Page 338 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

330 CHAPTER 13 Communications
with the vehicle—and the resulting tether size was huge! Later, as circuits further evolved, analog control of the vehicle and communication with the sensors led to digital circuits and control.
ROV-mounted sensors communicate with the surface through four separate modes (Figure 13.2):
1. Communication through the vehicle’s tether and telemetry system (a)
2. Communication through the vehicle’s tether but bypassing the telemetry system (b), for
example, with a separate fiber breakout within the junction box routed to a survey pod (or
directly to the sensor)
3. Communication outside of yet attached to the vehicle (c), for example, physically attaching (via
plastic or fiber wraps) a separate communication line to the tether
4. Communication separate and apart from the vehicle (d), that is, with a separate down line from
the vehicle and tether
Examples of each of these modes follow:
1. Mode 1: Most commercial ROV systems have accommodation for multiple serial ports within the telemetry system. A sonar mounted to one of these ports allows for communication through the telemetry system to an output on the vehicle’s control panel.
2. Mode 2: Most tethers of commercial ROV systems have extra conductors or fibers located within the tether specifically for spare capacity for sensor communication or for backup should a primary telemetry conductor fail. For instance, a typical fiber-based ROV system has four to eight fibers housed within a steel tube (or some other fiber stiffener arrangement for limiting microbending) located within the tether. One fiber is typically used for the telemetry system leaving three (or more) extra unterminated fibers available for use with sensors. These are easily broken out from the fiber bundle and routed from the vehicle’s junction box directly to the sensor through a fiber multiplexer/modem. But if the ROV will use a LARS or TMS (Launch and Recovery System or Tether Management System), another pass on the fiber-optic rotary joint (FORJ) will be required.
3. Mode 3: A sensor is typically mounted to the ROV’s frame. But for convenience, the sensor’s power and communications cable is mated to the outside of the tether, thus avoiding the time and trouble of integrating (or the sensor’s power and/or bandwidth needs exceed the ROV system’s capabilities) the sensor through the ROV’s tether. An example of this is a high- throughput 3D sonar (which exceeds the vehicle’s telemetry and power capabilities). The sensor is mounted to the vehicle’s frame, but the sonar’s cable is mated to the outside of the vehicle’s tether with tape or binder.
4. Mode 4: Some sensors are simply mounted to the ROV with no other attachment to,
or communications with, the vehicle. An example of this is a vehicle-mounted, battery- operated, acoustic transponder. The transponder is self-powered and the sensor’s transducer communicates directly with the hydrophone mounted to the vessel. The vehicle provides no power to the sensor and no sensor communication medium is linked to the vehicle.
13.1.4 Transmission versus communication
The data transmission versus communication question has a very close analogy with pipelines. The pipeline can carry water, gas, oil, gasoline, diesel, or a host of other gases or liquids. The actual process of transmission of the liquid is a completely different function from the fluid itself.