When Cascade Control Isn’t the Right Solution

cstr-cascade-bigSometimes looking for the right answer can be difficult especially within the realm of process control.  While it can be easy to identify solutions that satisfy some requirements it can often be a challenge to find options that meet multiple success criteria.  Something usually has to give.  Indeed, sometimes it’s easier to consider the reasons not to do something.  Applying a philosophy of ‘no’ can be particularly helpful when considering Cascade Control.

Previous posts covered the fundamentals of Cascade Control along with a comparison of the Pros and Cons associated with applying this advanced control strategy.  This post takes a contrarian’s view – the philosophy of ‘no’.  It offers reasons for eliminating Cascade Control as an option for improving process performance if only to postpone other difficult questions related to ROI.

Before delving into the economic benefits of Cascade Control, start by considering the following:

  • Secondary Variable

Lacking a secondary process variable that influences the primary control loop is a non-starter for implementing Cascade Control.

The secondary control loop provides the much-needed early warning variable to an otherwise slow process.  Through its influence on the control of a given process the secondary control loop speeds the response to disturbances.  Without access to such a secondary variable Cascade Control is not an option.

  • Faster Dynamics

If the secondary variable isn’t notably faster than its primary counterpart, then there is no performance gain to be realized.

A fundamental challenge of the primary control loop is its slow behavior whether due to an excessively large Time Constant and/or Dead-Time value.  In order to accelerate the process’ ability to correct for disturbances the secondary loop must offer a speedier response.  In fact, the secondary loop must have dynamics that are at least 3-5 times faster than those of the primary loop.

  • Shared Resource

Look elsewhere if the secondary variable doesn’t share the same Final Control Element (FCE) that’s used by the primary loop.

In order to function properly the secondary control loop in a Cascade Control schema needs to be nested within the primary loop.  While that secondary loop takes the lead in responding to disturbances by adjusting the FCE, the primary loop still makes meaningful adjustments as well.

If the answer to any of the three (3) criteria outlined above is ‘no’, then Cascade Control is not an option.

However, if each answer is ‘yes’, then it would be worthwhile to perform an economic assessment of applying this advanced control strategy.

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