The Pros and Cons of Cascade Control

As shared previously Cascade Control is an advanced application of Single Loop Control.  Through the use of a secondary and faster PID control loop, practitioners can improve a given process’ ability to correct for known disturbances.  Although it is considered an advanced strategy, Cascade Control is commonly used across the process industries.

Once the fundamental requirements are understood an important next step is to determine whether or not Cascade Control is the right solution for a given process.  The decision to implement Cascade Control can be evaluated simply in terms of Pros and Cons.  To help with an assessment consider the following:

Pros:

The goal of Cascade Control is to improve process performance by reducing – or even eliminating – the effects of a known disturbance through control of an early warning variable. The following benefits are achievable through the application of Cascade Control:

• Loops that correctly employ the cascade architecture respond more effectively to disturbances. This is because the inner loop is both closer to the source of the disturbance and faster than the outer loop. That combination allows the process to correct for upsets more quickly.

• The inner loop helps to correct for nonlinearities such as Stiction that are associated with the Final Control Element (FCE). Recall that the inner and outer loops rely on the same FCE.  Due to its faster dynamics the inner loop adjusts for FCE nonlinearities ahead of the outer loop, thereby minimizing negative affects to the process.

• A faster inner loop reduces the overall variability experienced by the process. Since the inner loop is able to respond more quickly to disturbances than the outer loop, it reduces the severity of a given disturbance and limits the degree of variability that would otherwise impact the process.

• The outer loop can be tuned more conservatively. Since the inner loop makes more rapid adjustments the outer loop no longer needs to be nimble. The steadier control benefits downstream processes.

Cons:

The downside of this advanced control schema can be summarized in terms of cost and complexity.  The following represent the negative aspects of Cascade Control:

• Instrumentation costs nearly double with the implementation of Cascade Control. The architecture requires the installation and use of a second sensor to measure the inner process variable.

• Configuration costs are nearly double as well. The added hardware costs must be complimented by increased installation and configuration costs.  Tuning the two PID controllers to deliver optimal responsiveness can take considerable time and effort.

• There is potential for increased wear and tear on the process’ FCE as the inner controller is typically tuned aggressively to enhance disturbance rejection. This cost can be difficult to calculate as it depends on the configuration of the two controllers (i.e. P-Only, PI, etc.) and the work that the FCE is required to perform due to cascade.

Beyond the potential for smoother final process performance there are other financial and support considerations that should be considered.  Most training workshops focused on process control cover this along with other basic and advanced applications of the PID.  They often provide other, valuable insights for assessing the pros and cons of Cascade Control.