# What is random vibration test? Vibration test standards and important indexes

Date：2021-07-27 18:21:00 Views：7004

Random vibration refers to a kind of vibration with chaotic vibration waveform, whose instantaneous value can not be determined in advance at any given time in the future, and its waveform change with time can not show a certain law, so it is impossible to explain its law with deterministic function. The single test results of random vibration have uncertainty, unpredictability and non repeatability, but the multiple test results under the same conditions have internal statistical laws. The motion law must be quantitatively described by probability and statistics. Random vibration is also composed of sinusoidal vibration, but the frequency of these sinusoidal vibration is not discrete, but continuously distributed in a certain range. The amplitude and displacement of each sinusoidal vibration are unpredictable and will change with time, but should be expressed by the average value, mean square value, probability density function and power spectral density of random vibration signal at a certain time.

The test conditions (severity level) of random vibration test are composed of four parameters: test frequency range (Hz), power spectral density (G2 / Hz), spectrum of power spectral density, total root mean square acceleration (GRMS) and test time.

1. Test frequency range

Range frequency range refers to the frequency between the highest frequency and the lowest frequency that the vibration of the product installation platform effectively excites the product. The typical low frequency is usually half of the lowest resonance frequency of the product or the lowest frequency of obvious vibration generated by its installation platform; The typical high frequency is twice the highest resonance frequency of the product, or the highest frequency of obvious vibration generated by its installation platform, or the highest frequency that can effectively and mechanically transmit vibration. It is generally believed that the highest frequency of mechanically transmitted vibration is 2000 Hz, although it is often lower in practice. If a frequency above 2000Hz is required, noise is usually required.

2. Spectrum of power spectral density (G2 / Hz) and power spectral density

Random vibration is characterized in the form of PSD power spectral density (ASD acceleration spectral density) and power spectral density spectrum defined in the relevant frequency range. Power spectral density (acceleration spectral density) refers to the energy per unit frequency, and the spectrum of power spectral density (acceleration spectral density) refers to the distribution of vibration energy in the whole frequency range.

3. Total root mean square acceleration (GRMS)

The total root mean square acceleration (GRMS) value is the integral of the spectrum area of the power spectral density in the full frequency band, that is, the root mean square value, which does not contain any frequency information. Therefore, GRMS value is usually used to control and detect the test error, and select the shaking table with how much thrust (power) is required according to the weight, volume and dynamic characteristics of the test sample.

4. Test time

Test time is the duration of random vibration, which is usually divided into function (performance) and strength (durability). For air vehicles and equipment used on air vehicles, the durability test time is usually 1.6 times of the function test time.

The key indexes affecting the vibration test are as follows:

Test thrust: the test thrust plays a decisive role in the test. If the required thrust exceeds the rated thrust, the test cannot be carried out, but the thrust is far less than the rated thrust, which is easy to cause a waste of resources,

Maximum displacement: during random vibration test, the maximum displacement of random vibration can not be seen from the vibration conditions, and its value is also uncertain. Therefore, it is necessary to estimate the maximum displacement before the test to avoid damaging the shaking table due to exceeding the travel

Root mean square value of acceleration: it is a statistical parameter representing the total energy of random vibration.

Frequency range: at present, the frequency of electromagnetic vibration table can reach 3000hz ~ 5000Hz, which can basically meet most test requirements.

Estimation of random vibration test parameters:

At present, the wideband random vibration is most used in the random vibration test. The test quantity is calculated according to the vibration requirements given by the test sample. By comparing with the limit value of the shaking table, it is estimated whether the test can be carried out.

Random vibration test needs to take into account: random vibration thrust and maximum displacement of random vibration. Generally, the relationship curve between power spectral density and frequency is given for random vibration. As shown in the figure, the root mean square value of acceleration can be calculated according to this curve and relevant calculation formulas.

The root mean square value of acceleration is obtained from the sum of the total area under the curve through open root operation, which is expressed by the following formula:

Where A1 is the area contained in the rising spectrum line, A2 is the area contained in the straight spectrum curve, A3 is the area contained in the falling spectrum curve, and the area contained in the spectrum curve is calculated by integrating the spectral density function in frequency.

Calculation formula of test thrust:

Where, f is the thrust (n) and M0 is the effective mass of the moving part of the shaking table (kg); M1 is the mass of auxiliary table (kg); M2 is the mass of the sample (including fixture and mounting screw) (kg); G is the test acceleration.

Calculation formula of displacement:

The accurate method should find out the displacement spectral density curve, calculate the root mean square displacement value, and then calculate the maximum peak displacement. In engineering business, it is often only necessary to estimate an approximate value. Here is a simple estimation formula. The estimated value is larger than the actual value.

Where, xp-p is the maximum peak to peak displacement (mm), F0 is the lower limit frequency (Hz); W0 is the power spectral density value (G ^ 2 / Hz) at the lower limit frequency (F0).

Note: this formula is applicable to the flat spectrum curve. For the curve with rising and falling spectrum, it may cause a large deviation between the calculated value and the actual value.

The inspection standards commonly used for vibration test are as follows:

Environmental testing of electric and electronic products - Part 2: Test methods, test FC and guidelines: vibration (sinusoidal) GB / T 2423.10-2008 / IEC 60068-2-6:1995

Environmental testing of electric and electronic products Part 2: Test methods Test Fh: broadband random vibration (digital control) and guidelines GB / T 2423.56-2006 / IEC 60068-2-64:1993

Environmental testing of electric and electronic products Part 2: Test methods Test fi: vibration mixed mode GB / T 2423.58-2008 / IEC 60068-2-80:2005

Packaging - Basic tests for transport packages - Part 2: temperature and humidity conditioning treatment (GB / T 4857.2-2005 / ISO 2233:2000)

Basic technical conditions for automotive electrical equipment QC / T 413-2002

General specification for military computer GJB 322a-1998

General specification for military communication equipment GJB 367a － 2001

General specification for military electronic test equipment gjb3947a-2009

Rail transit rolling stock electronic devices GB / T 25119-2010 / IEC 60571:2006

General specification for electronic measuring instruments GB / T 6587-2012

Test methods and procedures for microelectronic devices GJB 548b-2005

Test methods for electronic and electrical components GJB 360b-2009

Military equipment laboratory environmental test methods Part 16: vibration test GJB 150.16a-2009

Environmental stress screening method for electronic products GJB 1032-1990

Environmental test and vibration test of ship electronic equipment GJB 4.7-1983

Vibration test method for railway ground signal products TB / T 2846-1997

Packaging - Basic tests for transport packages - Part 23: random vibration test method (GB / T 4857.23-2012)

Impact and vibration test of rail transit rolling stock equipment GB / t21563-2008 / IE 61373:1999