Gyroscope values are not correct when changing the range

[wollewald]

Hi,

first I want to thank for this library. I tried the BNO055 library from Adafruit first (which is basically OK), but yours has much more features implemented. Great work.

I have found a small issue and a more complex one when I tried the example sketch config.ino. The smaller one is that the settings applied in this sketch do not have an effect because you switch into NDOF mode:

DFRobot_BNO055/examples/config/config.ino

Line 83 in f0912c8

bno.setOprMode(BNO::eOprModeNdof); // shift to other operate mode, reference datasheet for more detail

NDOF is a fusion mode and according to the data sheet chapter 3.5.1 - 3.5.3 the settings you apply are autocontrolled in fusion mode. You can easily test this. E.g. apply a range of 2G for the accelorometer and move the BNO055 fast. You will get values beyond 2G in NDOF mode. If you change to AMG, the limits are correct.

The solution is easy: Just change to a non-fusion mode. I suggest AMG.

I discovered the other issue when I tried the +/-125 dps range (bno.setGyrRange(BNO::eGyrRange_125);) for the gyroscope (in AMG mode). I get values between ~-2048 and ~+2048 when I rotate the module quickly and measured values are still too high if I rotate it slowly. It seems the results need to be scaled by a factor of 2000/range.

The reason for this bevavior is not really clear to me because setting different ranges for the accelerometer works fine, and I did not see differences in how you implemented the range settings in your library.

[wollewald]

I digged a little deeper and now I am sure that the factor you apply to the axes raw data needs to change for the gyroscope values if you change the gyroscope range.

This should be applied here:

DFRobot_BNO055/DFRobot_BNO055.cpp

Line 125 in f0912c8

case eAxisGyr: factor = 16.0f; break;

The challenge here are from my point of view:

1. How to know the gyroscope range
2. The change of the factor must no be applied in fusion modes.

The gyroscope range could be queried from the register (sReg.GYR_RANGE) like you do it in mapGyrAmThres(). But doing this with every call of getAxis() is not very efficient. Therefore I suggest to store the gyro range in a global variable. Same for fusion / non-fusion mode. Happy to do this via a pull-request.