Hand-in-glove

First the good bit so you can skip the boring explanation if you wish!

Low drift Phoebe

I did some experimentation to test which of the four different ways the IMU hardware interrupt and my GPIO code can work together to get the most efficient and accurate readings.

The IMU data ready interrupt can either generate a 50us pulse each time the data registers are updated, or it can produce a rising edge which latches high, only falling when the data is read over I2C.

My GPIO tweaks can either capture these interrupts continuously, or just capture the next one, and then capture no more until called again.

So I ran 4 tests to see which pairing worked together best based upon the number of I2C and data corruption errors I got and how accurate the timing was which is directly related to how many sensor readings are missed during motion processing.

Other than the latching, continuous option which just blocked (predictably), it was marginal between the other three in the lab testing, but with a slight advantage shown by 50us pulse, continuous reading.

And a few test flights with Phoebe confirmed this; much reduced drift and improved timing and at 500Hz sampling she missed only 2% of samples which is the best yet by a very long way – previously it’s been more like 20%

Two final tweaks: I loosened the tests for duff data and changed the butterworth filter values:

  • duff data can be identified when the sequence of ax, ay, az, temp, gx, gy, gz has some values set to -1 (0xFFFF) starting from the gz end.  Previously I’d been checking gy and gz.  Now I check just az, the reasoning being that errors in gyro of -1 have a very short term effect on the accuracy of angles, whereas -1 for az (which represents freefall in a vacuum) will plague a flight thereafter due to integration for velocity
  • The butterworth changes were experimental to see what settings might provide better flights by filtering more acceleration out of gravity; a 0.05Hz (20s) 6th order was used in these flights compared to the previous 0.1Hz (10s) 4th order resulting in much better drift control.

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