WO2010071607A1

WO2010071607A1 - Method and system for monitoring fastener loads

Info

Publication number: WO2010071607A1
Application number: PCT/SG2009/000476
Authority: WO
Inventors: Narasimalu Srikanth
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2008-12-17
Filing date: 2009-12-11
Publication date: 2010-06-24
Anticipated expiration: 2011-06-17

Abstract

A method and system for monitoring fastener loads, a fastener, a method of fabricating a fastener sensor, and a wind turbine. The fastener comprises a head for abutting a mounting surface of a medium; a shaft for engaging the medium; and a piezoelectric sensor integrated into the head for monitoring loads on the fastener.

Description

WIETHOD AND SYSTEM FOR MONITORING FASTENER LOADS

FIELD OF INVENTION

The present invention relates broadly to a method and system for monitoring fastener loads, to a fastener, to a method of fabricating a fastener sensor, and to a wind turbine.

BACKGROUND

Tensioned bolts under cyclic loads such as vibration, thermal variation and the like have been known to stretch and thus loosen over time. Looseness will result in reduction of the joint stiffness and the overall load carrying capacity of the bolted joint. It is thus required to monitor bolt loads.

Commonly, to monitor the looseness of a bolt, a torque wrench has to be manually used on every bolt. This is both tedious and time-consuming. Moreover, bolts that are located at inaccessible places make this process impractical.

Ia order to overcome the abovementioned disadvantages, bolt sensors may be employed instead. Presently, bolt sensors may be of a washer-type and are sandwiched between the bolt and the joint. However, this affects the stiffness of the joint as it directly affects the force path between the bolt and the joint.

Alternatively, external bolt sensors may be employed and brought into contact with, for example, the bolt head during manual monitoring. However, like manual monitoring using a torque wrench, this is oftentimes cumbersome and may not be suitable for applications where space constraint is a critical factor.

A need therefore exists to provide a method and system for monitoring fastener loads, a fastener, a method of fabricating a fastener sensor, and a wind turbine that seek to address at least one of the above-mentioned problems. SUMMARY

In accordance with a first aspect of the present invention there is provided a fastener comprising a head for abutting a mounting surface of a medium; a shaft for engaging the medium; and a piezoelectric sensor integrated into the head; for monitoring loads on the fastener.

Said piezoelectric sensor may be disposed in a cavity within the head.

Said piezoelectric sensor, in use, may generate a longitudinal standing wave in the shaft.

Said piezoelectric sensor may comprise a piezoelectric material sandwiched between a pair of electrodes.

§aid fastener may further comprise a preload element for compressing the piezoelectric material.

Said fastener may further comprise a plurality of wires integrated into the head for contacting the piezoelectric sensor.

Said fastener may further comprise a backing material disposed between the piezoelectric material and the preload element for facilitating acoustic coupling.

Said fastener may further comprise a matching layer disposed between the piezoelectric material and a bulk of the bolt for facilitating acoustic coupling.

In accordance with a second aspect of the present invention there is provided a method of monitoring fastener loads, the method comprising the steps of generating a longitudinal standing wave in a shaft of the fastener using a piezoelectric sensor; and determining a load to which a fastener has been subjected from a change in the frequency of the longitudinal standing wave.

The method may further comprise integrating the piezoelectric sensor into a head of the fastener. The change in the frequency of the longitudinal standing wave may be measured by performing a frequency response function measurement of the piezoelectric sensor.

The frequency of the longitudinal standing wave may be determined based on a minimum impedance measurement.

The frequency of the longitudinal standing wave may be determined based on a phase difference measurement between the input and output signals of the piezoelectric sensor.

The phase difference measurement may comprise using a phase lock loop circuit.

In accordance with a third aspect of the present invention there is provided a method of fabricating a fastener, the method comprising forming a piezoelectric sensor integrated into a head of the fastener for generating a longitudinal standing wave in a shaft of the fastener using the piezoelectric sensor.

In accordance with a fourth aspect of the present invention there is provided a system for monitoring fastener loads, the system comprising one or more fasteners as defined in the first aspect; wherein the fasteners are connected to an apparatus for performing frequency response function measurements of the piezoelectric sensors of the respective fasteners for monitoring loads on the fasteners.

The fasteners may be connected to the apparatus through wired or wireless connections.

In accordance with a fifth aspect of the present invention there is provided a wind turbine comprising one or more fasteners as defined in the first aspect.

In accordance with a sixth aspect of the present invention there is provided a wind turbine comprising a system for monitoring fastener loads as defined in the fourth aspect. BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure 1 shows a cross-sectional view of a bolt sensor according to an embodiment of the invention.

Figure 2 shows a cross-sectional view of a bolt sensor according to an alternate embodiment of the invention.

Figure 3 shows an example of a frequency response function plot from a bolt with integrated sensor according to an example embodiment.

Figure 4 is a flow chart illustrating one method of monitoring bolt loads according to an example embodiment. Figure 5 shows a schematic exploded view drawing of a sensor structure according to an example embodiment.

Figure 6 is a flowchart illustrating another method of monitoring fastener loads according to an example embodiment.

Figure 7 shows a schematic diagram illustrating a wind turbine comprising a system for monitoring bolt loads according to an example embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention may monitor bolt loads, non- destructively, without affecting the overall stiffness of a bolted joint, by means of a sensor and in addition, provides a means for real-time monitoring.

Figure 1 illustrates one embodiment of the present invention, which comprises of a sensor 101 that is physically integrated into a bolt head 100 of a bolt

102. The bolt 102 may be any type of threaded fastener, comprising a head and a shaft; and in the example embodiment is designed to be used in conjunction with a threaded portion 108a of one of the joint components 108. It will be appreciated that the present invention can also be applied to other fasteners, including, but not limited to screws, rivets, nails, etc. In the example embodiment, as the sensor 101 is positioned within the bolt head 100 such that, it will not affect the bolt's load capacity, strength and stiffness. More particular, in the example embodiment the cavity for _r

5

receiving the sensor 101 is positioned such that it lies outside of a force path, schematically indicated at 104 in Figure 1 , between bolt 102 and a joint structure between two joint components 106, 108 to be secured by the bolt 102.

The sensor 101 comprises a preload element, here in the form of a screw 105 with an Allen key notch 103, which may be screwed into the bolt 100 by means of threads 106. The sensor 101 further comprises a backing material 107; a pair of electrodes 109,113; a piezoelectric element 111 ; a matching layer 115 and +ve and - ve electrode wires 117,1 19. In the example embodiment, the electrode wires 117, 119 are accommodated in one or two grooves or recesses formed along a perimeter of the cavity for receiving the screw 105, backing material 107, electrodes 109, 113, piezoelectric element 11 1 , and matching layer 115, using for example drilling or forging techniques.

In this embodiment, the preload screw 105 is screwed down in order to compress the other components of the sensor 101. In particular, the piezoelectric element 111 is preferably compressed between the pair of electrodes 109,113 in order to have a preload for enhanced fatigue life. The Allen key notch 103 with the threads 106 may facilitate the fastening of the preload screw 105 in order to ensure compression of the sensor's components. The various components of the sensor are simply fixed in position by the compressional force applied by the preload screw. In this embodiment, the various components of the sensor 101 are assembled layer-by- layer within the cavity.

The piezoelectric element 111 may be of any piezoelectric material. By way of example and not limitation, the piezoelectric material could be quartz, lead zirconium titanate (PZT), etc. The thickness of the element 111 is such that it may operate in a thickness mode matched with a frequency of interest, for example about 2OkHz to 50OkHz. The longitudinal frequency (/) chosen is inversely proportional to the wavelength (A)₁ as the speed of sound (v) is a constant for a particular material. These variables are related by the following formula:

v = /Λ .

When a sinusoidal electric voltage is applied to the piezoelectric element 111 via the pair of electrodes 109,113, a mechanical longitudinal wave is generated, which reflects back from the free end 121 of the bolt 102. A mechanical longitudinal standing wave can be formed between the free end 121 and the top surface 122 of the screw 105. The frequency of a standing wave is related to the length of the bolt by: f - ^v - ^v(l + 0

' λ, 2£

where t is the length of the bolt 102 between the free end 121 and the top surface 122 of the backing material 107, and / = 0, 1 , 2, ... is the mode number, starting from the fundamental (/ = 0) mode, to the higher modes, as will be appreciated by a person skilled in the art.

In the example embodiment, the sensor 102 configuration is preferably chosen such that for a given overall dimension of the bolt 102, the frequency of the standing wave of interest is substantially matched to a thickness mode of the piezoelectric element 111 , such that the sensor 101 can advantageously operate with minimal energy loss. In the thickness mode, the piezoelectric deformation of the piezoelectric 111 is preferably purely a longitudinal deformation, or the other modes, i.e. radial and flextual modes, are at least minimised. In addition to facilitating operation at minimal energy loss, this can also advantageously minimise damage to/fatigue of the piezoelectric element 111 under operation. It is noted in that regard that in the example embodiment, the matching to the thickness mode takes into account the compression of the piezoelectric element 111 exerted by the preload screw 105.

In chosing the desired standing wave, i.e. fundamental or higher resonance modes, a balancing between different factors can be performed. For example, for higher resonance modes, the sensivity to structural defects such as cracks forming in the bolt 102 along the force path 104 is increased, due to the shorter wavelength of the standing wave. At the same time, energy damping increases with increasing frequency, resulting in losses which reduce the energy efficiency of the sensor 101 , and effectively heat the bolt 102. Thus can be additionally disadvantageous as it can contribute to fatigue of the bolt 102, and/or changing of material parameters of^the bolt 102, for example.

When the bolt 102 is stretched or compressed under load, the length will vary and accordingly the longitudinal resonance frequency for the standing wave will change. The longitudinal resonance frequency may be obtained from the zero point 300 of a Frequency Response Function (FRF) plot 302 illustrated in Figure 3, which shows the magnitude of the FRF as a function of frequency. The plot 302 may be generated from a sine sweep of the piezoelectric element 111 (Figure 1). Thus, the change in length Δ /Of the bolt 102 (Figure 1) can be measured from slifts in the zero point 300 and the axial load, P, can be derived:

I

where, A is the cross sectional area of the bolt 102 (Figure 1 ) along the force path 104 (Figure 1), and E is the elastic modulus.

It will be appreciated that the FRF, which measures the output over the input of the piezoelectric element 111 (Figure 1), includes both magnitude and phase information. In addition to obtaining the longitudinal resonance frequency, additional information may be obtained with reference to the phase information, including information for determining location of crack(s) along the bolt, which can be associated with occurrence of different set(s) of longitudinal resonance mode(s).

In the example embodiments, the electronics coupled to the sensor 101 (Figure 1) ensures to observe that the operating frequency is matching with the longitudinal resonant frequency of the overall bolt system. The benefits to operate at that resonant frequency point include that the monitoring can provide infromation not only about the change in bolt dimension, but advantageously also inromation about the presence of bolt damage (like crack, elongation, etc). Such damage can result in at leats partial reflection of the wave, and an associated resonance frequency can be detected. Also energy wastage is low and so no or littel heat up and corresponding fatigue or softening effects to the bolt material are encountered.

Additionally, monitoring for operating at a resonance condition does not require complex circuitry or processing. For example, in the example embodiment, the circuit applies a sinusoidal voltage and observes the current waveform. Two example methods that can be be used, but the invention is not limited to, are described below. _ o

The circuit uses the ratio of the voltage and current seen as impedence and is analysed for different frequencies. To find, indirectly, a resonance frequency, one can adopt a frequency sweep from a low frequency to high frequency and observe the minimal impedence condition.

In another eexample, a phase lock loop (PLL) concept is used to ensure that the circuitry looks only at the resonance frequency. In a bolt there are other vibration modes such as flexural modes, twisting modes, etc and they may get exited during the sensing measurements. Preferably, in example embodiments it is desired to focus to operate and monitor at the longitudinal frequencies. Using a PLL concept in may be preferred over using other types such as proportional control, etc.

The phase difference between the input and output signal to the piezo electric element 111 is a useful information to locate the resonance. A phase difference between 0° and -90° corresponds to a driving frequency below resonance, while a phase difference between -90° and -180° implies operation above resonance. Thus if the phase is controlled at -90° resonance can advantageously be ensured to take place. The relevant frequency of operation can be taken down as the resonant frequency and the impedence magnitude value can be taken down. Once the bolt parameters and load changes, the physical dimensions change and accordingly the resonant frequency, as mentioned above. Hence the PLL circuit will locate the new resonant frequency and one can derive the change in the bolt characteristics.

Figure 4 is a flow chart illustrating one method of monitoring bolt forces according to an example embodiment. The method begins 400 with applying a sinusoidal voltage on the piezoelectric element 111 , as shown at reference numeral 402. The following step involves conducting a sine sweep 404 of the piezoelectric element 11 1. Next, a FRF plot is obtained from the sine sweep, as shown at reference numeral 406. Thereafter, the longitudinal frequency of the standing wave is extracted from the FRF plot, as shown at reference numeral 408.

The change in bolt length is then derived 410. The next step 412 determines if the bolt length is within tolerance. If it is, the next step will determine if monitoring is to be done continuously 418. If it is, the system will go back to step 402.

Otherwise, monitoring ends 420. Should the bolt length fall out of the tolerance range, the subsequent step 414 will derive the axial load and thereafter determine the looseness of the bolt 416. Once completed, the monitoring process ends 420.

Figure 5 shows a schematic exploded view drawing illustrating the layer-by- layer built-up of a sensor structure 500 for incorporation into a bolt head (not shown) according to an example embodiment. A matching layer in the form of a sheet material 502 is provided, with the material chosen to provide good acoustic coupling/matching without reflection of energy at the interface to the bulk bolt material, in the assembled configuration. In this example embodiment, the sheet material 502 is further chosen to be electrically insulating. It will be appreciated that in different embodiments, a separate insulating layer may additionally be provided.

The next layer comprises the bottom electrode layer, here in the form of a metallic sheet material 504, for example BeCu alloy. The sheet material 504 includes a tail portion 506, to which a contact wire 506 is connected, in the example embodiment through a solder connection.

A piezoelectric material, here in a form of a disc 510 is provided, followed by the upper electrode, here in the form of a conducting material sheet 512, for example Be. The sheet material 512 includes a tail portion 513, to which a contact wire 515 is connected, in the example embodiment through a solder connection. This is followed by a packing material, here in the form of another sheet material 514, chosen for providing good acoustic coupling/matching to the material of the preload screw 516, which in this example embodiment is formed from the same material as the bolt (not shown).

As mentioned above, the wires 506, 515 are arranged for assembly in recesses or channels formed in the bolt head (not shown) along a cavity for receiving the sensor structure 500. At the surface of the bolt head (not shown), the wires 506, 515 can e.g. be terminated in suitable connector structures such as multi-pin co-axial cable connectors, for connection to external measurement equipment (not shown) via corresponding plugs.

Figure ^'2 shows an alternate embodiment of the present invention, a sensor

201 which is also integrated into a bolt head 200. However, a preload element, in the form of a screw 205, is in a cylindrical T shape. The preload screw 205 is screwed into the bolt 201 by means of an Allen key notch 203 and suitable threads 206. In this embodiment, the threads 206 are formed at a bottom portion 204 of the cavity for receiving the sensor 201.

The components of the sensor 201 further comprise a backing material 207; a piezoelectric element 211 ; a pair of electrodes 209, 213; a matching layer 215 and a +ve and -ve electrode wires 217, 219. In this embodiment, the backing material 207; piezoelectric element 211 ; pair of electrodes 209, 213 and matching layer 215 are of a cylindrical ring shape and surround the stem portion 210 of the preload screw 205. In this embodiment, because the thread 206 for the preload screw 205 is provided in the bottom portion 204, it will be appreciated that the formation of recesses or channels 220 for accommodating the electrode wires 217, 219 does not involve mechanical processing of a wall structure which itself incorporates a threaded portion, compared with the embodiment described above with reference to Figure 1. It will be appreciated that choosing between the configuration in the embodiment shown in Figure 2 and the embodiment shown in Figure 1 may involve a balancing between ease of manufacture on the one hand, and mechanical integrity consideration in relation to providing the desired preload compression via the treaded portion, for example. The choice may also depend on the physical dimension of the bolt.

Figure 6 is a flowchart 600 illustrating another method of monitoring fastener loads according to an example embodiment. At step 602, a longitudinal standing wave is generated in a shaft of the bolt using a piezoelectric sensor. At step 604, a load to which a fastener has been subjected is determined from a change in the frequency of the longitudinal standing wave.

A method of fabricating a fastener according to an example embodiment comprises forming a piezoelectric sensor integrated into a head of the fastener for generating a longitudinal standing wave in a shaft of the fastener using the piezoelectric sensor.

The embodiments described can be applied in many different application environments, where monitoring of tension and/or damage to fasteners is a critical issue. In one implementation, the example embodiments are individually connected to the relevant measurement equipment for performing the frequency response function measurements in an ad-hoc basis, for example conducted by a service engineer. In another application implementation, one or more fasteners according to example embodiments are "permanently" electrically interconnected to a suitable measurement equipment or system, for continued, real-time measurements or centralized ad-hoc measurements.

It will be appreciated that the interconnection between the fasteners according to example embodiments and the relevant measurement equipment, both for ad-hoc measurements as well as for a "permanently" interconnected configuration, can take many forms, including wired or wireless connections for performing of the FRF measurements.

Figure 7 shows a schematic diagram illustrating a wind turbine 700 comprising a system for monitoring bolt loads according to an example embodiment. It will be appreciated by a person skill in the art that the wind turbine 700 comprises numerous bolts the loads of which may be desired to be monitored. For clarity, only example bolts 704 to 706 are shown in Figure 7, representing bolts used to interconnect the respective tower segments e.g. 708, 710. It will be appreciated that the bolts 704 to 706 would typically be used in conjunction with nuts to secure the segments e.g. 708, 710 via adjacent inner flanges (not shown) of adjoining segments 708, 710. It will be appreciated that any number of the bolts required may be sensor bolts according to example embodiments, including a select number of bolts per joint, e.g. every nth bolt along the circumference of the flanges, n=1 , 2, 3,..,M, where M is the total number of bolts for a given joint..

The fasteners 704 to 707 are each connected to an apparatus for performing FRF measurements of the integrated piezoelectric sensors of the respective bolts 704 to 707 for monitoring loads on the bolts 704 to 706, here in the form of a spectrum analyzer 712 with PLL function located at a base station 714 of the wind turbine 700. In the example embodiment, the bolts 704 to 706 are connected to the spectrum analyzer 712 using wireless interconnections, using an RF transceiver 716 coupled to the spectrum analyzer 712, and corresponding transceivers (not shown) mounted at or near the heads of the bolts 704 to 706, and coupled to the piezoelectric sensors integrated in the respective heads of the bolts 704 to 706.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the embodiments without departing from a spirit or scope of the invention as broadly described. The embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. A fastener, comprising: a head for abutting a mounting surface of a medium; a shaft for engaging the medium; and a piezoelectric sensor integrated into the head for monitoring loads on the fastener.

2. The fastener according to claim 1 , wherein said piezoelectric sensor is disposed in a cavity within the head.

3. The fastener according to any of the preceding claims, wherein said piezoelectric sensor, in use, generates a longitudinal standing wave in the shaft.

4. The fastener according to any of the preceding claims, wherein said piezoelectric sensor comprises a piezoelectric material sandwiched between a pair of electrodes.

5. The fastener according to claim 4, wherein said fastener further comprises a preload element for compressing the piezoelectric material.

6. The fastener according to claim 1 , wherein said fastener further comprises a plurality of wires integrated into the head for contacting the piezoelectric sensor.

7. The fastener according to claim 5, wherein said fastener further comprises a backing material disposed between the piezoelectric material and the preload element for facilitating acoustic coupling.

8. The fastener according to claim 4, wherein said fastener further comprises a matching layer disposed between the piezoelectric material and a bulk of the bolt for facilitating acoustic coupling.

9. A method of monitoring fastener loads, the method comprising the steps of: generating a longitudinal standing wave in a shaft of the fastener using a piezoelectric sensor; and determining a load to which a fastener has been subjected from a change in the frequency of the longitudinal standing wave.

10. The method as claimed in claim 9, further comprising integrating the piezoelectric sensor into a head of the fastener.

11. The method as claimed in claim 9 or 10, wherein the change in the frequency of the longitudinal standing wave is measured by performing a frequency response function measurement of the piezoelectric sensor.

12. The method as claimed in any one of claims 9 to 11 , wherein the frequency of the longitudinal standing wave is determined based on a minimum impedance measurement.

13. The method as claimed in any one of claims 9 to 11 , wherein the frequency of the longitudinal standing wave is determined based on a phase difference measurement between the input and output signals of the piezoelectric sensor.

14. The method as claimed in claim 13, wherein the phase difference measurement comprises using a phase lock loop circuit.

15. A method of fabricating a fastener, the method comprising forming a piezoelectric sensor integrated into a head of the fastener for generating a longitudinal standing wave in a shaft of the fastener using the piezoelectric sensor.

16. A system for monitoring fastener loads, the system comprising: one or more fasteners as claimed in any one of claims 1 to 8; wherein the fasteners are connected to an apparatus for performing frequency response function measurements of the piezoelectric sensors of the respective fasteners for monitoring loads on the fasteners.

17. The system as claimed in claim 16, wherein the fasteners are connected to the apparatus through wired or wireless connections.

18. A wind turbine comprising one or more fasteners as claimed in any one of claims 1 to 8.

19. A wind turbine comprising a system for monitoring fastener loads as claimed in claims 16 or 17.