1. Field of the Invention
The invention relates to the field of micromachined gyroscopes and accelerometers, and in particular to designs for anti-phase devices to compensate for fabrication and environmental variations.
2. Description of the Prior Art
In recent years, the development of microelectromechanical systems (MEMS) and the improvement of fabrication techniques have opened new avenues for the development of low-cost sensors. This is especially true for inertial sensors, more specifically micromachined gyroscopes which have shown enormous potential for a wide range of applications.
When two masses are oscillating with a 180 degrees phase difference, they are considered to be moving in opposite directions. This type of motion is referred to as “anti-phase,” since they are oscillating out of phase with exact opposite motion paths. Typically, this technique is employed in the drive direction of vibratory gyroscopic devices which have come to be known as tuning fork gyroscopes. The advantage of such devices is that the induced sense response due to the input angular rate will also be an anti-phase oscillatory motion among the two masses. This means that inputs such as environmental noise and acceleration loads, which cause the masses to respond in phase (called common mode inputs), can be cancelled by utilizing differential sensing techniques.
One drawback to such a device is that the motion of both the drive and sense masses must be precisely in anti-phase so that the device can indeed reject common mode stimuli. In practice, due to inevitable imperfections in the mechanical structure, there are issues with maintaining precise anti-phase motion in both the drive and the sense directions. Methods for achieving this motion in the drive direction have been proposed including device designs and control architectures that force anti-phase motion, but this adds complexity to the system.
Another problem that is often overlooked in tuning fork devices is maintaining the anti-phase response of the sense mode, specifically the potentially large phase variations that can occur when operating at or very near resonant frequencies. A method of avoiding this phase stability issue in both drive and sense is to operate the device off resonance where the phase remains relatively unchanged for small frequency fluctuations. However, the sensitivity of the device is sacrificed drastically when advantage of sense-mode resonance is not utilized.
What is needed is a gyroscope design that alleviates problems commonly seen in conventional devices.