Control loop monitoring in SCADA systems - Step-by-Step Execution

Process Flow - Control loop monitoring

Start Monitoring

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Read Sensor Value

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Compare to Setpoint

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Calculate Error

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Adjust Control Output

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Send Output to Actuator

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Log Data & Status

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Check for Alarms

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Repeat Loop

This flow shows how a control loop continuously reads sensor data, compares it to a target, adjusts outputs, logs status, and repeats.

Execution Sample

SCADA systems

while True:
  sensor = read_sensor()
  error = setpoint - sensor
  output = controller(error)
  send_output(output)
  log_status(sensor, output)
  if alarm_condition(sensor):
    break

This code continuously monitors a sensor, calculates error, adjusts output, logs status, and stops if an alarm triggers.

Process Table

Step	Sensor Value	Setpoint	Error (Setpoint - Sensor)	Control Output	Alarm Check	Action
1	50	55	5	Increase Valve Opening	No	Send output to actuator, log data
2	53	55	2	Slight Increase	No	Send output to actuator, log data
3	55	55	0	Maintain	No	Send output to actuator, log data
4	58	55	-3	Decrease Valve Opening	No	Send output to actuator, log data
5	60	55	-5	Strong Decrease	Yes	Trigger alarm, stop loop

💡 Alarm condition met at step 5, loop stops monitoring

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
sensor	N/A	50	53	55	58	60
error	N/A	5	2	0	-3	-5
output	N/A	Increase Valve Opening	Slight Increase	Maintain	Decrease Valve Opening	Strong Decrease
alarm	False	False	False	False	False	True

Key Moments - 3 Insights

Why does the control output change even when the sensor value is close to the setpoint?

Because the error is still non-zero (see steps 1 and 2 in execution_table), the controller adjusts output slightly to fine-tune the process.

What causes the loop to stop monitoring?

At step 5, the alarm condition becomes true due to sensor value exceeding limits, triggering the loop to stop as shown in the exit_note.

Why is logging important in control loop monitoring?

Logging records sensor and output values each step (see Action column), helping operators track system behavior and diagnose issues.

Visual Quiz - 3 Questions

Look at the execution_table, what is the error value at step 3?

A 0

B 2

C -3

D 5

💡 Hint

Check the 'Error (Setpoint - Sensor)' column at step 3 in the execution_table.

At which step does the alarm condition become true?

A Step 2

B Step 3

C Step 5

D Step 4

💡 Hint

Look at the 'Alarm Check' column in the execution_table to find when it changes to 'Yes'.

If the setpoint was increased to 60, what would happen to the error at step 5?

A Error would be 5

B Error would be 0

C Error would be -5

D Error would be 10

💡 Hint

Error = Setpoint - Sensor; check sensor value at step 5 and calculate with new setpoint.

Concept Snapshot

Control loop monitoring:
- Continuously read sensor values
- Calculate error = setpoint - sensor
- Adjust control output based on error
- Send output to actuator
- Log data and check alarms
- Repeat until alarm triggers

Full Transcript

Control loop monitoring is a continuous process where a system reads sensor values, compares them to a target setpoint, calculates the difference called error, and adjusts outputs to keep the process stable. Each cycle logs data and checks for alarms. If an alarm condition occurs, monitoring stops to prevent damage or unsafe conditions. This ensures the system stays within desired limits by constantly correcting itself.