How a Control Loop Flows
// This is how your plant thinks and reacts in real time — click any node to explore
PROCESS
75°F
1. What's
happening
happening
RTD signal
▶
TRANSMITTER
11.2mA
2. Converts
to 4-20mA
to 4-20mA
4–20mA
▶
CONTROLLER
PID
3. Makes
decision
decision
4–20mA
▶
I/P
POSITIONER
POSITIONER
9 PSI
4. Converts
to air
to air
3–15 PSI
▶
CONTROL
VALVE
VALVE
50% open
5. Controls
flow
flow
flow changes
▶
PROCESS
CHANGES
CHANGES
→ 80°F
6. Process
responds
responds
FEEDBACK →
← Transmitter reads new value → loop repeats constantly
// STEP 1 OF 6
Process — The Physical Reality
The process is whatever you're trying to control — temperature, pressure, level, flow. A sensor (RTD, thermocouple, pressure element) is in physical contact with the process and measures what's actually happening right now.
At this point: Physical value → Sensor → Raw electrical signal (millivolts or ohms)
// TROUBLESHOOTING TIPS — FROM THE TRAINING NOTES
CHECK FIRST
Check 24V to transmitter — if no power, no signal
CHECK SECOND
Confirm mA signal isn't stuck — measure in series
CHECK THIRD
Verify air to I/P is live — check supply PSI at positioner
CHECK FOURTH
Look for I/O card signal lights — confirms PLC is seeing/sending signal
// TRACE A REAL LOOP — DO THIS IN YOUR PLANT
1
Find a transmitter in the plant
Pick any transmitter — level, temperature, pressure. Read the tag number on the nameplate (e.g., TT-101). This is your starting point.
↓
2
Trace wires to PLC or controller
Follow the two wires from the transmitter back to the panel. Find which analog input card terminal they land on. Note the card number and channel.
↓
3
Find the control valve it affects
In the PLC program or on the P&ID, find which control valve is in the same loop as your transmitter. The loop tag number ties them together (e.g., TT-101 controls TV-101).
↓
4
Identify the I/P and air lines
Find the I/P positioner on the control valve. Trace the 4-20mA wires from the PLC output card to the positioner. Find the air supply line and output line to the actuator.
↓
5
Sketch and label the signal flow
Draw it: Transmitter → wires → PLC AI card → PLC AO card → wires → I/P → air → actuator → valve → process → transmitter. Label every signal type: mA, PSI, mechanical. Now you own that loop.