Direct Measurement of Gyroscope Quadrature Error Using Interferometric Tools

A tuning fork gyroscope (TFG) functions by sensing a Coriolis acceleration. Proof masses are excited in plane, such that the motion is sinusoidal. A rotation, normal to the excitation direction, results in a Coriolis acceleration based on the cross product of the input rotation and the proof mass in-plane velocity. The Coriolis force displaces the proof mass, normal to the chip by an amount related to input rotation. The in-plane (drive) and out-of-plane (sense) motions of the proof masses are offset by 90°, placing these positions in quadrature. Quadrature error is present when the proof masses do not oscillate in plane, but rather have a small degree of out-of-plane motion. This error may cause a zero rate output in the sensor, also known as a bias error. A methodology is presented for using interferometry to directly measure the quadrature motion. Optical interferometry uses non-contact methods for shape measurement, with high accuracy normal to the viewing plane. Measurements may be made on the proof masses, while running, by utilizing a strobe illumination source. By matching the strobe frequency with the gyroscope resonance the motion is frozen to the camera, during which time quantitative measurements are made. This process is repeated at various positions of the in-plane proof mass motion and analyzed to reveal the degree of out-of-plane displacements of the proof masses while running.