PI Control Is Often the Default Form of the Controller Chosen by Practitioners. Given Its Simplicity and Effectiveness – That’s a Good Thing!
Studies show that when individuals are given a set of three options they are instinctively biased to prefer the middle one. When this finding is applied to purchasing behavior a common outcome is that consumers pick the middle priced option with little-to-no rationale other than a desire to avoid being viewed as either too cheap or too lavish. It’s known as the Compromise Effect. While the PID controller offers three options – P-Only, PI and PID – the rationale for selecting the middle option is generally clear. But PI Control is not only the instinctive choice, on many occasions it is also the superior and simpler one.
PI Control seems to be everywhere in process control applications and with good reason. As shared in previous posts P-Only Control with its propensity for Offset is most often used with select processes that favor simplicity and responsiveness above all else. The PID form with its use of the Derivative Term has similar application limitations. It frequently falls victim to noise which is common in industrial applications. What’s left is the middle option – PI Control. An interesting thing about this option is that PI Control provides superior control in a wide range of industrial applications.
Consider the following applications with which PI Control is use:
- Liquid Flow Control
Many processes such as Liquid Flow Control are ideal applications for PI Control. They are critical to maintaining production rates such that any extended periods of Offset associated with P-Only Control are counterproductive. They also possess exceptionally fast dynamics for which Derivative provides little to no benefit. For this and many other flow processes PI Control effectively mitigates Offset and it doesn’t induce excessive variability in response to Process Noise.
- Steam Pressure Control
Generally speaking, pressure is another process type that is often essential to production and that exhibits highly dynamic characteristics. With Steam Pressure Control the prevailing objective is to react quickly to changes in demand from downstream users while limiting any negative effects on sensitive process’ instrumentation. P-Only Control alone is unsuitable as it is unable to adequately track Set Point, and PID Control routinely succumbs to excessive variability due to noise. In contrast, PI Control delivers improved Set Point tracking and doesn’t subject instrumentation to unnecessary wear and tear.
- Heat Exchanger Temperature Control
Like the previous two examples Heat Exchanger Temperature Control is among the most common processes applied in industry. It’s typical that the exit temperature of a heat exchanger must be controlled within a relatively tight tolerance. That alone negates P-Only Control as an option. What’s more – whether plate-and-frame or shell-and-tube – the heat exchanger’s dynamics are sufficiently fast such that Derivative is unnecessary. The PID Form should be reserved for slower responding applications such as Furnace Temperature Control.
While the psychology of choice can be a curious thing the findings don’t necessarily apply to selection of a controller form. PI Control is the most prevalent form applied in industry for good reason – it delivers reliable performance and is also reasonably simple to configure.
The next post will delve into common applications of the third option – the full PID Form of the controller.