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Common Issues with Magnetic Compasses and How to Minimize Risk

Most common issues when using a magnetic compass

Calibration

Calibration has long been a source of error in the eventual heading readings of a magnetic compass. With many systems, the calibration is tedious and is often referred to as the “Chicken Dance” due to its complexity requiring the user to rotate to multiple positions while tilting the device upside-down. Most of the difficulty of these procedures lies in the fact that the user does not know which points are needed and which are not. In this case, there is the belief that a true and perfect calibration that will work best with all points if only these procedures are followed exactly. This is definitely not the case.

During the in-field calibration process, typically a system is required to rotate 360° in each of the three perpendicular planes: X/Y, Y/Z, and Z/X. A large number of devices will not actually be used in these specific pitch and roll orientations, but they are required to reach the best solution for the spherical accuracy of the system. Some host devices can only move in limited pitch and roll ranges and therefore cannot afford a compass calibration that requires flipping the device upside down. Some devices are too heavy to be moved a great deal only to have the system remain relatively stationary during usage.

Sparton systems have been reliably tested to be accurate at >45° pitch and roll after only being calibrated at +/- 15° during the in-field calibration process. This limited pitch and roll scenario means that the user only has to calibrate our system in an environment that matches their system as close as possible.

Integration and Alignment

When integrating a magnetic compass into a product it can be difficult to find the ‘correct’ spot to minimize the magnetic distortions that can impact the sensors. There are many passive and active electronic components in most systems these days. Even in the systems where only passive components are used, not all of them will be utilized or turned on during the usage of the device. It is these time-varying fields that cause the most issues when the system is integrated.

Once the least magnetically disturbing location has been found, it can still be difficult to place the compass in an orientation that aligns with the host device’s X-Y-Z axes. If the system is not aligned properly, the heading/pitch/roll during static and dynamic situations will be compromised.

Sparton currently has three methods to align the systems: a simple boresight matrix, InvokeTare, and the azimuth/pitch/roll (APR) tare procedure.

  • The boresight matrix is a simple rotation matrix held inside the compass that is only used on heading, pitch, and roll. This can be figured out mechanically ahead of time by the systems integrator.
  • The InvokeTare procedure has the user align the system with a known magnetic North and has the end device placed flat and level. It is important to realize not to use the compass data to measure north and level during this procedure since this procedure is used to align the compass with the host device.
  • The APR tare procedure allows a user to point to any known heading, pitch, and roll with the host device to calibrate the alignment.

These last two are typically used only when the true mechanical alignment may be too difficult to figure out.

Knowing when things are off

During and after in-field calibration

Selecting the magnetic calibration points is usually left to the compass manufacturer and requires a 4 to12-point calibration that includes flipping the device in odd orientations (the Chicken Dance) and usually gives a calibration score that is not readable by an end user. A typical calibration point selection procedure has the user point at 0°, 90°, 180°, 270° heading and -45°, 0°, 45° pitch during these procedures all while giving the unit from -30° to 30° of roll. Sometimes users are required to do three rotations implying the order of points taken matters in the data collection process.

Sparton systems outline a near-infinite number of calibrations:  the standard 4-point North, South, East, West calibration, the 6-point maximum magnetic field on each axis calibration using the possible magnetic point quality factor, the icosahedron 12-point calibration, and the ability to perform limited pitch and roll calibrations and still achieve the desired accuracy.

After calibration and alignment

Once the compass is aligned and calibrated inside the host device, everything is perfect right? Not really! Once the system is fielded, there will still be chances for external magnetic influences to affect and distort heading. If there isn’t any information that the system is off, then the user has no idea that the heading will be incorrect.

Sparton sensors have the ability to tell the user when the system sees a magnetic anomaly and also when that environment returns to normal. One can also use this data with external procedures to allow the user to find whether or not the compass needs to be re-calibrated or when the device is near to an external magnetic anomaly that needs to be avoided.

 

Conclusion

Magnetic compasses provide accurate platform heading and attitude information in a variety of applications and in many operational environments. However, these devices are susceptible to distortion with the presence of magnetic material. There are, however, techniques and best practices to minimize this risk to performance. The proper calibration of a device is critical in its performance capability. Where a device is implemented and how it is aligned into a system can further impact its performance. Continued monitoring of the environment and performance once in operation ensures the magnetic compass provides the best results possible. Manufacturers and users who adhere to these practices can better expect their magnetic compass to perform in otherwise challenging magnetic environments and applications.

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