Autopilots: Control Methods

• An autopilot is an automatic device that steers a vessel and maintains its heading in a intended direction
• Ship operators can appreciate the advantages of the autopilot as it steers the ship, freeing them to carry out navigational checks
• The autopilot compares desired course data, set by the helmsman, with actual course data from a gyro or magnetic compass
• It then applies rudder corrections to compensate for any error detected between these signals

Main Control Methods

• Vessels must be provided with data regarding its movement relative to the desired course line for accurate course holding
• Three main control functions affected by data inputs are proportional, derivative, and integral control

Proportional Control

• Proportional control means equal
• The rudder moves by an amount proportional to the positional error from the course line
• When only proportional control is applied, the vessel will oscillate on either side of the desired course
• The vessel eventually reaches its destination, but the erratic course increases fuel consumption

Derivative Control

• The rudder shifts by an amount proportional to the rate of change of the vessel's deviation from its course
• This is achieved by electronically differentiating the actual error signal
• Corrective starboard rudder is applied when course error is sensed
• The rate-of-change decreases, causing automatic rudder control to decrease
• The rudder returns to amidships position at Point X
• The vessel will continue parallel to the required course until the autopilot is acted on by external forces

Integral Control

• Combines proportional and derivative control for a more satisfactory return to course
• Data for integral control is derived by electronically integrating the heading error
• This offsets the effect of a vessel continuously moving off course
• Data signals are produced by continuously sensing the heading error over a period and applying an appropriate degree of permanent helm
• Autopilots normally include yaw, trim, draft, rudder limit, and weather controls

Comparator

• When a ship is on course, two signals are equal
• If the ship drifts off course, the difference between the primary heading and the set course changes proportionally
• This causes an imbalance at a comparator
• The comparator's output moves up or down depending on the difference in course
• The rudder turns the vessel to obtain a balanced condition at the comparator, then the drive to the rudder stops
• A rudder reference unit determines rudder position and direction, and it is continuously monitored by the processor unit

Block Diagram of Autopilot System

• An output from a gyro or magnetic compass is coupled to a differential amplifier with a signal from a manual course-setting control
• With no signal difference, the amplifier produces no output
• This means there will be no rudder movement
• An output signal that is proportional to the off-course error is produced if a difference exists between the compass and selected course inputs
• An output error signal, proportional in magnitude to the difference, is applied to the heading error amplifier
• The output of this amplifier causes the rudder actuator circuit to move the rudder in the direction of the output signal's sign

Alarm

• An off-course alarm circuit senses the error signal at the output of the heading error amplifier
• When signal amplitude falls outside of pre-determined limits, an audible alarm sounds

Basic Steering Modes

• Auto mode performs automatic course keeping using heading data from the gyrocompass and the operator's order setting
• Some devices monitor dynamic parameters, continuously adapting the steering control output
• This provides course keeping with minimal rudder motion and maximum efficiency.
• Nav mode performs automatic course keeping using inputs from an external navigation system to steer the ship toward a waypoint
• In NAV mode the external navigation system determines course to steer

Track Mode

• Track mode performs automatic course keeping using inputs from an external navigation system, corrected for cross track error by the autopilot
• This steers the ship toward a waypoint over a designated track over the ground.

Non-Follow Up (NFU) Mode

• This mode allows the operator to directly control the steering gear pump oil flow into the steering actuators
• Operating the NFU control causes the rudder to rotate portside or starboard side as long as the control is held
• When the control is released, the rudder remains stationary until the NFU controller is operated again, or the steering mode is changed
• The helmsman sets the desired rudder angle by observing the rudder angle indicator and operating the NFU control

Follow-Up (FU) Mode

• All autopilot functions apply

Advantages of Autopilot

• Reduce the number of deck crew
• Considerable saving in fuel is achieved
• Minimizes deviation of the steering course
• Decrease sailing time of the voyage.
• The OOW can concentrate on other issues (i.e. navigation).