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PLC programming challenges
Challenge 10 · Find the fault · Pro

One scan too late

The pump command outlives a trip by one scan.

In Auto (Run ON), PumpOn runs the pump while it is healthy: no Fault, and Permit ON. A Fault or a lost Permit must turn PumpOn OFF on the very it happens. Find why it is one scan late and fix it.

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The 3D pump skid this challenge runs on Pro challenge
Test replay

Operator report: the trip scan

Test 1 of 3
Set by the testChecked by the testStep 3 of 3 · 180 ms
Level
Medium · 20 points
Kind
Find the fault
Time
About 10 min
Pass rate
New
Machine
3D pump skid
Siemens
LAD and SCL
Rockwell
Ladder and Structured Text
Beckhoff
Structured Text
The operator’s report · pump skid
Every time the motor trips, the drive logs “run command present during trip”. The trend shows PumpOn still ON for one scan after the trip. Same thing when the outlet valve closes.
Automatic checks

What the checks test

  1. Run turns PumpOn ON while there is no Fault and Permit is ON.
  2. A Fault turns PumpOn OFF on the same scan.
  3. Losing Permit turns PumpOn OFF on the same scan.
  4. Turning Run OFF stops the pump.

The checks run 3 tests on the machine, scan by scan: Operator report: the trip scan; Losing Permit stops it on the same scan; Run starts and stops it.

On site

Where you’d find this

A pump on a variable speed drive that logs every run command it receives during a trip.

Drives and safety relays react within milliseconds. A command that lingers one scan after a trip can fault the drive or upset a safety function.

The machine

How it should behave

Run
Auto selected: run while healthy.
Healthy
No Fault, and Permit ON.

Pause, switch Run ON and step a few scans. Then switch Fault ON and press Step scan once: PumpOn must already be OFF.

Keep going

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