Double-shaft single-frequency fatigue testing method for wind turbine blades

Abstract

The invention relates to a method and system for fatigue testing of wind turbine blades using a test system. The test system includes a test stand for securing a wind turbine blade. A first excitation unit is connected to the wind turbine blade for applying a load in a flapping direction. A second excitation unit is connected to the wind turbine blade for applying a load in a shimmy direction. The load controllable unit is connected to the wind turbine blade for adjusting the resonance frequency of the test system. Loads in the flapping direction and the shimmy direction are applied at the same resonant frequency and are measured by a plurality of detection units electrically connected to the control unit. The control unit monitors and adjusts the amplitudes of the first harmonic motion and the second harmonic motion and the phase difference between the first harmonic motion and the second harmonic motion, so that the actual load polar coordinate graph is accurately matched with the target load polar coordinate graph.

Description

Technical Field

[0001] This invention relates to a biaxial resonant excitation method for fatigue testing of wind turbine blade structures. The method uses one or more excitation units to simultaneously apply loads in both the flaring and flapping directions. The fatigue test is performed at a single frequency. One or more load-controllable units are further mounted on the wind turbine blade to adjust the resonant frequencies of the test system in both the flaring and flapping directions to close to this single frequency. Background Technology

[0002] As is well known, wind turbine blades are designed with aerodynamic shapes extending along their length to optimize their wind energy capture efficiency at the installation site. Furthermore, wind turbine blades are designed with sufficient structural strength to withstand dynamic and static loads during operation, idling, and stationary states. Static testing of test blades verifies the wind turbine's design parameters' ability to withstand extreme loads. Fatigue testing, on the other hand, verifies the reliability of the design parameters for withstanding fatigue loads throughout its expected service life.

[0003] Existing technologies use resonant frequencies along both the flapping and teeter directions for blade fatigue testing. However, loads along the diagonal between the two main directions and in all other directions cannot be effectively tested. Furthermore, this method increases the overall testing time, and the applied loads may deviate from actual operating conditions—because in real-world environments, each rotation cycle generates combined loads in both directions simultaneously.

[0004] One approach to this problem is to conduct blade fatigue tests simultaneously at two resonant frequencies in both the flapping and yaw directions, with one frequency for each direction. This results in uncontrolled compound motion (forming a Lissajous curve) of the wind turbine blades during each load cycle. Without structural modifications, when the two resonant frequencies coincide, overloads are generated in the diagonal direction between the two principal directions and in other directions during the load cycle. In some special cases, even when structural modifications make the two frequencies multiples of each other (e.g., a 2:1 ratio forming a figure-eight motion), undesirable off-axis loads still exist. Therefore, this invention aims to eliminate these problems by employing a single, controllable frequency for simultaneous loading in both directions.

[0005] US Patent 2017 / 0241860 A1 discloses a biaxial fatigue testing system comprising a base for fixing and supporting the root end of a wind turbine blade, wherein a shimmy actuator assembly and a flapping actuator assembly are mounted on the blade. The shimmy and flapping actuator assemblies operate at two resonant frequencies. The shimmy resonant frequency can be tuned before testing by adjusting the position of a pulley along the length of a movable arm. The flapping resonant frequency can be tuned by adding a counterweight to the blade. However, it does not disclose a control scheme for controlling the blade tip flapping and shimmy motion.

[0006] US Patent 8621934 B2 discloses another biaxial fatigue testing system comprising a base for securing the root end of a wind turbine blade, wherein a shimmy actuator assembly and a flapping actuator assembly are mounted on the blade. The shimmy and flapping actuator assemblies operate at their respective resonant frequencies.

[0007] US Patent 11255744 B2 discloses another biaxial fatigue testing system operating at two resonant frequencies. This system adjusts the load distribution in the flapping and swaying directions by installing liquid dampers on the wind turbine blades. It requires not only a complex control scheme but also additional components on the blades.

[0008] US Patent 2023 / 0060931 A1 discloses a biaxial testing system that connects a wind turbine blade to a passive load introduction unit for adjusting the blade's resonant frequency. The blade is also connected to an active load introduction unit for applying cyclic loads. The resonant frequency of a first active load introduction unit in one direction is an integer multiple of the resonant frequency of a second active load introduction unit in another direction. The wind turbine blade is mounted in a load frame with clamping blocks. By rotating the load frame or clamping blocks about a longitudinal axis, the torque vector path generated by the active load introduction unit can be adapted to the torque vector path distribution that occurs under actual operating conditions.

[0009] PCT patent application WO 2020 / 089038 A1 discloses a biaxial testing system similar to US patent US 2023 / 0060931 A1. This system adjusts the configuration of the active load introduction units to maintain a constant ratio between the resonant frequency of the first active load introduction unit in one direction and the resonant frequency of the second active load introduction unit in another direction. It explicitly states that when a 1:1 ratio is selected, the phase angle between the two resonant frequencies must be set to 90 degrees. The phase angles and ratios of the different resonant frequencies are set before the biaxial test begins, and no phase angle adjustment is performed during the test.

[0010] Therefore, a fatigue blade testing method that can provide more accurate results for actual operating loads is needed. Summary of the Invention

[0011] One objective of this disclosure is to provide a method and system for solving the problems existing in the above-mentioned known technical solutions.

[0012] One objective of this disclosure is to provide a method and system that allows for simultaneous testing in multiple directions and reduces total testing time.

[0013] One objective of this disclosure is to provide a method and system for dynamically controlling the elliptical motion of wind turbine blades.

[0014] One objective of this disclosure is to provide a method and system that can provide loads that are more consistent with actual operating conditions than design loads or operating loads.

[0015] This invention achieves its objective through a wind turbine blade testing method, wherein the target load polar coordinate diagram is predetermined. The testing method includes the following steps: At least one first excitation unit is installed at a first longitudinal position on the wind turbine blade, and the at least one first excitation unit actively applies a load to the wind turbine blade along the flapping direction at a first excitation frequency; At least one second excitation unit is installed at the second longitudinal position of the wind turbine blade, and the at least one second excitation unit actively applies a load to the wind turbine blade along the oscillation direction at a second excitation frequency; At least one load controllable unit is installed at the third longitudinal position of the wind turbine blade, and the resonant frequency of the test system in the flapping direction and / or oscillation direction is adjusted by the at least one load controllable unit. By activating the first and second excitation units, loads are cyclically applied to the wind turbine blades, causing the wind turbine blades to produce elliptical motion. Using at least one detection unit positioned relative to or on a wind turbine blade, at least the relative motion or load on the wind turbine blade is detected, wherein the output signal of the at least one detection unit is transmitted to a control unit. The control unit monitors and controls the elliptical motion trajectory of the wind turbine blades by controlling the first and second harmonic motions and the phase difference between them in a closed-loop control manner.

[0016] This approach provides an improved method for testing wind turbine blades, enabling simultaneous biaxial fatigue testing in both the flapping and yaw directions. Preset test settings induce elliptical motion in the wind turbine blades, generating loads that better reflect actual operating conditions compared to design or operational loads. Furthermore, because fatigue testing is performed simultaneously in all directions, the overall test time is reduced.

[0017] According to the present invention, a wind turbine blade is placed in a test system to form a cantilever structure. The blade root or blade segment (e.g., the blade root section) is fixed to a rigid reaction block (also known as a test bench). This allows the blade tip to be displaced relative to the blade root in both the flapping and oscillation directions.

[0018] The excitation units are then installed at specific longitudinal positions on the wind turbine blades. Each excitation unit applies a load to the wind turbine blades along the flapping direction and / or the yaw direction. Preferably, a first excitation unit applies the load along the flapping direction at a first excitation frequency. A second excitation unit applies the load along the yaw direction at a second excitation frequency.

[0019] The load controllable units are installed at specific longitudinal positions on the wind turbine blades. Preferably, two or more load controllable units are installed at different longitudinal positions on the wind turbine blades. The load distribution along the length of the wind turbine blades is adjusted using these load controllable units. Furthermore, by adjusting the resonant frequency of the test system in the flapping and / or oscillation directions, the blade tip is made to undergo elliptical motion.

[0020] The detection unit is used to detect or measure the motion of a wind turbine blade relative to its initial starting position. This starting position can be defined by a centerline passing through the blade root and tip. The detection unit can also measure the load on the wind turbine blade relative to its initial no-load state. The output signal of the detection unit is transmitted to the control unit of the test system for data processing. This allows for the direct or indirect measurement of cyclic loads and / or elliptical motion of wind turbine blades using different types of detection units.

[0021] The control unit analyzes the received output signals from the detection unit to determine the cyclic load and / or elliptical motion of the wind turbine blades. The control unit also generates polar coordinate diagrams of the load in the flapping and oscillation directions, respectively. The received output signals from the detection unit can be processed using known algorithms.

[0022] This elliptical motion can be described by the major and minor axes passing through the center of the ellipse. The ellipse can be rotated such that the major axis is parallel to the polar coordinates. Figure X The axes form an angle. Elliptical motion can be characterized by a formula in polar coordinates that describes the harmonic motion coordinates of the waving and swinging directions, respectively. The output of this formula produces a first amplitude and a second amplitude, with a phase shift (i.e., a phase difference) between the two amplitudes. This allows elliptical motion to be converted into quantifiable parameters of the control scheme implemented within the control unit.

[0023] The control unit monitors and controls the elliptical trajectory of the wind turbine blades based on the output signals detected or measured by the detection unit. Compared to other known technical solutions, the control unit preferably employs closed-loop control to regulate the elliptical trajectory. This can significantly improve the control accuracy of biaxial fatigue testing of wind turbine blades.

[0024] In one embodiment, the control unit dynamically adjusts the elliptical motion trajectory during the test to match the polar coordinate diagram of the target load.

[0025] Furthermore, the setpoint can be input into the control unit. The control unit can automatically adjust the test system parameters to adjust and maintain the actual load at the desired setpoint. Preferably, the target load curve is pre-set into the control unit as the setpoint for the fatigue test. This allows the control unit to accurately match the elliptical motion of the wind turbine blades with the target load curve during the fatigue test. Compared to other known technologies, this adjustment can be completed within a few load cycles.

[0026] This function achieves its purpose by actively adjusting the configuration of the load-controllable unit to regulate the resonant frequency of the test system. Therefore, it allows for automatic and real-time changes to fatigue test parameters across different load curves without interrupting the test process to adjust the load-controllable unit configuration, thereby further reducing the overall test time.

[0027] In one embodiment, the first excitation frequency is equal to the second excitation frequency, so that the load is applied at the same resonant frequency.

[0028] Unlike known technical solutions, the method of this invention applies loads simultaneously in both the swinging and oscillating directions at the same resonant frequency. The cyclic load generated in this way (thus forming elliptical motion) allows the control unit to independently control the load peak values ​​in the swinging and oscillating directions. Furthermore, this technical solution enables the control unit to generate load curves with extremely high accuracy.

[0029] In one embodiment, the control unit performs closed-loop control on the phase difference between the first harmonic motion in the waving direction and the second harmonic motion in the swinging direction.

[0030] This method enables the control unit to control the phase difference between the first and second amplitudes. Preferably, the control unit can implement closed-loop control of the phase difference between the waving direction and the swinging direction, thereby generating a phase-locked offset between the first and second harmonic motions. This can be achieved by actively changing the load (e.g., inertia) introduced in the swinging direction. If the phase difference between the waving direction and the swinging direction is too large, the load (e.g., inertia) in the swinging direction is reduced, which will cause the second excitation frequency to increase, thereby changing the cycle period and ultimately adjusting the phase difference.

[0031] In one embodiment, the control unit independently controls the first amplitude of the first harmonic motion and the second amplitude of the second harmonic motion.

[0032] This method enables the peak load of elliptical motion to be separated and independently controlled by the control unit. The control unit can also be configured to independently control and adjust the first and second amplitudes, thereby allowing for more precise adjustment of the load in the swinging and oscillating directions.

[0033] In one embodiment, the load controllable unit is an inertial unit, and the change in inertia introduced by the oscillation direction can be caused by actively changing the inertia of the inertial unit.

[0034] The load introduced by the yawing direction can be actively adjusted using one or more inertial units mounted on the wind turbine blades. These inertial units employ a variable configuration structure, allowing their inertia to be automatically adjusted under the control of a control unit. The control unit changes the inertia by sending control signals to each inertial unit. The load introduced by the yawing direction can be measured by a detection unit. The control unit implements closed-loop control of the inertia of the inertial units based on a setpoint and measured load values.

[0035] In one embodiment, the load controllable unit is a stiffness unit, and the change in stiffness introduced by the swing direction can be caused by actively changing the stiffness value of the stiffness unit.

[0036] The load introduced by the flapping direction can be actively adjusted by one or more stiffness units installed on the wind turbine blades. These stiffness units employ a variable configuration structure, allowing their stiffness to be automatically adjusted under the control of a control unit. The control unit changes the stiffness by sending control signals to each stiffness unit. The load introduced by the flapping direction can be measured by a detection unit. The control unit implements closed-loop control of the stiffness of the stiffness units based on a setpoint and the measured load value.

[0037] By combining and adjusting the stiffness in the swing direction and the inertia in the oscillation direction, the test system can obtain the same resonant frequency.

[0038] In one embodiment, further loads are introduced onto the wind turbine blades via at least one passive load unit.

[0039] The passive load unit can further introduce load along the oscillation direction and / or the swinging direction. Unlike the load controllable unit, the passive load unit is not actively controlled by the control unit, but instead provides a constant load. The passive load unit can introduce a preset load along the oscillation direction or the swinging direction.

[0040] This invention achieves its objective through a wind turbine blade testing system, wherein the polar coordinate diagram of the target load is predetermined. The testing system comprises: A test bench is configured to receive and support wind turbine blades during testing; At least one first excitation unit is configured to be installed at a first longitudinal position on a wind turbine blade, the at least one first excitation unit actively applying a load to the waving direction of the wind turbine blade at a first excitation frequency; At least one second excitation unit is configured to be installed at a second longitudinal position on the wind turbine blade, and the at least one second excitation unit actively applies a load to the oscillation direction of the wind turbine blade at a second excitation frequency; At least one load controllable unit is configured to be installed at a third longitudinal position on the wind turbine blade, and the at least one load controllable unit is configured to adjust the resonant frequency of the test system in the flapping direction and / or oscillation direction. The control unit, electrically connected to the first excitation unit and the second excitation unit, is configured to control the cyclic load on the wind turbine blades by sending control signals to the first excitation unit and the second excitation unit. At least one detection unit is positioned relative to or on a wind turbine blade and connected to a control unit. The at least one detection unit is configured to detect at least the relative motion or load of the wind turbine blade and transmit at least one output signal to the control unit. The control unit is configured to monitor and control the elliptical motion trajectory of the wind turbine blades by controlling the first and second harmonic motions and the phase difference between the first and second harmonic motions in a closed-loop control manner.

[0041] This approach provides an improved test configuration that allows for simultaneous biaxial fatigue testing of wind turbine blades in both the flapping and oscillation directions. The preset test settings induce elliptical motion in the wind turbine blades, generating loads that more closely reflect actual operating conditions than design or operational loads. Furthermore, because fatigue testing is performed simultaneously in all directions, the overall test time is reduced.

[0042] According to the present invention, the testing system is configured to receive and hold a wind turbine blade in a test position, thereby forming a cantilever structure. The testing system includes a test rig that can be connected to the wind turbine blade. Preferably, the test rig includes a mounting interface configured to receive and fix the blade root end or blade segment of the wind turbine in a specific position. This allows the blade tip to be displaced relative to the blade root end in both the flapping and flaring directions.

[0043] One or more excitation units are configured to be installed at a dedicated loading position along the length of a wind turbine blade. Each excitation unit is configured to apply a load to the wind turbine blade along the flapping direction and / or the flaring direction, respectively. Preferably, the first excitation unit applies the load along the flapping direction at a first excitation frequency. The second excitation unit applies the load along the flaring direction at a second excitation frequency. Alternatively, a combined excitation unit can be used to apply loads simultaneously along the flapping and flaring directions at a specific excitation frequency. After the excitation unit is activated, it induces cyclic loads within the wind turbine blade along the flapping and flaring directions.

[0044] One or more load control units are configured to be installed at dedicated loading locations along the length of the wind turbine blade. A single or multiple load control units may be installed at one or more longitudinal locations on the wind turbine blade and are used to adjust the resonant frequencies of the test system in the flapping and / or oscillation directions, thereby causing the blade tip to follow an elliptical motion trajectory.

[0045] One or more detection units are used to detect the motion of wind turbine blades relative to their initial starting position, and can directly or indirectly measure the cyclic load on the wind turbine blades. Each detection unit is electrically connected to the control unit via wired or wireless means, and the control unit processes the measured or detected signals. Different types of detection units can be used to detect or measure different categories of signals.

[0046] The control unit contains a microprocessor connected to a database, which embeds executable computer instructions. Output signals from the detection unit and / or data generated by the control unit can be stored in the database. As mentioned earlier, the control unit incorporates formulas in polar coordinates describing the harmonic motion coordinates of the swing direction and oscillation direction, respectively. This allows elliptical motion to be converted into quantifiable parameters of the control scheme implemented within the control unit.

[0047] Furthermore, the control unit monitors and controls the elliptical motion trajectory of the wind turbine blades based on the output signals detected or measured by the detection unit. In the preferred configuration, the control unit employs closed-loop control to manage the elliptical motion trajectory, while known solutions use open-loop control, and the known solutions do not dynamically change the resonant frequency during fatigue testing. This can significantly improve the control accuracy of biaxial fatigue testing of wind turbine blades.

[0048] In one embodiment, the control unit is configured to dynamically adjust the elliptical motion trajectory during testing to match the polar coordinate diagram of the target load.

[0049] The control unit can be configured to receive setpoints (e.g., target load curves) and store them in a database, and can also automatically adjust the test system parameters in real time based on the setpoints. Preferably, the control unit can match the target load curve by adjusting the elliptical motion. This allows the control unit to accurately match the elliptical motion of the wind turbine blades with the target load curve during fatigue testing. Compared to other known technologies, this adjustment can be completed within a few load cycles.

[0050] This function adjusts the resonant frequency of the test system by actively changing the configuration of the load-controllable unit through the control unit. Therefore, it allows for automatic changes in fatigue test parameters between different load curves without interrupting the test process to adjust the configuration of the load-controllable unit, thereby further shortening the total test time.

[0051] In one embodiment, the first excitation unit and the second excitation unit are configured to apply loads to the wind turbine blades at the same excitation frequency.

[0052] As mentioned above, the method of this invention applies loads simultaneously in both the swinging and oscillating directions at the same resonant frequency. The cyclic load generated in this way (thus forming elliptical motion) allows the control unit to independently control the load peaks in the swinging and oscillating directions. Furthermore, this technical solution enables the control unit to generate load curves with extremely high accuracy.

[0053] In one embodiment, the control unit is configured to adjust the phase difference between the first harmonic motion in the waving direction and the second harmonic motion in the swinging direction via closed-loop control.

[0054] The control unit can be configured to adjust the phase difference between its output first amplitude and second amplitude. Preferably, the control unit can implement closed-loop control of the phase difference between the waving direction and the swinging direction. This can be achieved by the control unit actively changing the configuration of the second excitation unit, ultimately adjusting the load (e.g., inertia) introduced by the swinging direction. If the phase difference between the waving direction and the swinging direction is too large, the load (e.g., inertia) in the swinging direction is changed, which will cause a change in the second excitation frequency, thereby changing the cycle period and ultimately adjusting the phase difference.

[0055] In one embodiment, the load controllable unit is an inertial unit that is configured to allow the inertia introduced by the oscillation direction to be actively changed.

[0056] The load controllable unit can employ an adjustable inertial unit, allowing its inertia to vary. Preferably, the inertia of the inertial unit can be automatically adjusted under the control of the control unit. The inertia introduced by the oscillation direction can be adjusted using one or more inertial units.

[0057] The control unit sends control signals to each inertial unit, which adjusts its inertia accordingly. The load introduced by the oscillation direction is measured by the detection unit, and the control unit controls the inertia of the inertial units by combining the setpoint and the measured value.

[0058] The inertial unit may comprise a frame structure on which one or more mass blocks are movably mounted. Each mass block is positioned relative to a pivot point of the inertial unit to generate an inertial torque, and the mass blocks automatically approach or move away from the pivot point via actuators connected to the inertial unit. Furthermore, the inertial unit may have a vertical or horizontal rotation axis about which the mass blocks and / or actuators can rotate. Other known inertial unit configurations may also be used. This provides a simple and lightweight inertial introduction solution for wind turbine blades.

[0059] In one embodiment, the load-controllable unit is a stiffness unit whose configuration is variable so that the stiffness introduced by the swing direction can be actively changed.

[0060] Preferably, the stiffness of the stiffness unit can be automatically adjusted under the control of the control unit. The stiffness introduced by the swing direction can be adjusted by one or more stiffness units.

[0061] The control unit sends control signals to each stiffness unit, which then adjusts its stiffness accordingly. The load introduced in the swing direction is measured by the detection unit, and the control unit controls the stiffness of the stiffness units by combining the setpoint with the measured values.

[0062] By combining and adjusting the stiffness in the swing direction and the inertia in the oscillation direction, the test system can achieve the same resonant frequency. Both the stiffness unit and the inertia unit can be uniformly controlled by the control unit.

[0063] The stiffness element may include at least one deformable element, which is directly fixed between a ground reference and at least one saddle mounted on the wind turbine blade via one or more pivot connections. The deformable element may take the form of a spring, a cylinder with a movable piston, a flexible plate, or a plate-like structure, or may employ other known stiffness element configurations.

[0064] Plates or plate-like structures may be attached to two or more saddles and to a common pivot point on the ground reference.

[0065] In one embodiment, at least one passive load unit is further connected to the wind turbine blade, the passive load unit being positioned relative to at least one load controllable unit.

[0066] The load controllable unit can be used in combination with one or more passive load units. The passive load unit can be configured to introduce a predetermined or rated load onto the wind turbine blades along the oscillation direction and / or flapping direction. This passive load unit can be used as a passive follower with a movable pendulum, or it can adopt a similar structural configuration to the load controllable unit (but without actuators). In this way, the mass can remain in a fixed position relative to the pivot point. Other configuration options are also possible.

[0067] One or more load units installed on wind turbine blades can be in the form of load controllable units. Attached Figure Description

[0068] Several examples will be described below with reference to the accompanying drawings. Throughout the text, the same numbers refer to similar elements. Therefore, similar elements will not be described in detail in the description of each figure. It should be noted that the accompanying drawings are only for illustrative purposes and are not an exhaustive exposition of the claimed invention, nor do they constitute a limitation on the scope of the claimed invention. Furthermore, the illustrated examples need not possess all the aspects or advantages shown. Aspects or advantages described in connection with a particular example are not necessarily limited to that example and may apply to those examples even if they are not illustrated or explicitly described in other examples.

[0069] Exemplary embodiments of the present invention are shown in the accompanying drawings, wherein: Figure 1 An example of a wind turbine is shown; Figure 2 An exemplary embodiment of the test system according to the present invention is shown; Figure 3 An exemplary load polar plot of the elliptical motion of a wind turbine blade is shown; Figure 4 An exemplary graph showing the first harmonic motion and the second harmonic motion is shown; Figure 5 An exemplary load polar plot is shown before fatigue test calibration; Figure 6 An exemplary load polar plot after fatigue test calibration is shown; Figure 7 A first embodiment of the inertial unit according to the present invention is shown; Figure 8 A second embodiment of the inertial unit is shown; Figure 9 A third embodiment of the inertial unit is shown; Figure 10 An embodiment of the stiffness element according to the present invention is shown. Detailed Implementation

[0070] The exemplary embodiments will now be described more fully with reference to the accompanying drawings. It should be noted that these embodiments may take different forms and should not be construed as limited to the content described herein. Therefore, the following description uses examples only to explain various aspects with reference to the accompanying drawings.

[0071] In this specification, when referring to an element as being "connected" to another element, it may mean that the element is "directly connected," "electrically connected," "fluidically connected," or "communicationally connected" to the other element, or it may mean that the connection to the other element is achieved through one or more intermediate elements.

[0072] The terminology used herein is for the purpose of describing particular examples only and is not intended to be limiting. Unless otherwise expressly stated, the terms "comprising," "including," etc., as used in this specification are intended to indicate the presence of the stated features, integrals, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, and / or combinations thereof.

[0073] Unless otherwise defined, all terms used herein (including technical and scientific terms) shall have the meanings commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that, unless explicitly defined herein, terms defined in dictionaries shall be interpreted in accordance with their meanings in the relevant technical context, and not in an idealized or overly formalized manner.

[0074] Figure 1 An example of a horizontal axis wind turbine (HAWT) 1 is shown, comprising a wind turbine tower 2, a nacelle 3 mounted on top of the tower, and a rotor connected to a drive system within the nacelle. The rotor comprises a hub 4 and several wind turbine blades 5 connected to the hub. Three blades are shown here, but the hub 4 may have more or fewer blades connected to it.

[0075] The wind turbine 1 shown is an onshore type, but it can also be an offshore wind turbine. The wind turbine blades can adopt a continuous or modular structure. The wind turbine blade 5 extends from the root end 7 to the tip end 6 and includes a pressure surface and a suction surface.

[0076] Figure 2 An exemplary embodiment of a test system 8 for fatigue testing of a wind turbine blade 5 is shown. The blade root end 7 can be fixedly mounted on a test bench 9, and the rest of the wind turbine blade 5 extends longitudinally, allowing the blade tip 6 to be displaced in the flapping and oscillation directions.

[0077] The first excitation unit 10 and the second excitation unit 11 are arranged at specific positions along the length of the wind turbine blade 5. The first excitation unit 10 is configured to apply a load to the wind turbine blade 5 along the flapping direction. The second excitation unit 11 is configured to apply a load to the wind turbine blade 5 along the oscillation direction. Both the first excitation unit 10 and the second excitation unit 11 are electrically connected to the control unit 12, and the control unit controls the load input in the flapping and oscillation directions.

[0078] Multiple load units 14a and 14b are arranged or installed at specific positions along the length of the wind turbine blade 5. The load controllable unit 14b can introduce adjustable loads onto the wind turbine blade 5 along the flapping direction and / or oscillation direction. The load of the load controllable unit 14b is actively controlled by the control unit 12. By adjusting the load of the load controllable unit 14b, the control unit applies the loads in the flapping and oscillation directions at the same resonant frequency as the test system 8, thereby inducing an elliptical motion 13 of the wind turbine blade 5.

[0079] Alternatively, one or more of the aforementioned load units may be configured as a passive load unit 14a, which is used to introduce a rated load onto the wind turbine blade 5 along the flapping direction and / or oscillation direction.

[0080] Detection units 15, 16, and 17, electrically connected to control unit 12, are used to detect or measure the cyclic load or elliptical motion of the wind turbine blade 5. Sensor-type detection unit 15 (e.g., strain gauge) can be arranged on the pressure or suction surface of the wind turbine blade 5. Rangefinder-type detection unit 16 (e.g., laser rangefinder) can be positioned relative to the wind turbine blade 5 (e.g., aligned with a saddle on the blade). Camera-type detection unit 17 can also be arranged relative to the wind turbine blade 5. The output signals of all detection units 15, 16, and 17 are transmitted to control unit 12, where they are processed to determine the load distribution of the blade in the flapping and oscillating directions.

[0081] Figure 3 An exemplary load polar plot 16 shows the elliptical motion of a wind turbine blade 5. The X-axis 17 represents the displacement in the oscillation direction, and the Y-axis 18 represents the displacement in the flapping direction. This elliptical motion is described by the major axis 20 and the minor axis 19 passing through the center point of the ellipse, where the ellipse is rotated by an angle θ relative to the X-axis 17.

[0082] Figure 4 Example graphs of the first harmonic motion 21 and the second harmonic motion 22 of the wind turbine blade 5 in the flapping and oscillation directions are shown. The X-axis 23 represents time. There is a phase difference 24 between the amplitude Af of the first harmonic motion and the amplitude Ae of the second harmonic motion. The first harmonic motion 21, the second harmonic motion 22, and the phase difference 24 output by the control unit 12 together describe... Figure 3 The elliptical motion shown is 16.

[0083] Figure 5 A sample load polar coordinate diagram before fatigue test calibration is shown. X-axis 27 represents displacement in the oscillation direction, and Y-axis 28 represents displacement in the swing direction. The target load polar coordinate diagram 25 has been input into the control unit 12. The control unit 12 simultaneously determines the actual load polar coordinate diagram 26 of the test system 8.

[0084] Figure 6 An example load polar plot after fatigue test calibration is shown. Calibration is achieved by adjusting the load of the load controllable unit 14b and performing multiple runs and measurements. As shown, the actual load polar plot 26 can be adjusted to coincide with the target load polar plot 25 using a preset control scheme.

[0085] Figure 7 A first embodiment of the inertial unit 33 of the present invention is shown. The inertial unit 33 includes a frame structure 34 connected to a saddle 29 on a wind turbine blade 5 via a linkage mechanism 34a. The inertial unit 33 also includes at least one mass block 35 connected to a pivot point of the frame structure 34 via an actuator 36. The operation of the actuator 36 is controlled by a control unit 12, the operation of which can adjust the inertia of the inertial unit 33.

[0086] The inertia of the inertial element 33 can be introduced along the waving direction 32 and / or the swing direction 31.

[0087] Figure 8 A second embodiment of the inertial unit 33' is shown, wherein the inertial unit 33' includes a mass block 35 in a fixed position and an additional mass block 37 connected to an actuator 36. The additional mass block 37 is driven by the actuator 36 and ultimately controlled by the control unit 12.

[0088] Figure 9 A third embodiment of the inertial unit 33'' is shown, wherein both the mass block 36' and the actuator 37' are arranged vertically (distinct from...). Figure 7 and Figure 8 (Horizontal arrangement).

[0089] Figure 10 An embodiment of the stiffness unit 38 of the present invention is shown. The stiffness unit 38 includes at least one deformable element 39 (such as a spring), with its two ends connected to the saddle 29 and the ground reference, respectively. The spring stiffness of this stiffness unit can be adjusted by a control unit.

Claims

1. A test method for wind turbine blades (5), wherein a target load polar coordinate diagram (25) is predetermined, the test method comprising the following steps: At least one first excitation unit (10) is installed at a first longitudinal position on the wind turbine blade (5), wherein the at least one first excitation unit (10) actively applies a load to the wind turbine blade (5) along the flapping direction (32) at a first excitation frequency; At least one second excitation unit (11) is installed at a second longitudinal position on the wind turbine blade (5), wherein the at least one second excitation unit (11) actively applies a load to the wind turbine blade (5) along the oscillation direction (31) at a second excitation frequency; At least one load controllable unit (14b) is installed at the third longitudinal position of the wind turbine blade (5), wherein the at least one load controllable unit (14b) adjusts the resonant frequency of the test system (8) in the flapping direction (32) and / or the oscillation direction (31); By activating the first excitation unit (10) and the second excitation unit (11), loads are cyclically applied to the wind turbine blade (5), causing the wind turbine blade (5) to produce elliptical motion; Using at least one detection unit (15, 16, 17) positioned relative to or on the wind turbine blade (5), at least the relative motion or load of the wind turbine blade (5) is detected, wherein the output signal of the at least one detection unit (15, 16, 17) is transmitted to the control unit (12). The control unit (12) monitors and controls the elliptical motion trajectory (16) of the wind turbine blade (5) by controlling the first harmonic motion, the second harmonic motion and the phase difference (θ) between the first and second harmonic motions in a closed-loop control manner.

2. The method according to claim 1, wherein, The control unit (12) dynamically adjusts the elliptical motion trajectory (16) during the test to match the polar coordinate diagram (25) of the target load.

3. The method according to claim 1 or claim 2, wherein, The first excitation frequency is equal to the second excitation frequency, so that the load is applied at the same resonant frequency.

4. The method according to any one of claims 1 to 3, wherein, The control unit (12) performs closed-loop control on the phase difference between the first harmonic motion (22) of the waving direction (32) and the second harmonic motion (21) of the swinging direction (31).

5. The method according to any one of claims 1 to 6, wherein, The control unit (12) independently controls the first amplitude (Af) of the first harmonic motion (22) and the second amplitude (Ae) of the second harmonic motion (21).

6. The method according to any one of claims 1 to 5, wherein, The load controllable unit is an inertial unit (33), and the inertia introduced by the swing direction (31) is changed by actively changing the inertia of the inertial unit (33).

7. The method according to any one of claims 1 to 6, wherein, The load controllable unit is a stiffness unit (38), and the stiffness introduced by the swing direction (32) is changed by actively changing the stiffness of the stiffness unit (38).

8. The method according to any one of claims 1 to 7, further comprising introducing a further load on the wind turbine blade (5) by at least one passive load unit (14a).

9. A test system for testing wind turbine blades (5), wherein a target load polar coordinate diagram (25) is predetermined, the test system (8) comprising: A test bench (9) is configured to receive and support the wind turbine blades (5) during the test. At least one first excitation unit (10) is configured to be installed at a first longitudinal position on the wind turbine blade (5), and the at least one first excitation unit (10) actively applies a load along the flapping direction (32) of the wind turbine blade (5) at a first excitation frequency; At least one second excitation unit (11) is configured to be installed at a second longitudinal position on the wind turbine blade (5), and the at least one second excitation unit (11) actively applies a load along the oscillation direction (31) of the wind turbine blade (5) at a second excitation frequency; At least one load controllable unit (14b) is configured to be installed at a third longitudinal position on the wind turbine blade (5), and the at least one load controllable unit (14b) is configured to adjust the resonant frequency of the test system (8) in the flapping direction (32) and / or the oscillation direction (31); The control unit (12), electrically connected to the first excitation unit (10) and the second excitation unit (11), is configured to control the cyclic load introduced into the wind turbine blade (5) by sending control signals to the first excitation unit (10) and the second excitation unit (11); At least one detection unit (15, 16, 17) is positioned relative to or on the wind turbine blade (5) and connected to the control unit (12). The at least one detection unit (15, 16, 17) is configured to detect at least the relative motion or load of the wind turbine blade (5) and transmit at least one output signal to the control unit (12). The control unit (12) is configured to monitor and control the elliptical motion trajectory (16) of the wind turbine blade (5) by controlling the first harmonic motion, the second harmonic motion and the phase difference (θ) between the first and second harmonic motions in a closed-loop control manner.

10. The system according to claim 9, wherein, The control unit (12) is configured to dynamically adjust the elliptical motion trajectory (16) during the test to match the polar coordinate diagram (25) of the target load.

11. The system according to claim 9 or claim 10, wherein, The first excitation unit (10) and the second excitation unit (11) are configured to apply loads to the wind turbine blades (5) at the same excitation frequency.

12. The system according to any one of claims 9 to 11, wherein, The control unit (12) is configured to perform closed-loop control on the phase difference between the first harmonic motion (22) of the waving direction (32) and the second harmonic motion (21) of the swing direction (31).

13. The system according to any one of claims 9 to 12, wherein, The load controllable unit is an inertial unit (33), wherein the inertial unit (33) is configured to allow the inertia introduced by the swing direction (31) to be actively changed.

14. The system according to any one of claims 9 to 13, wherein, The load controllable unit is a stiffness unit (38), wherein the stiffness of the stiffness unit (38) can be changed so that the stiffness introduced by the swing direction (32) can be actively changed.

15. The system according to any one of claims 9 to 14, wherein, At least one passive load unit (14a) is further connected to the wind turbine blade (5), the at least one passive load unit (14a) being positioned relative to the at least one load controllable unit (14b).

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