Page 78 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

66 CHAPTER 3 Design Theory and Standards
3.3.3 Why the tether?
RF waves penetrate only a few wavelengths into water due to water’s high attenuation of its energy. If the RF is of a low frequency, the waves will penetrate farther into water due to longer wavelengths. But with decreasing RF frequencies, data transmission rates suffer. In order to per- form remote inspection tasks, live video is needed at the surface so that decisions by humans can be made on navigating the vehicle and inspecting the target. Full teleoperation (under current tech- nology) is possible only through a high-bandwidth data link.
With the UAV example above, full teleoperation was available via the RF link (through air) between the vehicle and the remote operator. In water, this full telemetry is not possible (with cur- rent technologies) through an RF link. Acoustic in-water data transmission (as of 2013) is limited to less than 100 kB/s (insufficient for high-resolution video images). A hard-wire link to the operat- ing platform is needed to have a full teleoperational in-water link to the vehicle. Thus, the need exists for a hard-wire link of some type, for the foreseeable future, for real-time underwater inspec- tion tasks.
3.3.4 Teleoperation versus remote control
An ROV pilot will often operate a vehicle remotely with his/her eyes directly viewing the vehicle while guiding the vehicle on the surface to the inspection target. This navigation of the vehicle through line of sight (as with the RC airplane) is termed “RC (remote control) mode.” Once the inspection target is observed through the vehicle’s camera or sensors, the transition is made from RC operational mode to teleoperation mode. This transition is important because it changes naviga- tion and operation of the vehicle from the operator’s point of view to the vehicle’s point of view. Successful management of the transition between these modes of operation during field tasks will certainly assist in obtaining a positive mission completion.
Going back to the UAV analogy, many kids have built and used RC model aircraft. The differ- ence between an RC aircraft and a UAV is the ability to navigate solely by use of onboard sensors. A UAV can certainly be operated in an RC mode while the vehicle is within line-of-sight of the operator’s platform, but once line-of-sight is lost, navigation and control are only available through teleoperation or preprogramming.
The following is an example of this transition while performing a typical observation-class ROV inspection of a ship’s hull: The operator swims the vehicle on the surface (Figure 3.8) via RC to the hull of the vessel until the inspection starting point is gained with the vehicle’s camera and then transitions to navigation via the vehicle’s camera.
3.3.5 Degrees of autonomy
An open-loop control system is simply a condition on a functioning machine whereby the system has two basic states: “On” or “Off.” The machine will stay On/Off for as long as the operator leaves it in that mode. The term “open-loop” (or essentially “no loop”) refers to the lack of sensor feedback to control the operation of the machine. An example of an open-loop feedback would be a simple light switch that, upon activation, remains in the “On” or “Off” condition until manually changed.