Document MT0605P.E
© Xsens Technologies B.V.
MTi User Manual
8
2.5 Installation
2.5.1
Transient accelerations
The 3D linear accelerometers in the MTi are primarily used to estimate the direction of gravity to obtain
a reference for attitude (pitch/roll). During long periods (more than tens of seconds) of transient “free”
accelerations (i.e. 2
nd
derivative of position) the observation of gravity cannot be made. The sensor
fusion algorithms take these effects into account, but nonetheless it is impossible to estimate true
vertical without added information.
The impact of transient accelerations can be minimized when you take into account a few things when
positioning 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/craft it is best to position the
measurement device at a position where you expect the least (smallest) transient accelerations. This
is typically close to the centre of gravity (CG) of the vehicle/craft since any rotations around the centre
of gravity translate into centripetal accelerations at any point outside the point of rotation, which is
usually close to the CG. The acceleration of the vehicle as a whole can of course not be taken into
account. For the MTi-G-700 however that have a valid GPS-fix, transient accelerations make the
orientation better observable.
The MTi 100-
series cope better with transient “free” accelerations because of the higher-class
gyroscopes in the MTi 100-series. Next to the better hardware, the algorithm in the MTi 100-series is
superior in detecting and coping with challenging conditions, such as transient accelerations.
2.5.2
Vibrations
Although the MTi samples at 10kHz and includes a strap down integration algorithm with
coning/sculling compensation and vibration rejection, for best results it is recommended that the MTi is
mechanically isolated from vibrations as much as possible: since vibrations are measured directly by
the accelerometers, the following 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.
This will cause the accelerometer to saturate, which may be observed as a “drift” in the zero-
level of the 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,
such vibrations are rejected, but in practice they can still give rise to aliasing, especially if
close to the bandwidth limit. This can be observed as a low frequency oscillation. Further, high
frequency vibrations 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-frequent
coning motion, the gyroscope readings may become invalid. The MTi 100-series features vibration
rejecting gyroscopes, designed to better cope with these specific conditions.
Note that the sleeve on the Fischer connector can move by design in order to enable unmating.
Vibrations on the MTi, espe
cially in the direction of the MTi’s x-axis, can make the sleeve vibrate
against the panel part of the connector. This may be visible in the accelerometer and gyroscope data.
To prevent this, the sleeve of the Fischer connector may be locked with the ring at the connector.
Xsens has tested a set of vibration dampeners on the MTi. Vibration dampeners are low-profile rubber
cylinders that allow the MTi to be mounted on an object without a direct metal to metal connection that
transduces vibrations from the object to the MTi. The vibration dampeners have been tested with
frequencies up to 1200 Hz that caused aliasing when the MTi was mounted directly on the vibration
table had no effect with the vibration dampeners fitted. The dampeners tested are manufactured by
Norelem and have part number 26102-00800855,
www.norelem.com
