Coarse alignment:
Coarse alignment adopts the multi-position method to obtain the output of the fiber optic gyroscope and accelerometer when the downhole probe tube is kept stationary. When the downhole probe tube rotates 180° around the main axis of the instrument, it reaches the designated position. After processing the output data, the initial attitude matrix and the zero drift of the sensor can be obtained. From the initial coarse alignment, the initial attitude angle and the initial attitude conversion matrix can be obtained. At the same time, the zero offset of the fiber optic gyroscope and the constant drift of the accelerometer can be obtained. It is a simple and effective initial alignment method.
Fine alignment:
During the initial coarse alignment, due to errors, such as motor rotation errors, errors in data processing, etc., the calculated initial posture will have a certain error, resulting in a difference between the navigation coordinate system and the real coordinate system. Angle of misalignment. This misalignment angle will be amplified in the subsequent process of strapdown inertial navigation calculation, which will cause a large deviation in the estimation of the attitude and must be eliminated. The precise alignment uses the Kalman filter method. Omit the scale factor error and installation error of the inertial device, the speed error equation and the attitude error equation can be obtained according to the error model of the strapdown inertial navigation. Since the repeatability error of the inertial device offset has the greatest impact on the accuracy of the system, the precision During the alignment process, only the gyro drift and the random constant part of the accelerometer zero bias are included in the state, and the state equation is listed. During the alignment process, the system's velocity output and angular velocity output are used as observations, because the output velocity at rest is the velocity error, and the angular velocity output is the error plus the component of the earth's rotation angular velocity, so an updated observation equation can be obtained.
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