Troubleshooting PLC Permissive Inputs

A engineer needs to write a PLC program to control a water pump driven by an electric motor. This water pump will be manually started and stopped by pushbutton switches, and shut down automatically by any one of several “permissive” switches.

Troubleshooting PLC Permissive Inputs

The operating statuses of these switches are listed here:

  • Start pushbutton (normally-open): open when unpressed, closed when pressed
  • Stop pushbutton (normally-closed): closed when unpressed, open when pressed
  • Low water level (normally-closed): closed when level is low, open when level is adequate
  • Low oil pressure (normally-open): open when pressure is low, closed when pressure is adequate
  • High vibration (normally-closed): closed when still, open when vibrating
  • Water leak detector (normally-open): open when dry, closed when wet (leak detected)

The engineer’s first attempt is shown here, but it contains a serious error. Identify and correct this error:

Troubleshooting PLC Permissive Inputs

Answer :

The Vib_high contact instruction should be drawn as normally-closed rather than normally-open as shown in the engineer’s first draft of the PLC program.

The program is designed to shut down the motor if ever the Shutdown bit goes to a 1 state. This means the motor will shut down if any of the permissive contact instructions become colored (i.e. “conductive” to virtual power).

We have been told that the realworld vibration switch is NC, which means its contact opens when vibration is excessive. This means high vibration causes that bit to be 0, which necessitates an NC contact instruction so that it will color under that condition.

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