Thursday, 8 February 2018

i.mx6sx - IMU Sensor fusion with the UDOO NEO

A key feature of the UDOO NEO is the hosting of a 9-DOF IMU through the inclusion of the NXP FXOS8700CQ and FXAS21002C sensors. The FXOS8700CQ provides a 6-axis accelerometer and magnetometer and FXAS21002C provides a 3-Axis Digital Angular Rate Gyroscope. Note that these sensors are only available on the Extended and Full models. In the video we demonstrate 9-DOF sensor fusion running on the cortex M4. The M4 fusion output is sent from the serial port and fed to a modified OrientationVisualiser application run on a PC. OrientationVisualiser is Processing application that displays a Arduino like board in 3D is part of the NXPMotionSense library refer to Teensys example and code.

The combination of the two sensors offers the ability to track absolute orientation with respect to a fixed Earth frame of reference with reasonable accuracy. Orientation can be modelled in 3 dimensions and most descriptions refer to the analogy of an aircraft. Where yaw represents movement of the nose of the aircraft from side to side, pitch represents the up or down movement of the nose of the aircraft and finally roll represents the up and down movement of the wing tips of the aircraft. Refer to this post for a detailed description. For our solution the sensor fusion algorithm implements a Kalaman filter. The filter smooths noise from the accelerometer/magnetometer and drift from the gyroscope. We also run magnetic calibration to reduce the effect of hard and soft iron magnetic interference.

Its important to note that the FXOS8700CQ is mounted on the underside of the pcb, however the axes should be aligned to a Cartesian coordinate system that follows the Right Hand Rule (RHR). Therefore its important that X,Y,Z values read from the FXOS8700CQ should to be adjusted accordingly before applying to the fusion algorithm. Although the FXOS8700CQ has been placed at the edge of the board its important to remember that the pcb spacer hole is about a 1 centimetre away. To reduce magnetic interference on the magnetometer a metal pcb spacer or screw shouldn't be used in this hole.

In this example we reallocated I2C4 to the M4 in order read data from both sensors. In our set-up the FXOS8700CQ output data rate (ODR) was 200Hz while the FXAS21002C was configured for 400Hz. As per previous posts the code was developed using the i.mx6sx FreeRTOS SDK.

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