High-proportion sub-modal test method for large flexible beam structure

Abstract

The invention provides a high-proportion sub-modal test method for a large flexible beam structure, which comprises the following steps: firstly, taking a part of the length of an original beam structure to establish a model, respectively taking the length size of a prototype beam to be 4-8 times of the length size of the taken sub-structure beam, and keeping the shape of the cross section of each position of the prototype beam to be consistent with that of the cross section of the sub-structure beam; the material attribute of the prototype beam is kept consistent with the material attribute of the substructure beam; then clamping the substructure beam in a manner that one end is fixed and the other end is free; the modal frequency of the substructure beam is measured in a knocking or frequency sweeping mode; and finally, obtaining the structural mode of the large flexible beam by means of a given mode frequency estimation formula, and averaging several groups of tests to obtain a final result. According to the method, the modal frequency of the beam under the large-size'free-free 'condition can be estimated by using a small-size cantilever beam model.

Description

Technical Field

[0001] The invention relates to the field of aerospace technology, and in particular to a high-proportion sub-modal test method for a large flexible beam structure. Background Art

[0002] Flexible beam structures are widely used in aerospace engineering, and their dynamic characteristics are directly related to the safety and performance optimization of the structure. In actual engineering, modal tests are required to analyze their modal parameters (including modal frequency, vibration mode, etc.).

[0003] In the prior art, the traditional modal test methods mainly have the following problems: 1. High test cost: modal testing usually needs to be performed on a full-size prototype. Due to the large size of the prototype structure and the complex operation, the test cost is significantly increased. 2. Large measurement error: The test is usually carried out in a suspension manner, but the suspension stiffness is close to the modal frequency of the beam structure, which can easily lead to distortion of the measurement results, especially in the low-frequency band. 3. Limited test conditions: The modal test of the real prototype beam requires a large-scale test space and high-performance experimental equipment, which is subject to many restrictions on the site and conditions. Summary of the invention

[0004] The purpose of the present invention is to provide a high-proportion submodal test method for a large flexible beam structure. By using a part of the original beam model for testing, and combining mathematical models and experimental formulas to deduce the modal characteristics of the entire beam, the experimental cost and complexity are greatly reduced, while the accuracy of the test results is improved. By testing a small-sized substructure beam, combined with formula derivation and boundary condition correction, the modal parameters of a large flexible beam under "free-free" boundary conditions can be quickly and accurately predicted.

[0005] According to one object of the present invention, the present invention provides a high-proportion submodal test method for a large flexible beam structure, comprising the following steps:

[0006] (1) Cut off a part of the length of the prototype beam as the substructure beam to ensure that the cross-sectional shape and material properties of the substructure beam are consistent with those of the prototype beam;

[0007] (2) Clamp the substructure beam in a manner where one end is fixed and the other end is free;

[0008] (3) Install sensors on the substructure beams and record sensor measurement data;

[0009] (4) Using fast Fourier transform (FFT) to process sensor measurement data and obtain the modal frequency of the substructure beam;

[0010] (5) Calculate the boundary correction coefficient a according to the formula:

[0011] a≈0.0022k 2-0.0340k+1.1521

[0012] Among them, a is defined as the boundary revision factor, k is the ratio of the length of the prototype beam to the substructure beam;

[0013] (6) Estimate the prototype beam modal frequency according to the formula:

[0014]

[0015] Among them, f 原型 is the first-order modal frequency of the prototype beam, f 子结构 is the modal frequency of the first-order beam of the substructure; a is defined as the boundary revision coefficient, and k is the ratio of the length of the prototype beam to the length of the substructure beam;

[0016] (7) Repeat the test on no less than three groups of substructure beams of different lengths and calculate the average value of the modal frequency as the final result.

[0017] Furthermore, the ratio of the prototype beam length to the substructure beam length is 4-8.

[0018] Furthermore, the difference in mass and stiffness distribution of the large flexible beam along the length direction does not exceed 20%.

[0019] Furthermore, the length of the prototype beam is not less than 20 meters and the first-order modal frequency is not higher than 1.5 Hz.

[0020] Furthermore, the stiffness of the fixed end of the substructure beam is not less than 20 times of its first-order modal frequency.

[0021] Furthermore, the installed sensor weighs no more than 5 g.

[0022] Furthermore, the types of sensors include acceleration sensors or strain gauges. The acceleration sensor is installed at the free end of the substructure beam, and the strain gauge should be installed at the fixed end of the substructure beam.

[0023] Furthermore, no less than three groups of substructure beams of different lengths are selected for testing, and the error between the test results is no higher than 10%.

[0024] Furthermore, the excitation modes include percussion excitation and frequency sweeping excitation, and the excitation should ensure sufficient coverage of the target frequency range.

[0025] Furthermore, after the test is completed, the modal frequency is obtained through fast Fourier transform analysis, and the average value is calculated based on multiple groups of test results to improve the measurement accuracy.

[0026] The technical solution of the present invention tests small-sized substructure beams, combines formula derivation and boundary condition correction, and quickly and accurately predicts the modal parameters of large flexible beams under "free-free" boundary conditions. Through small-sized substructure experiments, the high cost of large prototype beam testing is avoided; the accuracy is improved, and the correction formula corrects the boundary condition differences to improve the accuracy of modal parameter prediction; it is suitable for a variety of complex engineering models such as large-span beams and large flexible structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0028] Figure 1 It is a schematic diagram of the principle of the conventional test method of the present invention;

[0029] Figure 2 This is a schematic diagram of the structure of the method of the embodiment of the present invention;

[0030] Figure 3 It is a flow chart of the test method of the present invention;

[0031] In the figure:

[0032] 1. Suspension fixing surface; 2. Suspension cable; 3. Prototype beam; 4. Substructure beam; 5. Substructure beam fixing device. DETAILED DESCRIPTION

[0033] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0035] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "multiple" means two or more, unless otherwise clearly and specifically defined. In addition, the terms "installed", "connected" and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0036] Example 1

[0037] like Figure 1-Figure 3 As shown,

[0038] A high-proportion submodal test method for a large flexible beam structure comprises the following steps:

[0039] Substructure model construction:

[0040] The prototype beam is cut into parts according to the scale factor (4 to 8 times) as "sub-structure beams" to ensure that the cross-sectional shape and material properties are consistent.

[0041] Boundary condition settings:

[0042] One end of the substructure beam is fixed and the other end is free, and the end is fixed with a high-rigidity device to avoid the influence of boundary effects on the modal characteristic measurement.

[0043] Modal Excitation and Testing:

[0044] The substructure beam is subjected to knocking or sweeping excitation, and the dynamic response data is collected through sensors (such as accelerometers and strain gauges) installed on the beam.

[0045] Fast Fourier Transform Analysis:

[0046] The modal frequencies and related modal parameters of the substructure beams are obtained by processing the data using Fast Fourier Transform (FFT).

[0047] Boundary correction formula:

[0048] The following boundary correction formula is used to correct the difference between different boundary conditions:

[0049] a≈0.0022k 2 -0.0340k+1.1521

[0050] Among them, a is defined as the boundary revision factor, k is the ratio of the length of the prototype beam to the substructure beam;

[0051] Modal frequency derivation:

[0052] Based on the modal frequencies of the substructure beams, the modal frequencies of the prototype beams are estimated using the following formula:

[0053]

[0054] Among them, f 原型 is the first-order modal frequency of the prototype beam, f 子结构 is the modal frequency of the first-order beam of the substructure; a is defined as the boundary revision coefficient, and k is the ratio of the length of the prototype beam to the length of the substructure beam;

[0055] Take the average value of multiple groups of experiments:

[0056] Repeat the test and select substructure beams of different lengths to obtain the average value of the modal frequency to improve the reliability of the results.

[0057] Example 2

[0058] like Figure 1 As shown, the traditional method is to perform a modal test on an aircraft prototype, fix the prototype beam 3 by suspension, fix the prototype beam 3 on the suspension fixing surface 1 by a suspension cable 2, and then perform a modal test by tapping or the like.

[0059] This embodiment provides a high-proportion submodal test method for a large flexible beam structure, such as Figure 2 As shown, first take a part of the prototype beam 3 as the substructure beam 4, and then use the substructure beam fixing device 5 to fix one end of it, and leave one end free. Then use the sensor installed on the substructure beam 4 to perform a modal test, and convert the calculated result into the frequency of vibration, and use the conversion formula to obtain the modal frequency of the prototype beam 3. Select different lengths of the substructure beam 4 and repeat the above process 3 to 5 times, and calculate the average value of the calculated results as the final modal frequency of the prototype beam 3.

[0060] The invention discloses a high-proportion sub-modal test method for a large flexible beam structure; firstly, a part of the original beam structure length is taken to establish a model, and the length of the prototype beam is respectively 4 times, 5 times, 6 times, and 7 times the length of the sub-structure beam, and the cross-section of each position of the prototype beam is consistent with the cross-sectional shape of the sub-structure beam, and the material properties of the prototype beam are consistent with the material properties of the sub-structure beam; then, the sub-structure beam is clamped in a manner of fixing one end and freeing the other end; then, the modal frequency of the sub-structure beam is measured by tapping or sweeping; finally, the structural mode of the large flexible beam is obtained by relying on a given modal frequency estimation formula, and the average value of several groups of tests is obtained to obtain the final result. The invention can use a small-size cantilever beam model to estimate the modal frequency of a large-size beam under a "free-free" state.

[0061] like Figure 3 As shown, the embodiment of the present invention specifically includes the following steps:

[0062] Step 1: Take a part of the length of the prototype beam as the design model of the "substructure beam"; and ensure that the cross-sectional shape and material properties of the "substructure beam" are consistent with those of the prototype beam.

[0063] Step 2: Fix one end of the "substructure beam" and release the other end to free its degree of freedom.

[0064] Step three: install or set sensors on the "substructure beam". The types of sensors include "acceleration sensor", "strain gauge", "laser vibrometer" and the like.

[0065] Step 4: Record the sensor measurement data and use fast Fourier transform on the measurement data to find the modal frequency of the substructure model.

[0066] Step 5: Use the following formula to obtain the boundary revision coefficient between the "free-free beam" and the cantilever beam.

[0067] a≈0.0022k 2 -0.0340k+1.1521

[0068] Where: a is defined as the boundary revision coefficient, reflecting the error caused by changes in boundary conditions under different boundary conditions. k is defined as the cantilever measurement ratio, which means the ratio of the prototype beam length L to the substructure beam length l.

[0069] Step 6: Estimate the modal frequency of the prototype beam using the following formula:

[0070]

[0071] f 原型 is the first-order modal frequency of the large flexible structure, f 子结构 is the first-order modal frequency of the cantilever substructure, k is the ratio of the length of the large flexible structure to the length of the cantilever substructure, and a is the boundary revision coefficient obtained in step five.

[0072] Step 7: Select "substructure beams" of different lengths and repeat steps 1 to 6 to obtain multiple sets of measurement results.

[0073] Step eight, averaging the different measurement results obtained in step seven, and the obtained result is used as the modal frequency of the prototype beam.

[0074] In this embodiment, the difference in mass and stiffness distribution of the large flexible beam along the length direction should be less than 20%.

[0075] In this embodiment, the length of the beam structure is not less than 20 m, and the first-order modal frequency is not higher than 1.5 Hz.

[0076] In this embodiment, the ratio of the length of the prototype beam to the length of the "substructure beam" is preferably 4 to 8.

[0077] In this embodiment, the modal frequency of the end fixing structure is not less than 20 times the first-order modal frequency of the "substructure beam".

[0078] In this embodiment, the weight of the installed acceleration sensor should not exceed 5g.

[0079] In this embodiment, the weight for mounting the acceleration sensor should be mounted at the free end of the structure.

[0080] In this embodiment, the weight of the "strain gauge" should be installed at the fixed end of the structure.

[0081] In this embodiment, within the scope of claim 3, no less than 3 groups of "substructure beams" of different lengths should be selected, and the difference between different measurement results should not be higher than 10% of the measurement average value.

[0082] The invention can conduct tests on a local model, thereby reducing the requirements on test space and test equipment. The test is conducted in a fixed end manner, thereby avoiding interference with the test results due to the stiffness of the suspension rope.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high-proportion submodal test method for a large flexible beam structure, characterized in that: The following steps are involved: (1) Cut off a part of the length of the prototype beam as the substructure beam to ensure that the cross-sectional shape and material properties of the substructure beam are consistent with those of the prototype beam; (2) Clamp the substructure beam in a manner where one end is fixed and the other end is free; (3) Install sensors on the substructure beams and record sensor measurement data; (4) Using fast Fourier transform (FFT) to process sensor measurement data and obtain the modal frequency of the substructure beam; (5) Calculate the boundary correction coefficient a according to the formula: a≈0.0022k 2 -0.0340k+1.1521 Among them, a is defined as the boundary revision factor, k is the ratio of the length of the prototype beam to the substructure beam; (6) Estimate the prototype beam modal frequency according to the formula: Among them, f 原型 is the first-order modal frequency of the prototype beam, f 子结构 is the modal frequency of the first-order beam of the substructure; a is defined as the boundary revision coefficient, and k is the ratio of the length of the prototype beam to the length of the substructure beam; (7) Repeat the test on no less than three groups of substructure beams of different lengths and calculate the average value of the modal frequency as the final result.

2. The method according to claim 1, characterized in that The ratio of the prototype beam length to the substructure beam length is 4 to 8.

3. The method according to claim 1, characterized in that The mass and stiffness distribution of the large flexible beam along the length direction differ by no more than 20%.

4. The method according to claim 1, characterized in that: The length of the prototype beam is not less than 20 meters and the first-order modal frequency is not higher than 1.5 Hz.

5. The method according to claim 1, characterized in that The stiffness of the fixed end of the substructure beam is not less than 20 times of its first-order modal frequency.

6. The method according to claim 1, characterized in that The installed sensor weighs no more than 5g.

7. The method according to claim 1, characterized in that The types of sensors include acceleration sensors or strain gauges. The acceleration sensors are installed at the free ends of the substructure beams, and the strain gauges should be installed at the fixed ends of the substructure beams.

8. The method according to claim 1, characterized in that No less than three groups of substructure beams of different lengths are selected for testing, and the error between the measured results is no higher than 10%.

9. The method according to claim 1, characterized in that: The excitation modes include percussion excitation and sweep frequency excitation. The excitation should ensure that the target frequency range is fully covered.

10. The method according to claim 1, characterized in that After the test is completed, the modal frequency is obtained through fast Fourier transform analysis, and the average value is calculated based on multiple groups of test results to improve the measurement accuracy.

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