Entry & progression
Automation does not remove the engineer. It removes the engineer's eyes. The plant now tells you what it thinks is happening. The ETO is the person who can tell whether it is right.
The control loop
- A sensor measures the process variable — temperature, pressure, level, speed
- A transmitter converts it to a signal the controller understands
- A controller compares it with the setpoint and computes an output
- A final control element acts — a valve, a drive, a contactor
- The process responds, and the sensor measures the result. The loop closes.
Every fault in a loop is in one of those five places, plus the wiring between them. That is a very short list, and it is why loop faults should never be a mystery. The discipline is to establish which element lied, not to guess which one to replace.
PID, honestly
| Term | Acts on | Too much gives you |
|---|---|---|
| Proportional | The present error | Oscillation, hunting |
| Integral | The accumulated error over time | Slow overshoot; wind-up after a saturation |
| Derivative | The rate of change of error | Amplified noise; twitchy output |
Proportional alone leaves a standing offset — the loop settles near the setpoint, not on it. Integral removes the offset but adds lag. Derivative anticipates, and faithfully amplifies every bit of electrical noise on the signal. Most marine loops are PI. Derivative is used sparingly and on clean signals.
TIP — Before you tune, check the loop is not lying to you A large proportion of 'tuning problems' are instrument problems. A sticking valve, a fouled sensor, a transmitter drifting out of calibration, a loose terminal — all of these look exactly like a badly tuned loop from the screen.
Confirm the measurement is true and the final element actually moves where it is told. Then touch the gains. Tuning around a mechanical fault produces a loop that is broken in two ways.
PLCs and where they hide
Programmable logic controllers run the sequences: engine safety and start logic, fuel and lube transfer, bilge and ballast, HVAC plant, fire dampers, thruster and stabiliser permissives. A PLC scans its inputs, executes its program, and writes its outputs — thousands of times a minute, identically, forever.
It is a machine that does exactly what it was told. When a system 'behaves strangely', the PLC is almost never wrong; the input it is reading is wrong, the interlock it is honouring is one you have forgotten, or the logic was written for a plant that has since been modified.
NOTE — Software is a maintained item Programs, setpoints and drive parameters are part of the plant, and they are the part with no spare in the store. Keep a current backup of every PLC program and every drive parameter set, keep it off the vessel as well as on it, and record every change — what, when, why, by whom.
A controller that fails on a Sunday with no backup is not an electrical fault. It is a project.