Page 79 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

3.3 Autonomy plus: “why the tether?” 67
FIGURE 3.8
Surface swim in RC mode.
Beginning with pure teleoperation (which is no autonomy), the first step toward full autonomy is the point at which the vehicle begins navigation autonomously within given parameters. This is navigation through “closed-loop feedback.”
Closed-loop feedback is simply control of an operation through sensor feedback to the control- ler. A simple example of a closed-loop feedback system is the home air-conditioning thermostat. At a given temperature, the air conditioner turns on, thus lowering the temperature of the air sur- rounding the thermostat (if the air-conditioning is ducted into that room). Once the air temperature reaches a certain preset value, the thermostat sends a signal to the air conditioner (closing the con- trol signal and response loop) to “turn off,” completing this simple closed-loop feedback system.
The most common first step along this line for the ROV system is the auto heading and auto depth functions. Any closed-loop feedback control system can operate on an ROV system, manipu- lating control functions based upon sensor output. Operation of the vertical thruster as a function of constant depth (as measured by the variable water pressure transducer) is easily accomplished in software to provide auto depth capability. For example, consider an auto depth activation system on an ROV at 100 ft (30 m) of seawater. The approximate (gauge) pressure is 3 atmospheres or 45 psig (3 bar). As the submersible sinks below that pressure (as read by the pressure transducer on the submersible), the controller switches on the vertical thruster to propel the vehicle back toward the surface until the 45 psig (3 bar) reading is reacquired (the reverse is also applicable).
The same applies to auto altitude, where variation of the vertical thruster maintains a constant height off the bottom based upon echo soundings from the vehicle’s altimeter. Similarly, auto standoff from the side of a ship for hull inspections can be based upon a side-looking acoustic sen- sor, where variation of the sounder timing can be used to vary the function of the lateral thruster.
Any number of closed-loop variables can be programmed. The submersible can then be given a set of operating instructions based upon a matrix of “if/then” commands to accomplish a given