Autopilot Algorithms

In heading following mode the user selects the desired heading and the autopilot controls the appropriate thrusters, rudders, etc. in order to achieve the desired heading on the vessel's compass.

Under speed control the autopilot controls the appropriate thrusters, etc. in order to maintain the set speed as measured by the speed log.

In track following mode the user enters a set of waypoints, specifying a desired depth / height for each one. The autopilot will then control the track of the vehicle to make it move to each waypoint at the correct speed. Waypoints may be set up on an electronic chart system or ECDIS, and downloaded; alternatively they may be entered directly by providing coordinates.

The track controller effectively provides a high level mission control, including the following features:

Choice of track acquisition conditions - for example, starting at the nearest waypoint or at the start of the track;
Choice of operational modes on completion of the track;
Linkage between waypoints and events - for example, so that the speed or depth/altitude changes at a specific waypoint.

A communications link, between units on board the two vehicles, passes information about the current position, heading and velocity of the vessel to be followed. The vehicle autopilot then follows this vessel at a user-specified distance and bearing, all the time ensuring that a predefined safety zone around the vehicle is not encroached.

In order to hover on a Point the autopilot controls the position of the vehicle relative to a user-defined waypoint. Two modes are possible:

Position Control by Manoeuvre (PCM) in which the vehicle heading is chosen by the autopilot to minimise thruster activity and fuel usage. This mode is efficient if there are significant environmental forces (tidal stream and / or turbulence or waves);
Dynamic Positioning (DP) in which the operator specifies a heading to be maintained subject to environmental constraints. This might be used for example if a sensor is to be directed on a stationary target.

Hovering in an Area - the vehicle is controlled to maintain its position within a nominated region. Again, this is subdivided into Area PCM and Area DP (see PCM and DP, above). Area hovering is a very versatile mode and is the most fuel-efficient method of hovering, because it allows the vehicle to drift a controlled amount without making continual corrections and countercorrections. During Area Hover, several area conditions may be specified simultaneously.

These conditions may be superimposed simultaneously. At the same time, the operator (in Remote Control mode) may use a joystick to superimpose a controlled drift - for example, hover within a set zone alongside a pipeline, and slowly drift along it, while maintaining a desired heading so as to bring sensors to bear on the pipeline. This minimises the difficulty for the operator, as well as control communications bandwidth, because the autopilot takes care of the second-by-second control of the thrusters, and the operator only has to issue a simple "drift" command.

The autopilot system incorporates controllers for attitude (roll and pitch) and depth / height control. Roll control is built in to the attitude controllers, however, because some vehicles cannot achieve independant control of depth / height and pitch, two integrated controllers are provided:

Controller Type 1 uses independent depth / height and pitch control algorithms. This is particularly appropriate when the actuators for pitch and depth / height are separate units giving a capability for pitch to be adjusted independently of depth / height - for example, four vertical thrusters may be used together to thrust upwards, and differentially to adjust vehicle attitude;
Controller Type 2 uses pitch, in conjunction with vehicle forward motion, to adjust depth / height. This is appropriate, for example, for a vehicle equipped with forward hydroplanes.