Autopilot Algorithms

Last Updated on Thursday, 25 August 2011 10:24 Written by Alison Little Friday, 26 March 2010 16:01

H Scientific has designed and implemented self-tuning controller algorithms in the SPECTRE autopilot and simulators. The vehicle performs a sequence of manoeuvres from which it learns the vehicle's responsiveness. These algorithms have been thoroughly tested and have been licensed to Raymarine Ltd, who use them in their high-end autopilots.

Self-tuning algorithms for heading, speed, height/altitude, roll and pitch and lateral movement have been implemented.

Heading Following

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.

Track (Waypoint) Following

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.

Vehicle Following

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.

Hovering on a Point

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 within an Area

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.

Attitude (roll and pitch) and depth / height control

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.

Collision Avoidance / Obstacle Avoidance

The Collision Avoidance / Obstacle Avoidance module is integrated into H Scientific’s SPECTRE remote control system. The operator can specify exclusion zones surrounding static and dynamic obstacles, and extended zones ahead of moving targets. When engaged, the CA module then assesses obstacles and, when necessary, takes control of speed and steering, avoiding the exclusion zones and then returning the craft to its original mission track. The SPECTRE module also supports an “external CA” mode in which mission status is exported to a separate processor which maintains a database of static and dynamic obstacles and plots a modified track to avoid collisions. This “External CA” mode uses a simple interface which is designed to support third party external CA processors from other vendors.