The minimum frequency for a sinusoidal vibration test is 0.1 Hz according to the test specification for electrical and electronic products. However, the minimum operating frequency in tests such as earthquake simulation, wave simulation and road transport simulation may be lower than 0.1Hz, so the implementation of ultra-low frequency sinusoidal vibration testing is a special problem in sinusoidal vibration testing and requires special discussion.
Some on-board equipment installed on transport vehicles, boats or helicopters require not only a low lower working frequency of 1~3Hz, but also an upper working frequency of (1~2)*103Hz, and in order to meet these test frequency requirements an “electrodynamic shaker” (electrodynamic shaker for short) must be used. It is well known that the lower working frequency of an electrodynamic shaker depends on the resonance frequency of the air spring on the shaker table and the shaker body (including table, moving coil, excitation device, etc.), which is usually around 3Hz. If no special measures are taken with the electrodynamic shaker, vibration tests below 3-5Hz will occur due to the resonance of the shaker body as follows.
Problems with the implementation of low frequency sinusoidal vibration on electrodynamic shakers
1. Significantly increased errors in displacement and acceleration measurements at low frequencies
In the vibration test, the displacement detection is the relative displacement of the table to the shaker body. When the vibration shaker resonates and the shaker is in a state of sinusoidal vibration. At this time, due to the interaction between the moving coil and the excitation coil, the vibration table surface is in phase. For the table body is also in the state of sinusoidal vibration, the consequences of the superposition of these two sinusoidal vibrations are:
a. If the two vibrations are in the same direction (the phase difference is 0 degrees), the measured displacement value of the shaking table is less than the displacement of the shaker in a relatively static state.
b. If the two vibrations are in opposite positions (the phase difference is 180 degrees), the measured displacement value of the shaking table is greater than the displacement of the shaker in a relatively static state.
c. Even if the phase difference is between (0~180 degrees), the measured displacement value of the shaking table is not equal to the displacement of the shaker in a relatively static state.
On the other hand, the measurement of vibration acceleration is an absolute measurement of the vibration of the table surface relative to the earth. When the shaker table body resonates, the acceleration value detected by the accelerometer mounted on the table (or product) is a synthetic acceleration value of the superimposed type of vibration of the shaker table surface relative to the shaker body and the vibration of the shaker body relative to the ground. This value is obviously not the absolute amount of vibration with respect to the earth that is required in vibration testing. Please refer to the explanation of two-degree-of-freedom vibration systems and their extinction phenomena for the analysis and results of synthetic vibration.
To sum up, once the working frequency (fg) of the vibration test enters the interval less than 1.3 times the resonance frequency (fT) of the shaker(f is less than or equal to 1.3fT), the resonance of the vibration shaker body will directly affect the displacement in the vibration test and acceleration measurement accuracy.
2. The influence of the background noise of the electrodynamic shaker in the low frequency band increases
Generally, the double amplitude of the vertical electric vibrating table is ±25.4mm (2in), and the acceleration value in the low frequency band is very small. For example, the working frequency is 1Hz, and the maximum limit amplitude is ±25mm, and the absolute acceleration value is only 0.1g. Generally, the dynamic range of the electrodynamic shaker is 60dB. If the maximum peak acceleration is 100g, the background noise of most shakers is also in the order of 0.1g, and its background noise is of the same order as the test parameters of low-frequency vibration. The acceleration waveform is seriously distorted, and the detected acceleration data is questionable. If the electrodynamic shaker is used for a long time, or the vibration amplitude at low frequency does not reach ±25mm, the test magnitude of vibration acceleration will be lower than the background noise of the vibrating table, so that the test signal is overwhelmed by the background noise. It will lead the experiment did not run properly.
3, The relative displacement measured in the low frequency band may exceed the displacement limit of the shaking table
The protection value of the maximum displacement of the shaking table is that the displacement between the detection table and the shaker does not exceed the maximum displacement limit of the shaking table (such as ±25mm). Once the data detected by the limit sensor reaches this value, the control system of the shaking table will automatically protective shutdown, forcibly stop the test. The previous analysis has explained that when the working frequency of the vibration test is near the resonance area of the shaking table body (0.5fT≤fg≤1.3fT), since the shaker of the shaking table is near the “resonance” area. When the shaker displacement calculated according to the acceleration value of the test conditions may still be far less than the maximum displacement limit, the shaker table (i.e. the moving coil) may have reached or exceeded the maximum displacement limit for the relative displacement of the table body in resonance (in a static state during high-frequency vibration), and the shaker will automatically stop. This is the reason why the electrodynamic shakers often stops soon after starting in the low-frequency large displacement test, or suddenly stops when the sweep enters the low-frequency range during the frequency sweep test.
Measures for Realizing Ultra-low Frequency Sinusoidal Vibration Test with Electrodynamic Shaking Table
1. Build a foundation with large inertial mass and improve the dynamic characteristics of the low-frequency vibration of the electrodynamic shaker
The specific implementation method is:
A. The inertial mass (kg) of the foundation with large inertial mass should be (8~10) times the maximum exciting force (kgf) of the shaking table, and some data recommend 20 times.
B. Use bolts to firmly connect the bottom frame of the vibrating table to the steel skeleton of the foundation.
C. Lock the air spring between the shaker and the bracket, and firmly connect the shaker, the bracket and the foundation as a whole.
After the above work is completed, the resonance frequency of the shaking table body will drop to a very low level. As long as these connections are firm and reliable, the theoretical resonance frequency can be close to “zero”, so the electrodynamic shaker can be used to implement 0.5Hz or even lower vibration test at the operating frequency.
2. Build a good “ground wire” of the laboratory to reduce the background noise of the electrodynamic shaker
The purpose of reducing the background noise of the electrodynamic shaker is to improve the signal-to-noise ratio of the micro-level acceleration test, so as to improve the control accuracy of the vibration test. There are many ways to reduce the background noise of the vibration table. One is to require the vibration test equipment manufacturer to provide test equipment with a smaller signal-to-noise ratio, such as reducing the static noise of the power amplifier and improving the matching performance of the moving coil and the excitation coil, etc. . The second is to build a good “ground wire” in the laboratory, and the grounding resistance of the ground wire should be less than 4Ω (closer to 1Ω is the best). Both the shaking table body and the power amplifier should be directly and firmly connected to the ground wire of the laboratory, and during the vibration test, it is necessary to prevent other high-power electrical equipment from being connected to the ground wire unique to the laboratory.
3. Improve the sensitivity of acceleration measurement
Improving the sensitivity of acceleration measurement can obtain a larger acceleration signal value when there is a small acceleration vibration. This can be done by increasing the normalized sensitivity coefficient (mV/g) of the charge amplifier, or using the charge sensitivity coefficient (pC /g) Higher piezoelectric accelerometers. Although this method improves the perception sensitivity of vibration acceleration and can detect small acceleration values, it also increases the level of noise in the detected amount. Therefore, while adopting the above measures, the level of the interference noise must be reduced, or the level of the interference noise must be selectively filtered out during the signal acquisition process.
4. Maintain the centering state of the vibrating table
The electrodynamic shaker table (including moving parts such as the moving coil) is usually supported on the table body by rocker rolls and combined plate springs, and an air spring to balance the test load is also fitted underneath the moving coil skeleton. The inflation pressure of the air spring can be adjusted according to the size of the table load so that the shaker table is always stationary in the centre position, keeping the maximum displacement when vibrating sinusoidally relative to the centre position. In order to make full use of the maximum displacement limit for low frequency vibration, after the test load has been installed on the table, the table should be checked to see if it has deviated from its intended centre position and if so, the inflation pressure in the air spring should be adjusted so that the table returns to its centre position before the test is carried out.
5. Adjust the test frequency range
The above four measures are the fundamental solution for the low-frequency vibration test of the electrodynamic shaking table. In the case of a last resort, as a temporary measure, it is also possible to consider adjusting the low-frequency starting frequency of the test frequency. The premise of adopting this measure is:
A. The consent of the test client must be obtained (subject to the signed document)
B. The adjusted low frequency starting frequency fL must be lower than the lowest natural frequency f1n of the tested product. In theory, the minimum starting frequency value is required to meet the requirements of fL≤（0.4~0.6）f1n.