| Compared to omni-directional microphones, directional microphones have relatively worse internal noise, which limit their performance in some applications, such as hearing aids. This dissertation presents a scheme to lower the internal noise by introducing electrical active damping to the microphone system.;The noise analysis of the directional microphones shows the damping coefficient can be the major factor in affecting the microphones' internal noise; therefore, by designing the directional microphones with relative low passive damping and boosting the overall equivalent damping with an active damping scheme, the directional microphones can have low internal noise and desirable tracking performance.;The active damping scheme is achieved through electrostatic actuation. The nonlinear behaviors of the electrostatic actuation for the directional microphones are investigated and the stabilities caused by the actuation nonlinearity are also analyzed.;Three feedback control schemes for active damping are proposed. Although the "notch filter" and "derivative filter" approaches can satisfy the control objectives, they have intrinsic shortcomings in the noise performance and robustness issues. Finally, the robust controller design through Hinfinity method is presented. The resulting controller not only satisfies the control objectives but also has proven robustness. |
