Tuning Cascade Loops
One of the most common questions we are asked is, "I have a cascade loop, how do I tune it?" The simple answer is to tune the inner loop first and then tune the outer loop.
Typically, it is not the tuning of the PID loop that is an issue as much as it is a lack of understanding of what the cascade structure is and how it is designed to control a loop. The tuning of a cascade loop is quite simple as long as the concepts of the cascade loop are understood
Cascade Loop Application
To understand this, let's look at a common cascade loop application.
Figure 1. Standard PID Control of Tank Outlet Temperature
Figure 1 shows a tank that is heated via a steam valve. The ultimate goal is to control the outlet temperature T1 via the steam valve. We can attach a standard PID controller to this loop to control the temperature as shown. In this example, at least two types of disturbances can affect the process:
Inlet Disturbances - Fluctuations in the Infeed flow or Infeed temperature can cause disturbances to T1.
Supply Flow Disturbance - Fluctuations in the steam supply flow can cause a disturbance to T1 & T2
Steam Pressure Flow Variations With Simultaneous Temperature Control
We cannot tune our PID fast enough to react to both the steam pressure flow variations and control temperature at the same time. The fluctuation in the steam happens very quickly; the PID often does not react quickly enough to compensate for the flow disturbance.
A common way to resolve this problem is to use a cascaded PID configuration as shown in Figure 2.
Figure 2. Cascaded PID Control of Tank Outlet Temperature
PID 1 can be referred to as the Master, Primary, or Outer Loop.
PID 2 can be referred to as the Slave, Secondary, or Inner Loop.
Essentially what we have done is take our process response and split it into two pieces, a fast piece [G2(s)] and a slower piece [G1(s)]. Note that we still have only one control valve.
The temperature controller (PID 1) now determines the desired amount of flow to control the temperature. Instead of controlling temperature directly, the valve is now controlling the flow of the steam to the process from PID 2.
What does this do for our control? PID 2 can be tuned quickly and can respond to the flow disturbances. PID 2 will minimize any fluctuation disturbances in the steam supply. PID 1 still controls temperature, and is tuned relative to the temperature process, which is slower.
The benefit of using the cascade configuration is that we can minimize our fast process disturbances, and still control our temperature as best as possible with a PID. A disadvantage is that we now have 2 PID's to tune instead of the one PID as in our original configuration.
To learn more about tuning cascade loops, visit the Applications Section of our website and request TECH 16. For additional information on handling different tuning problems, select from the technical documents under "PID Tuning."
For a free copy of PID Loop Tuning Tips Pocket Guide, visit us on the web at www.controlsoftinc.com
