Bolt with integrated stress sensor

Abstract

The cylindrical shaft (110) of a bolt (101) is provided with a central bore (113), extending axially from the head (130) of the bolt (101 ) over at least a part of the axial length of the bolt (101). The bolt is provided with a measuring pin (120) located inside the bore (113) and extending substantially parallel to the shaft (110). The measuring pin (120) has an anchor end (122) fixated with respect to the shaft and has an opposite free indicator end (121 ). The bolt is further provided with a sensor element (140, 141 ) comprising a magnet (141 ) attached to the free indicator end (121 ) of the measuring pin (120) and a magnetic sensor (140) fixated with respect to the shaft (110) and aligned with the first sensor component (141 ).

Description

[0001] TITLE: Bolt with integrated stress sensor

[0002] FIELD OF THE INVENTION

[0003] The present invention relates particularly to a bolt with integrated stress sensor. Hereinafter, the present invention will be specifically explained in more detail for such embodiment.

[0004] BACKGROUND OF THE INVENTION

[0005] Generally speaking, a bolt comprises an elongate, cylindrical body that at least partly is provided with threading, at one extremity provided with a head on which a tool can engage for exerting torque. Typically, such head has non-cylindrical outer shape, for instance hexagonal, but an inner shaping such as for instance torx is also possible.

[0006] When the bolt has its threading engaging complementary internal threading of a mating object, rotating the bolt about its longitudinal axis will cause the bolt to be displaced longitudinally with respect to said mating object. The head, which typically has a diameter larger than the cylindrical body, also indicated as shaft, can engage a target object and exert longitudinal force to either pull that target object towards the mating object or clamp the target object and mating object together.

[0007] It is desirable to be able to measure the longitudinal stress inside the bolt. On the one hand, this stress is proportional to the longitudinal force exerted by the bolt. On the other hand, this stress is representative for the measure of loading of the bolt. Measuring and / or monitoring the stress can be useful in predicting an approaching failure of the bolt, and / or in ensuring that the bolt is applied to the correct stress, i.e. that the correct clamping force is exerted.

[0008] However, depending on use, a bolt may also be loaded in a direction perpendicular to its longitudinal axis, and hence be subjected to bending and / or shear. It is also desirable to be able to measure the corresponding deformation and / or stress.

[0009] SUMMARY OF THE INVENTION

[0010] Bolts with integrated stress sensor are already known. Typically, the operation of the integrated sensor is based on measuring the change of the length of the bolt. For instance, US-4553124 describes the use of strain gauges. However, strain gauges are difficult to install, and they require complicated and expensive electronics for outputting a useful measuring signal, which electronics are difficult to integrate within the bolt. GB-2372826 describes an optical measuring arrangement, which is complicated, bulky, expensive, and difficult to combine with wireless readout.

[0011] It is also possible to have a measuring pin extend in a longitudinal bore within the bolt: while the bolt becomes longer under increasing tension, the measuring pin is free of tension and will hence keep its length, causing a relative displacement of a free end of the pin with respect to the bolt, as for instance described in US-4636120, GB-2372826.

[0012] Practical implementations of the measuring pin as known in practice typically have one or more of the following disadvantages: difficult to integrate within a bolt, high costs for the sensor and required electronics, sensitive to vibrations and variations in temperature. Further it is often difficult to reliably calibrate the sensor, torsion and / or bending of the bolt may influence the sensor reading, and the sensors can not be read wirelessly. Known bolts with integrated sensor are expensive, and are only suitable for high-end application.

[0013] In bolts with internal measuring pin, it is a challenge that the maximum displacement to expect, i.e. the maximum length variation of the bolt, is in the order of 0.1 mm. GB-2469019 proposes the use of an eddy current sensor, aligned axially with the measuring pin, i.e. positioned in the path of the measuring pin, so that the eddy current sensor operates as a proximity sensor. Eddy current sensors typically have a measuring range in the order of 0.5 - 25 mm, which means that the displacement to be measured corresponds to only a small portion of the sensor's measuring range. Further, the entire surrounding of the eddy current sensor is mainly metal, namely the body of the bolt, and the displacing measuring pin constitutes only a very small fraction in this metal surroundings for the sensor. It is therefore difficult to obtain an accurate measuring signal with good resolution.

[0014] An object of the present invention is to provide a bolt with integrated stress sensor in which at least some but preferably all of the above-mentioned disadvantages are overcome or at least reduced.

[0015] Particularly, the present invention aims to provide a bolt with integrated stress sensor based on relatively low-cost components, making such bolts available for low- end applications.

[0016] Particularly, the present invention aims to provide a bolt with integrated stress sensor that is reliable, very accurate, and that does not require complicated and expensive amplification and signal processing equipment. Particularly, the present invention aims to provide a bolt with integrated stress sensor that is less sensitive to temperature variations, and less sensitive to torsion and / or bending of the bolt.

[0017] Particularly, the present invention aims to provide a bolt with integrated stress sensor that has small volume so that it can be easily integrated on or in the bolt, and that can be calibrated easily.

[0018] Particularly, the present invention aims to provide a bolt with integrated stress sensor that is easily capable of being adapted for wireless communication of the sensor signal, which in turn allows the sensor system to be watertight.

[0019] Furthermore, the present invention aims to provide a bolt with integrated stress sensor that can easily be equipped with a memory for data storage.

[0020] More generally, the present invention aims to provide a stress sensor having one or more of the above advantages, and designed to be attached construction parts in order to sense and / or monitor deformation thereof and / or stresses therein.

[0021] An important factor in the present invention is that the stress sensor comprises a magnetic field sensor, preferably a Hall sensor. Hall sensors of course are known per se, but it has not been recognized before that a Hall sensor can be used advantageously as a stress sensor is a bolt.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS

[0023] These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which the same reference numerals indicate the same or similar parts, and in which:

[0024] Figures 1 and 2 schematically illustrate measurement principles;

[0025] Figures 3A-C and Figures 4A-C schematically illustrate two implementing embodiments;

[0026] Figure 5 schematically illustrates a particular embodiment being a bolt;

[0027] Figure 6 is a longitudinal section of the bolt of figure 5;

[0028] Figure 7 schematically shows a detail of a bolt according to the present invention;

[0029] Figure 8 schematically shows another detail of a bolt according to the present invention;

[0030] Figure 9 schematically illustrates measuring tension in a cable. DETAILED DESCRIPTION OF THE INVENTION

[0031] Figure 1 schematically illustrates the principles of the measurement involved. Reference numeral 10 indicates a sensor element, comprising two cooperating sensor components 11 , 12. The two sensor components comprise a magnet 11 and a magnetic sensor, which for the sake of this basic explanation will be assumed to be a Hall sensor 12. Reference numeral 13 indicates a sensing face or front face of the Hall sensor 12; the magnet 11 is positioned in front of the sensing face 13. Since Hall sensors are known per se, a detailed description and explanation of the design and functioning of the Hall sensor 12 is omitted here. Nevertheless, it is noted that basic Hall sensors, due to their internal design, are sensitive to a magnetic field component in a sensing direction, which is perpendicular to their sensing face. Said sensing direction will be indicated as Zs direction in a coordinate system relative to the sensor. Directions perpendicular to said sensing direction, i.e. parallel to the sensing face, will be indicated as Xs and Ys.

[0032] It is further noted that also 2D and 3D Hall sensors exist, which are sensitive to two or three, respectively, mutually perpendicular magnetic field components. Except where explicitly stated differently, the following description and explanation relates to a basic sensor.

[0033] Suffice it to say that the Hall sensor 12 has an output 14, schematically shown as wiring, at which an output signal is provided that varies with magnetic field strength experienced at the sensing face 13, and hence varies with the relative positioning of the magnet 11 . If the magnet 11 is moved perpendicular to the sensing face 13 towards or away from the sensing face 13 (down I up in the figure, respectively), the variation of the magnetic field obeys a complicated formula, but the Hall sensor 12 may be provided with a processor designed to produce a linear output signal. If the magnet 11 is moved parallel to the sensing face 13 (left I right in the figure), the magnetic field varies linearly with the displacement in a good approximation, which makes it easier to provide a linear output signal.

[0034] In figure 1 , reference numerals 21 and 22 indicate two reference points. Reference numeral 23 indicates a sensing element that is attached to both reference points 21 and 22. Reference numeral 24 indicates a reference element, also indicated as measuring element, that is attached only to a first one 21 of the reference points 21 and 22. The magnet 11 is carried by the reference element 24, preferably at a free end thereof. The Hall sensor 12 is carried by the sensing element 23, at a position between the reference points 21 and 22.

[0035] With respect to the sensing element 23, a longitudinal direction X is defined in parallel to a virtual line connecting the reference points 21 and 22. The Hall sensor 12 is arranged with its sensing face 13 parallel to the longitudinal direction X. A transverse direction Y is defined perpendicular to the sensing face 13 and perpendicular to the longitudinal direction X. The reference points 21 and 22 may be part of, or attached to, a mechanical structure to be monitored.

[0036] If the reference points 21 and 22 are displaced longitudinally with respect to each other, the sensing element 23 is stressed (or compressed) longitudinally and the longitudinal position of the Hall sensor 12 changes in corresponding manner, while the magnet 11 remains in place, or better worded, its relative position with respect to the first reference point 21 remains constant. Thus, the relative longitudinal position of the magnet 11 with respect to the Hall sensor 12 changes, and a corresponding change in the output signal represents the relative displacement of the second reference point 22 with respect to the first reference point 21 and represents the change in the stress in the sensing element 23 between the reference points 21 and 22.

[0037] If the reference points 21 and 22 are displaced transversely with respect to each other, the sensing element 23 is bent and the transverse position of the Hall sensor 12 changes in corresponding manner, while the magnet 11 remains in place, or better worded, its relative position with respect to the first reference point 21 remains constant. Thus, the relative transverse position of the magnet 11 with respect to the Hall sensor 12 changes, and a corresponding change in the output signal represents the relative displacement of the second reference point 22 with respect to the first reference point 21 and represents the change in the bending of the sensing element 23 between the reference points 21 and 22.

[0038] Figure 2 illustrates a variation of the arrangement illustrated in figure 1 , where the sensor element 10 is mounted such that transverse displacement of the reference points 21 and 22 is sensed by a displacement of the magnet 11 parallel to the sensing face 13 of the Hall sensor 12, while longitudinal displacement of the reference points 21 and 22 is sensed by a displacement of the magnet 11 perpendicular to the sensing face 13 of the Hall sensor 12.

[0039] The above illustrates that, relative to the Hall sensor 12, the magnet 11 can be displaced perpendicular to the sensing direction Zs (with the distance between magnet and sensor remaining constant) or is a direction parallel to the sensing direction Zs (with the distance between magnet and sensor changing). For sake of convenience, the displacement of the magnet in a direction perpendicular to the sensing direction Zs of the Hall sensor 12 will also be indicated as a "shearing" displacement, and the displacement of the magnet in a direction parallel to the sensing direction Zs of the Hall sensor 12 will also be indicated as a "dipping" displacement.

[0040] Depending on the deformation of the sensing element 23, the displacement of the magnet can be dipping or shearing. The above also illustrates that the Hall sensor 12 may be positioned in line with the reference element 24 (figure 2), so that longitudinal stress in the sensing element 23 will result in a dipping displacement of the magnet 11 , while bending of the sensing element 23 will result in a shearing displacement. The above also illustrates that the Hall sensor 12 may be positioned alongside the reference element 24 (figure 1 ), so that longitudinal stress in the sensing element 23 will result in a shearing displacement of the magnet 11 , while bending of the sensing element 23 will result in a dipping displacement.

[0041] It is noted that the Hall sensor 12 may be attached to the sensing element 23 at any longitudinal position between the reference points 21 and 22, but best measuring signal is obtained when the positioning is as close to the second reference point 22 as possible, or as far away from the common first reference point 21 as possible.

[0042] It is possible that the positions of the magnet 11 and the Hall sensor 12 are interchanged.

[0043] In figures 1 and 2, the sensing element 23 is illustrated as an elongate bar, with the reference element 24 extending next to the bar. Figure 3A illustrates that the sensing element 23 may be hollow, i.e. tubular, with the reference element 24 extending inside the tubular sensing element 23. The reference element 24 may be attached to a first end wall 31 of the tubular sensing element 23. The Hall sensor 12 may be attached to an opposite second end wall 32 of the tubular sensing element 23, inside the tubular sensing element 23. These end walls can be considered to be the equivalent of the illustrative reference points 21 , 22 discussed above.

[0044] Figure 3B illustrates, in exaggerated manner, elongation of the tubular sensing element 23 if it is subjected to longitudinal stress P1 . The length of the reference element 24 remains constant. It can be seen that the longitudinal distance between the magnet 11 and the Hall sensor 12 has increased, and it will be understood that the longitudinal distance between the magnet 11 and the Hall sensor 12 will decrease if longitudinal stress is in the opposite direction (compression). In other words, the magnet / sensor configuration is such that a dipping displacement occurs in the case of elongation.

[0045] Figure 3C illustrates, in exaggerated manner, bending of the tubular sensing element 23 if it is subjected to transverse stress P2. The reference element 24 remains straight. It can be seen that the longitudinal distance between the magnet 11 and the Hall sensor 12 has substantially remained the same but the magnet 11 has been shifted in transverse direction with respect to the Hall sensor 12. In other words, the magnet / sensor configuration is such that a shearing displacement occurs in the case of bending.

[0046] In the embodiment of figures 3A-3C, the magnet 11 and the Hall sensor 12 are mounted inside the tubular sensing element 23. The relative arrangement of the magnet 11 and the Hall sensor 12 may be in accordance with the setup illustrated in figure 2, as shown, in which the Hall sensor 12 is positioned in line with the reference element 24, but this relative arrangement may also be in accordance with the setup illustrated in figure 1 , in which the Hall sensor 12 is positioned alongside the reference element 24, so that a shearing displacement occurs in the case of elongation and a dipping displacement occurs in the case of bending. Further, instead of being attached to the second end wall 32, the relevant component 11 or 12 of the sensor element 10 may be attached to the side wall of the tubular sensing element 23. These variations are however not illustrated for sake of simplicity.

[0047] Instead of being mounted inside the tubular sensing element 23, the magnet

[0048] 11 and the Hall sensor 12 may also be mounted outside the tubular sensing element 23. Figures 4A-4C are illustrations comparable to figures 3A-3C, respectively, of an embodiment where the second end wall 32 has a hole 33, where the reference element 24 extends through said hole 33, and where the Hall sensor 12 is mounted outside the second end wall 32.

[0049] The relative arrangement of the magnet 11 and the Hall sensor 12 may be in accordance with the setup illustrated in figure 1 , as shown, in which the Hall sensor

[0050] 12 is positioned alongside the reference element 24, but this relative arrangement may also be in accordance with the setup illustrated in figure 2, in which the Hall sensor 12 is positioned in line with the reference element 24, similar to the relative arrangement of figures 3A-3C. These variations are however not illustrated for sake of simplicity.

[0051] Figure 4B illustrates, in exaggerated manner, elongation of the tubular sensing element 23 if it is subjected to longitudinal stress P1 . It can be seen that the magnet 11 has been shifted in transverse direction with respect to the Hall sensor 12. In other words, the magnet / sensor configuration is such that a shearing displacement occurs in the case of elongation.

[0052] Figure 4C illustrates, in exaggerated manner, bending of the tubular sensing element 23 if it is subjected to transverse stress P2. It can be seen that the transverse distance between the magnet 11 and the Hall sensor 12 has increased, and it will be understood that the transverse distance between the magnet 11 and the Hall sensor 12 will decrease if bending is in the opposite direction. In other words, the magnet / sensor configuration is such that a dipping displacement occurs in the case of bending.

[0053] Figures 3A-3C and 4A-4C illustrate that, in the case of a tubular sensing element 23, depending on choices corresponding to intended use and convenience, the magnet and sensor can be placed inside or outside the tubular sensing element 23.

[0054] Further, figures 3A-3C and 4A-4C illustrate that, depending on choices corresponding to intended use, longitudinal stress may be measured by dipping displacement or shearing displacement of the magnet, while transverse stress (i.e. bending and / or shear) may be measured by shearing displacement or dipping displacement of the magnet, respectively.

[0055] As far as the relative displacement of the magnet and the sensor is concerned, shearing displacement will generally result in better linearity than dipping displacement. Nevertheless, any non-linearity can be corrected by a processor, for instance by calibration, a correction table, or similar. Further, there may be applications in which surpassing a trigger threshold is more important than linearity.

[0056] If, for the sake of easy processing, it is desired to have better linearity, linearity can be improved by setting the relative orientation of the magnet with respect to the sensor such that the magnetic axis is perpendicular to the sensing direction Zs of the sensor, and parallel to the expected displacement of the magnet.

[0057] The above explanation applies generally, irrespective of the nature of the device 1 of figures 3-4. In the following, the explanation will be specifically directed to a bolt with integrated stress sensor, indicated with reference numeral 101.

[0058] Figure 5 schematically shows a side view of an exemplary bolt 101 with integrated stress sensor, and figure 6 schematically shows a longitudinal cross section of this bolt 101 .

[0059] The bolt 101 comprises a cylindrical body 1 10, also indicated as shaft, which at one end is provided with a head 130, in the example shown a hexagonal head. The shaft 1 10 is provided with a threading 1 1 1 over at least a part of its length; in the example shown, a portion 112 is without threading.

[0060] The shaft 1 10 is provided with a central bore 1 13, extending axially from the head 130 over at least a part of the axial length of the bolt 101 . Inside the bore 113, a measuring pin 120 is located. The measuring pin 120, which corresponds to the reference element 24 discussed above, has one extremity 121 located in the vicinity of the head 130; this extremity 121 will be indicated as "indicator end". Opposite the indicator end 121 , the measuring pin 120 has a second extremity 122 that will be indicated as "anchor end".

[0061] The anchor end 122 is fixated with respect to the shaft 110. In a possible embodiment, this can for instance be done by a suitable glue. Apart from this fixation, it is preferred that the measuring pin 120 and the shaft 110 do not touch each other, i.e. the internal diameter of the central bore 113 is sufficiently larger than the external diameter of the measuring pin 120, which may for instance be about 3 mm. By way of comparison, the outer diameter of the shaft 110 can for instance be 10 mm or larger, although the invention is also capable for implementation in bolts with smaller diameter.

[0062] Figure 7 shows a detail of the head 130 at a larger scale, for a preferred embodiment of the bolt 101 . The play of the measuring pin 120 inside the central bore 113 is indicated at 119. The figure shows the indicator end 121 of the measuring pin 120 projecting out of the head 130. The figure shows a PCB 132 attached to the head's longitudinal end face 131 , having a through hole 133 aligned with the central bore 113, and carrying a sensor 140 positioned adjacent indicator end 121 of the measuring pin 120. It can be seen that the indicator end 121 of the measuring pin 120 extends through the hole 133 in PCB 132.

[0063] The sensor 140 is fixated to the head 130, either directly or, as in the example shown, via the intermediate PCB 132. The sensor 140 is configured to sense the relative position of the indicator end 121 of the measuring pin 120, which, as explained in the above, is indicative for the tension in the bolt 101 . Any change in this relative position is caused by elongation or shrinking of the portion of the bolt 101 extending between the axial position of the indicator end 121 of the measuring pin 120 and the axial position of the anchor end 122 of the measuring pin 20; this portion will be indicated as the length-variable portion.

[0064] In a preferred embodiment, the sensor 140 comprises a Hall sensor, and the indicator end 121 of the measuring pin 120 carries a small magnet or magnetisation 141 . Any shrinking or extension of the bolt 101 will translate to a relative longitudinal displacement of the magnet 141 along the Hall sensor 140. It will be recognized that the configuration of the magnet and sensor as shown is in accordance with the configuration illustrated in figures 1 and 4A-4C, with longitudinal displacement of the magnet being a shearing displacement. An output signal of the Hall sensor 140 will be linearly proportional to said displacement.

[0065] In a preferred embodiment, the measuring pin 120 is made from a nonmagnetic material having a magnet arranged at its tip. This will be advantageous as the magnetic field will be more concentrated and the measuring accuracy will be better. If the thermal expansion coefficient of the measuring pin 120 differs from the thermal expansion coefficient of the shaft of the bolt, this can be compensated for by measuring the temperature and calculating a correction in a processor or the like; however, it is preferred that the material of the measuring pin 120 is selected to have substantially the same thermal expansion coefficient as the shaft of the bolt. The material of the measuring pin 120 may even comprise a plastic. Considering that bolts are typically made from steel, the material of the measuring pin 120 preferably includes a non-magnetic stainless steel.

[0066] The Hall sensor 140 can be a standard component, which has properties of reliability and linearity while at the same time being small and relatively low-cost. This standard component only requires low-voltage supply, and already provides an output signal that can be directly processed by Voltage measuring means, or a computer or processor, without further electronics being necessary. For wired connection, a connector can be added to the PCB.

[0067] In a possible embodiment, the sensor 140 is provided with a battery or supercap to provide supply power. In such case, only a single output wire would be needed. The battery or supercap can for instance be charged via the communication wire.

[0068] In a possible embodiment, the sensor 140 is provided with a memory chip attached to the PCB. In such case, it is possible to continuously (or at fixed intervals) perform measurements and to store the results in the memory, for later output to the outside world.

[0069] In a possible embodiment, the sensor 140 is provided with a small coil attached to the PCB. In such case, it is possible to communicate wirelessly, for outputting measurement results and / or for charging the battery.

[0070] In a possible embodiment, the sensor 140 is provided with a microprocessor, to control the sensor and / or to perform calibration of the sensor and / or to compensate for temperature variations and / or to store measurement results and / or to output measurement results.

[0071] In a possible embodiment, the bolt 101 is provided with a housing 150 covering the sensor 140 and other possible electronic components, for protection. In a preferred elaboration, this housing is filled with fast-setting and / or UV-setting resin, such as to be watertight. It is noted that watertight resin may also be applied without such housing.

[0072] Figure 8 shows a detail of the anchor end 122 of the measuring pin 120 at a larger scale, for a preferred embodiment of the bolt 101 . In this preferred embodiment, the central bore 113 extends over the entire axial length of the bolt 101 , opening out into a free end face 114 of the shaft. Over a portion adjacent to the free end face 114, the central bore 113 is provided with internal threading 115. The anchor end 122 of the measuring pin 120 is, over a certain length, provided with external threading 123 that mates with said internal threading 115. The anchor end 122 may be provided with a shaping 118, for instance a central recess with hexagonal or torx contour, as shown, to allow a corresponding tool to engage with the measuring pin 120 to rotate it with respect to the shaft 110. Thanks to the mating threadings, it is possible to displace the measuring pin 120 axially in the shaft 110 to accurately adjust its axial position. If the position is considered correct, or optimal with respect to the sensor, it is possible to fixate the measuring pin 120 with respect to the shaft 110, if desired, for instance by glue, introduced into the central bore 113 from the side of the free end face 114.

[0073] It is noted that the rotation of the measuring pin 120 will entail a rotation of the magnet 141 (or magnetized tip portion) with respect to the Hall sensor 140. If the magnetic axis of the magnet 141 (or magnetized tip portion) is coaxially aligned with the measuring pin 120, in view of the rotational magnetic symmetry, the rotation as such has little or no effect on the sensor signal. Also, if the Zs axis of the sensor is coaxially aligned with the measuring pin 120, as in the illustration of figure 3A, rotation of the magnet has little or no effect on the sensor signal, even if the magnetic axis of the magnet 141 is perpendicular to the Zs axis of the sensor.

[0074] An important challenge of the present invention is to accurately measure small displacements of the indicator end of the measuring pin 120 with respect to the body of the bolt 101 . For this, it is important to have a sensor 140 that has high sensitivity to small displacements, which is provided by the combination of a Hall-sensor and a magnetized tip of the indicator end 121 of the measuring pin 120. The magnetization may be intrinsic in the indicator material, but may also be embodied in a separate magnetic element attached to the indicator end 121 of the measuring pin 20.

[0075] Further, it is important to be able to accurately position the sensor 140 and the indicator end 121 of the measuring pin 120 with respect to each other. This can be done by adjusting the axial position of the sensor 140 with respect to the body of the bolt, for instance by positioning one or more thin spacer sheets under the sensor. But the embodiment discussed above allows for very easy and accurate, continuous positioning without being restricted to positioning steps equal to the thickness of spacer sheets.

[0076] In some embodiments, especially when the measuring pin 120 is thin, it may be that the device is sensitive to vibrations which have influence on the output signal. If such is not desirable, it may be possible to avoid such vibrations by providing a low-friction bearing around the free end of the measuring pin 120. Such bearing may for instance be positioned in the hole 133. In order to reduce friction as much as possible, the measuring pin 120 may for instance have a portion of reduced diameter, for instance at a position close to the anchor end 122, which will reduce stiffness and hence reduce the force at which the measuring pin 120 can lean against the bearing.

[0077] The inventor has tested the measuring properties of a bolt as described above, and found that these properties are exceptionally good. A bolt of type M10x40 was prepared in accordance with figures 5-7, with the setup of figure 4A. The magnet used was a neodymium disc of 3 mm diameter and 1 mm thickness. The distance between magnet and Hall sensor surface was between 0.1 and 0.2 mm. The Hall sensor used was a standard linear programmable Hall sensor, which was supplied with 5 V supply voltage, and provided an output signal between 1 and 5 V. The bolt was arranged in a dedicated test bench capable of applying pulling force from 0 - 5 kN. A high quality force sensor was used to measure the actual force applied on the bolt. In a plot, the measuring result of Hall sensor output signal versus actual force appeared as a straight line with a sensitivity of 0.28471 V / kN and a correlation coefficient of 0.999816. The deformation of this bolt was calculated as 2.8 pm / kN, hence the sensor sensitivity can be expressed as 9.8 V / pm. The non-linearity, i.e. the deviation of the measured Hall sensor output signal with respect to said straight line, was never more than 5 mV, roughly corresponding to 17 N; this maximum deviation was achieved at a pulling force of 2 kN.

[0078] In the case of bolts, the stress normally is longitudinal stress, and the above description has been given mainly in relation to the desire to measure longitudinal deformation. Bolts can however also be subjected to transverse stress, and it is also possible to have a bolt in which the sensor arrangement is such as to be able to measure bending and shearing. As compared to the arrangement illustrated in figure 7, an alternative arrangement for sensing bending and shearing in particular can be achieved by rotating the sensor over 90 degrees and mounting it in alignment with the measuring pin 120, such as to achieve a configuration similar to the configuration illustrated in figures 3A-3C. If the Hall sensor used is a 2D sensor or a 3D sensor, it is even possible to determine the bending direction in 360 degrees around the central axis of the bolt. In another approach, again starting with the arrangement illustrated in figure 7, an alternative arrangement for sensing bending and shearing in addition to elongation can be achieved by adding a second Hall sensor and mounting it displaced over 90 degrees with respect to the first sensor. Although the inventor has made the present invention in the context of the desire to monitor the stress in bolts, the inventor realizes that his solution is not restricted to use in bolts but can be used in other fields as well.

[0079] More generally, there is a desire to measure displacements in a mechanical construction. Such displacements may involve the displacement of one component with respect to another component, or one position with respect to another position. The displacements may be representative for defects in the construction, such as cracks, which allow mutual displacement where normally no mutual displacement would be expected. The displacements may however also be representative for normal behaviour as caused by temperature, or for normal deformation as caused by stress. As such, measuring the displacement would be equivalent to measuring the stress, which in turn would be equivalent to measuring the forces exerted on or by the construction concerned.

[0080] The desire may be for a sensor device that is intended to be attached to the mechanical construction concerned.

[0081] The desire may also be for a sensor that is integrated in the mechanical construction concerned.

[0082] If the device 1 of figures 3-4 is a measuring instrument, the end walls 31 , 32 may be provided with attachment means for attaching the instrument to a mechanical structure to be monitored. For sake of simplicity, such attachment means are not illustrated. The attachment means may for instance comprise eyelets, for screwing the measuring instrument to the mechanical structure to be monitored, for instance a building. For attaching the instrument to a structure that comprises steel, or is primarily made from steel, for instance a bridge, the attachment means may for instance comprise welding tabs for welding to the structure.

[0083] It is however also possible that the device 1 is an object with integrated sensor element 10.

[0084] In the above explanation, the combination of an indicator pin and a Hall sensor is applied inside an object (i.e. a bolt or a tube) of which it is intended to measure longitudinal or transverse deformation as being representative for stress variations. Alternatively, it is also possible to have such an indicator pin extend at the outside such object. This is particularly if the nature of the object makes it inconvenient, difficult, or even impossible to arrange a longitudinal hole inside the object. As an example, a cable is mentioned here.

[0085] Cables, particularly steel cables, are used in various structures. By way of example, towing cables for towing ships, suspension cables for suspension bridges or cable cars or cranes. But also trunk cables for use in communication. For such cables, it may be desirable to monitor tension I elongation. In suspension bridges, the suspension elements may be steel bars or pipes instead of cables, and these may need to be monitored as well. In fact, in any structure where tension cables or tension bars are used, monitoring the tension may be useful. According to the invention, it is favourable to replace the measuring pin by a tube.

[0086] Figure 9 schematically illustrates an embodiment of the present invention specifically adapted to this situation. A measuring device is generally indicated at reference numeral 200. Measuring device 200 comprises a tubular body 220, indicated as measuring tube, which corresponds to the reference element 24 discussed above. The measuring tube 220 has one extremity 221 indicated as "indicator end". Opposite the indicator end 221 , the measuring tube 220 has a second extremity 222 that will be indicated as "anchor end". The measuring tube 220 is arranged around a cable C. The anchor end 222 is attached to the cable C. At the indicator end 221 , a Hall sensor 240 is attached to the measuring tube 220, for instance inside the measuring tube 220. Aligned with the Hall sensor 240, a magnet 241 is attached to the cable C. The magnet 241 may be held by an annular support arrangement around the cable C, and / or the magnet 241 may be annular itself.

[0087] Again, while the cable may be subjected to varying tension and thus varying elongation, the length of the measuring tube 220 will remain constant and the magnet will be longitudinally displaced with respect to the Hall sensor 240. The length of the measuring tube 220 may be many times more than the length of a bolt as described above, hence the expected measuring signal may be expected to have higher magnitude.

[0088] For attaching, servicing and removing the measuring tube 220, it would be convenient if the tube comprises two semi-circular tube segments attached to each other.

[0089] The object of which deformation (for instance length variation) is measured can be used as a weight sensor. It can be subjected to tensile stress by a suspended weight or to compression stress by a supported weight.

[0090] The object of which deformation is measured can be used as a deformation sensor, for instance for detecting deformation or even structural failure in structures such as buildings and bridges. If these measuring components are small and low- cost, collapses of such structures can more easily be predicted and hence prevented.

[0091] In the case of a bolt, as discussed above, the stress applied to a bolt will substantially be longitudinal stress only. A device with integrated sensor element may however also be subjected to substantial transverse stress, i.e. bending. By way of example, the device 1 may involve a handle bar of a push cart, emergency door, hospital bed, or the like, that may be provided with a motor. When the handle bar is pushed in transverse manner, it will slightly bend, resulting in a signal at output 14 allowing a processor to actuate the motor for driving the push cart, emergency door, hospital bed, or the like, respectively. Thus, the device 1 may be part of a control arrangement actuated by the measured deformation, be it longitudinal or transversal.

[0092] In another example, the device 1 may involve a robotic arm or an actuator. Typically, such arms or actuators are designed to be stiff to ensure positional accuracy. But with the integrated sensor arrangement proposed by the present invention, it is possible to measure any deformation of such arm or actuator and perform a necessary correction. Hence, stiffness requirements can be lowered, and the arm or actuator can be made from a cheaper material, for instance plastic.

[0093] A handle bar, as described above, may be mounted at its two opposite ends. In such case, the measuring sensor 140, 141 may be arranged approximately midway between those two opposite ends. In a possible example, however, an actuator arm is mounted at one end and is manipulated by a user at the opposite free end. In such case, the measuring sensor 140, 141 may be arranged at the free end, but it may be more convenient if the measuring sensor 140, 141 is arranged at the mounted end.

[0094] In all cases where it is desired to measure the transverse stress and / or deformation of a tubular device, that stress and / or deformation may in general have a direction perpendicular to the axial direction with a potential freedom of 360° about that axis. With a measuring arrangement such as illustrated in figures 4A and 4C, it is important that the orientation of the device corresponds to the deformation direction to be expected. However, with a measuring arrangement such as illustrated in figures 3A and 3C, the displacement of the magnet 11 is always parallel to the sensing face 13 of the Hall sensor 12, irrespective of the mounting orientation of the device 1 .

[0095] In a particular embodiment, it may be desirable that the response caused by the sensor does not depend on the radial direction of the transverse stress and / or deformation of the tubular device. In such case, it is convenient to apply a Hall sensor with a circular symmetrical sensitivity characteristic, i.e. a Hall sensor that has a single output of which the signal magnitude does not depend on the radial direction of the transverse stress and / or deformation of the tubular device.

[0096] It may however also be desirable to detect the radial direction of the transverse stress and / or deformation of the tubular device, so that a controlled response can have a corresponding direction. In such case, it would be convenient to apply a Hall sensor with a 2D sensitivity characteristic, i.e. a Hall sensor of which the output signal depends on the radial direction of the displacement of the magnet with respect to the sensor face. In a possible embodiment, the Hall sensor comprises an array of sensor units comparable to pixels. In another possible embodiment, the Hall sensor comprises an arrangement of a first central sensor unit, a second sensor unit displaced with respect to the first central sensor unit in a first direction, and a third sensor unit displaced with respect to the first central sensor unit in a third direction perpendicular to the first direction. In devices with such 2D Hall sensor, the mounting orientation of the device is not critical. It is also possible to apply a Hall sensor with a 3D sensitivity characteristic.

[0097] In a particular embodiment, the device with integrated sensor element is an axle in the drive train of a bicycle. Such axle may be the wheel axle of the driven wheel, typically the rear wheel. Such axle may also be the bottom bracket axle or pedal axle. Such integrated sensor allows to measure accurately the pedal force exerted by the cyclist. In an exercise bike, this allows to calculate the torque, and hence the power produced by the cyclist. In a bicycle with electric assist motor, this allows a control unit to control the assist motor to give assist power proportional to the pedal force exerted by the cyclist. Such bicycles are known, for instance from EP-1324913, but they involve piezo detectors or strain detectors mounted on the outside surface of the axle concerned. The present invention proposes to arrange a measuring pin inside the axle concerned.

[0098] It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. For instance, two or more functions may be performed by one single entity.

[0099] Although the use of a Hall sensor is preferred, it is possible to use other types of magnetic sensors, i.e. a sensor sensitive to magnetic fields, for instance a magneto-resistive sensor, a magneto-inductive sensor, a Fluxgate magnetometer, a GMR-sensor, a Fluxgate magnetometer. If it is only desired to detect when a bolt has been tightened to a certain desired stress, it is even possible to use a Reed-contact.

[0100] Even if certain features are recited in different dependent claims, the present invention also relates to an embodiment comprising these features in common.

[0101] For sake of convenience, an exemplary embodiment has been discussed and shown that has various features. The mere fact that these features are illustrated in one and the same embodiment illustrates that such features can be combined but does not mean that these features must always be present in combination with each other; unless explicitly stated otherwise, the present disclosure is to be considered as disclosing such features individually so that embodiments are possible in which one or more of these features are omitted.

[0102] Features which have not been explicitly described as being essential may also be omitted. Any reference signs in a claim should not be construed as limiting the scope of that claim.

Claims

CLAIMS1 . Bolt (101 ) with a cylindrical shaft (110), which at one end is provided with a head (130), and which is provided with a threading (111 ) over at least a part of its length; wherein the shaft (110) is provided with a central bore (113), extending axially from the head (130) over at least a part of the axial length of the bolt (101); wherein the bolt is provided with a measuring pin (120) located inside the bore (113) and extending substantially parallel to the shaft (110), the measuring pin (120) having an anchor end (122) fixated with respect to the shaft and having an opposite free indicator end (121 ); wherein the bolt is further provided with a sensor element (140, 141 ) comprising a first sensor component (141 ) in or at the free indicator end (121 ) of the measuring pin(120) and a second sensor component (140) fixated with respect to the shaft (110) and aligned with the first sensor component (141 ); wherein the first sensor component comprises a magnet (141 ) attached to the indicator end (121 ) of the measuring pin (120) or a magnetization of the indicator end(121 ) of the measuring pin (120); and wherein the second sensor component (140) comprises a magnetic sensor.

2. Bolt according to claim 1 , wherein the magnetic sensor (140) is positioned on the head (130), wherein the indicator end (121 ) of the measuring pin (120) protrudes out of the bore (113) to be aligned with the magnetic sensor (140).

3. Bolt according to claim 1 or 2, wherein the magnetic sensor (140) comprises a Hall sensor, possibly a 2D Hall sensor or 3D Hall sensor.

4. Bolt according to claim 1 or 2 or 3, wherein the sensor (140) has a sensing direction (Zs); wherein the sensor (140) is positioned adjacent the bore (113); and wherein the sensor (140) is positioned such that its sensing direction (Zs) is perpendicular to the axial direction of the bore (113).

5. Bolt according to claim 1 or 2 or 3, wherein the sensor (140) has a sensing direction (Zs); wherein the sensor (140) is positioned aligned with the bore (113); andwherein the sensor (140) is positioned such that its sensing direction (Zs) is coaxial with the measuring pin (120).

6. Bolt according to any of the previous claims, wherein the sensor (140) has a sensing direction (Zs); wherein the magnet (141 ) or magnetization, respectively, has a magnetic axis perpendicular to the sensing direction (Zs).

7. Bolt according to any of the previous claims, wherein the magnet (141 ) or magnetization, respectively, has a magnetic axis coaxial with the measuring pin (120).

8. Bolt according to any of the previous claims, wherein the central bore (1 13) is provided with internal threading (1 15), and wherein the anchor end (122) of the measuring pin (120) is provided with external threading (123) that is mating with said internal threading (1 15), to allow accurate adjustment of the axial position of the free indicator end (121 ) of the measuring pin (120) with respect to the shaft (1 10).

9. Bolt according to claim 8, wherein the free end face of the anchor end (122) of the measuring pin (120) is provided with a profiled recess or a profiled protrusion for engagement of a rotation tool for adjusting the axial position of the indicator end of the measuring pin (120) with respect to the shaft (1 10).

10. Bolt according to any of the previous claims, the bolt further comprising one or more of the following features:- the sensor (140) is mounted on a PCB (132) attached to the head's longitudinal end face (131 );- the sensor is provided with a battery or supercap to provide supply power;- the sensor is provided with a coil;- the sensor is provided with a memory chip;- the sensor is provided with a microprocessor, programmed to control the sensor and / or to perform calibration of the sensor and / or to compensate for temperature variations and / or to store measurement results and / or to output measurement results;- the output signal of the sensor is proportional to length variations of the bolt and / or to tensile stress or compressive stress in the bolt;-the output signal of the sensor is proportional to bending and / or shear of the bolt and / or to transverse stress in the bolt;- the bolt is provided with a housing (150) covering the sensor (140) and other possible electronic components, for protection;- at least the sensor (140) is covered by a resin, such as to provide a watertight protection;- the measuring pin is made from a non-magnetic material, preferably a nonmagnetic stainless steel.11 . Method for measuring deformation of a bolt and / or stress in a bolt, the method comprising the steps of: providing a measuring pin (120) having a free indicator end (121 ) that is magnetized or carries a magnet; arranging the measuring pin in a central axial bore (113); using a magnet sensor, preferably a Hall sensor, fixated with respect to the shaft (110) and / or the head (130) of the bolt, to sense displacement of the magnet or magnetized indicator end (121 ), respectively.

12. A tubular device having arranged therein a measuring device comprising:- a measuring body (220) having an anchor end (222) fixated with respect to the tubular body (C) and having an opposite free indicator end (221 );- a sensor element comprising a first sensor component (240) in or at the free indicator end (221 ) of the measuring body (220) and a second sensor component (241 ) attached to the tubular body (C) in alignment with the first sensor component.- wherein the first sensor component comprises a magnet attached to the indicator end (221 ) of the measuring body (220) or a magnetization of the indicator end (221 ) of the measuring body (220);- and wherein the second sensor component (140) comprises a magnetic sensor, preferably a Hall sensor; wherein the tubular device can be : a robotic arm, an actuator, a handle bar of a push cart that is provided with a motor actuated by the handle bar, or a handle bar of an emergency door that is provided with a motor actuated by the handle bar, or a handle bar of a hospital bed that is provided with a motor actuated by the handle bar.

13. Measuring device for measuring deformations in an elongate subject body (C), the measuring device comprising:- a tubular measuring body (220) adapted to be arranged around the elongate subject body (C), substantially parallel to the elongate subject body (C), having an anchor end (222) adapted to be fixated with respect to the elongate subject body and having an opposite free indicator end (221 ), wherein the tubular measuring body (220) preferably is implemented as two semi-circular tube segments attached to each other;- a sensor element comprising a first sensor component (240) in or at the free indicator end (221 ) of the measuring body (220) and a second sensor component (241 ) adapted to be attached to the elongate subject body in alignment with the first sensor component.- wherein the first sensor component comprises a magnetic sensor, preferably aHall sensor, attached to the indicator end (221 ) of the measuring body (220);- and wherein the second sensor component (240) comprises a magnet attached to the elongate subject body (C).

14. Suspension bridge or suspension crane, comprising at least one suspension cable and / or at least one suspension bar and / or at least one suspension tube, and a measuring device according to claim 13 of which the tubular measuring body (220) is arranged around the at least one suspension cable / bar / tube, and of which the magnet is attached to the at least one suspension cable / bar / tube.

15. Cable car arrangement comprising at least one suspension cable and a measuring device according to claim 13 of which the tubular measuring body (220) is arranged around the at least one suspension cable, and of which the magnet is attached to the at least one suspension cable.

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