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Understanding Sensor Bias (offset)

This article discusses the sensor parameter called Sensor Bias. For explanations of other sensor parameters, see Accelerometers or Gyroscopes.

Introduction

When looking at the inertial sensor data of gyroscopes and accelerometers you can see that there is often a small offset in the average signal output, even when there is no movement. This is what is also known as Sensor Bias.


Figure 1 - MT Manager screenshot of the inertial data (angular velocity for all 3 axes).

 


Figure 2 - Sensor bias of the Z-axis angular velocity.

 

Why is there Sensor Bias?

The gyroscopes and accelerometers used in the MTi's are MEMS (Micro Electro Mechanical Systems) sensors. The physical properties of these sensors change over time which results in different characteristics over time. Depending on sensor usage and time the internal sensor biases will increase.

What about the calibration of the MTi's that Xsens performs?

The physical property of each sensor is different and to correct for each sensor difference the MTi's are calibrated. If no calibration would be performed the biases would be much higher. Even though the MTi's are calibrated and the fact that the MTi uses advanced algorithms to correct for the bias, the physical properties of the sensors change which results in a sensor bias.

 How does the sensor bias affect my measurement?

There are two use cases:

  1. Outputting inertial data
  2. Outputting orientation

Outputting inertial data

When outputting the inertial data (gyroscope and accelerometer data), in most cases you will see sensor bias in your output. You can measure the biases when the MTi is laying still. Remember that the biases change, so there is no fixed value you can use to correct the bias for a longer period of time. 

For more information please see the Gyroscope and Accelerometer specifications.

Outputting orientation

If not corrected and you would use only the gyroscopes to calculate the orientation, the orientation would drift because of the sensor bias. For example; given a sensor bias of 0.1 deg/s as shown in Figure 2 it would mean that the orientation would drift 0.1 deg/s. See Figure 3 below.


Figure 3 - Orientation drift without bias correction of 0.1 deg/s over 60 seconds shown in MT Manager.

How does the MTi correct for sensor bias in orientation output?

The MTi's on-board filters use sensor fusion to correct for the sensor biases in the orientation. The MTi uses Gyroscopes, Accelerometers and the Magnetometers (as well as GNSS for MTi-G-710). Combining these sensors gives the MTi the possibility to detect and correct for sensor biases. It does this e.g. using accelerations, the gravitational pull of the earth, angular velocities and the magnetic field.

For more information see section Xsens Kalman Filter (XKF3i) in the MTi User Manual.

 

Using the magnetic field to estimate gyro bias

There are two situations:

  1. Stable magnetic field
  2. Unstable Magnetic Field

Stable Magnetic Field

The magnetometer plays an important role in correcting the sensor biases. In the case that the gravitational pull cannot be used to determine orientation (which is the case for the z-axis in Figure 3), the magnetic field can be used to correct for the sensor bias. A filter profile which uses the magnetometer needs to be selected and you would need to perform a Magnetic Field Mapping (magnetic calibration).

Important when using the magnetometer for correcting the orientation is that the magnetic field must be reliable enough. A non-homogeneous or disturbed magnetic field cannot be trusted. The MTi will correct the orientation based on the magnetic field even if it is incorrect.

Unstable Magnetic field

When the magnetic field cannot be used to correct the orientation there are other options to correct for sensor biases:

  1. Performing a No Rotation Update
  2. Select filter profile that does not use the magnetometers
  3. Enabling Active Heading Stabilization (AHS)

 

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