Vibration test is to simulate the various vibration environment effects encountered by the product in the transportation, installation and use environment. A vibration environment effect used to determine the ability of a product to withstand various environmental vibrations. Vibration test is to evaluate the resistance of components, parts and complete machines in the expected transportation and use environment.
Bader test believes that the most commonly used vibration methods can be divided into two types: sinusoidal vibration and random vibration. Sinusoidal vibration is a test method frequently used in the laboratory to simulate the vibration caused by rotation, pulsation, vibration (occurring on ships, aircraft, vehicles, space vehicles) and product structure resonance frequency analysis and resonance point residence verification Mainly, it is divided into two types: sweep frequency vibration and fixed frequency vibration. The severity depends on the frequency range, amplitude value, and test duration. Random vibration is used to simulate the overall structural seismic strength assessment of the product and the shipping environment in the packaged state. The severity depends on the frequency range, GRMS, test duration and axial direction.
The reciprocating motion of an object or particle relative to its equilibrium position is called vibration. Vibration is divided into sinusoidal vibration, random vibration, compound vibration, scanning vibration, fixed frequency vibration. The main parameters describing vibration are: amplitude, velocity, acceleration.
Vibration test standard:
GJB 150.25-86
GB-T 4857.23-2003
GBT4857.10-2005
WJ231-77
Tests on actual or model vibration systems in the field or in the laboratory. Vibration systems are mass-elastic systems that are excited by vibration sources, such as machines, structures or their components, living organisms, etc. Vibration testing was developed from the aerospace sector, and now it has been extended to various industrial sectors such as power machinery, transportation, construction, as well as environmental protection and labor protection, and its application is increasingly widespread. Vibration testing includes response measurement, dynamic characteristic parameter determination, load identification, and vibration environment testing.
Response measurement
Mainly the measurement of vibration level. In order to test the operation quality, safety and reliability of the machine, structure or its components and determine the environmental vibration conditions, it is necessary to measure the vibration magnitude of each selected point and selected direction of the vibration system under various actual working conditions. And record the relationship between vibration magnitude and time change (called time history). For periodic vibration, the vibration level (amplitude or effective value of displacement, velocity, acceleration or strain) and vibration period are mainly determined; for transient vibration and shock, the maximum peak value and response duration of displacement or acceleration are mainly determined; Vibration, mainly to measure the mean and variance of the time history of force and response; for non-stationary random vibration, the time can be divided into many small sections, the mean and variance of the time history in each small section can be measured, and the relationship between them and time can be found out. Use this as a measure of vibration level.
The vibration speed of many machines is almost constant over a wide frequency range, so the maximum effective value of the vibration speed measured at a selected point on the machine can be used as an indicator of the intensity of machine vibration (called vibration intensity).
Parametric determination
In order to design and trial-manufacture new machines or solve vibration reduction problems when retrofitting old machines, and in order to improve the efficiency of vibrating machines, it is necessary to understand the dynamic parameters of the system. There are many dynamic characteristic parameters. For linear systems, the most commonly used are modal parameters, including natural frequencies of each order, mode shape, modal mass or modal stiffness, and modal damping ratio. Modal parameters can be converted into mechanical parameters in physical coordinates (ie geometric coordinates), including lumped mass, stiffness and damping matrices.
test methods
In engineering design, sometimes you only need to know the low-order (such as first and second order) natural frequencies, mode shapes and damping coefficients. These parameters can be determined by simple methods:
① The natural frequency is determined by tapping or sudden unloading to make the system free to vibrate, record its attenuation waveform and compare it with the time scale signal in the instrument, or input the fixed frequency sine wave and attenuation waveform generated by the signal generator into the ray oscilloscope, and the oscilloscope The displayed Lissajous figure finds the first and second order natural frequencies. If there is an exciter or a shaking table, the system can be excited by step frequency or low-speed sweep frequency to find the resonance frequency, which is approximately equal to the natural frequency when the damping is small.
②Determination of mode shape: The hand-held wooden or aluminum probe touches each point of the system under test, and the position of all non-vibrating points, that is, the position of the nodal line, is determined by the impact sound (or by hand feel). For a flat plate system placed horizontally, sand can be sprinkled on the flat plate, and the sand particles will gather on the pitch line during vibration, and the mode shape can be roughly judged by the distribution of the pitch line.
③ Damping measurement The damping vibration method, resonance method and phase method can be used. The damping vibration method uses a recorder to record the damping waveform of free vibration, and calculates the damping value from the attenuation rate of two or several adjacent amplitudes in the same direction; the resonance method calculates the damping value from the amplitude at resonance and the frequency bandwidth of the resonance region; The phase method calculates the damping value from the relationship between the phase of the resonance region and the frequency.
Admittance method
Mechanical admittance is a characteristic parameter of the system in the frequency domain (see Mechanical Impedance). The natural frequencies of large and complex structures are numerous and dense, and the mode shapes are very complex, which cannot be determined by simple methods. However, it is possible to test the response of the system to the excitation force to obtain the mechanical admittance, and then use graphic identification (ie, graphical identification of mechanical admittance, such as amplitude frequency, phase frequency, real frequency, imaginary frequency or sagittal diagram) or computer identification. Determine modal or physical parameters.
time domain identification
The modal parameters of the system are obtained directly by using the time history of vibration. For free vibration, the modal parameters can be directly calculated by the relationship between the free vibration and the impulse response function (one of the time-domain characteristic parameters of the system, its Fourier transform is the mechanical admittance). For forced vibration, digital time series analysis methods or other methods (such as random reduction method, filtering method, etc.) can be used to calculate modal parameters. The advantage of the time domain identification method is that it can use the vibration signal of the machine in the running state, which is suitable for large structures that cannot be tested in the laboratory; recognition, so the accuracy is low.
load identification
It refers to analyzing and determining the nature of the vibration source (is it regular or random? Is it a constant force or a constant motion?…), the propagation path and the load spectrum (that is, the time history of the load) exerted by the vibration source on the system. Load identification, also known as environmental prediction, provides data for analyzing the system's dynamic response and causes of vibration. The loads borne by large structures are very complex and difficult to measure directly, but the loads borne by the system can be reversed through the response signals of the structure and the known mathematical models of the system, and then statistics and synthesis are carried out based on the data obtained under various working conditions. , and finally the load spectrum is obtained. The nature and propagation path of the vibration source can be obtained by power spectrum analysis or correlation analysis method.
Environmental testing
In order to understand the vibration resistance life of the product and the stability of the performance indicators, the weak links that may cause damage or failure are recorded, and the system is assessed and tested under the vibration and shock conditions that simulate the actual environment. Test specifications for finalized products are usually standardized, and appropriate test methods are developed for new products. The test methods are divided into two categories: ① Standard test, including resistance to predetermined frequency test, resonance resistance test, sine sweep test, broadband random vibration test, shock test, acoustic vibration test and transportation test, etc.; ② Non-standard test, including transient waveform Vibration test, narrow-band random vibration test, random wave reproduction test, sine wave and random wave mixed test, etc. (See Vibration Environment Test)
Vibration test data processing and analysis The large amount of raw data obtained from the test must be processed in various ways before it can be used as the basis for engineering design calculations. The original recorded data of the test is the time history of the parameters (the relationship between the displacement, velocity or acceleration and other magnitudes and time). Through intuitive analysis, the data can be divided into three types: transient, periodic, random or non-random continuous non-periodic , and then perform statistical analysis, correlation analysis and spectral analysis in the three domains of time domain (including time difference domain, that is, the independent variable is the time difference between two signals), frequency domain and amplitude domain, so as to obtain various functions that characterize time history. The processing method can be divided into analog processing method and digital processing method. The former has simple equipment, but poor precision and long processing time; the latter needs to convert the original recorded analog quantity into digital quantity and then process it with a digital computer. With the advent of data processors (such as fast Fourier analyzers), digital processing has gradually replaced analog processing.
Data processing and analysis
A large amount of original data obtained from the test must be processed in various ways before it can be used as the basis for engineering design calculations. The original recorded data of the test is the time history of the parameters (the relationship between the displacement, velocity or acceleration and other values and time). The data is divided into three types: transient, periodic, random or non-random continuous and non-periodic, and then in the time domain (including the time difference domain, that is, the time difference between the two signals as the independent variable), the frequency domain and the amplitude domain. Statistical analysis, correlation analysis and spectral analysis are performed to obtain various functions that characterize the time history. The processing method can be divided into analog processing method and digital processing method. The former has simple equipment, but poor precision and long processing time; the latter needs to convert the original recorded analog quantity into digital quantity and then process it with a digital computer. With the advent of data processors (such as fast Fourier analyzers), digital processing has gradually replaced analog processing.
Test Equipment
It can be roughly divided into vibration excitation equipment, vibration measurement equipment and analysis equipment, which correspond to three parts I, II and III in Figure 2, respectively. The single-line arrows in the figure represent the transmission paths of electrical signals, and the double-line arrows represent the transmission paths of mechanical quantities (force, velocity, acceleration, etc.). Some equipment or devices in the figure are described as follows: ① Vibration excitation equipment: It can be divided into two categories: vibration exciter and vibration table. At present, vibration excitation equipment with vibration controller has been gradually adopted, which can be excited according to the required waveform or spectral shape. vibrate. ②Sensor: It can be divided into three types: force measurement, movement measurement and impedance measurement (simultaneous measurement of force and movement at a point). ③ Filter: It can play the role of anti-interference, de-noise, and extract useful signals. Before processing with a digital computer, the signal must be passed through a low-pass filter (called an anti-aliasing filter) to avoid aliasing that may occur after the signal is discretized into digital quantities.