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Xsens MTi Series Reference Manual
Also see for MTi Series: User manual and technical documentation
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Contents
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29 Document MT1600P.2019.A© Xsens Technologies B.V. MTi Family Reference Manual6 Installation tips and tricks6.1 Transient accelerationsThe 3D linear accelerometers in the MTi are primarily used to estimate the direction of gravity to obtaina reference for attitude (pitch/roll). During long periods (more than tens of seconds) of transient “free”accelerations (i.e. 2nd derivative of position) the observation of gravity cannot be made. The sensorfusion algorithms can mitigate these effects to a certain extent, but nonetheless it is impossible toestimate true vertical without additional information.The impact of transient accelerations can be minimized when you take into account a few things whenpositioning the device when installing it in the object you want to track/navigate/stabilize or control.If you want to use the MTi to measure the dynamics of a moving vehicle it is best to position themeasurement device at a position close to the centre of rotation (CR) of the vehicle/craft. Any rotationsaround the centre of rotation translate into centripetal accelerations at any point outside the centre ofrotation. For a GNSS/INS device with a valid GNSS-fix, the detrimental effect of transient accelerationson orientation estimates is overcome by integrating GNSS measurements in the sensor fusion engine.6.2 VibrationsThe MTi samples IMU signals at high frequency per channel, processing them using a strapdownintegration algorithm with coning/sculling compensation. Proper coning/sculling compensation alreadymitigates errors that poorly designed signal processing pipelines introduce when the device is undervibration. For best results however, it is recommended that the MTi be mechanically isolated fromvibrations as much as possible: since vibrations are measured directly by the accelerometers, thefollowing two conditions can make the readings from the accelerometers invalid;1. The magnitude of the vibration is larger than the measurement range of the accelerometer. Thiswill cause the accelerometer to saturate, which may be observed as a “drift” in the zero-level ofthe accelerometer. This will show up as an erroneous roll/pitch.2. The frequency of the vibration is higher than the bandwidth of the accelerometer. In theory, suchvibrations are rejected, but in practice they can still give rise to aliasing, especially if close to thebandwidth limit. This can be observed as a low frequency oscillation. Further, high frequencyvibrations often tend to have large acceleration amplitudes (see item 1).There is an effect on the gyroscopes as well and especially when the vibrations include high-frequentconing motion, the gyroscope readings may become invalid.6.3 Magnetic materials and magnetsWhen an MTi is placed close to or on an object that is either magnetic or contains ferromagneticmaterials, the measured magnetic field is distorted (warped) and causes an error in the computedheading. The earth magnetic field is altered by the presence of ferromagnetic materials, permanentmagnets or power lines with strong currents (several amperes) in the vicinity of the device. The distanceto the object and the amount of ferromagnetic material determines the magnitude of disturbanceintroduced. Errors in estimated yaw due to such distortions can be quite large, since the earth magneticfield is very weak in comparison to the magnitude of the sources of distortion.By default, the AHRS and the GNSS/INS versions (when using the GeneralMag filter profile) stabilizeheading using the local Earth's magnetic field. In other words, the measured magnetic field is used asa compass. In addition, the gyroscope biases are continuously estimated by the MTi's on-board filter.For the rate of turn around the x-axis and the y-axis (roll and pitch axes), the gyroscope bias is estimated |
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