MEMS Inertial Sensors Characterization and Design

Abstract

MEMS gyroscopes suffer from mechanical quadrature error which results from mis-match in the geometry of the MEMS structure that happens during the fabrication process. The mechanical quadrature error causes direct coupling of the drive sig-nal to the sense mode. The coupled quadrature error can be even larger than the angular-rate signal to be measured and can have serious impact on the gyro per-formance, especially bias stability, bias temperature sensitivity and dynamic range. Therefore, several mechanical and electrical techniques were proposed to address the quadrature problem.

This work is driven by the need to eliminate the quadrature error, electrically, after the sensor has been manufactured. The objective is to prevent the quadrature error from limiting the dynamic range of the sense mode electronics and to eliminate the influence of the variations in quadrature error on the zero-rate output of the sensor which worsen the sensor performance regarding Bias Stability and Bias over Temperature.

Several electrical techniques for quadrature cancellation have been proposed. These techniques include synchronous demodulation, Open Loop and Closed Loop cancel-lation techniques. However, the effect of non-idealities on the effectiveness of these techniques was not reported. Since these techniques involve demodulation and mod-ulation process, phase-error is the non-ideality that can have a great impact on these techniques.

In this work, mechanical quadrature error gyroscope, and its effect on the gyroscope system is explained. Different techniques for quadrature error cancellation, in mis-matched mode gyroscopes, are presented and analyzed. The effect of phase error on the system performance in each technique is highlighted and closed form expressions for the phase error effect are derived.

Thereafter, quadrature cancellation system based on Open Loop, or Signal Injec-tion, technique is adopted and implemented. The system is based on canceling the quadrature error at the input of the front-end circuit of the sense mode. This tech-nique is very attractive for gyroscopes operating in open loop sense configuration and for gyroscopes that do not include electrodes for quadrature cancellation. The system-level analysis and design of the Open Loop quadrature cancellation loop are presented. In addition, circuit-level implementation of the key system blocks is conducted.

On the system level, the target overall system performance is defined, and accord-ingly the specifications and architecture of each block in the quadrature loop are defined. Stability analysis for the quadrature loop is studied with the aid of a MAT-LAB model. A SIMULINK model is implemented to study the transient behavior of the loop and evaluate the SNR of the system.