Data Sheet

October 2017 BNO080 Datasheet 1000-3927

3.1.1 Calibration Command

The SH-2 firmware in the BNO080 allows a host microcontroller to enable or disable the dynamic calibration of

the accelerometer, gyroscope and magnetometer. Refer to the SH-2 Reference Manual [1] for the command used

to control calibration of each sensor. The command allows the host to control when calibration is performed. Note

that the calibration settings do not persist across resets of the BNO080.

3.1.2 Accelerometer

Dynamic calibration for an accelerometer is the removal of zero-g offset (ZGO). An accelerometer at rest should

only report gravity, any deviation is the zero-g offset. This is most easily seen when the accelerometer is placed

with one axis perpendicular to the Earth, the other two axes should report zero. ZGO errors can manifest as tilts

or tilt corrections (e.g. the screen on a head mounted display may be tilted with respect to the expected horizon)

and this usually indicates that the accelerometer needs to be calibrated. The BNO80 provides two methods of

calibrating the accelerometer ZGO; a full 3-dimensional approach for devices that can move in freely in space and

a planar calibration for devices that are constrained to move in a plane (such as a robot vacuum cleaner). For

more information on calibrating the accelerometer, see section 3.2.

3.1.3 Gyroscope

Dynamic calibration for a gyroscope is the removal of zero rate offset (ZRO). A gyroscope at rest should report

zero rad/s on all axes. Any deviation from zero is the zero rate offset (ZRO). ZRO errors can manifest as drifts

(e.g. the screen on a head mounted display can continue moving even when the device is stationary). Note that

though the BNO080 can continuously attempt to calibrate the gyroscope for ZRO, it is possible to fool the

calibration algorithms through slow horizontal rotations (around the gravity vector). Placing the device on a stable

surface will force the ZRO calibration to converge rapidly. The gyroscope calibration algorithm attempts to remove

ZRO while the device is not on a very stable surface, for this to be successful the device to which the BNO080 is

mounted must have sufficient tremor such as the human hand. If there is insufficient tremor it is advisable to

disable the gyroscope calibration to prevent drift (i.e. by mis-calibrating ZRO). ZRO will always be corrected when

the device becomes very stable (such as when laid on a table).

3.1.4 Magnetometer

Magnetometers measure the magnetic field around them and the typical use case is to determine the position of

the Earth’s magnetic North pole. The magnetic field can be distorted by the presence of magnetic fields caused

by speakers, magnets etc. (hard-iron effects) or other ferrous materials (soft-iron effects). BNO080 can

dynamically calibrate the readings from the magnetometer to compensate for these distortions. Without calibration

the heading reported by BNO080 will be highly suspect so it is highly recommended to calibrate the

magnetometer. For more information on calibrating the magnetometer, see section 3.2.

3.1.5 Calibration Accuracy

The BNO080 provides input reports (see section 1.4.5) to send sensor data to the host microcontroller. The input

reports include a “Status” field that indicates the accuracy status of the sensor. This is especially useful when the

application needs to know the calibration status of the BNO080 outputs. Bits 1:0 of the Status field indicate the

accuracy as shown in Figure 3-1. Refer to the SH-2 Reference Manual [1] for more information.

Status Bits (1:0)

Description

Unreliable

Accuracy Low

Accuracy Medium

Accuracy High

Figure 3-1: Accuracy status of sensors