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INS & GNSS Configuration

Sensor Frame Rotation (IMU Rotation)

The sensor frame rotation allows the user to configure a rotation in multiples of 90 degrees to the IMU and magnetometer output. This is done by setting the SENSOR_CFG_SENSOR_ROTATION_MASK bits in the DID_FLASH_CONFIG.sensorConfig as defined in enum eSensorConfig. The rotation is from the sensor frame to the IMU and magnetometer output frame applied in order of 1.) heading, 2.) pitch, 3.) roll. The IMU rotation may be used to rotate the IMU and magnetometer output to match the vehicle frame.

INS Rotation

The INS rotation is used convert the INS output from the INS sensor frame (hardware frame) to the vehicle frame. This is useful if the sensor frame and vehicle frame are not aligned. The actual INS rotation parameters are DID_FLASH_CONFIG.insRotation[3] (Roll,Pitch,Yaw) in radians. The actual INS rotation values describes the rotation from the vehicle frame to the INS sensor frame in order of 1.) heading, 2.) pitch, 3.) roll.

Identifying INS Rotation

The INS rotation can be identified using the following method:

  1. Start by orientating the INS (or picturing the INS orientation) aligned with the vehicle frame. It can help to have the vehicle frame upright and level with the ground.
  2. Rotate the INS in heading (about Z axis) until the heading matches the sensor frame heading.
  3. Rotate the INS in pitch (about Y axis) until the pitch matches the sensor frame pitch.
  4. Rotate the INS in roll (about Z axis) until the roll matches the sensor frame roll.

Aligning the INS After Mounting

The following process uses the uINS to measure and correct for the uINS mounting angle.

  1. Set DID_FLASH_CONFIG.insRotation to zero.

  2. Set the sensor on the ground at various known orientations and record the INS quaternion output (DID_INS_2). Using the euler output (DID_INS_1) is not recommended as euler angles can have significant error when the pitch is near +-90 degrees.

  3. Find the difference between the known orientations and the measured INS quaternion orientations and average these differences together.

  4. Negate this average difference and enter that into the DID_FLASH_CONFIG.insRotation. This value is in euler, however it is OK for this step as this rotation should have just been converted from quaternion to euler and will be converted back to quaternion on-board for the runtime rotation.

If using software release 1.8.4 or newer, we recommend using the DID_FLASH_CONFIG.sensorConfig to rotate the sensor frame by 90 degrees to near level before following the steps above.

GNSS Antenna Offset

If the setup includes a significant distance (40cm or more) between the GPS antenna and the uINS central unit, enter a non-zero value for the GPS lever arm, DID_FLASH_CONFIG.gps1AntOffset (or DID_FLASH_CONFIG.gpsAnt2Offset) X,Y,Z offset in meters from Sensor Frame origin to GPS antenna. The sensor frame is labeled on the uINS EVB case.

IMU Sample and Navigation Periods

The IMU sample period is configured by setting DID_FLASH_CONFIG.startupImuDtMs in milliseconds. This parameter determines how frequently the IMU is measured and data integrated into the DID_PREINTEGRATED_IMU data. DID_FLASH_CONFIG.startupImuDtMs also automatically sets the bandwidth of the IMU anti-aliasing filter to less than one half the Nyquist frequency (i.e. < 250 / startupImuDtMs). The IMU anti-aliasing filter bandwidth can also be overridden to another frequency by setting bits SENSOR_CFG_GYR_DLPF and SENSOR_CFG_ACC_DLPF in DID_FLASH_CONFIG.sensorConfig.

The INS and AHRS kalman filter update period is configured using DID_FLASH_CONFIG.startupNavDtMs. This parameter also sets the integration period for the Preintegrated IMU or conning and sculling (delta theta, delta velocity) integrals. The DID_DUAL_IMU is the derivative of the DID_PREINTEGRATED_IMU value over a single integration period and serves as an anti-aliased moving average of the IMU value.

INS-GNSS Dynamic Model

The DID_FLASH_CONFIG.insDynModel setting allows the user to adjust how the EKF behaves in different dynamic environments. All values except for 2 (STATIONARY) and 8 (AIR <4g) are experimental. The user is encouraged to attempt to use different settings to improve performance, however in most applications the default setting, 8: airborne <4g, will yield best performance.

The STATIONARY configuration (insDynModel = 2) can be used to configure the EKF for static applications. It is a permanent implementation of the Zero Motion Command which will reduce EKF drift under stationary conditions.

Disable Magnetometer and Barometer Updates

Magnetometer and barometer updates (fusion) into the INS and AHRS filter (Kalman filter) can be disabled by setting the following bits in DID_FLASH_CONFIG.sysCfgBits.

Bit Name Bit Value Description
SYS_CFG_BITS_DISABLE_MAGNETOMETER_FUSION 0x00001000 Disable magnetometer fusion into EKF
SYS_CFG_BITS_DISABLE_BAROMETER_FUSION 0x00002000 Disable barometer fusion into EKF

These settings can be disabled using the General Settings tab of the EvalTool.


Disable Zero Velocity Updates

Zero velocity updates (ZUPT) rely on GPS and/or wheel encoder data. In some cases there can be a slight lag/deviation when starting motion while simultaneously rotating. This is because GPS data is updated at 5 Hz and it takes a few samples to detect motion after a period of no motion. When ZUPT is enabled, it acts as a virtual velocity sensor telling the system that its velocity is zero. It may conflict briefly with GPS velocity observation when starting motion. If a slight lag at the beginning of motion is an issue, ZUPT may be disabled. Generally it should be enabled (Default). It can be disabled using DID_FLASH_CONFIG.sysCfgBits or using the General Settings tab of the EvalTool.

Disable Zero Angular Rate Updates

Zero angular rate updates (ZARU) rely on analysis of either IMU (gyro) data or wheel encoders when available. When angular motion is very slow and no wheel encoders are available a zero angular rate may be mistakenly detected, which will lead to gyro bias estimation errors. In these cases it can be beneficial to disable ZARU if the applications has slow rotation rates (approximately below 3 deg/s). It is not encouraged to disable ZARU if there is no rotation or faster rotation. It can be disabled using DID_FLASH_CONFIG.sysCfgBits or using the General Settings tab of the EvalTool.