A Doppler Velocity Logger (DVL) is a sensor that provides velocity measurements by sending sound waves towards the sea bottom and analyzing the echoes. With high precision velocity measurements relative to the seafloor, and measurements from the Internal Measurement Units (IMU), the control system can estimate where it is with the use of “Dead reckoning”.
When we know where we are, we can start to control the vehicle automatically based on velocity and position updates. This enables a new control mode called “Station Keeping”. The mode will maintain the ROVs position relative to the seabed, even with ocean currents or other external forces acting on the vehicle.
Installing the DVL
The DVL kit comes with a mounting bracket, and the DVL sensor, with a Blueye smart connector ready to install.
The WaterLinked DVL A50 is compatible with all Guest Ports (GP1, GP2, GP3).
- Power “off” the drone.
- Open the side covers and remove the buoyancy.
- Slide off the ballast weights.
- Slide in the DVL mounting bracket with the DVL attached. (Cable should be pointing forwards, legs should be aft).
- Route the cable up the Guest port area.
- Check that the connector has o-rings and that they are clean and lubed.
- Select any of the 3 guest-ports and connect the cable with the screw.
- Use some small zip-ties to tidy up the cable routing.
- Put the buoyancy back on and close the side covers.
- That's it, due to our smart connectors, no configuration is needed.
New control modes and navigation features in the app
Station keeping is easily activated by cycling through the auto-heading button. First, it activates auto-heading, then station keeping, and last, manual mode. With Station Keeping mode active, you can still drive the ROV like normal, and once you let go of the joysticks, it will stay where you left it automatically. If the control system loses its trust in the position estimate, it will automatically switch over to the “Auto-heading” control mode. Also “Station-keeping” can always be deactivated by pressing the auto-heading button (B).
Pro tip: If you are standing by in some current with Station Keeping activated, it is recommended to turn your heading towards the current. This will save battery consumption and give a longer operation time.
The DVL also provides an altitude measurement, which is the distance in meters (or inches) to the sea bottom. The max altitude it can read is up to 50 m, (depending on the sea-bottom echo characteristics). With this measurement, we can keep a constant distance to the sea bottom, even though the water depth is changing.
There are numerous applications where it is desired to keep a constant distance while exploring an area. Imagine that you want to follow a pipeline with a 1-meter distance that goes from the shallow to a deeper area while recording. Or with a multi-beam equipped it is desired to know your distance to the bottom and keep this constant.
Once the altimeter has a valid reading, it will appear in the dive-view, right below the depth indicator. Once you have a reading, you can easily activate Auto-altitude by cycling through the Auto-depth button (A). With the Auto-altitude mode activated, you can still change the desired altitude the same way you do with Auto-depth. Thumb up to increase the distance, and down to get closer. If the altitude reading is invalidated, the drone will automatically switch to auto-depth. Then you just fly over to a new area where you get valid readings again and activate the mode.
The “Navigation” tab
Once you have powered on the drone and started a dive, you can head over to the dive menu in the top right corner, and select the new tab called “Navigation”. Here you can reset the "Position estimate" and get readings on where you are relative to where you hit reset last time.
When the drone is on land, it can drift quite a bit, so it is good practice to hit reset once you are in the water.
There is also a field called “Odometer”, which is measuring the traveled distance in any direction. This feature is particularly useful during a pipe inspection for instance. Then you can get an idea of how far down the pipe you found an area of interest. Or you can use it to approximate other distances you might need for navigating underwater. The Odometer also has a reset button that can be used to reset the accumulated distance, without resetting the position estimate.