SENSOR ARRANGEMENT FOR SENSING BENDING MOMENTS IN AN ELONGATE COMPONENT, ELONGATE COMPONENT, SENSOR SYSTEM AND WIND TURBINE

Description :

The invention relates to techniques for sensing bending moments in components, and more particularly to an improved sensor arrangement for sensing bending moments in an elongate component (such as a blade of a wind turbine), and to an elongate component, sensor system and wind turbine.

In equipment such as wind turbines, significant bending mo ^¬ ments are induced in elongate components, especially turbine blade and the tower of the turbine. Similar issues arise in other industries, e.g. with components made of glass fibre, metal etc.; and in order to ensure safe and satisfactory op ^¬ eration, it is desirable to accurately measure the bending moments arising in the components.

In particular, knowledge of the blade bending moments is sig ^¬ nificant as regards the following design aspects of a wind turbine: (i) provision of a substantial input for smart con ^¬ trol purposes, facilitating equipment weight reduction; (ii) direct estimation/valuation of the wind field in front of the rotor (wind speed, wind shear vertical & horizontal); (iii) rotor blade-pitch alignment; and (iv) count of cycles for lifespan purposes for the rotor blades and rotor shaft.

It is known to measure the bending moments arising in the blades of wind turbines by means of dynamic mechanical spec- troscopy (DMS) directly applied to the blades. However, the equipment for such techniques have a limited lifespan, and elaborate signal amplification is required.

It is also known to measure the bending moments on the basis of indirectly applied DMS. However, with such techniques there can be strong influence from the adhesive layer, which manifests as temperature influence and ageing phenomena; and selective attaching makes the sensor vulnerable to expansion gradients / bulging effects in the blade.

There are also known techniques involving the use of active or passive fibre-optic sensors and methods in which the glass fibres are laid on or in the blade wall. Such techniques are disclosed, for example, in EP2778602B1. However, these techniques require very expensive analysis units for realizing the optical signal, so far can have low reliability, and a temperature driven expansion proportion has to be compensated only by extensive learning.

Another known technique involves the use of laser or light beams as well as transmitters, receivers and, if applicable, mirrors in the blade in order to detect the movement of the blade tip (of an outer part of the blade) relative to the blade root. However, these techniques suffer similar drawbacks to the aforementioned fibre-optic based methods.

Yet another known technique involves integrating the sensed acceleration of a part/portion of the blade lying outside the blade root to the blade point speed or blade point deviation. WO2015/014366 discloses a wind turbine comprising at least one rotor blade and a load sensor located at a root end of the rotor blade. An optical accelerometer is located inside the rotor blade near a tip thereof, and a controller is con ^¬ nected to the accelerometer via one or more optical fibres that extend along the length of the rotor blade and is con ^¬ figured to control the wind turbine based upon the measured load and the measured acceleration to maintain the load on the rotor blade below a predetermined threshold level. EP2898216B1 discloses a method for monitoring the state of a rotor blade of a wind turbine, comprising: measuring an acceleration of the rotor blade with a first signal by means of a fibre-optic acceleration sensor, wherein the acceleration is measured at a first radial position at a predetermined distance from the rotor blade root in at least one direction comprising a first directional component orthogonal to the axis of the rotor blade; measuring a strain of the rotor blade with a second signal by means of a fibre-optic strain sensor, wherein the strain is measured at a second radial po ^¬ sition disposed in the area of the first radial position to the rotor blade root, in order to measure bending moments in two, typically orthogonal, directions; determining a first positional change by integrating the acceleration over time; determining a first value corresponding to the rotor blade stiffness by means of calculation on the basis of the first positional change and the strain; and determining the rotor blade state from the first value. The first radial position may be located approximately at half the blade radius or in between half the blade radius and a rotor blade tip and/or the second radial position may be located at a distance of 5 meters or less from the blade root. Further techniques based on acceleration signals are disclosed in DE102010032120A1.

However, with techniques involving integrating the sensed acceleration, due to the rotational movement of the blades, the Azimuth movement of the nacelle around the tower shaft/axis (active, passive) of the wind turbine, and the tower move ^¬ ment, as well as other errors occurring due to permanent in ^¬ tegration, the accuracy in particular of the deviation result is limited. Moreover, when using conventional acceleration sensors, there is a high risk of destruction/damaging of the electric lines and sensors in the rotor blade due to the ef ^¬ fect of lightning.

Accordingly, known solutions exhibit the following disadvantages or lead to the following problems:

robustness / reliability is not sufficient;

for electric components in the blade that are positioned at a distance from the blade root, there is the risk of destruction by lightning;

for expansion or deviation based methods, usually non- trivial methods for compensating the temperature influ ^¬ ence/effect on the blade performance/reaction need to be used or learned in practice, requiring in part separate sensors ;

for methods with fibre-optic sensors which are integrat ^¬ ed in blade walls during manufacturing, there is no pos ^¬ sibility of repair if the fibre-optic cables are faulty, damaged or non-functioning; and

costs of the known solutions can be extremely high.

The present invention seeks to overcome the aforementioned problems and provide an improved sensor arrangement for sens ^¬ ing bending moments in an elongate component, a sensor system and wind turbine.

SUMMARY

According to one aspect of the invention there is provided a sensor arrangement for sensing bending moments in an elongate component, for example a blade of a wind turbine, the sensor arrangement comprising: a measurement rod extending at least a predetermined distance parallel to the direction of elonga ^¬ tion of the elongate component and transverse to the axis of bending; a fixing element attached to one of a proximal end and a distal end of the measurement rod, the fixing element being configured for fixedly attaching the measurement rod to a surface of the elongate component; a support element at ^¬ tached to the other of the proximal end and the distal end of the measurement rod, the support element being configured to be fixedly attached to the surface and for supporting the measurement rod such as to allow axial displacement thereof; and a displacement sensor, disposed adjacent and spaced apart from the proximal end of the measurement rod, the sensor be ^¬ ing configured for outputting a signal indicative of the axi ^¬ al displacement of the measurement rod; wherein the measure ^¬ ment rods are made of the same material as the elongate com ^¬ ponent .

Preferably, the predetermined distance lies in the range 10- 200cm, more preferably 20-lOOcm, and more preferably 20-50cm.

Preferably, the measurement rod has a diameter that lies in the range 1-10mm, more preferably 2-8mm, and more preferably 4-6mm.

Preferably, the distance lies in the range 2-30%, more pref ^¬ erably 5-20%, and more preferably 10-15% of the length of the elongate component.

Preferably, the fixing element is configured to support an axis of the measurement rod at a predetermined spacing from the surface. Preferably, the predetermined spacing lies in the range 1-lOx, more preferably 2-5x, and more preferably 2- 3x the diameter of the measurement rod.

Preferably, the support element includes a bearing or bearing surface configured to permit sliding movement of the measure ^¬ ment rod relative thereto.

Preferably, a metal tip is disposed at the proximal end of the measurement rod. In one embodiment, the displacement sen ^¬ sor comprises an inductive sensor.

In another embodiment, the displacement sensor comprises a capacitive sensor, a magnetic sensor or an optical sensor.

Preferably, the displacement sensor is configured to sense axial displacements of the measurement rod lying in the range of ± 10 mm.

Preferably, the displacement sensor is configured to sense axial displacements of the measurement rod with a resolution of 0.1%.

Preferably, the measurement rod is made of glass fibre or carbon .

According to another aspect of the invention there is provid ^¬ ed an elongate component, for example a blade of a wind tur ^¬ bine, the elongate component having mounted thereon a sensor arrangement according to any one of claims 1 to 13 of the ap ^¬ pended claims.

Preferably, a plurality of the sensor arrangements are mount ^¬ ed on the elongate component, the displacement sensor of each sensor arrangement outputting a respective signal indicative of displacement sensed by that sensor arrangement.

Preferably, the sensor arrangements are mounted on the sur ^¬ face spaced apart around the cross-sectional periphery of the measurement rod.

Preferably, the sensor arrangements are mounted on the sur ^¬ face equally angularly spaced around the cross-sectional pe ^¬ riphery of the measurement rod.

In one embodiment, three sensor arrangements are mounted on the surface. Such sensors are preferably attached on the cir ^¬ cumference of the blade root to measure or calculate the two orthogonal blade root bending moments and the acting axial force .

In another embodiment, four sensor arrangements are mounted on the surface. Preferably, the sensor arrangements are ar ^¬ ranged as two pairs of diametrically opposing sensor arrange ^¬ ments. If four sensors, with two opposite each other, are im ^¬ plemented so as to be interconnected with signal summing, the two bending moments can be measured directly, wherein the ax ^¬ ial force influence is directly compensated.

Preferably, the elongate component is hollow and the surface is an internal surface.

Preferably, the sensor arrangements are mounted on a portion of the surface at one end of the elongate component. Preferably, the elongate component is a blade for a wind tur ^¬ bine. Preferably, the portion of the surface is at or adja ^¬ cent the root end of the blade.

Preferably, the elongate component is formed of glass fibre or carbon.

According to another aspect of the invention there is provid ^¬ ed a sensor system for sensing bending moments in an elongate component, for example a blade of a wind turbine, the sensor system comprising: a plurality of sensor arrangements accord ^¬ ing to any one of claims 1 to 13 of the appended claims or an elongate component according to any of claims 14 to 25 of the appended claims; and processing circuitry, coupled to receive the signal output by the displacement sensor of each sensor arrangement; wherein the processing circuitry is configured to determine, based on the signals indicative of the axial displacements of the measurement rods, the bending moment in an elongate component.

Preferably, each displacement sensor is coupled to the pro ^¬ cessing circuitry via an overvoltage protection circuit.

Preferably, the processing circuitry is coupled to one or more pitch sensors, each pitch sensor being configured to supply to the processing circuitry a signal indicative of a pitch angle of a respective elongate component, wherein the processing circuitry is configured to determine the bending moment, based on the signals indicative of the axial dis ^¬ placements and of the pitch angles.

According to another aspect of the invention there is provid ^¬ ed a wind turbine comprising: a plurality of sensor arrange- merits according to any one of claims 1 to 13 of the appended claims, an elongate component according to any of claims 14 to 25 of the appended claims or a sensor system according to any one of claims 26 to 28 of the appended claims.

An advantage of the invention is that robustness and/or reli ^¬ ability are significantly improved though the use of posi ^¬ tion/displacement sensors.

A further advantage is that no electric components are posi ^¬ tioned in the blade at a distance from the blade root, there ^¬ by lowering the risk of destruction by lightning.

A further advantage of the invention is that, in embodiments, there is direct compensation for the influence of tempera ^¬ ture .

A further advantage is that, with the devices used in embodi ^¬ ments, there is the possibility of easy retrofit ^¬ ting/refitting, or repair.

A further advantage of the invention is that, compared to known solutions, costs are significantly reduced, e.g. by two thirds in certain embodiments.

A further advantage is that the detection of the presence of ice on the turbine blade, and the identification of vibration modes and/or influence of wakes from upwind wind turbine gen ^¬ erators (WTGs), is enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS Further details of the invention will become apparent from the drawings according to the description. In the drawings:

Figure 1 (PRIOR ART) shows a known wind turbine with multiple blades ;

Figure 2 shows an axial cross-sectional view of a blade root of a blade of Fig. 1, showing sensor arrangements according to embodiment of the invention;

Figure 3 shows an enlarged view of detail B of Fig. 2 ;

Figure 4 shows an enlarged view of detail A of Fig. 2; and Figure 5 is a transverse cross-sectional view of a blade root of a blade of Fig. 1, in the case of (a) three equally spaced sensor arrangements and (b) two pairs of diametrically op ^¬ posed sensor arrangements.

FIG. 1 (PRIOR ART) shows a known wind turbine 1 with multiple blades. The wind turbine 1 includes a tower 3 and a nacelle 2 rotatably mounted on the tower 3. The nacelle 2 comprises a nacelle cover 4 mounted on a main frame (not shown) , which will be discussed in further detail below. On a rotor shaft

(not shown) inside the nacelle 2 a rotor 7 is arranged, which in turn comprises a hub 5 and at least one rotor blade 6

(here, three) .

Near the axis of the hub 5, an end portion or blade root 8 of blade 6 is retained in place, e.g. within jacket 9. It is de ^¬ sirable to provide devices for measuring bending moments about the y- and/or z-axis, and/or axial forces (along the x- axis), as described in further detail hereinafter.

Figure 2 shows a longitudinal cross-sectional view of a blade root 8 of a blade 6 of Fig. 1, showing sensor arrangements 10 according to an embodiment of the invention. In this embodi- ment, the blade 6 is made of glass fibre. However, it will be appreciated that the blade 6 may be made of other materials, such as carbon or carbon composites. Each sensor arrangement 10 is disposed on a surface portion 12 at or near the end 14 of blade 6, i.e. so as to substantially span the length of the blade root 8. With the sensor arrangement 10 according to the present invention, rather than measuring the expansion of the wall material of the blade 6 in the area of blade root 8, the elongation between two points which have a large enough distance between each other is directly measured, thereby keeping the influence of tension-expansion-gradients and bulging effects negligibly small.

Each sensor arrangement 10 is constructed as follows. A meas ^¬ urement rod 16 extends along the length of surface portion 12, and therefore substantially spans the length of the blade root 8. Measurement rod 16 is made of the same material as blade 6, i.e. glass fibre (or of other materials, such as carbon or carbon composite, where appropriate) . This serves to ensure "natural" temperature compensation with regard to temperature effects on the blade root 8. It is noted that the temperature distribution in the spatial expansion of a blade root 8 is not even and therefore that the temperature compen ^¬ sation has to be sensor position specific.

Figure 3 shows an enlarged view of detail B of Fig. 2. A fix ^¬ ing element 18 is fixedly attached to surface portion 12, e.g. by strong adhesive (not shown) . In addition, distal end 20 of measurement rod 16 is in turn fixedly attached to fix ^¬ ing element 18 via a rigid angle bracket 19 that tight ^¬ ly/rigidly grasps measurement rod 16 (which is, e.g., bonded into a recess thereof with the strong adhesive) . Returning to Fig. 2, at or near the proximal end 22 of meas ^¬ urement rod 16 a support element 24 is provided, that is fix ^¬ edly attached to surface portion 12, e.g. by strong adhesive. In this embodiment, the measurement rod 16 passes through an axial passage (not shown) in support element 24, with clear ^¬ ance, thus enabling axial/longitudinal movement of measure ^¬ ment rod 16 relative to support element 24. In embodiments, bearings, and/or lubricating material such as grease, may be provided between engaging surfaces of the support element 24 and measurement rod 16. A number of further support elements 24', 24'' may be provided (e.g. equally spaced) between fix ^¬ ing element 18 and support element 24.

Each sensor arrangement 10 further includes a posi ^¬ tion/displacement sensor 26. Referring now to Fig. 4, this shows an enlarged view of detail A of Fig. 2. The displace ^¬ ment sensor 26 is mounted on surface portion 12 of blade wall 27. In this embodiment, a metal (e.g. steel) tip 28 is pro ^¬ vided on proximal end 22 of measurement rod 16, and the dis ^¬ placement sensor 26 comprises an inductive sensor for sensing position or positional changes. In this way, movement/change of position of one point (e.g. proximal end 22) with respect to another (e.g. support element 24 or displacement sensor 26) can be measured by means of a simple, robust position sensor, usually without further signal processing. It will be appreciated that the displacement sensor 26 may alternatively comprise a capacitive sensor, a magnetic sensor or an optical sensor .

In this embodiment, signals generated by displacement sensors 26 on blade 6 (a "first blade") are fed via leads 28 and overvoltage protection unit 30 to first inputs 32 of pro ^¬ cessing circuitry 34. The latter further includes second in- put 36 and third input 38 for receiving signals indicative of movement/change of position from second and third blades (not shown) . The processing circuitry 34 further includes a fourth input 38 configured to receive signal indicative of a pitch angle of the first blade 6 from pitch angle sensor 40. The processing circuitry 34 further includes fifth input 42 and sixth input 44 for receiving signals indicative of move ^¬ ment/change of position from second and third blades (not shown) .

The processing circuitry 34 preferably comprises wind turbine generator programmable logic controller (WTG-PLC) circuitry, configured for calculating bending moments in the blade roots 8 (Fig. 1) about the y- and/or z-axis, and/or axial forces (along the x-axis) based on the various position/displacement signals and pitch angle signals, using techniques known in the art .

Figure 5(a) is an axial cross-sectional view of a blade root 8, in one embodiment - in the case of three the equally spaced sensor arrangements 10, 10' and 10' ' . The latter are arranged at equal (120 degree) angular spacings around the internal surface 12 of blade root 8. If three such sensors are attached on the circumference of the blade root 8, the two orthogonal blade root bending moments and the acting axi ^¬ al force, as mentioned above, can be measured or calculated.

Figure 5 (b) is an axial cross-sectional view of a blade root 8, in another embodiment (corresponding to Fig. 2) - in the case of two pairs of diametrically opposed sensor arrange ^¬ ments 10, 11. If four sensors are so used, with two within a pair opposite each other and interconnected, and signal addi ^¬ tion is employed, the two bending torques can be measured di- rectly, whereby the axial force influence is directly compen ^¬ sated .

While the invention has been described above in relation to rotor blades of wind turbines, it will be appreciated that the techniques are applicable to any slender/elongate struc ^¬ ture or structural component, such as the tower/shaft of a wind turbine.

List of reference signs

1 wind turbine

2 nacelle

3 tower

4 nacelle cover

5 hub

6 rotor blade

7 rotor

8 blade root

9 jacket

10 sensor arrangement

10' sensor arrangement

10' ' sensor arrangement

11 sensor arrangement

12 surface portion

14 end

16 measurement rod

18 fixing element

19 rigid angle bracket

20 distal end

22 proximal end

24 support element

24' support element

24'' support element

26 position/displacement sensor

27 blade wall

28 metal tip

29 leads

30 overvoltage protection unit 32 first input processing circuitry second input third input

fourth input pitch angle sensor fifth input

sixth input