Displacement sensor, force sensor, stress sensor, and bolt with integrated stress sensor

Abstract

A device (1) for measuring length variations comprises: - an elongate reference body (10); - a Hall sensor (40) fixated with respect to the elongate reference body; and - a measuring pin (20) extending substantially parallel to the elongate reference body, having an indicator end (21) aligned with the Hall sensor and having an opposite anchor end (22) fixated with respect to the elongate reference body, with the elongate reference body having a variable-length portion extending adjacent the measuring pin between the indicator end and the anchor end. TO PUBLISH WITH ABSTRACT: figure 6

Description

ref.: P 2024 TITLE: Displacement sensor, force sensor, stress sensor, and bolt with integrated stress sensor FIELD OF THE INVENTION The present invention relates in general to measuring 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. In one type of implementation, a sensor device is provided, that is intended to be attached to the mechanica! construction concerned. In a second type of implementation, a sensor is integrated in the mechanical construction concerned. In a particular embodiment of the second type of implementation, the present invention provides a bolt with integrated stress sensor. Hereinafter, the present invention will be specifically explained in more detail for such embodiment. BACKGROUND OF THE INVENTION 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. 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. 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. SUMMARY OF THE INVENTION 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. 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. 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. 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. An object of the present invention is to provide a bolt with integrated stress 5 sensor in which at least some but preferably all of the above-mentioned disadvantages are overcome or at least reduced. 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. 10 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 signa! 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 15 and / or bending of the bolt. 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. Particularly, the present invention aims to provide a bolt with integrated stress 20 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. Furthermore, the present invention aims to provide a bolt with integrated stress sensor that can easily be equipped with a memory for data storage. More generally, the present invention aims to provide a stress sensor having 25 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. BRIEF DESCRIPTION OF THE DRAWINGS 30 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: Figures 1 and 2 schematically illustrate measurement principles; 35 Figures 3A-C and Figures 4A-C schematically illustrate two implementing embodiments; Figure 5 schematically illustrates a particular embodiment being a bolt; Figure 6 is a longitudinal section of the bolt of figure 5; Figure 7 schematically shows a detail of a bolt according to the present invention; Figure 8 schematically shows another detail of a bolt according to the present invention, Figure 9 schematically shows another implementing embodiment, specifically suitable for measuring tension in a cable. DETAILED DESCRIPTION OF THE INVENTION 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 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. 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, 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, the variation of the magnetic field varies linearly with the displacement in a good approximation, which makes it easier to provide a linear output signal. 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. 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. 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 signa! 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. 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 signa! 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. 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. 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. It is possible that the positions of the magnet 11 and the Hall sensor 12 are interchanged. 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. Figure 3B illustrates, in exaggerated manner, elongation of the tubular sensing element 23 if it is subjected to longitudinal stress Pl. 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. 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 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, but may also be in accordance with the setup illustrated in figure 1. 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. 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. 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, but may also be in accordance with the setup illustrated in figure 2. Figure 4B illustrates, in exaggerated manner, elongation of the tubular sensing element 23 if it is subjected to longitudinal stress Pl. It can be seen that the magnet 11 has been shifted with respect to the Hall sensor 12. 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. 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 mechanica! 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. It is however also possible that the device 1 is an object with integrated sensor element 10. 5 An important embodiment of the present invention is a bolt with integrated stress sensor. Figure 5 schematically shows a side view of an exemplary bolt 101 with integrated stress sensor, and figure 6 schematically shows a longitudinal cross 10 section of this bolt 101. The bolt 101 comprises a cylindrical body 110, also indicated as shaft, which at one end is provided with a head 130, in the example shown a hexagonal head. The shaft 110 is provided with a threading 111 over at least a part of its length; in the example shown, a portion 112 is without threading. 15 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. 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 20 indicator end 121, the measuring pin 120 has a second extremity 122 that will be indicated as "anchor end". 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, 25 i.e. the internat 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. 30 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 35 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. 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 5 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. In a preferred embodiment, the sensor 140 comprises a Hall sensor, and the 10 indicator end 121 of the measuring pin 120 carries a smal! magnet or magnetisation 141. It will be recognized that the arrangement is in accordance with figure 4A. Any shrinking or extension of the bolt 101 will translate to a relative longitudinal displacement of the magnet 141 along the Hall sensor 140. An output signal of the Hall sensor 140 will be linearly proportional to said displacement. 15 In a preferred embodiment, the measuring pin 120 is made from a non- magnetic 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 20 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 25 includes a non-magnetic stainless steel. 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 30 computer or processor, without further electronics being necessary. For wired connection, a connector can be added to the PCB. 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 35 wire. 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. 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. 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. 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. 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. 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. 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. 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. 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 signa! 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. 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. 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 is inconvenient, difficult, or even impossible to arrange a longitudinal hole inside the object. As an example, a cable is mentioned here. Cables, particularly steel cables, are used in various structures. By way of example, towing cables for towing ships, suspension cables for suspension bridges 5 or cable cars or cranes. But also trunk cables for use in communication. For such cables, it may be desirable to monitor tension / 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 10 invention, it is favourable to replace the measuring pin by a tube. 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 15 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 20 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. 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 25 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 signa! may be expected to have higher magnitude. For attaching, servicing and removing the measuring tube 220, it would be 30 convenient if the tube comprises two semi-circular tube segments attached to each other. 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 35 weight or to compression stress by a supported weight. 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. In the case of the exemplary embodiment 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 5 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 10 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. In another example, the device 1 may involve a robotic arm or an actuator. 15 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. 20 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 25 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. 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 30 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 35 13 of the Hall sensor 12, irrespective of the mounting orientation of the device 1. 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. 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 centra! 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 centra! 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. In a particular embodiment, the device with integrated sensor element is an axie in the drive train of a bicycle. Such axle may be the wheel axie of the driven wheel, typically the rear wheel. Such axle may also be the bottom bracket axie or peda! 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 axie concerned. The present invention proposes to arrange a measuring pin inside the axle concerned. 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. Even if certain features are recited in different dependent claims, the present invention also relates to an embodiment comprising these features in common. 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. Features which have not been explicitly described as being essential may also be omitted. 5 Any reference signs in a claim should not be construed as limiting the scope of that claim.

Claims

1 Measuring device for measuring deformations, comprising: - an elongated reference body (110); - a measuring body (120) that extends mainly parallel to the elongated reference body (110), with an anchor end (122) fixed at relative to the elongated reference body and with an opposite located free indicator end (121); - a sensor element comprising a first sensor component in or near the free indicator end (121) of the measuring body (120) and a second sensor- component attached to the elongated reference body and is aligned with the first sensor component.

2. Apparatus in accordance with claim 1, where the sensor element is a Hall sensor includes, optionally, a 2D Hall sensor or a 3D Hall sensor.

3. Configuration in accordance with claim 1 or 2, where the second sensor component is a comprises a Hall sensor and where the first sensor component is attached to the The indicator end (121) of the measuring body (120) includes the attached magnet (141). or magnetization of the indicator end (121) of the measuring body (120).

4. Arrangement in accordance with any of the preceding claims, whereby the device a bolt (101) with a cylindrical shank (110) that the reference body, which is provided with a head at one end (130), and which over at least part of its length is threaded (111).

5. Layout in accordance with claim 4, where the shaft (110) is equipped with a central bore (113), which extends axially from the head (130) over at least one part of the axial length of the bolt (101), and where the measuring body (120) a is the measuring pin located inside the bore (113).

6. Apparatus according to claim 5, where the sensor (140) is located on the head (130) near the bore (113), where the indicator end (121) of the measuring pin (120) protrudes from the bore (113) to be aligned with the sensor (140).

7. Device in accordance with claim 6, where the sensor (140) is mounted on a printed circuit board (132) attached to the longitudinal end face (131) of the head.

8. Arrangement in accordance with any of the preceding claims 3-7, whereby the The sensor is equipped with a battery or supercapacitor to supply power.

9. Arrangement in accordance with any of the preceding claims 3-8, whereby the The sensor is equipped with a coil.

10. Arrangement in accordance with any of the preceding claims 3-9, whereby the The sensor is equipped with a memory chip.

11. Arrangement in accordance with any of the preceding claims 3-10, whereby The sensor is equipped with a microprocessor, programmed to connect the sensor to to send and / or to perform calibration of the sensor and / or to compensate for temperature variations and / or to store measurement results and / or to to deliver measurement results.

12. Arrangement in accordance with any of the preceding claims 3-11, whereby the output signal of the sensor is proportional to length variations of the bolt and / or to tensile or compressive stress in the bolt.

13. Arrangement according to any of claims 1-12, whereby the elongated reference body (110) is equipped with a central bore (13) which is provided with internal thread (15), and where the anchor end (122) of the measuring body (120) is equipped with external thread (123) that fits mentioned internal thread (15), for precise adjustment of the axial position of the free indicator end (121) of the measuring body (120) relative to to make it possible of the reference body (110).

14. Arrangement in accordance with any of the preceding claims, provided with a housing that the sensor (140) and any other electronic components covers, for protection.

15. Arrangement in accordance with any of the preceding claims, whereby at least the sensor (140) is covered by a resin, in order to ensure a waterproof to offer protection.

16. Measuring device for measuring deformations, comprising: - an elongated reference body (110); - a sensor fixed with respect to the elongated reference body (40); - a measuring body (120) that extends mainly parallel to the elongated reference body (110), with an indicator end aligned with the sensor and with an opposite anchor end that is fixed at relative to the elongated reference body, where the elongated reference body has a deformable part that extends next to the measuring body between the indicator end and the anchor end; where the sensor (40) is configured to generate an output signal that is proportional to the displacement of the indicator end of the measuring- body relative to the sensor and thus proportional to deformation of the deformable part of the elongated reference body; where the elongated reference body (10) and the anchor end of the measuring body (20) equipped with mutually cooperating screw- wires (15; 23), for precise adjustment of the axial position of the indicator end of the measuring body (20) relative to the to make elongated reference body (10) possible.

17. Apparatus according to claim 16, where the sensor (40) comprises a Half sensor.

18. Design according to conclusion 17, whereby the indicator end (21) of the measuring body (20) carries a small magnet (41).

19. Arrangement according to any of the claims 16-18, whereby the elongated reference body (10) is provided with a bore (13), which is axially extends over at least a portion of the axial length of the deformable part of the elongated reference body (10), and where the measuring body (20) a measuring pin located at least partially inside the bore (13) (20) includes 20. Arrangement according to any of the claims 16-19, whereby the mentioned interacting screw threads (15; 23) internal screw thread (15) of the bore (13) and external thread (23) of the anchor end of the include measuring body (20).

21. Arrangement according to any of the conclusions 16-20, whereby the free The end face of the anchor end of the measuring body (20) is provided with a profiled recess or a profiled projection for engagement with a Rotary tool for adjusting the axial position of the indicator end of the measuring body (20) relative to the elongated reference body (10).

22. Deformation sensor for detecting deformation of an object, comprising an arrangement according to any of claims 16-21, whereby it elongated reference body (10; 110) is equipped with fasteners for attaching the elongated reference body (10; 110) to the object whose deformation must be measured.

23. Distortion sensor within the meaning of claim 22, where the fixing agents are located axially aligned with the deformable part of the elongated reference body (10; 110), but outside this deformable part.

24. Weight sensor, comprising a measuring device of any of the claims 1-21 or a distortion sensor according to any of the claims 22-23.

25. Measuring device for measuring deformations in an elongated object body (C), which measuring device comprises: a measuring body (220) that is adapted to be set up in main thing parallel to the elongated object body (C), and that has an anchor end (222) that is adapted to be fixed relative to the elongated object body and an opposite located free indicator end (221); - a sensor element comprising a first sensor component (240) in or near the free indicator end (221) of the measuring body (220) and a second sensor component (241) modified to be attached to the elongated object body in alignment with the first sensor- component 26. Measuring device within the meaning of claim 25, where the elongated object body is a tubular body and the measuring body is adapted to be placed within the object body.

27. Tubular apparatus, comprising a measuring device according to a arbitrary of conclusions 1-21 or a distortion sensor according to an arbitrary of claims 22-23 or a measuring device according to any of the claims 25-26.

28. Tubular apparatus within the meaning of claim 27, where the apparatus is is a robot arm.

29. Tubular device within the meaning of claim 27, where the device is an actuator is.

30. Tubular apparatus within the meaning of claim 27, where the apparatus is handlebar is of a pushcart, or an emergency door, or a hospital bed, or such, equipped with a motor operated by the handlebar.

31. Tubular apparatus within the meaning of claim 27, where the apparatus is a shaft in the drivetrain of a bicycle, for example the wheel axle of the driven wheel, typically the rear wheel, or the bottom bracket or pedal axle.

32. Bicycle comprising pedals and a wheel driven by the pedals, whereby the driven wheel as a tubular device according to claim 27 implemented wheel axle comprises and / or where the pedals have a tubular shape device in accordance with claim 27 includes pedal shaft implemented.

33. Measuring device within the meaning of claim 25, where the measuring body (220) is a tubular body is that is adapted to be fitted around the object body (C).

34. Measuring device within the meaning of claim 33, where the tubular measuring body (220) is implemented as two semicircular tube segments that are joined together have been confirmed.

35. Measuring device within the meaning of claim 33 or 34, where the object body (C) a cable is.

36. Suspension bridge comprising at least one suspension cable and / or at least one support rod and / or at least one support tube, and a measuring device according to claim 33 or 34 of which the tubular measuring body (220) is fitted around the ten at least one support cable / rod / tube.

37. Cable cabin installation comprising at least one support cable and a measuring device according to claim 33 or 34 of which the tubular measuring body (220) is fitted around at least one suspension cable.

38. Overhead crane comprising at least one suspension cable and / or at least one support rod and / or at least one support tube, and a measuring device according to claim 33 or 34 of which the tubular measuring body (220) is fitted around the ten at least one support cable / rod / tube.

39. Arrangement in accordance with any of the preceding claims, whereby the 5 measuring body is made of a non-magnetic material, preferably non- magnetic stainless steel.

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