Portable north seeker based on MEMS inertial device

Abstract

The invention discloses a portable north seeker based on an MEMS inertial device, the portable north seeker comprises a circuit board, a motor, a rotary table and a display module, the circuit board is provided with a micro-control processor, an MEMS gyroscope and an MEMS biaxial accelerometer, the MEMS gyroscope and the MEMS biaxial accelerometer are distributed at two sides of the micro-control processor, and the micro-control processor is provided with a micro-control module. The sensitive axial direction of the MEMS gyroscope is kept to coincide with one single axis of the MEMS biaxial accelerometer; the circuit board is installed on the rotary table plane after being sealed and surrounded. The micro-control processor drives the rotary table to rotate through the motor and performs closed-loop control on the rotation angle of the rotary table through the position monitoring module; and the micro-control processor is used for collecting signals of the MEMS gyroscope and the MEMS biaxial accelerometer and carrying out attitude calculation and monitoring. According to the portable north seeker, the use of inertial devices can be reduced, and the north seeking function can be realized without local latitude under the condition that the platform has an inclination angle.

Description

Technical Field

[0001] This invention relates to a north finder, and more particularly to a portable north finder based on MEMS inertial devices. Background Technology

[0002] A north-finding instrument autonomously determines the true north direction of a platform by measuring the projection of the Earth's rotational angular velocity onto a horizontal plane using its own gyroscope. Currently, commonly used high-precision north-finding instruments employ two methods: one uses two fiber optic gyroscopes or laser gyroscopes and two quartz accelerometers; this method is relatively common, but it is bulky and expensive, and requires local latitude to perform the north-finding function when the platform is tilted; the other method uses MEMS gyroscopes and accelerometers, but there are few engineering prototypes of this method, and its accuracy compensation is not ideal. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a portable north finder based on MEMS inertial devices, which can reduce the use of inertial devices and achieve the north finding function without local latitude when the platform is tilted.

[0004] To address the aforementioned technical problems, this invention provides a portable north-finding device based on MEMS inertial devices, comprising a circuit board, a motor, a turntable, and a display module. The circuit board houses a microcontroller, a MEMS gyroscope, and a MEMS dual-axis accelerometer. The MEMS gyroscope and the MEMS dual-axis accelerometer are distributed on either side of the microcontroller, with the sensitive axis of the MEMS gyroscope coinciding with one single axis of the MEMS dual-axis accelerometer. The circuit board is sealed and mounted on the turntable plane. The microcontroller drives the turntable to rotate via the motor and performs closed-loop control of the turntable's rotation angle through a position monitoring module. The microcontroller acquires signals from the MEMS gyroscope and the MEMS dual-axis accelerometer for attitude calculation and monitoring.

[0005] Furthermore, the microcontroller is implemented using an STM32F407VET6 with a maximum clock frequency of 168MHz, providing multiple interfaces and a 12-bit ADC, and is connected to the EEPROM storage chip via I2C; the MEMS gyroscope, MEMS dual-axis accelerometer and the microcontroller communicate via SPI with a sampling frequency of 2kHz.

[0006] Furthermore, the circuit board is powered by 24V and consists of a DC-DC converter and a power filter. After being stepped down to 5V and 3.3V by an LDO, it powers the devices on the circuit board.

[0007] Furthermore, the MEMS dual-axis accelerometer has a single-axis range of 20g, a zero bias of 100ug, and completes startup within 5 seconds; the MEMS gyroscope has a range of 300° / s, a drift of 0.3 degrees, and temperature compensation is completed by ASIC within the MEMS inertial device.

[0008] Furthermore, the display module communicates with the microcontroller in real time via an RS232 serial port and displays the carrier's azimuth angle.

[0009] Furthermore, the microcontroller measures the horizontal angle between the load system and the geographic system using a MEMS dual-axis accelerometer and compensates for the horizontal tilt angle during the calculation process. It also measures the turntable rotation angle using a MEMS gyroscope, establishes the output characteristic equation of the inertial device, and compensates for the error terms in the output equation. When the turntable rotation angle is a fixed value, the true north direction can be output through navigation calculation.

[0010] Further, the microcontroller performs attitude calculation and monitoring according to the following steps: S1, Earth rotation angular velocity decomposition: Establish the carrier coordinate system b, with the gyroscope sensing axis as the X-axis, the axis perpendicular to the turntable plane as the Z-axis, and the Y-axis as the cross product of the X-axis and Z-axis, to obtain the component of the Earth's rotation angular velocity on the Z-axis and the gyroscope output; S2, Gravitational acceleration decomposition, to obtain the components of Earth's gravity on the X-axis and Y-axis; obtain the angle of the leveling equation through the MEMS dual-axis accelerometer, and solve for the pitch and roll angles; S3, when solving for the azimuth angle, use the calculated pitch and roll angles to compensate for the turntable's deflection relative to the horizontal plane; S4, input the determined angles 90 and 270 degrees, rotate within a certain time period, sample and mean filter the gyroscope output, and then calculate the gyroscope zero bias. S5. Place the north finder vertically on the ground and rotate it to four positions: 0, 90, 180, and 270. Calculate the zero offset and cross-axis coupling ratio coefficients at the current turntable angle. S6. Rotate the turntable 90 degrees to obtain four sets of gyroscope outputs and calculate the azimuth angle. S7. Model the error of the azimuth angle and simulate the azimuth angle accuracy of the north finder.

[0011] Furthermore, the output of the gyroscope in step S4 is: (1); In the formula: This represents the scale coefficient of the gyroscope. The white noise bias component of the gyroscope is used, and it is smoothed by mean filtering. For the scale coefficient error, This represents the component of the Earth's rotational angular velocity along the sensitive axis.

[0012] Furthermore, the output characteristic equation of the MEMS dual-axis accelerometer in step S3 is: (2); (3); In the formula: , For the calibration coefficients of a MEMS biaxial accelerometer, , The white noise bias component of the accelerometer. , The components coupled to the cross-axis of the accelerometer's sensing axis are smoothed using mean filtering. , This represents the local gravity component along the sensitive axis.

[0013] Furthermore, in step S5, the outputs of the four gyroscopes are... In step S3, the output of the MEMS dual-axis accelerometer is and Solving the simultaneous equations, we obtain the expression for the azimuth angle as follows: (4); in , This is the scale marking the direction downwards along this axis. This is the Earth's rotational angular velocity.

[0014] Compared with the prior art, the present invention has the following beneficial effects: The portable north finder based on MEMS inertial devices provided by the present invention compensates for errors such as zero bias and cross-axis coupling of MEMS inertial devices and implements closed-loop control of the turntable, thereby reducing the use of inertial devices and realizing the north finding function without local latitude when the platform is tilted. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the circuit of the portable north finder based on MEMS inertial devices according to the present invention. Detailed Implementation

[0016] The present invention will now be further described with reference to the accompanying drawings and embodiments.

[0017] It should be noted that if directional indicators (such as up, down, left, right, front, back, etc.) are involved in the embodiments of the present invention, these directional indicators are only used to explain the relative positional relationships and movement of the components in a specific posture (as shown in the attached figures). If the specific posture changes, the directional indicators will also change accordingly. Furthermore, the terms "horizontal" and "vertical" do not imply that the components must be absolutely horizontal or suspended, but rather that they can be slightly tilted. The terms "set," "install," "connect," and "link" should be interpreted broadly, referring only to the internal connection between two components.

[0018] Figure 1 This is a schematic diagram of the portable north finder based on MEMS inertial devices according to the present invention.

[0019] Please see Figure 1 The portable north finder based on MEMS inertial devices provided by this invention consists of a MEMS gyroscope, a MEMS dual-axis accelerometer, and a control system including a motor, turntable, microcontroller, and display module. During the calculation process, factors such as MEMS accelerometer bias, cross-axis coupling, and scale coefficients are compensated to calculate the relative tilt angle of the turntable. The control unit sends commands, and the motor drives the turntable to rotate sequentially by four angles. After compensating for MEMS gyroscope bias and scale coefficients, continuous sampling, filtering, and attitude calculation yield the vehicle's pitch angle θ, roll angle γ, and heading angle Ψ.

[0020] This invention is applicable to MEMS inertial devices and latitude-free north-finding systems. Specifically, it consists of two MEMS devices, a turntable, and a core circuit board. The main mechanism involves rotating the turntable to a fixed angle, measuring the horizontal angle between the carrier system and the geographic system using an accelerometer, and compensating for the horizontal tilt angle during the calculation process. The turntable rotation angle is measured using a gyroscope. Under the established output characteristic equation of the inertial device, the zero bias, cross-axis coupling, and scale coefficient error terms in the output characteristic equation (see Formulas 1-3) are compensated. When the turntable rotation angle is a fixed value, the navigation computer receives the angle and velocity increments output by the gyroscope and accelerometer, and outputs the true north direction through navigation calculation.

[0021] This invention employs a dual-axis MEMS accelerometer, which is small in size and has a high-precision MEMS accelerometer with a single-axis zero bias of up to 100ug. The two single-axis scale coefficients are consistent, making the sensitive structure of the MEMS accelerometer symmetrical, easier to fabricate, and convenient for data processing. The accuracy of the single-axis MEMS gyroscope after closed-loop control is close to that of a low-precision fiber optic gyroscope, reaching 0.1~0.5°. It can be embedded in the navigation core circuit board and communicate with the navigation computer via SPI. With the help of a turntable in closed-loop control, the north-finding function can be realized when the carrier platform is tilted, given a certain angle input.

[0022] The main modules and components of this invention are implemented as follows: The MEMS dual-axis accelerometer has a single-axis range of 20g and a zero bias of 100ug, and should be started within 5 seconds. The gyroscope has a range of 300° / s and a drift of 0.3 degrees. Temperature compensation is completed by ASIC inside the MEMS inertial device, and the power consumption of a single MEMS inertial device does not exceed 20mw. The communication between the MEMS gyroscope, accelerometer and microcontroller unit adopts SPI communication with a sampling frequency of 2kHz. The microcontroller is implemented using STM32F407VET6. This microcontroller has a built-in DSP, a maximum clock frequency of 168MHz, and provides multiple interfaces and a 12-bit ADC. It provides an EEPROM chip to store initial calibration parameters and communicates with the microcontroller via I2C; The circuit board is powered by a 24V power supply, consisting of a DC-DC converter and a power filter, which is then stepped down to 5V and 3.3V by an LDO before being used by other devices. The control logic mainly adopts closed-loop control to achieve precise control of the turntable angle. The CPU uses instructions to start and stop the turntable control module. The display unit mainly displays the carrier's azimuth angle in real time through RS232 serial communication.

[0023] The position monitoring module mainly measures the turntable angle and sends the result to the control logic to ensure that the deviation is 0. As a feedback module for the control logic, the control logic can be implemented by a discrete PID algorithm.

[0024] S1. Hardware Layout: The MEMS dual-axis accelerometer is soldered onto a 3mm rigid circuit board, with one axis parallel to the processor. The MEMS gyroscope is fixed on an outer plate perpendicular to the PCB, ensuring that the sensitive axis of the MEMS gyroscope coincides with one of the single axes of the MEMS dual-axis accelerometer, pointing east (E). This ensures that the other sensitive axis of the MEMS accelerometer is perpendicular to the sensitive axis of the gyroscope (N). The turntable axis points upward (U). The filter module, power supply, I / O interface, etc. are distributed on the circuit board, ensuring that the center of the circuit board is at the center of the processor. The circuit board is surrounded by an aluminum plate and mounted on the turntable plane.

[0025] S2. Decomposition of Earth's rotational angular velocity: Establish the carrier coordinate system b, with the gyroscope's sensitive axis as the X-axis, the axis perpendicular to the turntable plane as the Z-axis, and the Y-axis as the cross product of the Z-axis and X-axis. The component of Earth's rotational angular velocity along the Z-axis is: (5); in, This represents the celestial and northern components of the Earth's rotational angular velocity in the northeast-central geographic coordinate system. The roll angle of the turntable. The azimuth angle of the turntable. The pitch angle of the turntable. This represents the rotation angle of the turntable.

[0026] The output of the gyroscope is: (6); In the formula: This represents the scale coefficient of the gyroscope. For zero bias of the gyroscope, This represents the white noise bias component of the gyroscope. This represents the scale coefficient error.

[0027] S3. Gravitational Accelerometer Decomposition: The output of the MEMS dual-axis accelerometer in the b-frame can be composed of the following formula, which is used to compensate for the deflection angle of the turntable relative to the horizontal plane when calculating the azimuth angle. Therefore, the component of Earth's gravity on the Y-axis is: (7); The component of Earth's gravity along the X-axis is: (8); The output of the MEMS dual-axis accelerometer is: (9); (10) ； In the formula: This refers to the X-axis scale coefficient of a MEMS dual-axis accelerometer. This refers to the Y-axis scale coefficient of a MEMS dual-axis accelerometer. For the calibration coefficient error of the dual-axis accelerometer, , For the white noise of the accelerometer, , The component that is coupled to the sensitive axis of the accelerometer is cross-axis.

[0028] S4. Platform Deflection Compensation: This invention achieves north-finding by compensating for turntable deflection during turntable rotation. Since the latitude and longitude are unknown, the pitch and roll angles output from S3 are used as parameters for azimuth calculation. The turntable is rotated 90 degrees accordingly. Therefore, the pitch angle... and roll angle The expression is: (11); Similarly, the expression for the roll angle is: (12); During the rotation process, the output of the accelerator in the turntable coordinate system is: Cross-coupling components .

[0029] S5: Initial Calibration: Input specified angles of 90° and 270°, rotate the gyroscope within a certain time period, sample and average the gyroscope output, and then calculate the gyroscope zero bias. Place the north-finding instrument vertically on the ground and rotate it to four positions (0, 90, 180, and 270) for calibration. Calculate the zero offset at the current turntable angle. Coupling ratio with cross-axis The constant zero bias is stored in the circuit board's EEPROM, and the corresponding value is retrieved for compensation output during navigation calculation.

[0030] S6. Azimuth Angle Calculation: Rotate the turntable 90 degrees to obtain four sets of gyroscope outputs, denoted as follows: The accelerometer output is: and Solving the simultaneous equations, we can obtain the expression for the azimuth angle as follows: (13); in , .

[0031] Compared with traditional north finders, the small portable north finder provided by this invention has lower power consumption, smaller size, and lower cost. Moreover, since it does not require local latitude input, it has good application prospects in low-to-medium precision north finder devices such as vehicle-mounted ones.

[0032] S7: Error Characteristics Analysis During leveling on the static base, the true velocity v of the north-finding instrument is 0, and the true position is determined by the only vertically downward gravitational acceleration in the navigation system. Therefore, the leveling error mainly comes from the output of the accelerometer. Let's assume the azimuth attitude is 0, and also assume that the initial power-on zero bias of the accelerometer and gyroscope is constant. and The values ​​are approximately equal.

[0033] When the turntable is operating, the error in north-finding accuracy is the attitude error. Its north attitude is related not only to gyroscope sampling but also to the leveling accuracy of the accelerator, corresponding to the azimuth and east attitudes, and the pitch angle. and roll angle The measurement angle error can be expressed as: (14); (15); in, for , The average value.

[0034] (16); Based on equations 14-16, an error analysis is performed on the parameters in equation 13. Taking parameters A and B as examples, the above equation can be expanded as follows: (17); Based on Equation 17, the error modeling of the north finder is performed, and the alignment angle is also superimposed according to Equations 14 and 15. The north finding accuracy is 0.33 degrees within an alignment time of 8 minutes (one standard deviation is calculated from multiple measurements). Subsequently, high-precision north finding function can be achieved by combining GNSS data with Kalman filter for data fusion.

[0035] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications and improvements without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be defined by the claims.

Claims

1. A portable north-finding instrument based on MEMS inertial devices, characterized in that, The system includes a circuit board, a motor, a turntable, and a display module. The circuit board is equipped with a microcontroller, a MEMS gyroscope, and a MEMS dual-axis accelerometer. The MEMS gyroscope and MEMS dual-axis accelerometer are distributed on both sides of the microcontroller, with the sensitive axis of the MEMS gyroscope coinciding with one single axis of the MEMS dual-axis accelerometer. The circuit board is sealed and mounted on the turntable plane. The microcontroller drives the turntable to rotate via the motor and performs closed-loop control of the turntable's rotation angle through a position monitoring module. The microcontroller collects signals from the MEMS gyroscope and MEMS dual-axis accelerometer for attitude calculation and monitoring.

2. The portable north finder based on MEMS inertial devices as described in claim 1, characterized in that, The microcontroller is implemented using an STM32F407VET6 with a maximum clock frequency of 168MHz. It provides multiple interfaces and a 12-bit ADC, and connects to the EEPROM storage chip via I2C. The MEMS gyroscope, MEMS dual-axis accelerometer, and microcontroller communicate via SPI with a sampling frequency of 2kHz.

3. The portable north finder based on MEMS inertial devices as described in claim 1, characterized in that, The circuit board is powered by 24V and consists of a DC-DC converter and a power filter. After being stepped down to 5V and 3.3V by an LDO, it powers the devices on the circuit board.

4. The portable north finder based on MEMS inertial devices as described in claim 1, characterized in that, The MEMS dual-axis accelerometer has a single-axis range of 20g, a zero bias of 100ug, and completes startup within 5 seconds; the MEMS gyroscope has a range of 300° / s, a drift of 0.3 degrees, and temperature compensation is performed by ASIC within the MEMS inertial device.

5. The portable north finder based on MEMS inertial devices as described in claim 1, characterized in that, The display module communicates with the microcontroller in real time via an RS232 serial port and displays the carrier's azimuth angle.

6. The portable north finder based on MEMS inertial devices as described in claim 1, characterized in that, The microcontroller uses a MEMS dual-axis accelerometer to measure the horizontal angle between the load system and the geographic system and compensates for the horizontal tilt angle during the calculation process. It uses a MEMS gyroscope to measure the turntable rotation angle, establishes the output characteristic equation of the inertial device, and compensates for the error terms in the output equation. When the turntable rotation angle is a fixed value, the true north direction can be output through navigation calculation.

7. The portable north finder based on MEMS inertial devices as described in claim 6, characterized in that, The microcontroller processor performs attitude calculation and monitoring according to the following steps: S1. Decomposition of Earth's rotational angular velocity: Establish the carrier coordinate system b, with the gyroscope's sensitive axis as the X-axis, the axis perpendicular to the turntable plane as the Z-axis, and the Y-axis as the cross product of the X-axis and Z-axis to obtain the component of Earth's rotational angular velocity on the Z-axis and the gyroscope's output. S2. Gravitational acceleration decomposition to obtain the components of Earth's gravity on the X and Y axes; the angle of the leveling equation is obtained through a MEMS dual-axis accelerometer to solve for the pitch and roll angles; S3. When calculating the azimuth angle, use the calculated pitch and roll angles to compensate for the turntable's deflection relative to the horizontal plane. S4. Input a specified angle of 90 degrees or 270 degrees, rotate the gyroscope within a certain time period, sample and average the gyroscope output, and then calculate the gyroscope's zero bias. Place the north finder vertically on the ground and rotate it to four positions (0, 90, 180, and 270) for calibration. Calculate the zero offset and cross-axis coupling ratio coefficient at the current turntable angle. S5. Rotate the turntable 90 degrees in this way to obtain four sets of gyroscope outputs, and calculate the azimuth angle; S6. Perform error modeling on the azimuth angle to simulate the azimuth accuracy of the north-finding instrument.

8. The portable north finder based on MEMS inertial devices as described in claim 7, characterized in that, The output of the gyroscope in step S4 is: （1）； In the formula: This represents the scale coefficient of the gyroscope. The white noise bias component of the gyroscope is used, and it is smoothed by mean filtering. For the scale coefficient error, This represents the component of the Earth's rotational angular velocity along the sensitive axis.

9. The portable north finder based on MEMS inertial devices as described in claim 7, characterized in that, The output characteristic equation of the MEMS dual-axis accelerometer in step S3 is: （2）； （3）； In the formula: , For the calibration coefficients of a MEMS biaxial accelerometer, , The white noise bias component of the accelerometer. , The components coupled to the cross-axis of the accelerometer's sensing axis are smoothed using mean filtering. , This represents the local gravity component along the sensitive axis.

10. The portable north finder based on MEMS inertial devices as described in claim 7, characterized in that, In step S5, the outputs of the four gyroscopes are: In step S3, the output of the MEMS dual-axis accelerometer is and Solving the simultaneous equations, we obtain the expression for the azimuth angle as follows: （4）； in , This is the scale marking the direction downwards along this axis. This is the Earth's rotational angular velocity.

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