Not many processes are completely linear or follow a simple, straight-forward control narrative. Even so, the PID controller has proven to be a highly flexible tool for regulating industrial processes in most environments. Indeed, traditional applications of the PID satisfy the lion’s share of today’s process control requirements, applying different combinations of the Proportional, Integral, and Derivative terms as appropriate to maintain safe, efficient control.
More creative applications of the PID capitalize on its inherent flexibility and allow it to solve other, more complex control challenges. Consider how some processes like those involving both heating and cooling require a single PID controller to drive two different Final Control Elements (FCEs). The most common approach for this type of control challenge is to apply split-range control. The split-range strategy essentially breaks the Controller Output (CO) from a given PID loop into two separate ranges. It allows a portion of the output range to drive one FCE while allowing another portion of the output to direct a secondary FCE. Beyond Temperature control, this strategy has proven particularly effective in regulating other processes such as Pressure and Flow control.
When considering split-range control for your production environment, there are a few factors to keep in mind such as:
- Limitations of Traditional PID – The PID has proven its effectiveness though within a limited range. A traditional PID can be less than optimal when controlling a wide range of operation as most processes exhibit non-linear dynamic behavior. Processes demonstrate significantly different behavior at their operational extremes which forces a compromise in overall control.
- Two Ranges, Multiple Variables – Though split-range only accommodates the measurement of one Process Variable (PV), multiple Manipulated Variables (MVs) and associated FCEs can be involved. Under the framework of this strategy, the additional FCE resources are generally limited to controlling a process within two distinct ranges of operation.
- Drawing A Clear Line – When the output is split between competing dynamics (e.g., heating vs. cooling), it is generally recommended that one FCE should close completely before the other FCE begins to open. This prevents any waste of energy. In practice, it may be necessary to overlap the heating and cooling functions if the valve position exhibits deadband at the crossing point. If the split-range is using 2 different-sized valves to handle large and small flow conditions, then the cross-over point can be adjusted to establish some degree of linearity.
Split-range control is one among many alternative options for regulating a complex production process through the application of PID. To learn more about applications of PID – both basic and advanced – scan the various topics on our blog.
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