Hardware Sensor Fusion Solutions

One of the great innovations in motion control in the last few years is the embedding of a processor along with 6 and 9 DoF motion sensors in a single package to provide sensor fusion solutions where the resulting quaternions are read directly by a microcontroller via I2C or SPI. I call this hardware sensor fusion but in reality the algorithms are executed in software on the embedded processor rather than on the host microcontroller. The advantages are many: the sensor fusion code doesn't take up limited flash memory on the host, the sensor fusion is off-loaded from the host to the embedded processor, which is optimized for sensor fusion, and highly efficient and sophisticated filtering and autocalibration of the sensors can be accomplished automagically without the user having to become an expert in sensor fusion theory. I will study three of these hardware sensor fusion solutions here: the BNO055 from Bosch, the MAX21100 from Maxim Integrated, and the EM7180 from EM Microelectronics. Although Invensense was among the first to produce a hardware sensor solution with its MPU6050, I am ignoring the 9 DoF MPU9250 here since it can only provide a 6 DoF solution with its embedded DMP; a 9 DoF solution still requires host processing. Likewise, I just became aware of Fairchild's FIS1100 IMU with embedded motion co-processor but it looks like this also requires software run on the host to perform the full 9 DoF sensor fusion.

The BNO055 combines Bosch's flagship 9 DoF motion sensor the BMX055 (itself an agglomeration of the BMA055 accelerometer, BMI055 gyro, and BMM055 magnetometer, all packaged as separate sensors also) along with a ARM Cortex M0 processor. The sensor fusion relies on an extended Kalman filter for the fusion proper plus low- and high-pass filtering, autocalibration, temperature compensation, etc to fuse the three sensor data streams into a quaternion representation of absolute orientation as well as providing linear acceleration, gravity, and Euler angle outputs directly readable via I2C or UART from internal registers.

The MAX21100 is an accelerometer/gyro combination that has an embedded motion merging engine that does the same sensor fusion tasks as the BNO055 but, in this case, the magnetometer data must come from an external magnetometer. On the board I am using for the following tests I pair the MAX21100 with ST Microelectronics fine LIS3MDL 16-bit magnetometer.

The EM7180 is not a motion sensors, it is rather a motion sensor hub that takes external sensor data from a variety of motion sensors and performs 9 DoF sensor fusion using an extended Kalman filter, autocalibration, low-and high-pass filtering, magnetic anomaly detection all at a very low power consumption. For the following tests I am using a board with the BMX055 as the sensor data input to the EM7180. This allows a certain amount of cross comparison since for both the BNO055 and the EM7180 I am using the same BMX055 motion sensor for input data to the respective sensor fusion engines.

I am just beginning detailed comparative studies of these hardware motion sensor solutions and so I will begin with the simplest of tests. I will start the sensors in the hardware fusion mode, each being controlled by a Teensy 3.1 microcontroller and mounted on a breadboard, and capture the Yaw (Heading), Roll, and Pitch via Serial output to my laptop. The procedure is to orient the breadboard parallel to the edge of my desk (which should be 45 degrees from true North), then every 120 seconds rotate the breadboard by ninety degrees. The first series of test data is shown below. I'll start with the MAX21100 and since it is rather busy let's take some time to understand what is happening.