What is Proportional-Only Control? When Should P-Only Control be Used?

Don’t Let Simplicity Fool You.  Proportional-Only Control Has a Clear Place in Industrial Process Control.

As with most everything in process manufacturing, PID controllers require practitioners to make choices. One of those choices is which form of the controller to apply on a given regulatory control application. In a previous post the pros and cons of Derivative and the full PID were covered, and now the advantages and disadvantages of Proportional-Only control are explored. Due to performance characteristics the selection of a P-Only controller can be relatively straightforward and among a practitioner’s easier choices.

P-Only is the simplest form of intermediate value control. While it provides tighter control and less oscillation compared to the more basic On-Off control, Proportional-Only lacks the ability of PI and PID controllers to correct for error more thoroughly. Even so, there are industrial applications where a P-Only controller suits the purpose. It’s important not to confuse “simple” with ineffective.

Before choosing to apply – or not to apply – P-Only control be aware of these basic controller attributes:

Simplicity

Proportional-only control is the simplest of the PID controller forms as it is limited to a single tuning parameter – Controller Gain (K_C). With only one adjustable parameter the P-Only controller is easier to tune for “best” performance compared to other, multi-parameter forms of the PID. That said, P-Only control has its limits when tasked with meeting the complex requirements of many industrial production processes.

Offset

The primary drawback of P-Only control is its propensity for Offset. Offset is a sustained difference between a loop’s Set Point and its input. It typically results when the Set Point is changed without re-baselining or when the process encounters a sustained disturbance. Proportional-only control is not well equipped for handling prolonged changes to a control loop’s design level of operation.

Cascade

Offset notwithstanding, P-Only control can be ideal when applied to the inner loop of a cascaded control loop architecture. The characteristic responsiveness of an aggressively tuned P-Only controller quickly counters process disturbances. Configured for P-Only control the inner loop can deliver effective disturbance rejection and enable the outer loop to maintain steadier control.

Surge

Like the cascade application, Proportional-Only control can be highly effective when applied to surge tanks. The goal of a surge tank has little to do with maintaining tight control over the tank’s level and everything to do with providing steady liquid flow out of the vessel. P-Only level control allows the process to accommodate upstream volatility and thereby meet the process’ primary objective without the risk of inducing oscillations.

Choosing the right form of the PID should be based on the control objective of the process at hand.  In spite of its limitations, Proportional-Only control provides clear benefits when used in appropriate applications.  For additional insight into P-Only and other forms of the PID controller, consider taking a training course from a recognized authority on process control.