PLC

# Designing 2 out of 3 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 3 voting logic in control systems through a comprehensive video tutorial.

Discover the concept of 2 out of 3 voting logic, its significance in fault-tolerant systems, and its application in ensuring reliability and accuracy in various control mechanisms.

• “Voting 2 out of logic” refers to a decision-making process where a certain action or outcome is determined based on the agreement of at least two out of three inputs.
• This logic is commonly used in fault-tolerant systems, control systems, and redundancy mechanisms to ensure reliability and accuracy.
• Inputs A, B, and C represent three independent signals, conditions, or measurements.
• The output is determined based on whether at least two of the inputs agree (are either high or low).
• We’ll use three AND gates, each taking two inputs. These AND gates represent the combination of each pair of inputs (A and B, B and C, A and C).
• The first AND gate takes inputs A and B, producing a high output only if both inputs A and B are high.
• The second AND gate takes inputs B and C, producing a high output only if both inputs B and C are high.
• The third AND gate takes inputs A and C, producing a high output only if both inputs A and C are high.
• The outputs of these AND gates are then connected to an OR gate.
• The OR gate combines the outputs of the three AND gates. It produces a high output if any of its inputs are high.
• Therefore, if at least two out of the three inputs (A, B, and C) are high, the OR gate will produce a high output, indicating a “2 out of 3” voting logic.
• The output of the OR gate is the final output of the circuit. If at least two inputs are high, the output will be high; otherwise, it will be low.

Explore a practical example involving the design of a control circuit for monitoring steam turbine control oil pressure. Learn how 2 out of 3 voting logic is utilized to activate an Emergency Shutdown (ESD) control trip to protect the turbine in case of low oil pressure detected by multiple pressure switches.

• Let’s design a control circuit for monitoring steam turbine control oil pressure using three pressure switches connected in a “2 out of 3” logic configuration.
• This setup ensures that if at least two out of the three pressure switches detect low oil pressure, an Emergency Shutdown (ESD) control trip will be activated to protect the turbine.
• Three pressure switches (Pressure Switch 1, Pressure Switch 2, and Pressure Switch 3) are installed to monitor the steam turbine control oil pressure.
• Each switch is normally closed and opens when the oil pressure falls below a certain threshold.
• The outputs of the pressure switches are connected to voting 2 out of 3 logic.
• The outputs of voting 2 out of 3 logic is connected to the Emergency Shutdown (ESD) control trip mechanism.
• If at least two pressure switches detect low oil pressure (as determined by the AND gate), or if any two pressure switch detects low oil pressure (as determined by the OR gate), the ESD control trip will be activated, initiating a shutdown of the turbine to prevent damage.

View the equivalent PLC ladder logic diagram for implementing 2 out of 3 voting logic, created using Schneider Electric EcoStruxure Machine Expert Basic Software. Follow along with the step-by-step process demonstrated in the accompanying video tutorial.

Access the video tutorial demonstrating the creation of the PLC ladder diagram for 2 out of 3 voting logic using Schneider Electric EcoStruxure Machine Expert Basic Software. Gain insights into each step of the process and enhance your understanding of implementing this logic in control systems.

### 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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