Nortek offers the Vector with an integrated inertial sensor or IMU (MicroStrain 3DM-GX3-25-OEM). This sensor is intended to help you understand how the Vector’s own motion influences the measurements. The simple configuration allows you to basically record motion data simultaneously with the Vector velocity data. The data from the IMU sensor is sampled and stored in real time, so the velocity measurements and the motion and orientation measurements are tightly synchronized. When you think about this, there are several measurement possibilities and problems that may be better characterized for applications where motion is involved or influences the measurements themselves. This integrated sensor provides a first step for investigating such problems. Before continuing, be aware that the description below applies to 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 into the Vector simply by using the magnetometer contact interface. This means the standard tilt and compass are removed and replaced with the IMU, so there is no standard Nortek compass system with the IMU installed. Therefore, the IMU requires that the Vector collects data in XYZ coordinates.
The Vector and the IMU have different definitions of the XYZ and East-North-Up axes, and in addition, Nortek’s order of axis for the Earth frame of reference is East-North-Up while IMU uses the North-East-Down convention. Thus, in order to use the IMU coordinate transform matrix to estimate ENU velocity estimates in the Nortek Vector defined coordinates, coordinate conversion is required. To make this step a bit simpler, the Nortek software (from V1.39 onwards) performs this conversion for you, during the binary to ASCII conversion. This means that the data you get out are referred to ENU coordinates, relative to the Vector definition. The IMU can thus be used as an AHRS; the Vector with IMU can be used in any orientation (horizontally, vertically, etc.) while simultaneously converting the measured velocities to ENU, thereby providing 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 – this is left to the end user. We only correct for orientation; we do 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.
For the next processing step(s), 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:
To set the Vector to record data, use the Vector software and go to the Deployment Planning dialog:
To configure the Vector so that it uses and records the information from the IMU sensor, check the IMU box. The user is permitted to output (and record internally on the Vector) one fixed IMU format: dAng dVel Orient, which is short for Delta Angular Rate (change in angles), Delta Velocity (change in velocity) and Orientation matrix (orientation of the IMU with respect to fixed earth coordinate system (ENU) – you can read more about coordinate transformation and matrices in a separate FAQ).
When checking the IMU, observe also that the Coordinate system will be set to ENU/XYZ. What this means is that the instrument will collect velocity data in XYZ coordinates (head coordinate system) and calculate the corresponding ENU coordinates in the software.
Presentation in software:
When collecting online data you will see a presentation of both ENU and XYZ velocities. If you want to disable one of them, right-click in the graph, select Customization > Subsets and highlight the parameters you want to see.
There is also a tab in the lower part of the screen where you can observe the data coming from the IMU sensor directly. Note that these are referred to the IMU coordinate system and not the Vector’s.
The velocity data will be saved in two separate ASCII files: *.dat and *_ENU.dat, where the latter is the one containing converted data. Remember that we only correct for orientation; we do not manipulate velocity other than transforming the coordinate system to ENU.
The .dat files include the following (ref. *.hdr file):
1 Burst counter 2 Ensemble counter (1-65536) 3 Velocity (Beam1|X|East) (m/s) 4 Velocity (Beam2|Y|North) (m/s) 5 Velocity (Beam3|Z|Up) (m/s) 6 Amplitude (Beam1) (counts) 7 Amplitude (Beam2) (counts) 8 Amplitude (Beam3) (counts) 9 SNR (Beam1) (dB) 10 SNR (Beam2) (dB) 11 SNR (Beam3) (dB) 12 Correlation (Beam1) (%) 13 Correlation (Beam2) (%) 14 Correlation (Beam3) (%) 15 Pressure (dbar) 16 Analog input 1 17 Analog input 2 18 Checksum (1=failed) 19 Ensemble counter 20 Timer (s) 21 DeltaAngle X (deg) 22 DeltaAngle Y (deg) 23 DeltaAngle Z (deg) 24 DeltaVelocity X (m/s) 25 DeltaVelocity Y (m/s) 26 DeltaVelocity Z (m/s) 27 Orientation Matrix 1,1 28 Orientation Matrix 1,2 29 Orientation Matrix 1,3 30 Orientation Matrix 2,1 31 Orientation Matrix 2,2 32 Orientation Matrix 2,3 33 Orientation Matrix 3,1 34 Orientation Matrix 3,2 35 Orientation Matrix 3,3 36 Heading (deg) 37 Pitch (deg) 38 Roll (deg)
When converting to ENU coordinates in the software, the orientation matrix is used directly. Note: There is also a MATLAB script that performs this coordinate conversion (to ENU) – see the attachment below.
What orientation are the velocity, angular rate and matrix reported in? Refer to the header file example above.
- The velocity data is reported in either XYZ or ENU coordinates (see section above).
- DeltaAngle is reported in XYZ coordinates, relative to the Vector coordinate system.
- DeltaVelocity is reported in XYZ coordinates, relative to the Vector coordinate system.
- Orientation matrix: ENU, referred to Nortek convention.
Can I access the IMU sensor? Although it is not necessary, the end user may access the MicroStrain IMU by setting the Vector in a transparent mode. The steps are as follows:
- Send a break to establish standard contact with the Vector via the Terminal Emulator.
- Send [54 53 00 00 hex] to set in transparent mode (see screenshot below).
- Close the Vector software so that the comm port is available for the MicroStrain software (which you can find on their website – see link above).
- Connect to the IMU with the MicroStrain Monitor software.
NOTE: The software assumes that the IMU is oriented in the MicroStrain frame of reference in order to use the X, Y, Z, tilt, roll and heading quantities sensibly.
Do I need to run a compass calibration before instrument deployment? The IMU sensor has a magnetometer and may need calibration if you are to deploy it using materials that may influence the sensor. For most applications with a Vector with IMU, a compass calibration is superfluous. The following steps describe how the calibration should be carried out:
- Put the Vector in transparent mode, as described above.
- Check out MicroStrain’s video on how to do the soft and hard iron calibration – click here: http://www.microstrain.com/video/hard-soft-iron-calibration
- Follow the procedure described by MicroStrain.
Do I need to take the location of the IMU sensor into account? Yes, if you want to remove e.g. the effect of mooring vibrations in the data. The MicroStrain IMU is mounted on the Vector circuit board. The center of the sensor is approximately 16 cm from the end of the endbell where the probe stem is affixed.