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Electrical Troubleshooting Quiz, March 22, 2011

Your PLC-controlled process has recently begun to exhibit oscillations

Your PLC-controlled process has recently begun to exhibit oscillations. For example, a particular control valve that is normally in a steady position is now continually "seeking" plus or minus about 20% around the setpoint. Several batches have been ruined, and a shift supervisor is making loud noises about how you should "get a factory rep out here to kill the virus that's in the system." What are some first steps toward solving this technical problem and preventing a political one?

First, have an instrument tech check the entire control loop of the valve that's oscillating, including stroking the valve. While that's being done, take the supervisor for a walk. Thank him for his well-intended suggestion. Tell him he can be most helpful by making such suggestions only to the maintenance department. Perhaps make an analogy to the production process — just as production requires doing specific steps in a specific order, so does troubleshooting.

Solving the technical problem is really a matter of checking one control loop at a time. Start with those loops that relate to the oscillations. The first things to check include:

  • PLC power supply module outputs.
  • Sensor input cable integrity (look for inductive coupling).
  • Final control elements (valves, motors) and cabling/wiring.
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