There are many important features in autopilot electronics which will
contribute in a good working autopilot setup.
- Important is to have a
good rudder feedback unit, so the autopilot knows the position and speed
of the rudder and drive unit.
- A modern fluxgate compass, preferably with
movement and rotation sensors (rate sensing) to achieve a correct
magnetic course, even on rough seas.
- When the autopilot is adaptive
(self learning), the course computer will learn from its mistakes and
tries to use the best settings possible in various circumstances.
- Speed control (see explanation below)
- Dynamic braking (see explanation below)
Full speed control and dynamic braking are the
two most important features of a modern autopilot course computer.
Speed control: Some autopilots control the autopilot drive unit with an on-off signal. This
can result in
a bumpy autopilot steering and high peak forces in the system as all
movements are abrupt (no soft start and soft stop). The
autopilot has trouble to steer the boat as no real subtle
movements are possible and the pilot would often overshoot the required
rudder position. It’s like driving a car through a city with only full
throttle or full brake. You will get there, but it is a bumpy drive.
Often these on-off pilots are developed as power boat autopilot and
meant to drive a hydraulic cylinder (which is nearly always controlled
with an on-off signal).
More modern autopilots are able to control the speed of the autopilot drive
unit from 0 to 100% without any steps via so called pulse width
modulation. To be able to vary the power sent to the drive unit,
they don't sent a continuous signal, but pulses, and by adjusting the
width (duration) of the pulse from zero to hundred
percent they can accurately control the speed of the drive
unit. With this pulse width modulation the autopilot can make sure the start
is not with a bang (full 100% power at once) but a so called "soft
start" is performed. The same with the stop of the movement. This will
be done with a soft stop. As the autopilot is continuously starting and
stopping, this is very important. Autopilots without speed control often
start "hunting" at low boat speeds. Hunting is an oscillating movement
around the mid position where the steering wheel continuously swings
around the mid position. It can only be stopped by grabbing the wheel by
hand or turning the autopilot off. As the autopilot can't control the
speed properly it corrects the course, but continuously overshoots the
required position. It immediately responds by reversing the power and
powering the system again, but then the same overshoots happens. This
results in a wheel swing around the mid position.
Dynamic braking: "Why do I need
any braking? I want to power the rudder, not brake!". Modern sailing yachts
require rudder and steering systems and autopilot drives that are very
light running so the maximum amount of feedback from the rudder is
transmitted to the helmsman for the most optimal "tiller feel". On top
of that the tendency is to use bigger and bigger steering wheels with
larger moments of inertia (flywheel effect) or twin wheel systems that
also double the wheel inertia.
As no friction in the rudder and steering system is allowed any more and
all parts run on ball or roller bearings, there is no damping/braking
of the system any more. This means when you add energy to the system by
the autopilot drive, you should also have a way to get the energy out.
So in practice when the autopilot has to make a quick correction, the
system is speeded up and a few degrees before reaching the final
position of the rudder, the power is taken off. But due to the inertia
of the wheel and the fact that there is no damping, the rudder
overshoots the targeted end position and the autopilot starts to correct
this and reverses the power and this goes on and on and on. This is
called “hunting of the autopilot” an oscillation around the amidships
position. This causes high stresses in the construction, could cause
damage to the steering and rudder system, but above all will empty your
batteries very quickly! Modern autopilots have a feature called dynamic braking to
prevent this. It’s a very simple modification of the end transistors of
the autopilot. The feature is to shortcut the wires going to the
motor/drive unit as soon as there is no power given from the autopilot. As
the drive unit will act as a dynamo when back driven (the fraction of a
second before reaching the end position) it can really brake the system
very hard. This feature can be tested very well on board. For this you
need a battery and one of our autopilot drives. Put voltage on the
clutch (small wires) and try to back drive it (move output lever by hand
or turning the steering wheel). You will see that it is quite easy to
do. Now take both motor wires and connect them to each other (shortcut
the motor) and you will see that it is nearly impossible to back-drive
the unit.
This holding power is the second advantage of a dynamic braking feature.
While sailing on autopilot, the rudder needs a constant cut angle to the
water flow to keep the ship on course. This requires a constant force of
the autopilot drive driving the rudder. On systems without dynamic
braking, the autopilot commands the drive unit to a certain rudder angle
and takes the power off. But within seconds, the drive unit is pushed
back by the rudder forces to the neutral position. The autopilot will
have to drive the rudder to the required rudder angle again and when it
is reached, the rudder slowly turns back neutral again. This not only
gives a non constant course, but also uses a lot of power, causes high
wear in the drive unit and heats up the drive unit to immense
temperatures.
With dynamic braking, the autopilot can "grab and hold" the rudder fixed
when it is a certain position and hold it without any movement and
without using any energy. As shown in these points, speed control and
dynamic braking are very important and make or brake an autopilot
system.
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