Large flexible beam structure low-ratio sub-modal test method

Abstract

The application provides a large flexible beam structure low proportion sub-modal test method, which comprises the following steps: firstly, a part of the length of the original beam structure is taken to establish a model, the length of the original beam is 2 times, 2.5 times, 3 times and 3.5 times of the length of the taken sub-structure beam respectively, the cross section of each position of the original beam is consistent with the cross section shape of the sub-structure beam, and the material properties of the original beam are consistent with the material properties of the sub-structure beam; then the sub-structure beam is clamped in a way that one end is fixed and one end is free; then the modal frequency of the sub-structure beam is measured by knocking or sweeping; finally, the structural modal of the large flexible beam is obtained by relying on the given modal frequency estimation formula, and the final result is obtained by averaging several groups of test results. The application can estimate the modal frequency of the beam under the condition of large size "free-free" by using a small size cantilever beam model.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of aerospace technology, in particular to a low-scale sub-modal test method for large flexible beam structure. BACKGROUND

[0002] In the study of the dynamic characteristics of large flexible aircraft such as high-altitude long-endurance unmanned aerial vehicles, modal test is a crucial analysis method. The existing modal test is mainly carried out on full-size models, which has the following disadvantages: high cost: full-size model test requires large-scale test site and expensive equipment. Suspension stiffness interference: the traditional suspension method is prone to interference with the measurement results due to the stiffness of the rope during the test, resulting in higher modal frequency. SUMMARY

[0003] The purpose of the present application is to provide a low-scale sub-modal test method for large flexible beam structure, which uses a local substructure model for testing and calculates the modal characteristics of the prototype beam, thereby reducing the cost and improving the test accuracy; through the fixed support method, the low-cost and accurate prediction of the modal characteristics of the large flexible beam is realized, overcoming the suspension stiffness interference problem in the prior art, and adapting to the test requirements of various large flexible structures.

[0004] According to one object of the present application, the present application provides a low-scale sub-modal test method for large flexible beam structure, comprising the following steps:

[0005] (1) A part of the length of the prototype beam is taken as a substructure beam, and the cross-sectional shape and material properties are kept consistent;

[0006] (2) One end of the substructure beam is fixed and the other end is free to release its degrees of freedom;

[0007] (3) Acceleration sensors or strain gauges are installed on the substructure beam to collect dynamic response data;

[0008] (4) Calculate the modal frequency of the substructure beam by Fast Fourier Transform (FFT);

[0009] (5) Calculate the boundary correction coefficient using the formula:

[0010] a≈-0.2950k 2 +3.4111k-3.5124;

[0011] Where: a is defined as the boundary revision coefficient, which reflects the error caused by the change of boundary conditions under different boundary conditions; k is defined as the cantilever measurement ratio, which represents the ratio of the length of the prototype beam to the length of the substructure beam;

[0012] (6) Estimate the modal frequency of the prototype beam by the formula:

[0013] ;

[0014] wherein f is the modal frequency of the prototype beam; f1 is the modal frequency of the substructure beam; a is defined as the boundary modification coefficient; k is defined as the cantilever measurement ratio, representing the ratio of the length of the prototype beam to the length of the substructure beam;

[0015] (7) The final frequency is obtained by averaging the test results of multiple groups of substructure beams.

[0016] Further, the ratio k of the length of the substructure beam to the length of the prototype beam is 2-4.

[0017] Further, the modal frequency of the fixed end structure of the substructure beam is not less than 20 times of its first-order modal frequency.

[0018] Further, an acceleration sensor is installed at the free end of the substructure beam, and the weight of the acceleration sensor is not more than 5g.

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

[0020] Further, the test methods include the knocking method and the sweeping method.

[0021] Further, the difference in mass and stiffness distribution along the length direction of the substructure beam is not more than 20%.

[0022] Further, the error between the test results is not higher than 10%.

[0023] Further, the test of the substructure beam should select not less than 3 groups of different lengths.

[0024] Further, the calculated modal frequency of the prototype beam is based on the average of multiple groups of substructure beam test results to improve the accuracy.

[0025] The technical solution of the present application reduces the test cost and site requirement, avoids the influence of cantilever stiffness, and improves the measurement accuracy. It provides a flexible and repeatable test method, which is suitable for the modal characteristic research of various large flexible structures. The test can be carried out on a local model, which reduces the requirements for test space and test equipment. The test is carried out in an end-fixed manner, which avoids the interference of the suspension rope stiffness on the test results. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the specific embodiments or prior art of the present application, the drawings needed in the specific embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0027] Figure 1 A schematic diagram of the principle of the conventional test method of the present application;

[0028] Figure 2 A schematic diagram of the method structure of the embodiment of the present application;

[0029] Figure 3 A flow chart of the test method of the present application;

[0030] In the figure: 1, suspension fixing surface; 2, suspension cable; 3, prototype beam; 4, substructure beam; 5, substructure beam fixing device. DETAILED DESCRIPTION

[0031] The technical solutions of the present application will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0032] In the description of the present application, it should be understood that the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", and the like indicate the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation of the present application.

[0033] In addition, the terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "multiple" is two or more, unless otherwise explicitly and specifically limited. In addition, the terms "mounting", "connecting", "connection" should be broadly understood, for example, it can be fixed connection, or detachable connection, or integrally connected; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, or the communication between the two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0034] Embodiment 1

[0035] A large flexible beam structure low proportion sub-modal test method, comprising the following steps:

[0036] 1) Model sampling

[0037] A segment of the prototype beam is taken as a substructure beam, the length of the substructure beam is 1 / 2~1 / 4 of the length of the prototype beam, and the cross-sectional shape and material properties remain the same. For example, if the length of the prototype beam is 40 meters, a substructure beam with a length of 10 meters can be taken.

[0038] 2) Fixed support setting

[0039] The substructure beam is fixed at one end by a special clamping device, and the other end is free. The stiffness of the fixing device should be more than 20 times the first-order modal frequency of the substructure beam to avoid interference from the support.

[0040] 3) Sensor installation

[0041] Install an acceleration sensor at the free end of the substructure beam, with a weight of no more than 5g. Install strain gauges at the fixed end to supplement data collection.

[0042] 4) Modal test

[0043] Excite the substructure beam by knocking or sweeping, and record its dynamic response data.

[0044] Use Fast Fourier Transform (FFT) to process the data and extract the first-order modal frequency f_{\text{substructure}} f of the substructure beam.

[0045] 5) Boundary correction coefficient calculation

[0046] a≈-0.2950k 2 +3.4111k-3.5124;

[0047] Where: a is defined as the boundary revision coefficient, reflecting the error caused by the change of boundary conditions under different boundary conditions; k is defined as the cantilever measurement ratio, indicating the ratio of the length of the prototype beam to the length of the substructure beam;

[0048] 6) Prototype modal frequency estimation

[0049] ;

[0050] Where f is the modal frequency of the prototype beam; f1 is the modal frequency of the substructure beam; a is defined as the boundary revision coefficient; k is defined as the cantilever measurement ratio, indicating the ratio of the length of the prototype beam to the length of the substructure beam;

[0051] 7) Repeat the test and average

[0052] Select no less than 3 groups of substructure beams with different lengths, repeat the test and take the average of the measurement results, with an error of no more than 10%.

[0053] Example 2

[0054] like Figure 1 As shown, the traditional method is to conduct the test on the prototype of the aircraft by suspending the prototype beam 3. The prototype beam 3 is fixed to the suspension surface 1 by suspension cable 2, and then modal tests are conducted by means of hammering.

[0055] like Figure 2 As shown in this embodiment, a low-proportion submodal testing method for a large flexible beam structure is described. First, a portion of the prototype beam 3 is taken as a substructure beam 4. One end of the substructure beam is fixed using a substructure beam fixing device 5, while the other end remains free. Then, sensors are installed on the substructure beam 4 to conduct modal tests, and the calculated results are converted into vibration frequencies. The modal frequencies of the prototype beam 3 are obtained using a conversion formula. The above process is repeated 3 to 5 times with different lengths of the substructure beam 4, and the average of the calculated results is taken as the final modal frequency of the prototype beam 3.

[0056] This invention discloses a low-proportion submodal testing method for a large flexible beam structure. First, a model is established using a portion of the original beam's length. The prototype beam's length is 2, 2.5, 3, and 3.5 times the length of the selected substructure beam, while the cross-sectional shape and material properties of the prototype beam at all locations are consistent with those of the substructure beam. Next, the substructure beam is clamped with one end fixed and the other end free. Then, the modal frequencies of the substructure beam are measured using tapping or frequency sweeping methods. Finally, the structural modes of the large flexible beam are obtained using a given modal frequency estimation formula, and the average value of several sets of tests is calculated to obtain the final result. This invention can use a small-sized cantilever beam model to estimate the modal frequencies of a large-sized beam in a "free-free" condition.

[0057] Specifically, such as Figure 3 As shown, the low-proportion submodal testing method for flexible beam structures of the present invention specifically includes the following steps:

[0058] Step 1: Take a portion 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.

[0059] Step 2: Fix one end of the selected "substructure beam" and free the other end to allow it to have its degrees of freedom.

[0060] Step 3: Install or set up sensors on the "substructure beam". The types of sensors include "accelerometer", "strain gauge", "laser vibration meter" and so on.

[0061] Step 4: Record the sensor measurement data and use Fast Fourier Transform on the measurement data to determine the modal frequencies of the substructure model.

[0062] Step five, the boundary modification factor between the "free-free beam" and the cantilever beam is obtained by the following formula.

[0063] a≈-0.2950k 2 + 3.4111k - 3.5124

[0064] Wherein: a is defined as the boundary modification factor, reflecting the error due to the change of boundary conditions under different boundary conditions. k is defined as the cantilever measurement ratio, indicating the ratio of the length L of the prototype beam to the length l of the "substructure beam"

[0065] Step six, the modal frequency of the prototype beam is estimated by the following formula:

[0066]

[0067] Step seven, select "substructure beams" of different lengths, repeat steps one to six, f is the modal frequency of the prototype beam; is the modal frequency of the "substructure beam"; a is defined as the boundary modification factor; k is defined as the cantilever measurement ratio, indicating the ratio of the length L of the prototype beam to the length l of the "substructure beam".

[0068] Obtain multiple sets of measurement results.

[0069] Step eight, average the different measurement results obtained in step seven, and the obtained result is taken as the modal frequency of the prototype beam.

[0070] In this embodiment, the difference between the mass and stiffness distribution along the length direction of the large flexible beam should be less than 20%.

[0071] In this embodiment, the length of the beam structure is not less than 20m, and the first order modal frequency is not higher than 1.5Hz.

[0072] In this embodiment, the ratio of the length of the prototype beam to the length of the "substructure beam" is preferably 2-4.

[0073] In this embodiment, the modal frequency of the end fixed structure is not less than 20 times the first order modal frequency of the "substructure beam".

[0074] In this embodiment, the weight of the installed acceleration sensor should not exceed 5g.

[0075] In this embodiment, the weight of the installed acceleration sensor should be installed on the free end of the structure.

[0076] In this embodiment, the weight of the installed "strain gauge" should be installed on the fixed end of the structure.

[0077] In this embodiment, not less than 3 sets of "substructure beams" of different lengths are selected, and the difference between different measurement results should not be higher than 10% of the average value.

[0078] The application can be tested in a local model, which reduces the requirements for test space and test equipment, and uses an end-fixed mode for testing, avoiding interference with the test results due to the stiffness of the suspension rope.

[0079] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A large flexible beam structure low-ratio sub-modal test method, characterized in that, The method comprises the following steps: (1) a part of the prototype beam length is taken as a substructure beam, keeping the cross-sectional shape and material properties consistent; (2) one end of the substructure beam is fixed, and the other end is free to release the degree of freedom; (3) an acceleration sensor or a strain gauge is installed on the substructure beam to collect dynamic response data; (4) the modal frequency of the substructure beam is calculated by fast Fourier transform (FFT); (5) the boundary correction coefficient is calculated by the formula: a ~ -0.2950k 2 + 3.4111k - 3.5124; wherein a is defined as the boundary revision coefficient, reflecting the error caused by the change of the boundary condition under different boundary conditions; and k is defined as the cantilever measurement ratio, representing the ratio of the length of the prototype beam to the length of the substructure beam; (6) the modal frequency of the prototype beam is estimated by the formula: ； wherein f is the modal frequency of the prototype beam; f1 is the modal frequency of the substructure beam; a is defined as the boundary revision coefficient; and k is defined as the cantilever measurement ratio, representing the ratio of the length of the prototype beam to the length of the substructure beam; (7) the final frequency is obtained by averaging the test results of multiple groups of substructure beams.

2. The method of claim 1, wherein, The ratio k of the length of the substructure beam to the length of the prototype beam is 2-4.

3. The method of claim 1, wherein, The modal frequency of the fixed end structure of the substructure beam is not less than 20 times of the first-order modal frequency.

4. The method of claim 1, wherein, An acceleration sensor is installed at the free end of the substructure beam, and the weight of the acceleration sensor is not more than 5g.

5. The method of claim 1, wherein, The length of the prototype beam is not less than 20 meters, and the first-order modal frequency is not higher than 1.5Hz.

6. The method of claim 1, wherein, The test methods include the knocking method and the sweeping method.

7. The method of claim 1, wherein, The difference between the mass and stiffness distribution of the substructure beam along the length direction is not more than 20%.

8. The method of claim 1, wherein, The error between the test results is not higher than 10%.

9. The method of claim 1, wherein, The test of the substructure beam should select not less than 3 groups of different lengths.

10. The method of claim 1, wherein, The calculated modal frequency of the prototype beam is based on the average value of multiple groups of substructure beam test results to improve the accuracy.

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