An Inertial Measurement Unit (IMU) is the principal sensor for directly sensing vehicle motion in an INS. It consists of a set of triaxial orthogonal accelerometers and a set of triaxial orthogonal gyroscopes. The accelerometers measure translational acceleration on the x, y, and z axes in the instrument frame. The gyroscopes measure rotational velocities about the x, y, and z axes (φ, θ, and ψ respectively).
IMUs are generally grouped by the fundamental sensing technologies used in the accelerometers and gyroscopes. The differences in these technologies present trade-offs in size, power, cost, and performance the system integrator has to consider when selecting a sensor for use in an INS. Generally speaking, common IMUs used in the subsea industry fall into two basic categories, MEMS and optical, with several categories within each that address the specific technology used. These categories usually speak to the type of gyros used in the IMU rather than the types of accelerometers used.
What data does an IMU provide?
Greensea's workspace shows the following information from the IMU.
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Timestamp: The Unix time stamp of the most current IMU reading.
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Heading: The heading relative to the topside of the vehicle.
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Heading Rate: The velocity rate of the current heading for the vehicle.
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Forward Acceleration: The acceleration value for the ROV in the forward direction.
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Starboard Acceleration: The acceleration rate for the starboard side of the ROV.
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Down Acceleration: The rate at which the vehicle is descending.
