Subsystem/imu

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Currently possible InertialMeasurementUnit subsystems are

type architectures firmwares defaults1 notes
analog all all no
apogee stm32f4 all yes mounted on the Apogee autpilot board
aspirin_v1.0 all all yes AspirinIMU via SPI with DMA
aspirin_v1.5 all all yes AspirinIMU via SPI with DMA
aspirin_v2.1 all all yes AspirinIMU via SPI with DMA
aspirin_v2.2 all all yes AspirinIMU via SPI with DMA
aspirin_i2c_v1.0 all all yes AspirinIMU via I2C
aspirin_i2c_v1.5 all all yes AspirinIMU via I2C
aspirin2_i2c all all no AspirinIMU v2 via I2C
b2_v1.0 all all no
b2_v1.1 all all no
b2_v1.2 all all no
drotek_10dof_v2 all all yes Drotek IMU board with MPU6050, HMC5883 (MS5611 not used in this driver)
gl1 all all yes I2C IMU with L3G4200, ADXL345, HMC5883 and BMP085 from GoodLuckBuy
yai all all yes
krooz_sd stm32f4 all yes mounted on the KroozSD autopilot board
navgo lpc21 all yes mounted on the NavGo autpilot board
umarim lpc21 all yes mounted on the Umarim autpilot board
crista no
crista_hmc5843 no
ppzuav lpc21 fw no

1. Has default GYRO_x and ACCEL_x values from datasheet.


Add the imu subsystem with the appropriate type to your firmware section:

File: conf/airframes/myplane.xml
  <firmware name="fixedwing or rotorcraft">
     ...
    <subsystem name="imu"       type="aspirin_v2.1"/>
  </firmware>

Other IMUs can be used through modules or you can just add a subsystem makefile for your own.

Defining the calibration

In the IMU section the IMU calibration is defined.
Some IMUs (see table above) have defaults defined. You may leave out the GYRO_x and ACCEL_x defines to use the datasheet values.

Magnetometer

The magnetometer always needs to be calibrated (Unless you want to disable it).
Here is a resulting minimal example:

File: conf/airframes/myplane.xml
  <section name="IMU" prefix="IMU_">
    <define name="MAG_X_SENS" value="3.17378921476" integer="16"/>
    <define name="MAG_Y_SENS" value="3.14663275967" integer="16"/>
    <define name="MAG_Z_SENS" value="3.26531022727" integer="16"/>

    <define name="MAG_X_NEUTRAL" value="2059"/>
    <define name="MAG_Y_NEUTRAL" value="1944"/>
    <define name="MAG_Z_NEUTRAL" value="2099"/>
  </section>

45deg Mag hack

If your magnetometer axes are rotated by 45deg around z. (e.g. with external mag on booz imu v1.0)

File: conf/airframes/myplane.xml
  <section name="IMU" prefix="IMU_">
    ...
    <define name="MAG_45_HACK" value="1" />
    <!--define name="MAG_X_SENS" value="5.14821844457 * sqrt(2)/2" integer="16"/-->
    <!--define name="MAG_Y_SENS" value="5.11810156597 * sqrt(2)/2" integer="16"/-->
    <define name="MAG_X_SENS" value="3.640340173" integer="16"/>
    <define name="MAG_Y_SENS" value="3.619044324" integer="16"/>
    <define name="MAG_Z_SENS" value="5.07618333556" integer="16"/>
  </section>

Gyro and Accel

Defining these values will override any defaults that most IMUs already have (see table at top for which drivers provide defaults that work unless you want to change the cofiguration).
See the ImuCalibration page for the actual calibration instructions.

File: conf/airframes/myplane.xml
  <section name="IMU" prefix="IMU_">
    <define name="GYRO_P_NEUTRAL" value="32362" />
    <define name="GYRO_Q_NEUTRAL" value="32080" />
    <define name="GYRO_R_NEUTRAL" value="32096" />

    <define name="GYRO_P_SENS" value="1.1032765" integer="16" />
    <define name="GYRO_Q_SENS" value="1.1360802599" integer="16" />
    <define name="GYRO_R_SENS" value="1.1249874614" integer="16" />

    <define name="ACCEL_X_SENS" value="2.45932966" integer="16"/>
    <define name="ACCEL_Y_SENS" value="2.45106376" integer="16"/>
    <define name="ACCEL_Z_SENS" value="2.47825717" integer="16"/>

    <define name="ACCEL_X_NEUTRAL" value="31886"/>
    <define name="ACCEL_Y_NEUTRAL" value="32162"/>
    <define name="ACCEL_Z_NEUTRAL" value="32941"/>

    ...
  </section>

Defining the orientation of the IMU

Paparazzi has parameters to define the orientation of the IMU with respect to the body of the vehicle.

Important Not all algorithms properly deal with any BODY_TO_IMU orientation. See the notes for the respective implementation at Subsystem/ahrs and the issue on github. For the algorithms not supporting that, the only way to properly deal with 90deg rotations is to just change the sensors channels (e.g. so that what are normally the x-axis gyros and accels become the y-axis sensors, etc.)

Quad-top.png

Not defining these values is the same as setting them to zero.

File: conf/airframes/myplane.xml
  <section name="IMU" prefix="IMU_">
    ...
    <define name="BODY_TO_IMU_PHI"   value="0.0" unit="deg"/>
    <define name="BODY_TO_IMU_THETA" value="0.0" unit="deg"/>
    <define name="BODY_TO_IMU_PSI"   value="0.0" unit="deg"/>
  </section>

Note: Angles are always defined in radians, but by setting unit="deg" you can define them in degrees and they are automatically converted to rad.

  • The positive 90 degree offset, parallel mount example:

Quad-top IMU-90.png

Configured as:

File: conf/airframes/myplane.xml
  <section name="IMU" prefix="IMU_">
    ...
    <define name="BODY_TO_IMU_PHI"   value="0.0" unit="deg"/>
    <define name="BODY_TO_IMU_THETA" value="0.0" unit="deg"/>
    <define name="BODY_TO_IMU_PSI"   value="90.0" unit="deg"/>
  </section>

In Flight Tuning

  • Switch to AHRS telemetry mode and look for the fields that are prefixed with imu_

AHRS telemetry.png 90 degree positive offset example

  • Open a Real_time plotter from the paparazzi console tools tab and drag int32 body_phi and int32 body_theta into the plotter box.
  • Hover rotorcraft (preferably indoors) to record data.
  • Copy the (best fit) recorded values into the airframe BODY_TO_IMU defines:
File: conf/airframes/myplane.xml
  <section name="IMU" prefix="IMU_">
    ...
    <define name="BODY_TO_IMU_PHI"   value="1.5"  unit="deg"/>
    <define name="BODY_TO_IMU_THETA" value="-0.5" unit="deg"/>
    <define name="BODY_TO_IMU_PSI"   value="90."  unit="deg"/>
  </section>
  • Ignore 'on the ground' resting values as depicted in the above image. They will change once the vehicle is in flight. The aim is to get the 'in flight' values of "int32 body_phi" & "int32 body_theta" as close to zero as possible.