Sliding Mode for an All-Digital Control and Readout of MEMS Gyroscopes

Abstract

Mobile devices and sensor networks advances have sharply increased the demand for inertial MEMS sensors with a direct digital output. This paper reports on the first implementation of a MEMS-based gyroscope system with all-digital control and readout, using sliding mode and bitstream processing for both the driving and sensing modes. The in-plane vibrating MEMS gyroscope, sensitive to out-of-plane external angular rates, was designed in a custom 50um SOI technology. A band-pass sliding mode control (SMC), equivalent to a digital PLL control, was used for electrically driving one of the resonant modes at its resonant frequency, and track its drift (due to environment variations). Simulations indicate a fast capture time in response to step variations (high tracking speed of 11,486 rad/s2) and small continuous tracking errors, below 0.0083 %. A regular (low-pass) sliding mode control was used for the sensing mode, in order to cancel its vibration and thus improve its linearity. The feedback loop action was proven to attenuate the displacement in the sensing mode by more than 100 times. The resulting bitstream output (that generates in the SMC loop the high-frequency cancelling electrostatic forces) contains the relevant information about the Coriolis force, and a bitstream synchronous demodulation technique is used to reconstruct the input angular rate to be measured, with a reconstruction error of ±0.2 rad/s. The overall system enables an easy future implementation on a MEMS+FPGA of a high-performance angular rate sensing microsystem with digital output.