There have been increasing demands for the long-term performance retention of the Quartz Flexible Accelerometers (QFA), extensively employed in the Inertial Navigation System (INS). Considering the convenience of conducting the parameter identification and control algorithm, the Digital Close-Loop QFA (Digital-QFA) is established to improve the long-term performance. Based on the model of the entire close-loop system, the interfering factors relating to time are analyzed in detail, both in the forward channel and the feedback channel. Two schedules are proposed to estimate the gain variation, utilizing the modulating signal and the feedback signal separately. According to the reference gain settings in ideal conditions, the forward and the feedback gain drifting of the system are calibrated online through the adjustment of the digital controller and the reference voltage of the DA-chip, separately. The simulation experiments show that about 90.26% of the gain-drifting could be estimated and about 87.56% of the output error of the accelerate could be restrained. Consequently, the proposed schedules could promote the long-term stability of the Digital-QFA significantly.

Directional drilling utilizes the Measurement While-Drilling (MWD) to provide attitude for drilling bit. Traditional MWD system, consisting of magnetometers and accelerometers, rely on expensive and huge nonmagnetic drill collar (NMDC) to shield from magnetic interference to obtain accurate attitude angles. A fiber optic gyroscope-based MWD (FOG-based MWD) that is based on inertial navigation is an alternative survey scheme. But random vibration environment of underground introduces high amplitude random noise to the FOGs. Besides, the auxiliary measuring equipment (such as GPS) will not be available in underground environments, which will bring massive attitude error under pure inertial navigation. Therefore, combined with engineering application conditions, this paper proposes a modified attitude precision improvement approach of FOG-based MWD. Firstly, an autoregressive moving average (ARMA) error model for FOG is established to restrain the random vibration environment interference of underground. Secondly, Combined with FOG error model, a continuous backtracking navigation error model is developed to improve the azimuth accuracy. Moreover, the continuous backtracking algorithm frame is designed to estimate the attitude. Finally, Semi-physical experiment and drilling test verify that the error model has an obvious effect on reducing the amplitude of FOG random noise. The proposed backtracking algorithm make the azimuth angle achieve a high level estimation in short time. The result of drilling test show that the azimuth has an improvement of 48.79%, as compared to the traditional ZUPT.

The quartz flexible accelerometers (QFAs) have been widely utilized in inertial navigation systems (INS) due to their low cost, compact structure, and relatively high accuracy. However, the indicated QFAs do have high-order nonlinear parameters, which have significantly impact on the navigation performance of the INS, particularly in high-g input environments. Therefore, the suppression of the output nonlinearity is of great importance to enhance the performance of QFA. The digital closed-loop QFA (Digital-QFA) provides a convenient solution to compensating for such nonlinear error without altering the header structure by means of the digital control processor. In this paper, a method based on the maximum likelihood estimate is proposed to compensate for the output nonlinearity of Digital-QFA. The mechanism of the output nonlinearity has been analyzed based on the pendulous unbalancing and elastic properties of the pendulum component. A revised model equation is derived to describe the total output nonlinear errors. The precision centrifuge test for Digital-QFA has been conducted to verify the effectiveness of the proposed method. The experiments show that the output linearity of Digital-QFA could be improved effectively. The maximum output nonlinear error is restricted from over 2% to about 0.1% within an acceleration range of ±50 g , which is of great assistance to enhance the performance of Digital-QFA under high-g input environments.

In this paper, the performance drift of digital closed-loop quartz flexible accelerometer (QFA) in start-up period is attributed to the heating of the feedback circuit and coil. Compared to the drift compensation by temperature model which is built by statical rule, the gain stabilization of closed-loop channel is an essential and direct method to restrain the drift. In this paper, a high accurate current detection circuit is designed to evaluate the parameters drift and to stabilize the gain of the feedback channel. A stability experiment is conducted to prove the efficiency of the proposed stabilization strategy. The result shows that the stability of output in start-up period improves by 37.6% at most. Therefore, the proposed stabilization strategy can effectively improve the start- up performance of QFA, which could be promoted to other sensors whose performance suffer from the drift of the channel.

Two key factors for calculating the trajectory of a wellbore are the attitude and interval of the survey stations. A measurement while drilling (MWD) system that relies on magnetometers and accelerometers may fail to measure the wellbore attitude in an environment with magnetic anomalies. In addition, in the case of a wellbore trajectory with large variations in the attitude, the commonly used minimum curvature method (MCM) can result in large deviations from the planned wellbore trajectory. In this study, a novel dead reckoning method was developed that is constrained by drill pipe length subdivisions for improved tracking accuracy of wellbore trajectories with a large variety of attitudes. A gyro MWD system based on fiber optic gyroscopes and accelerometers is utilized to continuously calculate the wellbore attitude. The variations in the attitude angle within the wellbore can be tracked by using the proportional relationship between increments in the attitude and drill pipe length subdivisions, which can be synchronized with the gyro MWD update frequency. Simulations and experiments were performed to verify that the proposed method could accurately track wellbore trajectories with large variations in the attitude. In the simulation, the proposed method demonstrated a mean trajectory deviation of less than 0.5 m over a distance of 30 m, which was markedly lower than the mean deviation of 2.5 m by the MCM. In the slope experiment, the proposed method demonstrated substantially better tracking accuracy of the wellbore trajectory than the MCM. Measurements from an actual wellbore with large variations in the attitude confirmed that the proposed method reduced the tracking error by up to 3 m compared to the MCM.

In the strap-down inertial navigation system (SINS), the initial attitude matrix is acquired through alignment. Though there were multiple valid methods, alignment time and accuracy are still core issues, especially regarding the condition of the motion carrier. Inspired by the idea of constructing nonlinear vectors by velocity in a different coordinate frame, this paper proposes an innovative alignment method for a vehicle-mounted SINS in motion. In this method, the core issue of acquiring the attitude matrix is to calculate the matrix between the inertial frame and the initial body frame, which can be constructed through the nonlinear velocity vectors’ information from the GPS and the odometer at different moments, which denominate the multi-vector attitude determination. The possibility of collinearity can easily be avoided by a turning movement. The characteristic of propagation of error is analyzed in detail, based on which an improved method is put forward to depress the effect of random noise. Compared with the existing alignment methods, this method does not use the measurement information of accelerometers. In order to demonstrate its performance, the method is compared with the two-position alignment method and the traditional two-stage alignment method. Simulation and vehicle-based experiment results show that the proposed alignment method can establish an attitude reference in 100 s with an azimuth error of less than 0.06°, and that the accuracy does not have a strong correlation with the accelerometer.