PLC系统

Alex, we need the control logic for the cooling water pump station — three pumps, duty-standby-standby configuration. The pumps should auto-rotate every 168 hours to even out the wear. And if the duty pump trips, the standby pump must start automatically within 3 seconds.

Got it. Three pumps — P-301A, P-301B, P-301C. I'll write this in ladder logic first, then we can discuss the sequence. Carla, what are the start permissives?

Five permissives: suction valve open, discharge valve open, cooling water basin level above 2.0 meters, no motor overload, and no emergency stop active. If any permissive is lost while running, the pump must trip and an alarm must be generated. Also, minimum run time is 30 minutes — we don't want short-cycling.

(typing on the programming workstation) Alright, let me structure this. I'll create three function blocks: FB_PumpControl, FB_AutoSequence, and FB_AlarmHandler. FB_PumpControl handles the individual pump — permissives, start/stop, run feedback, trip logic. FB_AutoSequence handles the duty selection, rotation timer, and auto-start on trip. FB_AlarmHandler generates the alarm tags.

Walk me through the auto-start sequence. If P-301A is running and trips, what exactly happens?

(displaying the ladder logic) Step 1: P-301A run feedback drops. The PLC detects this as "duty pump stopped unexpectedly." Step 2: A 2-second debounce timer starts — this filters out transient signal drops that aren't real trips. Step 3: If run feedback is still missing after 2 seconds, the PLC sets P-301A as "faulted" and immediately sends a start command to the next pump in the sequence — let's say P-301B. Step 4: P-301B starts. If its run feedback doesn't come back within 5 seconds, it's marked as "failed to start" and P-301C gets the start command. Step 5: Once a pump is running successfully, an alarm is raised for the tripped pump so maintenance knows to investigate.

What about the rotation timer? Does it reset if a pump trips and a standby starts?

Good question. The rotation timer only runs when the system is in normal operation — duty pump running, no alarms. If a trip occurs and a standby takes over, the rotation timer resets. The idea is: once the system returns to stable operation, we want the newly running pump to run for a full rotation cycle before switching again. This prevents rapid switching that could mask an underlying problem.

I like it. One more thing — the HMI needs to show which pump is currently duty, which is standby, total run hours per pump, and the time until next rotation. Can we do that?

Absolutely. I've mapped the internal tags: PUMP_DUTY_NUMBER for the current duty pump, PUMP_RUN_HOURS array for each pump's accumulated hours, and ROTATION_TIME_REMAINING for the countdown. These all read directly into the HMI interface. Tom, when we do the SAT, we'll test all the HMI display tags as part of the functional test.

Sven, we lost a bunch of field signals — all the instruments on the raw water intake system. The HMI is showing "???" for all readings, and the intake valve won't respond to commands. The operators are running blind. We need this fixed — the plant can't run without the raw water system.

(checking the engineering station) The PLC diagnostic buffer shows a fault on Rack 2, Slot 4 — that's the AI-08 module, an 8-channel analog input card. Status is "module failed — hardware fault." That explains why we lost all eight signals — the entire card went down. This is a critical module — it reads the raw water flow, pressure, level, and the intake valve position feedback.

Let me check the hardware. (opens the PLC cabinet) The module's fault LED is solid red — no blinking. That means it's a permanent fault, not a recoverable error. Power LED is off. Let me measure the backplane power... (uses multimeter) 5.1 volts on the backplane — that's within spec. So power to the rack is good, the module itself is dead. We need to replace it.

OK. We have a spare AI-08 module in the warehouse. Rita, go grab it. Klaus, I need you to bypass the raw water intake interlock in the control logic so the downstream processes don't trip while we swap the module. Can you do that from the HMI?

Give me two minutes — I'll put the intake system in "maintenance override" mode. The downstream pumps will keep running on the water already in the system. We've got about 15 minutes of buffer before the storage tank drops below minimum. So let's move fast.

(returning with the replacement module) Got it. Part number matches — AI-08, Rev 3.2. Sven, do we need to power down the rack, or is this hot-swappable?

This model supports hot-swap, but I'm going to play it safe. Let me first save the current module configuration — the channel settings, scaling, alarm limits. (uploads config from the PLC) Config saved. Now, remove the failed module. Gentle — don't force it. The backplane connector has guide pins.

(carefully removing the module) Old module out. Inserting the new one... click — it's seated. Power LED is green, OK LED is flashing green — that means it's initializing. Now it's steady green. Module is healthy.

Downloading the saved configuration to the new module... done. Let me verify the channel values. Channel 1 — raw water flow, 4.02 mA, that corresponds to 12.5 m³/h on the calibrated range. Channel 2 — intake pressure, 8.15 mA, reading 2.8 bar. These values look normal. HMI should be updating now. Klaus, what do you see?

(at HMI screen) All readings back to normal! Intake valve position feedback is live again. I'm removing the maintenance override. System is back to automatic. Total downtime: 11 minutes. Excellent work, team.

Rita, tag the failed module with the date, failure description, and send it back to the vendor for root cause analysis. We want to know why an 18-month-old module failed. If it's a manufacturing defect, we might have more of these in the plant and we need to plan proactive replacement.