Air-Gap Detection Circuit Using Equivalent Capacitive Changes for Inductive Micromotor

In this paper, we present an innovative capacitive gap-sensing readout circuit to replace the traditional gap sensor embedded in an inductive micromotor (IMM). Twelve equivalent capacitor pairs are constructed to detect the position deviation of the eccentric disc, which is rotating. As the position deviation of disc occurs, the capacitances of the corresponding capacitor pairs are altered due to air gap changes. The major functions of the proposed capacitive gap-sensing readout circuit are: 1) to amplify a micrometer-scale signal to be of millimeter scale; 2) to suppress the low-frequency noises caused by exerted three-phase ac power; 3) to rectify the induced ac gap-sensing signal into a dc output voltage; and 4) to eliminate the undesired dc bias caused by parasitic capacitances. In addition, by applying the effects of inertial force and centrifugal force, an innovative noncontact measurement method to quantify the unbalance degree of the microdisc, i.e., eccentricity, due to imperfect fabrication of IMM, is also proposed. By intensive computer simulations and intensive realistic experiments undertaken, the performance of proposed capacitive gap-sensing readout circuit and the unbalance degree of the disc have been successfully evaluated, respectively. From the simulation results, the minimum resolution of 1.22 mV/pF is obtained for the proposed capacitive gap-sensing readout circuit. The maximum sensitivity and bandwidth are ~118 mV/μm and 51.604 kHz, respectively.