CN114001757B - Zero offset calibration method for magnetometer - Google Patents
Zero offset calibration method for magnetometer Download PDFInfo
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- CN114001757B CN114001757B CN202111304003.0A CN202111304003A CN114001757B CN 114001757 B CN114001757 B CN 114001757B CN 202111304003 A CN202111304003 A CN 202111304003A CN 114001757 B CN114001757 B CN 114001757B
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- G—PHYSICS
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Abstract
The invention discloses a zero offset calibration method of a magnetometer, which comprises the steps of collecting initial data of a triaxial gyroscope, a triaxial accelerometer and a triaxial magnetometer before a drill string rotates; when the drill string rotates at a constant speed, acquiring real-time output data of the triaxial magnetometer; calculating the initial attitude of the underground measurement system by the initial data of the triaxial gyroscope, the triaxial accelerometer and the triaxial magnetometer; and calibrating zero offset of the magnetometer through the initial posture and real-time output of the triaxial magnetometer when the triaxial magnetometer rotates at a constant speed. The invention analyzes the mathematical relationship between zero offset of the magnetometer and the output of the magnetometer in the underground rotation measurement in detail, and provides a zero offset real-time calibration method of the magnetometer while drilling in single-degree-of-freedom rotation based on the analysis, thereby improving the measurement accuracy of the underground magnetic heading angle.
Description
Technical Field
The invention belongs to the technical field of inertial measurement, and particularly relates to a zero offset calibration method of a magnetometer.
Background
The petroleum drilling technology requires the adoption of an accurate track control technology to meet the requirements of directional wells, horizontal wells, large-displacement wells and other applications. Rotary steerable drilling may achieve steering control of the wellbore trajectory as the drill string rotates. However, downhole control systems are very sensitive to the environment at different moments in time. Thus, real-time measurement of downhole attitude parameters is critical. Current attitude while drilling measurement systems are different combinations of magnetometers, accelerometers and gyroscopes. Furthermore, the inertial sensor needs to be calibrated before use to compensate for its own error effects on the measurement results.
Currently, the calibration method of magnetometers is mainly divided into multi-position calibration, ellipsoid fitting, other sensor assistance and the like. The multi-position calibration method is not real-time and the calibration step needs to be performed before the measurement. While rotary drilling, the sensor can only rotate around the rotation axis and cannot provide multiple degrees of freedom, which is not applicable downhole. The ellipsometry method also requires arbitrary rotation of the magnetometer, which is difficult to achieve for rotary drilling measurement systems with only one degree of freedom. Other sensor-assisted calibration methods also do not allow real-time calibration of magnetometer zero-bias with only one degree of freedom.
Aiming at the problems, the invention analyzes the mathematical relationship between zero offset of the magnetometer and output of the magnetometer in underground rotation measurement in detail, and provides a zero offset real-time calibration method of the magnetometer while drilling in single-degree-of-freedom rotation, thereby improving the measurement accuracy of the underground magnetic heading angle.
Disclosure of Invention
Aiming at solving the problem that the zero offset calibration of the magnetometer while drilling under the single-degree-of-freedom rotation can not be realized by the existing method, the invention analyzes the mathematical relationship between the zero offset of the magnetometer and the output of the magnetometer during the downhole rotation measurement, and provides the zero offset real-time calibration method of the magnetometer while drilling under the single-degree-of-freedom rotation, which can improve the measurement precision of the magnetic heading angle under the well. The specific technical scheme of the invention is as follows:
a method of zero offset calibration of magnetometers, providing static data of a tri-axial accelerometer and tri-axial gyroscope by a downhole attitude measurement system, the calibration comprising the steps of:
s1: defining a coordinate system;
defining a local geographic coordinate system as a navigation system, and marking as an n system, wherein X, Y and Z axes of the navigation system point to the east and north respectively, and the direction of the sky, namely along the opposite direction of gravity, respectively marking as Xn, yn and Zn;
defining a three-axis magnetometer coordinate system as a body system, and marking as a b system, wherein X, Y and Z axes of the system respectively point to the right, front and upper along the center of gravity to form a right-hand system, and the right-hand system is respectively marked as Xb, yb and Zb;
the three-axis accelerometer and the three-axis gyroscope have the same direction as the three-axis magnetometer;
s2: before the drill string rotates, acquiring static data of a triaxial accelerometer, a triaxial magnetometer and a triaxial gyroscope;
s3: the initial roll angle gamma and the initial tilt angle beta are calculated through data of the triaxial accelerometer, and the calculation method comprises the following steps:
wherein A is x,out 、A y,out 、A z,out The outputs of the accelerometers in the X direction, the Y direction and the Z direction respectively, g being the local gravity vector;
s4: the initial course angle is calculated by the data of the triaxial magnetometer, and the calculation method comprises the following steps:
wherein H is xb 、H yb 、H zb Initial of three axis magnetometers in X-direction, Y-direction and Z-direction, respectively
Outputting;
s5: calculating a transfer matrix from n system to b system through the initial attitude angles obtained in the step S3 and the step S4
S6: when the drill string rotates at a constant speed, real-time output data of the triaxial magnetometer is collected, cosine fitting is carried out, and offset values from the central axis of an output fitting curve of the triaxial magnetometer to the abscissa axis are obtained and respectively recorded as N x 、N y And N z ；
S7: calibrating zero offset of the triaxial magnetometer;
wherein b x 、b y And b z Respectively in the X directionZero offset of the three-axis magnetometer of (2), the three-axis magnetometer of the Y direction and the three-axis magnetometer of the Z direction;
B U the component of the geomagnetic field in the current position of the triaxial magnetometer can be obtained through inquiry of a magnetic intensity distribution official website.
The invention has the beneficial effects that:
1. the invention analyzes the mathematical relationship between the zero offset of the magnetometer and the output of the magnetometer in the underground rotation measurement in detail, and provides the zero offset real-time calibration method of the three-axis magnetometer while drilling under the single-degree-of-freedom rotation, which can realize the zero offset real-time calibration of the magnetometer while drilling under the single-degree-of-freedom rotation;
2. the method can improve the measuring precision of the underground magnetic heading angle;
3. the method can avoid frequent calibration of the magnetometer on the well due to error accumulation, and improves the working efficiency of drilling.
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For a clearer description of an embodiment of the invention or of the solutions of the prior art, reference will be made to the accompanying drawings, which are used in the embodiments and which are intended to illustrate, but not to limit the invention in any way, the features and advantages of which can be obtained according to these drawings without inventive labour for a person skilled in the art. Wherein:
FIG. 1 is a schematic diagram of a triaxial magnetometer output fit;
FIG. 2 is a flow chart of a magnetometer zero offset calibration method of the present invention;
FIG. 3 is a detailed configuration diagram of a calibration experiment of the three-axis magnetometer zero offset according to an embodiment of the invention;
FIG. 4 is an output of a three axis magnetometer of an embodiment of the invention at 30/s.
Detailed Description
In order that the above-recited objects, features and advantages of the present invention will be more clearly understood, a more particular description of the invention will be rendered by reference to the appended drawings and appended detailed description. It should be noted that, without conflict, the embodiments of the present invention and features in the embodiments may be combined with each other.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, however, the present invention may be practiced in other ways than those described herein, and therefore the scope of the present invention is not limited to the specific embodiments disclosed below.
As shown in fig. 1-2, the basic principle of the zero offset calibration method of the magnetometer of the invention is as follows:
s1: first, a coordinate system is defined: defining a local geographic coordinate system as a navigation system, and marking as an n system, wherein X, Y and Z axes of the navigation system point to the east and north respectively, and the direction of the sky, namely along the opposite direction of gravity, respectively marking as Xn, yn and Zn;
defining a three-axis magnetometer coordinate system as a body system, and marking as a b system, wherein X, Y and Z axes of the system respectively point to the right, front and upper along the center of gravity to form a right-hand system, and the right-hand system is respectively marked as Xb, yb and Zb;
the three-axis accelerometer and the three-axis gyroscope have the same direction as the three-axis magnetometer;
s2: the output of the triaxial magnetometer is:
wherein H is xb 、H yb And H zb Initial outputs of the magnetometer in the X direction, Y direction and Z direction, respectively; b (B) E 、B N And B U The components of the geomagnetic field in the east direction, the north direction and the sky direction of the current position of the triaxial magnetometer are respectively; b x 、b y 、b z Zero offset of the triaxial magnetometers in the X direction, the Y direction and the Z direction respectively;
s3: let the bit rotate around Zb axis at a uniform angular rate ω, the magnetometer output at this time is rewritten as:
wherein the angular rate omega is a known value, and is measured by human setting or gyroscopes; h x ′ b 、H y ′ b And H z ′ b Is the output of the magnetometer when the drill bit is rotated about the Zb axis at a uniform angular rate ω;
a transition matrix from a navigation system to a body system, wherein t is rotation time;
wherein,,
wherein θ is an initial magnetic heading angle, γ is an initial roll angle, and β is an initial tilt angle;
the calculation method of gamma and beta comprises the following steps:
wherein A is x,out 、A y,out 、A z,out The outputs of the accelerometers in the X direction, the Y direction and the Z direction respectively, g being the local gravity vector;
the method for calculating theta comprises the following steps:
s4: order the
Obviously->
Only related to the initial attitude of the downhole measurement system, which is a constant value, the formula (2) is expanded as:
and has the following relation:
combining (6) and (7) yields:
performing triangular transformation on the step (8) to obtain the following components:
wherein:
from equation (12), it can be seen that when N1, N2, N3, C13, C23, and C33 are known, offset calibration of the three axis magnetometer can be achieved. N1, N2, N3 are deviations of the respective central axes of the three curves described by formula (9) from the axis of abscissa, which can be obtained by cosine fitting, as shown in fig. 1.
Equation (9) shows that the magnetometer output follows the cosine law, as shown in fig. 1. The cosine curve represents the magnetometer output, the dashed lines are the central axes thereof, and then N (N1, N2 and N3) are the offsets of the dashed lines to the X axis, and the numerical values are obtained by fitting.
B U For the natural component of the geomagnetic field at the position of the magnetometer, the geomagnetic field can be queried through a magnetic intensity distribution official website, and the website is as follows: https:// www.magnetic-decilination.
C13, C23 and C33 are related to the initial pitch and roll angles, calculated by the local gravity g and the X, Y and Z outputs of the three-axis accelerometer, calculated by equations (3) and (4). Obviously, the method of the invention realizes the offset calibration of the triaxial magnetometer under the single-degree-of-freedom rotation.
The calibration method of the invention is thus obtained:
a method of zero offset calibration of a magnetometer, providing static data of a tri-axial accelerometer and tri-axial gyroscope by a downhole measurement system, the calibration comprising the steps of:
step1: defining a coordinate system;
defining a local geographic coordinate system as a navigation system, and marking as an n system, wherein X, Y and Z axes of the navigation system point to the east and north respectively, and the direction of the sky, namely along the opposite direction of gravity, respectively marking as Xn, yn and Zn;
defining a three-axis magnetometer coordinate system as a body system, and marking as a b system, wherein X, Y and Z axes of the system respectively point to the right, front and upper along the center of gravity to form a right-hand system, and the right-hand system is respectively marked as Xb, yb and Zb;
the three-axis accelerometer and the three-axis gyroscope have the same direction as the three-axis magnetometer;
step2: before the drill string rotates, acquiring static data of a triaxial accelerometer, a triaxial magnetometer and a triaxial gyroscope;
step3: the initial roll angle gamma and the initial tilt angle beta are calculated through data of the triaxial accelerometer, and the calculation method comprises the following steps:
wherein A is x,out 、A y,out 、A z,out The outputs of the accelerometers in the X direction, the Y direction and the Z direction respectively, g being the local gravity vector;
step4: the initial course angle is calculated by the data of the triaxial magnetometer, and the calculation method comprises the following steps:
wherein H is xb 、H yb 、H zb Initial of three axis magnetometers in X-direction, Y-direction and Z-direction, respectively
Outputting;
step5: calculating a transfer matrix from n system to b system through the initial attitude angles obtained in the step S3 and the step S4
Step6: when the drill string rotates at a constant speed, real-time output data of the triaxial magnetometer is collected, cosine fitting is carried out, and offset values from the central axis of an output fitting curve of the triaxial magnetometer to the abscissa axis are obtained and respectively recorded as N x 、N y And N z ；
Step7: calibrating zero offset of the triaxial magnetometer;
wherein b x 、b y And b z The zero offset of the three-axis magnetometer in the X direction, the three-axis magnetometer in the Y direction and the three-axis magnetometer in the Z direction are respectively adopted;
B U the component of the geomagnetic field in the current position of the triaxial magnetometer can be obtained through inquiry of a magnetic intensity distribution official website.
In order to facilitate understanding of the above technical solutions of the present invention, the following detailed description of the above technical solutions of the present invention is provided by specific embodiments.
Example 1
The attitude measurement while drilling system is fixed on a single-shaft electric turntable, the rotation shaft of the attitude measurement while drilling system is overlapped with the Zb shaft, fig. 3 is a detailed experimental configuration, the system is powered by a direct-current power supply, and initial data are collected by a serial port and a collecting computer.
The antenna magnetic field component at the position of the experiment is obtained by inquiring the https:// www.magnetic-decrystallization.com/, wherein the https is:
B U ＝0.548×sin(5911″)≈0.4706(Gauss)
with six different rotational speeds, the output of the three-axis magnetometer at 30 DEG/s is shown in FIG. 4, and the output of the three-axis magnetometer shows a cosine curve, which can be seen as H zb The average value of (2) is about 0.3 Gaussian, and H xb And H yb Approximately 0.25 gauss and 0.2 gauss, respectively.
Cosine fitting operations were performed on the outputs of the three axis magnetometers to calculate deviations, the results being shown in table 1.
TABLE 1 constant rotation triaxial magnetometer offset calibration results
As shown in the table, the fluctuation of the 6 calibration results is within 5%, which proves that the calibration method is very effective.
The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (1)
1. A method of zero offset calibration of magnetometers providing static data of a tri-axial accelerometer and tri-axial gyroscope by a downhole attitude measurement system, the calibration comprising the steps of:
s1: defining a coordinate system;
defining a local geographic coordinate system as a navigation system, and marking as an n system, wherein X, Y and Z axes of the navigation system point to the east and north respectively, and the direction of the sky, namely along the opposite direction of gravity, respectively marking as Xn, yn and Zn;
defining a three-axis magnetometer coordinate system as a body system, and marking as a b system, wherein X, Y and Z axes of the system respectively point to the right, front and upper along the center of gravity to form a right-hand system, and the right-hand system is respectively marked as Xb, yb and Zb;
the three-axis accelerometer and the three-axis gyroscope have the same direction as the three-axis magnetometer;
s2: before the drill string rotates, acquiring static data of a triaxial accelerometer, a triaxial magnetometer and a triaxial gyroscope;
s3: the initial roll angle gamma and the initial tilt angle beta are calculated through data of the triaxial accelerometer, and the calculation method comprises the following steps:
wherein A is x,out 、A y,out 、A z,out The outputs of the accelerometers in the X direction, the Y direction and the Z direction respectively, g being the local gravity vector;
s4: the initial course angle is calculated by the data of the triaxial magnetometer, and the calculation method comprises the following steps:
wherein H is xb 、H yb 、H zb Initial outputs of the three axis magnetometers in the X-direction, Y-direction and Z-direction, respectively;
s5: calculating a transfer matrix from n system to b system through the initial attitude angles obtained in the step S3 and the step S4
S6: when the drill string rotates at a constant speed, real-time output data of the triaxial magnetometer is collected, cosine fitting is carried out, and offset values from the central axis of an output fitting curve of the triaxial magnetometer to the abscissa axis are obtained and respectively recorded as N x 、N y And N z ；
S7: calibrating zero offset of the triaxial magnetometer;
wherein b x 、b y And b z The zero offset of the three-axis magnetometer in the X direction, the three-axis magnetometer in the Y direction and the three-axis magnetometer in the Z direction are respectively adopted;
B U the component of the geomagnetic field in the current position of the triaxial magnetometer can be obtained through inquiry of a magnetic intensity distribution official website.
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