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Crossing frequency and crossing point
August 14, 2013 source: navigation instruments Author:

Crossing frequency and crossing point

For the test with only one crossing point, it is usually to move the position below the crossing point, because the displacement scene is the main influence on the product at low frequency. Because the displacement above the crossing point is very small and the acceleration amplitude is large, according to the principle that the acceleration is proportional to the force, at this time, the product is mainly affected by the acceleration. For the test with multiple crossing points, it can be positioned to move a certain acceleration, a certain displacement, a certain acceleration, etc., and can also be positioned to move a certain speed and acceleration.

As long as you know two of them, you can work out the third. In the sweep frequency test, below the crossover frequency is the positioning shift sweep frequency. In other words, the displacement is constant in the process of sweeping frequency, and the acceleration amplitude will increase with the increase of frequency, and it will increase to the acceleration amplitude of constant acceleration sweep frequency at the crossover frequency. After that, the frequency continues to increase, the displacement amplitude becomes smaller and smaller, while the acceleration amplitude remains constant. It can be seen that the displacement amplitude and acceleration amplitude reach the specified value at the crossover frequency. That is to say, at the crossing point, the acceleration amplitude calculated by substituting the positioning displacement amplitude and crossover frequency is exactly equal to the acceleration amplitude when the constant acceleration sweeps the frequency. In the same way, the value obtained by substituting the constant acceleration amplitude and crossover frequency is exactly equal to the amplitude of the positioning displacement. So when the displacement amplitude and acceleration amplitude are known, the crossover frequency can be calculated.

? Control technology of single point and multi-point test

The vibration test can adopt single point control or multi-point control.

It is more suitable to control the single point of the same phase with the single point which is known. In single point control, it is necessary to monitor the signals on other detection points in time to see whether the error between the signal and the control point is within the tolerance range specified in the test or within the tolerance range required by the detection point.

Each control point is selected as the control signal from the virtual control point. The minimum value control is often used in important military products or primary effective equipment, because it is to ensure that the vibration stress of any part of the sample can not be lower than the requirements in the test specification. The sensor signal of the most sensitive part of the sample to vibration should be selected to control the maximum value. Compared with the shaking table (including the horizontal sliding table), the samples with large volume, heavy weight and complex dynamic characteristics should be controlled by multi-point average control as far as possible, and the signals from the sensors on the fixture and the sample can not be used for control one One is installed on the platform, otherwise it is easy to damage the vibration table.

? Tracking filter technology

In the sinusoidal vibration test, if the vibration waveform distortion is large, the measurement system will display the incorrect amplitude value, because it includes not only the required basic frequency (fundamental wave), but also many unnecessary other frequency components and higher harmonic components. This will result in a lower than specified amplitude at the required fundamental frequency. If it is not lower than the tolerance range, it is allowed; if it is exceeded, it must try to restore the fundamental amplitude to the required value, otherwise it is easy to produce under test. There are many ways to recover the fundamental amplitude to the desired value, and tracking filter is one of them. The characteristic of this instrument is that its center frequency can change synchronously with the excitation frequency of sweeping frequency, so as to carry out effective filtering, so as to control the amplitude of fundamental wave to the specified severity level, so as to ensure that the sample can be subjected to the specified vibration stress. However, the use of tracking filter is also limited. The reason is that when the amplitude is restored to the normal value, those unnecessary high-order harmonic components will also increase. If they have an impact on the sample, it will inevitably produce additional high stress on the product, and the result will make the sample over test. So it can only be used in the sample with little influence of higher harmonic. In addition, with the recovery of the fundamental wave and the increase of the high-order harmonic components, the distortion will inevitably increase.

? Test of sample with shock absorber

Generally, the products with shock absorber should be tested together with the shock absorber. If the product and other products are installed in a public system, which makes it impossible to use the original shock absorber for test; or when the product with shock absorber has no suitable shock absorber available during the test, the original vibration magnitude must be modified and the test without shock absorber must be conducted with different amplitude. The amplitude can be determined according to the transmission characteristics of the shock absorber on each axis in the whole test frequency range. If the characteristics of the original shock absorber are unknown, it can be determined according to the general characteristic curve of the shock absorber.

The method is to select one of the four shock absorber characteristic curves which is most suitable for the sample, and then multiply the specified vibration value by the transmission coefficient in the whole frequency range. The vibration magnitude obtained in this way may not be reproduced in the laboratory. If this is the case, the maximum vibration magnitude of the shaking table can be taken as the vibration magnitude within the whole test frequency range, and it can also be carried out in different frequency bands. The four transmission characteristic curves can also be used as the basis of shock absorber design, development and production, and also can be used as the basis for selecting shock absorber in anti vibration design of products. When the curve is applied D He'er represents the natural frequency at 10Hz The following shock absorbers are suitable for shock absorbers of products installed on vehicles. curve C Represents the natural frequency at 20?30Hz The shock absorber is within the frequency range, and the frequency range falls within the frequency range of the product during transportation and use. It is recommended that the shock absorber should only be used in some fixed occasions.