To complete a sine vibration test according to the specifications, the capabilities of the vibration test table are divided into three categories: vibration functional indicators, technical indicators and auxiliary technical indicators.
1)Functional indicators
It refers to the functions required by the vibration table to complete the sinusoidal vibration, as shown below:
a) Fixed frequency vibration: implement sinusoidal vibration at a certain fixed frequency.
b) Vibration dwell: usually refers to the vibration implemented at the resonant frequency point of the product. When the resonant frequency point of the product drifts, the vibration control system can automatically track the change of the resonant frequency and always “reside” at the resonance of the product. vibrate at frequency.
c) Sweeping frequency vibration: Sweeping frequency vibration refers to a vibration test in which the frequency of vibration changes with time according to a certain pattern during the vibration test. Sweep vibration is divided into linear sweep and logarithmic sweep.
d) Beat frequency vibration:Beat frequency vibration uses a sine wave signal with a lower vibration frequency to modulate another sine wave signal with a higher vibration frequency. Mathematically it can be understood as the product of two sinusoidal vibrations of different frequencies, namely:
x(t)=x0sin(ωt)xsin(ωt/m)
In the formula: x0–amplitude of sinusoidal vibration, cm;
ω–circular frequency of sinusoidal vibration, rad/s;
m–The modulation magnification, that is, the ratio of the modulation frequency to the basic signal frequency.
2) Technical indicators
Technical indicators refer to the basic performance parameters of the shaking table, as follows:
a) Exciting force: the force transmitted by the vibration table to the test specimen. The maximum force (N; kN) usually given is divided into sinusoidal force and random force values;
b) Vibration amplitude: refers to the macro motion parameters of the vibrating table. Including the maximum displacement or represented by peak-to-peak displacement and maximum stroke (cm), maximum speed (m/s), maximum no-load acceleration (m/s2; g=9.81m/s2);
c) Load capacity: Maximum inertial mass that the vibrating table can bear (kg):
d) Frequency range: the lowest to the highest frequency value at which the vibration table can work (Hz);
e) Frequency sweep rate: the rate at which the vibration table implements linear frequency sweep or logarithmic frequency sweep (Hz/min; oct/min).
A frequency sweep whose ratio of frequency change to frequency sweep elapsed time is equal to a constant m is a linear frequency sweep:
m=(fh -f1)/Δt
In the formula: m–linear sweep rate, Hz/min;
f1–Start frequency of linear sweep, Hz;
fh–termination frequency of linear sweep, Hz.
Δt–the time elapsed from the start frequency f1 frequency sweep to the end frequency fh
In a logarithmic frequency sweep, the ratio of the logarithmic value of the frequency change to time is a constant. Logarithmic sweep rate is usually expressed as the number of octaves swept(oct) per unit time.
The rated performance of sinusoidal vibration is usually expressed in terms of the maximum value (peak value), but it can also be expressed in terms of the root mean square value, with the peak value being 1.414 times the root mean square value. Since the square of the root mean square value of the current flowing through the moving coil of the electric vibrating table multiplied by the impedance of the moving coil is the total heat generation of the moving coil, the root mean square value is an important parameter in the structural design of the electric vibrating table and the calculation of cooling air volume and cooling temperature.
3) Auxiliary technical indicators
a) Stability of vibration parameters S
The stability of vibration parameters refers to the change in vibration output parameters (frequency, displacement, speed, acceleration) with the vibration time history, usually expressed as a percentage “%” value.
In vibration tests, the more stable the vibration parameters are, the more credible and effective the test results are.
b)Vibration waveform distortion degree Y
The smaller the waveform distortion of the shaking table, the more conducive it is to the waveform reproduction of sinusoidal vibration and the improvement of the accuracy of frequency sweep vibration. It is also conducive to the reproduction and simulation of transient waveforms and “spectrum” in random vibration and shock tests.
c) The first-order resonant frequency ω of the vibration table The first-order resonant frequency of the vibrating table refers to the basic natural frequency of the mass-spring system composed of the moving parts of the vibrating table and its suspension and support structures in the no-load state. The higher the value of this parameter, the more advantageous it is for the user to perform vibration tests.
d) Fixed vibration accuracy table for sweep frequency vibration
e) The dynamic range alarm test of the vibration test system During the vibration testing, due to the resonance of the sub-vibration table, fixture and test piece assembly, in the open-loop state, the measured acceleration-frequency response characteristics of the control point will have many steeply rising convex peaks and sudden depressions. During closed-loop control, the vibration control system must have the ability to compensate and flatten the steep changes in acceleration between “peaks and valleys” (referred to as voltage control capability) to meet the fixed vibration accuracy requirements of sweep frequency vibration, and is also conducive to the vibration power spectrum and Reproducible simulation of shock response spectrum. This ability to define the “peak-to-valleys” voltage control of the vibration table system is the dynamic range of the vibration test system.
The dynamic range of the vibration test system cannot be regarded as the same as the dynamic range of the vibration test control system itself. The former index is also related to the background noise of the vibration table body and the power amplifier. The dynamic range of the control system only directly connects its output and input ports, and detects the ratio of the maximum output of the control system itself to the background electrical noise, which has nothing to do with the working conditions of the vibration table and power amplifier.
f) Shaking table acceleration signal-to-noise ratio M
The acceleration signal-to-noise ratio of the shaking table refers to the ratio of the highest output acceleration to the lowest output acceleration under the no-load state of the shaking table. This indicator indicates the relationship between the highest acceleration output of the shaking table and the background noise. The higher this index is, the higher the dynamic range of the vibration test system will be, and it will also help improve the distortion of the waveform, and will be more conducive to the reproduction and simulation of random vibration signals and shock signals.
g) Vibration table transverse vibration T
The lateral vibration of the vibrating table refers to the percentage of the maximum vibration amplitude (displacement, acceleration) in the plane perpendicular to the main vibration direction and the vibration amplitude in the main vibration direction, usually expressed as a percentage “%” value.
h) Vibration table acceleration uniformity N Vibration table acceleration uniformity refers to the non-uniformity of the vibration acceleration output of each point on the vibration table in the no-load state. The calculation is based on the maximum of the acceleration of each point on the table and the acceleration of the center point of the table. The ratio of the difference to the acceleration of the center point, expressed as a percentage “%”.
i) Offset load capacity The eccentric load capacity refers to the degree to which the center of gravity (i.e., the center of mass) of the test piece installed on the vibration table is not allowed to coincide with the central axis of the vibration table. This type of vibration table is usually marked with “eccentric moment” (N·m or kN·m). ability.
j) Magnetic leakage on the working table
Table magnetic leakage refers to the intensity (T) of the magnetic leakage magnetic field generated above the central axis of the vibration table (that is, the position of the test piece during the vibration test) when the vibration table is working.