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Page 3-2 Robertson AP45 Autopilot
Design and theory of operation
Simrad Robertson AS
Egersund - Norway
In order to stop the rudder movement, a feedback signal is produced from the
feedback unit. The feedback signal will be compared with the compass signal and
when there is a balance between the two, the solenoid will be de-energised. The
rudder has now been moved to a position that makes the vessel turn. This turn is
picked up by the compass and causes a new unbalance between the feedback
signal and the error signal which energises the opposite solenoid. Now the rudder
will start to move back towards the previous position, and the feedback signal will
again cancel out the unbalance and de-energise the solenoid to stop the rudder.
By utilising digital technology to perform the function of an autopilot, the typical
block diagram will be slightly changed. Even so, the basic operation should be
recognised on Fig. 3-2.
As we know, a microprocessor can only do what it has been programmed to. This is
called software. The program can be either fixed or partly adjustable to adapt the
microprocessor to the individual type of vessels. In Robertson terminology this is
called "setting parameters", and it will determine the performance of the autopilot.
The AP45 Control Unit contains two electronic boards, the Control Board and the
Illumination Board. An optional navigation interface board (N40) may be supplied.
The Control Board contains the micro-computing circuitry, and a plug-in PC-board
for the Course Display. The alarm circuit is also mounted on this board.
The Illumination board consists of the display backlighting- and the Information
display electronics.
All interconnecting plugs for heading sensors, junction unit and remote controls
are mounted on the Control Board.
The two boards are interconnected with a ribbon cable which are soldered to the
illumination board and plugged into the control board.
All parameter settings and operation of the autopilot are made using the push
buttons on the front panel.
CD109 is a magnetic compass sensor in moulded plastic. It is mounted to a vessel's
magnetic compass to transfer the heading information to the control unit. The
primary windings are excited by a pulsating signal. Dependant of the magnetism
induced by the position of the compass card magnets (Heading), pulses of variable
amplitude will be generated in the secondary sine and cosine windings. These
pulses are filtered through the R/C network and amplified before entering the A/D
converter.
AP45 Control Unit
CD109 Course
Detector
N
S
Exitation
Digital HeadingC1
C2
R1
R2
A/D
CONV.
2.5V (Ref.)
HEAD 2 (COS)
HEAD 1 (SIN)
Fig. 3-3
Course Detector principle