This time I’ll show you the first results of incorporating a 3-axis magnetometer (or compass) into my IMU filter.
A quick round-up first. I have a IMU sensor that houses a 3-axis accelerometer and a 3-axis gyro sensor. Well, actually I got two, one that I built myself and one from Dexter Industries. To use this type of sensor I have written a non-linear complementary filter (NLC-filter) who’s job it is to fuse the output of the two sensors to a drift free low noise attitude signal. The filter tells me the tilt over the x-axis and tilt over the y-axis. It also tells me the heading. The heading signal is not drift-free. The reason for this is that the accelerometer can provide tilt data but it cannot provide heading data. For this you need a compass.
A compass sensor should really be called an magnetometer because it doesn’t give you heading like a sailors compass does. Instead it gives you the force of the magnetic field over three axis. From this one can calculate heading.
It was technically not very difficult to incorporate the magnetometer data into the filter. Although it took me a long time to figure out how to do this. The main thing was to tune the PI-controller for the compass. I’m not finished with this but I can show you some first results of the filter anyway. The measurements were taken with the setup that you see in the picture. The NXT is mounted on a platform some 20 cm above the ground. This is to minimize effects of the steel enforced concrete floor. It can rotate endlessly as it is mounted on a turn table and driven by a NXT motor and a worm wheel. The worm wheel in combination with a regulated motor gives a very stable rotation speed. The sensors are mounted 10 cm from the NXT to prevent disturbances from the NXT. Measurements are sent over bluetooth to the PC in real time.
this first graph was taken while the NXT was stationary. It shows how stable the filter signal is. Note that the scale of the vertical ax is 0.1 degree, the vertical axis ranging from -4 to 0 degrees. The roll and pitch signals are very stable. During the 15 minutes of the test these signals stayed within a bandwidth of 0.4 degrees. This is all that is left of the gyro drift. The yaw signal (that is controlled by the compass) is less stable, it floats between -0.8 and -2.0 degrees. But also in the yaw signal the gyro drift is eliminated to a large extend. I am very, very pleased with these noise levels as the bandwidth of noise from an accelerometer is around 5 degrees.
The second graph compares the output signal from the compass with that of the filter. This time the NXT was spinning around the Z-axis. The graph shows the results of a full spin. . You basically see two descending lines, indicating a constant movement of the sensors. The signal from the compass, in blue, is not exactly over the signal from the filter. There is a slight difference in the heading that these two report. The reason for this is that the setup of the NXT is not exactly level. The compass signal suffers from this. Its line is not really straight but it is like an S-curve due to this. The filter signal on the other hand is tilt compensated and almost straight. The smoother red line also indicates a lower noise level.
This last image shows the effect of a magnetical disturbance on the compass sensor and on the filter. While taking this graph I slowly moved a head phone along side of the sensor.
The compass suffers from this disturbance, at the peak the signal is over 10 degrees of. The filter on the other hand is not affected by the disturbance. It detects the disturbance and ignores the compass signal while it is disturbed. During this time the filter relies solely on the gyro to keep track of heading.
I think the low noise levels of the filter are nice. But the true advantages are in the tilt compensation and the robustness to magnetic disturbances. The filter is equally robust to disturbances in acceleration I think. This however I cannot show you using this setup.