Page 298 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

288 CHAPTER 11 Vehicle Sensors and Lighting
11.1.1.3 Tether in/out
On TMS-based ROVs, a linear counter is added to the tether management system for measuring the amount of tether payed out from the reel. This measurement is typically displayed on the pilot con- sole to advise the pilot of the tether status.
11.1.1.4 Pressure-sensitive depth gauge
As further discussed in Chapter 12, there are several technologies available to measure ambient pressure. On OCROV systems, inexpensive pressure gauges measure some nominal amount of local pressure (in psi or bar) and then scale the results into water depth. Care must be exercised to accu- rately calibrate the depth sensor based upon the water density (e.g., fresh water or salt water and cold or warm). The calibration technique typically has the technician setting the sensor pressure to surface reference and then to some nominal reference pressure (e.g., 100 psi or 7 bar). The sensor then scales the output to feet or meters based upon these scaling and density parameters.
11.1.1.5 Rate gyro
As discussed more in-depth in Chapter 17, the rate gyro is primarily used for heading hold routines based upon auto stabilization functions.
11.1.1.6 Obstacle avoidance sonar
As explained in Chapter 15, vehicles are usually equipped with a mechanically scanning single- beam sonar for sensing of obstacles and locating major anomalies surrounding the vehicle. The out- put is typically displayed on the pilot’s console.
11.1.1.7 Altimeter
Along with the pressure-sensitive depth gauge is the vehicle’s altimeter, which is used to measure the vehicle’s height above the bottom. This sensor comes in a variety of sensitivities. It works much like a boat’s fathometer and typically transmits a vertical pencil-beam acoustic signal to bounce off the seafloor and measure the time of flight to determine the distance measurement between the transducer and the reflective surface (e.g., the bottom). Bottom type (e.g., sand, mud, and clay) as well as topography (e.g., flat bottom, canyons, and sea mounts) all play into the mea- sured distance of the echo return, thus affecting the signal strength of the acoustic return (and, hence, the distance resolved). These factors must be considered when interpreting the readings from the altimeter.
11.1.1.8 Inclinometer
Also discussed in Chapter 17 is inclinometer theory. These sensors are used for sensing vehicle ori- entation for pitch/trim functions. The vehicle orientation will directly affect the interpretation of sensor output including camera angle (e.g., zero camera angle is referenced to the vehicle, but the vehicle could be off of the horizontal plane), sonar interpretation (e.g., your sonar could paint a huge obstacle (i.e., a clear flat bottom) if the vehicle is oriented nose down), and altimeter output. It could also affect vehicle performance (e.g., forward thrusting with the vehicle-oriented nose- down could drive the vehicle into the bottom). The sensor’s output is to the pilot console.