In this demo, Pat Hickey from Invensense does a demo on a turntable showing how much error you can expect from an IMU (in this case the MPU 3000 using the onboard Kalman filter) in a turn if you don't have a GPS or magnetometer to correct it. The answer is less than 10 degrees at .2G (12 inch turntable turning at 33rpm), which is a reasonable guess of what a plane would experience in a prolonged turn.
This is relevant because the forthcoming Invensense MPU 6000 sensor with 3-axis gyros and 3-axis acclerometers on a single chip has a great onboard sensor fusion processor, which is attractive to use. But it doesn't have an on-board magnetometer or a good way to input GPS data into the sensor fusion processing, which could correct the inertial errors from centrifugal forces.
So what if you go without that correction? This demo suggests that the consequences might not be too bad.
(demo filmed by me at the Wired offices)
Comments
To correct for the centripetal acceleration effect on the MPL sensor fusion, you have two options:
- Calculate the magnitude of the radial force from your GPS/airspeed and the current rotation rate, find the resulting angle between gravity and apparent gravity (centripetal acceleration + gravity), pass that calculation through a low pass filter to matching the transient response of this error term in the MPL output (you can find a first order approximation experimentally with a rate table), and finally rotate the MPL output by this correction term.
- Calculate the accelerometer output due to centripetal acceleration using GPS/airspeed and current rotation rate, pass that calculated value to MPL to be set as the accelerometer bias (zero point) used during sensor fusion, and the MPL output will be corrected to remove the unwanted portion of the accelerometer measurement.
Magnetometer will not (by itself) help with centripetal correction. A speed measurement is what is needed. Vector speed, really. With APM in a plane we make the assumption that the motion is along the fuselage axis (a reasonable assumption) and can then use a scalar speed measurement.
I am not enthused by the result presented, and we don't really need a turntable demo to tell us what will happen in a sustained turn in an airplane. If the turn is coordinated then the load factor will always be perpendicular to the wings, the acceleration vector will depart from downward by the bank angle, and the steady state error in the IMU will equal the bank angle. So, for example if you flew APM in Stabilize mode and held a 45 degree bank by hand for a while, then let the stick go the IMU would be off by 45 degrees and your "stabilization" would be shot. As another approach to viewing the problem and a sanity check on the turntable "experiment"; I have flown the SkyFun with APM at bank angles up to 60 degrees. At 60 degrees bank the load factor is 2g;s, translating to centripetal acceleration of about 1.7g's. If the relationship from the turntable was linear, then you could expect an error of 80 degrees. (It won't be that bad, just 60 degrees)
Looked like it, but thought USB physical layer might be too sensitive for something like that.
Stupid question, but... how was the cable connected to the module?
(also, poor poor turntable :)
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