All-digital MEMS tuning-fork self-excited vibration control by phase-relation using TAD-based ADPLL

For achieving an all-digital resonant MEMS gyroscope, this paper presents an all-digital MEMS tuning-fork self-excited vibration control method, using TAD (Time-A/D converter)-based all-digital PLL (TAD-ADPLL) by applying a unique control algorithm based on entirely time-domain processing, which uses no conventional analog method such as automatic gain control (AGC) or automatic level control (ALC). The proposed algorithm involves three-step processing: 1) driving a tuning-fork using the ADPLL for searching its self-resonant frequency, 2) comparing the phase difference between drive-pulse signal and monitor-pulse signal, which should be 90^o (π/2-radian) each other, and 3) keeping 90^o-relationship between them even with any drift factors such as temperature, supply voltage, etc. In this method, TAD-type TDC (time-to-digital converter) digitizes the resonant frequency and phase difference alternately in order to realize self-excited vibration condition along with TAD-type DCO (digitally-controlled oscillator) without the need for any analog circuit method. By using a conventional piezoelectric MEMS tuning-fork element, we experimentally confirmed its self-excited vibration, resulting in its resonance jitter level of σ = 52.6ns at 37μs-self-resonance period. Finally, we propose an all-digital synchronous detection of angular-rate signal for achieving a digital-type gyro sensor.