Using a Velocimeter 35
© 2018 Nortek AS
2.1.9 Optional IMU
Nortek offers the Vector with an optional integrated Intertial Motion Unit (IMU). This sensor is
intended to help understand how the Vector's own motion influences the measurements. The simple
configuration allows one to record motion data simultaneously with the Vector velocity data. The data
from the IMU sensor are sampled and stored in realtime, so the velocity measurements and the
motion and orientation measurements are tightly synchronized. Before continuing; be aware that the
description below applies for Vectors using firmware/software versions from V3.42/V1.39 and onward.
If you use older versions, an update is recommended.
Physically, the IMU is incorporated in the Vector by simply using the magnetometer contact
interface. This means the standard tilt and magnetometer are removed and replaced with the IMU.
So there is no standard Nortek compass system with the IMU integrated.
Note that the use of IMU demands the use of 4GB recorder, due to the amount of data collected.
Figure: The IMU sensor replaces the com pass board
The velocity measurements and the IMU data will be presented in the coordinate system selected.
The IMU can be used as an AHRS; since the Vector with IMU can be used in any orientation
(horizontally, vertically,..) while simultaneously converting the measured velocities to ENU, - thus a
full 3D solution. We also indicate the heading, pitch and roll to the direction the instrument had
during the measuring period.
Note that there is no advanced internal processing as this is left with the end user. The software
does only correct for orientation, it does not manipulate velocity other than transforming it to ENU.
Correcting for movement will have to be done as a next processing step by the user, using e.g. delta
velocity/delta angle. But the good news is that the orientation will be correct even when there is
movement.
The end user would benefit by understanding the sensor itself. This may be done by visiting the
product page and looking at the Documentation tab: http://www.microstrain.com/inertial/3DM-GX3-
25-OEM
The location of of the IMU is relevant if e.g. removing the affect of mooring vibrations are of interest.
The center of the sensor is 16 cm from the end of the endbell where the probe stem is affixed.
Figure: Location of IMU relative to stem
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