文档介绍：Cascade Control
Handle Processes that Challenge Regular PID Control
Arthur Holland, Holland Technical Skills
In previous columns we have named lags in a process as major obstacles to good
temperature control. When they are inconveniently long e in multiple stages, first
try to determine where changes to process design can avoid or reduce lags. Then do your
best with PID control and if you fail to obtain the response you hoped for you can turn to
cascade control.
Let’s look at two kinds of lags that you encounter in the process heating industry.
Dead time
This is when an input occurs and you see no output until a time called the dead time has
elapsed, after which the output appears, unchanged except for the delay. This is often
associated with transport of material from one place to another.
An example is heating an air stream and sensing the temperature some distance
downstream. This is sometimes called transport lag or distance-velocity lag. Before
you consider an advanced control solution see if you can place the sensor close to the
heater to give the controller the earliest possible opportunity to respond.
First order lag.
This is also called an exponential lag. One example is how a temperature sensor
responds to a temperature change. First in a linear manner, then at an ever-decreasing
speed approaching the final value. Here agai