A dual-loop waveform distortion suppression method for electromagnetic vibrators at low frequencies

The total harmonic distortions (THDs) of output acceleration waveforms from an electromagnetic vibrator are always large due to the nonlinear stiffness, damping and magnetic intensity, which will affect the vibration calibration precision. To suppress THDs of output acceleration waveforms, a dual-loop waveform distortion suppression method based on current and velocity feedback for electromagnetic vibrators at low frequencies is presented. A counter electromotive force compensator is adopted in the current loop to remove the coupling between the current loop and velocity loop, which improves the performance of the current loop. Besides, a damping adjuster is adopted in the velocity loop to increase the damping ratio, which can improve dynamic characteristics and reduce waveform distortions when the multiple working frequencies coincide with the resonant frequency. Finally, the cooperation of the outer velocity loop and the inner current loop reduces THDs of acceleration waveforms effectively. Especially, the vibration velocity for the velocity loop is extracted by a secondary coil instead of commercial sensors, which is coaxially and separately wound around the driving coil of the electromagnetic vibrator. The proposed method is analyzed in simulation and verified by experiments, whose results show that the THDs of output acceleration waveforms are well restrained from 25% before feedback to less than 3% after feedback at low frequencies.