Autopilot system for a vessel

An autopilot system for a vessel such as a boat receives a pre-set heading direction set by e.g. the user and also receives the apparent orientation (heading) from a heading sensor. The difference between the pre-set heading direction and the actual heading is then determined and a correction signal is generated in dependence on that difference, which correction signal is then used to control the orientation of the vessel by e.g. generating a rudder correction signal which varies the position of the rudder. In the present invention, the correction signal is further dependent on the difference between the actual heading and a reference direction such as magnetic North or South. In this way errors due to differences between the apparent orientation as detected by the heading sensor and the actual orientation of the vessel may be corrected. The correction value may alternatively, or in addition, be dependent on magnetic dip angle, and/or the vertical and/or horizontal component of the Earth's magnetic field, and/or latitude.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an autopilot system. 
2. Summary of the Prior Art 
It is now common for even relatively small boats or other vessels to have 
an autopilot which permits automated steering of the vessel. The autopilot 
maintains the boat or other vessel on a pre-set heading (orientation), 
which pre-set heading is compared with the heading (orientation) of the 
boat as determined by a heading sensor (which outputs a signal called the 
Compass Heading). That heading sensor may be, for example, a flux gate 
system such as disclosed in our European patent number 0113221. 
SUMMARY OF THE INVENTION 
On this basis, a Heading Error can be calculated where: 
EQU Heading Error=Compass Heading-Locked Heading 
A gain is then applied to the Heading Error in order to determine the 
change in rudder required to correct the course. The gain is generally 
expressed as degrees of rudder angle per degree of Heading Error (other 
forms of expression can be reduced to this), so that, for example a gain 
of 0.1 corresponds to 0.1.degree. of rudder for 1.degree. of Heading 
Error. The difference between the required rudder position and the actual 
rudder position (this difference being the Rudder Position Error) can then 
be calculated as follows: 
EQU Rudder Position Error=(Gain.times.Heading Error)-Rudder Position 
A suitable processing unit receives the Rudder Position Error as an input, 
and has processing algorithms to derive a drive signal which is applied to 
the drive electronics of the rudder to change the position of the rudder 
and so reduce the Rudder Position Error to within acceptable limits. 
As described above, it would appear that, since the Heading Error can 
always be derived directly from the difference between the Compass Heading 
and Locked Heading (pre-set orientation), an autopilot system operating 
according to this principle should in theory be able to steer the boat or 
other vessel correctly. However, this has been found not to be the case; 
and the Heading Error is direction sensitive with the Compass Heading 
sometimes departing from the actual heading of the vessel especially when 
the vessel is turning and lateral forces are acting on the gimbals of the 
compass. Thus, the compass system provides an apparent orientation for the 
vessel. Therefore, the system for deriving the Heading Error described 
above does not operate correctly, because additional factors (to be 
discussed later) have been found to influence the Heading Error. 
Therefore, according to the present invention, it is proposed the control 
arrangement of the autopilot system contains a suitable processing means 
for correcting one or more of the orientation dependent errors which 
occurs. 
In a first aspect, the applicants have found the factor which primarily 
determines the compass error is the direction of the boat and this error 
is maximum when travelling North in the Northern Hemisphere and when 
travelling South in the Southeren Hemisphere. Therefore, in this first 
aspect the difference between the compass heading (apparent orientation) 
and a reference direction e.g. magnetic North or South is determined, and 
the heading error signal (correction value) is varied in dependence on 
that difference. Preferably, this is achieved by adjusting the gain of the 
autopilot system on the signal corresponding to the difference between the 
compass heading (apparent orientation) and the pre-set heading direction, 
in dependence on the difference between the compass heading (apparent 
orientation) and the reference direction. 
Such an arrangement has the further advantage that it can also correct for 
other direction sensitive errors. As the vessel is turning, lateral 
acceleration causes the compass to heel, and this causes a further error 
in a system in which the compass heading (apparent orientation) is used 
directly. Such a further error increases with increasing heel angle, but 
again is direction sensitive, and so this error is at least partially 
corrected by the present invention. 
Another factor which may affect the compass error is variation due to 
changes in the orientation of the Earth's magnetic field relative to the 
vessel, which can be determined in terms of variation in the magnetic dip 
angle, the vertical component of the Earth's magnetic field, and/or the 
horizontal component of the Earth's magnetic field, each of which change 
with changes in latitude, or can be determined approximately in terms of 
the latitude itself. During turning of the vessel, the compass will swing 
on its gimbals due to lateral acceleration and the effect of this swing 
will change in dependence on the magnetic dip angle and/or the vertical 
field component, and/or the horizontal field component and/or latitude. 
Therefore, since magnetic dip at various latitudes is known, it is 
possible for the autopilot system to store in a suitable memory the values 
of the dip. Alternatively, the horizontal and/or vertical field components 
may be measured or stored, and/or the latitude itself may be determined. 
Then, the heading error signal (correction value) may be varied in 
dependence on the dip angle and/or horizontal field component, and/or 
latitude. Such a variation may be performed in combination with a 
variation in dependence on the difference between the compass heading 
(apparent orientation) and the reference direction. This is particularly 
useful because the error due to variation in magnetic dip angle, and/or 
vertical field component, and/or horizontal field component, and/or 
latitude, is direction sensitive. However, the variation of the heading 
error signal (correction value) with dip angle, horizontal and/or vertical 
component, and/or latitude may be applied independently of direction 
sensitive correction and thus represents a second, independent aspect of 
the present invention. 
A further factor which may affect the compass error is the speed of the 
vessel. The greater the speed, the greater the forces on the gimbal 
mounting of the compass due to lateral acceleration. Therefore the 
autopilot system of the present invention may detect the speed of the 
vessel and apply a corresponding variation in the heading error signal 
(correction value). This may be independent, or in combination with the 
variation due to the difference between the compass heading (apparent 
orientation) and a reference direction, and/or the variation dependent 
magnetic dip angle. 
The variation of the heading error signal (correction value) in dependence 
on one or more of the factors of: the difference between the compass 
heading (apparent orientation) and a reference direction, the magnetic dip 
angle, and/or vertical field component, and/or horizontal field component, 
and/or latitude, and/or the speed of the vessel, and may be proportional 
to those factors. However, the variation may alternatively be stepwise. 
The latter is easier to achieve using digital electronics in the autopilot 
control system, but the former avoids sudden changes of turn rate. 
In practice, the effect of the above factors may cause significant error in 
the direction of the vessel only when they exceed particular values 
related to the vessel. Therefore, it is possible for the autopilot system 
to operate, so that, unless these factors do exceed those values, the 
system operates without these direction sensitive corrections being 
applied. 
The present invention relates to both apparatus and method aspects of the 
present invention.

DETAILED DESCRIPTION 
Referring first to FIG. 1, the rudder 10 of a vessel is connected via a 
suitable linkage 11 to a steering wheel 12, or other steering system, to 
enable the rudder 10 to be controlled manually. The link 11 can be 
mechanical, using rods or cables, or could be hydraulic. When the steering 
wheel 12 is turned, the link 11 transmits the movement to the rudder 10 
and this steers the vessel. It can be noted, that for some vessels, the 
action of the rudder 10 is provided by swiveling the propellers 
themselves. As illustrated in FIG. 1, the link 11 passes via a connection 
13 to the rudder 10. The connection 13 is connected to a drive unit 14, 
which drive unit 14 is connected to an autopilot processing unit 15. When 
the vessel is being steered by the autopilot, the autopilot processing 
unit 15 transmits signals to the drive unit 14, which moves the connection 
13 so as to cause the appropriate rudder movement. In practice, the drive 
unit 14 will normally be a motor system, and it may also be noted that 
parts of the link 11 and connection 13 may be common to each other. 
As illustrated, the autopilot is connected to a suitable operator control 
16 which inputs commands to the autopilot processing unit 15 via suitable 
operator inputs. An audible alarm may be incorporated in the operator 
control 16. Furthermore, there is a heading sensor 17 which is a 
transducer generating information concerning the orientation of the 
vessel, and which in turn generates signals to the autopilot processing 
unit 15. Furthermore, signalling between the autopilot processing unit 15 
and the drive unit 14 will depend on the particular electronic arrangement 
envisaged. For example, if motor drive amplifiers are contained within the 
autopilot processing unit 15, then full power signals may be transmitted 
directly to the drive unit 14, in which case the drive unit 14 can be 
relatively simple. On the other hand, if only low level signals are to be 
transmitted between the autopilot processing unit 15 and the drive unit 
14, then full power electronics system is required in the drive unit 14. 
The present invention is particularly concerned with the control of the 
rudder 10 by the processing unit 15 so as to correct errors due to the 
effect of the movement of the boat or other vessel on the heading sensor 
17. Before describing an embodiment of the present invention in detail, 
however, explanation will first be given of how an autopilot system may 
operate in a stable or an unstable way. FIG. 2 shows two curves a, b of 
Heading Error with different gains. At an optimum gain setting (e.g. curve 
a) the course-holding ability of the autopilot is acceptable and the 
Heading Error is allowed to wander within an acceptable dead band. Any 
excursion from the dead band is quickly corrected. However if the gain is 
significantly greater than the optimum setting (e.g. curve b) the 
performance of the autopilot system is unstable, which means the Heading 
Error is large and could become cyclic. 
Referring now to FIG. 3 the autopilot processing unit 15 receives a compass 
heading input 110 (apparent heading) from the heading sensor 17, and a 
locked heading 112 (i.e. the pre-set heading) set by the operator at the 
operator control 16. The locked heading 112 is subtracted from the compass 
heading 110 to produce a heading error signal 114. The heading error 
signal 114 is multiplied by a system gain 116, and a signal 118 related to 
the rudder position derived e.g. from rudder position sensor 18, is then 
subtracted to generate a rudder position error signal 120. The rudder 
position error signal 120 is then processed by error signal processor 122 
to generate a drive signal 124 which controls a power electronics system 
126 which drives the rudder system 128. Of course the gain 116 can be 
applied at any suitable stage. 
In a first embodiment of the present invention, the system gain 116 is 
varied in dependence on the heading of the vessel. Of course, whilst it 
would be desirable to provide a variation dependent on the actual heading 
(actual orientation) of the vessel, it is necessary to use the compass 
heading (apparent orientation) as only this is measurable. Referring to 
FIGS. 4 and 5, suppose that the vessel is sailing in a direction more than 
45.degree. away from due north, i.e. is in region B or C in FIG. 5. If the 
operator inputs a course change taking the vessel into region A in FIG. 5 
then the step 130 in FIG. 4 will detect this fact. As shown in step 132 in 
FIG. 4 the gain 116 of the system is reduced by a predetermined number of 
steps, X. This ensures that the gain is within the range in which the 
system remains stable. 
Now consider the situation where the vessel is turned away from the North. 
When the vessel's Heading enters region C, the gain is not reset (Step 
134) to avoid the gain "hunting" between settings when the vessel is on a 
course approximately coincident with the boundary between A and C (L1 and 
L2). The gain 116 may not revert to normal until the heading enters region 
B. However the gain 116 is not necessarily immediately restored on 
entering region B. A time delay is imposed (step 138), so that the gain 
116 is only restored (step 138) when the rudder reverts to an 
approximately neutral position. This avoids an undesirable sudden change 
in the rudder position, which would cause a sudden change in turn rate. 
Thus, in the first embodiment, the system gain 116 is varied in dependence 
on the direction of the vessel relative to North (or South) derived from 
the heading sensor 17. 
In an alternative embodiment, the gain is increased and decreased 
gradually, rather than in steps, thus avoiding sudden change of turn rate. 
In the above described embodiments the amount of gain correction applied 
and the regions (A,B,C) in FIG. 5 are fixed, and the autopilot system is 
thus limited to the geographical location for which it has been 
configured. In order to increase the versatility of the system in further 
embodiments, the gain change may be adjusted in response to other 
variables. 
Thus, the vessel's position may be established either by being manually 
input or from an automatic position sensor. This may then be used to look 
up the magnetic dip angle from a table stored in a memory 140 of the 
system (Magnetic dip angle data is published by the Hydrographic Office). 
Alternatively, the dip angle may be derived from information from a 
fluxgate heading sensing system (not shown). Such a sensor may provide 
information relating to the vertical component of the Earth's magnetic 
field from which the dip angle may be calculated. The dip angle may also 
be manually entered into the system. This information may then be used to 
adjust the system gain 116, the correction being according to 
predetermined algorithms. 
Indeed, as mentioned previously, variation of gain in dependence on dip 
angle may be valuable independent of variation relative to North or South. 
Since dip angle is latitude dependent, the gain will then be adjusted in 
dependence on the geographical position of the vessel. 
As was mentioned previously, a similar effect may be achieved by 
consideration of the horizontal component of the Earth's magnetic field, 
and/or the vertical component of the Earth's magnetic field, and/or the 
latitude, at the location of the vessel. The respective component and/or 
latitude may be stored in memory 140 together with a gain adjustment 
appropriate for different values of those components and/or for different 
latitudes. 
The characteristics of the vessel may also be stored in the memory of the 
system and used to ensure a suitable gain. For instance, the speed of the 
vessel may be entered; high speed vessels with high turn rates require 
more adjustment of the gain than vessels that turn more slowly. This idea 
may also be embodied using a speed sensor 142 which determines the speed 
of the vessel and supplies a suitable signal 144 to the system gain 116, 
to permit variation in dependence on speed.