DVL - Doppler Velocity Log is an acoustic sensor used to measure the velocity of a vehicle in relation to the environment with precision. With the acquired data, it is possible to determine the position of the vehicule (in relation to a defined initial position), along with it’s depth, altitude, etc. As you can see, this sensor is a very important component in underwater vehicles (AUV, ROV, USV, ASV, etc.).
This component sends ping signals to the outside at a known frequency from 4 transducers. The acoustic propagation of the signal undergoes a change in frequency, perceived by the DVL, allowing it to measure both the speed of the medium and the speed itself in relation to a non-stationary reference.
Transducers are components that convert a signal from one nature to another. Those used in DVL are the transceivers that convert optical signals into electrical signals and data signals. In this case, the water environment functions as a “fiber optic cable” connected to the sensor, that is responsible for capturing the variation by the Doppler Effect. The conversion takes place via a transceiver.
Doppler effect: described as an effect caused by a non-stationary observer in relation to a wave (Both the wave and the observer may be in motion). This observer perceives different frequencies emitted by a source because of the existence of a relative speed between them. For example, when an ambulance is approaching you, it seems that the siren's sound becomes higher (higher frequency) and the opposite happens if the ambulance is moving away. This fenomeni is called Doppler Effect.
With the obtained data, the speed is converted to the Cartesian axes (XYZ), making it possible to determine the position of the vehicle, since knowing the ping interval can be calculated by derivating velocity in relation to time in each dimension. With the three-dimensional position, data such as altitude and depth can be obtained.
The main examples of DVL are:
Pathfinder, which has the advantage of being versatile to updates, low cost, high compatibility with other equipment, along with its small size;
Tasman, which has greater practicality than the others because of its size and because it is easily replaceable, has high compatibility and also allows the measurement of temperature;
Pioneer, more mechanically resistant which allows its use in depths with high pressure and has leak detection;
Workhorse navigator, has high accuracy in measuring altitude, temperature and depth; and it also measures the error in measuring speed.
The main application of DVLs is in building submarine vehicles. It’s used by companies related to this branch, such as C&C Technologies, Copenhagen Subsea, SeaSam, among others. Although many companies use it, this device still has a high price on the market, which results on the search for alternatives such as INS, ADCPs and IMUs, which can be adapted to perform some functions that the DVL performs. However, as it is a refined device and provides accurate information, it is not always possible to replace it. Nautilus' AUV does not yet use DVL, which makes vehicle location algorithms harder.