[Richard100]

"Rotate robot 360 deg in *horizontal plane*. Best data results from rotating *about actual sensor axis*". (Dialog box instruction, * for emphasis.)

Try not to stray from the horizontal plane. Once the IMU is on robot, this is probably easier to ensure, since you're spinning the entire bot on the ground (and the robot wants to stay on the ground).

Don't make a big wandering loop, spin the sensor about it's vertical axis. If the sensor is in your hand (off robot), you would place it flat on a (non-metal) table (and away from metal fasteners - i.e. look under your table!), and simply rotate the sensor in place without 'orbiting' anything. I usually mount the board on a small block of wood with squared-up sides which facilitates experimentation. If sensor is on-robot, stare at the sensor while you manually spin the robot, adjusting it's motion to maintain the sensor over the same spot on the ground, as much as you can - it won't be perfect.

Make a complete 360 degree rotation, it's ok to go over some.

I usually take about 20 seconds to complete the rotation when the sensor is on-robot, the software is collecting data samples during this period. IOW, don't try to go too fast. It's ok to partially rotate and stop long enough to readjust your grip, then start rotating again. The data analysis process is insensitive to this - you'll just see some data concentration on some locations of the data plot.

If you receive an ellipse on the plot, instead of a circle, your sensor is experiencing soft iron effects for which the code does not compensate. Ensure the graph axes are scaled the same (equally sized ranges), prior to deciding that you see an ellipse, because the graph is set to auto range and might visually distort even though it's really a circle. Follow the guidance under the XY Mag Plot on the IMUOpen Front Panel should you get an ellipse.

After trying all these tricks, if you still can't achieve a fairly noise-free circle, try moving your operation to a completely different location. You may be attempting the cal in an Earth field that is disturbed. I had no trouble with the calibration in my stick-framed workshop, but a metal-sided building might disturb the Earth field. (If you have access to an airport, you could perform the cal on their compass rose, as that area is certified to be magnetically clean ... joking - that's definitely going overboard). I don't expect this to be a real issue, just a possibility.

Bottom line is to use the example plot for reference. This gives you a simple way to show you what you're trying to produce. The reference is typical of what I achieve after some practice. Like many things in metrology, technique can influence the result.