In instrument design, what is the difference between two-of-three selection and median selection, and where are they used respectively?

Choosing two out of three is generally used in interlock logic, and is often applied in SIS systems; it improves availability while ensuring reliability. It is typically used in SIL2 applications or in situations with special requirements. Choosing the middle value out of three is more often used in control systems, in order to obtain a more reasonable measurement value and thus achieve smoother control. Averaging values is also used for similar purposes; all these methods are related to safety and control – with process operations being the core focus

Two-out-of-three voting primarily addresses hardware reliability issues, while majority voting addresses data reliability issues.

The main considerations are hardware reliability and data types. In the case of processing boolean data, that is, True and False – a choice between two options – the result of the two votes becomes the final outcome. When dealing with floating-point data, such as the values measured by pressure sensors, it is possible that all three sensors may provide different results. In such cases, deciding which two of these results to use depends on the voting logic in place. If a specific voting logic is designed, for example, to select the two values that are closest to each other and average them as the output, then it’s clear what to do when three values are available: simply choose the middle value. For example, when measuring an actual pressure of 1 Mpa, the three sensors report values of 0.96, 0.99, and 1.01 respectively. In this case, a majority vote among the three may result in a value of 1.0, while if all three agree, the output will be 0.99. Similarly, when the sensor readings are 0.95, 0.97, and 0.99, a majority vote might yield 0.96, whereas if all three agree, the output will be 0.97. In the event of faults, with readings of 0.96, 0.98, and 0, a majority vote might result in 0.97, while if all three agree, the output will be 0.96. It is difficult to say which method yields a value closer to the accurate value under different circumstances; however, the logic behind a majority vote is relatively simple, as it only requires comparison, whereas a majority decision might involve calculations. To summarize: for logical data types, it’s a choice of two out of three; for floating-point data types, usually all three are selected, but a choice of two out of three is also possible