Pigeon 2.0
User’s Guide
___ www.ctr-electronics.com ______________________ 1/31/2022
Cross The Road Electronics Page 13 ____________________________ 1/31/2022
2.4. Avoid Magnetic/Ferromagnetic materials
Most IMUs have some type of compass calibration procedure, including Pigeon 2.0. However, if the IMU is placed
in an area with severe magnetic disturbance, then minimally the benefit of compass is reduced, and maximally the
robot heading may be severely incorrect. Although many IMUs (including Pigeon 2.0) will automatically detect
invalid magnitude field strengths, depending on the type and direction of the distortion, the compass can still be
affected. For this reason, compass-features are often utilized in an outdoor setting in a chassis designed to
accommodate compass features.
The common disturbances are due to proximity to
ferromagnetic materials such as iron/steel and proximity to
magnetic materials (motors). Additionally, if an IMU is placed
within an extreme magnetic field (e.g., proximity to
neodymium, rare-earth magnets), the hard-iron offsets within
the magnetometer may permanently change, requiring a new
calibration. Care should be taken to ensure such magnets are
not placed near the IMU PCB (e.g., CTRE Mag
Encoder/CANcoder magnets).
This impact can be root-caused by waving a simple
mechanical compass in the presence of where the IMU is
intended to be placed. Look for any major changes in the
needle position as the compass is waved near the critical areas of the robot, while maintaining consistent
orientation between compass and Earth.
Next place the mechanical compass in the location where the IMU is meant to be placed. While the robot is
placed on blocks, drive all motors, chains, and articulators while watching the compass. If the compass needle
reacts to robot actuation, then IMU should be relocated, or simply avoid relying on compass features if the
application will allow it.
Phone compass applications (software) can be used to a lesser degree; however, a mechanical compass is best,
and are widely available for <$5.
Another means of confirming this source of error is to print/plot the compass magnitude as reported by the IMU
while moving the robot into various poses. This can be compared with the expected magnetic field strength which
can be determined with a Phone compass application, or with an online search such as…
http://www.ngdc.noaa.gov/geomag-web/
Any massive dip or rise in the magnetic norm may indicate magnetic disturbance.
However, it should be noted that there can be magnetic disturbance despite the norm not changing by much. For
this reason, the mechanical compass testing above is recommended.