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Troubleshoot Allen Bradley PLC Logic

A engineer needs to write a program for an Allen-Bradley CompactLogix PLC to control a pressure relieving solenoid valve in a gas processing system.

A pair of high-pressure control switches signals the PLC when to open the solenoid valve: one telling the PLC to open the valve after a 3-second time delay and the other (called the “high-high” switch, with a higher trip setting) telling the PLC to open the valve immediately.

A pushbutton switch serves as a manual override to open the solenoid valve immediately when pressed. In all cases, the solenoid vent valve will remain open (energized) until pressure falls below the setting of a low-pressure gas switch.

The operating statuses of these switches are listed here:
  • Override pushbutton (normally-open): open when unpressed, closed when pressed
  • Low gas pressure (normally-closed): closed when pressure is less than 10 PSI, open when pressure exceeds 10 PSI
  • High gas pressure (normally-closed): closed when pressure is less than 30 PSI, open when pressure exceeds 30 PSI
  • High-high gas pressure (normally-open): open when pressure is less than 40 PSI, closed when pressure exceeds 40 PSI

Allen Bradley PLC Logic

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

Troubleshoot Allen Bradley PLC Logic

Answer :

The High pressure contact instruction should be drawn as normally-closed rather than normally-open as shown in the engineer’s first draft of the PLC program. This particular switch signals a high-pressure condition by opening its contacts, since the real-world switch is normally-closed (NC).

We want the 3-second timer to begin timing when this switch senses a high pressure, and so we need the contact instruction to color when the real-world high-pressure switch trips (opens). A PLC contact instruction that colors when it senses a “0” bit condition is a normally-closed (NC) instruction.

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