PLC

Designing 2 out of 4 Voting Logic in Control Systems: A Step-by-Step PLC Ladder Diagram Tutorial with Video

In this content, you’ll discover how to implement 2 out of 4 voting logic in control systems(PLC) through a comprehensive video tutorial.

Voting 2 out of 4 logic is a redundancy scheme utilized in control systems to enhance reliability. It operates by evaluating four input signals and generating a single output based on the majority vote of those inputs. Specifically, the output signal becomes active only when at least two of the input signals are active.

• Inputs: The voting logic requires four input signals (A, B, C, and D), each originating from distinct sources or sensors.
• Output: A solitary output signal (designated as Y) is produced based on the majority vote result of the inputs.
• Logic Gates: The inputs are connected to a series of AND and OR gates that implement the 2 out of 4 voting logic.
• Voting: The logic gates are arranged in such a way that the system requires at least two inputs to be in the same state (either high or low) to consider the input valid.

The truth table provides a breakdown of the relationship between the various combinations of inputs and the output that is produced:

To implement a 2 out of 4 (2oo4) voting logic using 6 AND gates and 4 OR gates, you can follow this logic diagram:

1. Label the inputs as A, B, C, and D.
2. Connect the inputs to the AND gates to create combinations that represent the condition where at least two inputs are true:
• AND Gate 1: A and B
• AND Gate 2: A and C
• AND Gate 3: A and D
• AND Gate 4: B and C
• AND Gate 5: B and D
• AND Gate 6: C and D
3. The outputs of these AND gates are then fed into the first set of OR gates to combine the conditions:
• OR Gate 1: Output of AND Gate 1 or Output of AND Gate 2
• OR Gate 2: Output of AND Gate 3 or Output of AND Gate 4
• OR Gate 3: Output of AND Gate 5 or Output of AND Gate 6
4. Finally, the outputs of these OR gates are fed into the last OR gate to produce the final output:
• OR Gate 4: Output of OR Gate 1 or Output of OR Gate 2 or Output of OR Gate 3
5. The final output of OR Gate 4 will be true if at least two of the inputs A, B, C, or D are true, fulfilling the 2oo4 voting logic.
• Take into consideration a situation that occurs within the scope of centrifugal gas compressors, where the operational integrity of such equipment is of the highest priority.
• Instrumentation that is not functioning properly could lead to unexpected downtime as well as severe production losses.
• Radial vibration monitoring has emerged as an essential component of compressor health, with high vibrations having the ability to indicate bearing wear or other underlying problems.
• Within this framework, the implementation of voting logic based on two out of four ensures strong and dependable monitoring of radial bearing vibrations, hence reducing the likelihood of unfavorable operational results.

The equivalent ladder logic diagram for 2 out of 4 voting logic comprises rungs that correspond to the input signals and their associated logic. Typically, each rung represents the combination of AND and OR logic gates necessary to implement the desired redundancy scheme effectively.

For a comprehensive understanding of how to implement 2 out of 4 voting logic using a PLC ladder diagram, we recommend referring to the accompanying video tutorial. This tutorial will guide you through the step-by-step process, providing visual demonstrations of creating the ladder logic and its subsequent deployment within a PLC programming environment.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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