Random vibration experimental conditions Random vibration experimental conditions generally give a reference curve (PSD pattern) of power spectral density and frequency. The solid line shown is the PSD reference spectrum pattern given by the experimental conditions. The experimental requirement frequency range PSD reference spectrum pattern is within 50-1000 Hz, and the vibration table mesa signal power spectrum is a flat spectrum. The solid line shown is the power spectrum of the vibrating table signal measured in the experiment, that is, the control spectrum curve. The upper and lower dotted lines in the sum are the tolerance alarm lines of the reference spectrum, and the magnitudes are 3dB and -3dB respectively. At the same time, the time of each experiment is specified as: 20s in the pre-experimental level -6dB and -3dB; The stage 0dB runs for 60s. In the broadband random experiment, since the input energy of the vibrating table is very small, the power spectral density value is 0.0025g2/Hz in the range of 50-1000Hz, so the structural response is considered to be a stationary random vibration in this experiment. <8>, the dynamic characteristics of the support before and after the loosening of the connecting bolt can be described by a linear model, and the analysis of the response signal uses a power spectrum analysis method.
The spectrally reproducible wideband digital random vibration control system is roughly divided into four components, namely: A/D and D/A conversion, forward and reverse conversion in the time-frequency domain, spectrum equalization, and randomization of the excitation signal. Several key problems in random vibration experiments (1) PSD calculations In theory, an infinitely long time record should be used to describe a random vibration process, and in practice only a finite time record or data can be obtained. For a random vibration experienced by each state, this finitely long record is called a "sample" or "frame" and contains all of the statistical information in the entire random vibration process (parent). The experiment uses the windowed segmentation average periodogram method, that is, the Welch method to perform PSD calculation of discrete random signals.
(2) Spectrum Equilibrium In the random vibration control experiment, an important problem is to achieve the equilibrium of the vibration experiment process. In the laboratory, a random signal source can be used as a vibration table input signal to achieve random vibration, but the response on the vibration table depends on the dynamic characteristics of the vibration test system composed of the vibration table and the test piece. It is difficult to meet the reference spectrum Sr(w) required in the experimental specification.
The experimental results and the arrangement of the analysis points are as shown. In the experiment, the structural acceleration response signals of different measuring points (2, 3, 4) are first measured under the normal tightening condition of the 12 bolts connecting the support belt and its trough support (measurement point 1 is used to control the vibration table). , not marked); then by loosening the bolt in the reverse direction, the tightening force of the bolt is reduced, and the distance between the upper and lower straps is increased to simulate the looseness of the bolts of different severity, and the experiment is carried out according to the above experimental method and measured. data.
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