Why Vibration Testing Like IEC , MIL-STD , ISO , ASTM , GB / T ?

Why Vibration Testing?
Due to the demands of high speed operation and the use of light structures In modern machinery, static measurements of stress/strain properties are not sufficient. Dynamic measurements are necessary and vibration 'testing has therefore found widespread use.

In the environmental laboratory, vibration testing is per·formed as part of a company's quality assurance programme together with for example temperature and humidity tests to ensure product reliability. The test object is exposed to a certain vibration level according to a
procedure specified by national and Internatrional standards,

To find the dynamic properties of a structure, the response to a vibrational force Is of interest rather than the actual vibration level. This concept is found for instance in determination of the ability to transmit or damp vibrations or In the description of the vibrational modes of a structure at resonances. In the calibration of vibration transducers a comparison is made between the transducer to be calibrated and a reference transducer at a prescribed vibration level. To produce a defined vibration an electromagnetic vibration exciter (also called a shaker) Is used. This converts an electric signal Into a mechanical movement, controlled to maintain a certain vibration level or force.

How Does an Exciter Work?
In principle the electromagnetic vibration exciter operates like a loudspeaker, where the movement is produced by a current passing through a coli in a magnetic field. The force used to accelerate the moving element is proportional to the drive current and the magnetic flux.
Therefore by controlling the current, the vibration level of the exciter can be controlled. In small exciters the magnetic field is produced by a permanent magnet, whereas in the larger ones electromagnets are necessary. The maximum current and the load determines the acceleration level which can be obtained. At low frequencies, however, this acceleration level will decrease due to displacement limitations of the
moving element. Resonances in the moving element will set the upper frequency limit. The performance of an exciter is presented in a diagram, showing the maximum acceleration as a function of frequency. With double logarithmic scales the displacement limit will be represented by a straight line with a slope of 12 dB/octave. A velocity limit is often also found, especially with the larger exciters, and this is Indicated by a line with a slope of 6 dB/octave.

The Power Amplifier
The frequency response for an exciter driven by a constant current will show three regions of different nature. The first two regions represent the spring-mass system of the moving element and Its suspension with a resonance of typically 20 Hz. In the third region, typically above 5 kHz for big exciters, axial resonances in the moving element will occur, set1lng the upper operational frequency of the exciter. A response curve for an exciter with a constant voltage input will show the same regions of control, but the lower resonance is considerably damped, giving an easier control of the level. The voltage control, obtained by it low Impedance amplifier is normally preferred. In some cases, however, a current control will be advantageous, primarily when the exciter is used as a force generator or where non-feedback control is required using the mid frequency range of the e)(citer. This demands a high impedance output and therefore amplifiers will often have selectable impedance outputs.

The EXCiter Control
The use of a vibration exciter assumes a constant vibration level at the table. The frequency response curve is not flat. it contains resonances, and other resonances will be introduced when a test object Is mounted on the exciter. When used throughout a frequency range the gain of the amplifier must consequently vary with frequency. This gain is set by a controller, receiving feedback information from the test object. The main elements of an exciter control must therefore be a frequency generator, a vibration meter and a level controlling circuit

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