常常在一个用模拟调节器组成的混合加料系统中,其中某个物料的供料可能不足,以至使得它的调节阀开得很大,甚至全开还会不能满足需要。在这种情况下,这个调节回路已失去控制,系统将偏离规定的混合比。这个问题可以用二个单元的选择性控制系统来解决。如图所示,单元1是一个阀位调节器,它的手动给定是在某个固定的高的数值(多半是95％)。单元2是一个主物料流量调节器,它也有一个手动给定,是放置在所期望的主物料流量值上。在图中所见到的所有调节阀都是气开的。 Often in a mixed feed system with an analogue regulator, the feed of one of the materials may not be enough so that its regulating valve can be widened or even fully opened. In this case, the control loop is out of control and the system will deviate from the specified mixing ratio. This problem can be solved with a two-unit selective control system. As shown, Unit 1 is a valve positioner whose manual reference is at a fixed, high value (mostly 95%). Unit 2 is a master material flow regulator, which also has a manual reference that is placed on the expected value of the master material flow. All of the control valves seen in the picture are open.