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PLC programming challenges
Challenge 08 · Find the fault · Free

Overflows by a drop

Fill to 80%. At exactly 80%, the valve stays open.

Start opens FillValve. Filling stops when Level reaches 80%, or when Stop is pressed. Find why the valve stays open at exactly 80% and fix it. A tank already at 80% or above must not start filling.

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The 3D process tank this challenge runs on
Test replay

Operator report: level held at exactly 80%

Test 1 of 3
Set by the testChecked by the testStep 5 of 5 · 380 ms
Level
Easy · 10 points
Kind
Find the fault
Time
About 6 min
Pass rate
New
Machine
3D process tank
Siemens
LAD and SCL
Rockwell
Ladder and Structured Text
Beckhoff
Structured Text
The operator’s report · process tank
The batch sheet says fill to 80%. On the test panel I held the level at exactly 80.0% and the inlet valve stayed open. On the real tank it overfills a little every batch.
Automatic checks

What the checks test

  1. Start opens FillValve while Level is below 80%.
  2. At exactly 80.0%, FillValve closes.
  3. Start does nothing at 80% or above.
  4. Stop closes FillValve.

The checks run 3 tests on the machine, scan by scan: Operator report: level held at exactly 80%; Start does nothing at 80%; Stop closes the valve.

On site

Where you’d find this

A batching tank fills to the level on the batch sheet before the next ingredient goes in.

A comparison one value out fills past the setpoint every batch. On a product tank that is waste; on a chemical tank it can be a spill.

The machine

How it should behave

Start
Opens FillValve below 80%.
80%
Reaching 80% closes it.
Stop
Closes it at any time.

Pause the simulation. In the Inputs tab set Level to 80 and press Start, then Step scan. FillValve must stay closed.

Keep going

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