Since the inception of airplanes, pilots have used the magnetic compass for navigation. Magnetometers are used in flying vehicles to measure the earth’s magnetic field in order to determine orientation. There are two types of magnetometers based on their method of calibration. They are:
- Absolute magnetometers: These devices are calibrated from the perspective of their internal constants.
- Relative magnetometers: These are calibrated to an existing external magnetic field.
There are two kinds of commonly used magnetometers in modern flying aircraft: Simpler heading magnetometers and Three-Axis Magnetometers (TAMs). While in flight, smaller distortions may occur in the readings of the magnetometer. The local magnetic field vector forms a significant angle with the local ellipsoid surface while the drone is in flight. If you want to obtain magnetic measurements from both hemispheres of the ideal magnetic sphere, your drone must be able to:
1. Fly with a pitch angle that is greater than the angle that the local magnetic vector forms with the respective ellipsoid surface.
2. Fly with a greater roll angle than the angle that the local magnetic vector forms against the local ellipsoid surface.
3. The addition of roll angle and pitch angle should be equal to the angle of plane XY in the aircraft body. While the local magnetic field vector forms a significant angle with the local ellipsoid surface.
The simplest absolute magnetometer that was designed by the great German mathematician, Carl Friedrich Gauss in 1832, has a permanent bar magnet that is suspended by a gold fiber. The unit of magnetic induction is thus named after him.
Magnetic fields can be determined in several ways. However, the simplest technique to measure the magnetic field is to use a permanently magnetized needle. The needle is mounted in such a way that it can pivot in the horizontal plane. If there is no external influence such as gravity, the needle will align itself exactly along the local magnetic field vector. But if the setup is influenced by gravity, the needle’s measurements would be affected by it. So, now you can understand why magnetometers are used to measure heading.
Combining the magnetometer with GNSS and IMU sensors, an extremely accurate heading can be calculated.
The horizontal component of the earth’s magnetic field is called magnetic north. The magnetic north is considered true north except near the poles. The angle between the local magnetic north and true north is known as magnetic variation. Remember that the earth’s magnetic field goes through various changes.
Orthogonal components of the magnetic vector are calculated to estimate the heading. The formula is:
Heading = arctan(Hx/Hy)
This is applicably true when the aircraft is completely leveled with the ground. When the aircraft is tilted, this calculation does not return a precise value. The magnetic compass is usually a reliable device due to its simplicity, it is prone to errors and can be difficult to interpret sometimes. Note that, the presence of electrical devices near your magnetometer can affect the true readings.
Dip error: The error created by the ‘dip’ or downward slope of Earth’s magnetic field is known as a dip error. Due to this effect, the magnetic compass gives error-ridden measurements whenever the aircraft is accelerating, decelerating, or banking. This is why the use of a magnetic compass is problematic in ideal aircraft. A magnetic compass will produce accurate readings in a perfect, uniform, and level flight.
Tilt error: 3-axis magnetic sensors and additional accelerometers are used to avoid tilt errors. The 3-axis magnetic sensor reads the earth’s magnetic vector coordinates while the accelerometer measures the angle between the compass and gravity. In this way, the heading vector components are estimated and the error is eliminated from the reading at the same time.
The role of a magnetometer in drones
All drones must have a method of measuring heading accurately for a safe flight. The heading information in a drone is supplied by a magnetometer. The accuracy and reliability can be amplified by using GNSS and IMU devices.
The magnetometer provides information about the orientation of the platform as the drone hovers over it. Therefore, magnetometers are critical for rotary wing platform operations. Drone navigation implements several methodologies to minimize drift and errors in a magnetometer. The external influences such as magnetic fields created by electrical devices are also kept in check to obtain accurate measurements from the magnetometer in a drone.