Page 109 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

increases, fine thruster control becomes less critical due to the amount of mass requiring movement (larger mass vehicles react much more slowly to thruster output due to simple inertia).
Vehicle manufacturers use a variety of techniques for gross versus fine control of vehicle move- ment to conform to the operating environment. One vehicle manufacturer makes use of a horizontal and vertical gain setting to vary the power versus joystick position. Another vehicle manufacturer allows for variable power delivery scaling via the controller’s software. The reason this power scal- ing is necessary is that when towing the tether and vehicle combination to the work site, the full power complement is needed for the muscling operation. Once at the work site, finer adjustments are needed to ease the ROV into and out of tight places. If the power were set to full gain in a con- fined area, a quarter joystick movement could over-ramp the power so quickly that the vehicle could ram into a wall, damaging the equipment and causing some embarrassing personnel reviews.
Unlike underwater vehicles built for high speed (examples of high-speed underwater vehicles are a torpedo or a nuclear attack submarine), most ROV submersibles are designed for speeds no greater than 3 knots. In fact, somewhere in the speed range of 6
8 knots for underwater vehicles, interesting hydrodynamic forces act upon the system, which require strong design and engineering considerations that address drag and control issues. At higher speeds, small imperfections in vehicle ballasting and trim propagate to larger forces that simple thruster input may not overcome. As an anecdote to unexpected consequences for high-speed underwater travel, during trials for the USS Albacore (AGSS 569), it was noted with some surprise (especially by the Commanding Officer) that the submarine snap rolled in the direction of the turn during high-speed maneuvering!
For the ROV at higher speeds, any thruster that thrusts on a plane perpendicular to the relative water flow will have the net vector of the thrust reaction move in the direction of the water flow (Figure 4.4). Also, one current vehicle manufacturer makes use of vectored thrust for vehicle con- trol, which mitigates the thrust vector problem at higher speeds.
At what point is control over the vehicle lost? The answer to that is quite simple. The loss of control happens when the vehicle’s thrusters can no longer counter the forces acting upon the vehi- cle while performing a given task. Once the hydrodynamic forces exceed the thruster’s ability to counter these forces (on any given plane), control is lost. One of the variables must be changed in order to regain control.
4.1.2 Autostabilization
With sensor feedback fed into the vehicle control module, any number of parameters may be used in vehicle control through a system of closed-loop control routines. Just as dogs follow a scent to its source, ROVs can use sensor input for positive navigation. Advances are currently being made for tracking chemical plumes from environmental hazards or chemical spills (although this is con- sidered a higher, logic-driven control level). A much simpler version of this technique is the rudi- mentary auto-depth/altitude/heading.
Auto-depth is easily maintained through input from the vehicle’s pressure-sensitive depth trans- ducer. Auto-altitude is equally simple, but the vehicle manufacturer is seldom the same company as the sensor manufacturer (causing some issues with communication standards and protocols between sensor and vehicle). The most common compass modules used in observation-class ROV systems are the inexpensive flux gate type. These flux gate type compasses have a sampling rate (while accurate) slower than the yaw swing rate of most small vehicles, which cause the vehicle to “chase
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