Variable Sampling Verification Protocols for High Rate MEMS Gyroscope Lots
Variable sampling verification for high-rate MEMS gyroscopes optimizes lot acceptance by deriving continuous quality indices from dynamic rate table test samples.
Coriolis Acceleration
Inertial Effect
A kinematic force appears in a rotating frame of reference that acts on an object moving with a velocity relative to that frame. This coriolis acceleration is the physical principle that allows vibratory gyroscopes to detect angular velocity in inertial navigation systems. It creates a secondary motion perpendicular to both the primary vibration axis and the axis of rotation.
The magnitude of this effect is proportional to the product of the mass velocity and the angular rate. This interaction defines the sensing mechanism for microelectromechanical systems and larger mechanical rate sensors. The effect vanishes when the object is at rest relative to the rotating frame or when the velocity is parallel to the rotation axis.
Sensing Mechanism
Measurement of this force requires a drive system that maintains a constant oscillation of a proof mass. Inside a sensor utilizing coriolis acceleration, the secondary vibration is detected as a change in capacitance or a piezoelectric voltage. This signal is typically much smaller than the primary drive motion, requiring high precision electronics to isolate the rotational information.
The sensitivity of the instrument depends on the mass of the vibrating element and the frequency of the drive loop. Calibration involves spinning the sensor on a rate table to establish the relationship between the applied rotation and the resulting output. Drift in the drive frequency can lead to errors in the calculated acceleration.
Verification of the scale factor occurs by comparing the sensor response to a known reference standard.
Interference Factors
Errors in the measurement arise from mechanical imbalances or imperfections in the fabrication of the vibrating structure. When detecting coriolis acceleration, quadrature error occurs when the drive motion leaks into the sensing channel due to non orthogonal alignment. This interference is often larger than the actual signal and must be rejected through phase sensitive detection.
Environmental vibration or acoustic noise can also excite the sense mode, leading to false readings. Temperature changes alter the stiffness of the support springs and the viscosity of the damping gas, which shifts the resonance. Protective packaging and vacuum sealing are used to minimize these external influences.
Technicians set the zero rate offset by measuring the output when the sensor is perfectly stationary.
Navigation Boundary
Performance of the system is limited by the noise floor of the electronics and the stability of the mechanical resonance. While coriolis acceleration provides a direct measurement of rate, it must be integrated over time to find the total change in angle. This integration process accumulates bias errors that lead to heading drift in the navigation solution.
High grade gyroscopes use materials like quartz or silicon with high quality factors to reduce these errors. The verification of the bias stability is performed over long durations to ensure the sensor meets the requirements for the intended application. Integration into a full inertial measurement unit requires aligning the axes of the sensor with the vehicle frame.
Final accuracy depends on the combination of the mechanical design and the signal processing algorithms.