What are triple majority, triple average, and triple two-out-of-three in a DCS system?
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In a redundant configuration, \"two-out-of-three\" refers to the voting system used in digital control; the same signal is evaluated using three redundant signals, and the condition is considered met if two of them satisfy it. Taking two out of three is intended to ensure accuracy and reliability in judgment, to prevent false operations, and is commonly used in protection circuits. “\"Three-out-of-three selection\" refers to the use of three redundant inputs for analog signals, with the middle value being taken as the actual value to be used; this approach is commonly employed in primary control loops. Generally, when there are excessive deviations among three measurement points at the same location, it is necessary to identify the problem and address it safely and quickly; one must never rely on luck. In addition to the median value, standard function blocks also offer the option to select: high value, low value, average, or one of the three. “\"Average of three readings\" is also applied to analog values, by taking the average of three numbers; this can help to some extent overcome systematic errors, but random errors caused by instrument failures are difficult to avoid. The difference between them is that for digital signals, a \"three-out-of-two\" principle is used, primarily regarding the way in which the protection signals are selected. Analog quantities use the \"median of three\" or \"average of three\" method, and are generally used in control and regulation circuits; however, the average of several analog quantities should in principle not be used in critical regulation or protection circuits. Precautions in logical configuration 01 For three-way selection, ① the middle value is taken under normal conditions ； ②When a point is identified as defective or the deviation exceeds the limit, it is automatically removed, and the average of the remaining two points is taken ； ③A well-designed logic is capable of performing calculations between any arbitrarily specified single value, median, or average. The three-way selection method is still the most commonly used in control systems. It allows for the comparison of three signals to identify any incorrect ones; the average of the remaining two signals is then taken. If another signal becomes faulty, the third signal is automatically used. If all signals fail, the value corresponding to normal operation is maintained, along with an alarm signal 02. For the three-average method, a decision logic can be applied: when all three signals are normal, the average of the three is used; when one signal is abnormal, the average of the two remaining signals is used; and when two signals are abnormal, the value of the remaining normal signal is used. This method is the most widely used in automatic regulation. 03 For two-out-of-three voting, it is generally used for logical judgment to protect the sampled signal; to ensure the reliability of the signal, logic such as quality assessment and rejection can be incorporated. For motion protection in three-choice scenarios, generally speaking, it is important to use appropriate signals, either for interlock protection or for regulation and control. So some judgment conditions and removal conditions will be added. In logical design, it is often necessary to have two or even three sets of switchable protection logic. Two-out-of-three, majority voting – these may be used in a protection logic.
In DCS (Distributed Control Systems), triple majority voting, triple equal voting, and triple two-thirds voting are common signal processing methods used to improve the reliability and accuracy of the system. Below is a detailed explanation of these three methods: 1. **2-out-of-3 Voting**: - This is a method for processing digital signals, commonly used in protection circuits. - The system collects three redundant inputs of the same signal, and then determines the final signal state based on the majority voting principle (i.e., at least two signals being identical). - This method can effectively prevent malfunction caused by single-point failures. 2. **Median Select**: - This method is used to process analog signals, especially in the main control loops. - The system also collects three redundant signals, but it processes them by selecting the middle value among the three as the final output. - When one of the three signals shows an anomaly, the abnormal signal is usually automatically excluded, and the average of the remaining two signals is used. - This method helps reduce random errors and improve the stability and accuracy of the signal. 3. **Average Select**: - This is also a method for processing analog signals, in which the averages of three signals are taken. - This method can smooth out some random fluctuations, providing a more stable signal output. - However, if one of the instruments fails, it may cause the output value to deviate from the true value. These three methods each have their own advantages and application scenarios; the choice of the appropriate method depends on the specific system requirements and the importance of the signal. When designing the logic, it is necessary to consider the importance of the signals, possible types of failures, and the overall reliability requirements of the system. .