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Published 2026-05-02
This article briefly introduces the basic working principles of ship autopilot, also known as autopilot, to help quickly understand how it can replace manual steering.
The key principle of the automatic steering gear is "closed-loop control". It uses the heading sensor to sense the current heading, compares it with the set heading to obtain the deviation, and then sends instructions to the steering gear based on calculations based on the control unit, prompting the rudder blades to rotate to correct the deviation, and ultimately allows the ship to automatically maintain on the set route.。
This process of constantly experiencing "measurement", then "comparison", then "execution", and then "measurement" again is the essence of achieving automatic steering by the ship's autopilot.。
The working principle of the automatic steering gear can be fully understood from three consecutive links:
To sense the current heading, use the gyrocompass or magnetic compass installed on the ship as a heading sensor to continuously measure the real-time heading of the ship with high precision.
Obtain the set sailing direction, which is the sailing direction expected to be achieved by manual input by the driver on the autopilot control panel.。
The control system automatically calculates the difference between the "set heading" and the "actual heading", that is, the yaw angle, to calculate the heading deviation. For example, if the set heading is 090° and the actual heading is 087°, then the resulting deviation is -3°.
The control unit, usually a dedicated microprocessor or PLC, calculates the required steering order signal based on the heading deviation and a preset control algorithm. The most common algorithm is PID control.
byPID controlFor example, it combines three factors to output instructions:
Proportion (P), it outputs the rudder command in proportion based on the current deviation. The larger the deviation, the larger the rudder angle command will be. For example, when the deviation is 3°, it is possible to command a rudder angle of 5°.
Integral (I), which accumulates continuous deviations over a period of time, is used to solve the "steady-state error" produced when the ship is subjected to constant lateral forces (forces such as constant current and constant cross wind). The integral effect will gradually increase the rudder command until the deviation is completely eliminated.
In terms of differential (D), adjustments are made in advance based on the speed of deviation change. If the bow of the ship quickly deviates from the course, the differential effect will output a larger reverse rudder command in advance to suppress excessive yaw.
The most basic autopilot operating mode is course maintenance. In this mode, the PID controller stably outputs the rudder command so that the bow of the ship is always aligned with the set course.
The steering gear action is driven by the rudder command signal output by the control unit, which is usually an electrical signal or a hydraulic signal.
The hydraulic steering gear controls the electromagnetic reversing valve through signals, causing the hydraulic pump to start, using high-pressure oil to push the cylinder piston, and then drive the tiller, so that the tiller drives the rudder blades to rotate.
Electronically controlled steering gear: The signal directly drives the servo motor and drives the rudder blade through the reduction mechanism.
After the rudder blade rotates to the specified angle, the feedback unit on the steering gear sends the actual rudder angle signal back to the control unit to form a closed-loop confirmation.. Fully displaying the entire process from "command output" to "actual arrival" is the key to ensuring automatic control accuracy.
Take a cargo ship setting a route from point A to point B as an example. The complete workflow is as follows:
1. Set route: The driver enters the desired heading on the autopilot panel, such as 090°.
2. For real-time monitoring: the gyro compass measures the actual heading within the time range of each second through multiple frequencies. For example, the current state is 091° and so on.
3. The calculated deviation performed by the control system is by subtracting 091° from 090°. The result is -1°, which means that the ship's bow is 1° to the right.
4. PID operates, and the controller calculates based on the -1° deviation and the preset PID parameters to obtain a command that requires a left rudder, such as 3° left rudder.
5. Drive steering gear: The electrical signal is sent to the hydraulic steering gear, driving the rudder blade to turn 3° to the left.
6. ship response: The bow of the ship begins to deflect to the left under the action of hydrodynamic force.
7. Measure again: The gyro compass detects that the heading has changed to 090°.
8. Deviation to zero: The deviation is 0, the PID output rudder command returns to zero, and the rudder blade returns to 0°.
9. The cycle is maintained: Once there is external interference, including wind, waves and currents, causing the ship to yaw, the system will immediately repeat the steps mentioned above.
There is such a common case: there is a 10,000-ton cargo ship sailing in sea areas with level 5 cross wind and 1.5 knot cross current. When the autopilot is turned on, the average yaw distance of the ship's position is less than 10 meters per nautical mile. The frequency of steering gear action is about once every 3 to 5 minutes, which is far lower than the multiple times per minute of manual steering, thus significantly reducing fuel consumption.
Q1: Will the autopilot suddenly fail and cause a collision?
A1: It will not completely lose control all of a sudden. Modern autopilots have fault self-diagnosis and alarm functions, and regulations require that the driver on duty must monitor the vehicle at all times and can immediately switch to manual emergency steering.
Q2: Is automatic steering more fuel-efficient than manual steering in strong winds and waves?
Yes, the autopilot can accurately control the rudder angle and reduce ineffective rudder with the help of PID algorithm. The overall fuel saving effect usually reaches 3 to 8%, and the advantage is more significant in wind and waves.
Q3: What basic conditions do ships need to meet to install an autopilot?
It is necessary to have a reliable gyrocompass to provide heading signals, and it must be equipped with a hydraulic steering gear that meets the standards, or an electronically controlled steering gear that meets the standards.
Q4: Can small fishing boats be equipped with similar autopilot systems?
Small ships have a simplified version of the autopilot specially designed for them. It is low-priced and easy to install. However, its accuracy and functionality are lower than those of the commercial ship system. A4 said it is OK.
Q5: Do I need to adjust the PID parameters of the autopilot myself?
5: It requires initial setting. It is recommended that the manufacturer or professionals conduct debugging according to the type and loading conditions of the ship. Ordinary drivers can use the preset modes such as "light load/heavy load".
The principles described in this article are based on the autopilot performance standards of the International Maritime Organization (IMO) Resolution A.342(IX), the International Electrotechnical Commission (IEC 62065 "Ship Autopilot" standard), and the commonly used PID control theory. The specific fuel saving data is quoted from the measured average value of 10,000-ton bulk carriers in "Analysis of Autopilot Energy Saving Effects Based on Real Ship Data" (this document is from China Navigation, 2023).
For the ship's automatic steering gear, it relies on such a closed-loop control principle to achieve automatic correction navigation. The principle is to measure the actual heading, and then compare the actual heading with the set heading. Then use the PID algorithm to calculate the rudder order, and then drive the steering gear to perform the corresponding action based on the calculated rudder order. After the action is completed, the measurement is performed again.
Suggestions for action：
1. Conduct theoretical verification, go to the bridge, observe the readings displayed by the gyrocompass, check the actual rudder angle displayed on the autopilot panel, and the heading deviation, and verify the closed-loop process on site.
2. Put it into practice: In an open and safe water area, switch from manual steering to automatic mode, and pay attention to how the autopilot uses its own ability to correct small-angle yaw conditions.
3. Understand the parameters, consult the manual of the ship's autopilot, find the PID parameter setting interface, and then understand the role of proportion in course maintenance, the role of integral in course maintenance, and the role of differential in course maintenance.
The core interface of human-computer interaction is the rudder command unit, which integrates all functions such as course limitation, mode change, and parameter control. All instructions received by the autopilot system are issued by the driver through this unit.
Understanding this closed-loop control principle is the foundation for safe and effective use of ship autopilot. It is recommended that the captain and driver organize regular training on the working principles and emergency operations of the ship's autopilot system to ensure that every person on duty can operate it correctly when needed.
Update Time：2026-05-02
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