Page 296 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

286 CHAPTER 11 Vehicle Sensors and Lighting
the surface) for their survey bottle along with flotation for the extra payload. The vehicle sensors are normally integrated through the vehicle’s telemetry system and displays on the vehicle’s pilot console graphical user interface (GUI).
Typically, survey sensors are of a higher quality than vehicle sensors due to surveys’ inherent need for better accuracy. Basic vehicle navigation usually does not require surveying’s higher toler- ance measurements for simple orientation and positioning. Please refer to Chapter 17 for a deeper explanation of this division. For this discussion, the vehicle sensors will be divided into vehicle navigation sensors and vehicle health monitoring sensors.
11.1.1 Vehicle navigation sensors
Vehicle navigation sensors are normally integrated into the vehicle’s telemetry system and are ported to the pilot’s control console to actively manage the position, orientation, and physical status of the vehicle. Such sensors are typically positional in nature (e.g., compass, depth gauge, and tether in/out). The smaller OCROV systems feature only basic items while the larger MSROV and WCROV integrate increasingly more complex sensors for a full situational status of the vehicle. Table 11.1 details the various levels of “housekeeping” sensors by ROV classification.
For a deeper explanation of sensor theory as well as survey sensors, review Chapters 12 (Sensor Theory) and 17 (Navigational Sensors). Some of the more common vehicle navigation sensors are examined below:
11.1.1.1 Flux gate compass
This simple device is used to measure the ambient magnetic field vector and intensity surrounding the sensor. It is hoped that the only magnetic field surrounding the sensor is that of the Earth’s field (so as to measure orientation with regard to magnetic North), but that is not always the case.
The basic sensor of a flux gate compass is a simple coil surrounding the core of some high- permeability magnetic material for measuring the ambient magnetic field. This sensor then becomes either 2D or 3D by orienting a series of coils along two or three (or more) axes. As these magne- tometer arrays are rigidly attached to the chassis of the vehicle, they are termed “strapped down” magnetometers. In a perfect world (i.e., one where the host vehicle had no self-induced magnetic field and did not operate near magnetic anomalies such as steel offshore platform legs or ship hulls), the magnetometer would only measure the Earth’s magnetic field as there would be no local interference. But that is seldom the case.
In order to calibrate a strapped down magnetometer, the total field must be measured (Earth’s magnetic field along with all of the local magnetic noise from the vehicle). Once the field is mea- sured along a single axis (from all sources of magnetic flux), the vehicle must then be rotated along all relevant axes in order to subtract out the variations due to local (i.e., vehicle induced) interfer- ence from the variations due to orientation with the Earth’s magnetic field.
Earth0s magnetic field 1 vehicle0s magnetic field 5 total measured field
It is assumed that the vehicle’s magnetic noise is a fixed sum (although it may vary somewhat due to magnetic noise from electric motors, vehicle frame, and/or power transformers). Once the total magnetic field is measured on each orientation (North/East/South/West), the total variation is