uNavAHRS

uNav Attitude and Heading Reference System 7 State EKF Arduino Library.

Description

The uNav Attitude and Heading Reference System (AHRS) is a 7 state Extended Kalman Filter (EKF) to estimate attitude and heading from IMU data. The 7 states comprise a quaternion and three gyro biases. It uses gyro measurements to propogate the state; accelerometers are used as a measurement update on the pitch and roll channels and magnetometers as a measurement update on the yaw channel to constrain drift.

Usage

Installation

This library requires Eigen to compile. First download or clone Eigen into your Arduino/libraries folder, then download or clone this library into your Arduino/libraries folder. Additionally, this library requires IMU measurements. For the included examples, an MPU-9250 IMU is used, and its library will need to be installed as well. Finally, because this library is using accelerometers and magnetometers as a measurement update, the IMU used should be well calibrated.

Function Description

Object Declaration

This library uses the default constructor. The following is an example of declaring a uNavAHRS object called Filter.

uNavAHRS Filter;

Setup Functions

(optional) void setInitializationDuration(uint32_t duration) Statistical data are gathered from the IMU measurements for automatically setting up the filter. By default this initialization takes 2 minutes. Optionally, this function can be used to set initialization durations other than the default value. The input is initialization time in microseconds.

Filter.setInitializationDuration(60000000);

Data Collection Functions

bool update(float ias,float p,float q,float r,float ax,float ay,float az,float hx, float hy, float hz) updates the filter with new IMU measurements. Inputs are:

float p: gyro measurement in the x direction, units are rad/s.
float q: gyro measurement in the y direction, units are rad/s.
float r: gyro measurement in the z direction, units are rad/s.
float ax: accelerometer measurement in the x direction, units need to be consistant across all accelerometer measurements used
float ay: accelerometer measurement in the y direction, units need to be consistant across all accelerometer measurements used
float az: accelerometer measurement in the z direction, units need to be consistant across all accelerometer measurements used
float hx: magnetometer measurement in the x direction, units need to be consistant across all magnetometer measurements used.
float hy: magnetometer measurement in the y direction, units need to be consistant across all magnetometer measurements used.
float hz: magnetometer measurement in the z direction, units need to be consistant across all magnetometer measurements used.

Please note that all measurements need to be given in the defined axis system. Measurements need to be provided in integer rates of each other, which the gyro measurements always updated. The update function checks each set of measurements to see if they've been updated and uses the following logic:

Gyro, accelerometer, and magnetometer measurements updated: filter time update, accelerometer and magnetometer measurement update.
Gyro and accelerometer measurements updated: filter time update, accelerometer measurement update.
Gyro measurements updated: filter time update.

The filter automatically initializes itself. Calls to update return false if the filter has not completed its initialization and return true after initialization is complete. The duration of initialization can be optionally set with the setInitializationDuration function, otherwise, the filter takes 2 minutes to initialize by default.

// read the sensor
Imu.readSensor();
// update the filter
Filter.update(Imu.getGyroX_rads(),Imu.getGyroY_rads(),Imu.getGyroZ_rads(),Imu.getAccelX_mss(),Imu.getAccelY_mss(),Imu.getAccelZ_mss(),Imu.getMagX_uT(),Imu.getMagY_uT(),Imu.getMagZ_uT());

float getRoll_rad() returns the roll angle in units of rad.

float roll;
roll = Filter.getRoll_rad();

float getPitch_rad() returns the pitch angle in units of rad.

float pitch;
pitch = Filter.getPitch_rad();

float getYaw_rad() returns the yaw angle in units of rad.

float yaw;
yaw = Filter.getYaw_rad();

float getHeading_rad() returns the heading angle in units of rad.

float heading;
heading = Filter.getHeading_rad();

float getGyroBiasX_rads returns the current gyro bias in the x direction in units of rad/s.

float gxb;
gxb = Filter.getGyroBiasX_rads();

float getGyroBiasY_rads returns the current gyro bias in the y direction in units of rad/s.

float gyb;
gyb = Filter.getGyroBiasY_rads();

float getGyroBiasZ_rads returns the current gyro bias in the z direction in units of rad/s.

float gzb;
gzb = Filter.getGyroBiasZ_rads();

Axis System

This library expects IMU data to be input in a defined axis system, which is shown below. It is a right handed coordinate system with x-axis pointed forward, the y-axis to the right, and the z-axis positive down, common in aircraft dynamics. Pitch is defined as a rotation angle around the y-axis with level as zero and roll is defined as a rotation angle around the x-axis with level as zero. Yaw is defined as a rotation angle around the z-axis with zero defined as the starting orientation. Heading is defined as a rotation angle around the z-axis with zero defined as magnetic north.

Example List

uNavAHRS-with-MPU9250: demonstrates using this filter with an MPU-9250 IMU. CalibrateMPU9250.ino is used to calibrate the MPU-9250 IMU and store the calibration coefficients in EEPROM. uNavAHRS_MPU9250.ino uses the MPU-9250 IMU as measurement input to the uNav AHRS filter, which is run at a rate of 100 Hz.

FlyTheThings/uNavAHRS