GB2483659A

GB2483659A - Strain sensor with strain attenuating substrate

Info

Publication number: GB2483659A
Application number: GB1015342.7A
Authority: GB
Inventors: David William Richards
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2010-09-14
Filing date: 2010-09-14
Publication date: 2012-03-21
Also published as: GB201015342D0

Abstract

A strain sensor for a component 100, e.g. wind turbine blade, comprises strain sensing element 70 supported by substrate 80 having a mounting surface 90 for mounting to the component. The strain sensing element, e.g resistance strain gauge, is arranged on a side of the substrate opposite the mounting surface. The substrate may be elastomeric or the mounting surface comprise at least two spaced apart regions (fig. 4). When the component to which the strain sensor is mounted undergoes strain or deformation (fig. 2 unstrained, fig. 3 strained), the deformation is transmitted to the mounting surface of the substrate. The substrate attenuates the strain and the strain sensing element sees less strain than the mounting surface and therefore less strain than that experienced by the component. The strain sensor may be calibrated so that the strain on the component can be calculated by mounting the strain sensor and a duplicate strain sensing element to a surface and determining a relationship between the strain sensed by the duplicate strain sensing element and the strain sensed by the strain sensor. Allows use of resistance strain gauge on components subject to large deformations.

Description

Strain Sensor for a Component The present invention reiates to a strain sensor for a high elongation component, in particular a deformable wind turbine component such as a wind turbine rotor blade.

It is known to monitor strain or deformation of a wind turbine component such as a rotor blade. Wind turbine components are subject to large amounts of strain from for example the accumulation of particulates like dirt or ice, their weight and the force exerted by the wind itself. Such strain can lead to fatigue and it is therefore important to monitor the strain on the components so that one can ensure the components remain fit to operate over their working lives.

A number of ways of detecting strain on a wind turbine component are known.

These include using Fibre Bragg Grating (FBG) sensors. An FBG sensor is an optical fibre in which an optical grating is formed. When the sensor is deformed the spacing of the grating changes which causes a detectable change in the wavelength of light reflected back by the grating. A problem with such known sensors is that they are complex and consequently costly.

We have appreciated that it would be desirable to use conventional strain gauges to sense strain on wind turbine components. Such conventional strain gauges typically comprise electrically conducting materials which change resistance as they deform. These sensors are far cheaper than FBG sensors and, in addition, the hardware required to convert the core data into strain measurements is more developed and therefore these sensors are easier to use. However, these sensors can not reliably be used to monitor strain on wind turbine components such as wind turbine rotor blades which are subject to large deformation or strain. Often the conducting materials in the strain gauges fatigue and break before the wind turbine component whose strain they are supposed to be sensing. Thus the gauges often need to be re-gauged or replaced. This is particularly problematic for wind turbines as they are often erected in hostile, inaccessible environments where servicing or replacing a sensor would be difficult, time consuming and expensive.

According to a first aspect of the invention there is provided a strain sensor for a component comprising a strain sensing element supported by a substrate, the substrate having a mounting surface for mounting on the component such that deformation experienced by the component is transmitted to the mounting surface, wherein the substrate is elastomeric whereby the deformation transmitted to the strain sensing element through the substrate is less than the deformation experienced by the mounting surface of the substrate.

According to a second aspect of the invention, there is provided a strain sensor for a component comprising: a strain sensing element supported by a substrate, the substrate having a mounting surface for mounting on the component such that deformation experienced by the component is transmitted to the mounting surface, wherein the mounting surface comprises at least two spaced apart mounting regions, whereby the deformation transmitted,to the strain sensing element through the substrate is less than the deformation experienced by the mounting surface of the substrate.

The component may be a high elongation component.

By high elongation we mean that the component has a potential elongation greater than that which the strain sensing element could reliably sense if mounted directly to the component. The present invention may be advantageous whenever it is desired to use a strain sensing element to monitor strain on a component having an elongation higher than that which the strain sensing element is designed to monitor.

Embodiments of the invention have the advantage that a conventional strain gauge, such as a resistance strain gauge, can be used to more reliably measure strain on high elongation materials such as the composites used in the manufacture of wind turbine rotor blades.

The materials for wind turbine components are chosen taking into account the large strain under which the components must operate. More recently the composites used for wind turbine rotor blades have included carbon fibre and the blades have become more flexible and thus have increased elongation. Generally wind turbine rotor blades can repeatedly deform by a large amount without suffering fatigue damage. For example, glass fibre reinforced composite materials (GFRP) are presently preferred for wind turbine blades. The glass fibre may be reinforced with epoxy resin or polyester materials.

Advantageously the substrate of embodiments of the invention attenuates the strain experienced by the component to which the sensor is mounted. This means that the strain sensing element sees less strain than the component. Thus, conventional strain gauges such as those described above can be used to measure strain on a high elongation component such as a wind turbine rotor blade more reliably. Generally, since the strain sensing element is subject to less strain it is less likely to fatigue and thus costs and time which would have been expended servicing or replacing the sensor can be saved.

In the first aspect of the invention the substrate may comprise natural rubber.

In the second aspect of the invention, when the strain sensor is mounted to the component, the portion of the substrate between the mounting regions is raised from the component. The mounting regions may be separated by a reduced thickness substrate portion. Preferably, the reduced thickness substrate portion is arcuate. Preferably, the reduced thickness substrate portion extends from one side of the substrate to another and the at least two mounting regions are parallel to one another.

In a preferred embodiment the strain sensing element is an electrical conductor.

Preferably the electrical conductor is made of metal. Thus advantageously the strain sensing element, and therefore the strain sensor, can be low cost.

The strain sensing element may be supported by a flexible backing and may be part of a pre-existing strain gauge. This can simplify manufacture of the strain sensor.

Preferably the strain sensing component is located on a surface of the substrate opposite the mounting surface. Preferably, the surface on which the strain sensing element is located and the mounting surface are parallel.

The invention also resides in a high elongation component, which may be a deformable wind turbine component and particularly a wind turbine rotor blade, having a strain sensor as described above. The invention further resides in a wind turbine comprising a wind turbine component having a strain sensor as described above.

There is also provided a method of calibrating the strain sensor embodying the present invention comprising: mounting the strain sensor and a duplicate strain sensing element to a surface; measuring the strain sensed by the strain sensor and the duplicate strain sensing element when a strain is experienced by the material; and determining a relationship between the strain sensed by the duplicate strain sensing element and the strain sensed by the strain sensor.

In a preferred embodiment, the strain sensor is mounted on a surface and a duplicate strain sensing element is mounted on the surface adjacent the strain sensor.

There is further provided a system for monitoring strain on a component, comprising: a strain sensor embodying the present invention mounted to a component; and a controller for calculating the strain experienced by the component based on measurements from the strain sensor and a calibration factor determined by the method above.

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying Figures, in which: Figure 1 shows a schematic view of a wind turbine; Figure 2 illustrates a strain sensor embodying the first aspect of the invention; Figure 3 illustrates a strain sensor embodying the first aspect of the invention when the wind turbine component is under strain; Figure 4 illustrates a strain sensor embodying the second aspect of the invention; Figure 5 is a flow chart of the steps of a method of calibrating a strain sensor embodying the invention according to a preferred embodiment of the invention; and Figure 6 is a graph showing cyclic strain against fatigue life of a WK-series strain gauge.

The embodiments to be described are given with reference to wind turbine rotor blades. However, it is to be understood that embodiments of the invention may also be used with other components, in particular high elongation components.

The wind turbine 10 of Figure 1 generally comprises a nacelle 20 mounted for rotation on a tower 30. A rotor 40 comprising a plurality of rotor blades 50 and a hub 55 is mounted to the nacelle 20. A generator (not shown) is housed within the nacelle 20 and has a rotor shaft extending from the nacelle front which is turned by rotation of the rotor blades 50 to generate power.

The strain sensor 60, 110 described below may be mounted to the inside or outside of one of the rotor blades. Presently preferred rotor blades have a cyclic strain level of approximately +1-2500 microstrain up to +1-3000 microstrain. The strain sensor 60, 110 may also be mounted to any other wind turbine component which has a similar elongation.

Figure 2 illustrates a first embodiment of the invention. The strain sensor 60 comprises a strain sensing element 70 supported by a substrate 80. In this example, the strain sensing element 70 is part of a pre-existing conventional strain gauge. The strain sensing element 70 comprises an electrical conductor which changes resistance as it is deformed. The electrical conductor is made of metal.

The gauge comprises a flexible backing supporting the strain sensing element 70.

A common type of conventional strain gauge is a foil strain gauge which comprises a flexible backing supporting a metallic foil pattern. The foil pattern comprises a long thin conductive strip generally arranged in a zigzag pattern of parallel lines.

When the electrical conductor is stretched within its limits of elasticity it becomes narrower and longer and its resistance end to end increases. Conversely, when the conductor is compressed before buckling it becomes broader and shorter and its resistance end to end decreases. Having a plurality of parallel conductors is advantageous as it multiplies the change in resistance which can be observed.

The change in electrical resistance is related to the strain experienced by the strain gauge and therefore the object to which the strain gauge is attached by a quantity known as the gauge factor which is defined as GF = where AR is the change in resistance caused by strain, Rg is the resistance of the undeformed gauge and e is strain. Many ways of obtaining strain data from such sensors are known.

In this case both the flexible backing and the strain sensing element is supported by the substrate 80 and the strain sensing element 70 is indirectly attached to the substrate 80 via the flexible backing, for example by an adhesive. In an alternative embodiment the strain sensing element 70 may be attached directly to the substrate 80.

The substrate 80 comprises a mounting surface 90 for mounting the strain sensor to a wind turbine component 100 such that the deformation experienced by the wind turbine component is transmitted to the mounting surface 90. in this case the substrate 80 is rectangular and the strain sensing element 70 and the mounting surface are located on opposite sides of the substrate 80. Other substrate shapes are possible and will occur to the skilled person.

The mounting surface 90 of the substrate may be mounted directly or indirectly to a wind turbine component 100. For example the mounting surface 90 may be mounted directly using an adhesive.

The substrate 80 is elastomeric. This means that where one side of the substrate is subject to a force the substrate does not behave uniformly. For example, when one side of the substrate is stretched, the opposite side will noticeably be stretched by a smaller amount.

The substrate may comprise natural rubber. A substrate of natural rubber having a thickness of 2mm has been found to attenuate strain of a wind turbine blade by one half. Figure 6 illustrates a graph showing cyclic strain against fatigue life of a WK-series gauge which is one of the best presently known gauges for measuring strain of high elongation materials. If the strain level is +3000 microstrain, fatigue life is about io cycles. If the strain is attenuated by a third or more, fatigue life will be greater than 108 cycles. It will be appreciated that the amount of attenuation desirable will depend on the particular gauge chosen. Different gauges, including those which are better suited to the environmental operating conditions of wind turbines, have slightly different fatigue life graphs. The thickness of the elastomeric substrate may be adjusted accordingly. Other elastomeric materials can be used as the substrate and will occur to the skilled person.

Figure 3 illustrates the strain sensor 60 when the wind turbine component 100 to which it is mounted is under strain. The component is stretched and deformed as indicated by the large arrows in Figure 3. The deformation experienced by the wind turbine component is transmitted to the mounting surface 90 of the substrate 80 and the mounting surface 90 of the substrate 80 similarly stretches and deforms.

However, this deformation is not seen by the strain sensing element 70. Because the substrate 80 is elastomeric the layers of the substrate 80 further from the mounting surface experience noticeably less deformation. Thus the substrate 80 attenuates the strain experienced by the wind turbine component. The strain sensing element 70, which in this example is located on the uppermost layer, therefore experiences less deformation than the mounting surface 90 of the substrate 80 and in turn less deformation than the wind turbine component 100.

This lesser strain is indicated by the small arrows in Figure 3.

Figure 4 illustrates a second embodiment of the invention. The strain sensor 110 comprises a strain sensing element 70 supported by a substrate 120. In this embodiment the substrate 120 may comprise an elastomeric material or a non-elastomeric material, provided the substrate 120 is sufficiently deformable to experience deformation transmitted from the high elongation component to which it is mounted. For example the substrate 120 may comprise a matrix of composite similar to that of the high elongation component to which it is to be mounted. For example in this case where the high elongation component is a wind turbine component such as a wind turbine rotor blade the substrate 120 may comprise epoxy resin. This choice of material is advantageous as such composites are known to be particularly thermally stable. Therefore the sensor 110 can be more easily assembled ofisite, in a controlled environment where it is easier to access the sensor. Thus, set up time can be reduced and subsequent installation on-site is much easier.

The strain sensing element 70 may be part of a pre-existing conventional resistance strain gauge as described above.

Contrary to the first embodiment the mounting surface 130 of the substrate 120 does not extend over the entire bottom face of the substrate 120, but is restricted to two or more spaced apart mounting regions 140. Thus, when the sensor 110 is mounted to the wind turbine component, the face of the substrate 120 comprising the mounting surface 130 is not entirely flush against the component. The portion between the mounting regions 140 is raised from the component.

The mounting regions 140 may be separated by a reduced thickness substrate portion 150.

In this example the substrate 120 is substantially rectangular and the strain sensing element 70 is mounted to a surface of the substrate 120 opposite and parallel to the mounting surface 130. The strain sensing element 70 is arranged along the length of the upper surface of the substrate 120. Portions of the upper surface at either end of the strain sensing element 70 taper downwards. The mounting surface 130 comprises eight mounting regions 140. Seven reduced thickness substrate portions 150 are provided therebetween. The shape of the portions 150 is arcuate.

The portions 150 extend from one side of the substrate 120 to another, in this case, in a straight line across the width of the substrate 120. The portions 150 are uniformly shaped and evenly spaced. Thus, the mounting regions 140 are parallel.

This arrangement is advantageous as the substrate 120 may thereby attenuate strain in a uniform manner.

Although in this embodiment there are several mounting regions 140, it will be appreciated that different numbers of mounting regions 140 are possible. The shapes of the portions 150 are also not limited to arcuate. Other shapes are possible and will occur to the skilled person.

When the sensor 110 is mounted to a wind turbine component, and a strain is experienced by the wind turbine component, the deformation experienced by the wind turbine component is transmitted to the mounting surface 130. Due to the spaced apart mounting regions 140, the upper portion of the substrate 120 is deformed less than the mounting surface 130 of the substrate 120, and thus similarly to the first embodiment, the deformation transmitted to the strain sensing element 70, which is located on the upper surface of the substrate 120, through the substrate 120 is less than that experienced by the mounting surface 130 of the substrate 120. Thus, the strain sensing element 70 sees less strain than the mounting surface 130 and in turn less strain than the wind turbine component.

In both embodiments, since the strain sensed by the strain sensor will be less than the strain experienced by the wind turbine component, the measurements from the strain sensor must be calibrated if the strain on the wind turbine component is to be determined.

Calibration may take place on a test material. Figure 5 is a flow chart showing the steps of calibrating the strain sensor 60, 110, according to a preferred embodiment.

The strain sensor 60, 110 and a duplicate strain sensing element 70, that is the same strain sensing element 70 as that of the strain sensor 60, 110, are mounted to the material. The components may be mounted to the material in any order. In the preferred embodiment illustrated in the flow chart, at step 160 the strain sensor 60, is mounted to the material adjacent a duplicate strain sensing element 70.

Then, at step 170 the strain sensed by the strain sensor 60, 110 and the duplicate strain sensing element 70 is measured when a strain is experienced by the material. Several different loads may be applied to the material. At step 180 a relationship between the strain sensed by the duplicate strain sensing element 70 and the strain sensed by the strain sensor 60, 110 is determined using these measurements. A calibration factor may be derived from this relationship. In this embodiment the strain sensor 60, 110 and the duplicate strain sensing element 70 are present on the material simultaneously. However, this need not be the case.

For example the strain sensor 60, 110 may be mounted on the material first and measurements taken and then the duplicate strain sensing element 70 may be mounted on the material and measurements taken.

As in the described embodiment where the strain sensing element 70 is part of a pre-existing strain gauge comprising a flexible backing supporting the strain sensing Is element 70, mounting a duplicate strain sensing element 70 to the material may include mounting the pre-existing strain gauge to the material.

This calibration factor may be used by a wind turbine controller (not shown in Figure 1) to calculate the strain experienced by the wind turbine component to which the strain sensor is mounted.

Embodiments of the present invention have the advantage that the amount of deformation or strain experienced by the sensing element is less than that experienced by the mounting surface of the substrate and, where the substrate is mounted to a wind turbine component, the wind turbine component. This means that lower cost sensors such as resistance strain gauges can be used to monitor strain on a wind turbine component such as a wind turbine rotor blade more reliably.

Embodiments of the invention have been described in relation to wind turbine components. However, it will be appreciated that the invention is also applicable to other components. For example the sensor described above may be used on components in the aerospace industry which comprise similar composites to those of wind turbine rotor blades. Moreover, the invention is not limited to a strain sensor for a component of particular elongation. The present invention may be advantageous whenever it is desired to use a strain sensing element to monitor strain on a component having an elongation higher than that which the strain S sensing element is designed to monitor.

Various modifications to the embodiments described are possible and will occur to those skilled in the art. For example many different elastomeric materials may be used as the substrate. The scope of the invention is defined solely by the following claims.

Claims

Claims: 1. A strain sensor for a component comprising: a strain sensing element supported by a substrate, the substrate having a mounting surface for mounting on the component such that deformation experienced by the component is transmitted to the mounting surface, wherein the substrate is elastomeric whereby the deformation transmitted to the strain sensing element through the substrate is less than the deformation experienced by the mounting surface of the substrate.
2. A strain sensor for a component comprising: a strain sensing element supported by a substrate, the substrate having a mounting surface for mounting on the component such that deformation experienced by the component is transmitted to the mounting surface, wherein the mounting surface comprises at least two spaced apart mounting regions, whereby the deformation transmitted to the strain sensing element through the substrate is less than the deformation experienced by the mounting surface of the substrate.
3. A strain sensor according to claim 2, wherein the substrate is elastomeric.
4. A strain sensor according to claim 1, 2 or 3, wherein the substrate comprises natural rubber.
5. A strain sensor according to any of claims 2 to 4, wherein, when the strain sensor is mounted to the component, the portion of the substrate between the mounting regions is raised from the component.
6. A strain sensor according to any of claims 2 to 5, wherein the mounting regions are separated by a reduced thickness substrate portion.
7. A strain sensor according to claim 6, wherein the reduced thickness substrate portion is arcuate.
8. A strain sensor according to claim 6 or 7, wherein the reduced thickness substrate portion extends from one side of the substrate to another.
9. A strain sensor according to any of claims 2 to 8, wherein the at least two mounting regions are parallel to one another.
10. A strain sensor according to any preceding claim, wherein the strain sensing element is an electrical conductor.
11. A strain sensor according to claim 10, wherein the electrical conductor is made of metal.
12. A strain sensor according to any preceding claim, wherein the strain sensing element is supported by a flexible backing.
13. A strain sensor according to any preceding claim further comprising a pre-existing strain gauge, wherein the strain sensing element is part of the pre-existing strain gauge.
14. A strain sensor according to claims 12 or 13, wherein the substrate is thicker than the flexible backing.
15. A strain sensor according to any preceding claim, wherein the substrate is located on a surface of the substrate opposite the mounting surface.
16. A strain sensor according to claim 15 wherein the surface on which the strain sensing element is located and the mounting surface are parallel.
17. A strain sensor according to any preceding claim, wherein the component is a high elongation component.
18. A strain sensor according to any preceding claim, wherein the component is a deformable wind turbine component.
19. A strain sensor according to claim 18, wherein the wind turbine component is a wind turbine rotor blade.
20. A strain sensor according to any preceding claim, wherein the component has a cyclic strain level of over +1-2500 microstrain.
21. A component having the strain sensor of any preceding claim.
22. A deformable wind turbine component having the strain sensor of any preceding claim.
23. A wind turbine comprising a wind turbine component according to claim 22.
24. A method of calibrating the strain sensor of any of claims I to 20 comprising: mounting the strain sensor and a duplicate strain sensing element to a surface; measuring the strain sensed by the strain sensor and the duplicate strain sensing element when a strain is experienced by the surface; and determining a relationship between the strain sensed by the duplicate strain sensing element and the strain sensed by the strain sensor.
25. A method according to claim 24, wherein mounting the strain sensor and the duplicate strain sensing element to a surface comprises mounting the strain sensor on a surface and mounting the duplicate strain sensing element on the surface adjacent the strain sensor.
26. A method according to claims 24 or 25, when dependent on claim 12, wherein mounting the duplicate strain sensing element to the surface includes mounting the flexible backing to which the strain sensing element is attached to the surface.
27. A method according to any of claims 24 to 26, when dependent on claim 13, wherein mounting the duplicate strain sensing element to the surface includes mounting the pre-existing strain gauge of which the strain sensing element is a part to the surface.
28. A system for monitoring strain on a component, comprising: a strain sensor according to any of claims I to 20 mounted to a component; and a controller for calculating the strain experienced by the component based on measurements from the strain sensor and a calibration determined according to the method of any of claims 24 to 27.