Background of the invention

The invention relates to a windmill blade of the type disclosed in the preamble to claim 1, and a method for the reduction of the oscillations in a windmill blade of the kind disclosed in the preamble to claim 13.

During normal operation, a windmill blade will be exposed to a large number of forces which are dependent upon para¬ meters such as the construction of the windmill and the conditions of wind and movement.

Under certain conditions, in the plane of the blade there will arise oscillations transversely to the length of the blade, so-called transversal oscillations. These trans¬ versal oscillations stem among other things from a number of resonances which are primarily transverse resonances. During operation in strong winds, this manifests itself in a particularly complicated oscillation pattern with com¬ ponents not only at right-angles to the blade but also in the plane of the blade. Moreover, the oscillation pattern becomes more complicated due to the fact that the excita- tion of the resonances varies with both place and time, as is the case for example for a transient excitation. Furthermore, a strong turbulence arises especially at the trailing edge of the blade. Finally, the wave dispersal in the windmill blade is a combination of several wave types, these being mainly transversal, longitudinal, torsional and flectional waves.

This is a great disadvantage, the reason being that it not only gives rise to a limitation of the utilization of the windmill during operation, but also results in a reduction of the lifetime of the windmill and of the blade, in that

among other things the blade itself, the windmill hub, the main shaft, the tower and bearings are all more or less ex¬ posed to strong vibrations. The blade will thus be exposed to oscillations which can result in a deterioration of, among other things, the glass fiber structure. This in turn will mean that service has to be carried out on the wind¬ mill or the windmill blade, which is both time-consuming and costly. Moreover, the load on the movable mechanical parts will result in considerable wear.

The above-mentioned vibration and oscillation phenomena have been known for many years, but the disadvantages con¬ nected with these conditions have hitherto been limited, the reason being that the windmill blades have hitherto been of relatively small dimensions. However, the problem has now grown to greater proportions as a consequence of the constantly-increasing dimensions of the windmills. The character of the disadvantages ascertained are no longer only of a dimensional nature, but a physical obstacle to further progress.

Advantages of the invention

According to the invention, by incorporating an oscilla- tion-reduction mechanism in the blade, said mechanism being coupled to the blade or a part of same, and consisting of one or more oscillation-reduction elements which are mutu¬ ally connected in series or in parallel, and where at least one oscillation-reduction element is made up of at least two of the components resilience, mass and suppression, and where any further oscillation-reduction elements are made up of one or more of the components resilience, mass and suppression, said oscillation-reduction elements having one or more degrees of freedom and preferably moving in a straight line or executing angular displacement around a position of equilibrium, an oscillation-reduction mechanism

under ideal conditions with regard to dimensional and ex- citational states will cancel out the undesired oscilla¬ tions. The disadvantages with which windmills have been encumbered by these detrimental oscillations are hereby avoided in a surprisingly simple manner.

The decisive aspect of the invention is thus the discrete suspension of the oscillation-reduction elements in the blade, which after measurements on a certain windmill blade or type can in a simple manner be dimensioned for the ac¬ tual application, all depending on the orientation and size of the undesired oscillation components. There can thus be achieved a great reduction of undesired oscillations re¬ gardless of the magnitude and direction of the oscilla- tions.

As disclosed in claim 2, by allowing the kinetic components of at least one of the oscillation-reduction elements to be oriented mainly in the blade's transversal direction, a reduction is achieved in precisely those oscillations which stem from the transversal oscillations in the blade. As a consequence of this reduction, a reduction is achieved in other wave phenomena in the blade. However, it will with advantage also be possible to orient all or some of the damping elements in a direction other than that of the transversal oscillations. Under certain conditions, it can be advantageous to reduce the follow-on oscillations rather than the directly-occurring transversal oscillations, the reason being that under certain conditions these can be easier to locate and herewith to reduce.

As disclosed in claim 3, by configuring the oscillation- reduction mechanism or at least one oscillation-reduction element with a natural frequency which corresponds substan- tially to or is the same as the dominating natural fre¬ quency of the blade, the most effective reduction of the

vibrations is achieved, in that the oscillation-reduction mechanism or the individual oscillation-reduction element's counter-oscillation is also frequency dependent, and as such can best be utilized at its maxium value. By a coup- ling of these oscillation-reduction elements it is advant¬ ageous that the respective natural frequencies are adjusted and oriented in relation to the natural frequencies of the blade, so that the total oscillation reduction of the oscillation-reduction mechanism corresponds to but is in opposition to the oscillation pattern demonstrated by the blade. If a strong reduction in oscillation is desired, this will typically best be achieved within the oscilla¬ tion-reduction element's 3dB bandwidth.

As disclosed in claim 4, by coupling the oscillation- reduction element or the oscillation-reduction elements to the blade in that or those areas of the blade where its greatest vibrations in relation to the non-activated state are generated, a further optimization of the oscillation reduction is achieved, in that the oscillation-reduction element has the greatest possible influence on the blade where the oscillation amplitudes actually arise. An oscil¬ lation-reduction element placed at a point of no oscilla¬ tion will thus have little or no influence on the oscilla- tions in the blade structure.

As disclosed in claim 5, by the incorporation and coupling of at least one of the oscillation-reduction elements to the blade in the vicinity of the blade tip, the most ef- fective oscillation reduction is achieved in relation to the transversal oscillations in the blade, in that these frequently have their maximum components in the blade tip. The oscillation-reduction element or the oscillation- reduction elements will typically be located in the vicin- ity of the blade tip, but in certain situations it will also be possible to locate the oscillation-reduction ele-

ment or some of the elements further down towards the blade root. The optimum location of the oscillation-reduction elements will be determined by the blade's oscillation pattern under given or assumed excitations. The oscillation reduction will typically be most effective by locating the oscillation-reduction elements in or in the vicinity of those areas where the blade displays its maxium vibration.

As disclosed in claim 6, by allowing the oscillation- reduction mechanism to consist of a common, suppressed oscillator, a damping of the undesired oscillations is achieved in a simple manner. Among the advantages afforded by the relevant embodiment, it can be mentioned that the parameters involved are temperature independent.

As disclosed in claim 7, by allowing the oscillation- reduction mechanism to consist of a closed container filled with fluid and containing a simple oscillator, a particu¬ larly compact embodiment is achieved.

As disclosed in claim 8, by further providing the flow- through path of the oscillation-reduction mechanism with a pressure valve which can preferably be regulated, a partial independence of the viscosity is achieved. The pressure valves will also have the advantage that they can be ad¬ justed so that the acceleration of the mass necessary to move the damper is greater than the gravitational accelera¬ tion, and hereby reduce the wear on the system during op¬ eration in situations where the system is not subject to oscillations.

As disclosed in claim 9, by allowing the suppression to consist of a pre-stressed friction blanket against the mass, a purely mechanical embodiment is achieved.

As disclosed in claim 10, by providing the flow-through

path with an adjustable throttle valve, a dynamic and ad¬ justable damping of the oscillation-reduction element is achieved.

As disclosed in claim 11, by allowing the oscillation- reduction mechanism to consist of just one oscillation- reduction element, where the oscillation-reduction element comprises an aluminium disk to which a mass is connected, said aluminium disk being rotatably mounted on an axle and having a magnet arranged around its edge, so that the mag¬ net, upon movement of the aluminium disk in relation to the magnet, induces eddy currents, a flexible and durable con¬ struction is achieved whereby the system's parameters can easily be adjusted without any special demands with regard to the mechanical parts. It will thus be possible, for example, to carry out running adjustments and control the system's parameters with an electrical control circuit. Under certain conditions, it can be an advantage for this circuit to be remotely controlled.

As disclosed in claim 12, by allowing the oscillation- reduction mechanism to consist of just one oscillation- reduction element, where the oscillation-reduction element comprises a closed container filled with fluid, said con- tainer being divided into two sections by a perforated dividing wall, where the mass consists of fluid, where a possible resilience consists of a resilient gas, and where the damper consists of the velocity-dependent resistance directed against the movement which stems from the movement of the fluid through the perforated dividing wall, an ef¬ fective reduction is achieved of the undesired oscillations in the blade. Further advantages of this embodiment are that the oscillation-reduction mechanism, by the selection of a suitable fluid for same, will not suffer from ageing and hysteris phenomena, which in turn results in a blade which does not place great demands with regard to mainten-

ance.

As disclosed in claim 13, by orienting the oscillation- reduction element or elements in the oscillation direction or oscillation directions of the undesired oscillation(s), and by tuning the resonant frequency of the individual oscillation-reduction element(s) more or less to the fre¬ quency of the undesired oscillation(s), a reduction in the undesired oscillations in the blade is achieved in a simple and effective manner. This method thus makes it possible, with subsequent mounting on known blades, to achieve all of the advantages of the invention.

As disclosed in claim 14, by orienting the oscillation- reduction elements mainly in the blade's transversal direc¬ tion, a particularly effective reduction is achieved in the blade's transversal oscillations.

As disclosed in claim 15, by placing the oscillation- reduction element or elements mainly in those places in the blade structure which have the greatest vibration(s) in relation to the blade's non-activated state, a particularly effective reduction in the undesired oscillations is achieved, the reason being that the oscillation-reduction means have greatest effect on the blade where this has its greatest vibrations.

The drawing

In the following section, some example embodiments accord¬ ing to the invention will be described in more detail with reference to the drawing, where

fig. 1 shows an example of a principle configuration of an oscillation-reduction element, with the basic elements mass, damper and resilience

built into a windmill blade,

figs. 2-6 show examples of embodiments of an oscillation reduction element according to the invention, and

fig. 7 shows a preferred embodiment of the oscilla¬ tion-reduction mechanism consisting of one oscillation-reduction element.

Description of the example embodiments

Any oscillation system in a structure can in principle be configured or simulated as a combination of three elements; a resilience 2, a damper 3 and a mass 4.

In fig. 1 is shown how the direction of movement of the transversal oscillations in a windmill blade 1 are orient¬ ed, in that these are indicated by the arrow 90. In fig. 1, the three elements are coupled together to form an oscilla¬ tion-reduction element according to the invention. This consists of a damper 4, a resilience 2 and a mass 3, which in the given example are coupled to the blade 1 as a simple, suppressed oscillator with a degree of freedom.

In fig. 2 is shown an example embodiment according to the invention, where the oscillation-reduction element consists of a swinging mass 8, which is integrated in a damper hous¬ ing, which in turn consists of a closed container 9 con- taining a viscous medium 11. The mass is placed between two springs 7 and 12 which are arranged in the direction of movement 91 of the mass 8. The mass 8, which thus separates the container into two sections, is provided with- a fluid path 10 through which fluid can flow. The shown and the following oscillation-reduction elements can thus be mount¬ ed firmly in a blade as an oscillation-reduction mechanism

or as a part of an oscillation-reduction mechanism accord¬ ing to the invention. The orientation of the oscillation- reduction element will thus depend on which oscillations the oscillation-reduction element is intended to counter- act.

In fig. 3 is seen a further example embodiment of an oscil¬ lation-reduction element which in principle is configured in the same manner as the example described above. A mass 14, with fluid paths 17 which connect the sections divided by the mass 14, is built into a damper housing 5 which con¬ tains fluid 16. The mass 14 is connected to the damper housing 5 by two springs 13 and 15, and the fluid paths 17 are provided with a pressure valve 18, which provides the advantage that they can be adjusted so that the necessary acceleration of the mass in the direction of movement 92, which is required in order to activate the oscillation- reduction element, is greater than the gravitational accel¬ eration. This reduces wear on the oscillation-reduction element during operation in situations where the blade is not excited.

In fig. 4 is shown another oscillation-reduction element which is made up of a mass 19 which is built into a damper housing 20 containing fluid. The mass 19, which divides the inside of the damper housing 20 into two sections, is con¬ nected to the damper housing in each section by two springs 21 and 22. The two sections are mutually connected by a fluid path 80. By inserting a throttle valve 23 in the fluid path 80 which connects the two sections of the damper housing 20, an adjustable damping of the fluid flow is achieved when the mass is excited in the direction 93.

In fig. 5 is shown a purely mechanical embodiment of an oscillation-reduction element, where a mass 33 is held in a movement path in the direction 94 by two plates 27 with

friction mats 28, said friction mats being pre-stressed by means of springs 25, 26, 31 and 32. The whole system is placed in a housing 29. The damping of the oscillations in the direction of the arrow will thus depend of the elasti- city constant of the springs 25, 26, 31 and 32 and on the friction characteristic of the felt mats 28.

Fig. 6 shows an electromagnetic version of an oscillation- reduction element. This element, which executes angular oscillations in the direction 95 during a given excitation, consists of an aluminium disk 34 with a mass 35. This disk is suspended on an axle 36, and during movement of the system a pole shoe with strong magnet 37 will induce eddy currents in the aluminium disk 34, and will thus produce a resistance to movement.

Fig. 7 shows a preferred embodiment according to the inven¬ tion, where the oscillation-reduction mechanism consists of a single oscillation-reduction element which in turn con- sists of a closed container 40. This container 40 is divid¬ ed into two chambers 44, 45 by a perforated plate 41 through which, all depending on the degree of perforation and the friction which arises at the individual holes in the plate, fluid 42 can flow from the one chamber to the other. The container 40 also contains a gas 43, for example air. During acceleration in the direction 96, the fluid will flow through the perforation in towards the chamber 44, all depending on the amount of acceleration and the degree of perforation. The friction stemming from the per- forated dividing wall 41, which serves as a number of re¬ strictions, will act against the movement of the fluid. Furthermore, the gas 43 acts as a resilience which partly counteracts the movement of the fluid and partly stores the movement energy.

Under certain conditions, however, it can be an advantage

to omit the gas 43, so that the oscillation system consists solely of the basic elements, i.e. mass and damper.

With this embodiment, the oscillation-reduction mechanism consists of one oscillation-reduction element, as described above, which is placed in the vicinity of the tip 6 of the windmill blade, where the natural frequency of the oscilla¬ tion-reduction element corresponds to or is +/- 10% of the natural frequency of the blade. This provides a maximum utilization of the oscillation-reduction mechanism.

According to the invention, all of the examples of oscilla¬ tion-reduction elements mentioned can be coupled both mutu¬ ally and to the blade to form an oscillation-reduction mechanism whereby the oscillation-reduction elements to¬ gether produce the desired magnitudes and orientations of oscillation reduction.