If you can only use an Electrodynamic vibration test system to perform low-frequency vibration tests, it is recommended to adopt the following technical measures.
1) Construct a foundation with large inertial mass and improve the dynamic characteristics of the low-frequency vibration of the ED shaker.
The specific implementation methods are:
a) The inertial mass (kg) of the foundation with large inertial mass should be 8 to 10 times the maximum thrust (kgf) of the vibration machine, and some data recommend up to 20 times;
b) Securely connect the bottom frame of the shaking table to the steel frame of the foundation with bolts;
c) Lock the air spring between the table body and the bracket to firmly connect the table body, bracket and foundation into one body.
After completing the above work, the resonance frequency of the vibration 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”. Therefore, the vibration test of 0.5Hz or even lower operating frequency can be carried out with the electrodynamic vibration test system.
2) Build a good laboratory “ground wire” to reduce the background noise of the ED shaker.
The purpose of reducing the background noise of the ED shaker is to improve the signal-to-noise ratio during micro-level acceleration tests, so as to improve the control accuracy of vibration tests. There are many ways to reduce the background noise of ED shaker. One is to require vibration test equipment manufacturers to provide test equipment with a smaller signal-to-noise ratio, such as reducing the static noise of the power amplifier, 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. The ground resistance of the ground wire should be less than 4Ω (close to 1Ω is best). Both the vibration machine body and the power amplifier should be directly and firmly connected to the ground wire of the laboratory, and during the vibration test, other high-power electrical equipment should be specifically prevented from being connected to the laboratory’s unique ground wire.
3) Improve the sensitivity of acceleration measurement
Improving the sensitivity of acceleration measurement can obtain a larger acceleration signal value during small acceleration vibrations. This can be done by increasing the normalized sensitivity coefficient (mV/g) of the charge amplifier or by using the charge sensitivity coefficient (pC /g) Higher piezoelectric accelerometer. Although this method improves the sensitivity of vibration acceleration and can detect small acceleration values, it also increases the level of noise in the detection volume. Therefore, while taking the above measures, the level of interference noise must be reduced, or the level of interference noise must be selectively filtered out during the signal acquisition process.
4) Maintain the centering of the vibrating table
The electric vibration table (including moving parts such as moving coils) is usually supported on the table body by rocker rollers and combined leaf springs. An air spring is also installed under the moving coil frame to balance the test load. According to the size of the table load The inflation pressure of the air spring can be adjusted so that the vibration table is always stationary at the center position. The maximum displacement when maintaining sinusoidal vibration is the vibration relative to the center position. In order to make full use of the maximum displacement limit during low-frequency vibration, after installing the test load on the table, check whether the table deviates from the original center position. If there is a deviation, the inflation pressure in the air spring should be adjusted to return the vibration table to the original position. After the center position is reached, the test can be carried out again.
5) Adjust the range of test frequency
The above four measures are the fundamental solution for implementing low-frequency vibration tests on electric vibrating tables. As temporary measures when absolutely necessary, you can also consider adjusting the low-frequency starting frequency of the test frequency. The prerequisite for adopting this measure is :
a) Must obtain the consent of the trial client (subject to signed documents);
b) The adjusted low-frequency starting frequency fL must be lower than the lowest natural frequency f1n of the product being tested. Theoretically, the lowest starting frequency value is required to meet the requirement of fL≤(0.4~0.6)f1n.