Use material from https://docs.google.com/document/d/1GHzjsPTfSUAxYGPNlXSL7FYkxFcq76jTfsaquc57Rww/edit
PX4 conains functionality to calibrate and compensate rate gyro, accelerometer and barometric pressure sensors for the effect of changing sensor temperature on sensor bias. Calibration refers to the process of measuring the change in sensor value across a range of internal temperatures and performing a polynomial fit on the data to calculate a set of coefficients stored as parameters that can be used to correct the sensor data. Compensation refers to the process of using the internal temperature to calculate an offset which is subtracted from the sensor reading to correct for changing offset with temperature.
The inertial rate gyro and accelerometer sensor offsets are calculated using a 3rd order polynomial, whereas the barometric pressure sensor offset is calculated using a 5th order polynomial.
With the existing parameter system implementation we are limited to storing each value in the struct as a separate entry. To work around this limitation the following logical naming convention for the parameters is used:
TC_<type><instance>_<cal_name>_<axis> , where
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<type> is a single character indicating the type of sensor where G = rate gyroscope, A = accelerometer and B = barometer
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<instance> is an integer 0,1 or 2 allowing for calibration of up to three sensors of the same <type>
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<cal_name> is a string identifyng the calibration value with the following possible values:
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Xn : Polynomial coefficient where n is the order of the coefficient, eg X2 * (temperature - reference temperature)^2
SCL : scale factor
TREF : reference temperature (deg C)
TMIN : minimum valid temperature (deg C)
TMAX : maximum valid temperature (deg C)
<axis> is an integer 0,1 or 2 indicating that the cal data is for X,Y or Z axis in the board frame of reference. for the barometric pressure sensor, the _<axis> suffix is omitted.
Examples:
TC_G0_X3_0 is the x^3 coefficient for the first gyro x-axis
TC_A1_TREF is the reference temperature for the second accelerometer
The correction for thermal offsets using the calibration parameters is performed in the sensors module. The reference temperature is subtracted from the measured temperature to obtain a delta temperature where:
delta = measured_temperature - refernece_temperature.
The delta temperature is then used to calculate an offset, where:
offset = X0 + X1*delta + X2*delta**2 + ... + Xn*delta**n
The offset and scale factors are then used to correct the sensor measurement:
corrected_measurement = (raw_measurement - offset) * scale_factor
Scale factors are assumed to be temperature invariant due to the difficulty associated with measuring these at different temperatures. This limits the usefulness of the accelerometer calibration to those sensor models with stable scale factors. In theory with a thermal chamber or IMU heater capable of controlling IMU internal temperature to within a degree, it would be possible to perform a series of 6 sided accelerometer calibrations and correct the acclerometers for both offset and scale factor. Due to the complexity of integrating the motion control with the calibration algorithm, this capability has not been included.
Add step by step procedure for calibration using the inbuilt calibrator and the SYS_CAL_* parameters.
Add step by step procedure calibration using the SYS_LOGGER and SDLOG_MODE parameters and the process_sensor_caldata.py script file.