3.3 Sensors
THIS SENSOR IS THE MOST EXPENSIVE PART OF THE AUV. PLEASE DO NOT TOUCH IT WITHOUT A SENIOR MEMBER PRESENT
A Doppler Velocity Log (DVL) is a sophisticated and highly accurate underwater navigation instrument. It operates by emitting acoustic beams downwards towards the seafloor. When these acoustic beams bounce back, the DVL analyzes the Doppler shift in their frequency. This shift is a result of the relative motion between the DVL and the underwater objects. By interpreting this Doppler shift, the DVL calculates the vehicle's velocity in three dimensions: surge (forward/backward), sway (sideways), and heave (vertical).
DVLs are known for their exceptional precision and are capable of providing real-time velocity data, even in challenging underwater conditions. This level of accuracy is essential for tasks such as maintaining a fixed altitude above the seafloor, following precise paths, or conducting detailed underwater surveys.
Dead reckoning is a navigation algorithm used to estimate the position of an AUV. It begins with the assumption that the underwater vehicle's starting point is at the origin (0,0,0) and relies on continuous velocity measurements from the DVL.
The algorithm integrates these velocity measurements over time to estimate the vehicle's position as it moves through the underwater environment. By multiplying the measured velocity by the time interval and adding these increments to the previous position estimate, the algorithm calculates the position change in all three dimensions: surge (forward/backward), sway (sideways), and heave (vertical).
The driver code can be found here. It uses pyserial to read from the DVL. The data from the DVL is formatted according to the serial protocol specified here.
- dead_reckon_report: All of the data from the dead reckon report given by the DVL (lines that begin with wrp)
- velocity_report: All of the data from the velocity report given by the DVL (lines that begin with wrv)
An IMU is a multi-purpose sensor equipped with accelerometers, gyroscopes, and a magnetometer. These sensors work together to provide real-time data on the robot's movement, orientation, and magnetic field readings.
The accelerometers measure changes in velocity, allowing the robot to understand its linear movements and accelerations. Gyroscopes track angular velocity and orientation, helping the robot maintain its orientation and stability. The magnetometer adds another layer of information, providing data on the Earth's magnetic field. This helps the robot determine its absolute heading or orientation relative to magnetic north.
- sbg_ekf_quat: Orientation in North, East, Down interms of a quaternion refined using the Extended Kalman Filter algorithm
- sbg_imu_data: Linear acceleration, angular velocity.
A device that estimates the depth of the AUV in metres by measuring the pressure of the surrounding water.
- depth: Depth measurement in North, West, Up