Sensor Head Side-Tracking Mounting Alignment

If you plan to use the DVL for tracking on surfaces other than the bottom, you may want to mount it in other than a downward-facing attitude. While it is possible to mount the DVL as normal (i.e., pointed down) and just pitch or roll the ROV to point the DVL at the target surface, this, violates some DVL design assumptions. The dead reckoning may not work as expected.

The figure below shows a DVL mounted in standard neutral position.

Bottom view -- DVL sensor head pointed in standard down-facing neutral position. Note in this case the sensor head cable (red circle) is exiting the sensor head toward the stern, which means the +X axis of the DVL is facing the bow.

Usually, side-tracking is used with the DVL pointed off vertical. The only constraint on DVL alignment is to set it up so it can parallel the target surface normal when the ROV is level, and it should also be set up in such a way that the orientation with respect to the ROV coordinate frame can be accurately determined.

The figure below shows an example of a forward-pointing DVL using a DVL with a separate sensor head. Note: if you are using an all-in-one or most-in-one DVL, the discussion is analogous.

DVL sensor head pointed in parallel with the ROV’s forward direction of travel. Note in this case the sensor head cable (red circle) is exiting the sensor head toward the port side of the ROV.

To determine the mounting orientation of the DVL, refer to the coordinate frame of the DVL as shown in the figure below. Note that pitch and roll are right-hand rule and heading is left-hand rule. This is consistent with compass-based headings familiar to sailors and Boy Scouts.

The neutral position of a standard DVL is the +X axis of the DVL parallel to the forward axis of the ROV, and the roll and pitch of the DVL are zero when the ROV is level. The second figure back shows a DVL in standard neutral position.

Refer to the previous figure with the forward-facing DVL. To determine the mounting orientation, you need to determine the rotations needed to get the sensor head from the neutral position to the as-mounted position. The order of the rotations is important, they are applied heading first, then pitch, then roll (in conventional notation they are listed in the reverse order of roll, pitch, yaw just to keep life interesting). Remember the DVL sensor head orientation frame also rotates when the sensor rotates. For example, when you apply a heading rotation (which is around the Z axis) the X and Y axes rotate with the sensor head.

In the figure above, to get from standard neutral orientation to the orientation shown, the heading should rotate 90 degrees, followed by a 0 degree change in pitch, followed by a roll of +90 degrees. This corresponds to sending the DVL command:

SET-SENSOR-ORIENTATION 90,0,90

Beware: You might think you can also achieve the orientation in the figure above by pitching up -90 degrees and then rotating about Z by 90 degrees – and you can. BUT, this ignores the heading-pitch-roll rotation order. If you were to send the DVL a SET-SENSOR-ORIENTATION 90,-90,0 command, the DVL would think the sensor was pointed directly to starboard, parallel to the -Y axis.

The DVL sensor head coordinate frame for $DVEXT messages