Eddy current system for vibration testing of cantilevered nonferrous articles

Claim 1

An eddy current-induced vibration testing device for automatically maintaining and controlling the amplitude of selfexcited resonant frequency vibration of nonferrous cantilevered articles, comprising: means including a high mass mount for holding said article to be vibrated in a cantilevered fashion; an electromagnet having first and second pole ends positioned relative to the free end of said article so as to form an air gap between each of said pole ends and said article to induce eddy currents in said article; a permanent magnet having one pole end disposed adjacent one side of said article diametrically opposed to one of said pole ends of said electromagnet disposed adjacent an opposite side of said article so as to induce vibration in said article upon actuation of said electromagnet; a vibration sensing transducer having an AC signal output whose frequency is equal to the frequency of vibration of said article and propOrtional in amplitude to the amplitude of vibration of said article; a variable phase-shifting circuit connected to the output of said transducer; a converter circuit coupled to the output of said transducer for providing a DC control voltage proportional to the amplitude of vibration of said article at an output thereof; an amplitude regulator circuit including a dual gate field effect transistor having a source, drain, and first and second gate electrodes, said first gate electrode connected to the output of said converter circuit, said second gate electrode coupled to the output of said phase-shifting circuit and said source electrode connected to ground potential so that a signal at the output of said drain electrode is an AC signal whose amplitude is regulated in accordance with said DC control voltage; a power amplifier having an input coupled to said drain electrode of said transistor and an output connected to said electromagnet; and means for selectively controlling the amplitude of the input signal to said converter circuit, thereby selectively controlling the desired amplitude of vibration of said article.