Projectile Attitude Determination via Magnetometer and Accelerometer
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Solution Overview
Problem
Inexpensive rate gyroscopes are unable to accurately measure the spin rate, attitude, and heading angle of projectiles spinning at rates greater than 100 Hz, necessitating despun projectiles to reduce spin rates, which limits their accuracy and effectiveness.
Innovation Solution
A method involving a magnetic unit vector, velocity unit vector, and algorithms to calculate the roll angle, pitch angle, and heading angle of a spinning projectile, using a three-axis magnetometer and single-axis accelerometer, enabling accurate attitude determination regardless of spin rate, and can be retrofitted into existing dumb munitions to create smart munitions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If inexpensive rate gyroscopes are used to measure projectile attitude, then cost is reduced, but measurement precision deteriorates at spin rates over 100 Hz
Solution Approach 1:
The patent replaces the mechanical rate gyroscope system with a magnetometer-based attitude determination system. The magnetometer measures the magnetic field vector in the body frame, which is then transformed to the inertial frame to calculate attitude angles (roll, pitch, heading) through vector operations, eliminating the need for mechanical gyroscopes and their associated high-spin-rate limitations.
Solution Approach 2:
The patent changes the measurement parameter from angular rate (which fails at high spin rates) to magnetic field vector orientation. By measuring the magnetic field vector components (Bx, By, Bz) in the body frame and transforming them to the inertial frame, the system determines attitude independently of spin rate, thereby resolving the measurement precision deterioration at high frequencies.
2Measurement precision
If despun projectiles are used to reduce spin rate below 100 Hz, then measurement precision improves, but device complexity increases due to additional despunt mechanisms
Solution Approach 1:
The patent eliminates the mechanical despunt mechanism by substituting it with a magnetic field-based attitude determination system. The magnetometer, combined with mathematical transformation of the magnetic field vector from body frame to inertial frame, provides accurate attitude measurement without requiring any mechanical intervention to reduce spin rate.
Solution Approach 2:
The patent extracts and removes the despunt mechanism from the projectile system. By taking out the mechanical complexity of despunt wings or buffers and replacing it with a purely sensor-based solution (magnetometer plus computational transformation), the system achieves attitude measurement without the added device complexity.
3Measurement precision
If magnetometer and accelerometer are used for attitude determination, then measurement precision is maintained at high spin rates, but device complexity increases compared to simple gyroscopes
Solution Approach 1:
The patent makes the magnetometer serve multiple functions: it measures the magnetic field vector for attitude determination, and when combined with accelerometer data, provides both attitude angles and spin rate information. This multi-functionality reduces the need for separate dedicated sensors for each measurement, thereby offsetting the increased device complexity with functional consolidation.
Solution Approach 2:
The patent uses low-cost MEMS magnetometers and accelerometers that replicate the functionality of expensive mechanical gyroscopes through software-based attitude determination algorithms. The computational transformation of sensor data creates a virtual gyroscope system that achieves comparable precision without the high cost and complexity of mechanical gyroscopic mechanisms.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables accurate and reliable guidance and navigation for projectiles, even those spinning at high rates, by providing a low-cost attitude and heading reference system that stabilizes the projectile's attitude and trajectory, enhancing targeting precision and reducing collateral damage.
Implementation Method 1
determining a magnetic unit vector in a body frame and in an inertial frame of the spinning projectile during the flight of the spinning projectile
Implementation Method 2
determining a velocity unit vector in the body frame and in the inertial frame of the spinning projectile during the flight of the spinning projectile
Data Source
AI summary
A method to determine roll angle, pitch angle, and heading angle of a spinning projectile during a flight of the spinning projectile is provided. The method includes providing a magnetic unit vector in an inertial frame of the projectile at a projectile launch location prior to launch of the projectile; determining a magnetic unit vector in a body frame and in an inertial frame of the spinning projectile during the flight of the spinning projectile; determining a velocity unit vector in the body frame and in the inertial frame of the spinning projectile during the flight of the spinning projectile; and calculating the roll angle, the pitch angle, and the heading angle of the spinning projectile during the flight of the spinning projectile, regardless of the spin rate of the projectile. The roll angle and the pitch angle of the spinning projectile form an attitude of the spinning projectile.