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 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.
"How Do I . . ." Contact Information |
For more information about tuning cascade loops, contact Paul J. Botzman, Marketing & Sales, ControlSoft Inc., Avion Park Dr., Highland Heights OH 44143. Phone: 440-443-3900 x101; fax: 440-443-0249; 5387.