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Mission 51 · Pro PLC programming exercise

21 °C, fan proven, coil safe

Start an air handler in order and write its PI temperature loop.

Start the unit in order with the coil warmed, prove airflow within 10 s, hold the supply air at 21 °C with a PI loop that cannot wind up, and stop safely on frost.

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The 3D air handling unit this mission runs on Pro mission
Test replay

Starts in order, then PI

Test 1 of 5
Set by the testChecked by the testStep 8 of 8 · 2.1 s
Level
Advanced
Time
About 50 min
Reward
+600 XP
Rig
3D air handling unit
Siemens editors
SCL, LAD
Rockwell editors
Structured Text, Ladder
Course stage
28. Machine lab 3: measurements and batches
Automatic tests

What the tests check

  1. Damper first, with the valve at 100 % to warm the coil; the fan only at DamperOpen.
  2. Airflow not proven within 10 s, or lost while running, latches FanFail.
  3. PI control with Kp, Ki, and the integral held between 0 and 100.
  4. Frost: fan off, damper closed, valve 100 %, until Reset.
  5. Unoccupied: all off, valve closed.
  1. Try it Use the buttons below the model and watch the rig respond.
  2. Test solution Every check runs on your code in one press.
  3. Replay If one fails, step through it one scan at a time to see why.
Why it matters

Where you’d see this on site

Building management systems run this on thousands of AHUs; chemical plants use the same PI loop and anti-windup.

A fan started against a shut damper can implode ductwork. A coil that freezes splits and floods the plant room. A PI loop without anti-windup swings the temperature for minutes after every disturbance.

Where you'd find thisFresh air at 21 °C, without freezing the coil

Every office, hospital and factory has air handling units on the roof or in a plant room. The controller starts them in a safe order, proves the fan is really moving air, and holds the supply temperature with a PI loop.

  1. DamperOpens first
  2. FilterCleans the air
  3. CoilPI heatingYou are here
  4. FanProven by airflow
On a real site: places, parts and safety
  • Offices, schools and hospitals
  • Factories and warehouses
  • Clean rooms and laboratories

Fan starts are interlocked to the damper: a fan against a closed damper can collapse ductwork.

Frost protection overrides everything: stop the fan, close the damper, open the valve.

The sizes, timings and cutaways are teaching choices, not a drawing of any one real installation.

Keep going

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