High-performance inertial sensors, such as ring laser gyroscopes or fiber optic gyroscopes, have sufficient performance to enable “dead reckoning” navigation for adequate periods of time. Smaller microelectromechanical system (MEMS) inertial sensors, such as MEMS gyroscopes and MEMS accelerometers, typically have relevant performance characteristics that are 10 or 100 times worse than high-performance inertial sensors. As a result, these small MEMS inertial sensors must be aided by a global positioning system (GPS) if they are to be used for navigation.
An improved gyroscope uses interferometric techniques with diffractive gratings to measure the ultra-small displacements of the proof mass in said gyro. This sensing technique enables the use of a large proof mass with very low thermomechanical noise while maintaining very high-resolution sensing of the proof mass displacement. This high-resolution sensing also enables gyroscope designs that have large separations between the proof mass resonant frequency and the sense resonant frequency, improving the stability of the device.
The MEMS gyroscope uses a single light source with a diffractive interferometric sensor. A single diffractive interferometric sensor is coupled to the proof mass and a single light source interrogates this sensor to produce the interferometric optical signal that represents the proof mass motion. In this and related prior art, inertial sensors with interferometric sensing use a single interferometric sensor with a single light source, typically a VCSEL (Vertical Cavity Surface-Emitting Laser).
This approach with a single-ended measurement has a significant disadvantage when compared to the differential capacitive sensing method. The disadvantage is that no mechanical drift or thermal drift of the sensor can be compensated for with this approach. Thus, ambient temperature changes severely affect the performance of the sensor. None of the prior art, at the time of this reporting, describes using a single light source in conjunction with an optical beam splitter and with two interferometric sensors for measuring proof mass motion. By using a single light source with an optical beam splitter and two interferometric sensors in a MEMS gyroscope, laser intensity noise can be canceled using a differential approach, noise from motion of the sense element due to thermal drift can be canceled, noise from motion of the sense element from mechanical drift can be canceled, and noise from common motion of the sense element from unwanted inertial forces (such as off axis acceleration) can be canceled.
This work was done by Matthew D. Ellis of Fine Structure Technology Inc. for Kennedy Space Center. KSC-13894