Improving the Dynamic Range of a Digital Random Equalizer

The electrodynamic exciter is widely used to perform vibration testing. Commonly, the exciter is energized with wide-band audio random noise. The spectral density of the random vibration is made to conform to a desired plot, through automatic servo techniques. The spectrum is split into contiguous filter bandwidths, with an individual control loop for each band. Thus, any specimen resonance is equalized. Dligital equalizers have recently been introduced. Their control dynamic range is liable to be curtailed by the number of significant figures possible in the digital computations. A method of improvement is described, whereby the roundoff effect is used as a dither signal. Some control schemes have been reported using the specimen transfer-function as the basis of control. These schemes are also analyzed here and the effect of roundoff error in limiting their function is explained. Finally, the technique proved effective for simple equalization is applied to the more sophisticated multiple-signal control situation, through the application of random-flight theory. The technique maintains equalizer performance equal to that of the single-acceleration case, with negligible increase in system complexity: a long-standing goal of vibration technology.