P-Only Control Plays an Important Role in Equipping Industrial Processes for Effective Disturbance Rejection
“When your only tool is a hammer, every problem looks like a nail” – a concept attributed to Abraham Maslow. Such can be the situation with process control and the PID controller. For decades practitioners have applied the PID to tackle the majority of challenges related to process control. Fortunately for practitioners the PID is more like a Swiss Army Knife as it can take different forms. The focus of this post is on a pair of applications of P-Only Control – or Proportional-Only Control.
A general description of P-Only Control was provided previously. As noted in that post the primary challenge is Offset – a sustained difference between a control loop’s Set Point and its input. While Offset presents a challenge when changes to a control loop’s design level of operation are prolonged, there are a range of process applications where the effect of Offset is negligible. In those applications P-Only Control can shine due to the PID controller form’s simplicity and speed of response. It’s in these applications where the advantages outweigh the disadvantages…or when other elements of the control strategy provide suitable compensation.
Consider the following examples of P-Only Control:
Inner Loop Cascade Control
P-Only Control is well suited for many cascade applications as it provides an effective means for counteracting upstream process disturbances. Within the cascade architecture it’s important to note that the Controller Output of the outer loop serves as the Set Point of the inner loop. The inner loop is able to see and react to disturbances ahead of the outer loop. When tuned aggressively using the P-Only configuration the controller is able to react faster than the outer loop and thereby take the brunt of disturbances. While overshoot – even significant overshoot – can be expected its impact is confined to the inner loop. Any offset associated with use of a P-Only controller is typically corrected by the outer loop as long as the outer loop is not excessively slow.
In the cascade architecture the Controller Output of the outer loop serves as the Set Point of the inner loop, enabling the inner loop to react to disturbances ahead of the outer loop.
Surge Tank Level Control
Level control of a surge tank is similar to that of the inner loop cascade control example above. A surge tank is designed to protect other downstream processes from the negative effects of upstream disturbances. Surge tanks routinely apply level control to mitigate harmful variability of liquid as if flows downstream. When configured with a P-Only controller a surge tank is able to respond effectively to large incursions of liquid and it is easily tuned for suitable level control. While the precise level in the tank is not a critical concern, it is essential that the surge tank’s level is maintained near the middle. By doing so the negative effects attributed to Offset are not a factor.
These are but a pair applications where P-Only Control can prove advantageous. While there are limitations to the P-Only form of the controller in its ability to control certain processes, it should be clear that there are others where the advantages outweigh the disadvantages. What’s more it should be evident that P-Only Control can be part of a larger strategy that compensate for issues such as Offset.
Other posts have explored the nuances of the Integral Term and the Derivative Term. The next pair of posts will cover industrial applications for both the PI form and PID form of the controller.
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