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What is the working principle of the autopilot system?

  • P accounts for present values of the error. For example, if the error is large and positive, the control output will  also be large and positive.
  • I accounts for past values of the error. For example, if the current output is not sufficiently strong, error will      accumulate over time, and the controller will respond by applying a stronger action.
  • D accounts for possible future values of the error, based on its current rate of change

With a Proportional, Integral and Derivative steering control system, the oscillation is minimized by modifying the error signal produced as the difference between the selected heading and the compass heading. Figure  shows that a three-input summing-amplifier is used, called a dynamics amplifier, to produce a resultant output signal equal to the sum of one or more of the input signals.

The demanded rudder error signal is inspected by both the differentiator and the integrator. The differentiator determines the rate of change of heading as the vessel returns to the selected course. This sensed rate of change, as a voltage, is compared with a fixed electrical time constant and, if necessary, a counter rudder signal is produced. The magnitude of this signal slows the rate of change of course and thus damps the off-course oscillation.

Obviously, the time constant of the differentiation circuit is critical if oscillations are to be fully damped. Time constant parameters depend upon the design characteristics of the vessel and are normally calculated and set when the vessel undergoes initial trials. In addition, a ‘counter rudder’ control is fitted in order that the magnitude of the counter rudder signal may be varied to suit prevailing conditions

About the author

Amit Sharma

Graduated from M.E.R.I. Mumbai (Mumbai University), After a brief sailing founded this website with the idea to bring the maritime education online which must be free and available for all at all times and to find basic solutions that are of extreme importance to a seafarer by our innovative ideas.

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