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三取二是指开关量控制时的投票制，同一信号采用三冗余信号进行表决，二个满足即认为条件满足。三取二是为了判断的准确、可靠，防止误动，常用于保护回路。三取中是指模拟量信号采用三冗余输入，取其中值为实际使用值，常用于主要调节回路。一般，三个同一位置的测量点出现偏差过大时，就要查清楚问题，并安全、快速的处理掉，万万不可抱侥幸心理。除了取中，一般功能块还提供选择：高选、低选、平均或三个中的一个。三取均也是针对模拟量，三个数取平均值，能有限克服系统误差，但仪表故障造成随机误差难以避免。
二、区别
开关量采用“三取二”，主要是保护信号的取法。
模拟量采用“三取中”或“三取均”，一般用于控制调节回路，但几个模拟量“取均”原则上不能用于重要的调节或保护回路。
三、逻辑组态中的一些注意事项
3.1、三取中设计时应该做到：
1、正常时取中间值2、当出现一个点为坏点或偏差超限时自动剔除坏点变为剩余二点取平均3、一个设计良好的逻辑是能够做到在指定单点值、中值、平均值间任意指定的。三取中在调节系统中还是用的最多，可以做成三个信号相互作偏差检查，剔除其中一个不正确的信号，剩余两个信号取均值，如果再坏一个，自动取第三个信号，如果全坏，就应该保持正常输出时的值，同时发出报警。可以加入判断，当三信号全正常时，取三个平均值，当有一信号不正常时取二个数的平均值，当两个信号不正常时，取另一个正常的数值。这种方法在自动调节中应用属于最广泛的。
3.2、三取二，一般用于保护取样信号的逻辑判断，为确保信号的可靠性，可以加入质量判断、切除等逻辑。
对于三选的动作保护，一般来说是比较重要的信号采用，不是连锁保护就是调节控制。所以会加上一些判断条件和切除条件。逻辑设计中常常需要可切换的两组甚至三组保护逻辑。三取二、三取中等，这些可能在一个保护逻辑中用到。
专栏：行业资讯
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