ONE OF the most common questions we get asked is, "I have a cascade loop, how do I tune it?” 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 and how the cascade structure 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.
In order to understand this further, let us look at a common cascade loop application.
FIGURE 1 – Standard PID Control of Tank Outlet Temperature
Figure1 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. For this example, two types, among many, of disturbances that 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 pressure causes flow to fluctuate which cause a disturbance to T1
The PID can only be tuned as fast as the process will allow. There is a significant amount of time (both dead time and lag time) in the process. In controlling such process, the steam pressure changes which results in the reduction of steam flow, controller will not detect such pressure variation until it affects the temperature, this will take sometime, as it is a slow process. 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 taken our process response and split it into two pieces – a fast piece (G2(s)) and a slower piece (G1(s)). Note that we still only have one control valve. Instead of manipulating steam valve directly for temperature control, the valve is now controlling the flow of the steam to the process from PID 2. The temperature controller (PID 1) now determines the desired amount of flow to control the temperature.
What does this do for our control? PID 2 can be tuned relatively fast and can respond to the flow disturbances. PID 2 will minimize any fluctuation disturbances in the steam supply. PID1 still controls temperature, and is tuned relative to the temperature process, which is slower. The benefit of using the cascade configuration is that any disturbances within the inner (fast) loop, can be corrected (by PID2), without waiting for it to show up in the temperature loop (PID1), so that better (tighter and faster) temperature control can be achieved. A disadvantage is that we now have 2 PIDs to tune instead of the one PID as in our original configuration.
FIGURE 3 – Outline of PID Control compared to Cascaded PID Control
What does this mean to our configuration of the PIDs? In looking at Figure 3, one of the things we notice is that the CO output of PID 1 is feed directly into the Setpoint of PID 2. PID 1 is dependent on PID 2 being in control in order to control the temperature. In our control logic, we may want to ensure that PID 1 is put to track mode if PID2 is in manual control. Also, it is important to stop PID1 to increase (or decrease) if PID2 saturates, otherwise it will cause controller windup. Hence it requires some efforts to implement cascade control correctly. Some controller manufacturers have a Cascade PID block that is designed to handle the coordination logic required to run the cascade correctly.
Let’s put all this information together. The simple answer to our question “I have a cascade loop, how do I tune it?” is:
- The PID 2 must be tuned first before tuning can be done on PID 1. Often, a simple Proportional control is suffice
- PID 2 needs to be in control (typically AUTO mode) when tuning PID 1
That’s it! One more point we should also make. We mentioned before that the inner loop (PID2) is faster than the outer loop (PID 1). This condition must be true. If the inner loop is not faster than the outer loop, then the cascade will not offer any significant improvement in the process.
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