Modern ships are no longer operated entirely through manual control. From maintaining boiler water level and steam pressure to controlling engine parameters, tank levels, temperatures and flow rates, automation plays a critical role in safe and efficient ship operation. At the heart of many of these automated systems are two important technologies: PID controllers and PLCs (Programmable Logic Controllers).
For a marine engineer, understanding these systems is not just about theory. PID control and PLC logic are encountered regularly in the engine room, automation systems, boiler control, pumps, compressors and propulsion machinery.
PID stands for Proportional, Integral and Derivative.
A PID controller is a feedback control system that continuously compares the desired value (set point) with the actual measured value (process variable) and calculates how much corrective action is required.
The difference between the desired and actual value is called the error.
For example, suppose the desired boiler drum water level is:
Set Point = 60%
But the actual measured level is:
Process Value = 55%
The controller detects a 5% error and sends an appropriate output to the feed-water control system.
This process continuously repeats:
This closed-loop operation allows the system to automatically maintain the desired operating condition.
The uploaded reference describes PID as one of the most widely used forms of feedback control and notes that PID controllers are used across control applications, including complex automation systems.
The PID controller consists of three control actions.
The proportional component responds to the present error.
The larger the error, the greater the controller’s corrective output.
For example:
If the boiler water level falls significantly below the set point, the proportional action increases the feed-water valve opening.
Simple idea:
More error = More corrective action
However, proportional control alone normally cannot completely eliminate steady-state error.
The reference material notes that proportional control can reduce rise time and reduce steady-state error, but does not necessarily eliminate it.
Integral action responds to the accumulated error over time.
Even if the error is small, if it continues for a period of time, the integral component keeps increasing the corrective action until the steady-state error is removed.
Simple idea:
Error continuing over time = Increasing correction
This makes integral control particularly useful when the system needs to return precisely to its set point.
However, excessive integral action can negatively affect the transient response and may contribute to overshoot or oscillation.
Derivative action responds to the rate at which the error is changing.
It essentially asks:
“How quickly is the error changing?”
If the system is approaching the set point very rapidly, derivative action can reduce the controller’s response and help prevent excessive overshoot.
Simple idea:
Fast-changing error = Anticipatory correction
According to the reference material, derivative control can improve stability, reduce overshoot and improve transient response.
Imagine a ship’s cooling-water temperature needs to be maintained at:
Set Point = 80°C
The temperature sensor measures:
Actual Temperature = 75°C
The PID controller detects the error:
Error = 80 − 75 = 5°C
The controller then determines the appropriate output based on:
The controller then sends an output to an actuator such as a control valve.
The valve position changes the cooling/heating process, the temperature changes, and the sensor sends the new value back to the controller.
This happens continuously.
PLC stands for Programmable Logic Controller.
A PLC is an industrial computer designed to control machinery and processes.
Instead of using large amounts of conventional hard-wired relay logic, a PLC can use programmable logic to monitor inputs, execute a control program and operate outputs.
A simplified PLC system looks like:
Sensors → PLC Inputs → PLC Program → PLC Outputs → Actuators
This makes PLCs extremely useful in shipboard automation.
No. This is one of the most important concepts for cadets to understand. A PID controller is a control algorithm, whereas a PLC is a programmable control device/system.
A PLC can execute many different types of logic, including:
Therefore, a PLC can contain a PID control function as part of its program.
PLC = The platform/controller
PID = One of the control algorithms running within the system
One of the most important marine applications of process control is boiler drum water-level control.
A boiler drum is not as simple as maintaining a fixed water level using only one sensor.
The reference material explains the swell and shrink effect that occurs because changes in steam demand affect steam bubbles and the apparent water level inside the drum.
When steam demand suddenly increases, drum pressure can fall. Water flashes into steam, increasing the volume of steam bubbles.
The apparent drum level may therefore rise, even though the actual quantity of water is decreasing.
This is called Swell
If a simple level controller sees the rising level and reduces feed-water flow, it could make the actual situation worse.
When steam demand decreases, pressure and steam-bubble conditions change.
The apparent drum water level can fall.
This is known as Shrink
A simple level-control system could respond incorrectly if it only considers drum level.
This is why more advanced boiler control systems use multiple measurements.
A three-element control system uses:
These measurements provide the controller with much more information about what is actually happening inside the boiler. The reference material explains that incorporating steam flow and feed-water flow helps overcome the problems associated with swell and shrink.
This is a great example of how control engineering theory becomes practical marine engineering.
Ships operate continuously and often under changing loads.
A vessel may experience:
Manual adjustment of every parameter would be inefficient and potentially unsafe.
Automatic control allows machinery to respond continuously to these changes.
PID control provides:
The performance of a PID controller depends heavily on its tuning parameters:
Kp — Proportional Gain
Ki — Integral Gain
Kd — Derivative Gain
Changing these parameters affects characteristics such as:
The reference material specifically notes that the effects of changing Kp, Ki and Kd are interdependent, so the summarized relationships should be treated as a guide rather than an absolute rule.
PID stands for Proportional, Integral and Derivative. It is a feedback control method used to maintain a process variable close to its desired set point.
A PLC (Programmable Logic Controller) is a programmable industrial control device, while PID is a control algorithm. A PLC can execute PID control along with logic, sequencing, alarms and interlocks.
PID control can be used for applications such as boiler water-level control, temperature control, pressure control and flow control, depending on the ship’s automation system.
It is a boiler control system that uses three measurements: drum water level, steam flow and feed-water flow. It helps address the swell and shrink effects associated with changes in steam demand.
Proper tuning of Kp, Ki and Kd helps achieve the desired response, stability and accuracy. Poor tuning can result in excessive overshoot, oscillation or slow system response.
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