brought into contact with the detector  404 . This may comprise moving the second portion  436  with respect to the first portion  432  until the abutment surface of the second portion  436  contacts the detector  404 . By applying pressure to the second portion  436  once the abutment surface is in contact with the detector  404 , the axial position of the second portion  436  and the detector  404  is fixed. 
     The first portion  434  may then be moved axially with respect to the detector  404  (and with respect to the second portion  436 ) until the first portion  434  disengages from the detector  404 . In the exemplary arrangement of  FIG.  4   , the first portion  434  disengages from the detector  404  once the detector  404  exits the recess. Once the detector decouples from the locator  422 , the locator  422  may be removed, leaving the detector  404  mounted to the surface in the predetermined position and/or orientation with respect to the indicator  402 . 
     The skilled person will appreciate that in alternative arrangements and methods, the locator may not comprise a second portion  436 . In such arrangements, the first portion  432  may be decoupled from the detector  404  by the user applying a force directly to the detector  404  (for example, using a hand) and then sliding the first portion  436  off of the detector  404 . 
     Decoupling the detector  404  and the locator  422  may simultaneously disengage the locator  422  and the indicator  402 . In the exemplary method described above, the locating features  424 ,  426  may be disengaged as a result of the axial movement of the first portion  434  with respect to the detector  404 . 
     The skilled person will appreciate that the above-described locator  422  enables accurate placement of the detector  404  with respect to the indicator  402 , and specifically, the sensor  416  of the detector  404  with respect to the indicator element  412 . The locator  422  provides a further advantage of allowing a detector to be moved and secured to a surface without the user having to directly touch the detector. In applications in which the detector comprises an adhesive to secure the detector to a surface, this is particularly advantageous since this ensures that there is no human contamination of the adhesive as a result of accidental contact with the adhesive. The detector may, for example, be provided on a reel of tape (for example where the adhesive is a double sided tape), and the locator allows the detector to be picked off of the tape and mounted to the surface without the user having to touch any component other than the locator. 
     A computer program may be configured to provide any of the above described methods. The computer program may be provided on a computer readable medium. The computer program may be a computer program product. The product may comprise a non-transitory computer usable storage medium. The computer program product may have computer-readable program code embodied in the medium configured to perform the method. The computer program product may be configured to cause at least one processor to perform some or all of the method. 
     Various methods and apparatus are described herein with reference to block diagrams or flowchart illustrations of computer-implemented methods, apparatus (systems and/or devices) and/or computer program products. It is understood that a block of the block diagrams and/or flowchart illustrations, and combinations of blocks in the block diagrams and/or flowchart illustrations, can be implemented by computer program instructions that are performed by one or more computer circuits. These computer program instructions may be provided to a processor circuit of a general purpose computer circuit, special purpose computer circuit, and/or other programmable data processing circuit to produce a machine, such that the instructions, which execute via the processor of the computer and/or other programmable data processing apparatus, transform and control transistors, values stored in memory locations, and other hardware components within such circuitry to implement the functions/acts specified in the block diagrams and/or flowchart block or blocks, and thereby create means (functionality) and/or structure for implementing the functions/acts specified in the block diagrams and/or flowchart block(s). 
     Computer program instructions may also be stored in a computer-readable medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable medium produce an article of manufacture including instructions which implement the functions/acts specified in the block diagrams and/or flowchart block or blocks. 
     A tangible, non-transitory computer-readable medium may include an electronic, magnetic, optical, electromagnetic, or semiconductor data storage system, apparatus, or device. More specific examples of the computer-readable medium would include the following: a portable computer diskette, a random access memory (RAM) circuit, a read-only memory (ROM) circuit, an erasable programmable read-only memory (EPROM or Flash memory) circuit, a portable compact disc read-only memory (CD-ROM), and a portable digital video disc read-only memory (DVD/Blu-ray). 
     The computer program instructions may also be loaded onto a computer and/or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer and/or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions/acts specified in the block diagrams and/or flowchart block or blocks. 
     Accordingly, the invention may be embodied in hardware and/or in software (including firmware, resident software, micro-code, etc.) that runs on a processor, which may collectively be referred to as “circuitry,” “a module” or variants thereof. 
     It should also be noted that in some alternate implementations, the functions/acts noted in the blocks may occur out of the order noted in the flowcharts. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Moreover, the functionality of a given block of the flowcharts and/or block diagrams may be separated into multiple blocks and/or the functionality of two or more blocks of the flowcharts and/or block diagrams may be at least partially integrated. Finally, other blocks may be added/inserted between the blocks that are illustrated. 
     The skilled person will be able to envisage other embodiments without departing from the scope of the appended claims.