Advanced PID Architecture for Improving Disturbance Rejection and Enhancing System Performance
As they say: Timing is everything. The lag between upset and response directly impacts a system’s performance along with the performance of other processes with which it interacts. Fortunately, there are a variety of approaches available that leverage the PID’s capabilities. These approaches improve a system’s ability to reject the negative impact of disturbances. Cascade Control is among them.
Cascade Control is an advanced application of the PID that can improve control of systems that are subject to significant lag. Since such systems are slow to respond to disturbances their performance can suffer with each upset. The Cascade architecture can be applied effectively to such sluggish processes when a related and faster responding loop is available. When applied in concert the faster loop serves as an early warning mechanism that buffers the impact on its slower counterpart, allowing for smoother control and enhanced performance.
Some thoughts to keep in mind when considering Cascade Control:
- Mind Your P&ID
A variety of term pairings are used to describe the Cascade configuration. Whether Primary-Secondary, Master-Slave, or Outer-Inner, these pairings may not jibe with a process’ physical attributes. For instance, the loop controlling the temperature of a jacket that surrounds a reactor vessel would not be the Outer loop in a Cascade structure. Although the jacket is on the outside, it serves as the Inner Loop. The correct configuration can be determined by referencing a Piping & Instrumentation Diagram (P&ID).
- Faster, Faster!
In order to function properly Cascade requires a loop – the Inner, Secondary or Slave – that is capable of responding to changes at a rate 5x-10x faster than the Outer loop. When architected properly, the speedier Inner loop uses the Controller Output from the Outer loop as its Set Point. This allows the Inner loop to respond directly to disturbances that affect the Outer loop. Given its responsiveness, the Inner loop reacts more rapidly to changes than the Outer loop and it can help to counter the negative impact of disturbances.
- Sprechen Sie Deutsch?
Effective communication between and among production staff is critical and so is the communication between the Inner and Outer loops. When configuring the Outer loop it’s essential to scale the controller based on the range of the Inner loop’s Set Point (i.e. 0% – 100%). The two loops are linked and they must communicate properly if any performance improvement is to be achieved. Improper scaling inhibits the Outer loop from taking full advantage of the Inner loop’s range.
- Industry Best-Practice
Use of Cascade has grown dramatically across the process industries, and it has become best-practice to apply it to most Temperature and Level loops where a corresponding Flow loop is available. The dynamics of a typical Flow loop allow for an appropriately fast response to the slower changing Temperature/Level loop. When configured such that the Outer loop’s Controller Output drives the Inner loop’s valve position, the Cascade structure allows for improved disturbance rejection and for smoother performance.
There are numerous industrial processes – too many to list – that suffer from poor performance due to extensive lag. The slow responding nature of such processes leaves them vulnerable to disturbances. Cascade Control is one advanced application of the PID that enables them to react more quickly to disturbances and to limit the negative impact.