Wen He

A long-stroke horizontal electromagnetic vibrator for ultralow-frequency vibration calibration

A novel long-stroke horizontal electromagnetic vibrator with maximum stroke of 1?m is proposed. To reply to the strong nonlinearity arising from long stroke, a closed double-magnetic circuit with optimal air gap, an electro-viscoelastic-suspension device and a track following device are adopted in the vibrator. Also, a compact moving component with a higher first-order natural frequency is designed to increase the operating frequency of the vibrator. Finally, experimental results show that the vibrator could output low distortion acceleration on its working platform from 0.002?Hz to 100?Hz, which verifies the validity of the proposed technologies and the applicability of the vibrator for an ultralow-frequency vibration calibration system.

A novel vibration-level-adjustment strategy for ultralow-frequency vibration calibration based on frequency-shifted method

Aimed at the time-consuming process of an ultralow-frequency vibration calibration, a strategy of rapid vibration-level-adjustment for ultralow-frequency vibration exciters is put forward. First, an ultralow-frequency vibration exciter system based on a displacement feedback control technique is analyzed, whose frequency response function between its input voltage and output displacement is established and calculated based on MATLAB. It is found that the output displacement of the system is only dependent on the input voltage rather than the input signal frequency. Then, a novel rapid vibration-level-adjustment strategy on the basis of the frequency-shifted method is proposed, whose process is that the vibration exciter is first adjusted to the target vibration level at a higher pre-adjustment frequency and then the input signal frequency is gradually decreased from the pre-adjustment frequency to the target ultralow frequency. The method is applied in an ultralow-frequency vibration automatic calibration system. The experimental results indicate that the new strategy can obviously reduce the time of the vibration-level adjustment of ultralow-frequency vibration exciters, and improve the efficiency of the calibration system. The study is helpful for the widespread use of ultralow-frequency vibration calibration.

A dedicated pistonphone for absolute calibration of infrasound sensors at very low frequencies

Aimed at the absolute calibration of infrasound sensors at very low frequencies, an upgraded and improved infrasonic pistonphone has been developed. The pistonphone was designed such that a very narrow clearance between the piston and its guide was realized based on an automatically-centered clearance-sealing structure, and a large volume rigid-walled chamber was also adopted, which improved the leakage time-constant of the chamber. A composite feedback control system was applied to the electromagnetic vibrator to control the precise motion of the piston. Performance tests and uncertainty analysis show that the leakage time-constant is so large, and the distortion of the sound pressure is so small, that the pistonphone can be used as a standard infrasound source in the frequency range from 0.001 Hz to 20 Hz. The low frequency property of the pistonphone has been verified through calibrating low frequency microphones. Comparison tests with the reciprocity method have shown that the pressure sensitivities from the pistonphone are not only reliable at common frequencies but also have smaller uncertainties at low frequencies.

Self-Sensing Waveform Control for a Low-Frequency Electromagnetic Vibrator

In this paper, a self-sensing model of an electromagnetic vibrator is analyzed to pick up linear velocity directly without any extra sensors. A feedback control system is then established based on the self-sensed velocity. As a result, the bandwidth of the constant-velocity segment of the vibrator can be expanded and the total harmonic distortion (THD) of the output waveform is decreased in the low-frequency range. The influence of different parameters in the feedback control system on the THDs is experimentally analyzed. Comparing with the conventional feedback control system with an extra displacement sensor, the proposed control system is more powerful in reducing the THDs of acceleration waveform to be lower than 1% at the frequencies from 0.1 to 10 Hz, but without any other feedback sensors.